Face-centered cubic (FCC) alloys such as nickel and austenitic stainless steels are important to many industries, most notably nuclear power generation, turbine manufacturing, and petrochemical. These alloys are prone to ductility-dip cracking (DDC), a high-temperature, solid-state cracking phenomenon. Susceptible metals experience an abnormal intermediate temperature ductility loss which leads to cracking upon applying sufficient restraint, such as the restraint introduced by large, multipass welds. A unified mechanism for DDC has been elusive. One micro-mechanistic detail in need of clarification is the precise role sulfur plays in the exacerbation and/or formation of DDC. Various literature sources do not agree on whether sulfur causes DDC or if it simply deteriorates a metal’s resistance to DDC. To learn more, a method of introducing sulfur into nickel-based alloy 690 has been developed for use in future studies. The procedure involves melting of 690 base metal in a small, argon-shielded crucible along with controlled additions of iron sulfide powder. Two levels of sulfur were desired: 0.003<wt% S<0.05 and wt% S>0.1. Sample composition was evaluated on an optical emission spectrometer (OES) to determine the sulfur levels before and after melting. The new, sulfur-containing samples will be used in future work characterizing the effect of this sulfur content on alloy 690 during fixed-displacement thermal cycling.
These sulfur studies were conducted with alloy 690 owing to its relative simplicity, high purity, and known susceptibility to DDC. First, buttons of pure 690 were melted in a copper crucible with an argon shielding gas atmosphere. These buttons had controlled sulfur additions introduced in the form of iron sulfide powder. After attempting to introduce sulfur in this way, the buttons were analyzed using an OES to verify sulfur was absorbed. Once the procedure reliably produced buttons at a “medium” (0.003<wt% S<0.05) and “high” (wt% S>0.1) level of sulfur, these buttons were cast into pins. This was accomplished on OSU’s cast pin tear testing (CPTT) machines. Buttons were used as charges in a miniature induction furnace, and they were levitation-melted then dropped into a mold. This mold has a cylindrical pin region in the center. The pins were sectioned in small, 0.25” diameter nuggets which were implanted in the gauge section of alloy 690 dog bone tensile specimens. This was accomplished by drilling a 0.25” diameter counterbore into the sample, then dropping the pin section into the depression. Next, a small spot weld was centered on the pin which melted both pin and part of the surrounding base metal. In this way samples with varying sulfur concentrations were created. In addition, every tensile specimen received a control spot weld which has no sulfur addition on the opposite side of the gauge section (i.e. two spot welds per sample). Once the samples were spot welded, they will be evaluated again with the OES to verify the sulfur contents.
Results and Discussion
The samples were divided into two series, the medium sulfur and high sulfur samples. Both series tested with compositions which fell in their designated ranges; however, the levels were slightly lower in the spot welds for both sets of samples. This is likely because of dilution with the high-purity base metal. The medium level of 0.003<wt% S<0.05 was selected based on recommendations from literature which suggest sulfur below 0.003 wt% is significantly less likely to cause DDC. The objective here is not to determine whether sulfur has a causal relationship with DDC, but rather to introduce it into a once-pure alloy system for use in future studies. Likewise, this value simply serves as a low-end cutoff which has a basis in experimental work involving sulfur in nickel-based alloys. 0.05 serves as the upper end of the medium sulfur series because it is just over the maximum solubility limit of sulfur in nickel at elevated temperatures. The low end for the high sulfur series is 0.1 wt% S, which is a more arbitrary value based on observations of high-sulfur heats of metals produced by common suppliers of alloy 690 plates. An alternative to this approach of sulfur implantation would be sealing the tensile samples in a heat-resistant container along with some sulfur powder and then putting the container in a furnace for several days to allow for sulfur diffusion into samples. This has been accomplished in past research, but the drawback is the locality of sulfur concentration is much less controlled. The researchers in the present study desired samples with local sulfur concentration for future testing. As mentioned previously, this allows for a control spot weld in addition to the sulfur-containing spot weld on every sample.
This method of sulfur implantation in alloy 690 works for the purposes of which it was initially intended. A medium and high series of samples were created with distinct sulfur levels in both button form and final spot-welded tensile samples. The precision of sulfur content is kept in a range instead of a single target composition based on literature findings and observations from manufactured alloy 690 plate. Future work will involve the use of the sulfur-doped spot-welded tensile samples. These samples will undergo fixed displacement thermal cycling with elevated temperature straining, a testing procedure designed to simulate multipass welding. The effect of sulfur will be observed and quantified through EBSD, EDS, and metallography.