Carbide Formation and Growth Kinetics Enhanced by Tensile Stress and Elevated Temperature Intergranular Cracking in 2.25
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eat or stress relief cracking and premature cracking in heat-resistant steels are types of intergranular cracking that occur after postweld heat treatment and during exposure to elevated temperatures. The cracking usually occurs in the heat-affected zone and is usually confined to the coarse-grained regions.[1] However, reheat cracking is also possible in coarse-grained weld metal. The cracking mechanism was explained by the combination of a precipitation-strengthened matrix and a soft denuded zone formed adjacent to prior austenite grain boundaries[2–4] and by the prior austenite grain boundary segregation of impurities containing P.[5–7] In the former mechanism, M23C6 carbides are formed along the prior austenite grain boundaries, resulting in a C- and Cr-depleted zone. Also, the grain interior is strengthened by the precipitation of fine MC carbides. Therefore, most of the strain, which results from residual or thermal stress, is concentrated in the soft denuded zone, causing intergranular cracking. In the latter mechanism, the segregation of impurities to the prior austenite grain boundaries is known to cause the intergranular cracking by lowering a cohesive strength of grain boundaries. It has been also reported that the intergranular reheat cracking takes place by the normal creep failure mechanism in which cavities open along
N.H. HEO, Principal Researcher, and J.C. CHANG, Senior Researcher, are with the Power Gen Lab., KEPCO Research Institute, Daejeon 305-380, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted July 9, 2010. Article published online October 8, 2011 3562—VOLUME 42A, DECEMBER 2011
grain boundaries and then coalesce to form a continuous crack.[8–13] It is the purpose of this study to investigate effects of tensile stress on carbide formation and growth kinetics and elevated temperature intergranular cracking in heatresistant steels. Three types of 2.25Cr1.5W heat-resistant steel ingots of 6 kg were prepared using a vacuum induction melting process. The chemical compositions of the steels are shown in Table I. The ingots were solution treated at 1473 K (1200 °C) for 1 hour and forged to 12-mm-thick plates. Un-notched and cylindrical rupture test specimens with a thread head of 11 mm in diameter and a gage with a length of 15 mm and a diameter of 6 mm were machined from the plates. The specimens were austenitized at 1323 K (1050 °C) for 1 hour under a vacuum of about 10 2 Torr and water quenched for obtaining a martensitic structure. Rupture tests were performed in air using conventional creep test machines in a temperature range of 848 K to 973 K (575 °C to 700 °C) and in a stress range of 75 to 300 MPa. The specimens to which the N-type thermocouple was attached were heated to the test temperature at 1200 °C/h. The rupture test was carried out with no soaking at the test temperature. Fracture surfaces of ruptured specimens and reduction in area were examined using a scanning electron microscope (SEM) after ultrasonic cleaning. Changes in hardness with test condition were inv
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