Investigating the effect of heat treatment on the mechanical properties and microstructure of AISI431 martensitic stainless steel

Martensitic stainless steels applications increase sharply in these decades. Applications cover a big variety of areas including vapor generators، food industries and any application in which an acceptable corrosion resistance and mechanical properties are the main factor in the material selection. In this study the effect of austenizing temperature، tempering period and tempering temperature on the microstructural and mechanical properties of AISI431 martensitic stainless steel were observed using optical microscopy (OM)، X-ray diffraction (XRD)، tensile، impact and hardness tests. To do this، double austenizing (primary austenizing at 1030 ˚C، oil quenching، tempering at 200 ˚C followed by secondary austenizing at 970˚C، oil quenching and tempering at 650 ˚C) was performed on the samples to gain the highest absorbed impact energy and tensile strength. Results showed that with increasing the austenizing temperature from 970 to 1030 ˚C، the absorbed energy in impact test and the tensile strength did not change appreciably. In double austenizing، the maximum absorbed energy in impact test and the highest value in the tensile strength and hardness were achieved as a consequence of dissolving the carbide in the austenite matrix during the primary austenizing treatment. Tempering at low temperatures (200 ˚C) leads to the carbide precipitation in the final structure. Lowering the temperature of second austenizing process prohibited the austenite grains growth and subsequently a small grain sized structure produced. These consequences of heat treatments produce an optimum mechanical property which consists of maximum tensile strength and absorbed energy during the impact test. Furthermore، secondary hardening was observed in AISI431 martensitic stainless steel between 350 to 420 ˚C.