Creep crack growth of HK40 steel: Microstructural effects
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I.
INTRODUCTION
C R E E P properties of smooth specimens of HK40 reformer tubes which are used in petrochemical plants have been widely investigated in the as-cast and postservice conditions, t1-4] As in many other practical engineering alloys, the final creep fracture of HK40 is generally very brittle ~2] and does not just occur by the simple growth of cavities until they impinge upon each other and cover the bulk of grain boundary interfacial area. Instead, final fracture occurs by the relatively brittle propagation of a few intergranular cracks which form by the growth and interlinkage of smaller cavities, tSl Furthermore, it has been shown that longitudinal and circumferential cracks form in the matrix and heat-affected zones of welds when the tubes have been used for one-third of their design life. t41 Therefore, an investigation into the creep crack growth behavior of HK40 steel was carried OUt. [6'7'8] The reformer tubes are put into service in the as-cast condition. One tube is constituted with four sections by welding. Because the different parts of the tube are at different temperatures, their microstructures are different. Therefore, research into the effects of aging and solution treatment on creep crack growth is significant for engineering practice. This paper is concerned with the effects of intergranular and transgranular carbides obtained by aging directly from as-cast condition and after solution treatment at 1250 ~ on creep crack growth behavior. II.
EXPERIMENTAL PROCEDURE
The materials investigated are commercially produced centrifugal casting tubes which have an almost total columnar grain structure across the tube wall. The chemical composition of the material tested in this study is listed as follows (weight percent): C 0.47, Cr 24.91, Ni 21.14, Mn 0.84, Si 0.87, P 0.017, and S 0.013. The heat treatments used are shown in Table I. Constant load, tensile creep crack growth tests were conducted on these materials using 9-mm-thick compact
S.J. ZI-IU, Doctor, is with the Laboratory of Fatigue and Fracture for Materials, Institute of Metal Research, Shenyang 110015, People's Republic of China. J. ZHAO, Assistant Professor, is with the Department of Mechanical Engineering, Fushun Petroleum Institute, Fushun, People's Republic of China. F.G. WANG, Associate Professor, is with the Materials Engineering Department, Dalian University of Technology, Dalian 116024, People's Republic of China. Manuscript submitted September 20, 1988. METALLURGICAL TRANSACTIONS A
tension specimens at 871 ~ The temperature was maintained within +--3 ~ The shape and dimensions of the specimens are shown in Figure 1. The specimens were oriented so that the crack ran in the actual crack propagation direction of the reformer tube. Crack lengths were measured using a direct current (DC) electrical potential drop technique in which a stabilized DC of 4 amps was passed along the axis of the specimen. The resolution of the crack growth monitoring system was about 0.06 mm. [9] A calibration equation for crack length as a funct
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