Effect of filler alloys on heat-affected zone cracking in preweld heat-treated IN-738 LC gas-tungsten-arc welds

摘要

The effect of filler alloys C-263, RENÉ-41, IN-718, and FM-92 on heat-affected zone (HAZ) cracking susceptibility of cast IN-738 LC, which is a high-temperature Ni-based superalloy used at temperatures up to 980 °C and is precipitation hardened by the γ′ (Ni3Al,Ti) phase, by gas-tungsten-arc (GTA) welding was studied. In addition, autogenous welds were also made on the IN-738 parent material. The preweld treatments consisted of the standard solution treatment at 1120 °C for 2 hours followed by air cooling, and a new heat treatment, which was developed to improve the HAZ cracking resistance of IN-738 LC. This heat treatment consisted of solution treating at 1120 °C followed by air cooling then aging at 1025 °C for 16 hours followed by water quenching. Welds were observed to suffer intergranular HAZ cracking, regardless of the filler alloy; however, the autogenous welds were most susceptible to HAZ cracking. In general, the cracking tendency for both heat treatments was maximum for C-263 and RENE-41 fillers and decreased with the use of FM-92 and IN-718 filler alloys. The HAZ cracking was associated mainly with constitutional liquation of γ′ and MC carbides. On some cracks, liquated low melting point containing Zr-carbosulfide and Cr-Mo borides were also observed to be present. The cooling portion of the weld thermal cycle induced precipitation hardening via γ′ phase in the γ matrix of the weld metal. The HAZ cracking increased as the weld metal lattice mismatch between γ′ precipitates and γ matrix of the weld and its hardness (Ti + Al) increased. However, the weld-metal solidus and solidification temperature range, determined by high-temperature differential scanning calorimetry, did not correlate with the HAZ cracking susceptibility. It is suggested that the use of filler alloys with small γ′-γ lattice mismatch and slow age-hardening response would reduce the HAZ cracking in IN-738 LC superalloy welds.