August 2021 Volume 3

MATERIALS

embrittlement as their martensitic/ferritic counterparts. It is paramount to understand that austenitic stainless steels are NOT hardenable by heat treatment. These alloys can only be stress relieved to relieve cold working or solution annealed to improve corrosion resistance and maximize ductility. Solution annealing becomes a necessity for austenitic stainless forgings due to sensitizationwhich describes the precipitation of chromiumcarbides into the grain boundaries of the material when it is cooled relatively slowly from forging temperatures. If left unchanged, a sensitized microstructure will rob the passive oxide film of chromium atoms (as they are now tied up in carbides) and result in intergranular corrosion and ultimately stress corrosion cracking of the component. These carbides also inhibit ductility and machinability if present in final form. In a solution anneal treatment, the forgings are re-heated to the range of 1850-2050°F (varies by specific alloy) and thoroughly soaked to ensure full heat-through of parts. It is often advisable to use the high side of any specified range for these alloys in order to maximize Cr-carbide dissolution into the austenitic matrix. After soaking, the forgings would typically be water quenched with high severity. The goal of this operation is to cool quickly through the range of 800-1600°F as this is the zone of carbide precipitation. Moving too slowly through this range risks re-formation of the carbides that were previously dissolved upon heating. Additionally, there is no risk

of quench cracking these alloys as no bulk microstructural change (and thus no volumetric change) is taking place to induce tensile stress on the part surface. Care should still be taken to avoid parts warping under their own weight, however. In the next issue, this column will be taking a deeper dive into duplex and precipitation hardening stainless steels along with their heat treatment parameters. In the meantime, don’t hesitate to reach out to the author with any questions or ideas for upcoming editions!

References: 1. ASMMetals Handbook - Desk Edition, 2nd Edition ■

Chuck Hartwig is the Director of Operations for Carburizing and Batch Hardening at ThermTech in Waukesha, WI. He holds a B.S. in Metallurgical Engineering from Colorado School of Mines. Email: chuck.hartwig@thermtech.net

FIA MAGAZINE | AUGUST 2021 41