Many foodservice equipment manufacturers rely heavily on stainless steel. The shiny material is widely used to make commercial coolers, dishwashers, fryers, ovens, refrigerators and other products because of universal demand for corrosion resistance, durability and hygiene. “Because of its corrosive resistance, stainless steel makes sense for applications that require constant wash down and food safety,” said Jay McKenna, global product manager at PennEngineering in Danboro, Pa.

However, not all stainless steel has the same corrosive resistance properties. For instance, 400-series has lower corrosion resistance than 300-series. “We manufacture parts from both 300- and 400-series, in addition to A286,” McKenna said. For applications requiring high-corrosion resistance, McKenna doesn’t recommend 400-series, because it is comparable to plated steel. The most common grades or stainless steel used for applications in this industry are series 301 and 304.

“Finish and appearance are critical in the commercial appliance industry,” said David Parham, marketing manager at Orbitform Group, Jackson, Mich. “The unique characteristics of stainless steel provide ease of assembly with a nice appearance.”

To make stainless steel, AK Steel Corp., West Chester, Ohio; Carpenter Technology Corp., Reading, Pa.; ThyssenKrupp Stainless USA, Mobile, Ala.; and other suppliers process steel with metals such as chromium, manganese and nickel. For instance, adding small quantities of nickel to steel increases its durability and strength.

“During the steelmaking process, carbon steels and low alloy steels undergo substantial post processing to develop their microstructures in a specific manner such that their chemical composition (alloy) is optimized for their end application,” explained Nate Ames, engineering team leader in the materials group at the Edison Welding Institute (EWI), Columbus, Ohio. “Stainless steels undergo similar post processing. However, unlike their carbon and low-alloy steel cousins, they also undergo significant preprocessing to ensure that their chemical composition is ideal for them to remain corrosion-resistant and mechanically sound for their desired application.”

Cost Fluctuation

One of the biggest challenges to using stainless steel, however, is constantly fluctuating raw material prices. For instance, demand for stainless sheet dropped 18 percent in 2008. The price of 304-series material was more than 25 percent lower in late 2008 than during the same period in 2007.

Early in 2009, nickel costs were 50 percent less than they were in 2008. As a result, stainless steel prices dropped dramatically in the spring of 2009. Currently, prices appear to be on the way back up. In fact, mid-last year, AK Steel announced that the base price of all its stainless flat-rolled steel products was increasing by $60 per ton.

“Stainless steel cost affects everybody in this industry,” said Michael French, president of MF&B Restaurant Systems Inc., Dunbar, PA, a company that specializes in pizza ovens. “We have changed some of our parts to a 400-series stainless to help keep costs down. By producing all the stainless steel components at our facility, we are able to have complete control over production and quality.”

Cutting Costs Cuts Quality?

Three years ago, when the cost of stainless steel rose rapidly, commercial appliance manufacturers were forced to scramble. Some companies began to use less expensive grades of stainless steel that use less nickel. According to McKenna, less expensive grades of stainless steel, such as 201-series, can be as much as 50 percent less than 304-series, because nickel content is much lower.

The amount of nickel used in stainless steel can affect the mechanical properties of the material, however. As a result, some end users are concerned that the use of lower nickel grades may impact fabrication and assembly operations.

“I don't believe that using lower grades of stainless has much impact on their functionality or ‘weldability,’” EWI’s Ames said. “What is more difficult is the dissimilar material joints that result in having lower cost alloys in some areas and not in others. It is hard enough to weld any stainless and keep it performing like a stainless material. When we mix different grades or types of material, this difficulty is usually exacerbated.”

Because stainless steel is different than other types of metal, it presents unique challenges during the manufacturing and assembly processes. For instance, stainless steel’s polished finish and high strength make it more difficult to use than carbon steel. According to McKenna, many appliance companies use threaded fasteners because repairability is important. “Fasteners allow field service technicians to easily access electronic controls,” he added.

Ease of welding during the manufacturing process is also a consideration when designing with stainless steel. Resistance spot welding, flux-cored arc welding and gas metal arc welding are the leading processes used to join stainless steel sheets, according to Ames. “[However], I don't believe that the appliance market—commercial or otherwise—has really taken advantage of the advanced welding technologies like lasers,” he said. “In their current form, lasers can be used to weld stainless steels with exceptional quality at speeds over 1.5m/sec.

Lower-Priced Alternative

Because of severe price volatility in the market, several organizations, including EWI, are looking at cladding and coating as an alternative to full thickness stainless steel. These hybrid materials allow the exposed surface of a product, such as oven countertops, to be stainless, with the inner core manufactured from a less expensive material.

“Material thickness is only an optional variable for an undefined thickness range,” explained Ames. “Higher strength materials are great in tension, but dents and buckling are directly attributed to material thickness.”

One newer technology that holds great potential is multi-layer, honeycomb-like materials. They offer higher strength-to-weight ratios, can be made of different materials to reduce cost, and can be purpose-built to meet the unique needs of foodservice appliance applications. One potential issue, however, is that the internal pattern (which provides their strength) often shows through to the exterior.

“Very soon, this will be considered avant garde, much like bolt-on fender flares and carbon fiber,” Ames said. “[At one time, automotive engineers] would hide the bolts and paint the carbon fiber so they looked more original. Nowadays, car manufacturers use fender flares with exposed fake bolts on them and ‘paint’ body and interior panels to look like carbon fiber. Soon, [consumers] will be standing in line to purchase a refrigerator with a faint honeycomb pattern showing through.”