The effect of weld Heat-Affected zone (HAZ) liquation kinetics on the hot cracking susceptibility of alloy 718
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I.
INTRODUCTION
CONSTITUTIONAL liquation o f precipitate particles and the subsequent formation o f grain boundary liquid in the weld heat-affected zone (HAZ) have been the subjects o f several studies,rl-61 In alloy 718, the formation o f grain boundary liquid has been attributed to the liquation o f carbides, I7J Laves phase, 18,9,1°] silicides, t~°l and borides.rl°,~iI Since the volume fractions o f minority phases, such as borides and silicides, were extremely small in wrought alloy 718 o f nominal composition, the primary mechanism for the formation o f grain boundary liquid in wrought alloys was considered to be the constitutionai liquation o f carbides, lm Using a pseudo-ternary o f y, Laves, and NbC,lj21 it was shown that the liquation o f carbides in wrought alloys and Laves phase in the cast alloys could be explained by the presence o f a 3 / + N b C + Laves terminal ternary eutectic in the alloy system, tl31 The development o f thermal stresses in the HAZ during cooling o f the weld caused the separation o f the liquated boundaries, resulting in intergranular cracks termed microfissures o r hot cracks. The extent o f hot cracking in alloy 718 was found to be directly related to the volume fraction o f the liquating phases, tl4] However, it was found that for an alloy o f given composition, the hot cracking susceptibility measured by the nil ductility temperature (NDT) range in a hot ductility test was related to the thermal history o f the alloy p r i o r to the test.f1,7,1°l High-temperature homogenization heat treatments (1050 °C to 1100 °C) and aging treatments following homogenization (650 °C to 700 °C) were both known to result in a l a r g e r NDT range than the solution heat treatment (927 °C). An identical result was obtained when the hot cracking susceptibility o f alloy 718 was characterized by the total c r a c k length (TCL) in a spot v a r e s t r a i n t t e s t . [9,I4]
Several explanations have been offered in the literature to account for the effect o f p r i o r heat treatment on the extent of hot cracking in the HAZ o f alloy 718. The heat treatments described above caused the precipitation
B. R A D H A K R I S H N A N , R e s e a r c h Associate, and R . G . THOMPSON, Professor, are with the Department of Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL 35294-4461. Manuscript submitted A u g u s t 29, 1990. METALLURGICAL TRANSACTIONS A
or dissolution o f Nb-rich phases in the alloy matrix. The homogenized material, in which the only precipitates were the N b C particles, had the highest matrix Nb content. However, precipitation reactions reduced the matrix Nb content in both the solution-treated and age-hardened materials. The solution treatment resulted in the precipitation o f Ni3Nb plates (6 phase), while aging resulted in the precipitation o f 3/'[Ni3(Nb, Ti)], both o f w h i c h depleted Nb in the matrix. The volume fraction o f intergranular liquid produced by N b C liquation was assumed to be directly related to the intergranular Nb co
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