Simulation of the Precipitation Kinetics of Maraging Stainless Steels 17-4 and 13-8+Mo During Multi-pass Welding
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UCTION
MARAGING stainless steels 17-4 and 13-8+Mo are candidate alloys for high-strength military applications.[1,2] The low carbon content in these alloys results in a relatively ductile martensite, low retained austenite content, and minimal secondary carbide formation. Thus, their high strength is largely derived from the growth of fine nanometer scale precipitates that form during heat treatment. Many applications involving these materials require welding, necessitating an investigation of the microstructural and mechanical property
ROBERT J. HAMLIN and JOHN N. DUPONT are with Lehigh University, Bethlehem, PA, 18015. Contact e-mail: [email protected] CHARLES V. ROBINO is with Sandia National Laboratories, Albuquerque, NM, 87185. Manuscript submitted March 29, 2018. Article published online December 3, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A
changes that occur during welding. Previous studies on 17-4 and 13-8+Mo have demonstrated significant softening occurs in the heat-affected zone (HAZ) during welding due to the dissolution of the strengthening precipitates, and a post weld heat treatment (PWHT) was required to restore the properties.[3,4] However, PWHTs are expensive and impractical when welding on a large scale or making field repairs. Thus, improved welding procedures for these alloys are required. Multi-pass welding provides an opportunity to reform the strengthening precipitates that dissolve during primary welding thermal cycles by using secondary thermal cycles from subsequent weld passes. For the final weld passes, additional weld metal can be deposited to strengthen the final pass and the excess material can then be machined away. A concurrent study on 17-4 and 13-8+Mo has demonstrated that the times and temperatures associated with secondary welding thermal cycles are sufficient to restore the hardness lost during primary thermal cycles, indicating precipitate formation and VOLUME 50A, FEBRUARY 2019—719
growth is occurring.[5] In the current investigation, the precipitate kinetics during primary and secondary thermal cycles were analyzed using isothermal aging data and Avrami/Arrhenius analysis. Evaluation of precipitation hardening can be performed using the impinged volume Avrami equation[6–10] x ¼ 1 expðktn Þ:
½1
For which x is the fraction transformed, t is time and k and n are constants. The impinged volume equation is used to represent the interference of growing particles due to removal of solute from the matrix by neighboring particles. Thus, Eq. [1] has been found to be useful for estimating growth in precipitation strengthened systems such as maraging stainless steels.[10,11] The strength increase due to ordered precipitates has been described by Gerold and Haberkorn[12] as Ds / f1=2 r1=2 ;
½2
where Ds is the change in flow stress, f is the volume fraction of precipitates, and r is the precipitate radius. It has also been presented by Tabor[13] that the change in flow stress is proportional to the hardness. These relationships were used by Robino et al.[10] to further develo
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