Name: Kevin Jacob
Department: Metallurgical Engineering and Materials Science
Program: Ph. D.
Name of supervisors: Prof. Nagamani Jaya Balila
Description of Research:
High Pressure Torsion Processing of Maraging Steel 250: Microstructure and Mechanical Behaviour Evolution
Figure 1 Engineering applications of Maraging steel vary widely a) Rocket motor casings b) Fencing blades
Figure 2a) Hardness vs HJ plot showing shift in the peak aging in case of HPT processed samples b) APT map of the HPT + overaged sample (shown in orange box in a) showingelemental distribution with isosurface concentrations of Fe 70%, Ni 20%, Mo 8% and Ti4%
Precipitation hardening/age hardening is a technique used to increase the strength of the materials through heat treatment. We work with Maraging steels which are a class of high strength Fe-Ni martensitic steel whose strength increases by nearly 75% due to age hardening. These steels are extensively used in rocket motor casings and landing gears due to their excellent combination of strength and ductility. Exceptional formability, high strength to weight ratios, good weldability are additional advantages of this steel which make it distinct from other conventional martensitic type steels resulting in a range of applications as shown in Figure 1. The superior properties associated with these steels are mainly due to the complex microstructure they possess which includes: a) a hierarchical martensitic lath structured matrix b) nanometre sized precipitates that form on aging c) reverted and retained austenite which is produced by the dissolution of the Ni based precipitates on overaging.
The current study follows a quantitative and comprehensive approach to explore the influence of severe plastic deformation (SPD) processing parameters on the precipitation kinetics of maraging 250 grade steels and the resulting microstructural stability and mechanical response. High Pressure Torsion (HPT) has been used for the first time to obtain a finer grain size as well as a high dislocation density in the as-solutionized sample. An increased density of dislocations and grain boundaries is expected to provide additional new nucleation sites for precipitation, leading to changes in the kinetics of phase transformations. The study aims to determine the change in aging kinetics of HPT samples and correlate it to the microstructure and mechanical behaviour.
Methodology and Key Results
HPT processing was done on 30 mm diameter disc of the as received (AR) samples at room temperature under an applied pressure of 5.6 GPa for a total of 5 turns. The aging kinetics were characterised using hardness measurements. It is clear from Figure 2a that the peak aging in case of the HPT processed sample has significantly shifted to an earlier time and temperature, proving that there is acceleration in kinetics of precipitation for this condition due to increased density of precipitation nucleation sites. Atom Probe Tomography (APT) was done on the HPT overaged samples in a Cameca® LEAP 5000 XR instrument. From the isosurface plots it can be seen that Mo (along with Fe) aligns itself in the form of sheets/plates along what appears to be lath boundaries. The major precipitate that is expected in the overaged condition is of Fe-Ni-Mo which takes up a sheet/plate morphology instead of the spherical morphology seen in the AR + overaged samples. The distinct change in morphology of the Fe-Ni-Mo type precipitates (Figure 2b) resulted in an increase in tensile strength by 70% but a decrease in ductility from 26% to 2% was observed.
An overall peak hardness of ~8.7 GPa (890 HV, 64 HRC) was achieved after HPT processing followed by aging which is nearly 1.5 times higher than conventional 250 grade (AMS 6512) maraging steel. High strengths also resulted in poor ductility. The reason for reduced ductility in the HPT + aged sample was correlated to the changed morphology of Fe-Mo-Ni precipitates to a plate like form, acting as stress-concentration points, along with the high dislocation density, preventing further plastic accommodation at the tip of micro-cracks. This understanding gives a template for improved engineering of precipitate morphologies and distributions in maraging steels through other thermo-mechanical routes, which needs to be explored and can result in high strength grades of maraging steels to be deployed in service.
More details regarding the same can be found in:
K. Jacob, D. Yadav, S. Dixit, A. Hohenwarter, B. Nagamani, Mater. Sci. Eng. A 802 (2021) 140665.