Effects of M 23 C 6 on the High-Temperature Performance of Ni-Based Welding Material NiCrFe-7
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THE application of Ni-based alloys has been increasing due to industry’s demands for improved properties of materials, such as high-temperature strength and corrosion resistance.[1–3] However, the use of such alloys has been limited by ductility-dip-cracking (DDC) appearing in the weld metal and its intermediatetemperature embrittlement (ITE) under high-temperature tensile conditions.[4–8] DDC is a solid-state hot cracking defect, occurring in the high temperature range of 0.5 to 0.8 Tm, and DDC often occurs at grain boundaries (GBs) because of the poor ductility of GBs.[5,9–11] To test and observe the sensitivity of DDC in Ni-based alloys, especially in the NiCrFe-7 (also known as FM 52, a filler metal (FM) for Inconel 690) weldment, researchers have developed several test methods, such as the strain-to-fracture (STF) test based on the Gleeble test,[12] thermo-mechanical in-situ scanning electron microscopy (SEM) observation,[13] and the high-temperature tensile test.[14] However, there remains no uniform understanding of the mechanism of DDC. Researchers have proposed a variety of mechanisms, such as impurity elements
WENLIN MO, Doctoral Student, and SHANPING LU, DIANZHONG LI, and YIYI LI, Professors, are with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] Manuscript submitted October 9, 2013. Article published online July 8, 2014 5114—VOLUME 45A, OCTOBER 2014
embrittlement, in which S, P, and other impurity elements segregate at GBs, causing the GB loss of ductility, thereby leading to DDC[15–18]; GB migration, i.e., migrated GBs, which migrate from the solidification grain boundaries (SGBs) and form straight GBs that may promote GB sliding, thus assisting in the formation of DDC[5,9,19–22]; and precipitation-induced cracking (PIC), such as M23C6 (M = Cr, Fe) PIC.[23] The occurrence of ITE indicates that the maximum elongation and reduction in area during tensile tests at intermediate temperatures are much lower than those at low and high temperatures.[6,7,24–26] ITE only appears in polycrystalline metals and alloys because of GB embrittlement, which cannot be found in single crystals.[27] The factors affecting ITE include GB embrittlement caused by impurity elements and PIC.[6,7,24–29] Both DDC and ITE are ductility dip phenomena caused by intermediate-temperature GB embrittlement. Understanding ductility dip phenomena at intermediate temperatures is of practical interest, because such phenomena limit the hot workability of Ni-based alloys. DDC and ITE have a tendency to make welding, forging, and casting of the alloys difficult, thus making the minimization of the effects of DDC and ITE an urgent problem to be solved. M23C6 is a commonly existed precipitate in Ni-based alloys. The influence of M23C6 precipitation on ductility dip phenomena has been discussed, but remains an issue with no common agreement. Nissley et al.[30,31] inferred that M23C6 pins the GBs, blocks GB migration,
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