Fig. 9 — SEM/EDS analysis of the region around a backfilled crack from Fig. 8. Spots 1–3 represent the composition of the backfilled cracks. Fig. 10 — SEM/EDS analysis of the region around a backfilled crack in a 6Nb4Mo alloy. Spots 1, 2, and 6 represent the composition of the backfilled cracks. function of temperature (Ref. 43). The average solute solidification profiles calculated by the module and the resulting mass fractions were compared to ImageJ analyses of the volume fractions from metallographic analysis and to the energy dispersive spectroscopy (EDS) composition data. The significance of the Scheil simulations in this study was in the correlation made between the measured volume fraction eutectic and the calculated mass fraction eutectic, as well as the relative composition changes that were predicted during solidification based on compositional changes to Alloy 690. Cast Pin Tear Test The cast pin tear test (CPTT) can be used to evaluate susceptibility to solidification cracking through a controlled variation of restraint levels imposed by solidification shrinkage. It can be classified as a “self-restraint” test since the strain that promotes cracking results from contraction stresses that accumulate in the pin during solidification. Detailed information regarding the test as well as the procedure can be found elsewhere (Refs. 44, 45). Pins ranging from 0.75 to 1.625 in. in length were cast at 1460°C after a 90-s purge in argon gas. The CPTT has been previously demonstrated to effectively rank the solidification cracking susceptibility of a number of Ni-based alloys (Refs. 45, 46). In this study, the shortest pin length that exhibited no visible cracking was used to quantify cracking susceptibility. This value is designated the threshold pin length for solidification cracking. At pin lengths above threshold, WELDING RESEARCH there tends to be more scatter in the cracking response, making comparison among alloys more difficult. Determination of the threshold pin length for each alloy was based on a minimum of four “acceptable” tests. Acceptable pins were those that exhibited no visible casting defects. Casting defects result from improper mold filling and leave behind swirl-like indentations on the pin surface and sometimes visible voids. Casting defects were minimized through extensive mold cleaning between casts and by ensuring that the molten button was heated to the appropriate casting temperature and dropped properly into the pin mold. The judgment regarding “acceptable” pins was based on visual inspection only. Sectioning of multiple pins that were considered “acceptable” did not reveal any subsurface voids. JULY 2016 / WELDING JOURNAL 233-s
Welding Journal | July 2016
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