Date

2015

Document Type

Thesis

Degree

Master of Science

Department

Materials Science and Engineering

First Adviser

DuPont, John N.

Abstract

Cast precipitation hardened (PH) stainless steels 17-4 and 13-8+Mo are used in applications that require a combination of high strength and moderate corrosion resistance. Many such applications require fabrication and/or casting repair by fusion welding. The purpose of this work is to develop an understanding of microstructural evolution and resultant mechanical properties of these materials when subjected to thermal cycles characteristic of the heat affected zone (HAZ) in a fusion weld. Samples of each material were subjected to weld thermal cycles in solution treated and aged condition (S-A-W condition) and solution treated condition with a post weld thermal cycle age (S-W-A condition). Dilatometry was used to establish the onset of various phase transformation temperatures. Light optical microscopy (LOM) and microhardness measurements were used to characterize the microstructures and comparisons were made to gas tungsten arc welds (GTAW). Tensile testing was also performed. MatCalc thermodynamic and kinetic modeling software was used to predict the evolution of copper (Cu) rich body center cubic (BCC) precipitates in 17-4 and β-NiAl precipitates in 13-8+Mo. The yield strength was lower in the simulated HAZ samples of both materials prepared in the S-A-W condition when compared to their respective base metals. Samples prepared in the S-W-A condition had higher and more uniform yield strengths for both materials. Significant changes were observed in the matrix microstructure of various HAZ regions depending on the peak temperature, and these microstructural changes were interpreted with the aid of dilatometry results and microhardness indentations. Despite these significant changes to the matrix microstructure, the changes in mechanical properties appear to be governed primarily by the precipitation behavior. The decrease in strength in the HAZ samples prepared in the S-A-W condition was attributed to the dissolution of precipitates, which was supported by the MatCalc modeling results. MatCalc modeling results for samples in the S-A-W condition predicted uniform size of precipitates across all regions of the HAZ, and these predictions were supported by the observed trends in mechanical properties. These findings indicate that welding these PH stainless steels in the solution treated condition and using a post weld age will provide better and more uniform mechanical properties in the HAZ that are more consistent with the base metal properties.

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