Stainless steel is the material of choice for food processing equipment, both because of food safety regulations and because it has an oxide layer that hinders the formation of rust.
But stainless steel is an inherently soft material that doesn’t hold up well against prolonged abrasion. Typically, protective coatings and finishes are applied to stainless steel to help it survive the harsh food plant environment. But there’s another option — surface hardening. To learn more about this process, we spoke with Christian Dalton, the North American Sales Manager at Expanite.
Expanite specializes in the surface hardening of stainless steel parts like pumps, valves, nuts and bolts, and knives. “We aren’t a coating company,” Dalton emphasizes. “You could put us in the same category as coatings, but we're not actually adding anything. We’re only diffusing nitrogen and carbon, so it’s called surface hardening or case hardening.”
The company is headquartered in Denmark. Dalton came to Expanite after his experience testing and analyzing surface hardening technologies for the Navy while pursuing a degree in material science, and he was instrumental in helping the technology cross the pond. “I knew whatever recipe they had was a good one,” he says. “I approached them after graduation to see if they wanted to bring this technology to the United States, and, lo and behold, Expanite’s first customer was in my backyard of Cleveland in Twinsburg, Ohio.” Today, Expanite has treatment centers in Denmark, Germany, China, and the United States.
Coatings vs. surface hardening
The softness of stainless steel makes the material vulnerable to abrasive wear, as well as galling, which is a type of wear caused by adhesion between sliding surfaces. “It’s like butter,” Dalton explains. “If you were to put two sticks of butter together and twist them, there isn’t a clean separation. You're going to have some material transfer. Stainless steel can behave the same way.”
That adhesive quality is part of what makes coatings risky. “You have to worry about adhesion of the coating to the surface of the part,” Dalton says. He adds that coatings can often chip, crack, or flake, all of which can lead to product contamination.
That’s why surface hardening is emerging as an alternative. The process doesn’t add anything to the material (besides nitrogen and carbon), it just makes the surface harder — as much as 10 times harder. Dalton explains: “We have a patented gas phase nitrocarburizing process. The process begins with baking stainless steel parts under certain atmospheric conditions, which diffuses nitrogen and carbon atoms into the surface of the part.”
Another reason surface hardening is becoming more common is because hard chrome plating is being phased out. This older technology uses chromium-6 (of Erin Brockovich fame), which is a known carcinogen and air pollutant. “There are significant environmental hazards with hard chrome plating,” Dalton says. “It’s being phased out in Europe and I suspect it’s only a matter of time before the same happens here in the United States. So, manufacturers are looking for alternatives.”
Long-lasting corrosion resistance to keep food safe
Stainless steel can also break down over time, especially when it’s exposed to the harsh sanitation methods used in the food industry. This can cause food safety problems. “I've seen some old stainless steel sanitary pumps that have been worn down to the bones,” Dalton says. “And I always think to myself, where did that material end up? Over time, small amounts of this has been going into product.”
Different stainless steel alloys provide different levels of corrosion resistance. The 316L stainless steel alloy is common in the food and beverage industry, due to its ability to withstand high temperatures and corrosive materials. It’s also preferable where welding is necessary, as it has a lower carbon content.
Dalton says that although food and equipment manufacturers know they should be using 316L, they sometimes opt for a harder material, hoping it will last longer. “They can find themselves in trouble with severe corrosion while playing this alloy selection game. Or maybe they start using a particular anti-wear material that is very, very expensive for them.”
Surface hardening can help them go back to more preferable and affordable grades. Because the metal itself is left unchanged, the surface hardening process is completely safe for food and beverage applications. It passes ASTM tests, and the resulting products meet all FDA regulations and are ISO 9001:2015 compliant.
Although Dalton has attended PROCESS EXPO in the past, this will be Expanite’s first time as an exhibitor. “We see the show as a good opportunity to educate and network with companies who are both exhibiting and walking the floor, and hopefully meet some new customers,” he says.
To learn more about Expanite’s surface hardening process and its benefits, visit Booth #863 at PROCESS EXPO.