Date of Award

Fall 1967

Degree Type

Thesis - Restricted

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Jansen, Richard J.

Second Advisor

Blumenthal, Robert N.

Third Advisor

Hirthe, Walter

Abstract

Austenite reversion and stabilization in 18% nickel maraging steels has recently become of practical significance with the increasing application of these alloys to high temperature service, welded structures and cold formed parts. A number of investigations have been carried out to determine the percentage of stable austenite present after elevated temperature exposure, but little data is available on the microstructural changes which occur during reversion and the effects of reversion on mechanical properties. High and low temperature impact, and room temperature hardness and tensile tests, along with microstructural studies were conducted on an 18% nickel 250 grade maraging steel which had been aged for times ranging from 1 minute to 9 weeks at temperatures between 700 and 1300°F. The test specimens were cut from a 1/2 " thick hot rolled plate that had initially been solution treated at 1500°F. It is shown that austenite reversion takes place more rapidly as time at temperatures above 900°F is increased. A maximum in the amount of stable reverted austenite was found after aging for 1 hour at 1200°F. Small quantities of reverted austenite have little effect on mechanical properties. Results are consistent with the view that stable austenite is formed by a diffusion reaction. In addition, other phases form as a function of aging time and temperature and a qualitative model has been proposed to explain the sequence of transformations.

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