Identify and quantify the thermodynamics and kinetics aspects of fast tempering in martensite low-alloy medium carbon steel used for aerospace application

Abstract

Induction heating, a fast and popular localized heat treatment process is increasingly used in aerospace industries that require good fatigue and wear resistance. Due to the induction highheat exposure, transformation towards equilibrium state of martensite in over-tempered zone (loss of hardness) happens between the hardened surface layer and the core of the part by a series of thermally activated phenomena. In order to understand over tempering behavior, the kinetics of cementite coarsening in tempered martensite AISI 4340 steel system with initial hardness 460 HV has been investigated. Microstructural investigation and modeling have been undertaken to understand the material response during tempering at 550 °C, 650 °C, and 700 °C. Different methods to reveal precipitates have been tested and compared. A statistical analysis was performed on microstructural measurements such as size, volume fraction, and morphology. On the other hand, the coarsening behavior of this multicomponent alloy system has been investigated using kinetics and thermodynamics thanks to DICTRA software. This computational tool can handle complex alloys and was used to model the precipitation resulting from tempering effect. Modeling results indicate that the coarsening resistance of 4340 alloy system is achieved not only by Cr and Mn partition to cementite at the early stage of tempering but also at later stage of tempering by partitioning element Mo in cementite, and also provided by partitioning of Ni and Si in the matrix at later stage of tempering. These results highlight the remarkable coarsening resistance of AISI 4340 steel even for prolonged tempering. Also due to low temperature 653 K (380 °C) chosen for initial tempering, only C can diffuse at this temperature, resulting a thermodynamics paraequilibrium stability, and any further high temperature exposure (over-tempering) will require to reach a new thermodynamic condition preventing rapid cementite growth. An extension of Björklund model is used to validate DICTRA model results. At the end, based on microstructure parameters strength and hardness of over-tempered zone are measured.

Date	6 Apr 2018
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Philippe Bocher (Supervisor)