Design for welding: Letter to an engineer
|Drawing Guide Notes courtesy of www.roymech.co.uk, a U.K.-based Web site. Editors Note: The image is presented solely for the purpose of illustrating typicaldrawing notes.|
My Dear Design Engineer,
I sometimes am baffled by the way some of your colleagues deal with projects that require welding design.
In a recent posting in an engineering forum on the Internet, a design engineer with unspecified previous experience, requested information regarding weld joint detail examples. It was as if the author said, "Just show me those details somewhere, and I will produce the best welding design quickly."
Another design engineer, who probably has some experience, asked the following question in a private communication: "I'm looking for general notes to place on a drawing. I'm making my first weldment drawing, and I don't have any background on the proper notes to place on the drawing to indicate grinding levels or filling the weld for painting or finishing processes."
Do these questions sound familiar to you? With all due respect, I believe that, unfortunately, both express a fundamental lack of insight or basic understanding of essential welding design requirements by professionals who may be highly competent in other related design activities .
A lack of proficiency in welding design most likely is rooted in engineering schools' complacency about the subject or in outdated management concepts. It is not surprising that when faced with their tasks, designers are pressed to produce, even if they are in good faith unaware of their shortcomings, and even when the process has no built-in quality check.
You may have gained a superb education in modern engineering design requirements and display an enviable expertise, having learned the trade from outstanding professionals, but can you honestly maintain to be an expert in everything –
Are You Sure You Know Enough About Welding?
In particular, would you consider yourself an expert in the following important subjects that need be considered when you design for welding?
- Base and filler material properties and selection
- Metallurgy and weldability
- Restraint's influence on stress concentration
- Design of joint details for fatigue resistance
- Design for corrosion resistance
- Design for durability in operating conditions
- Design for manufacturability
- Process expertise and influence on properties
- Need for additional thermal treatments
- Finish treatment selection
- Weld sequences and distortion control
- Weld joint dimensioning
- Formulation of appropriate quality requirements
- Design for safety
- Design for inspectability
- Design for maintainability
- Integration of design and procedures to ensure minimum cost
- Use of weld and inspection symbols and pertinent notes
- Use of appropriate code requirements
Important Facts to Consider
Overwelding is not an asset; it is an expensive liability in terms of cost, residual stresses, and distortion .
The effect of joint design on fatigue behavior is important and cannot be overemphasized. Detail geometry and joint configuration are the most important variables affecting the fatigue life of a structural detail .
The fatigue strength of welded joints is relatively unaffected by the material's tensile strength. Although tensile strength may be affected by changes in alloys, heat treatment, or temper, the crack propagation rates are relatively insensitive to such changes .
Process selection can simplify the need for joint preparation. As an example, consider that electron beam welding is suitable for simultaneous double- or multiple-tier welding in separate positions .
Examples of recommended joint details are not the full story. It is important to review case histories of welded assemblies and of the modifications introduced to simplify and ease production, or to improve productivity and reliability .
Good design practices require knowledge of process requirements for easy and free access to joint faces. Learning and assimilating the basic principles involves being familiar with practical applications and having actual experience. Simply copying a few joint configurations in a drawing will not suffice.
You may be aware of concurrent engineering, a modern approach to design based on cooperative decision-making that harmonizes knowledge and experience of all relevant specialties. The central concept of concurrent engineering is that the multifunctional team brings together all of their specialized knowledge in making each decision .
A properly formulated engineering program should strive to engage a small team of professionals with a vast range of knowledge and experience in auxiliary disciplines to produce the best and most economic design .
It might be nice to adopt concurrent engineering that integrates all forms of specialization and uses relevant expertise in making each decision, but local and present constraints may not let you overcome the difficulties in adopting this policy .
Unfortunately, your company may press you to undertake wide and complex tasks for which you are not specifically trained. Your boss has confidence in you. How could you disappoint him or her and propose that you need to consult with experts, especially if those experts are not available in-house –
It is well-known that important programs, such as those involving aerospace hardware and industrial mass production of consumer items, rely on experts. What probably is not adequately appreciated with more limited projects is that a cooperative effort by a few individuals with complementary experience has a far greater chance of producing a balanced work than a single individual, however endowed.
Even when it is impossible to set up multidisciplinary task forces and design teams, a simple program should include reviews of all new design documents by experts in related professions or functions .
Heed these words from the AWS Welding HandbookChapter Committee on Design for Welding: "... designers should refrain from relying entirely upon their own knowledge and experience, which may be generalized. They are encouraged to consult with welding experts whenever appropriate."
Does all this make some sense to you? Can you build on some of these concepts and try to produce better designs for welding?