Stainless Steel plays a unique role in metallurgy. Stainless steels are first and foremost used for their corrosion resistance. They are created by mixing iron and carbon with chromium, and sometimes nickel and other alloys.
“Chromium is the most critical component of stainless,” says John Dobek, V.P. Commercial, Stainless Steel and Aluminum at Kloeckner Metals. “Essentially you can’t have a stainless steel without the addition of chromium.”
The addition of chromium, usually 10-20 percent, makes the resulting metal alloy highly resistant to corrosion. The degree of corrosion resistance required is one factor that decides the overall required composition of the stainless steel. Malleability, temperature resistance, and aesthetics are other factors.
Types of Stainless Steel
There are three main families of stainless steel. Ferritic stainless steels are non-nickel bearing metals and are typically highly corrosion resistant. Martensitic stainless steel usually doesn’t contain much nickel, but contains a higher amount of carbon and unlike ferric grades, is heat treatable. Austenitic stainless steels are the most common, contain nickel, and are more formable and ductile than other stainless steels. They also are non-magnetic, largely due to the nickel content. Duplex and Precipitation Hardening are smaller, specialty groups making up the balance of the standard stainless grades.
“A typical austenitic or nickel bearing grade is something that might take a lot of deep draw, for example deep bowls,” Dobek says. “Sinks for instance are quite often made from austenitic steel. Many require a tremendous amount of draw and need great elongation in order to perform without cracking.”
On the other side of the kitchen, refrigerator panels are often made of simple ferritic stainless steel. In this case product demand and performance is more centered on aesthetics.
“Basically you want the look and feel of stainless steel, but you may not necessarily need that extra level of corrosion protection that comes with a highly anticorrosive alloy,” Dobek says.
Temperature resistance also comes into play, such as in stove parts or muffler systems. Each stainless grade is alloyed to meet a specific demand, for example heat. Exhaust systems are routinely 409 or 439.
“Different metallurgical alloys and different processes all play a role in determining the right grade to use in meeting the demands of a specific part,” Dobek says.
The price of nickel can also factor into grade choice and the use of nickel bearing grades. This commodity can be extremely volatile in the market. As such, some companies will hedge their spending to counter wide transaction variances. One of the principle reasons for the growth of ferritic stainless has been to offset nickel price variability, especially when nickel prices topped $20 per pound.
“Nickel has a lot of positive benefits, but when the pricing goes out of whack in the commodity markets, it creates a lot of uncertainty for the end user, and they seek alternatives,” Dobek says.
PVD coating has added an extra dimension to stainless steel. Now you can color the metal while maintaining the classic stainless steel look. This opens up many aesthetic possibilities, in gourmet kitchens for example.
“If you’re handling food, it usually is on a stainless steel surface,” says Matt Meyer, VP Digital Innovations Kloeckner Metals Corporation.
As such, the aesthetics of many commercial and residential kitchens have been based around the classic stainless steel look. PVD has opened up the door for more design variety in addition to the functional benefits. Anti-fingerprint and antimicrobial treatments can add even more options for the designer.
Stainless steel can also be polished into a mirror-like finish. As such, it is often used in food display cases instead of glass mirrors. Correctional facilities have mirrors also made of highly polished stainless steel.
Working with Stainless Steel
When working with stainless steel, it is important to maintain cleanliness in the production process. You do not want to damage the surface with nicks and dings, nor do you want your metal exposed to carbon contamination.
“You can contaminate it with little particles of iron, and it can wreak havoc on the end use resulting in rust or pitting,” Meyer says.
It is important to either use dedicated tooling and equipment, or have detailed cleaning procedures and audits. This is going to be a key factor in determining the ultimate quality of the end product.