QUENCHING AND PARTITIONING (Q&P) PROCESSING OF MARTENSITIC STAINLESS STEELS AISI 414

Abstract

Heat treatment consisting quenching of steel to a temperature inside the martensitic transformation interval with subsequent carbon partitioning between quenched martensite and retained austenite (Q&P treatment) is applied to martensitic stainless steel AISI 414 (Fe – 0.14 % C – 12.5 % Cr – 2.3 % Ni). Q&P processing was modeled using the thermomechanical simulator Gleeble 3800. Phase transformations during processing were studied using a contact dilatometer, which measures the change in the specimen diameter at the thermocouple spot.
After austenitization at 1150 °C for three minutes, the temperature of the onset of the martensitic transformation of the steel under study was 270 °C. To implement Q&P processing, the samples were quenched to temperatures of 250 °C, 200 °C, 150 °C and 100 °C. The carbon partitioning between the quenched martensite and untransformed austenite was carried out in the process of 3-minute exposure at 450 °C.
It has been shown that quenching to temperatures of 200–250 °C with subsequent annealing at 450 °C does not ensure the enrichment of unconverted austenite with carbon, sufficient to suppress martensitic transformation upon subsequent cooling to room temperature. Lowering the quenching temperature to 150 °C with subsequent annealing at 450 °C leads to complete thermal stabilization of the untransformed austenite up to temperatures of minus 60 °C, which ensures a high lowtemperature toughness of the steel under investigation through the TRIP effect, exceeding the values laid down in the requirements for high-strength pipe steel. The conclusion is made about the prospect of Q&P processing of the investigated steel for obtaining a high-strength state with a high level of low-temperature viscosity.