Failure analysis of a 316 stainless steel heat exchanger cell

Instruments used

EDAX^® Element detector and APEX™ software for EDS

Background

A burst section of a 316 stainless steel heat exchanger cell was analyzed to determine the root cause of unexpected failure during routine pressure testing. Based on the analysis, it was quickly determined that the 316 stainless steel had become heavily sensitized during a previous brazing operation.

Sensitization occurs when a susceptible material, such as austenitic stainless steel, is held within a critical temperature range for prolonged time periods. Chromium precipitates out of the parent matrix at the sensitization temperature, forming chromium carbides along the grain boundaries. The resulting lack of chromium in the material adjacent to the grain boundaries leads to rapid intergranular corrosion and failure through the compromised alloy.

It is important to select low-carbon alloy varieties to avoid sensitization. In this case, a 316L stainless steel would be more desirable than a typical 316 stainless steel with standard carbon allowances. Adding preferential carbide forming elements can assist in scavenging free carbon prior to the formation of chromium carbides. Lastly, it is important to implement the correct heat treatment to allow for uniform chromium diffusion in cases where exposure within the critical temperature range cannot be avoided.