Correlation Between Intercritical Heat-Affected Zone and Type IV Creep Damage Zone in Grade 91 Steel
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IT is well established that a heat-affected zone (HAZ) with heterogeneous structures forms in the base metal close to the fusion zone after arc welding.[1–3] Figure 1 shows a schematic fusion welding peak-temperature profile and typical microstructures in the as-welded heat-affected zone of Grade 91 steel pipe weldments. The intercritical heat-affected zone (ICHAZ) experiences a peak temperature between Ac1 and Ac3, and has a mixed structure of fine re-austenitized grains and tempered martensite retained from the base metal.[3–5] The 100,000-hour creep strength of Grade 91 base metal at 873 K (600 C) can be greater than 100 MPa.[2] However, dramatic creep strength degradation of Grade 91 steel weldments has been reported, reducing the time to rupture to less than 10,000 hours at 873 K (600 C), when subjected to loads less than 100 MPa.[6,7] The majority of these creep failures are identified as the infamous Type IV cracking in the fine-grained heat-affected zone (FGHAZ)/ICHAZ.[3,4,8–12] The total creep
YIYU WANG, RANGASAYEE KANNAN, and LEIJUN LI are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada. Contact e-mail: [email protected] Manuscript submitted April 14, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
strain may be small, but a large local strain is observed in the FGHAZ/ICHAZ of the ruptured crossweld specimens. This short-term Type IV cracking has also been reported in many other creep strength-enhanced ferritic steels.[2,4] The Type IV cracking is more likely if the creep condition involves a higher temperature and a lower stress.[2] It is well recognized that multiple factors, including initial state of base metal, welding parameters, and postweld heat treatment (PWHT), contribute to the Type IV cracking. Many efforts[13–17] have been made to understand the creep rupture mechanisms of the Type IV cracking. Coarse, equiaxed grains with low dislocation densities and large precipitates along the grain boundaries were often observed in the creep-damaged regions. Creep cavities (voids) usually nucleated in the FGHAZ/ ICHAZ. However, it is not clear whether these cavities initiate in the FGHAZ or ICHAZ.[4] One common observation is that the region where the Type IV cracking occurred also has a lower hardness.[18–20] The location of this soft zone has been correlated to the FGHAZ, ICHAZ, or overtempered base metal by different researchers.[4,12] These conflicting observations may be caused by the narrow width of the HAZ itself, the sample preparation method (polishing and etching of the specimen), the limited resolution of optical microscopes, or the use of a too large loading force in hardness measurements. In addition, it is notable that
most studies are based on observing the structures of the weldment after a PWHT or a creep test. The PWHT parameters (temperature and hold time) and the creep conditions (sample size, creep temperature, and applied stress) introduce further microstructural changes that may shift the location of the soft zone
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