Doctoral Dissertations

Date of Award

3-1983

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Committee Members

E. E. Stansbury, Ben F. Oliver, C. R. Brooks, A. Mathews

Abstract

The primary purpose of the present research was to investigate the influence of the alloying elements and multiple thermal cycling on the hot cracking resistance of austenitic stainless steel weld metals. In addition, the effect of composition and multiple thermal cycling on the hot ductility was also determined and correlated with the hot cracking tendency. To more clearly define the variables, the following experimental methods were developed and experiments conducted : (1) a method to develop laboratory-made alloys with variations in composition, (2) a method to evaluate the hot ductility and the hot cracking behavior of commercial weld metals in addition to experimental compositions, and (3) a test to separate the Influence of strain and thermal history on the hot cracking tendency of the alloy.

Both the Fissure Bend Test and the Gleeble Hot Cracking Test: were utilized to evaluate the hot cracking resistance of austenitic weld metals. Optical metallography, SEM fractography and energy dispersive X-ray analysis were employed to define the cause and mechanism of hot cracking in the austenitic weld metals.

It was found that fissures occur primarily along grain boundaries in the HAZ from the previous weld deposit, and fissuring is enhanced by multiple thermal cycling in the HAZ. Increasing Mn and Mo content and reducing Si content can reduce the hot cracking tendency of fully austenitic stainless steel weld metals. Liquation of the Si-segregated grain boundary regions produced during thermal cycling is a dominant factor which leads to an enhanced fissuring propensity.

A theory of "Reheat Hot Cracking" is proposed for both ferrite-containing and fully austenitic welds. A region, called the "Hazard HA2", exists near the fusion zone of subsequent bead upon multipass or repair welding. This "Hazard HAZ" region exhibits a relatively low ductility and fissures occur in this region upon welding under conditions of sufficient restraint. The cause of the low ductility in the "Hazard HAZ" is the embrittlement of austenite grain boundaries which are enriched with "harmful" elements such as Si, P, and S. The extent of the "Hazard HAZ" region depends primarily on the compositions of weld metal as well as the Cr/Ni ratio and Mn plus Mo level.

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