Insu Woo

Metallurgical factors contributing to HAZ cracking susceptibility in cast alloy 718 welds and its improvement by cerium addition

A study was carried out to determine the effect of cerium addition on HAZ cracking susceptibility in cast alloy 718 welds. The cause of HAZ cracking was also investigated using commercial cast alloy 718 varying in grain size at three levels. The hot cracking test results for commercial cast alloy 718 indicated that the fine-grained alloy was less sensitive to HAZ cracking. Furthermore, cerium addition of up to 0.3 wt.% was found to have a beneficial effect in reducing HAZ cracking susceptibility. When cerium addition exceeds 0.3 wt%, HAZ cracking susceptibility increased further. The mechanism of HAZ cracking was found to be related to intergranular liquation caused by the eutectic melting of sulfur containing a laves cluster at the grain boundaries. The degree of grain boundary liquation in the fine-grained specimen was less than that of the coarse-grained specimen due to a decrease in the amount of laves cluster and sulfur segregation in it at the grain boundaries, which seems to be responsible for a reduction in hot cracking susceptibility in the fine-grained specimen. Microscopic observation suggested that the improved HAZ cracking susceptibility in cerium containing alloy could be attributed to a reduction in the amount of laves cluster and sulfur segregation in it at grain boundaries due to the grain size reduction and sulfur-scavenging effects of cerium.

Effect of grain size on heat affected zone cracking susceptibility.Study of weldability of Inconel 718 cast alloy (2nd Report)

Introduction A previous report has shown that Inconel 718 cast alloy faces the problem of liquation cracking in the heat affected zone (HAZ) and clarifies that HAZ cracking is mainly due to grain boundary liquation associated with constitutional liquation of the Laves cluster. These results suggest that, to improve the HAZ cracking susceptibility, it is basically necessary to find a way of reducing the grain boundary liquation caused by constitutional liquation of the Laves cluster. For this purpose, it is effective to reduce the amount of Laves cluster present at the grain boundaries in the base metal. The extent to which the Laves cluster is formed at the grain boundaries varies depending on the alloy chemistry, granular structure, heat treatment, etc. To improve HAZ cracking in Inconel 718 cast alloy, it is necessary to clarify the effects of these factors on the HAZ cracking susceptibility. Previous research intended to establish the relationship between the HAZ cracking susceptibility and grain size among these factors has been mainly focused on the forged alloy. That is to say, Thompson et al, Lucas et al and Robinson et al have shown that the HAZ cracking susceptibility of Inconel 718 forged alloy can be improved by grain refinement. However, relatively few quantitative research efforts have so far been mounted to determine the relationship between the HAZ cracking susceptibility and grain size of Inconel 718 cast alloy, and many obscure aspects concerning the mechanism of HAZ cracking susceptibility improvement remain to be clarified. To obtain fundamental data relating to improvement of HAZ cracking, Inconel 718 cast alloy specimens with different grain sizes are subjected to the spot varestraint test and the isothermal liquation test, and the effect of the grain size on the HAZ cracking susceptibility is investigated. Based on these results, the mechanism of HAZ

Characteristics of Heat-Affected Zone Cracking in Cast Alloy 718-A Study on Weldability of Cast Alloy 718 (Report 1).

Characteristics of the heat-affected zone cracking of cast alloy 718 has been investigated using the bead on plate test and microscopic observation for isothermally processed specimens.The results obtained in this study are summarized as follows :(1) It was clarified that cracking in HAZ of cast alloy 718 was intergranular cracking which was caused by liquation of grain boundary phases.(2) According to the microscopic observation of specimens heated at various temperatures, incipient melting of the grain boundaries was found to occur at the temperature of 1383 K and above.(3) The Laves cluster was observed to liquate at the ternary eutectic temperature of 1383 K, which was lower than the NbC liquation temperature of 1413 K.(4) The liquation cracking in HAZ of cast alloy 718 was considered to be caused by constitutional liquation of Laves cluster at the grain boundaries.

Influence of electron beam welding parameters and metallurgical factors on intergranular liquation cracking susceptibility of cast alloy 718

The factors affecting intergranular liquation cracking susceptibility in electron beam welds were investigated for cast alloy 718. The materials used were as-received plates and heat-treated plates with three different levels of grain size. Liquation cracking susceptibility in HAZ was evaluated by a bead-on-plate test and a restraint/relaxation U-type hot cracking test. The penetrated shapes in the welds were classified into wine cup-like Type W and nail head-like Type N. For a given beam current, Type w and Type N were observed at the lower and higher welding speeds, respectively. Welding defects, i.e., underfills and microcracks were seen in the electron beam welds. Compared with Type W, the liquation cracking was more sensitive for the Type N bead cross sectional shape. Furthermore, it easily occurred at grain boundaries in Region II, i.e., very near the nail head necked part. According to the restraint/relaxation U-type hot cracking test, the liquation cracking susceptibility decreased with decreasing grain size or with homogenization heat treatment. These results suggested that the liquation cracking susceptibility in cast alloy 718 electron beam welds could be improved by using the Type W bead cross sectional shape, a decreasing the grain size and using appropriate heat treatment before welding.

Mechanical properties of lanthanum-containing inconel cast alloy 718 welds. Study of weldability of inconel cast alloy 718 (Report 9)

Previous articles in this series 2 have shown that, through addition of 0.2–0.3 mass% lanthanum and cerium to Inconel cast alloy 718, the HAZ cracking susceptibility of laser welds can be appreciably improved. Lanthanum and cerium are also shown to be effective for cracking susceptibility improvement. To evaluate the effectiveness of lanthanum and cerium addition to Inconel cast alloy 718, it is important to understand the mechanical properties of its welded joints. This article is intended to clarify the mechanical properties of welded joints as part of an effort to evaluate the weldability of Inconel cast alloy 718 containing the rare earth metal lanthanum, describing tensile tests of laser welded joints at room and elevated temperatures. For reference, the mechanical properties of La-free TIG, laser and electron beam welded joints are also investigated. On the basis of the results obtained, the effect of lanthanum on improvement of the tensile properties of the welded joints is further discussed.

Effect of homogenisation heat treatment on heat affected zone cracking susceptibility.Study of weldability of Inconel 718 cast alloy (3rd Report)

Introduction A previous report has shown that the heat affected zone (HAZ) cracking susceptibility of Inconel 718 cast alloy can be improved by grain refinement. To identify alternative ways of improving the HAZ cracking susceptibility of Inconel 718 cast alloy, the present paper describes an investigation of the effect of homogenisation heat treatment before welding. The relationship between the HAZ cracking susceptibility and homogenisation heat treatment of Inconel 718 cast alloy has been mainly researched by Thompson et al and Huang et al. Thompson et al have shown that homogenisation heat-treated specimens heat-treated at 1363 K x 3.6 ksec have a higher HAZ cracking susceptibility than corresponding as-cast specimens. Huang et al, on the other hand, have reported that the HAZ cracking susceptibility of electron beam welds can be improved by homogenisation heat treatment. These examples indicate that there is no conclusive evidence of any consensus having been reached regarding the effect of homogenisation heat treatment on the HAZ cracking susceptibility. To obtain fundamental data relating to ways of improving the HAZ cracking susceptibility of Inconel 718 cast alloy, this paper describes an investigation of the effect of homogenisation heat treatment on the HAZ cracking susceptibility. As-cast specimens and homogenisation heat-treated specimens heat-treated at 1368 K x 5.4 ksec as the homogenisation heat treatment conditions conventionally applied in practice and at 1378 K x 10.8 ksec established in a preparatory investigation forming part of the present study as being suitable homogenisation heat treatment conditions are subjected to the longitudinal varestraint test and the isothermal liquation test to evaluate the HAZ cracking susceptibility. Based on these results, the mechanism of HAZ cracking susceptibility improvement by homogenisation heat treatment is discussed.