The Reliability of Single-Step and Double-Step Quench and Partitioning Heat Treatments on an AISI 420A Low Carbon Martensitic Stainless Steel

The microstructural and mechanical effects of various single-step (SS) and double-step (DS) quench and partition (Q&P) heat treatments applied to an AISI 420A low carbon martensitic stainless steel (MSS) has been studied. The goal with this work is to reach a total elongation (E pct) of 12 pct and an ultimate tensile strength (UTS) above 1200/1300 MPa, but ultimately to achieve a superior strength-ductility balance in comparison to its traditional Quench and Temper (Q&T) counterpart. This is being done by retaining austenite within the steel’s martensitic matrix at room temperature (RT) using novel SS and conventional DS Q&P heat treatments. Considerable work has been done to optimize DS Q&P heat treatments, but little has been done to understand the effects of removing a subsequent heating cycle through SS Q&P heat treatments has on MSSs. With that being said, partitioning is performed at the same quench interruption temperature for the SS Q&P heat treatments, and reheated to a higher temperature for the DS Q&P heat treatments. Experimental investigations were carried out on 1 mm thick, sheet samples to increase the number of potential applications for this steel and heat treatment. The microstructure of different SS and DS Q&P heat treatments was investigated through X-ray diffraction (XRD) and transmission electron microscopy (TEM) while mechanical property investigations were carried out using tensile and fracture toughness testing. DS Q&P heat treated samples quenched to 130 °C and partitioned for industrially relevant times of 10 and 30 minutes featured the highest values in terms of total elongation, tensile strength and fracture toughness. The SS Q&P heat treatments, on the other hand, were able to achieve improved mechanical properties to its Q&T counterpart. Overall, this work opens up the possibility of increased MSS usage for reliable, thin-walled component production with improved properties through Q&P heat treatment methods. The best results achieved in this study are a UTS of 1585 MPa, E pct of 22 pct, and a fracture toughness of 77 kJ/m2. Their lower total elongation of 9.6 pct is balanced by high tensile strength of 1812 MPa, ensuring higher toughness compared to traditional Q&T samples.