Microstructural Evolution and Mechanical Properties of Simulated Heat-Affected Zones in Cast Precipitation-Hardened Stai
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TRODUCTION
MARTENSITIC precipitation-hardened (PH) stainless steels 17-4 and 13-8+Mo have been used for a number of applications in the aerospace, nuclear, and military industries due to their high strength, corrosion resistance, and relatively good ductility.[1,2] Both of these materials solidify as d-ferrite, transform almost
ROBERT J. HAMLIN, Research Assistant, and JOHN N. DUPONT, Professor, are with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. Contact e-mail: [email protected] Manuscript submitted May 27, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
entirely to austenite upon cooling, and finally to martensite upon further cooling. The matrix microstructures, therefore, consist of martensite with approximately 10 to 20 pct remnant d-ferrite and less than a few percent retained austenite.[3–6] The typical heat treatment for these alloys consists of a homogenization step to reverse microsegregation from casting, followed by a solution treatment and quench to produce martensite that is supersaturated, and a final aging step. The aging step is typically conducted at temperatures between 723 K and 923 K (450 C and 620 C) for 1 to 5 hours to promote the formation of fine nanometer-scale precipitates. Alloy 13-8+Mo is strengthened by b-NiAl precipitates and 17-4 is strengthened by BCC copper (Cu)-rich precipitates.[6] The evolution of the matrix
microstructure and precipitates through the HAZ will determine the final mechanical properties. Therefore, it is necessary to understand the evolution of phase transformations within the matrix microstructure during heating and cooling as well as potential changes to the morphology of precipitates. Dilatometry experiments were performed at various heating rates for PH 17-4 and 13-8+Mo by Kapoor et al. to identify precipitation temperatures of these alloys.[7] It was determined that precipitation begins at temperatures between 673 K and 823 K (400 C and 550 C) for both 17-4 and 13-8+Mo and ends at temperatures between 773 K and 873 K (500 C and 600 C), depending on the heating rate. It was also observed that the martensite to austenite transformation began at temperatures between 923 K and 1173 K (650 C and 900 C) and finished at temperatures between 1023 K and 1223 K (750 C and 950 C), depending on heating rate.[7] These findings suggested that significant microstructural changes can be expected in the HAZ of these alloys during welding. MatCalc thermodynamic and kinetic modeling software is a useful tool that can be used to predict the evolution of the precipitates in these materials as they are subjected to weld thermal cycles. In the work performed by Povoden-Karadeniz and Kozeschnik,[8] the modeling of b-NiAl precipitates in PH 13-8+Mo was investigated. The mean radius, number density, and phase fraction of b-NiAl were calculated as a function of aging time at 848 K (575 C). It was determined that the critical precipitate radius, after which strength in the material decreased, was 3.8 nm. These findings compared we
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