Heat treatment of investment cast PH 13-8 Mo stainless steel: Part I. Mechanical properties and microstructure
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I.
INTRODUCTION
M A R A G I N G stainless steels are low-carbon, precipitation hardenable martensitic stainless steels which can achieve high strength, good corrosion resistance, and reasonable ductility, t~-Sl These alloys typically contain between 13 and 17 pct Cr, in addition to other alloying elements, to achieve these properties. Wrought PH 13-8 Mo stainless steel has been found to have superior properties when compared to other maraging stainless steels such as wrought 15-5 PH and 17-4 PH alloys. [5-H1 These alloys are usually heat-treated in three steps: (1) solution treating in the fully austenitic region at temperatures greater than 850 ~ (2) quenching to produce a completely martensitic matrix; and (3) aging at intermediate temperatures to allow for hardening by uniform precipitation of intermetallic compounds. Most available information concerning the alloy PH 13-8 Mo was developed for material in the wrought condition. However, recent interest in this material as a casting alloy for structural components has motivated an investigation of the heat treatment, mechanical properties, and corresponding microstructures of investment cast PH 13-8 Mo alloy. Little detailed information exists concerning the mechanical properties and microstructure of cast PH 13-8 Mo stainless steel. [~zi This investigation was undertaken to study the effects of various heat treatments on the P.W. HOCHANADEL, Graduate Research Assistant, and G.R. EDWARDS, Professor, are with the Center for Welding and Joining Research, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. M.J. CIESLAK, Manager, and C.V. ROBINO, Senior Member of the Technical Staff, are with the Physical and Joining Metallurgy Department, Sandia National Laboratories, Albuquerque, NM 87185-5800. Manuscript submitted February 25, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
microstructure and to relate the observed microstructural development to corresponding mechanical properties.
II.
EXPERIMENTAL PROCEDURE
Investment cast PH 13-8 Mo stainless steel was obtained from Amcast, Inc. (Rancho Cucamonga, CA) in the form of cast tensile and Charpy impact samples. The Charpy impact specimens were machined to standard proportions in accordance with ASTM E 23-88, and the tensile specimens were machined to have a 25.4-mm gage length and a 5-mm diameter at the reduced parallel gage section (similar, but smaller, than the ASTM Standard A 370). The chemical composition of the asreceived material is given in Table I. The heat treatments used in this investigation varied either the homogenizing or the aging conditions. All samples were homogenized in a vacuum atmosphere furnace. The homogenizing step for the "standard" heat treatment was performed at 1038 ~ for 5.4 ks (1.5 hours). The samples subjected to the "alternative" heat treatments were homogenized either at 1038 ~ or at 1121 ~ for times of either 5.4 or 14.4 ks (1.5 or 4 hours). The samples were then furnace-cooled to room temperature. All samples were solutionized in
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