Doctoral Dissertations

Chang H. Lee

Date of Award

12-1988

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Committee Members

Ray A. Buchanan, Charlie R. Brooks, Dominic A. Canonico, John D. Landes

Abstract

This study evaluated the hot ductility response, and hot cracking susceptibility (fusion zone solidification cracking and HAZ liquation cracking) of modified nuclear grade and standard austenitic stainless steels. Extensive microstructural characterization using state-of-the art analytical electron microscopy (TEM and STEM) as well as SEM (EDAX) and OLM was performed to correlate the material behavior with metallurgical characteristics. In addition, studies of the effect of Si, N, and rare earth elements on hot cracking susceptibility, significance of the ductility dip phenomena and backfilled solidification cracks were also performed. Furthermore, based on the metallurgical evaluation, the possible mechanisms involved in solidification cracking and HAZ liquation cracking of the modified alloys are proposed. Finally, the optimized chemical specifications and requirements for nuclear grade stainless steels are also suggested.

The hot ductility and weldability behavior of the modified 316NG and 347NG alloys were found to be superior to, or at least equivalent to, the standard AISI 304, 316 and 347 alloys. Hot ductility response and hot cracking susceptibility of stainless steels were dependent upon major and minor element content (F, S, Si and Nb), ferrite content (potential), grain size, and solidification mode (Creq/Nieq ratio). Among them, the primary solidification mode was the most important factor such that the primary ferritic alloy exhibited a better behavior than that solidified in a primary (fully) austenitic mode when other variables were constant. The influence of the primary solidification mode was found to be more important for crack propagation than for initiation.

The effect of impurity level (P + S) on the extent of cracking was found to be dependent on the Creq/Nieq ratio, such that when the ratio is greater than approximately 1.6, the P + S content can be as high as 0.06%. However, when the ratio is less than about 1.5, a P + S content as low as 0.02% can exert significant influence on solidification cracking. The relative amount of Nb to C and N (1/2 %Nb / (30 X %C + 50 X %N) appeared to be a predominant factor for the hot cracking in type 347. When this ratio is less than about 0.1, P, S and Si played a significant role in solidification cracking.

In order to predict material behavior more quantitatively and definitively, "new" parameters; critical strain temperature range (CSTR) and ductility recovery ratio (DRR) for Gleeble hot ductility testing and the cracked HAZ length (CHL) for the Varestraint base metal HAZ cracking, were developed and analyzed as a function of metallurgical variables and chemical composition. The correlation among these parameters indicated that when the CSTR is smaller than 70 ± 10 F° and DRR is greater than 40 ± 5%, the material is less susceptible to HAZ liquation cracking (virtually zero CHL).

The ductility dip phenomenon occurs by fracture along original but embrittled grain boundaries (by melting and resolidification due to impurity element segregation) at temperatures above the equicohesive temperature but below the dynamic recrystallization temperature. Grain boundary embrittlement may be further enhanced by the precipitation of chromium rich carbides and an increased amount of liquid formation by exposure to a higher peak temperature. The ductility dip phenomena may be characteristic of the testing method which involves significant deformation and dynamic recrystallization (under fast strain rate condition at high temperatures) and may not be likely to occur in the actual weld HAZ.

The addition of nitrogen to high silicon (1.25X) 304 decreases the solidification cracking susceptibility, when the primary solidification mode is maintained ferritic. The decreased cracking susceptibility was found to be due to a decrease in amount of low melting sulfides by the formation of high sulfur containing silicates. For a fully austenitic 316 stainless steel, the extent of solidification cracking decreased with an increase in the nitrogen content from 0.047% to 0.12%. The decrease in solidification cracking in a material containing higher nitrogen was due to a decrease in the solidification cell size and a narrower "brittleness temperature range" (BTR).

The extent of backfilled solidification cracks appeared to be a function of the primary solidification mode, length and width of the crack, location of crack in the weld and the applied strain level. The backfilling phenomenon is a result of capillary action of liquid. The liquid source for backfilling is that just ahead of solidification front which is enriched in Cr and/or Ni depending upon the solidification behavior and the liquid existing during the last stages of solidification.

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