The scientific processes behind successful welding and fabrication are a complex combination of metallurgy, welding techniques and managing stressors and temperatures. Achieving extremely tight tolerances is only half the battle. Ensuring that weldments are structurally sound, free of defects, and won’t crack once cooled is of equal importance.
Weld cracking is unacceptable in any application, but especially so in critical components produced for the defense department. Here, we’ll cover common causes of weld cracking and prevention techniques, as well as proper inspection processes.
Weld Cracking Causes and Preventative Measures
Material Selection
A welder doesn’t require a degree in metallurgy, but a basic understanding of how various materials perform under certain conditions is helpful. Material sensitivities and yield strengths will differ depending on several factors. If you’re welding two types of metal together, one will be inherently stronger than the other and will require special techniques and filler materials to create a strong bond.
The type of alloys and filler metal being deposited needs to be carefully considered. Refer to a filler metal selection chart for guidance on choosing proper filler metals. In general, steel, stainless steel, and aluminum are all prone to cracking, but for different reasons, which we explain in the following sections.
Chemical Makeup
Related to material selection is the chemical makeup of stainless steel vs. aluminum. The introduction of hydrogen during the weld process can cause steel or stainless steel weldments to crack. Aluminum, however, is impacted more by environmental stressors. It has much more springback than steel and conducts about five times more heat. As such, the probability of hot cracking is more prominent in aluminum than steel. In-depth knowledge of how each metal reacts is critical to preventing issues.
Temperature Changes
Expansion and contraction of weldments is a major cause of cracking. As a component is being welded, the heat will cause a weld joint to expand. As it cools, the filler material will solidify and contract or shrink, creating internal stresses. There are many techniques to mitigate the risk of weld cracking and weld shrinkage, including clamping and fixtures, weld mapping, sub weldments, and more. Using less heat in the weld and/or preheating the components being joined can also mitigate the risk of cracks. At times, a heat treatment is applied once a weld is finished to remove stress.
Residual or Applied Stresses
Weld cracking can occur due to stress risers such as improper joint preparation and the introduction of contaminants or tungsten inclusions. The overuse of intermittent welds or stitch welds also increases the likelihood of defects, as each start and stop is an opportunity for another weld crack. Preventing these types of unnecessary stressors requires strict processes, clean workstations, and attention to detail — the mark of true craftsmanship and welding experience.
If weld cracking occurs, it may be able to be repaired by grinding and excavating the area down to sound metal and laying another weld bead. Needless to say, the process is labor intensive and costly. Most notably, if the original issue that led to the problem in the first place is not corrected, it may fail to pass inspection yet again.
The Importance of Qualified Weld Procedures
One of the best ways to ensure the integrity of a weldment is to have proven and repeatable processes. Qualified weld procedures must be created based on various material types and thicknesses.
A Procedure Qualification Record (PQR) is used to help create these processes. The PQR indicates what materials and filler metals to use, what position to weld a component in, the amperage and voltage range, the type of gas to use, and other essential variables. Once a procedure is qualified for the specified material and thickness and all the nondestructive and destructive tests are completed, a weld procedure specification is written for use in production. The welder must stay within the parameters noted in the specifications or they run the risk of creating a faulty weld. Welders are monitored while performing their work to confirm that they’re adhering to the specifications to avoid noncompliance.
NAVSEA also has a technical publication outlining welder workmanship training. It’s a program that welders must complete every three years to ensure that they understand NAVSEA welding requirements. These defense department requirements combined with a fabricator’s qualified weld procedures help ensure top quality craftsmanship.
Inspection Processes
Military welding code (NAVSEA) specifies inspecting weld joints using unaided methods, meaning that no cracks should be visible to the naked eye. The timing of inspection is important as well; some materials require a 48-hour waiting period after completion because they are prone to delayed cracking.
Inspectors pay extra close attention to where a weld starts and stops, as that is where it is most prone to cracking, especially with aluminum. The beginning and end of a weld bead has a higher likelihood of more heat being applied or pooling of the filler material. In the end, however, every inch of welding must be inspected 100%.
At Fox Valley Metal-Tech, we have a team of weld inspectors, including:
- Four American Welding Society (AWS) certified weld inspectors (CWI)
- Seven American Society for Nondestructive Testing (ASNT) Visual Testing (VT) Level II inspectors (three of which are AWS CWI's)
To comply with defense fabrication codes, VT Level II inspectors are required. That’s why it’s important for defense contractors to ask a prospective metal fabrication company about their inspection process and their team’s credentials. To achieve various certifications can take hundreds of training hours, multiple exams, and years of experience. Finding a metal fabrication company that has in-house certified inspectors demonstrates that they are committed to quality and are willing to invest in their people and processes to prove it.
When issuing a request for quote (RFQ), it’s important to list any requirements for certifications, compliance, inspections, and testing. When vetting metal fabrication companies, also be sure to inquire about their processes, written procedures, and whether their welders and inspectors are certified.
For additional tips on developing your next RFQ, be sure to download our helpful RFQ guide below, and contact our team of metal fabrication experts with any questions.
