The metal finishing industry has a long working relationship with stainless steel.
Stephen Rudy CEFNumerous parts for many uses are processed, from passivation to plating, electropolishing, and black oxide, as some popular treatments. Indeed, much of the equipment used is also stainless steel. We utilize baskets, tanks, racks, heaters, and other items.
A finisher would be hard-pressed to find suitable alternative equipment. The lower alloy stainless steel exhibits excellent corrosion resistance, while higher alloy stainless steels are corrosion-resistant to most acids, chlorine-containing, and alkaline solutions. Stainless steel is closely associated with our daily lives. It is found in the home (appliances and utensils), commercial buildings, automotive, aircraft, military, and many other applications. How can such a useful and critical material like stainless steel become so prominent?
Let us review some facts and useful information associated with stainless steel.
Corrosion Resistance Of Iron-Chromium Alloys
Almost 200 years ago, a French metallurgist, Pierre Berthier, discovered the corrosion resistance of iron-chromium alloys. In subsequent decades, notable researchers and metallurgists expanded on the initial findings, developing stainless steel alloy.
They found that the alloy containing at least 11.5% chromium does not corrode, stain, or rust like plain steel. Different grades and finishes were developed, meeting particular requirements for wear resistance and applications. Stainless steel is generally made using an electric arc furnace. In the furnace, recycled stainless scrap, along with chromium and/or nickel alloys and molybdenum, are melted by passing an electric current. Once molten, the material is treated to remove excess carbon. No other additives are generally used.
Aside from working with very high temperatures and specific process parameters, this is a relatively simple process. Stainless steel is cast into ingots, billets, or slabs. The material can be cold or hot-rolled. The final product can be formed into wire, rods, sheets, bars, or rolls. This results in simpler fabrication cycles overall. In many cycles, stainless steel is annealed for surface softening. Descaling is achieved by acid pickling.
This step promotes naturally occurring surface passivation. Traditional nitric acid or nitric acid/dichromate methods have achieved additional chemical passivation that meets various specifications. However, in recent years, these have been supplanted by environmentally safer citric acid proprietary passivation, which meets ASTM A 967, A 380, and AMS 2700.
In the current economy, metals generally have experienced steep price increases. Due to its nickel and chromium content, stainless steel has certainly experienced higher prices. The good points about stainless include its excellent corrosion resistance, high and low-temperature stability, overall strength, aesthetically pleasing appearance, cleanliness, and hygiene. These benefits and others make it a tough material to replace. For economic reasons, stainless steel may also slow down market price increases. This is because it is recyclable. A 100% turnaround signifies no waste of the material.
Over half of the newly made stainless steel is derived from older scrap material. An added important fact: a full life cycle of recyclable material would comply with RoHS and other similar environmentally driven directives.
Types of Stainless Steel
Austenitic
Microstructure of 321.Over three-quarters of annual stainless steel production is of the austenitic type. A popular designation for austenitic is the 300 series. Austenitic alloys are nonmagnetic. They exhibit excellent hardness in a wide range of exposure temperatures. The material derives its name from the sufficient addition of nickel, changing the alloy crystal structure to austenite. The alloy generally contains a minimum of 16% chromium and a maximum of 0.15% carbon. The following term may also clarify why we sometimes refer to particular designations. 18/10 stainless contains 18% chromium and 10% nickel. Several of the austenitic alloys consist of 18/8. Improved corrosion resistance is achieved by adding 2-3% molybdenum to the particular alloy. Popular applications include building & architecture along with food & beverage processing equipment and jewelry (316L). Generally, austenitic steels are the most widely used stainless steels.
Series designations:
- Excellent welding characteristics, ductile, with good wear resistance.
- Similar corrosion resistance to 302, but higher strength.
- The additions of sulfur and phosphorus improve machining characteristics.
- The previously described 18/8 material. This is the most common of the series of 300 alloys.
- Essentially, 304 has been modified to improve welding characteristics.
- An improvement to the temperature resistance of 304.
- Second most common alloy after 304. Specialized for application in food and pharmaceutical industries. Also, it is more tolerant to chloride corrosion; therefore, it is a preferred marine grade.
Ferritic
These are highly corrosion-resistant alloys, commonly referred to as the 400 series. They are magnetic and contain chromium ranging from 12% to 18%. Some also contain increased chromium, up to 27%, for higher corrosion protection. The carbon level is typically low. At most, there is very little nickel in this alloy series. Inclusions are molybdenum and either aluminum or titanium. Alloys are magnetic. Applications include architectural and builders’ hardware, appliances, liners, and deck plates.
Martensitic
These are the first commercial stainless steels, also known as plain chromium stainless steels, part of the 400 alloy series. The high carbon level is alloyed with 12-18% chromium. These alloys are machineable, heat-treatable for hardness, and extremely strong and durable. Commercial applications include cutlery, fasteners, surgical instruments, aerospace, engineering, shafts, springs, and nozzles.
Series designations:
- Recommended for welding applications. Ferritic.
- Better heat resistance.
- Most economical. Highly visible as common automobile exhausts.
- Good wear resistance. Martensitic.
- Good machining characteristics, with the addition of sulfur.
- Martensitic. This grade is similar to the original formulation for rustless steel. It has good polishing characteristics and is mainly used in cutlery.
- Ferritic. Decorative automotive trim.
- Higher carbon content improves hardness. This alloy, exhibiting Rockwell 58, is one of the hardest stainless steels.
Stainless steel is very important for equipment used in metal finishing and assorted industries, such as consumer, medical, technical, and military.
One successful method of opening clogged arteries has been the insertion of stainless steel stents. Stainless steel materials are obvious if one acknowledges their presence. Many structures worldwide incorporate stainless steel in their fabrication.
Stainless steel's anti-weathering effect and strength make it a shining tribute. Two famous architectural landmarks stand out: the Gateway Arch, clad in alloy 304, and the top of the Chrysler Building, clad in alloy 302.
Stephen F. Rudy, CEF, is president of Chem Analytic and has written extensively about the finishing industry. Visit www.chemanalytic.com or call him at 917-604-5001.