Effect of thermal compensation treatment on the microstructure and mechanical properties of IN738LC joint by electron beam welding

Abstract

Thermal compensation treatment (TCT) obtained sound IN738LC joint by electron beam welding (EBW), increasing the characteristic dimension of fusion zone (FZ) and heat affected zone (HAZ) prominently. But no microstructure change occurred in the FZ of IN738LC joint subjecting to TCT, consisting of nanoscale γ′ particles, discrete MC carbide and isolated γ/γ′ eutectic. The content of γ/γ′ eutectic and γ′ size in FZ increased as the number of TCT increased, attributed to the decreased cooling rate of FZ. When adopting TCT, the dissolution of γʹ particle in HAZ was more evident, resulting in an increase of liquid film thickness. As a result, the total length of HAZ liquation cracking rapidly decreased. With the increased number of TCT, the micro-hardness of FZ and HAZ in IN738LC joint increased gradually. All the joints fractured in the BM, but obtained a decreased elongation at room temperature (RM) and 900ºC, mainly associated with the increased content of MC carbides and γ′ particles in FZ and HAZ. The fracture surfaces of IN738LC joints showed quasi-cleavage fracture mode and ductile fracture feature at RM and 900ºC, respectively. In addition, the rupture life of IN738LC joint at 900ºC/350MPa was improved significantly from 3.1h to 4.25h after TCT. All the joints fractured in FZ and showed a typical intergranular fracture feature.