Masters Theses

Date of Award

6-1983

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

E. Eugene Stansbury

Committee Members

Carl D. Lundin, J. E. Spruiell

Abstract

The sensitization behavior of CF and CFM type cast duplex stain less steels has been examined to provide a basis for the evaluation of potential sensitization damage due to weld repair thermal cycles. The major variables in the study include carbon content, ferrite number, molybdenum content (inherent in the choice of casting types) and heat treatment including simulated weld repair thermal cycles.

Two new electrochemical techniques, termed the EPR and JEPR tests, for assessing degree of sensitization (as a result of grain boundary chromium depletion) have been investigated and the results compared with those resulting from the standard oxalic acid etch test. The EPR test provides a sensitive quantitative measure of the degree of sensitization. For example, values for as-cast material exceed those for solution annealed material by approximately three orders of magnitude. Although molybdenum seemed to be the strongest compositional variable governing the EPR response, the test was also sensitive to carbon, chromium and ferrite content as well as heat treatment.

An adaptation of the EPR test, designed to be equally responsive to degree of sensitization but capable of reducing effects of variations in the bulk alloy content, was also investigated. This test, called JEPR, also detected the influence of carbon, chromium and molybdenum as they affected sensitization but not as they affected general passivation characteristics (general passivation controlled by total alloying present). This test indicated that molybdenum retards carbide formation by indicating Mo bearing alloys are superior to alloys that do not contain Mo for short heat treatments.

The oxalic acid etch test was shown to be a very severe test for cast duplex stainless steels resulting, almost without exception, in etch structures "unacceptable" according to the ASTM standard. This test appears to be capable of providing only limited qualitative differences between low and high levels of sensitization in the cast duplex stainless steels.

Sensitization measurements on samples subjected to thermal cycles simulating weld repair indicate that for some alloys, the requirement of post weld repair heat treatment may be unnecessary from a sensitization standpoint. The effects of energy input (con trolling the cooling rate), heat affected zone peak temperature, multiple thermal cycles and strain (simulating weld restraint) were studied. The results indicate that the Mo bearing CFM alloys with carbon contents less than .08% C may be reasonably immune to weld repair sensitization. The CF alloys with intermediate and high ferrite numbers, and with carbon contents less than .06% C, are only slightly sensitized by the simulated weld repair procedures. The low ferrite CF alloys appear to be the most susceptible to sensitization resulting from weld thermal cycles. However, this is at least in part related to a pitting phenomenon observed in these alloys which is not understood.

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