Welding-design

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Exceptional Welding-design is in fact the precondition for outstanding weldments.

Important as it may be, this is not always self-evident.

Designers and Engineers may be expert in their profession.

Successful welding design requires involvement in all aspects of welding technology.

That, however essential, may be lacking because of their standard preparation.

Why is Good Design so important?

The required knowledge, including metallurgy and technology, is extremely vast.

Designers may be exposed only occasionally to welding as one of many other fabrication technologies,

Therefore it is quite possible for professionals, to neglect inadvertently fundamental details too specialized to be of common knowledge.

This in turn may cause more costly welding operations.

Or difficulties in obtaining defect free welded constructions.

This might cause even welding failures that could have been prevented.

In the Conclusion at the end of Chapter 5 - Design for Welding - of the American Welding Society (AWS) Welding Handbook, Ninth Edition, Volume 1, at page 236 one reads:

"Many issues are related to the process of designing for welding. In addition to component performance, service, intended life and safety, the designer should have a good understanding of the fundamentals of welding, metallurgy, fabrication technology and inspection techniques".

In the introduction to the above Chapter the authors list fifteen items of specialized knowledge whose effects on Welding-design should always be evaluated.

And they add that designers should "refrain to rely only on their own knowledge and experience" but should be "encouraged to consult with welding experts wherever appropriate".

This exigency, when coming from welding professionals, risks usually of being dismissed as "self serving", by confident designers who brag, to themselves and to their management, that they don't need any additional welding advice.

It seems to be increasingly accepted the modern trend indicating a growing need in Welding-design team activity, to satisfy requirements covering different disciplines that cannot possibly be mastered by a single individual.

At a minimum, the chief designer should make sure to submit new design to professionals with welding experience for their review of welding issues.

A short page on Welding-design like this one can only alert receptive and curious professionals to the large mass of specialized knowledge available.

One can get a partial idea of the mass of specific welding knowledge available, just by browsing through the many pages of this website's Site Map.

That should be tapped in every instance of involvement with any project calling for welding.

Responsible professionals should be encouraged to check the issues in depth, with the help of experts of the matter.

Individuals responsible for Welding-design should dedicate time and study to learn and assimilate at least the basic principles of the different subjects.

Such study will make them receptive to the input available from welding specialists.

That is because those principles are essential in establishing effective weldments.

Conversely welding experts should familiarize themselves with Welding-design to the point of understanding construction and manufacturing requirements, even if it is not their piece of cake.

They should develop an effective communication language to make their points clear without generating resistance or hostility.

To illustrate this point I would like to call readers' attention to an article of mine published on this subject and available online at
"Design for Welding - Letter to an Engineer".

The Role of Experience in Welding-design

There is no substitute for experience. Welding-design should be based on knowledge of the principles, refined by experience, distilled in quality examples of successful, cost effective realizations.

Welding-design, as well as any other engineering design not involving welding, starts as an approximate idea or sketch of what is needed.

Once a general idea is conceived about the form and functionality of the new construction, the application of estimated and service loads will help with further elaboration of the project.

That will provide a provisional approach as to which elements must be used and of their tentative dimensioning.

The design process must undergo successive iterations and refinement, that permit to determine with increasing precision the loads acting on single members.

Stability is traditionally assured by calculation of the applied loads or forces, those deriving from the weight of the structure itself and from environmental conditions (wind, snow etc.).

These sum up with those produced by the performance of the intended functions (i.e. hoisting, moving materials or whatever action is done).

Loads and a provisional selection of shape and dimensions for force sustaining members define stresses and strains operating on the structural elements and on the joints.

The largest Stresses to be sustained by the structure allow to select tentatively the materials to be employed, based on tables of mechanical properties.

At some point one has to check the service conditions (normal and extreme temperatures, exposure to atmospheric or corrosive media) and to verify if the provisional selection is adequate.

The final selection is most of times a compromise solution, as usual for many other engineering decisions, acceptable in terms of functionality, economy and building time.

It is the designer's responsibility to verify that working stresses are well within acceptable limits for safe operation. In definite cases the Welding-design of certain structures must, by law, meet Code Requirements.

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Properties of Materials

Review an Article on Mechanical Properties in section (8) of Issue 142 of Practical Welding Letter for June 2015.
Click on PWL#142 to see it.

It should be remembered that material properties include both those insensitive and those sensitive to metallurgical structure.

To the first group pertain physical properties like thermal expansion and conductivity, melting point (or range), specific heat and emissivity, density, Vapor pressure and electrical conductivity.

One should remark that the Modulus of Elasticity, valid only in the elastic range, is structure insensitive but temperature dependent, while the Elastic Limit, delimiting the elastic behavior and the start of plastic deformation, is structure sensitive.

To the second group pertain mechanical properties like Yield and Ultimate Strength, hardness, ductility and elastic limit, Toughness, Fatigue Strength, Creep and Rupture Strength.

As affected by microstructure, these are the properties most likely to be modified by welding processes, usually (not always) performed by applying considerable amounts of heat.

Therefore the properties of as welded metals may be strikingly different from those of the base metal before welding application.

Furthermore one should be alert to factors that, without being properties, affect mightily the suitability of weldments to perform safely for their service life.

Among these are notch effects, multi-axial and residual stresses, surface finishes and special coatings.

If welding is needed for fabrication, weldability of proposed materials must be verified.

It should be understood that properties and characteristics of materials are influenced and changed by welding processes.

Therefore at the Welding-design stage one must specify the required processes while taking into account their consequences on integrity and stability.

Metallurgy and weldability are essential considerations for Welding-design, especially when it appears that special procedures have to be implemented for successful welding.

In certain cases suitable controls should be specified in drawings and implemented during welding to avoid brittle fracture.

The requirements may well be established in detail by experts, but the designer is responsible for specifying their adoption on the drawing or other binding document.

The Mechanical properties to consider are those displayed at actual service temperature.

When considering Welding-design involving the use of aluminum alloys, it is important to remember that the strongest alloys are not weldable by regular fusion processes, as explained on the Materials page.

Only specific processes can be used to preserve those high properties (like strength and corrosion resistance), as explained in a page on Aluminum Welding.

Material selection should follow a complex procedure.

It has to consider:

  • standard mechanical properties,
  • manufacturing constraints,
  • requirements for resistance to corrosion or weathering agents,
  • need for protection,
  • finishing requirements,
  • ease of maintenance,
  • availability of materials and consumables
  • economy of production.

Special Welding-design instructions may be needed, like minimum ambient temperature required for load testing of structures whose toughness may be affected by very cold conditions.

Fracture toughness is a very important property to be studied thoroughly because it is likely to depend on ambiental conditions (temperature), load application (rate and directionality) and presence of notch (stress concentrating feature).

Projected savings in maintenance along the planned life of the structure may suggest the selection of a more resistant material even if more expensive at purchase time. This concept is sometimes called the Total Life cost.

Similarly, if weight is at premium (for high rise constructions or for transport), higher properties may permit weight reduction and savings, even if more expensive materials are selected.

It is not uncommon to examine different designs to find out maximum advantages at minimum costs or compromise solutions.

For Welding-design the following Factors should be considered:

  • Review of Experience with existing products and eventual modifications
  • Review of prevailing load conditions, to provide for fatigue resistance if required, for the design service life
  • Review of interaction with other members or service media
  • Review of Metallurgy and weldability of proposed materials
  • Weld joint design geometry, dimensioning and preparation that takes into account the welding process to be used
  • Tolerances on Size of welds
    Note: - If mechanized and robotic fabrication is considered, Welding-design should pay particular attention to set realistic joint tolerances, especially for sheet metal parts.
  • Subassemblies, provisions for fool proof assembly
  • Availability, cost
  • Ease of fabrication, manufacturing constraints and economy of production
  • Safety and stiffness without over-designing, resistance to weld distortion
  • Heat flow, internal constraints, residual stresses
  • Accessibility for manufacturing and ease of maintenance
  • Closed sections for torque resistance, esthetic requirements, use of standard sections
  • Inclusion of welding aids like attachments for hoisting, handling, overturning
  • Accessibility for inspection, provisions for use of positioners
  • Welding procedures
  • Weld Map establishing placement and sequence of welds
  • Need of preheating and post weld heat treatment
  • Inspections requirements
  • Cleaning, needs of protection and Finishing

Unacceptable defects are those compromising stability and/or functionality.

They should be established by design, specifying limits for acceptable discontinuities, detected by visual and other non destructive inspection methods, each technique with its requirements.

When using radiography (X-Ray inspection), reference is usually made to the volumes of Reference Radiographs (see in Welding Inspection), according to materials and thickness.

The designer has to select the maximum size and density of defects that is acceptable for the service conditions of the item on hand, and establish on the drawing a note to this effect.

If resistance to fatigue straining is an issue, then the assessment of the relative fatigue life of welds should be researched and, if applicable, fatigue life improvement (i.e. shot peening) methods should be implemented.

Drawings of weldments should use the standard Symbols that express completely the essential features and requirements of welded joints.

ANSI/AWS A2.4:2012
Standard Symbols for Welding, Brazing, Nondestructive Examination
American Welding Society / 01-Jan-2012 / 152 pages

Drawing Notes

As a consequence of the final configuration of the design adopted, definite instructions representing essential information must be included in welding drawings.

This is done by adding suitable notes to the drawings, as listed hereafter. The notes assure the transmission to the fabrication personnel of the practical ways to be employed for realizing the designer's intentions.

  • Material and condition as per Specification,
  • Joint design and dimensions as described by the use of standard symbols,
  • welding process to be selected or authorized alternatives,
  • Joint dimensions, preparation and fixturing,
  • Cleaning procedures,
  • Tolerances for fit-up,
  • Filler material, size and Specification,
  • Minimum and acceptable joint size, (as it can influence quality and cost)
  • Special precautions as Welding Procedure Specification (WPS) if required,
  • Inspection requirements and acceptance limits,
  • Heat Treatment if required,
  • Code Requirements if applicable like:
    • WPS (Welding Procedure Specification),
    • PQR (Process Qualification Record) and
    • WPQ (Welder Performance Qualification).

References

Most books on Engineering Design reserve only a limited place to the description of popular welding processes.

A good course in Welding-design would provide important insight to the students if detailing and discussing proven quality examples of successful implementation of the basic principles briefly introduced in this presentation.

Dedicated Welding-design courses are, at times, available at
http://www.ewi.org/
and, among others, at The Welding Institute,
http://www.twi.co.uk/

A page with links to Welding Calculations is available at
http://www.gowelding.com/calcs/

The following publications provide essential information on Welding-design.

AWS ARE-5
Design for Welding
American Welding Society, 01-Jan-2001, 80 pages

A few essential books on Design for Welding can be purchased at a nominal price from The Lincoln Electric Company at
https://ssl.lincolnelectric.com/lincoln/apdirect/store.asp?PID=16&cat=8
In particular we would recommend:
Design of Welded Structures

Handbooks, the Aluminum Design Manual, and a few of the important Welding Codes can be found in the page on Welding Books.

In conclusion, Welding-design is a most complex subject, because it involves knowledge and experience in a lot of disciplines that cannot be adequately found in a single individual, however brilliant and thorough.

The best provision to ensure adequate accomplishment would be to involve in the task a small team of excellent professionals.

An Article on Integrated Computational Materials Engineering (ICME) was published (7) in Issue No.122 of Practical Welding Letter for October 2013.
Click on PWL#122 to see it.

An Article on Computer Modeling was published (2) in Issue No.123 of Practical Welding Letter for November 2013.
Click on PWL#123 to read it.

An Article on Weld-design-review(NEW) was published (8) in Issue No.136 of Practical Welding Letter for December 2014.
Click on PWL#136 to see it.

An announcement on pages reporting on new materials, High Entropy Alloys, was published (8) in Issue No. 141 of Practical Welding Letter for May 2015.
Click on PWL#141 to see it.

An Article on Advice to the New Welding Engineer, was published (2) in Issue No. 143 of Practical Welding Letter for July 2015.
Click on PWL#143 to see it.

An Article on Testing Automotive Composites, was published (3) in Issue 152 of Practical Welding Letter for April 2016.
Click on PWL#152.


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To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics.

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