Understanding High-Temperature Distortion of Metals

Austenitic stainless steels expand at nearly twice the rate of low alloy steels and four times the rate of ceramics. These differences in thermal expansion can create significant tensile or compressive stresses when the materials are combined for use in a cyclic thermal environment.

Another consideration is that heating and cooling are usually not uniform. Uneven temperatures, also known as thermal gradients, create stresses within the metal, often causing distortion and the poor thermal conductivity of stainless steel contributes to more significant thermal gradients. Thermal gradients of as little as 200°F from one side of a plate to the other are sufficient to create stresses that can exceed the yield strength of the material. Under cyclic conditions, distortion will become more severe with each cycle. Higher-strength alloys often permit thinner cross-sections, reducing distortion due to thermal gradients.

Distortion of components can also arise from relieving residual internal stresses. When mill products are produced, they are straightened or flattened after annealing. These residual stresses are what keep products straight and flat. There may also be residual stresses from fabrication. These stresses can be relieved once the components are exposed to elevated temperatures, resulting in potential distortion.

Lastly, it is crucial to distribute heat as evenly as possible to prevent distortion when welding. Welding consecutive tacks will cause the joint to close. Tack welds should be staggered over the joint to reduce distortion. A similar approach is necessary when making circumferential welds.