Most 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. However, it presents unique assembly challenges to manufacturing engineers.
Most 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.
“Stainless steel is ideal for foodservice applications,” says Jay McKenna, global product manager at PennEngineering (Danboro, PA). “Because of its corrosive resistance, stainless steel makes sense for applications that require constant wash down and food safety.”
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 from both 300- and 400-series, in addition to A286," says McKenna. "We recommend to our customers who need high corrosion resistance that 400-series is not appropriate, because it is comparable to plated steel."
“Finish and appearance are critical in the commercial appliance industry,” adds David Parham, marketing manager at Orbitform Group (Jackson, MI). “The unique characteristics of stainless steel provide ease of assembly with a nice appearance.”
To make stainless steel, AK Steel Corp. (West Chester, OH), Carpenter Technology Corp. (Reading, PA), ThyssenKrupp Stainless USA (Mobile, AL) 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 postprocessing to develop their microstructures in a specific manner such that their chemical composition (alloy) is optimized for their end application,” explains Nate Ames, engineering team leader in the materials group at the Edison Welding Institute (EWI, Columbus, OH). “Stainless steels undergo similar postprocessing. 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.”
Most applications in the foodservice equipment industry use stainless steel series 301 and 304, which is the most common grade used. However, one of the biggest challenges to using stainless steel is constantly fluctuating raw material prices.
For instance, demand for stainless sheet dropped 18 percent last year. The price of 304-series material was more than 25 percent lower in late 2008 than during the same period in 2007.
Earlier this year, nickel costs were 50 percent less than they were in 2008. As a result, stainless steel prices dropped dramatically in the spring.
But, prices now appear to be on the way back up. In fact, AK Steel recently announced that the base price of all its stainless flat-rolled steel products is increasing by $60 per ton.
“Stainless steel cost affects everybody in this industry,” says 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.”
Two 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.
However, the amount of nickel used in stainless steel can affect the mechanical properties of the material. 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,” notes EWI’s Ames. “The reality is that we don't take advantage of the strength that these materials have.
“What is more difficult is the dissimilar material joints that result in having lower cost alloys in some areas and not in others,” Ames points out. “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 assembly challenges to manufacturing engineers. For instance, stainless steel’s polished finish and high strength make it more difficult to use than carbon steel.
According to McKenna, many foodservice equipment manufacturers use threaded fasteners because repairability is important. “Fasteners allow field service technicians to easily access electronic controls,” he explains.
Welding is also commonly used in the industry According to Ames, resistance spot welding, flux-cored arc welding and gas metal arc welding are the leading processes used to join stainless steel sheets. “[However], I don't believe that the appliance market (commercial or otherwise) has really taken advantage of the advanced welding technologies like lasers,” he warns. “In their current form, lasers can be used to weld stainless steels with exceptional quality at speeds over 1.5m/sec.
“The greatest challenge of joining stainless steel is to ensure that the joint and adjacent material is stainless after joining,” adds Ames. “Meeting the mechanical property requirements of strength and toughness in stainless steel welds is generally easy to accomplish. [However], maintaining corrosion resistance is not so straight forward.”
Ames claims that many stainless steel joining challenges are relatively easy to overcome. “The most critical [steelmaking] applications require the most stringent melting practices,” he points out. “Many times, the critical stainless steels are melted and remelted multiple times in controlled environments to produce the desired chemistry. Then we (manufacturers) take these beautiful, compositionally and microstructurally clean materials and weld them together hoping that our weld will meet the same properties of the base material.
“Unlike the base material, we do not weld them in a vacuum, we don't submerge them in a ladle where the composition can be corrected through minute alloy additions,” adds Ames. “Rather, we weld them in air, hoping that the shielding gas or slag from the welding process will adequately protect them.
“The filler metal and stainless steel alloy companies have done an amazing job designing systems that enable us to get acceptable results from our welded stainless steel components,” claims Ames. “But, we need to remain diligent and follow the advice of the people that developed the materials and filler metals. Complacency [will lead to] rust.”
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 material allow the exposed surface of a product, such as oven countertop, 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,” explains Ames. “Higher strength materials are great in tension, but dents and buckling are directly attributed to material thickness.”
One new technology that holds tremendous 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 equipment applications. The only issue 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,” predicts Ames. “[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.”
Stainless Steel Challenges
July 6, 2009