Masters Theses

Date of Award

6-1984

Degree Type

Thesis

Degree Name

Master of Science

Major

Materials Science and Engineering

Major Professor

Carl D. Lundin

Committee Members

Roy A. Vandermeer, E.E. Stansbury

Abstract

This research program was undertaken to provide fundamental and basic metallurgical information on the behavior of the heat-affect zone (HAZ) in Cr-Mo steel welds, as well as practical information on their relative weldability. The principal work was the evaluation of the stress-relief cracking (SRC) and hydrogen-assisted cracking (HAC) susceptibility of Cr-Mo steels ranging in Cr content from 2 1/4%-12%. Differences in observed cracking behavior were contrasted with differences in composition, on-cooling transformation behavior and weld microstructure.

The test methods were varied to investigate the effect of preheat, post-weld heat treatment (PWHT) , metallurgical structure and restraint (externally applied) on the weld cracking susceptibility. Stress-rupture testing of the Gleeble simulated HAZ was employed in the HAC determination. In addition, the Gleeble apparatus was employed to determine the continuous-cooling transformation (CCT) behavior of the coarse grained heat-affected zone. (This region of the weld is known to be the region most susceptible to SRC and HAC.) The samples generated during testing provided test data as well as metallographic specimens for examination by optical and electron optic techniques (SEM). Evaluation of the metallographic specimens defined the effect of microstructure on the weld cracking susceptibility.

The basic metallurgical information resulting from this investigation made clear the reasons for the relative cracking behavior of different Cr-Mo materials. The practical data from this investigation provides a basis for the determination of suitable welding conditions as well as a guide for the selection of materials from the standpoint of fabricability.

The literature review provides a concise historical review and basis of theories for SRC and HAC in Cr-Mo steels which were employed to explain the weld cracking susceptibility of various Cr-Mo steels. It is apparent that this weldability investigation has answered many questions neglected by prior investigations.

The results of the CCT determination for the coarse grained HAZ in 12Cr-2Mo-6Ni, 9Cr-2Mo-V-Nb, 9Cr-2Mo,9Cr-1Mo-V-Nb and 9Cr-1Mo show that martensite or martensite plus delta-ferrite is the predominate microstructure in the coarse grained HAZ. The microstructure of these materials is independent of cooling rates normally attendant upon welding. The Ms and Mf temperatures in the 9Cr-2Mo-V-Nb, 9Cr-1Mo , 9Cr-1Mo-V-Nb and 9Cr-1Mo are higher than the Ms and Mf temperatures determined for the 12Cr-2Mo-Ni materials. The lack of a detectable Mf temperature in the 12CR-2Mo-6Ni and 12Cr-2Mo-3Ni suggests that austenite may be present in the HAZ of these two materials.

Results of the CCT behavior determination for the coarse grained HAZ of 3Cr-1.5Mo and 2 1/4Cr-1Mo show that the on-cooling microstructure is dependent on the cooling rate. Over the range of cooling rates associated with commonly employed energy inputs the following types of HAZ microstructures may be observed: (1) for a fast cooling rate the microstructure is acicular martensite and lenticular martensite; (2) the microstructure at a slower cooling rate consists of lenticular martensite and bainite; (3) a further reduction in the cooling rate produces a microstructure which contains bainite or bainite plus ferrite. The 3Cr-1.5Mo HAZ is more likely to contain martensite than the 2 1/4Cr-1Mo HAZ because of its higher alloy content.

The SRC susceptibility of the Cr-Mo steels employed in this investigation is related to the extent of grain matrix strengthening which occurs during PWHT. An increase in grain matrix strength increases the susceptibility to SRC. During PWHT the grain matrix of 2 1/4Cr-1Mo and 3Cr-1.5Mo is strengthened more than that of 12Cr-2Mo-Ni, 9Cr-2Mo-V-Nb, 9Cr-2Mo, 9Cr-1Mo-V-Nb and 9Cr-1Mo and as a result, the 3Cr-1.5Mo and 2 1/4Cr-1Mo is more susceptible to SRC. During PWHT at 1150oF (620oC) the grain matrix of the 3Cr-1.5Mo and 2 1/4Cr-1Mo is strengthened more than at 1250oF (680oC) PWHT. This increase in grain matrix strength at the lower PWHT temperature results in an increased SRC susceptibility. Apparently the amount of Cr present in the 3Cr-1.5Mo imparts improved ductility to the precipitate strengthened matrix, and as a result, this material is less susceptible to SRC than 2 1/4Cr-1Mo. An increase in the C or Si content in the nominal 2 1/4Cr-1Mo increases the likelihood of strengthening the matrix, thus increasing the SRC susceptibility.

The HAC susceptibility of the Cr-Mo steels is dependent on the microstructure, carbon content, on-cooling transformation behavior and to a certain degree on the level of restraint. The 12Cr-2Mo-6Ni, 12Cr-2Mo-3Ni and 12Cr-2Mo-1Ni is more susceptible to HAC than the 9Cr-2Mo,9Cr-1Mo-V-Nb and 9Cr-1Mo because the austenite transforms to martensite at a lower temperature. The diffusivity of hydrogen in martensite is reduced at the lower temperature of transformation, thus hydrogen is less able to diffuse out of the HAZ, thereby increasing the cracking susceptibility. Among the 9Cr-2Mo, 9Cr-1Mo-V-Nb and 9Cr-1Mo materials, the HAC susceptibility is dependent on carbon content and delta-ferrite content. In general, an increase in carbon content results in an increase in HAC susceptibility. The 9CR-2Mo is more susceptible to HAC than the 9Cr-1Mo material of a comparable carbon content because the increased amount of delta-ferrite present in the 9Cr-2Mo partitions carbon to the martensite, increasing both hardness and HAC susceptibility. For high levels of simulated restraint (2% and 4% augmented strain) the HAC susceptibility of 3CR-1.5Mo and 2 1/4Cr-1Mo is approximately the same as 9Cr-2Mo and 9Cr-1Mo, while at a lower simulated restraint level (1% augmented strain) , the situation is reversed and the 3Cr-1.5Mo and 2 1/4Cr-1Mo materials are more susceptible to HAC. For an equivalent carbon content (0.11% C), the bainitic 2 1/4 Cr-1Mo is less susceptible to HAC than the martensitic 9Cr-1Mo. The 3Cr-1.5Mo is more susceptible to HAC than the 2 1/4Cr-1Mo because martensite is more prevalent in the 3Cr-1.5Mo HAZ.

In general, the relative SRC and HAC susceptibility of Cr-Mo steels can be categorized in terms of subclasses of Cr-Mo steels. With respect to SRC, the 12Cr-2Mo-Ni, 9Cr-2Mo and 9Cr-1Mo materials are not as susceptible to SRC as the 3Cr-1.5Mo and 2 1/4Cr-1Mo. With respect to HAC, the 12Cr-2Mo-Ni alloys are more susceptible to HAC than the 9Cr-2Mo and 9Cr-1Mo. For an equivalent carbon level the 3Cr-1.5Mo and 2 1/4Cr-1Mo materials are less susceptible to HAC than the 9Cr-2Mo and 9Cr-1Mo.

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