Forging Materials: Stainless Steels
Like all steels, stainless steels are iron-based alloys. They are considered a special class of steels because of their high alloy content and their special properties as compared to plain-carbon and low-alloy steels. As their name indicates, this class of metals has good corrosion resistance, which comes from the formation of an adherent chromium-oxide film on the surface of the metal. This film, though very thin, prevents the degradation of the base iron. On the whole, stainless steels have fairly high strength and relatively good ductility.
Figure 1. Various components made from stainless steel
Stainless steels are composed of 55-90% iron and from 10-28% chromium. Nickel is often added to the alloy ranging from 0-22%. Because of the high cost of nickel, such steels are more expensive. Carbon is often quite low in these alloys. In fact, some of the alloys are designated with an “L” after their number to indicate that they are very low carbon. The low carbon content of these alloys permits them to be more easily welded. Manganese between 1% and 2% is also found in stainless steels.
Types of Stainless Steels
Figure 2. The high flow stress of stainless steel results in higher forging loads relative to carbon and alloy steels. Stainless is sensitive to strain rate during hot forging. During cold forming, work hardening is very significant.
There are four primary types of stainless steels: austenitic, ferritic, martensitic and precipitation-hardening. They are classified by the type of microstructure they possess. The 200- and 300-series alloys comprise the austenitic grades, which have a microstructure that is a face-centered cubic phase (i.e. austenite). Austenitic stainless steels are both strong and ductile. Nickel is needed to produce these austenitic grades, hence their higher cost. Some of the 400-series are ferritic and some are martensitic. The microstructure in the ferritic grades is a body-centered cubic structure (i.e. ferrite). Ferritic stainless steels do not possess quite the strength or the ductility of the austenitic grades.
The 500-series denotes martensitic steels, which have the highest strength but also have the lowest toughness and ductility. Carbon in these martensitic grades can be as high as 1.2%. The higher the carbon level, the stronger the steel will be. The precipitation-hardening stainless steels make up the PH grades, which require special post-forging heat treatments to allow the precipitation reactions to occur. The service temperatures of these grades are also more limited.
Ferritic and austenitic stainless steels do not undergo a phase transformation during cooling, which is different than the plain-carbon and low-alloy steels. The ferritic grades will be forged as ferrite and remain ferrite on cooling. Similarly, the austenitic grades will be forged as austenite and remain so on cooling. However, the martensitic grades will be forged as austenite and need to be quenched to achieve the high-strength martensitic structure.
Some of the austenitic grades are metastable at room temperature. If these grades are deformed (i.e. cold worked) at room temperature, the austenite will transform into martensite. This transformation will cause a local increase in strength but can be difficult to control.
In service, forged stainless steel components can often operate up to temperatures of about 800°F. Since they are single-phase metals, they will not undergo any major microstructural changes when used at these elevated temperatures.
Forged stainless steels are used in valves, bolts, shafts, kitchen equipment and food service. Because of its corrosion resistance, this alloy class is well-suited to these applications. In highly corrosive environments such as oil refining, chemical processing, mining and drilling, stainless steel components are often used to prevent rapid part degradation.
Another corrosive environment that we sometimes overlook is the human body. Medical instruments and some biomaterial implants are made from stainless steel, which will not readily degrade inside the human body. Figure 1 shows a number of stainless steel components.
Forging Stainless Steels
Figure 3. Microstructure of a austenitic 304L stainless steel forging. Only grain boundaries are observed in the microstructure.; Figure 4. Microstructure of austenitic 309 stainless steel; Figure 5. An annealed ferritic 409 stainless steel
The hot forging temperature for stainless steels is 1700-2300°F, depending on the specific grade. The hot forgeability of stainless steels is generally reasonable, but they require relatively higher forging loads and energy. While not as resistant to deformation as nickel-based superalloys, forging equipment size can become an issue for forge shops accustomed to forging plain-carbon or low-alloy steels.
In addition to the strength resulting from higher chromium and nickel contents, the forging temperatures for stainless steels are limited by their lower melting temperatures. Because of the higher flow stress and lower thermal conductivity, deformation heating is higher for stainless steels relative to plain-carbon and low-alloy steels. Thus, the risk of overheating must be considered. Some stainless grades will work-harden (increasing strength during deformation) during hot forging, which is uncommon for other steel alloys. While work-hardening readily occurs in almost all metals at room temperature, stainless steels exhibit more hardening than most other metal alloys. This means that “cold” forging requires very high forging loads. Figure 2 shows the stress-strain curves for hot-forged and cold-forged stainless steels.
Overheating can result in ductile fracture (hot shortness) or microstructure issues in service. For example, if austenitic stainless steels are heated too high during forging, a deleterious phase called delta ferrite can form, which will not only cause degradation in properties but can adversely affect the forgeability of the metal. In all cases, stainless steels are very sensitive to temperature.
Stainless steels are more sensitive to strain rate than plain-carbon and low-alloy steels. The speed of the press will also affect the metals’ resistance to flow at high temperatures for most stainless steels. So, if there is difficulty in filling the die, a slow press speed may be beneficial in achieving the desired fill.
Scale is light and tight on most austenitic stainless steels. In some forge shops, the workpiece is pre-coated with a lubricant, which can reduce friction, reduce the tendencies for stickers and act as a scale retardant.
Heating is generally performed in gas-fired furnaces since austenitic stainless steels are not ferromagnetic, a requirement for efficient induction heating.
The microstructures of most stainless steels are composed of a single phase, thus revealing only grain boundaries. Figure 3 illustrates the microstructure of a forged 304L stainless steel. It was forged at 1600°F and allowed to air cool. The only visible structure is the boundaries between the austenite grains. Figure 4 is the microstructure of another austenitic stainless steel. It is essentially a single-phase material with a few carbides visible. The straight grain boundaries are indicative of a long anneal after forging. Figure 5 is a photo a 409C ferritic stainless steel after an anneal. The grains are relatively equiaxed, which is to be expected from an annealed single-phase metal.
Because most stainless steels are single-phase metals, the grain size of the component needs to be properly controlled. Unlike plain-carbon and low-alloy steels, the attainment of appropriate properties often must be accomplished in the forge shop rather than in a subsequent heat treatment.
Figure 6. The temperature ranges for stainless steels
Softening or annealing of some grades can be accomplished after forging by heating to temperatures between 1700° and 2000°F. This process will make the final component more ductile but with a decrease in its strength.
Some grades of austenitic stainless steels can become “sensitized.” Sensitization means that in service they will be susceptible to intergranular corrosion. Such degradation is very undesirable. To avoid sensitization, one needs to finish-forge above the sensitization temperature (1700°F) and rapidly cool to below 900°F. Figure 6 shows the temperature ranges for stainless steels.
Stainless steels are an important class of engineering alloys because of their ability to resist degradation in corrosive environments. Most grades have reasonable forgeability, although they require higher loads and are a bit more difficult to flow as compared to plain-carbon and low-alloy steels. Because most stainless steels are single-phase materials, consideration of how they are forged and cooled needs to be given.
The support for this work from the PRO-FAST Program is appreciated. The PRO-FAST Program is enabled by the dedicated team of professionals representing both the Department of Defense and industry. These teammates are determined to ensure the nation’s forging industry is positioned for challenges of the 21st century. Key team members include: R&D Enterprise Team (DLA J339), Logistics Research and Development Branch (DLS-DSCP) and the Forging Industry Association (FIA). This work was originally prepared for the FIA Theory & Applications of Forging and Die Design course by Scientific Forming Technologies Corporation.
Co-author Dr. Chet Van Tyne is FIERF Professor, Department of Metallurgical Engineering, Colorado School of Mines, Golden, Colo. He may be reached at 303-273-3793 or firstname.lastname@example.org. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at 614-451-8330 or email@example.com.