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Welding Titanium to Stainless Steel
[From PWL #33]

Q: How is Titanium welded to Stainless Steel?

A: Titanium and Titanium alloys cannot be fusion welded to Stainless Steel. However special solid state processes that do not resort to fusion, can be used to join the two materials for preparing Transition Elements.

Among these processes are friction welding, explosive welding, ultrasonic welding, magnetic pulse welding, coextrusion or roll welding and other methods.

Regular fusion welding processes can then be used to weld each end of the transition element to the part of the same material needed to accomplish the required assembly.

Welding Titanium Clad Steel
[From PWL #23]

Q: - How is Titanium Clad Steel welded on itself?

A: - From the Article reported in Section 11 in PWL #023 we quote: "Fusion welding [the new product] is not critical to the same degree [as conventional explosively clad plate] and conventional methods of fusion welding the joints of titanium and zirconium [clad steel] plates are significantly less critical.

It has even been possible to make a welded joint by stripping back a minimum amount of titanium, fusion welding the steel substrate, covering the exposed steel with a weld deposited layer of silver and weld depositing titanium onto the silver. This gives a continuous and smooth joint profile on the titanium surface and an unbroken, continuous bond at the cladder/substrate interface."

Welding Titanium to Cobalt base Alloy
[From PWL #36]

Q: How are welds performed between Cobalt base Alloy ASTM F75 and Titanium Grade 2 and how are the joints tested?
Note - The question, actually asked by one of our readers, refers to research on implants for medical applications.

A: Cobalt and Titanium cannot be successfully fusion welded to each other but they are currently joined by friction welding, provided that the shape is favorable for this type of process. Other solid state welding processes may also be suitable.

Both Cobalt and Titanium are being used for medical implants and it is reasonable to research their joint properties to exploit their specific advantages for special applications.

A recent paper dealt with "Investigations on the galvanic corrosion of multialloy total hip prostheses". An abstract can be read at
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12659138&dopt=Abstract

The usual testing procedures that are used for any welding are also applicable here. In particular bend testing and tensile testing are standard. In case the joints to be tested are suspected of being brittle, they can be tested by notched bend test and by impact if required. Metallographic examination is usually conducted on sectioned, ground, polished and etched specimens.

Readers interested in Materials and Processes for Medical Devices are advised that a periodic publication of the same name is issued by ASM International and included in their renowned Advanced Materials and Processes.

Gas Metal Arc Welding Titanium
[From PWL #58]

Q - Does anyone GMA(W) weld titanium on a regular basis? It sounds good in theory but I don't remember ever seeing anything in print about Joe's Weld Shop using GMAW on some heavy titanium structure.

A: - From a quick search I found that Titanium GMAW is done but still somewhat experimentally. Special means have to be employed and the technique has to be developed.

The abstract from an article found at: DTIC,states:

"There are initiatives to develop low-cost titanium materials supplies; however, low-cost and high-rate fabrication processes are sorely lacking. Welding and joining technologies enable improved manufactured components by reducing the weight, production time, and cost of joining parts. Improved welding technology increases product lifetimes and makes possible the fabrication of large structures. Gas Metal Arc Welding (GMAW) has the potential to significantly improve the quality, speed, and penetration depth of titanium welds, while reducing the cost per part. However, this result can only be achieved if proper weld parameters are selected and dynamically maintained during the welding process due to the nature of titanium."

See also the following:

Titanium Welding Technology
http://www0.nsc.co.jp/shinnihon_english/kenkyusho/contenthtml/n95/n9515.pdf

New Joining Technology for Titanium
http://www.ewi.org/uploads/document_library/white_papers/WJ-May06.pdf

Pulsed GMAW of Titanium
http://files.aws.org/wj/supplement/Zhang02-01.pdf

Novel Titanium Wire
http://www.twi.co.uk/j32k/psg/review.xtp?id=3

Welding Tin to Stainless Steel
[From PWL #46]
Note - The question was actually asked by one of our readers

Q: How do you weld tin to stainless steel?

A: You don't, due to the huge difference in melting point. If you need soldering on stainless you might pre-electroplate the stainless with tin, possibly by duplex coating (first nickel, then tin). Finally you may solder your connection to the plated tin layer.

Snapping Sounds from the Roof
[From PWL #26]

Q: In general terms, what would cause significant popping and snapping sounds from a metal deck system over a steel joist roof system?

What corrective measures would remedy that activity? The event occurs when the roof heats up in the morning and then cools down at days' end.

A: You would hear the same sounds in a yard of empty steel barrels. As you correctly assumed it is due to heating and cooling. Consider the sheet metal surfaces emitting the snapping sounds as a membrane welded or otherwise fixed along the periphery.

When cold, the metal lays in a stable position. To simplify we will call it the concave position. Upon heating up the sheet metal tends to expand, but it is restrained mechanically, by welding or otherwise, at the borders that block its expansion.

The thermal stresses generated by heating will cause its central area, that is not limited in transversal movement, to bulge, in order to reach an equilibrium position, that we will call convex.

The situation where a mechanical system is stable with minimum internal stresses, in two different positions, is called a bi-stable equilibrium. The passage from concave to convex position occurs suddenly, upon heating, when the internal stresses are just right. The reverse movement occurs upon cooling. That is the movement generating the snapping sounds.

To avoid the snapping activity one has to permit the free thermal expansion of the sheet, by freeing at least two out of the four sides. Upon expanding freely, no stresses will build up causing the sheets to bulge and no sounds will be heard.

In order to avoid compromising stability one should provide suitable restraints in other directions while permitting free expansion and contraction along the plane of the sheet itself.

Welding in cold Weather
[From PWL #30]

Q: What is the coldest ambient temperature allowed for welding carbon steel and stainless steel pipes and structure?

A: Whenever ambient temperature causes water vapor condensation upon metals, it is recommended good practice to preheat before welding up to 120 C to make sure the joints are dry.

A few authorities, specifying the conditions for welding of Bridges and similar Structures, put the coldest ambient temperature limit, below which welding is not allowed, at 0 ⁰F or -18 ⁰C. See:
http://www.thruway.state.ny.us/business/design-manual/appendixf.pdf
(page 12) and
http://www.dot.state.oh.us/testlab/StructuralSteel/Field-Welding-Inspection-Guide.pdf
(page 9)

Other authorities, dealing with welding requirements for Piping and Pressure Vessels, put the coldest ambient temperature limit, below which welding is not allowed, at 32 ⁰F or 0 ⁰C. See:
http://www.wbdg.org/ccb/NASA/NASAASC/NS15055.pdf
(page 6)

For ASME Codes and Standards, the minimum temperature for welding is generally specified at 50 ⁰F or 10 ⁰C.

Minimum temperature and preheat requirements for welding on pressure retaining items are also referenced in the National Board Inspection Code (2004 Edition), Appendix B.

Semi Automatic Ultrasonic Inspection
[From PWL #31]

Ultrasonic Testing is a mature and robust non destructive inspection technology capable of detecting small and dangerous flaws within material bodies. As such it is widely used within the welding industry for providing safe and secure proofs of acceptable process performance meeting exacting requirements.

It is based on the properties of propagation of sound waves, of such a high frequency as to be inaudible by the human ear, and on their capacity to be reflected by geometric features or by internal imperfections.

When combined with radiographic inspection both technologies supplement each other due to their somewhat different sensitivity to specific geometric details of lack of soundness.

For all of its practical success, ultrasonic inspection has one major drawback.It is labor intensive. Moreover ultrasonic inspectors require a long formal education and preparatory apprenticeship consisting in theoretical studies covering specific chapters of acoustics, a branch of physics, and a long practical training under the supervision of experienced instructors.

Finally the trained personnel must take complex examinations and obtain a Certification demonstrating their capacity to perform successful ultrasonic inspections in order to be cleared for employment by industrial contractors, according to requirements of binding Specifications or Codes.

Various attempts are being done to automate as much as possible the ultrasonic inspection of welds in specific joint configurations. Such efforts are reported in an article published in the Issue 23 of Practical Welding Letter for July 2005. It can be read by clicking on PWL#023.

When the inspected items are repetitive and the requirements, relative to acceptance conditions, are clearly cut, some manufacturers find that it is more attractive to use equipment set up only for the specific configurations needed.

In particular this attitude is successfully applied for inspecting mechanized or robotic welds as they are performed, in line, by a welder or a helper with no specific ultrasonic training.

The body to be inspected is set-up by placing it in a well defined position relative to the sensor (called transducer), and starting the test. A mechanical scanning movement may be used if necessary.

The automatic answer or inspection result is either to accept or to reject. This is made possible by electronic manipulation of the visual signal, on a screen, representing the behavior of the ultrasonic beam.

Apart from the entrance and back peaks signaling echoes from known geometric features of the body inspected, any further peak is suspected of being caused by reflections from unwanted discontinuities.

Its intensity is measured as height from the baseline (on the screen) and its position or depth in the body is inferred from the linear relationship between the position of the reflector in the body and the horizontal distance of its signal trace from the entrance peak.

A "gate" that limits the inspected volume to the location of interest for the inspected body is superposed on the signal.

Any signal appearing in the volume of interest (that is within the gate) and of intensity higher than that of an established threshold for that specific inspection will trigger rejection.

The threshold is established by reference to ultrasonic reflections from known discontinuities of definite dimensions, introduced on purpose in special reference test pieces.

Failed items can be then subjected to additional inspection by certified ultrasonic inspectors for further decision if needed.

Specific applications of the above principles were successfully employed also for routine examinations of critical aircraft details, where the integrity of certain components must be assured by periodic inspection.

The possible economic advantage of such a solution relative to a general ultrasonic testing performed by a certified inspector, has to be examined case by case by comparing the costs of equipment and operation.

Thickness Range of a Plasma Cutter
[From PWL #43]

Q: The capacity of a certain cutter is noted as 1 1/2" Manual and 3/4" mechanized. Why would the performance be cut in half when using a machine torch rather than a hand torch?

A: Plasma machines that can handle both manual and mechanized tasks usually have a lower thickness rating for mechanized cutting rather than manual. This is for several reasons.

Starting and ending the cut on materials close to the maximum thickness properly requires some operator technique that is easy for a human operator but difficult or impossible to program into a machine. Hence a lower rating for satisfactory cuts.

Mechanized cutting requires piercing in most cases rather than edge starts. A machine generally can not pierce successfully as thick as it can cut.

Customers using a machine in an automated set up would generally find the cut speeds on the thickest materials too slow to be acceptable for mechanized operation.

Note: This answer was kindly supplied by B. Fernicola from ESAB, USA.

Oxyfuel Gas Bevel Cutting
[From PWL #68]

Q: In carbon steel, welding preparation of bevel with gas cutting is advisable?

A: All suitable cutting processes must be used with correct parameters for providing bevels of acceptable quality.

Depending on the amount of carbon in the steel and on the thickness, oxyfuel gas cutting can be advisable if it permits to achieve a suitable cut quality, sufficiently smooth bevel, without excessive oxidized scale or deep decarburized layer.

It is the careful balancing of all cutting variables, more than 20 according to certain counts, that help obtain a smooth edge.

In general if bevels are finish machined there is more tolerance for eventual imperfections. Finally it is the result of further welding that helps to decide if the cut quality is acceptable or not.

Welding Effects on Aluminum Structures
[From PWL #44]

Note: The following is an actual question sent to us by a correspondent.

Q: This application is structural. It is a large (42 feet in length) box cover structure. This structure is to have 2x6x1/8 inch wall rectangular aluminum chords running the full length of the box. This 2x6 tube only seems to come in 6063-T52. This aluminum box structure also has 6061-T6 sheet for the sides and top and also 2x2x1/8 inch tubes and angles that are 6061-T6.

Problems: I cannot seem to find any allowable strength values for the 6063-T52 when it is welded - can you point me in the right direction? Also and more important is the 6063-T52 readily weldable to the 6061-T6 and is it an excellent weld as this application is structural in nature? Thank you for any help you can give me.

A: In principle welding heat will destroy the mechanical properties of heat treated aluminum alloys and revert them to those of annealed condition.

The Minimum Tensile Strength of Welded Aluminum Alloys with no post-weld heat treatment, is listed in Table 5.16 at page 232 of AWS Welding Handbook 9th Edition. For the above materials the following data are reported:

6063-T52: 17 ksi = 115 MPa
6061-T6 : 24 ksi = 165 MPa

It is true that some of the properties of heat treatable alloys like 6061 and 6063 could be recovered by accurate reheat treatment if correct filler metals were employed, but this is not a feasible option for a large structure.

The welded solution is therefore applicable if the lower strength is considered adequate in design. The allowable load on the joint is established by the orientation of the heat affected zone relative to the stress direction and by its percentage relative to the whole section.

The only way out, permitting to exploit the improved mechanical properties of the above products in their as received heat treated condition, would be to implement proper mechanical joints without welding.

If it is decided to design suitable joining elements to be assembled later in the larger structure by mechanical fastening without welding, the joints themselves could be fabricated in heat treatable aluminum alloys by welding, and should be heat treated before mounting in the structure.

The last warning is that one should beware from having aluminum making contact with other materials because of the danger of galvanic corrosion, except if proper insulation materials are employed.

Welding with Robots
[From PWL #48]

Q: - I have a setup of 6 Mig Welding Robots for welding Car Seats Frames. I have lots of down time because of Arc outs, changing tips. It gets worse during cold weather. What can be done to improve?

A: - It is not uncommon for industry at large to experience breakdowns of robot lines. One way to overcome the difficulties is to grow in-house expertise of a few technicians. That is done either by sending them to follow special Mig (GMAW) courses or by hiring an instructor for the needed time.

The personnel involved should master welding expertise before starting to program robot cells because a sound theoretical background in GMAW is essential.

It is usually recommended to develop first a robust manual welding procedure for the robot parts, and then to prepare the robot program while adapting the optimized techniques used by the manual welder.

This assertion, although quite true, should be improved by making sure that the basic manual welding cycle is cautiously but firmly modified in order to exploit the robot increased productivity capabilities relative to manual processing.

In particular, while assuring quality as required, one should make skillful use of the parameters that robots can accommodate like larger wire size, higher current, higher weld speed and swift relocation between welds. Attention should be paid also to control of part distortion, minimizing spatter and establishing the most suitable sequence.

It would be a costly mistake to entrust the job of running robot cells for GMAW to personnel skilled primarily or only in robot programming. To achieve the maximum benefit from robot installations, Management should understand the special requirements of robot cells in welding environments.

Among these are parts design, part and gap tolerances, fixture design, weld process expertise and quality of the operating programs. Furthermore expert maintenance personnel should be readily available whenever needed.

Cut Pie Welding
[From PWL#054]

Q: - I have a situation where several plates are coming together at a point in the fashion of a cut pie.
The plate thickness is 5/8" ABS (American Bureau of Shipping) Grade A steel.
This design has all points of the pie being welded together and I know it will crystallize at the vertex, due to overheating.

Where is it noted that this design should never occur, due to the crystallization of the material at all points of intersection?

A: - The design you propose looks to me problematic, not because of crystallization (which is not a metallurgically defined defect but means other things, see further down 9.2 [in PWL#054]) but because it may be difficult to assure full weld penetration and because a triaxial state of stress (to be avoided) risks being formed in the place.

It would be better, if possible, to stop the plates short of the vertex and butt weld each plate to those on its sides, with bevels as needed, leaving a circular hole in the center of the pie, to be covered if needed by a suitable independent cover or cap.

Fumes from Welding Zinc Coated Steel
[From PWL#055]

Q: I need to know how to find the type of fumes which are created when spot welding steel sheets with zinc coating.

A: You may have a look at

Metal Fume Fever
http://files.aws.org/technical/facts/FACT-25.PDF

and at two other publications:

Resistance Spot Welding of Zinc Coated Steels
http://www.spotweldequip.com/FFA%20PDF/15.pdf

Handbook for Resistance Spot Welding
http://www.millerwelds.com/pdf/Resistance.pdf

Although not lethal, it seems better not to breath those fumes...

Welder Qualification
[From PWL#059]

3 - How to do it well: Welder Qualification

Q: - I have a query related to welder qualification criteria addressed as below:

Background: A welder has deposited E 7018 (F4) electrode on a 6" X Sch 40 test coupon with backing. According to QW-452.1(b), this welder is qualified to use F4 electrodes up to 14.22mm with backing. According to QW-433, this welder is also qualified to use F1, F2 & F3 with backing.

Question: If a production joint having 14.22 mm thickness with backing strip is to be deposited using E7024 (F1), E6013 (F2) & E6010 (F3) electrodes, do we have to re-qualify the above welder in all the three "F" numbers individually?

A: -

1) - 17 Jun 08 13:49
As you point out, per QW 433, F4 qualifies for F1- F4

2) - "do we have to re-qualify ...?"
17 Jun 08 14:05
No. See QW-353 in Article 2 and QW-404.15 in Article IV in the 2007 Edition of ASME B&PV Code, Section IX.

3) - 20 Jun 08 18:36
The only case in which the electrodes do NOT qualify for the lower F Number is in cases in which there is NO backing. F4 open roots qualify for F4 open roots only and F1,2,3 with backing.
See QW-433

Note: The above question from a reader was submitted by us to:
http://www.eng-tips.com/index.cfm
whose Contributors provided the above answers which are gratefully acknowledged.

Threaded Hole Repair
[From PWL#059]

Readers called our attention to their need to repair damaged or badly worn out threads inside holes.

While a common cure for this type of damage cannot be of general character, because each case depends on the materials involved and on the application, it is possible to hint to a few solutions that must be studied in depth to check their applicability to the prevailing service conditions.

Welding should not be considered as the first and foremost solution. On the contrary, it should be appreciated that usual fusion welding introduces much heat, with consequent development of considerable stresses and deformations.

Furthermore one should have complete knowledge on materials and condition, otherwise the part to be restored might be damaged beyond repair, so that welding should be avoided whenever possible.

Some investigation should be devoted to understand the causes of failure, which may be due to galvanic corrosion, to wear from long use or from vibration, and every effort should be applied to avoid the occurrence of the same failure again.

Apart from the obvious application of oversize bolts, which may fit oversize holes with new threads, one could possibly consider the application of HeliCoil which are commercially available Screw Thread Inserts. See: http://www.emhart.com/products/helicoil.asp

In this page on Welding FAQ, under the title: EBW Repair of a rejected Casting we reported on a case that could have some similarity with the problems addressed in this section.

Depending on application and service conditions a new threaded sleeve might be fixed in place by suitable adhesive bonding instead of EBW.

How to Attach Nuts to a Zinc Alloy Hub
[From PWL#060]

Q: I am looking to attach some hex nuts to a handwheel hub made from ASTM/ANSI DIN1743 Z410 zinc alloy. Can this material be welded? If so, what material should the nuts be and what type of rod should be used? This is a fairly low-strength application using a 3/8" hex nut being driven with an electric screwdriver. Any help?

A: Try any suitable structural adhesive.
See our pages on Adhesive Bonding and on Joining Lead Tin Zinc.

Designing a Hinge
[From PWL#062]

Q - How to design a hinge for the rear of a platform dump, if running into clearance issues with the swinging part?

A - Take a clear new sheet of paper. Mark a small cross in the center. Use your compass to draw a circle representing your hinge. Draw the fixed section of your application either at 1:1 or in scale.

On a separate transparent paper, draw the section of the rotating part at the same scale, starting from the circle that represents the hinge.

Now take the transparent paper with the drawing of the moving part and put it on the drawing of the fixed part so that the circles of the hinge match exactly. Pierce with a pin the center of the hinge of the moving part so that it can move around the pin at the hinge center.

You can now swing carefully the moving on the fixed part. Examine for eventual interferences and unwanted clearances. Modify the drawing until you are satisfied with the result, and then build it.

Welding High Tensile Bolts
[From PWL#063]

Q: I am trying to find out if you loose tensile strength of threaded rod/bolts if you weld them end to end (to gain extra length)? The rod is high tensile steel.Also what sort of weld should be carried out?

A: Don't weld high strength bolts. As you suspect you will loose strength. Welding heats the material and destroys the mechanical properties obtained through heat treatment.

There is no suitable welding method that will preserve strength. If welding is performed however, depending on the type of steel, the mechanical properties could be partly restored only by repeating completely the original heat treatment.

Cutting Pipes and MPI
[from PWL#065]

Q: Should one perform Magnetic Particles Inspection before welding, after cutting pipes by any abrasive process?

A: The use of abrasive discs for cutting pipes (or any other item)(but not abrasive waterjet cutting) can heat the edges to quite high a temperature, unless cooling water is used to flood the place.

While mild steel will not harden (and will not crack) upon being heated by abrasive cutting even if not cooled, alloy steels subjected to uncontrolled heating can crack because of self quenching that generates untempered martensite.

Therefore, for materials susceptible to cracking, the absence of cracks due to this process should be controlled by MPI if quality production has to be assured.

Even if this requirement is not spelled out by applicable codes, it would be a matter of good workmanship to establish safe practices for the process and/or conservative measures for inspection.

A: You could try to use through passing rivets or bolts as per sheet
Tips for Processing.

But how will the joints behave under load?
Probably adhesive bonding, also mentioned in the above sheet, is likely to be more suitable, as it is used in aircraft and in car manufacturing. Even Laser welding is proposed there for joining, although without details.

I see that data on Bending Stiffness against Surface Density are reported, more important for crashworthiness properties than classic tensile testing data.

What Tungsten is used for Titanium Tig?
[From PWL#071, Section 3]

As it is widely known, available tungsten electrodes used in Gas Tungsten Arc (or Tig) Welding can be made of a few different types of powder metal compacts.

For Titanium welding the conventional thoriated Tungsten electrodes known as EWTh-1 and EWTh-2 are used (with 1 or 2%Thorium respectively), ground to a point.

Welding-titanium is done with straight polarity direct current (tungsten electrode connected to the negative pole).

See also Welding Titanium.

Welding Lead
[From PWL#073, Section 3]

The following comment was sent by Mr. Timothy Lynch
President of Kenneth Lynch & Sons from the United States.

Date: 12 Aug 2009

Sir:
I read with interest your treatise on welding lead.
Well done, except that we use oxygen and hydrogen mix to weld lead (statuary).

The reason is that oxy-acetylene will cause the lead puddle to "pop" or blow out, sometimes causing burns if the spatter lands on your skin.

This never occurs with oxy-hydrogen mix. All other problems remain as described.

s/ Tim Lynch

(The reference is to my page on Joining Lead Tin Zinc).

Brazing Flux Removal
[From PWL#075, Section3]

Q - Ran into an horrendous problem removing brazing flux - is there a process that would reduce the level of aggravation and labor? The metals involved were German silver and a stainless that is extremely tough. The object was to braze a guard to the blade.

A - You probably used a flame for brazing. Overheating should be avoided. In general flux removal should be done immediately after brazing, and the method used is generally hot water rinsing, possibly with soft brushing.

Immersing the brazed joint in water before full cooling, helps flux removal, if not objectionable for other reasons. Leaving the flux on, causes it to oxidize making it a form of glass, more difficult to remove.

Pickling solutions or chemical cleaning is available. Mechanical means are a possibility, including fiber brushing, wire brushing, blast cleaning and steam jet. Next time you may consider using a suitable stop off, to limit the area where flux is spread, heating as low as possible and removing flux immediately.

Selecting Carbide for Hardfacing
[From PWL#077]

Q: We use Tungsten Carbide to hard face our steel and I recently heard from someone that Vanadium Carbide cost 30% less, you get 30% more coverage per pound. Is there any truth to this and if so is the abrasion resistance as good as tungsten carbide. Any info you have would help us a lot. Thanks.

A: While it is essentially a good thing to try to improve processes and reduce expenses, one has to be very cautious before changing processes working satisfactorily. In particular one has to be extremely skeptic when someone (not better defined, could have hidden interests?) throws in an idea whose reckless adoption might cause much damage.

There is more to Hardfacing than the carbide powder used. The abrasion resistance of the final application depends much on the type of thermal spray process, on actual spraying parameters, on hardness but also on adhesion and on the type of materials your equipment is called to work on.

Tungsten carbide is called the ultimate in abrasion resistant qualities. If you are interested in comparing two different carbide powders, you should set up an experimental study for your specific applications and draw conclusions from practical results.

Braze-Welding of Steel
[From PWL#078]

Q: We have to complete various tests on "coupons" fitted together. On 2 pieces of very thin 1/16" metal, I sheared them off at 8 inch lengths (3" wide) and went into the grinding room and ground everything that the braze-weld would touch. I tacked both outer sides (which looked excellent) and then proceeded to braze-weld. I had a thin layer of bronze on the top side, no penetration and then after 6 inches the whole length of the weld cracked right down the middle of the weld.

A: The above description does not mention essential steps of the process. Both sides of the joint should be generously covered with flux before even starting to braze-weld. The groove gap between the parallel coupons should be between 1/32 and 1/16". The oxyacetylene flame should be slightly oxidizing as explained in the following ESAB Handbook-Braze Welding page at
http://www.esabna.com/euweb/oxy_handbook/589oxy14_1.htm
and then use the right arrow to see the following pages.

Finding the right base metal temperature for successful wetting (ESAB calls this "tinning") of the base metal by the brass filler metal may be the most tricky part of the exercise. They suggest to try a bead on plate for getting the feeling.

To obtain penetration of the filler metal in the joint one should have the bottom side of the joint reachable by the flame, and then alternatively manipulate the torch from both sides without overheating the base metal.

Trying to do this again and again is the only way to reach the needed skill.

Inside Tube Inspection
[from PWL#082 - Section 3]

Q: How can I perform visual inspection of welds or corrosion condition from the inside of tubes or small containers with no direct line-of-sight access?

A: You should procure long and thin optical instruments called Borescopes. There are many types, portable, rigid or flexible if made with optical fibers, and they contain an autonomous light source to illuminate the area to be inspected. Sophisticate equipment may include a camera and transmit the view to a portable TV screen.

There use is invaluable but the inspector needs to get training and some experience before being able to perform reliable inspections.

Interested readers can browse manufacturers' catalogs to find the instrument best adapted to their requirements.

Safety in Hydrogen Furnaces
[from PWL#083 - Section 3]

The Hydrogen protective atmosphere has many useful applications in furnaces for metal processing, because of the reducing properties of this light gas. It is therefore widely used for bright annealing processes and for brazing stainless steels and nickel alloys.

However it has a number of safety issues because Hydrogen gas at 1 atm is flammable in the concentration range 4–74% (volume per cent of hydrogen in air) and is explosive in the concentration range 18.3–59% (volume per cent of hydrogen in air).

Therefore precise procedures must be followed when starting up and when shutting down, because it is in those transition periods that air may find a way to enter the furnace and form an explosive mixture with hydrogen.

Security alarms and features should inhibit the start of any operation if the gases are low in volume and/or pressure in their respective containers, or if any of the valves is found faulty.Leaks from the Hydrogen line are particularly dangerous and therefore frequent checks should verify their complete absence. Hydrogen sensors must be used for rapid detection of hydrogen leaks.

The air present in the furnace space must be removed before admitting Hydrogen gas. That is done by purging the furnace with an inert gas, usually nitrogen. Depending on the furnace build and function (batch type or continuous) a slight overpressure will always be maintained to avoid air leaking in.

Hydrogen excess and that portion expanding due to heat, will be vented at the highest point and lit with a pilot flame to burn quietly in air (producing drops of water).

Also at shut down, the inert gas is admitted to displace Hydrogen, heat is removed and the furnace is let cool down. Only then doors are opened and air is admitted, to unload the treated parts.

The security issues to take care of are non programmed black out occurrences (interruption of electricity flow), interruption of cooling water flow, if used to cool down gaskets and doors, interruption of Hydrogen gas supply or sudden loss of availability of purge inert gas. A spare inert gas container must be easily accessible and operational with a few valve manipulations.

For each of these emergencies it is imperative that automatic shut down procedures step in without manual intervention and overtake any other standard operation. The automatic planned shut down must include positive electric power interruption, positive hydrogen flow interruption, and admission of inert gas through a normally open valve (that opens when power goes off).

As accidental explosions may be extremely dangerous, one should have in place back up systems, a thorough maintenance plan of periodic equipment tests, and a good training program frequently rehearsed.

If properly planned, executed, maintained and controlled, hydrogen furnaces are not any more dangerous than other industrial equipment and can provide essential contribution to production facilities.

See:
Hydrogen Safety
http://www.hydrogen.energy.gov/pdfs/doe_h2_safety.pdf

Disposing of Arc Strikes
[from PWL#084 - Section 3]

Arc Strike is a surface discontinuity caused by a localized application of an electric arc. It appears as remelted or heat affected material or as a surface change. During welding it may be caused by arc initiation not exactly where the weld puddle is formed.

Depending on the material and the application, the blemishes appearing on the spot need to be cleaned and removed because they include remelt material, hard spots and possibly cracks. In case of low carbon steel it will be enough to grind out the surface lightly to remove the apparent discontinuity.

For medium or high carbon steel, removal of the heat affected zone is required by grinding to some depth, to be sure that no spots with untempered martensite will remain near the affected surface.

An interesting point was clarified by Damian J. Kotecki in his Stainless Q&A note published at page 14 in the July 2010 issue of the Welding Journal. There the reference is to the Duplex Stainless Steel type 2205 which is not hardenable.

Answering to a concerned reader who was annoyed by the insistence of an inspector who requested the removal of the complete heat affected zone, Mr. Kotecki justified the inspector's request explaining that in the said material arc strikes generate locally almost 100% ferrite in the HAZ.

By a complex chain of events, the nitrogen (austenite former) which has no time to reach the austenite and diffuse there, precipitates as chromium nitrides in the large ferrite grains. Therefore the ferrite grains remain depleted of chrome and prone to corrosion.

Interested readers are urged to see the original note. In conclusion arc strikes should be avoided but, if present, they should be thoroughly removed.

Change of Material
[from PWL#085 - Section 3]

A very interesting note was published on page 18 of The Welding Journal for August 2010. A reader reported on a high leak rate found in manually brazed distribution systems and asked for possible causes. Originally the systems were made of copper tubing. Recently however, in order to save on material costs, a switch had been made in some systems, substituting copper plated steel tubing for copper tubing.

In his very detailed and reasoned answer, Tim P. Hirthe, the brazing expert, while conceding that the material substitution could have been a good idea, warns on the differences in thermal conductivities, that could cause steel tubes to overheat. In that case the copper coating could become readily damaged or removed, compromising the brazing application.

Additional warnings concern the presence of phosphorus in certain silver base brazing filler metals, intended to deoxidize the copper surface, but producing brittle compounds if let to alloy with iron, and also the difference in Coefficient of Thermal Expansion (CTE)(between copper and steel) making it quite difficult to maintain a suitable clearance between elements at brazing temperature.

The problem is multiplied by having a team of 15 persons performing manual brazing, difficult to train for the special requirements and difficult to supervise. A possible substitution of filler metal is proposed, one suitable for larger than usual clearances.

Summing up, the brazing expert, considering the stricter brazing procedures changes that should be made and the increased risks of failure, asks if it would not be more cost effective after all to stick with the old material.

While it is true that no progress would be possible if old procedures were never questioned, looking only for savings at the material level without taking into account possible process implications, risks to procure more damage than benefit.

Interested readers are urged to seek the original publication, whose details are described above.

Proper Water Cooling of Spot Welding Electrodes
[from PWL#087 - Section 3]

Practical notes are often most useful in that they address common problems. I usually pay attention to the Q&A appearing in the Welding Journal because there is frequently something to learn. In the November 2010 issue at page 16 there is an explanation on the importance of the correct mounting of water tubes in resistance welding electrodes.

The adjustable tube extensions should be so mounted, by sliding them on their support as necessary, that they reach the bottom of the electrode internal cavity. The tube end should be cut at 45 degrees to make sure that water flows unhindered without forming steam pockets.

Cooling water should be circulated as near as possible to the tip to be effective in reducing the copper temperature, to keep its strength as needed during the forging cycle when large pressures are applied.

Keeping electrodes cool prevents copper softening and tip mushrooming, maximizing electrodes life and improving spot weld quality. Either sliding or spring loaded tubes are used. When becoming distorted or otherwise damaged they must be substituted with new ones and regularly maintained.

The note recommends to use straight electrodes whenever possible. If not, offset tip holders with straight tips should be preferred. To further enhance the importance of the subject, the author of the note, Tim Snow, refers readers to a research paper titled "Influence of Water Temperature and Flow on Electrode Life" available at
www.unitrol-electronics.com from the Download section.

Interested readers are urged to seek the original article from the source mentioned above.

Increasing the Weld Deposition Rate
[From PWL#088 - Section 3]

Weld Deposition Rate directly affects the productivity of the operation. Therefore it should be considered as one of the most important factors for satisfactory financial results.

For the processes employing continuous wire as filler metal, like Gas Metal Arc Welding (GMAW or Mig), Flux Cored Arc Welding (FCAW) and Submerged Arc Welding (SAW), there is an easy way to increase the Deposition Rate, with minimum adjustment of parameters.

One recommended but under-used way of increasing the Weld Deposition Rate without incurring in unacceptable defects like burn through, is to increase the Electrode Extension called also Wire Stick Out (WSO), representing the free length of wire between the contact tip and the arc.

The reasons underlying the dramatic change in performance produced by this simple change are explained in some detail in the article in section 11 in Issue 88 of Practical Welding Letter for December 2010.
Click on PWL#088 to see it.

That note refers in particular to SAW, but the principles are applicable also for the other mentioned processes.

Slight adjustments may be needed for other parameters, in particular the voltage may need a small increase to make up for the additional voltage drop along the added length of electrode consequent to the increased electrode extension.

Increased deposition rate, due to the higher resistance heating of the electrode between the contact tip and the arc, is the main advantage. Other advantages include lower heat input, higher impact properties, narrower heat affected zone, decreased penetration and lower dilution levels.

If burn through was a problem before increasing the extension, it may well be solved with this technique. It is true that some experimentation is needed to find the best set of parameters but understanding the basics should improve the performance.

In any case it should be one of the first changes to be attempted when working on improving the productivity of any welding operation, large or small.

Find if an aluminum alloy is weldable or not
[From PWL#089 Section 3]

Good that you asked. It is better to pause and to inquire to get the answer before welding, otherwise one may make irreversible damage to whatever was incautiously welded.

Weldable is to be understood by fusion welding, that is by processes that use an electric arc or a flame to cause melting of the alloy.

Aluminum alloys are also classified as being or not Heat Treatable by Solutioning and Precipitation Hardening. For those alloys that are both heat treatable and weldable like 6061 and similar alloys, additional heat treatment may be needed to restore mechanical properties after welding.

Heat treatable high strength aluminum alloys like 2024 and 7075 which are not successfully welded by fusion welding, are currently welded by resistance spot welding.

One should know which is the material one wants to weld. In case the material is new and identified by a standard designation, it is easy to find in handbooks or from the supplier its welding characteristics.

If you consider repairing an aluminum object whose designation you ignore, first look on it to see if it shows signs of original welding. If it was welded it is weldable. If it has only spot or seam resistance welds, it is probably not fusion weldable.

If the original material is not known, the family to which it belongs should be determined at least by qualitative analysis, preferably by X-ray fluorescence methods. See Material Identification.

Hardness testing should also be performed to determine the material condition.

See also:
Aluminum Welding
and
Welding Aluminum, Reprint from HIWT

Please be advised that special aluminum alloys, called Metal Matrix Composites, including reinforcing particles of various types are not considered weldable by common means. See Joining Aluminum MMC.

Also a different class of aluminum base materials for moderately elevated temperatures, called Dispersion Strengthened Aluminum Alloys and introduced further down in Section 7 in this issue of PWL, are not considered weldable unless special procedures are developed and applied. To the same subjects are devoted the links presented in the Mid Month Bulletin, the section appended at the end of PWL#089.

Welding Unknown Materials
[From PWL#091 - Sect. 3]

Q: I am attempting to tig weld together two apparently dissimilar aluminum alloys.
Also, one of them has a 3mm wall thickness and the other one a 10mm thickness.
Neither piece is cast, but both are billet machined.
I cannot start an arc pool on the thicker material, do I need to preheat the part, and how do I avoid oxidation during the pre-heating process?
I am worried about using higher current flows because I do not want to destroy the more delicate piece.

A: Why would not you try the right way to do welding?
You have unknown materials. Please identify them.
Only then, once you know what materials they really are, can you look for a suitable welding method.
See my page:
http://www.welding-advisers.com/Material-identification.html

I recommend that you try to locate a source of x-ray fluorescence analysis (qualitative) and that you request also hardness test for both parts.

P.S. - It is known and should be always remembered that certain aluminum alloys are not fusion weldable, although they may be welded by resistance welding or by other methods. Typical examples are the alloys 2024 and 7075 that should never be fusion welded.
See Aluminum Welding.

Hard Facing of Austenitic Manganese Steel
[From PWL#092 - Section 3]

Austenitic Manganese Steel contains 1-1.4 %C and 10-14 %Mn. This composition stabilizes the austenitic structure that is maintained even upon rapid quenching from high temperature. However, upon reheating to a moderate temperature, the material is embrittled by a partial transformation of austenite.

Manganese steel, obtainable in cast or wrought form, is a low-strength, high-ductility material. This material is tough and wear resistant, with the capability to work-harden from an initial hardness of 240 BHN (Brinell Hardness Number) (23 Rc = Rockwell C) to well over 500 BHN (51 Rc).

Work hardening occurs naturally as this steel is subjected to impact conditions under normal work in demanding applications such as primary rock crushing. This process increases the hardness of the affected metal and its abrasion resistance. If cracking of the work hardened layer occurs, crack propagation is quickly arrested and prevented by the tougher original (not work hardened) core.

As this material is selected primarily for impact and wear resistance, it is not uncommon that working surfaces of various implements wear out in time to the point where rebuilding become imperative. This is normally achieved by hard facing using arc welding to deposit new wear resistant layers.

To avoid the embrittlement of the base metal, welding and hard facing require procedures that result in minimum heat buildup. Severely worn or cracked material is first removed and replaced by welding using electrodes of austenitic manganese steel. Small parallel stringer beads are usually recommended, with thorough cooling between them. When much material must be added, round or square section bars of austenitic manganese steel can be embedded within the welded build up material.

Heat building should be minimized by skip welding, or moving from a weld area to another one relatively far away, before coming back to continue welding in the previous place. The same procedure is followed when applying, on the top, wear resistant alloys. A huge selection of materials is available, to be selected with the help of experts or manufacturers and according to previous experience.

In all welding operations base metal temperature should not exceed 260 ⁰C (500 ⁰F). Limiting the temperature reduces metal shrinkage, stress buildup, distortion and surface cracking of deposits. Preferably two or more hard facing layers are applied to minimize dilution effects of base material on the working surfaces.

Accurate bookkeeping of time to wear out and of costs both for the original implements and for repairs can help in improving the economic performance and in maintaining tools and assemblies in working condition.

Stress Relieving Test
[From PWL#093 - Section 3]

Recent queries addressed stress relieving test. As the specific conditions referring to the cases confronting the inquirers are not known in any details, the answer here refers to general cases.

There is no practical mechanical test capable to determine, by measuring a given property (like hardness), if the process was applied or not. Therefore in case of doubt if the process was applied or not, the only resort would be to make it again.

Whenever stress relieving procedures (see Stress Relieving) are specified in engineering requirements, there must be an established record keeping method that certifies for every workpiece submitted to this procedure, that the process was indeed done as prescribed.

The record, normally written and signed by an authorized inspector, must define the item, by serial number if necessary, the facility used, the parameters (date, time, temperature), the operator, and include furnace graphs if available. Special remarks like interruptions, must also be included.

As sometimes stress relieving operations may cause deformations, dimensional checks should be performed to make sure that the items are still acceptable. Normally hardness test are not required except in special cases.

If stress analysis is required for development programs, it is not generally a routine inspection, and will be performed on experimental basis.

Steel parts undergoing stress relieving at elevated temperature may develop a surface scale that should be removed before applying paint, by such a process as specified in engineering drawings. See Abrasive Blast Cleaning.

Complying with Welding Codes
[From PWL#094 - Section 3]

Readers are referred to my page on Quality, presenting the main issues governing all aspects related to this most important subject.

It is not uncommon that from time to time questions arise referring to interpretation of Code requirements. One such question was formulated as follows:

"I was told that in VA (Virginia) there are requirements for weld inspections of pressure vessels. Max pressure in this reactor will be 40 PSI. Is there a federal reg requiring the welds to be x-rayed?"

The answer cannot be exhausted only with regard to radiographic inspection of welds. Depending on the actual design and function of the referred reactor, it will probably become under the jurisdiction of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC), whoseSection IX regulates "Welding and Brazing Qualifications".

While the comprehensive requirements refer also to design, materials, installation and periodic inspection, exemptions may be applicable in well defined specific cases. To provide a correct and complete answer capable to stand Court inquiry, with reference to specific paragraphs of the applicable Code, a study of the case must be done after all details are established and known.

Authorized Inspectors, certified by insurance companies or by jurisdictional authorities are endowed with the capacity to perform inspections and examinations of components built to the ASME Code.

Manufacturers intending to fabricate components to the ASME Code must obtain an ASME Certificate of Authorization, also called Code Stamp, before initiating the work. Furthermore they must have an operational and approved quality control system and a manual describing it.

The language of Codes and Specification is highly technical and the correct terms must be used to convey the exact intention. As the application is required by law however, the discussion of specific paragraphs may be open to discussion in certain borderline case, to the delight of lawyers who specialize in the matter.

When requests for clarification are referred to the committees who released the Code or Standard, the agreed upon answers are generally collected in Interpretation Appendixes that become the official meaning of the issue from the date of their publication.

To be complying with Codes one has to check if the project on hand in any given location, is under a jurisdictional authority. To get a feeling for what it means meeting Code Requirements, readers are invited to see the following post on a valuable Forum:
Distillation column manufacturing - right grade of steel?
http://www.eng-tips.com/viewthread.cfm?qid=298462&page=1

The Mid June Bulletin #62 appended to issue #094 of Practical Welding Letter, provides Resources on Welding Codes and Standards. Please see it past the end of the first part of PWL#094.

Avoiding Porosity in Aluminum Welding
[from PWL#095 - Section 3]

Looking for a quick answer for resolving problems of porosity in aluminum welding may prove disappointing. Unfortunately a number of conditions relative to material, preparation, contamination, consumables, welding technique or equipment can cause porosity.

In any specific case, it is necessary to evaluate each of the possible problem areas in order to identify the culprit.

The main causes of porosity to be investigated to avoid porosity in aluminum welding are listed hereafter.

• Hydrogen gas becoming entrapped within the solidifying aluminum, from contaminants within the welding area,
• Inadequately pure gas,
• Insufficient gas shielding, as when drafts are present,
• Spatter buildup inside the gas nozzle, restricting shielding gas flow,
• Incorrect or variable nozzle distance that disturbs flow rate, coverage and efficiency,
• Inaccurate material preparation and cleaning.
• Contamination of base and filler metal, not completely removed by inadequate cleaning,
• Use of oily compressed air on the material,
• Anti-spatter compound for protecting the welding nozzle: should not be used for aluminum welding,
• Moisture presence from condensation at low temperatures or absorbed in aluminum oxide not removed.

The list provides quite a number of instructive information on the subject, readily available online. Readers can collect the files interesting them in a special folder to be saved in their computer for further reference.Click on PWL#095 to find it. Bookmark this page.

Fillets on Beam Reinforcements
[from PWL#096 - Section 3]

Q - I'm a brand-new subscriber...and need some feedback onthe issue of using transverse welds on structural beams.
Legend has it that such an application is bad practice, but I need to understand the physics behind it. Typical application is welding a plate (clamp base, tap pad, etc.) with simple edge fillets onto the side of a structural beam (rectangular tube, mostly), which is loaded in bending. Some individual welders insist on full weld-arounds, others only weld the axial sides and then caulk to seal the unwelded, transverse sides. Textbooks seem to avoid the subject.
Any help would be appreciated!

Thanks very much.

A: - Thank you for your interesting question.
If I understand the situation you describe I think that the problem has to do not with physics but with metallurgy.
And it may be that both opinions are valid.

If you are considering mild steel in a construction not heavily loaded and not severely subjected to fatigue, I would not think there are serious dangers of crack development in the transverse loaded fillet weld. Therefore I would not consider welding all around bad practice.

If however you are dealing with low alloy steels, possibly hardened and tempered, with elevate carbon equivalent, which needs precise procedures of preheating and post weld treating, then maybe you should be cautious with those transverse welds which might develop cracks expanding later to the whole structure.
I hope this helps.

Failure of Stainless Welded Studs
[from PWL#097 - Section 3]

Q - Recently I was sub-contracting to a company that was building an acoustic smoke stack for a gas turbine. The outside casing was 3Cr12 stainless and the acoustic panels were held in by 304 stainless studs (8 mm dia) which had been welded with a stud welder. These studs failed in service. The temperature rises to 500 deg [C? F?] rapidly. It has cost the company a lot of money in re-work. Could this be carbide precipitation?

A - The company should have learned that suitable consultancy might have actually saved a lot of money in re-work. A failure investigation conducted by professionals would have easily spotted its cause. Carbide precipitation is not probable, due to the low carbon content of the steel. Furthermore carbide precipitation would probably affect corrosion resistance, not mechanical failure.

The probable causes of failure should be looked at in the reduced toughness and ductility because of grain coarsening and in formation of martensite in the 3Cr12 Heat Affected Zone following stud welding.

Wailing Structures
[from PWL#098 - Section 3]

Q - We manufacture hydraulic press machines. We fabricate a top and bottom block which we hold together with two pillars of about 9.5" (240 mm) diameter.

The top and bottom blocks are hollow box girders. The vertical section is 70mm and the horizontal section is 50mm. These structures are stress relieved and then machined.

My problem is that when we assemble the machine and we put pressure on the structures, a crackling sound comes. We thought this sound should not be there because we have stress relieved it after welding, but it is still there. How can I get rid of the sound?

A - Thank you for your question. On the contrary, you should listen carefully to the crackling sound. The structure is wailing, signaling to you that it is in distress. In fact a crack is propagating and the elastic energy liberated by the crack progress is heard as sound. If at any time the sound seems to stop it can be due to loading to a lower stress level. To get sounds you need to exceed the last maximum load.

You should search Universities or Metallurgical Laboratories to find a provider of Acoustic Emission Analysis services. They will come with their instruments and place a few sensors (microphones) in strategic locations on the structure to test.

When the sensors are in place you should load sufficiently the structure again so as to repeat the sounds. Then the emission point will be found by triangulation.

Acoustic Emission Analysis may point you in the right direction as to the reason of failure and let you avoid defective materials or marginal welding practice. A thorough and correct metallurgical investigation, will solve the problem for all your future production.

An introduction to Acoustic Emission was published (2) in Issue 62 ofPractical Welding Letter for October 2008. Several links to additional information are offered there.

Assuming this is standard production I would think it advisable to find the reason and to avoid liabilities for the future even if the test costs money.

Most probably, using Magnetic Particles Inspection, you will find at that place a small crack. Then you will have to develop a repair procedure. You will get rid of the sound after the repair is done.

You should then inquire why the crack appeared in the first place. Is that the material at fault? Is it an inadequate procedure? Was the preheat correct? Were the electrodes dry and moisture free?

Assuming the plates are of mild steel, it is not the composition that matters most as the absence of internal defects like laminations, that cannot be found except by ultrasonic testing.

If defective areas are detected on heavy plates by a knowledgeable inspector with suitable equipment and experience in Ultrasonic Inspection, they can be scrapped, before welding them in stressed structures.

The quality improvement will be remarkable and valuable, and the Crackling Sounds will be eliminated.

Furnace Brazing Copper
[from PWL#099 - Section 3]

An interesting note at page 16 of the October 2011 of the Welding Journal advices against furnace brazing in a Hydrogen atmosphere, suggesting instead to select vacuum furnace with partial pressure of argon (to prevent outgassing of the copper).

The inquirer complained of the blisters appearing on the hydrogen furnace brazed parts. The material, which should have been "oxygen free" copper (C10200), was found after the mishap to be actually "tough pitch copper" (C11000) with 0.02-0.06% oxygen.

It appears that the material was erroneously misidentified and mis-certified.

The Author, Dan Kay, explains that blistering occurred when hydrogen combined with the cuprous oxide at the grain boundaries, forming steam that bubbled away cracking the grain boundaries, in what is called hydrogen embrittlement.

He also tries to appease the inquirer to think that such errors are rare and should be taken with compassion. Interested readers are urged to look for the original article indicated above.

Automatic Radiographic Evaluation
[from PWL#100 - Section 3]

Radiographic evaluation is a critical step in nondestructive inspection of welds. The traditional approach involved the employment of highly educated inspectors, capable to take expositions, to develop images and to evaluate them visually.

This was before the development of digital radiography. However the personal interpretation stage remains even presently on the inspectors' shoulders. There is a certain measure of personal judgement in every evaluation, based on inspectors' education and experience.

The ability to perform automatic evaluation of digital radiographs taken from repetitive welds as those obtained in mass production, has a remarkable economic interest, in that it makes processing more speedy and independent of costly human professionals.

To be effective, such evaluation must be reliable, capable of sorting out all unacceptable flaws, but also robust, minimizing the instances of false positive, that is the number of parts falsely classified as rejects.

A recent article, published at page 29 in the November 2011 of the Welding Journal, reports on advances in automated image processing that make evaluation less expensive and more reliable.

An automatic radiographic inspection system is described that completely eliminates human intervention. After digital acquisition of X-Ray images, automatic image processing follows.

Image processing consists in computer performed operations for contrast optimization, noise suppression and image modification using mathematic filters to eliminate confusion and to sharpen the view for easier automatic evaluation.

These operations are necessary to enhance features that allow measurement and classification of flaws. Successful applications will promote more numerous instances in which similar automated systems perform automated radiographic evaluations.

Interested readers are urged to see the complete article indicated above.

Substitution of Lead Based Solders
[from PWL#101 - Section 3]

When confronted with the need to eliminate lead based solders from industrial processes, one knows that any replacement of filler material will be more expensive. Also the process, requiring higher temperature, will be more costly.

An alternative worth checking, may be adhesive bonding, that in certain cases might be suitable, depending on the requirements, except possibly for electronic connections.

One should first explore if any of the lead-free filler metals available on the market has the potential to be acceptable for the application involved. In particular filler metals including silver and/or indium in large proportions are likely to be significantly more expensive.

Apart from commercial publications that may be useful, interested readers are referred to a Report issued by the National Institute of Standards and Technology (NIST) for critical evaluation of mechanical property data of lead free solders. See:
http://www.metallurgy.nist.gov/solder/clech/Introduction.htm

Users ready to consider silver containing soldering filler metal for the substitute process, have to deal with higher temperatures. More energy must be supplied and more time will it take to heat uniformly the parts, than was needed for the original soldering.

The flux has to be examined if its action is adequate and if its elimination after use can be performed properly enough that will not cause corrosion.

Finally to evaluate the new process and materials against the former, the mechanical properties should be compared, assuming acceptable discontinuities. Tables of published mechanical properties, if available, may be difficult to find, and the data may not be readily applicable.

Establishing Squeeze Time
[from PWL#102 - Section 3]

This is not the first time I pick up useful hints from the always interesting Q&A notes published by The Welding Journal (January 2012, page 20). I always recommend these informative pages.

A reader had asked where to find squeeze time data to spot weld cold rolled and galvanized steel, as it is not published along other schedule data in the RWMA Resistance Welding Manual. The Author, Roger Hirsch, suggested in his answer to find it by trial and error.

One should remind that squeeze time is the time interval between timer initiation and first application of current. It is designed to allow electrode movement and full force action deployment before performing the spot weld.

It appears that this interval is dependent on design details of each machine: because of this reason it cannot be a fixed datum to be published on tables.

Too short a squeeze time will manifest itself by the appearance of sparks from under the electrode. By simply increasing this time in parallel tests until sparking (almost) stops one can find the correct value (in cycles) needed for correct operation.

Note that any further increment does not add any advantages and wastes production time.The alternative answer that involves calculations seems less attractive.

If the equipment has a differential pressure transducer that monitors the air pressure on both sides of the cylinder, one can establish the time when full pressure is applied to the electrodes. If a signal can then be used to start the current cycles, squeeze time is redundant and can be bypassed (set to zero).

Interested readers are referred to the original article indicated above.

How many weld repairs are allowed?
[From PWL#103 - Section 3]

You may recall having seen requirements limiting the number of allowed repairs performed on the same location, for the purpose of protecting the construction from weak spots likely to develop due to repeated weld repairs.

If you are bound by such a customer requirement you can either argue on its necessity or abide to it. But now you can at least find a reference to a research program intended to test if there is a reason supporting this request.

A group of researchers, knowing the limitations but unable to find on the subject precise indication of accepted Standards, undertake a testing program intended to verify, one way or the other, the influence of actual simulated cut and repair cycles on the properties of a welded joint.

You will find the summary in the February 2012 Issue of the Welding Journal at page 25.In the specific case the test was performed on low carbon steel, ASTM A 283 GrB, flat plates 3/8 in. (9.5 mm) thick.

Test pieces were welded together using direct current, with the gas metal arc welding (GMAW) process, in the flat (1G) position, with a wire of 1.2 mm diameter, type ER 70S-6 per AWS Specification A5.18, recommended for welding low-carbon steel. The shielding gas was 75% argon and 25% carbon dioxide.

Six equal test pieces 200 mm wide ׳ 440 mm long were welded after having been prepared with a 60 deg. bevel, manually cut with an oxyacetylene torch and cleaned with a grinding disk. After the face of the specimens was welded, the root was backgauged with a file and rewelded.

The first specimen was put aside for testing. The remaining specimens were cut with the same methods and rewelded. The second specimen having one weld and one repair weld after cutting was put aside for testing. The other specimens followed the same procedure. The third had one weld and two repairs, the fourth one weld and three repairs, the fifth one weld and four repairs and the last one had one weld and five repairs.

Test pieces removed from the six specimens were tested for bending, ultimate tensile, impact, elongation, average grain size, and metallographic structure of the HAZ.

The results showed a remarkable homogeneity, without dramatic conditions likely to alert the researchers of dangerous worsening of basic properties. The conclusion reached in this program was that welding can be performed safely on the same area at least six times (one weld and five repairs) on low-carbon steel.

My comment is that the conclusions may be correct for applications reflecting in all details the procedures of this research. It would be risky to extend them to different situations.

Interested readers are urged to seek the original article reported above.

Controlling Distortion
[From PWL#104 - Section 3]

From time to time I get queries trying to locate parameter changes capable of reducing distortion while welding. The following, besides other queries, illustrate the kind of questions on this argument:

1) - "I need to do some extensive aluminum welding on a cast outboard block for modification purposes. I have a Lincoln square wave tig 255. I'm experienced in welding from thin Alu sheets to the much thicker ones and different parts etc.

I've been told to watch for warping/distortion when welding Aluminum that extensively. My question now is what can I do to reduce these risks of distortion or warping?"

2) - "We have a storage tank (18000m3, floating roof, type of metal A283C, diameter of tank 44m approximately, height 13m). Thickness of bottom 9mm. Thickness of floating roof 7mm. Thickness of shell (18,16,14,12,8,8,8)mm.

When we weld the tank by using submerged arc welding technique (electrode spec. EH12K, DIA 3mm,4mm), we notice distortion in welding joint in bottom 9mm and floating roof 7mm andshell at thickness of 12,10&8 mm. Please advise me what is the proper electrode and flux to weld this tank without causing distortion in welding joint."

My standard answer is as follows: "Unfortunately, distortion problems are among the most difficult and intractable of welding practice. Contrary to what you may think they will not go out by a simple change of electrode or flux.

You may wish to see my page
http://www.welding-advisers.com/Welding-distortion.html
In general the problem is attenuated by careful planning of the weld sequence and by limiting heat input."

What this means in practice is that a complete description of the structure must be secured and that a logical sequence must be established to limit concentrating heat input in a limited area.

This analysis is best done by an experienced welding engineer capable to calculate heat input based on used parameters and to exploit fabrication sequences so as to minimize residual stresses. In certain situations it may be possible to straighten out deformed plates or profiles.

See also PWL#104B.

Brazing with Nickel AWS BNi-2/AMS 4777
[From PWL#105 - Section 3]

A quite common question from shops using AWS BNi-2/AMS 4777 nickel base filler brazing alloy to join elevated temperature resistant alloys in vacuum furnaces, refers to the frequent appearance of a center crack right through the brazing material.

ANSI/AWS A5.8M/A5.8:2011
Specification for Filler Metals for Brazing and Braze Welding Edition: 10th
American Welding Society / 17-Jun-2011 / 62 pages
Click to order.

The brazing range of this filler metal is given as 1010-1175 ⁰C (1850-2150 ⁰F), relatively low, due to melting point depressants included in its composition.

The added elements performing this function (boron, silicon, phosphorus) also reduce the ductility of the brazing alloy. As the solidification occurs in the direction from the outside toward the central part of the brazing gap, and as these elements are the last ones to freeze, it so happens that their concentration is higher in the center.

The location of concentration of those elements is also a weaker point because of their reduced ductility, prone to cracking in service but also, occasionally, during solidification.

If the capillary clearance existing at the brazing temperature results larger than 0.1mm (0.004"), then there are good chances that the brittle constituents solidify at the joint center, visible in a metallographic section as a dark line, indicating a continuous layer likely to crack.

If however the said clearance is smaller than that, the brittle parts solidify preferably as separated islands in a more ductile matrix, capable of sustaining minor strains without cracking.

Therefore, for successful brazing with this brazing alloy, besides all other precautions, it is essential to control the actual capillary gap at brazing temperature, to keep it consistently smaller than 0.05mm (0.002").

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