Advances in arc and laser welding as well as new solid-state approaches offer opportunities to substitute less expensive, lower content nickel stainless steels in certain types of mechanical assemblies.
“With recent increases in nickel surcharges, the stainless steel market is experiencing significant changes in product usage, particularly involving 304 stainless material,” says Alex Millar, a representative of corporate metallurgical services at Ryerson Tull, a major metal service center based in Chicago, IL.
There was a big move to stainless 10 years ago in consumer goods applications, particularly appliances, when stainless cost only 80¢/lb. “Stainless is now double that, and there’s no real end in sight,” says Millar.
Some OEMs have moved to lower nickel materials for refrigerators, but the look is significantly different. Some engineers are now looking at an ultra-lean duplex grade of 2101 as a lower-cost alternative to 304 (L) stainless. “The other approach is to use cladding and coatings as a means to utilize the same alloys at the active surface, while having the bulk strength derived for a non-corrosion resistant alloy,” says Nate Ames, a business development manager at the Edison Welding Institute in Columbus, OH.
The first major rub is that while many OEMs want to move to more cost-stable materials, they also want to drive up the efficiency of their products for use at higher temperatures in more corrosive environments and for an extended length of service. One example has been the slow transition of carbon steel in oil and gas applications to materials that were first all stainless, then clad with stainless and ore and recently have been clad with Inconel, austeniticnickel-based superalloys.
The second major rub is that lower energy density welding processes may result in large grains and significant distortion in high-nickel alloys. “The higher energy processes are less susceptible to distortion and grain growth, but bring with them problems associated with high cooling rates, such as solidification, cracking and tight fit-up requirements,” says Ames. “The solid state process, like Spinduction welding and resistance welding (which tend to be fast and lower temperature) tend to yield the highest quality joints; however they typically make use of full automation and thus can be more costly to deploy.”
New processes are now in production-ready commercial packages that can address some of the issues.
These new processes allow faster welding speed, reduced joint preparation, use of less filler wire and reduced distortion correction costs. Reasons: increased welding speed in the case of laser welding or through higher joint preparation in the case of laser welding and arc welding.
“These results an be achieved through use of penetration-enhancing compounds with gas tungsten arc welding, use of keyhole plasma arc welding and the use of high-power density in laser welding to increase joint penetration,” says Randy Dull, a technical expert at the EWI. One of the disadvantages of plasma arc welding is the requirement for expensive and complex equipment. Torch maintenance is also critical.
Welding issues that allow stainless metal replacement will be the topic of a seminar that will be held by the EWI Jan. 30-31.