Automatic Sample Preparation of Austenitic Stainless Steels for Microstructural Analysis

Wednesday, September 15, 2021: 9:00 AM
242 (America's Center)
Ms. Kelsey Torboli , Struers Inc., Westlake, OH
Austenitic stainless steels (2xx and 3xx) are the most common of the four types of stainless steels, both in usage and number of alloys. The corrosion resistance of stainless steels is based on alloying chromium with iron and is dependent on the formation of a passive surface oxide layer. Austenitic stainless steels contain 17-24% chromium and display high ductility, corrosion resistance, and resistance to chemical attack. To ensure stainless steel pipes and tanks are suitable for their intended use in chemical, pharmaceutical, and food industries, manufacturers perform metallography.

The high ductility of stainless steels presents difficulties during sample preparation. Austenitic steels are soft, ductile, and prone to mechanical deformation. Sectioning the hollow geometries of pipes and tanks can require special considerations as the thin walls may deform under the load of the cutoff wheel. Thin walled samples may undergo hot compression or cold mounting prior to grinding and polishing to make the process easier for the metallographer. When preparing 304 stainless, avoid coarse grinding steps and high pressures as this can result in deep deformation. Some companies will use diamond suspensions to polish the sample, while others will use electrolytic polishing and etching to reveal the microstructure. Electrolytic polishing gives excellent results for checking the microstructure, but is not suited to identify carbides as they will appear washed out or enlarged. Traditional etching can be difficult with stainless steels as they are highly corrosion resistant and require very strong acids to reveal their microstructure. If the stainless steel sample contains a weld, various image analysis programs can be used to measure features of the weld, like throat, penetration, and porosity. This presentation will demonstrate how automatic sample preparation can be used to achieve reproducible results, suitable for microstructural and weld measurement in austenitic stainless steels.