Sequential nucleation of phases in a 17-4PH steel: Microstructural characterisation and mechanical properties

Guma Yeli,Paul A.J Bagot,George D.W Smith,Michael P Moody,Keith Wilford,Maria A Auger

doi:10.1016/j.actamat.2016.11.052

Guma Yeli, Paul A.J Bagot + Show 4 more

Open Access

https://doi.org/10.1016/j.actamat.2016.11.052

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Abstract

A major concern for 17-4PH steels operating at elevated temperatures is embrittlement due to Fe-rich (α) and Cr-enriched (α′) phase separation, along with precipitation of other detrimental phases. In this study the sequence of microstructural changes at the atomic scale in a 17-4PH steel is characterized by atom probe tomography (APT) at two different ageing temperatures, 480 °C and 590 °C. In the earliest stages of heat treatment at both temperatures, APT reveals that dislocations and matrix defects are highlighted by the segregation of NbN/CrN ionic species, providing heterogeneous nucleation sites for subsequent Cu-rich precipitates (CRPs) and Nb-rich precipitates, respectively. At the lower temperature, Cr-rich α′-phase also nucleates and ultimately a Mn, Ni, and Si-rich (MNS) phase was observed to form. The evolution in number density and fraction of CRPs and Cr-rich α′-phase, the latter of which was not observed at the higher temperature, has been quantified and their respective contributions to the overall precipitation hardening of the material has been estimated.

Highlights

A major concern for 17-4PH steels operating at elevated temperatures is embrittlement due to Fe-rich (a) and Cr-enriched (a0) phase separation, along with precipitation of other detrimental phases
Miller and Burke made a direct observation of Cr-rich a0 precipitates in 17-4PH aged at 480 C utilizing atom probe field ion microscopy (APFIM) [5]
A 17-4PH steel was heat treated at two precipitation hardening temperatures (480 C and 590 C)

Summary

Introduction

17-4PH is a precipitation hardened steel that normally contains 15.5e18.5 at.% Cr and 2.5e4.5 at.% Ni. After solution treatment (typically at 1038 C) and quenching, it can be heat treated at different temperatures to develop a wide range of mechanical properties. Miller and Burke made a direct observation of Cr-rich a0 precipitates in 17-4PH aged at 480 C utilizing atom probe field ion microscopy (APFIM) [5] They reported that no a0-phase was present in samples aged at a higher temperature (635 C). The present study is focused on using APT to provide detailed analysis of the sequence of precipitation at the atomic scale in different stages of ageing in 17-4PH. The nucleation, growth and coarsening of precipitates at the various ageing stages has been characterized and correlated to results from Vickers hardness testing, linking microstructure to a key mechanical property

Experimental

Hardness variation

Precipitation sequence

Precipitation hardening effects

Conclusion