Aging behavior of 17-4 PH stainless steel studied using XRDLPA for separating the influence of precipitation and disloca
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The aging behavior of precipitation hardenable 17-4 PH stainless steel is studied by analyzing the changes in microstrain, crystallite size, and dislocation density derived from the modified Williamson–Hall (mWH) method and the Fourier analysis of XRD profiles. Aging treatment of this steel at 380, 430, and 480 °C for 0.5, 1, and 3 h durations leads to changes in the microstrain due to precipitation and substructural changes caused by dislocation annihilation. The microstrain estimated from the mWH method is dominated by the precipitate-induced effects. The influence of precipitates and dislocations on the mean squared strain he2(L)i are separated by fitting the variation of he2(L)i with an expression P0 1 P1/L 1 P2/L2, where the parameter (P0)0.5 and P1 are shown to be related to the precipitate-induced and dislocation density-induced microstrain, respectively. The study shows that the XRD profile analysis can be used to separate the combined effects of precipitation and dislocation annihilation.
I. INTRODUCTION
17-4 PH Stainless Steel (SS) is a precipitation hardened martensitic stainless steel containing approximately 3–4 wt% Cu and is widely used in applications requiring high strength and modest level of corrosion resistance. This steel is used in different heat treated conditions to achieve the desired strength and toughness. Mostly, 17-4 PH stainless steel is used as the structural material in nuclear, aerospace, naval, and chemical industries where high strength, toughness, wear resistance, good fabrication characteristics, and corrosion resistance are essential. The microstructure of the solution annealed (SA) 17-4 PH SS is found to have parallel lath martensites, having a high dislocation density.1 The age hardening behavior of 17-4 PH steel has been studied extensively by various authors by using different analytical techniques.2–6 The precipitation of highly dispersed nano size copper particles in the martensite matrix leads to an increase in the microstrain, which in turn leads to an increase in strength of the material. The earlier studies on thermally aged 17-4 PH SS by TEM, atom ion probe microscopy, electrical resistivity, and hardness show evidence of copper precipitation, but prolonged aging leads to coarsening of the precipitates. From changes in resistivity, Viswanathan et al.1 inferred that there is no incubation period for the formation of precipitates, and the sequence of Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.396
precipitation is similar to that of Fe–Cu binary alloys. Rearrangement of dislocations within martensite laths was found in samples aged at a temperature of 510 °C for 0.5 h.2 The high-temperature aging studies on 17-4 PH steel revealed the formation of the austenite phase even at shorter durations, and the austenite phases were observed by different authors at 580 °C for 0.25 h,5 above an aging temperature of 599 °C for 1 h1 and at 620 °C for 4 h.7 While the microstrain increases due to coh
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