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PWL#038 - Fluidized Bed Furnaces, Furnace Brazing, Soldering Magnesium Alloys, Low Temp C Supersatur
September 28, 2006
We hope you will find this Letter interesting and useful.
Let us know what you think of it.

Fluidized Bed Furnaces, Furnace Brazing of Tubular Joints, Filler Metal for Soldering Magnesium Alloys, Low Temperature Carbon Supersaturation, Resonant Acoustic Method, Stress-Strain Microprobe (SSM) based on Automated Ball Indentation (ABI), Projection Welding and more...

This publication brings to the readers practical answers to welding problems in an informal setting designed to be helpful and informative. We actively seek feedback to make it ever more useful and up to date. We encourage you to comment and to contribute your experience, if you think it may be useful to your fellow readers. Click on Contact Us (opens new page).

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October 2006 - Practical Welding Letter - Issue No. 38


TABLE of CONTENTS

1 - Introduction

2 - Article: Fluidized Bed Furnaces

3 - How to do it well: Furnace Brazing of Tubular Joints

4 - Filler Metal for Soldering Magnesium Alloys

5 - Online Press: recent Welding related Articles

6 - Terms and Definitions Reminder

7 - Article: Low Temperature Carbon Supersaturation

8 - Site Updating: Projection Welding

9 - Short Items

10 - Explorations: beyond the Welder

11 - Contribution: RAM and SSM (ABI)

12 - Testimonials

13 - Correspondence: a few Comments

14 - Bulletin Board


1 - Introduction

Our 38th issue of Practical Welding Letter for October 2006 opens with a presentation of a furnace type, based on Fluidized Bed technology that, depending on conduction heat transfer, is very effective for heating rapidly the immersed bodies.

In the Questions and Answers section a few basic facts are detailed for performing successful Furnace Brazing of tubes and fittings.

The Filler Metals department reports on recent research on soldering of Magnesium Alloys with new solders and an innovative application of ultrasonic vibration to rupture the surface layer of oxides in order to dispose of flux while preserving corrosion resistance.

A newly researched Low Temperature Carbon Diffusion process capable of hardening stainless steel and other alloys is being introduced. It may be able to gain in the future more meaningful applications as it improves mechanical properties and does not impair corrosion resistance.

For the Site updating department we report, in section 8 further down, on a new page dedicated to Projection Welding, a process having definite advantages that make it cost effective in certain cases and on two older pages (on Quality and on Design) completely revised and edited.

In the Contributions department, No. 11, we provide some details on a new method of individual nondestructive testing. Best used for inspection of mass produced items, it uses the Resonant Acoustic signature to single out defective items from a population of acceptable ones. The method can detect the presence of rejectable defects from different origins.

Another new nondestructive inspection method, employing an Automated Ball Indentation device, potentially useful for studying the residual serviceability of welded structures, is presented. It provides a way to correlate mechanical properties tested on the site, with sets of data usually obtainable on specimens only by destructive testing.

The data collected can be used to perform fitness-for-purpose studies using the principles of fracture mechanics, to reach critical decisions on pipeline and other structures integrity, or on the need of immediate maintenance provisions.

Other departments show up at their regular places. We hope you will enjoy this issue. Please let us know what you think of it. Let us have your comments by clicking on Contact Us.


2 - Article: Fluidized Bed Furnaces

The remarkable fluidized bed technology employs heat transfer in a very effective way. It has been used for several decades for industrial processes, including gasification and incineration of organic waste.

A typical utilization that may interest the metallurgical processing community is that of special furnaces presenting interesting properties that recommend it in certain special cases.

Recirculating fluidized bed furnaces use a retort filled with sand or alumina particles. A controlled stream of air or gas conveyed vertically upward, creates a lifting or buoyancy effect on the particles that float in the gaseous medium and move around in turbulent motion.

The fluidized media displays the physical characteristics of a boiling liquid bath. When heated and used with a suitable fluidizing gas mixture it provides an excellent heat treatment furnace for components that are immersed into it.

Heat is transferred from the particles to the immersed components at a rate two to ten times higher than that provided by other means.

Conduction assures higher heat transfer rate and shorter soak time in fluidized bed furnaces compared to atmosphere furnaces that exploit convection and vacuum furnaces based on radiation. Temperature uniformity is exceptional.

Fluidized Bed Furnaces are flexible in that they permit to change over quickly from one gas to another, to adjust the temperature as needed up to 1200 0C (2200 0F), and are considered safe and not polluting.

Uniform temperature assures lower distortion, while short cycle times enhance economy and high productivity. When applicable this technology might be preferred to that of vacuum furnaces because of much lower equipment and maintenance costs.

Using proper enriched atmospheres, this technology permits to perform surface modification treatments like carburizing or carbonitriding without external gas generators.

Due to its advantages this technology is being used in heat treating furnaces for tool steels and aluminum alloys components, for brazing and other processes. It should be explored when new opportunities arise.

Please let us have your comments on this article by clicking on Contact Us. Pertinent comments will be published in the next issue of PWL.


3 - How to do it well: Furnace Brazing of Tubular Joints

Q: What are recommended practices for furnace brazing of tubular joints?

A: Cleanliness is the single most important prerequisite for successful brazing. The use of suitable preforms, that is of prepared units of formed brazing alloy of definite shape and weight for a specific application, may contribute to improve quality and reduce costs.

The selection of the suitable filler metal should be adequate for joint and process requirements. An indication on usability of different silver brazing alloys was given in a note published in section 4 of Issue 003 of Practical Welding Letter for November 2003. Click on PWL#003.

Radial clearance of the capillary space should be calculated to be between 0.050 to 0.125 mm (0.002 to 0.005") at brazing temperature.

The brazing filler metal preform should be possibly located at the recessed side of the joint, so that the free side of it can be visually inspected for uninterrupted presence of the flown brazing alloy, indicative of an adequate brazed joint.

For designing Brazed joints with confidence one should learn the lessons of the following publication.

ANSI/AWS C3.3:2002
Recommended Practices for Design, Manufacture, and Inspection of Critical Brazed Components
American Welding Society, 01-Jan-2002
32 pages
Click to Order.

For an informative list of useful Brazing Resources available online, interested readers are reminded to consult again the Mid July Bulletin of the Practical Welding Letter at PWL#035B.


4 - Filler Metal for Soldering Magnesium Alloys

Recent developments of new and improved light weight magnesium alloys of higher purity and better corrosion resistance spurred research in better joining procedures.

Due to the limitations in weldability of magnesium with aluminum alloys, because of the formation of intermetallic phases like Al12Mg17 that reduce mechanical properties even with laser or friction stir welding, alternative joining procedures have been explored.

Soldering of magnesium alloys had problems because wetting is hindered by the presence of surface oxides. The fluxes used to improve wettability impaired the corrosion resistance properties of the material.

Therefore research was aimed at developing new soldering alloys capable of permitting fluxless soldering of magnesium to dissimilar materials like steel and aluminum alloys.

New soldering alloys composed of Magnesium and Zinc, with or without Aluminum were developed, while the best results of soldering magnesium alloys to mild steel is reported as having been achieved with a solder of the following composition: 96%Sn, 4%Ti, 3%Ag.

It is interesting to remark that to dispose of the use of flux, an ultrasonic mechanical activation procedure was developed, operated at 20kHz as soon as the filler metal melted, and held for 5 seconds. The process is made possible as the oxide layer is being destroyed by the ultrasonic vibrations.

An article on this research was published at page 48 of the Welding Journal for September 2006. It includes details on compositions, on melting behavior as investigated by Differential Scanning Calorimetry, on microstructures that revealed the formation of diffusion zones in the base metals, on mechanical properties and on corrosion resistance.

Interested readers are urged to seek the original article above.

As we inquire on usefulness and interest of this publication for our readers, we would ask you to please let us have your comments on the presentation above by clicking on Contact Us. Pertinent comments will be published in the next issue of PWL.


5 - Online Press: recent Welding related Articles

From Case Western Reserve University (CWRU)
Low-Temperature Carburization of Ni-base Alloys
http://dmseg5.case.edu/Groups/Ernst/Research/Ni-C.html

From TWI
Hybrid Nd:YAG Laser-AC MIG welding
of thin section automotive aluminium alloy

http://www.twi.co.uk/j32k/protected/band_8/spcmajun2006.html
Recent Developments in Welding Technology
http://www.twi.co.uk/j32k/protected/band_8/spgsbmay2006.html
Needs no cost registration.

From AWS
AWS D1 - Codes Interpretations
http://www.aws.org/technical/d1interps.html

From ASM International
Sneak Preview
Welding Metallurgy - Part 1 - Introduction
by Sindo Kou
http://www.asminternational.org/Template.cfm?Section=SneakPreview&template=Ecommerce/FileDisplay.cfm&file=74575G_CH.pdf


6 - Terms and Definitions Reminder

Burn Through indicates excessive melting through a joint to the point of piercing a hole or otherwise failing to produce a weld, locally.

Flaw means an unwanted discontinuity to be avoided or removed.

Nozzle is the cylindrical or conical portion of a torch through which the shielding gas is directed to the weld.

Pilot Arc is a low power arc maintained continuously to ionize the gas and to provide an easy way to establish instantly the full power welding arc when needed.

Staggered Intermittent Weld is deposited alternatively from two sides of an element so that to one stretch of weld from one side corresponds an unwelded interval on the other.

Underbead Cracks generally not visible on the surface are formed in the brittle heat affected zone.

Weld Interface is called the bordering surface between base metal and weld metal if there is fusion, or between two solid state welded surfaces.

Weld Root is the location where the back of the weld contacts the base metal.


7 - Article: Low Temperature Carbon Supersaturation

Diffusional thermal treatments meant to improve surface properties of stainless steels have long been known to reduce their corrosion resistance.

A recent cooperative research effort, funded in part by the Department of Energy, and involving Case Western Reserve University (CWRU), Oak Ridge National Laboratory, the Naval Research Labs and the commercial Company Swagelock, produced the development of a new quite promising diffusional surface hardening process.

When applied to Stainless Steel Type 316 it provides an increase in surface hardness, improved resistance to wear, corrosion and fatigue without impairing ductility.

The uniform hardness gradient avoids any risks of delamination or peeling. The microstructure remains austenitic and non magnetic throughout the process. The treatment is performed at the relatively low temperature of 470 0C (878 0F) that does not alter dimensions or produce distortions.

The process involves two surface activation cycles in a carbon enriched gaseous atmosphere and a long carbon atom diffusion cycle. At the specified temperature that constrains substitutional diffusion, carbon in atomic form is drawn into the metal interstitially preventing the formation of carbides. The carbon solid solution shows a Vickers hardness often in excess of 1000 HV with expanded metal lattice and consequent compressive stresses.

The microstructural characterization and the behavior of the material so hardened were the object of dedicated research programs to assess the properties enhancement available.

Interested readers should explore a short review of the experimental work that is presented in the article published in the ASM International publication Advanced Materials and Processes at Page 32 of the September 2006 Issue. A related article from CWRU can be seen online from the address shown in the first item of Section 5 above.

The study was further expanded to cover precipitation hardenable stainless steels, duplex alloys, nickel based and cobalt based alloys. The new process should be followed closely by whoever is interested in significant improvements in the surface properties of the alloys investigated.

As we inquire on usefulness and interest of this publication for our readers, we would ask you to please let us have your comments on the presentation above by clicking on Contact Us. Pertinent comments will be published in the next issue of PWL.


8 - Site Updating: Projection Welding

The website new Page of this Month deals with the process known as Projection Welding.

It is a kind of Resistance welding performed with equipment so modified as to allow the creation of spot welds in correspondence with points of contact between two items.

The embossed or machined projections in at least one of the elements to be welded permit the use of simpler electrodes and the simultaneous welding of several spots.

While providing economical advantages in certain cases, projection welding should be compared and critically evaluated with alternative processes which might be more convenient in other circumstances.

The new page can be read by clicking on Projection Welding.

You can always see new addition to the website by checking the Site Map.

Please be informed that two other existing pages were recently thoroughly rewritten and edited. Readers are invited to review the page dealing with
Quality and that concerned with Welding Design.

Although these revised pages were duly announced in our Blog, the extent of revision is not evident other than by reading those pages.


9 - Short Items

Stress Relaxation is the time-dependent decrease in stress (e.g. of fastened bolts) in a solid under constant constraint at constant elevated temperature.

Tempering is a heat treatment consisting in reheating hardened (austenitized and quenched) steel (martensitic structure) to some intermediate temperature to decrease hardness and increase toughness.

Thermal Shock usually generated by sudden heat change is the development of a steep temperature gradient within a material or structure (generally because of significant differences in thickness) and accompanying high stresses.

White Iron in cast form is essentially free of graphite, and most of the carbon content is present as separate grains of hard Fe3C. Upon fracturing along the iron carbide platelets, white iron exhibits a white, crystalline fracture surface that gives the material its name.

Work Lead is the electrical cable connecting one pole of the source of arc welding current to the work. The other cable connects to the electrode for establishing the welding arc.

Young's modulus is the same as modulus of elasticity. It represents the ratio of tensile or compressive stresses to the resulting strain.

Zone Melting is done by melting locally by induction heating a small volume of a solid metal piece, usually a metal rod, within a quartz crucible. The melted zone can be transferred from one end of the rod to the other by moving the induction coil along the rod. In certain alloys one can refine the solid to high purity by concentrating one of the constituents in the liquid as it moves along the rod. This procedure is called Zone Refining.


10 - Explorations: beyond the Welder

Incan Knots
http://web.mit.edu/newsoffice/2006/knots.html

How to Blow Up a Star
http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID= 000160CC-A71B-150E-A26183414B7F0000

Bosses Who Bully http://sciencecareers.sciencemag.org/career_development/previous_issues/ articles/2006_09_22/bosses_who_bully/(parent)/68

Fractography of Metals and Plastics
http://www.asminternational.org/Template.cfm?Section=SneakPreview&template= Ecommerce/FileDisplay.cfm&file=5902998_PFA_ch.pdf

Birthday Parties
http://www.coolest-kid-birthday-parties.com


11 - Contribution: RAM and SSM (ABI)

A proven technique that provides accuracy, reliability and cost effective high throughput has been presented for use in online nondestructive testing of mass produced items. The technique, applicable to cast, forged or powder metal sintered products can detect the presence of structural defects.

It is called Resonant Acoustic Method (RAM). It measures the structural response of a component's mechanical resonances by comparing the unique signature of a single item against the statistical variation of the same signature from a control set of good parts.

The method consists in giving each part a controlled and repeatable impact blow that provides broadband input energy causing the part to vibrate, and recording the acoustic spectral signature obtained through a microphone.

The frequency spectrum displays peaks corresponding to the part's mechanical resonances at its specific natural frequencies. Any difference in the recorded spectrum denotes a structural change indicating the presence of defects.

One may recall that clay wares were long ago inspected by listening with human ear to the sound emitted upon striking them lightly with a proper tool. The above method is an automated extension basing the judgement on objective, automated and scientific principles.

An article on this technique was published in ASM International publication Advanced Materials and Processes at Page 35 of the September 2006 Issue. Interested readers are urged to seek the original article and to find more information at:
http://www.modalshop.com/industrial_ndt.asp

Another unrelated interesting innovation, called Stress-Strain Microprobe (SSM) is based on Automated Ball Indentation (ABI). This nondestructive test can be performed on existing structures to measure stress-strain curves and fracture toughness properties.

The ABI test applies progressive indentation (similar to that used for hardness testing) but with partial unloadings, until maximum depth (strain) is reached. At that point the tungsten carbide spherical indenter is fully unloaded.

The indentation load data and the corresponding depth points are collected continuously. The incremental values of load and plastic depth are converted to incremental values of true stress and true plastic strain values.

The complete fully automated test lasts about two minutes. Also initiation fracture toughness, not tearing modulus, can be determined from ball indentation.

The great advantage of this method is that it is localized and non destructive, permitting its application in place ("in situ"), to limited volumes like those of Heat Affected Zones. Extensive correlations of the method have been substantiated with different laboratories and against results from destructive testing.

Therefore the nondestructive ABI test is considered a true and robust mechanical testing method, now recommended by the US Office of Pipeline Safety for use in the pipeline industry.

The test allows the deterministic structural integrity assessment of fitness-for-service evaluation based on fracture mechanics analysis.

Interested readers are encouraged to seek the original article at page 41 of the above mentioned publication or to download a sample Test Data file at
http://www.atc-ssm.com/pdf/ABI-test-data-sample.pdf

As we inquire on usefulness and interest of this publication for our readers, we would ask you to please let us have your comments on the presentation above by clicking on Contact Us. Pertinent comments will be published in the next issue of PWL.


12 - Testimonials

From: Sukhendu Samanta (e-mail address removed for security)
To: Welding Advisers
Date: 05 Sep 2006, 10:47:15 AM
Subject: Re: PWL #033B - Mid May Bulletin - Online Resource List

Sir,
thanks for such a great support [...]


From: "David Wynn" (e-mail address removed for security)
To: Welding Advisers
Date: 14 Aug 2006, 09:31:00 AM
Subject: Re: books

Thank You! Your Web Site was a big help and I plan on using it more. Thanks Again!!


13 - Correspondence: a few Comments

One of our faithful long time readers was unfortunately struck by a relapse of his dreadful illness and, thinking as he was told, to have but a few more weeks to live, he wrote to take his leave from our community, even finding time and strength to thank us for the information we provide.

I hastened to answer that doctors can make errors of judgement and I reported that, after a person in my family was sentenced by his doctors as having no more than a few months to live, he lived on for a few more years and could still enjoy his life. Although I had no real argument with which to comfort him, I expressed my sympathy and wished him to overcome the worse of this difficult period.

He was kind enough to answer that my letter had made him some good, putting him in a more relaxed attitude and in a fighting back mood. I was pleased to hear that and confirmed that the personal attitude of a patient has much to do with his/her own chances of recovery.

His latest answer was that he had been busy in looking on the Internet for natural cures and had found something encouraging enough to convince him to print out the whole booklet and to plan to start some new therapy.

I was pleased to hear of his initiative and wished him all the best.

Even if my whole correspondence would come up to encourage only a single person, this friend, and to comfort him in his hard time, I would be glad to have done it.

If you like to comment, we would appreciate it. Please let us know what you think and write us by clicking on Contact Us.


14 - Bulletin Board

14.1 - Developments and Applications -
Low Pressure Vacuum Carburizing and High Pressure Gas Quenching
November 1-2, 2006 - Worcester Polytechnic Institute - Worcester, MA
http://www.asminternational.org
Click on "Events".

14.2 - Instituto Argentino de Siderurgia (IAS) organizes the 16th Rolling Conference and 3rd IAS Conference on Uses of Steel for November 6-9, 2006.
The venue is the Hotel Colonial, San Nicolas, Argentina. An exhibit with 32 booths will run at the same time.

Program and registration at
www.siderurgia.org.ar/en/index.php

14.3 - In case you did not yet download the previews of the important book "A Welder's Mate" by David J. Keats on Underwater Welding, please click on
A Case for Wet Welding.

14.4 - If you wish to stay updated with new offerings from our site, may we suggest, after having concluded that RSS feeds are not quite reliable, that you either click on the Blog button in anyone of our website pages or that you bookmark for frequent visits the following page:
http://www.welding-advisers.com/Welding-plan-blog.html

14.5 - If you wish to browse through our other website on a completely unrelated subject, click on Home Surveillance Guide.


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