Masters Theses

Date of Award

12-1985

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Committee Members

Charlie Brooks, E. E. Stansbury

Abstract

The goal of this program was to investigate the effect of carbon content and post-weld heat treatment (PWHT) on the stress rupture strength of 2-1/4Cr-1Mo weld metal. In order to establish the correlation between carbon level and rupture strength, rupture testing of weld metals with carbon contents in the range of 0.02 to 0.13 weight percent was conducted. For this phase of the study, the PWHT selected was 1300°F/25 hrs. Secondly, the effect of PWHT on rupture strength was evaluated by rupture testing 0.08-0.09 carbon weld metal PWHT 1175°F/25.5 hrs. and comparing the results to those obtained for the same material PWHT 1300°F/25 hrs.

To gain further insight into the relationship between the rupture strength of 2-1/4Cr-1Mo weld metal and both carbon content and PWHT, additional rupture data were collected from the literature with the restriction that only data from tests of 100 percent weld metal specimens in the PWHT condition be considered. The resulting data base, consisting of data from this program as well as from the literature, was partitioned by both temper parameter and carbon content and analyzed using three separate techniques.

The results of the analysis indicated that the 100,000 hour rupture strength of 2-1/4Cr-1Mo weld metal increases as the temper parameter which characterizes the PWHT decreases. This effect is particularly pronounced for weld metal PWHT with P<32.0 which, at 750°F, has a 100,000 hour rupture strength 40 to 45 KSI higher than that of weld metal PWHT with 32.0

The effect of carbon content on rupture strength depends on the PWHT given the material. For weld metal PWHT with P<37.5, the 100,000 hour rupture strength increases with carbon level over the entire range of carbon contents typical of 2-1/4Cr-1Mo steel. In contrast, the 100,000 hour rupture strength of weld metal given a more extensive PWHT (P>37.5) appears to increase with carbon content only for levels up to about 0.05 weight percent. Weld metals having higher carbon contents have similar rupture strengths. It should be noted, however, that the strengthening effect of carbon diminishes as the temperature increases.

In support of the rupture testing effort, extensive microstructural examination of the above weld metals in the as-welded, PWHT, and rupture tested condition was performed using the SEM and STEM. This microstructural work was aimed at characterizing the microstructure as a function of carbon content and PWHT with the ultimate goal of correlating the microstructure with the stress rupture properties. SEM examination of the weld metals revealed the dependence of microconstituent morphology on prior material history. Furthermore, to obtain information on the type, size, morphology, distribution, and composition of the alloy carbides present, STEM examination of carbide extraction replicas taken from weld metal specimens was also conducted.

Efforts to correlate the weld metal microstructure with observed rupture behavior were successful. Microstructural analysis indicated that the superior rupture strength of the 0.08-0.09 carbon weld metal PWHT 1175°F/25.5 hrs. is primarily a consequence of the strengthening effect of the fine acicular M2C carbides present within the matrix. In contrast, the rupture behavior of weld metal PWHT 1300°F/25 hrs. is determined primarily by the major constituents present in the microstructure. The 0.02-0.03 carbon weld metal has a fully ferritic structure and also has the lowest rupture strength. The higher carbon materials (0.04-0.05, 0.08-0.09, and 0.12-0.13 weight percent) have a tempered bainite structure and have similar rupture strengths despite the variations in carbon content.

Weld metal specimens were thermally cycled on the Gleeble using a variety of thermal cycles designed to simulate the thermal history occurring in the coarse-grained region of the weld metal HAZ during welding. Cooling conditions were chosen to span the normal range of weld energy inputs characteristic of welds in heavy-section Cr-Mo materials. A dilatometric technique was employed to detect the weld metal transformation during cycling and the data obtained used to construct CCT diagrams. SEM examination of the resulting microstructures was then performed to determine the microconstituents produced as a consequence of the thermal history.

Both the dilatometric studies and the microstructural analyses indicated that the as-transformed microstructure of the 0.08-0.09 and 0.12-0.13 carbon weld metals is fully bainitic except in the material thermally cycled 150 KJ/IN (2400°F peak temperature, 400°F preheat, 2 inch plate) which contain traces of polygonal ferrite. The bainite morphologies observed include lath-like, granular, and massive. The lath bainite predominates at faster cooling rates and is gradually replaced by granular bainite as the cooling rate decreases. Some massive bainite is present at all cooling rates. Comparison of the as-transformed microstructures of the 0.08-0.09 and 0.12-0.13 carbon weld metals indicated that increasing the carbon content lowers the bainite start and finish temperatures and thus results in a finer bainitic structure.

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