Investigation of the effects of cryogenic treatment on the mechanical properties of AISI 431 stainless steel

This study investigated the effects of shallow and deep cryogenic treatment on the mechanical and microstructural properties of AISI 431 martensitic stainless steel. Steel samples were first kept at 980 °C for 30 minutes, then cooled in oil, and then tempered at 650 °C for 2 hours. After this process, shallow cryogenic treatment was applied at -80 °C for 12(SKİ12T) and 24 hours(SKİ24T), and deep cryogenic treatment was applied at -180 °C for 6 (DKİ6T), 12 (DKİ12T), 18 (DKİ24T), 24 (DKİ6T), and 36 (DKİ36T) hours on steel samples. It was observed that the hardness, tensile, yield, and abrasion resistance of the sample tempered at 200 °C increased by approximately 3.89%, 1.44%, 6.95%, and 62%, respectively, compared to the conventional hardening heat-treated sample. It was observed that the hardness, tensile, yield, and abrasion resistance of the samples tempered at 300 °C increased by approximately 3.52%, 1.18%, 7.03%, and 56%, respectively, compared to the untreated sample. It has been found that samples tempered at 200 °C generally exhibit beter mechanical properties. All other cryogenically treated samples were tempered at 200 °C for 2 hours. As a result of the tests, it was observed that the hardness, tensile, yield, and abrasion resistance of the cryogenically treated samples at different holding times increased compared to the untreated sample, and these increases were directly proportional to the increase in the holding times in both cryogenic treatment types. Compared to the untreated sample, the hardness, tensile, yield, and abrasion resistance of the SKİ24T sample increased by approximately 5.37%, 1.59%, 7.11%, and 78%, respectively. Hardness, tensile, yield, and abrasion resistance of the DKİ36T sample increased by approximately 5.87%, 1.66%, 8.17%, and 88%, respectively, compared to the untreated sample. As a result of the study, it has been determined that both cryogenic treatment types and all holding times increase mechanical properties compared to the untreated sample, while deep cryogenic treatment provides superior mechanical properties. In microstructural studies, a lath-type martensite structure with residual austenite was observed for all samples. However, the martensitic laths in the cryogenically treated samples are smaller than in the untreated sample. The highest mechanical properties were obtained by deep cryogenic treatment applied for 36 hours. The study observed that cryogenic treatment increased other mechanical properties (hardness, tensile,and yield) of AISI 431 martensitic stainless steel, but especially a significant increase in wear resistance. Keywords:AISI 431 martensitic stainless steel, Deep cryogenic treatment, Mechanical properties, Microstructure, Shallow cryogenic treatment