Producing components with stainless steel is a smart choice for durability and product longevity because of its natural oxide protection layer. However, warping, corrosion, or even metallurgical changes can occur if best practices are not used.
Metalworkers new to stainless, as well as experienced welders and fabricators in a hurry, must pay careful attention to correct stainless steel handling methods and processes to protect the metal’s durability to hold the finished product in-spec.
Find out why introducing surface pollutants is one of the riskiest mistakes in stainless steel handling and how to prevent this and other mistakes during stainless steel fabrication.
Metallurgical Changes
How it happens:
Since stainless steel has a lower thermal diffusivity (4.2 mm2/s) than most metals, the Heat Affected Zone (HAZ) during the welding or thermal cutting process is greater.
This can result in a grade change (austenitic stainless steel being martensitic, more brittle and harder) or heated metal weakening.
Cutting Options:
In his article on the Heat Affected Zone, the four methods of cutting and the considerations to consider while using them during stainless steel fabrication are discussed.
Shearing, manual cutting, and waterjet cutting do not produce a Heat Affected Zone because they do not overheat the sheet metal.
Since it spreads heat to a relatively small area, laser cutting has the lowest HAZ among any other thermal cutting technique.
Since a plasma pulse is wider than a laser wave, it produces an intermediate HAZ. Higher currents make for faster cutting, minimizing exposure time and the width of the Heat Affected Zone.
Because of the extreme fire, sluggish motion, and wide flames, oxyacetylene cutting has the widest HAZ of any thermal cutting system.
Weld Decay:
Welding can cause chromium depletion in the base metal around the weld due to intense concentrated heat. Because of this change in the metal, it is more prone to corrosion. ASM International released a paper outlining these stainless steel sensitization control options.
A post-weld high-temperature anneal and quench to redissolve the chromium at grain boundaries to prevent the formation of chromium carbide on cooling.
To avoid carbide forming, use a low-carbon stainless steel grade (e.g., 304L or 316L). A stabilized stainless steel containing titanium (alloy 321) or niobium (alloy 327), which preferentially form carbides and leave chromium in solution. A high-chromium alloy (e.g., 301ln plates)
Distortion
How it happens:
Because of its poor thermal conductivity and high expansion rate, stainless steel is an easy target for distortion during welding.
Welding Tips:
Use the lowest allowable amperage without jeopardizing weld efficiency. Temperatures at the interpass can be regulated. Regulated tack welding, clamping jigs, or a copper backing plate should be used.
Introducing Surface Contaminants
How it happens:
As stainless steel comes into contact with iron filings and other chemical pollutants, The surface may be contaminated by grease on palms, chalk, and other objects introduced during handling. Both of these conditions can result in localized corrosion.
Avoiding Surface contamination:
Use a stainless steel grinding, mixing, and deburring abrasive.
Separate stainless steel abrasives from other abrasives used on various metals. Never use an abrasive that has previously been used on a different form of metal.
Separate stainless steel from carbon steel and other metals.
Keep stainless steel in its packaging and away from the elements for as long as possible.
Remove all transport adhesives from the stainless steel; these will harden and induce crevice corrosion on the job site.
Proper welding and cutting methods, knowledge of the HAZ, and commitment to proper quality management and anti-contamination. Procedures can result in a well-finished stainless steel product that meets specifications.
