Precipitation-Induced Changes in Microstrain and Its Relation with Hardness and Tempering Parameter in 17-4 PH Stainless
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PRECIPITATION-HARDENED stainless steels are widely used as structural materials for chemical and power plants because of their balanced combination of good mechanical properties, wear resistance, and adequate corrosion resistance. These steels are used in different heat-treated conditions in nuclear, aerospace, naval, and chemical industries where high strength, toughness and wear resistance, good fabrication characteristics, and corrosion resistance are essential. The general principle of precipitation hardening is to produce supercooled solid solution from which on aging, secondary phases precipitate. The 17-4PH (precipitation hardening) stainless steel used for the present study, is a martensitic stainless steel containing approximately 3 to 4 wt pct Cu and is strengthened by the precipitation of highly dispersed copper particles in the martensite matrix. This material has been studied extensively by
S. MAHADEVAN and R. MANOJKUMAR, Scientific Officers, and B.P.C. RAO, Head, are with the Nondestructive Evaluation Division, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Contact e-mail: [email protected] T. JAYAKUMAR, formerly Director with the Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, is now Visiting Professor with the Metallurgical and Materials Engineering Department, National Institute of Technology, Warangal 506004, India. C.R. DAS, Scientific Officer, is with the Materials Technology Division, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research. Manuscript submitted March 5, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
various authors using different material characterization techniques.[1–7] Viswanathan et al.[1] discussed the kinetics of precipitation in 17-4 PH stainless steel using changes in electrical resistivity and microstructures. In solution-treated condition, the microstructure of the alloy is found to have parallel lath martensite having a high dislocation density. On aging the alloy at 668 K, 728 K, and 772 K (395 C, 455 C, and 499 C), the resistivity decreased drastically and reached a steady value on prolonged aging. When the alloy was aged above 773 K (500 C), the resistivity changed at a slower rate with a higher value of final resistivity on prolonged aging, as compared to the lower temperature heat treatments. On aging at 872 K (599 C) for 1 hour, the authors found, in addition to the fine copper-rich precipitates, an interlath fcc precipitate with ribbon-like morphology, which was identified as austenite. Murayama et al.[2] reported that the solution annealed specimen of 17-4 PH stainless steel consists of largely martensite with a small fraction of d ferrite. However, Hsiao et al.[3] found that on solution treatment, the steel produces a uniform matrix, which consists chiefly of lath martensite with micro-twins. A peak Vickers hardness of 450 Hv is reported at 753 K (480 C) after an hour of aging and remains constant up to 10 hours of aging. At 838 K (565 C) and 893 K (620 C), the
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