Page 49 - MetalForming-Jun-2018-issue

Introduction to Stainless Steels for Stamping Applications
Stainless steels can corrode— they’re called stain-less after all, not stain-free. They offer many grades from which to choose, providing a wide spectrum of uses and challenges.
The five main categories of stain- less steels, designated by their pre- dominant microstructural phases and characteristics:
• Austenitic • Ferritic
• Martensitic • Duplex
• Precipitation-hardened
Like all steels, each of these have iron as the primary element. Corrosion resistance of stainless steels results from the reaction of microstructural chromi- um with the atmosphere, forming a tenacious oxide layer only one-mil- lionth of a millimeter thick. This reac- tion begins when the iron-based alloy contains at least 10.5-percent chromi- um, making 10.5 percent the minimum amount of chromium possible in stain- less steels. Corrosion resistance typically improves with increasing chromium content. Formability, strength, tough-
Danny Schaeffler, with
30 years of materials and applications experi- ence, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solu- tions (EQS). EQS provides product-applications assistance to materials and manufacturing com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bach- elor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
ness and other properties of individual grades within the five categories result from the type and distribution of addi- tional alloying elements.
According to the International Stain- less Steel Forum, the combined market share of martensitic, duplex and pre- cipitation-hardened stainless steels totals less than 5 percent of all stainless applications. Martensitic stainless steels, like their carbon-steel equiva- lents, offer high strength and limited formability. Duplex grades blend the merits and challenges of their austenite and ferrite component phases. Precip- itation-hardening stainless steels can maintain corrosion resistance after heattreating, enabling them to reach strengths of 1800 MPa.
Nearly three-quarters of all stainless applications use austenitic grades. These 300-series stainless steels are made from alloying iron with chromi- um (16 to 26 percent), nickel (6 to 12 percent) and other alloying elements such as molybdenum. Adjusting the alloy content can maximize corrosion performance in different service envi- ronments, such as marine or those with high or low temperatures.
Austenitic grades constitute the most formable stainless steels. These steels strengthen when formed, as their high n-values lead to work-hardenability. Austenitic stainless steels, though not magnetic when produced, become slightly magnetic when formed into parts.
SS304, the most frequently used austenitic grade, has a composition of 18-percent chromium and 8-percent nickel, and sometimes is referred to as 18-8 stainless. Another common austenitic grade, SS316, has similar chromium and nickel content in addi- tion to about 2-percent molybdenum for enhanced corrosion resistance.
Increasing nickel content allows the austenite phase to form more readily
at room temperature, and is associated with increased ductility. However, the commodity price of nickel can vary greatly, from $50,000/ton in 2007 to one-quarter of that today. Nickel price is a key driver of 300-series stainless- steel pricing as it comprises about 10 percent of the alloy content. To get around high nickel prices, 200-series austenitic stainless steels were devel- oped, where various amounts of man- ganese, nitrogen and molybdenum replace some nickel content.
During cooling from welding or annealing temperatures, chromium in austenitic stainless steels combines with carbon to form chromium carbide. These precipitates occur at the microstructural grain boundaries. In a process called sensitization, chromi- um feeds the carbide formation at the expense of the surrounding metal. With now-lower chromium content, the grain boundaries are at risk for corro- sion. Using grades with reduced carbon content of 0.03 percent rather than the standard 0.08 percent reduces the ten- dency for chromium-carbide precipi- tation, as will alloying with titanium and/or niobium, which combine pref- erentially with carbon. Austenitic grades with a lower carbon content are designated with the suffix L, such as SS304L or SS316L. Sensitization in fer- ritic stainless steels is minimized with specific thermal profiles.
Ferritic stainless steels comprise part of the 400 series, and contain chromium (12.5-17 percent) as the pri- mary alloying element. These stainless steels, ferromagnetic and generally having adequate formability, are essen- tially nickel-free, making them a lower- cost option to 300-series austenitic grades. Ferritic stainless steels are at risk of grain growth with an associated loss of properties when welded in thick- er sections. Unlike austenitic stainless
The Science of Forming By Daniel J. Schaeffler, Ph.D.
www.metalformingmagazine.com
MetalForming/June 2018 47