Carbide precipitation in 12Cr1MoV power plant steel

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I.

INTRODUCTION

T H E vast majority of creep-resisting steels used in power plant or in the petrochemical industry are based on low-carbon, low-alloy steels containing carbideforming elements, such as chromium, molybdenum, and vanadium, as deliberate additions. I1,2~ In addition to creep resistance, prolonged service at elevated temperatures also requires good oxidation and hot-corrosion resistance, possibly in environments containing hydrogen and sulfur. In the United Kingdom, the steels are often used within the temperature range 480 ~ to 565 ~ the service stresses being of the order of 15 to 30 MPa over time periods of some 30 years. There is currently considerable research in progress to implement higher alloy steels with the aim of improving the creep strength so that the service temperature can be increased. [3.4] Alternatively, the higher strength can be exploited by reducing section size, which can be beneficial from the viewpoint of welding, thermal fatigue, and the reduced cost of support structures. A lot of the effort is focused on the 12CrlMoV steel, which is the subject of this article. Given the prolonged and rather severe service conditions, it is of interest to examine the microstructural stability of all power plant steels.J5] A particularly important aspect of the microstructure is the carbide particles, t6] For low-alloy steels, there has been a lot of research to indicate that the initial carbide phase found immediately after the stress-relief heat treatment (typically at 700 ~ for a few hours) is cementite. In these alloyed steels, the cementite is not, of course, the stable carbide. Furthermore, it does not at this stage have a chemical composition which would be expected by considering the metastable equilibrium between ferrite and cementite.t7] Instead, the ratio of substitutional solute (X) to Fe atoms is initially the same as that in the ferrite matrix, Is] perhaps because the particles grow rapidly by paraequilibrium ~9,1~ transformation. During service, the cementite therefore enriches in elements such as chromium and molybdenum ~6,11-~3]at the expense of its iron content. This process of enrichment is interesting in several respects.

R.C. THOMSON, Research Student, and H.K.D.H. BHADESHIA, Lecturer, are with the Department of Materials Science and Metallurgy, University of Cambridge/JRDC, Cambridge, United Kingdom CB2 3QZ. Manuscript submitted July 24, 1991. METALLURGICAL TRANSACTIONS A

For example, it is of importance in determining the point where carbide transitions are expected. Since the enrichment process is thermally activated, it can also be used to estimate the thermal history experienced by the steel, especially where direct data are not available. As a follow-up to some studies done on low-alloy s t e e l s , 16A1-13] the purpose of the present work was to examine the effect of representative heat treatments on the chemistry and some other characteristics of the carbides that are found in 12CrlMoV-type steels.

II.

EXPERIMENTAL

DETAILS

A. Materials and Heat Treatment T