Masters Theses

C. H. Lee

Date of Award

3-1985

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Committee Members

E. E. Stansbury

Abstract

The primary purpose of this study was to determine the range of welding variables for which the weld HAZ sensitization resistance of wrought austenitic stainless steels will not be reduced to an unacceptable level. In addition, a comparison of weld HAZ sensitiza tion behavior of the recently proposed BWR alternate alloys (304LN, modified 347, and 316NG) to the standard type 304 and AISI 347 alloys was also conducted. Furthermore, a comparison of the three new electrochemical methods (EPR, JEPR, and constant potential etch test in perchloric acid) to the conventional ASTM A262 practice A for detecting sensitization is included in this study. To accomplish precise determination of HAZ sensitization, a quick and accurate method, HAZ profiling combined with a bead-on-edge weld method, was developed. The results of the HAZ profiling technique were compared to the Gleeble HAZ simulation technique and field weld conditions.

Both the EPR and ASTM A262A (oxalic acid etch) tests were utilized to measure the degree of sensitization of the actual weld HAZ and Gleeble simulated HAZ. The optical microscopy and SEM were employed to correlate the measured degree of sensitization with the microstructure.

It was found that the degree of the HAZ sensitization strongly depends on the material composition, especially carbon content, and is a function of location in the HAZ and the peak temperature reached at the particular location. For a low carbon alloy, the allowable preheat and interpass temperature levels can be significantly greater than those for the higher carbon containing materials without causing a significant increase in the sensitization susceptibility. The weld restraint (thermomechanical cycle) affected the degree of sensitization. When a single thermomechanical cycle is employed, at least 10% strain is required to increase the degree of sensitization as compared to the thermal cycle alone. However, when multiple cycles are utilized, a strain as low as 5% affects the degree of sensitization provided the peak temperature is approximately 1600°F.

Multiple thermal cycles (five cycles) with a high interpass temperature (800°F) and a low temperature sensitization (LTS) treatment (500°C/24 hours) did not significantly affect the HAZ sensitization resistance of the BWR alternate alloys. The modified 347 alloys having a low carbon content (<0.03% C) and a high niobium-carbon ratio (Nb/C > 15) are susceptible to the knife line attack after a long-time sensitizing heat treatment (600°C/24 hours).

The new electrochemical methods (EPR, JEPR, and constant potential etch method in 1 N HC104 + 0.2 N NaCl) have a high discrimination power in detecting the degree of sensitization. Among them, the EPR test is most sensitive to the low degree of sensitization which occurs in the HAZ.

The HAZ profiling technique combined with the bead-on-edge weld method can precisely define the HAZ sensitization as a function of location from the fusion line and peak temperature reached, and the results are applicable and comparable to full section field welded HAZ.

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