Tensile and Fracture Toughness behaviour of Thermo-Mechanically Rolled Direct Quenched and Partitioned(TMR-DQP) Steels

About

My work focuses on understanding the effect of the underlying mechanisms influencing the tensile and fracture toughness properties of TMR-DQP steel and the role of retained austenite in it.

This is a collaboration project between The Indian Institute of Technology Bombay , India and The University of Oulu, Finland.

The TMR-DQ and TMR-DQP steel (the as received material) is manufactured and processed at University of Oulu and the Mechanical and Microstructural characterisation is done at IIT Bombay.

Overview

Thermo-mechanical Rolling followed by Direct Quenching & Partitioning (TMR-DQP) heat treatment route consists of recrystallization-controlled rolling in the recrystallization regime followed by controlled rolling in the non-recrystallization regime. This results in a higher dislocation density and hence improves the strength. TMR step is then followed by direct quenching and partitioning resulting in a final microstructure consisting of a martensitic matrix along with evenly distributed and finely divided retained austenite. Martensite provides the high strength and austenite provided the enhanced ductility. Addition of alloying elements like Al, Si and P suppresses the competing reactions of carbide precipitations (unlike in the case of tempering)

The Quenching & Partitioning process: It is a novel heat treatment process which involves the heating of the steel to the fully austenized or the inter-critical temperature followed by Quenching to temperature between Ms (Martensite Start) and Mf (Martensite finish) temperature. This is then followed by the process of Partitioning : Partitioning may be carried out as a one step partitioning or a 2 step partitioning :

1. One step partitioning : Holding at the Quench temperature for some Partitioning time.

2. Two step partitioning: Holding at slightly higher temperature than the quench temperature for some partitioning time .

Objectives

Steel is an all versatile alloy and there is no end to the extent to the applications of it, but when it comes to Structural Applications, one always tries to seek both high strength and high ductility. The thermomechanical rolling followed by Quenching and Partitioning imparts this High strength and High ductility combination. Along with the the tensile properties, fracture toughness and understanding of the threshold stress intensity and associated fatigue crack growth rate of these steels are also very important for structural applications.

Here are the research objectives from this project.

• Understanding the effect of Thermo-Mechanical Rolling with Direct Quenching & Partitioning (TMRDQP) process on the microstructure and mechanical properties under monotonic and fatigue loading.

• Estimating the Fracture toughness of these steels and investigate whether the Linear Elastic Fracture Mechanics(LEFM) or the Elastic Plastic Fracture Mechanics (EPFM) prevails.

• Understanding the Fatigue Crack Growth Rate response of the steel as a function of the microstructure of the TMR-DQ and TMR-DQP steels