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

Abstract

The effect of filler alloys C-263, , 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 ␥Ј (Ni 3 Al,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 ␥Ј 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 ␥Ј phase in the ␥ matrix of the weld metal. The HAZ cracking increased as the weld metal lattice mismatch between ␥Ј precipitates and ␥ matrix of the weld and its hardness (Ti ϩ 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 ␥Ј-␥ lattice mismatch and slow age-hardening response would reduce the HAZ cracking in IN-738 LC superalloy welds.

The Evolution of Cast Microstructures on the HAZ Liquation Cracking of Mar-M004 Weld

“…The induced microcracks were strongly related to inherent metallurgical factors of the Mar-M004 superalloy. As reported in previous works [20,21,24,25], constitutional liquation of MC carbides, M 3 B 2 borides, M 2 SC sulfocarbides, γ particles, and γ-γ eutectic accounts for the HAZ liquation cracking of IN 738 superalloy weld. The cracks were prone to propagate along the interfaces between the MC carbide and the matrix (Figure 5b), owing to substantial amount of MC carbides in the cast Mar-M004 superalloy.…”

“…Grain boundary liquation results from incipient melting of MC carbides, Cr-Mo borides, γ-γ eutectic and Ni-Zr intermetallics along the solidified boundaries [18][19][20][21][22][23][24]. The use of filler metals with (a) slower aging response, (b) smaller lattice mismatch between the precipitates and the matrix, (c) lowered (Ti + Al) concentrations and (d) softer weld metal have been reported to reduce the HAZ cracking susceptibility of IN 738LC welds [25]. Moreover, preweld heat treatments [10,14,15,[22][23][24] have been conducted to modify the microstructures of precipitate-strengthened Ni-based superalloys to decrease the sensitivity of liquidation cracking in the HAZ.…”

Liquation Cracking in the Heat-Affected Zone of IN738 Superalloy Weld

“…To date, many efforts have been made to eliminate or reduce the HAZ cracking of precipitation-hardened Ni-base superalloys through the control of primary microstructures by preweld heat treatment [18,19,[22][23][24], by laser [12,18,20,23,[25][26][27] or electron beam [28,29] welding processes, or by using filler with lower Al + Ti content [30][31][32]. In practical applications, the use of under matched filler or low heat-input welding processes to reduce weld-shrinkage stress are able to lower cracking in those Ni-base superalloys with poor weldability.…”

A Study of the Weldability of Gamma Prime Hardened Superalloys

“…However, γ' precipitation-strengthened alloys are very difficult to weld in a crack-free manner due to their inherent susceptibility to heat affected zone (HAZ) cracking during welding and during post-weld heat treatment [1,2,3]. Despite the difficulties regarding crack-free welding of superalloys, welding is the most essential joining and repair process for turbine parts [4].…”

“…Various combinations of these parameters can lead to different cracking mechanisms such as · solidification (hot) cracking · constitutional liquation cracking along grain boundaries · sub-solidus / ductility dip cracking and · strain age cracking during post weld HT, see [1,2,3] for details.…”