tions beyond the peak, the eutectic liquid has a backfilling, or crack healing, effect. The crack healing approach has been shown to be quite effective with aluminum alloys (Refs. 24–36), but there is limited data for other alloy systems. The cast pin tear test (CPTT) was used in this investigation since it provides a more effective means of accounting for the effect of backfilling on solidification cracking susceptibility than the Varestraint test, or other externally loaded tests. Because it is, in effect, a “self-restraint” test, it provides a means to determine and obtain cracking data at imposed strains low enough as to not overwhelm the backfilling effect. The CPTT may provide better predictions regarding cracking susceptibility in actual practice compared to the Varestraint test since the test is more effective in identifying the critical strain required to produce solidification Fig. 4 — The average circumferential cracking in pins containing 2 or 4 wt% molybdenum along with 2, 4, or 6 wt% niobium additions is compared to that observed in pins with only niobium additions. Each point represents at least 4 acceptable pins. A B cracking. Field testing involving weldments of varying degrees of restraint and comparison to CPTT data may provide insight on the practical significance of the test, as well as the backfilling effect in various Nibased alloys. The objective of this study was to determine the effect of variable Nb and Mo content on the susceptibility of Ni- 30Cr filler metals to solidification cracking as a function of the fraction eutectic. The potential for a reduction in susceptibility due to a crack backfilling effect with increasing fraction eutectic was investigated using an Alloy 690 base alloy that contains no Nb or Mo. A more fundamental understanding of Nb and Mo additions in terms of solidification cracking will potentially influence filler metal development for these high- Cr alloys and allow filler metals to be developed that are resistant to both DDC and solidification cracking. WELDING RESEARCH Experimental Methods and Procedures Materials and Sample Preparation Alloy 690 was used as the base material for this study. The composition (wt-%) of the alloy used was Ni- 29.45Cr-9.13Fe-0.15Mn-0.32Ti- 0.24Al-0.02Mo-0.02Nb-0.029C- 0.002S-0.0048P-0.014N. Additions of Nb and Mo were made using a button melting technique. Niobium additions in this study varied from 0 to 8 wt-% and were in the form of 99.8% pure 1.0-mm-diameter wire. Molybdenum additions of 2 and 4 wt-% were made to select niobium additions and were in the form of 99.94% pure, annealed 1.0-mm-diameter wire. The Alloy 690 and pieces of Nb and Mo wire were carefully weighed and JULY 2016 / WELDING JOURNAL 231-s Fig. 3 — Cracking susceptibility based on CPTT results compared to measured and ThermoCalcpredicted fraction eutectic values. Error bars represent the standard deviation of the measured fraction eutectic values. Fig. 5 — Optical micrographs of sectioned pins representative of the CPTT data in Fig. 2. Fig. 6 — Low and highmagnification optical micrographs of backfilled cracks. A — 4Nb0Mo; B — 8Nb0Mo pins.
Welding Journal | July 2016
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