Quantification of the Dislocation Density, Size, and Volume Fraction of Precipitates in Deep Cryogenically Treated Martensitic Steels

Ajesh Antony,Daniel Fabijanic,Reza Mahjoub,Natalya M Schmerl,Nikki E Stanford,Anna Sokolova

doi:10.3390/met10111561

Ajesh Antony, Daniel Fabijanic + Show 4 more

Open Access

https://doi.org/10.3390/met10111561

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Journal: Metals	Publication Date: Nov 23, 2020
Citations: 7	License type: CC BY 4.0

Affiliation: Deakin University, University of South Australia

Abstract

Two groups of martensitic alloys were examined for changes induced by deep cryogenic treatment (DCT). The first group was a range of binary and ternary compositions with 0.6 wt % carbon, and the second group was a commercial AISI D2 tool steel. X-ray diffraction showed that DCT made two changes to the microstructure: retained austenite was transformed to martensite, and the dislocation density of the martensite was increased. This increase in dislocation density was consistent for all alloys, including those that did not undergo phase transformation during DCT. It is suggested that the increase in dislocation density may be caused by local differences in thermal expansion within the heterogeneous martensitic structure. Then, samples were tempered, and the cementite size distribution was examined using small angle neutron scattering (SANS) and atom probe tomography. First principles calculations confirmed that all magnetic scattering originated in cementite and not carbon clusters. Quantitative SANS analysis showed a measurable change in cementite size distribution for all alloys as a result of prior DCT. It is proposed that the increase in dislocation density that results from DCT modifies the cementite precipitation through enhanced diffusion rates and increased cementite nucleation sites.

Highlights

Deep cryogenic treatment (DCT) is a sub-zero treatment where materials are exposed to temperatures below −125 ◦ C with liquid nitrogen being the preferred cooling media [1]
This analysis showed that the alloys with a low Ni concentration were largely martensite, but the highest Ni alloy was fully austenitic at room temperature
The Fe11CR sample shown here was of particular interest because in the literature [7,9,46], it had been suggested that after cryogenic treatment, Cr carbides are redistributed throughout the microstructure, thereby enhancing the wear properties

Summary

Introduction

Deep cryogenic treatment (DCT) is a sub-zero treatment where materials are exposed to temperatures below −125 ◦ C with liquid nitrogen being the preferred cooling media [1]. The transformation of retained austenite (RA) to martensite is one of the primary objectives of DCT, and it is one of the predominating proposed mechanisms for improvement in wear resistance, as the martensitic phase is much harder than the parent austenite phase [2,4,19]. This transformation has regularly been quantified and verified via X-ray diffraction (XRD) [3,4,19,22,24,25,26,27,28,29,30]. It has been observed that alloys that are already fully martensitic can exhibit markedly improved wear resistance after DCT, even though there is no change to the martensite volume fraction [31]; this is commonly referred to as the conditioning of the martensite [32]

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