uous-wave laser welding. Given that solidification rate is proportional to travel speed, a range of solidification rates were measured, and are discussed below. The results are three separate weldability diagrams that capture the shift in solidification mode and critical Creq/Nieq with solidification rate. The Creq/Nieq, impurity content, solidification mode, and cracking behavior of the Sigmajig samples used to generate the diagrams are given in Table 2. The primary solidification modes were identified by their different characteristic microstructures, shown and discussed in Part 1. When calculating the Nieq of the alloys for plotting the weldability diagrams, nitrogen loss of 10% in the weld metal was used based on previous results (Ref. 23). Note that the equivalents presented in Table 2 are based on the base metal without any nitrogen loss. The weldability diagram developed for 21 mm/s travel speed includes a larger number of alloys with the large number of compositions used for the impurity content variation in Part 1. Of the laboratory melted alloys produced with variations in S and P levels, only Alloys 40, 49, and 53 were repeated at 42 and 85 mm/s travel speed for the weldability diagrams. The cracking behavior and solidification mode for welds at 21 mm/s is shown in Fig. 4, with impurity content as a function of Creq/Nieq. Solid symbols indicate cracking was observed in at least one of the three transverse cross sections. All alloys with primary ferrite solidification were crack free. A critical Creq/Nieq of approximately 1.55 was observed for primary ferrite solidification. All alloys greater than 1.55 Creq/Nieq solidified as primary ferrite with the exception of one alloy that was not a 21-6-9 type. Alloy 35, a Nitronic 60-type indicated with the solid red to the right of the cracking boundary, with Creq/Nieq of 1.70 solidified as primary austenite and exhibited cracking. The cracking boundary was drawn excluding the Nitronic 60 alloy considering that Type 21-6-9 was the focus of this work. For 21-6-9 alloys (all shown in black on the plot) with primary austenite solidification, the minimum impurity content to exhibit cracking susceptibility was 0.03 wt-%. Two 21-6-9 alloys with primary austenite solidification A B Fig. 3 — Astested Sigmajig sample welded at 21 mm/s travel speed. A — Photograph; B — stereoscope image of cracking at the end of the weld. The box in 3A indicates the area for the image of 3B. Fig. 4 — Weldability diagram for 21 mm/s (50 in./min) with impurity content as a function of Creq/Nieq. Note Nieq was calculated assuming 10% N loss. A is primary austenite solidification, F primary ferrite solidification, and D dualmode solidification. Other indicates non2169 type. and greater than 0.03 wt-% impurity content were crack free; however, the majority of alloys in this composition range exhibited cracking. The 21-6-9 heats exhibiting cracking were all experimental alloys. No commercial 21- 6-9 alloys showed primary austenite solidification or solidification cracking at 21 mm/s. The horizontal cracking boundary for 21 mm/s is below the minimum impurity content for cracking in 21-6-9 alloys at approximately 0.02 wt-% to be conservative, based on the cracking observed below 0.03 wt- % impurity content at higher travel speeds. The two alloys exhibiting cracking at Creq/Nieq of approximately 1.1 and 1.4 and impurity content of less than 0.03 wt-% were both Nitronic 50 alloys. The Nitronic 50 alloys showed cracking at lower impurity levels than the experimental 21-6-9 alloys tested. The difference in minimum impurity content to cause cracking could be an indication that other elements within Nitronic 50 contribute to its crack susceptibility. At constant Creq/Nieq of roughly 1.55, there is a change in solidification mode from primary ferrite to primary austenite as impurity content is increased. The increased levels of P and S could be affecting the solidification mode. The weldability diagram developed for 42 mm/s travel speed is shown in Fig. 5. As expected, a larger number of alloys showed primary austenite solidification at the increased travel speed. A number of alloys also showed the WELDING RESEARCH 412-s WELDING JOURNAL / NOVEMBER 2016, VOL. 95
Welding Journal | November 2016
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