Masters Theses

Cong Yue Qiao

Date of Award

12-1988

Degree Type

Thesis

Degree Name

Master of Science

Major Professor

Carl D. Lundin

Committee Members

Charlie R. Brooks, Joseph Danko

Abstract

The results of this study are discussed in four parts. 1) Statistical test for hot ductility testing evaluation. 2) Statistical evaluation of the proposed subscale Varestraint test method. 3) Evaluation of nuclear grade austenitic stainless steels by the Gleeble hot ductility and Varestraint hot cracking methods. 4) The technique correlations between four hot cracking test methods.

The hot ductility test standardization was undertaken in order to provide comparable test conditions between laboratories and individual researchers. The various parameters which can affect the results were experimentally and statistically evaluated from data obtained single heats of type 304, 316 and 347 stainless steel. They are the Thermocouple attachment method, Thermal cycle. Peak temperature. Cross-head speed, and Holding time. For each parameter, more than two variations were tested and each variation using minimum of 10 specimens at selected test temperatures during both the onheating and the on-cooling portion of the thermal cycle. Based on the results of the above study, a standard for hot ductility testing was developed and was proposed for research community adoption.

Thirteen heats of nuclear grade stainless steel were evaluated by using the newly developed hot ductility test standard (Gleeble device). The criteria for the hot ductility test evaluation were reviewed and new concepts were developed. The statistical evaluation of the subscale moving torch Varestraint test method was undertaken to determine the validity of the test parameters. The results showed that the method provides significant statistical data. The hot cracking behavior of the same thirteen heats of the nuclear grade stainless steel were also evaluated by the Varestraint hot cracking test. The test results showed that the solidification mode was the dominant factor controlling weld metal hot cracking susceptibility. The materials with a primary austenite solidification mode exhibited a higher hot cracking susceptibility than those with a primary ferritic solidification mode. The susceptibility of the weld metal HAZ in these materials can be satisfactorily explained by Lundin- Chow's Reheat hot cracking theory. The susceptibility of the base metal HAZ to hot cracking was found to depend on grain boundary liquation. The content of the "harmful elements" was considered as a significant factor affecting the base metal HAZ hot cracking susceptibility since the "harmful elements" (such as S and P) may decrease the liquation temperature and thus, increase grain boundary liquation during welding.

Selected materials from the above mentioned thirteen heats of nuclear grade stainless steel were employed to conduct the Sigmajig test, Finger test and Dummy Bead-On-Plate tests. Among the three hot cracking tests, the Sigmajig test is an external restraint test method as is the Varestraint method. The Finger test method is a semi-self restraint hot cracking test method and the Dummy test is a completely self restraint type test. The test results from these three hot cracking test methods showed a good correlation with the results from Varestraint hot cracking test. It is concluded that the Varestraint hot cracking test is a reliable method for evaluating the hot cracking susceptibility of nuclear grade austenitic stainless steels.

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