of 0.02 to 0.03 wt-%. Alloy 10 showed cracking at 42 and 85 mm/s travel speeds, but Alloys 11, 13, and 40 with similar impurity content and primary austenite solidification were crack free. Small differences in alloy chemical composition other than S and P could cause the variable cracking behavior. Looking at the alloy compositions, the only noticeable differences for Alloy 10 are higher Si and Cu contents. Silicon is known to be detrimental to solidification cracking resistance with primary austenite solidification (Refs. 31, 32), and could have contributed to the difference in cracking behavior of Alloy 10 relative to the other commercial 21-6-9 alloys. However, further investigation is required to clarify the cracking mechanisms for Alloy 10 with primary austenite solidification and the change in crack susceptibility with chemical composition variation other than P and S. In general, for Type 21-6-9 or other similar alloys, it must be considered that changes in levels of minor alloying elements (i.e., Mo, Nb, Si) or other residual elements (possibly Cu) could cause significant changes in solidification behavior and crack susceptibility relative to the results observed here. The 21-6-9 alloys with primary austenite solidification and impurity contents greater than 0.02 wt-% that tested crack free included Alloys 11, 13, and 40–45. Based on results from previous weldability diagrams, cracking would be expected in those conditions. The explanation for the difference in cracking behavior for these 21-6-9 alloys compared to the 300 series stainless steels used in previous weldability diagrams is unknown. The high Mn level of 21-6-9 may allow 21-6-9 to tolerate higher S content, which could contribute to the difference in cracking behavior. Considering the S levels are low in the commercial 21-6-9 heats, the difference in cracking behavior is likely not related to the propensity for 21-6-9 to form MnS. Excluding Alloy 10, the commercial 21-6-9 alloys showed lower crack susceptibility than 300 series alloys for given impurity contents in the range of 0.02–0.03 wt-%. Ogawa and Tsunetomi (Ref. 31) showed that increasing weld metal N content decreased crack susceptibility for a given impurity level in fully austenitic 310 weld metal, and attributed the difference to reduced enrichment of Si at boundaries with higher N content. The higher N levels in 21-6-9 alloys relative to 300 series alloys could influence partitioning behavior of Si or other elements that could contribute to crack susceptibility. It is also possible the high Mn content may be beneficial in aspects other than just forming MnS, which would support the observations from Honeycombe and Gooch (Ref. 33) of Mn significantly decreasing solidification cracking when added to 310 arc welding filler metal. Additional work is necessary to confirm the possibility and mechanism of either such hypothesis. Effect of Restraint No solidification cracking was observed in the alloys with primary ferrite solidification for the given Sigmajig testing conditions. The stress level for this work was chosen to create reproducible cracking in a known cracksusceptible heat of 309 stainless steel with primary austenite solidification. Cracking may be observed at higher stress levels in alloys even with primary ferrite solidification. However, in practical use, it is known that reduction in crack susceptibility with primary ferrite solidification is sufficient to avoid solidification cracking weldability issues. In this work, the travel speed was varied to change the solidification rate to observe changes in solidification mode. The change in heat input with travel speed and subsequent changes in local stress on the solidifying weld pool were unintentional. Goodwin (Ref. 34) showed that decreasing heat input increased the stress level required to initiate cracking for Sigmajig testing. However, the changes in crack susceptibility with heat input are considered negligible compared to the changes in crack susceptibility with solidification mode. It is known that immediately behind the weld pool the transverse (relative to the welding direction) stresses are compressive, and only at some point away from the pool does the transverse stress change to tensile (Ref. 35), where solidification cracks can then initiate in a susceptible microstructure. Also, as discussed when describing the location for the cross sections taken from the Sigmajig samples, WELDING RESEARCH modeling work by Feng et al. (Ref. 21) showed the transverse tensile stress is only expected to develop in the second half of the sample. Varying travel speed during the Sigmajig test shifts the locations at which the tensile transverse stress is present. In the work by Feng et al., at low travel speeds (4.2 mm/s), the tensile region only develops in the final 20% of the weld length, and at high travel speeds (14.8 mm/s) the tensile region was present along 50% of the weld length. For the roughly constant cracking observed with variation in travel speed in this work, the increase in the amount of the sample subject to transverse tensile stress as travel speed increases could be offset by the reduced heat input at higher travel speeds. The modeling work also showed that the longitudinal stress that could promote transverse cracking is present in a much larger portion of the sample at high travel speeds compared to only at the end of the weld at low travel speeds. The large increase in the number of noncenterline cracks observed at 85 mm/s travel speed relative to the lower travel speeds in this work may be due to changes in the longitudinal stress within the weld. The lower travel speeds and arc welding used in previous modeling of the Sigmajig test are certainly different than the conditions used to develop the weldability diagrams here. Compared to arc welding, the laser welding thermal cycle, weld pool shape, and increased travel speeds would change the stress state and the transient nature of the stresses. Modeling work of the Sigmajig testing with laser welding would be beneficial to better quantify the changes in restraint between the three weldability diagrams. Conclusions Weldability diagrams to predict solidification crack susceptibility for laser welding of 21-6-9 were developed at 21, 42, and 85 mm/s travel speeds. The minimum Creq/Nieq for primary ferrite solidification of 1.55 at 21 mm/s travel speed using Espy equivalents shifted to 1.75 Creq/Nieq at 42 and 85 mm/s travel speeds due to the increase in solidification rate as travel speed increased. The Creq/Nieq ratios were calculated using a Nieq that accounts for 10% nitrogen loss NOVEMBER 2016 / WELDING JOURNAL 417-s
Welding Journal | November 2016
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