The Science of Forming


 

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Introduction to Stainless Steels for Stamping Applications

By: Daniel J. Schaeffler, Ph.D.

Danny Schaeffler, with 30 years of materials and applications experience, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solutions (EQS). EQS provides product-applications assistance to materials and manufacturing companies; 4M teaches fundamentals and practical details of material properties, forming technologies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bachelor 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 Tel. 248/66-STEEL E-mail ds@eqsgroup.com or Danny@learning4m.com

Friday, June 1, 2018
 

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 stainless steels, designated by their predominant 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 chromium with the atmosphere, forming a tenacious oxide layer only one-millionth of a millimeter thick. This reaction begins when the iron-based alloy contains at least 10.5-percent chromium, making 10.5 percent the minimum amount of chromium possible in stainless steels. Corrosion resistance typically improves with increasing chromium content. Formability, strength, toughness and other properties of individual grades within the five categories result from the type and distribution of additional alloying elements.

According to the International Stainless Steel Forum, the combined market share of martensitic, duplex and precipitation-hardened stainless steels totals less than 5 percent of all stainless applications. Martensitic stainless steels, like their carbon-steel equivalents, offer high strength and limited formability. Duplex grades blend the merits and challenges of their austenite and ferrite component phases. Precipitation-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 chromium (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 environments, 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 addition 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 developed, where various amounts of manganese, 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, chromium feeds the carbide formation at the expense of the surrounding metal. With now-lower chromium content, the grain boundaries are at risk for corrosion. Using grades with reduced carbon content of 0.03 percent rather than the standard 0.08 percent reduces the tendency for chromium-carbide precipitation, as will alloying with titanium and/or niobium, which combine preferentially with carbon. Austenitic grades with a lower carbon content are designated with the suffix L, such as SS304L or SS316L. Sensitization in ferritic 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 primary alloying element. These stainless steels, ferromagnetic and generally having adequate formability, are essentially 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 thicker sections. Unlike austenitic stainless grades, the ferritic grades become brittle at low temperatures.

SS430 is the most widely used ferritic stainless steel, with SS409 having greater corrosion risk owing to a lower chromium content. SS439 offers greater resistance to corrosion and improved high-temperature stability, making it suitable for exhaust systems. Using titanium and niobium to tie carbon and nitrogen into fine precipitates results in improved formability–the same mechanism employed in the production of interstitial-free extra-deep-drawing ultra-low-carbon steels.

Greater shear strength in annealed austenitic stainless steels as compared to carbon steels leads to more force being required to shear stainless alloys of equal thicknesses. Press and die sections should be built with greater rigidity to account for the increased shear strength. Austenitic grades workharden to a greater extent than do ferritic grades, giving higher strength to the rollover section of a cut edge. Flanging or otherwise expanding a poorly cut edge results in a greater likelihood of edge cracks. Minimizing rollover, by using well-aligned cutting tools with tighter clearances, improves the cut edge. However, tight clearances accelerate the wear of shear knives, making it difficult to keep cutting tools sharp and sufficiently aligned.

Computer-simulation models used for low-carbon steels are insufficient to model the forming and structural performance of stainless steels. Ferritic grades have a relatively constant n-value, whereas austenitic grades have an n-value that changes with strain, test speed and temperature. Austenitic grades have a “TRIP-effect,” converting to martensite during forming, which must be incorporated into any prediction involving austenitic stainless steels. MF

Want to learn more about different sheetmetals and their formability? Plan to attend PMA’s Sheetmetal Formability of Steel, Aluminum and Stainless Steels seminar in Cleveland, OH, June 26-27. Visit www.pma.org to register, or contact Marianne Sichi at msichi@pma.org for information.

 

See also: 4M Partners, LLC, Engineering Quality Solutions, Inc.

Related Enterprise Zones: Materials/Coatings


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