Author: Daniel J. Schaeffler, Ph.D.

First published in the June 2018 issue of MetalForming Magazine

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 heat treating, 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 Stainless Steels

The most malleable types of steel are the austenitic grades. These steels strengthen when formed, as their high n-values lead to work-hardenability. Austenitic grades of steel, although non-magnetic in their initial state, develop a slight magnetic property after being shaped into components.

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 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. Manufacturers designate austenitic grades with a lower carbon content with the suffix L, such as SS304L or SS316L. They minimize sensitization in ferritic stainless steels using specific thermal profiles.

Ferritic Stainless Steels

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.

The most widely used ferritic stainless steel is SS430, while SS409 has a greater corrosion risk due to its 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.

Processing Stainless Steels

Annealed austenitic stainless steels have greater shear strength than carbon steels, which results in requiring more force to shear stainless alloys of equal thicknesses, necessitating greater rigidity in the press and die sections to account for this increased strength. Austenitic grades work harden more than other grades. This gives them higher strength, especially in the 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. Austenitic grades, on the other hand, have an n-value that changes with strain, test speed, and temperature. Austenitic grades have a “TRIP-effect,” which involves converting to martensite during forming. Incorporating this effect is necessary for making any prediction involving austenitic stainless steels.