Stainless steel is the material of choice for forging because of its corrosion and heat resistance. The process of forging stainless steel, nearly any grade of stainless steel, improves and accentuates the present properties of stainless steel. Forging may also improve the strength-to-weight ratio. However, unlike carbon steel which can be welded, forged stainless steels cannot be joined by conventional welding techniques. They must be fusion-bonded instead.
Stainless steel can be forged to a greater degree than carbon steel because of its higher yield point. Therefore, parts made from stainless steel can be more severely formed before failure. When forming stainless steel, like other metals, it is important not to go beyond its yield point or else you might damage the metal.
After being forged, parts should be heat treated to restore their strength. Heating to very high temperatures will "stress-relieve" the part, making it easier to form in subsequent steps. Also, heating relieves residual stress that comes from forming.
Forgings can be used to create new shapes or changes in direction within components. These are called "transitional forms". Forgings are commonly used in industrial applications where weight or size limitations require use of a lighter material. For example, airplane wings are often made from aluminum alloys because they are light weight yet strong enough to support the weight of an aircraft.
Because of its resistance to heat and corrosion, stainless steel is widely used in forging. 304/304L, 316/316L, and ferritic stainless steels are the most commonly used in forging. Some specialized alloys are also used for specific applications.
Stainless steel forging involves heating the metal above its recrystallization temperature while pressing it under pressure. The main advantage of this process is that it can produce articles with complex shapes. Also, because there is no grain growth during forging, the material retains its strength and hardness properties after forming. Finally, because there is no tendency toward formation of oxides or other surface contaminants, the finished part will not require post-processing treatment such as polishing or cleaning.
The choice of alloy for stainless steel forging depends on the required property profile. For example, 304 and 316 alloys are used when maximum strength and hardness are needed. Ferritic stainless steels are used for tools that must withstand high temperatures or corrosive environments. Special alloys such as BALASTRONIC and YELLOWINN are designed for use at elevated temperatures above 450°F (232°C) while QUENCHERSTEEL is used for water cooling systems.
Stainless steel forging is usually done using a hydraulic press. However, mechanical presses can be used as well.
Materials for Forging
304 Stainless Steel Forging is frequently utilized in the production of shafts, valve bodies, valve trim, and food processing equipment. 316 (L) stainless steel is the second most often used stainless steel grade (after 304 stainless steel) and is widely utilized in food and surgical stainless steel applications. 317 (H) stainless steel is a low-cost alternative to 316 (L) stainless steel and can be used in similar applications.
All three grades of stainless steel can be hardened by heat treatment or case hardening. Hardened stainless steel is known as "stainless steel armor". Case hardening involves heating the metal above its recrystallization temperature while it is still in its molten state and then rapidly quenching it with oil or some other non-corrosive liquid. This process creates fine grains of carbide within the metal that provide extra hardness. The metal is heated to 1100 degrees Fahrenheit (550 degrees Celsius) or more and plunged into a vat of oil before being rapidly cooled down.
Stainless steel can also be hardened by carburizing - heating the metal in air or water to approximately 1800 degrees Fahrenheit (1000 degrees Celsius) and allowing it to cool overnight. The surface layer of carbon forms a protective layer that prevents further oxidation or corrosion. Carburized stainless steel is harder than plain stainless steel but cannot be welded. It must be heat treated after welding to restore its toughness and ductility.
Forging 304 Stainless Steel Because type 304 has more hot strength than carbon, alloy, and even martensitic stainless steels, it requires considerably higher forging pressures or more hammer strokes to forge it and other austenitic stainless steels. The metal may be forged into shapes using conventional metal-working techniques, but because of its hardness, it is difficult to work with hand tools only. Machining or otherwise modifying 304 stainless steel parts after they are hardened is difficult because the heat used during machining can cause the metal to reharden, making it impossible to remove afterward.
Welding/Bonding/Annealing Of 304 Stainless Steel Because of its high resistance to corrosion, welding is often used to join sections of pipe, ductwork, and tanks. The most common types of welding used for stainless steel are gas tungsten arc welding (GTAW) and submerged arc welding (SAW). Both processes require that the metal be preheated before welding to prevent cooling below the recrystallization temperature. The heat from the welding process also anneals the metal, removing any residual stress from cold working. Post-weld treatment of stainless steel components includes quenching them in water or oil to prevent surface oxidation from occurring. This is particularly important when welding across material differences (such as between carbon and stainless steels), since there is a risk the weld will contain carbide if done improperly.