Stainless steels are available in numerous grades that provide a wide variety of uses and challenges.

The five main categories of stainless steels, each designated by the metallurgical characteristics and phases within its microstructure, are:

1. Austenitic
2. Ferritic
3. Martensitic
4. Duplex
5. Precipitation-hardened.

Each of these steels is iron-based and alloyed with at least 10.5 percent chromium, which is what gives the metal its corrosion resistance (see Figure 1). The type and distribution of other alloying elements give each grade its unique properties.

The duplex grade is a combination of austenite and ferrite, so it offers the blended characteristics of those two grades. Martensitic grades, like their carbon steel equivalent, maintain extremely high strength at room temperature. Precipitation-hardening grades have good room-temperature formability and can reach 260 KSI in strength after heat treating while maintaining corrosion resistance. Combined, however, the duplex, martensitic, and precipitation-hardening grades have a market share of less than 4 percent. Let’s look at austenitic and ferritic stainless steels more closely (see Figure 2).

Austenitic Stainless Steels (200 and 300 Series)

Austenitic stainless steels are the most common family of stainless steels in use, with a market share of 75 percent as recently as 2004. As the name suggests, the microstructure is composed of the austenite phase. In the 300 series, this is achieved with about 16 to 22 percent chromium and 8 to 14 percent nickel. Although the nickel adds ductility, it is prone to a fluctuating commodity price, reaching $50,000 a ton in 2007 but now closer to $10,000 a ton. The 200 series, developed to get around the high price of nickel, replaces some nickel content with manganese and nitrogen.

The most commonly used austenitic grade is SS304. With its composition of 18 percent chromium and 8 percent nickel, it is sometimes referred to as 18-8 stainless. However, this designation isn’t recommended for general use, as there are tolerances in the allowable range of these elements that overlap with other grades. For example, SS316 is similar in chromium and nickel content, but it also has about 2 percent molybdenum for additional corrosion resistance.

These grades are prone to sensitization, a loss of alloy integrity. During cooling from welding or annealing, chromium carbide precipitates form at the microstructural grain boundaries. In these areas, the chromium feeds the carbide formation at the expense of the surrounding metal. With lower chromium content, the grain boundaries are at risk for corrosion.

Carbide precipitation can be reduced through the use of grades with lower carbon content (about 0.03 percent rather than 0.08 percent). Lower-carbon versions of austenitic grades are designated with the suffix L, such as 304L or 316L. Another way to prevent sensitization is to add titanium and/or niobium, which combine preferentially with carbon.

Ferritic Stainless Steels (400 Series)

About 20 percent of all stainless steel grades have a ferritic microstructure, with SS430 being the most widely used. SS409 has lower chromium and, therefore, reduced resistance to corrosion. SS439 has greater resistance to corrosion and improved high-temperature stability.

Additions of titanium and niobium combine with the carbon and nitrogen in a manner similar to super-soft. interstitial-free. extra-deep-drawing carbon steel. Tying up carbon and nitrogen in fine precipitates results in better welding and formability.

 

Figure 2. Although similar in nature, these stainless steels do display some significant differences.

Depending on the alloy chosen, the properties and performance can be comparable to that of SS304. Because the ferritic stainless grades do not have nickel, they are generally lower cost than the 300 series grades.

Properties and Performance

Neither the austenitic nor ferritic steels are heat-treatable. The 200, 300, and some of the 400 series stainless steels all work-harden, getting stronger during forming, but the austenitic grades do so more rapidly and to a greater extent.

Even though the austenitic grades typically have better general corrosion resistance, formability, and weldability, fluctuating nickel prices have caused some companies to make processing changes to accommodate forming ferritic alloys.

In elevated-temperature applications, such as exhaust systems that can reach 1,650 degrees F, ferritic grades provide better tensile-property stability and thermal fatigue resistance. They have lower thermal expansion and higher thermal conductivity than austenitic grades.

Ferritic stainless steels become brittle as the temperature decreases. The transition temperature is about 32 degrees F, although it depends on the alloy composition. Austenitic stainless steels are not at risk of becoming brittle at low temperatures.

According to the Specialty Steel Industry of North America, the shear strength of annealed austenitic stainless steel is about 65 to 70 percent of its ultimate tensile strength; for carbon steels that number is about 55 to 60 percent. For this reason, shearing of stainless alloys requires more force and heavier equipment than shearing of carbon steels of equal thickness, so press and die sections need to have greater rigidity.

Ferritic stainless steels tend to fracture after being cut about halfway through their thickness, similar to carbon and low-alloy steels. Austenitic steels allow greater penetration before fracturing. Large clearances, either from poor setup or from dull tools, can lead to greater rollover, which results in a poor cut. Austenitic grades work-harden to a greater extent than ferritic grades, so the rollover section of the cut edge has higher strength. Flanging or otherwise expanding this cut edge is more likely to cause edge cracking. Tighter clearances accelerate wear of the shear knives. Per-side clearances of 5 percent are recommended, with the percentage increasing as the sheet metal thickness increases.

Ferritic stainless steels are magnetic, while austenitic stainless steels in the annealed condition are not. However, when austenitic stainless grades are formed into engineered shapes, they undergo a microstructural transformation to martensite in the same way as the transformation-induced plasticity (TRIP) family of advanced, high-strength steels. When the austenite converts to martensite, strength increases, ductility increases, and the structure becomes magnetic. The strain-hardening exponent known as the n-value exceeds 0.4 in austenitic grades, which is double that of ferritic stainless steel grades.

Using computer simulation to predict forming and structural behavior involves additional challenges, as models used for low-carbon steels are insufficient. The tensile properties of austenitic stainless steels depend strongly on temperature and test speed. These grades have an n-value that increases with strain, while the 400 series ferritics have a relatively constant n-value. The TRIP effect needs to be incorporated in any prediction involving austenitic stainless steels.

As with any engineered material, it is up to the user to specify what is needed for the application—not overengineered, which will cost more, or under-engineered, which exposes some degree of risk during usage. Knowing the limitations and constraints associated with the many choices is a good step in determining the optimum material for each application.

• [Editor:Wang Linyan]