What Is Precipitation Hardening Stainless Steel?
Precipitation hardening stainless steel is a type of stainless steel often used in environments where high strength and corrosion resistance are required.
Alloying elements such as chromium, nickel, and manganese in precipitation hardening stainless steel improve the corrosion resistance, strength, and wear resistance of stainless steel. Very high strength and corrosion resistance can be achieved by hardening treatment at high temperatures. The crystalline structure of stainless steel is changed and a special reinforcing phase is precipitated, resulting in increased strength.
Uses of Precipitation Hardening Stainless Steel
1. Nuclear Power Plants
Often used for control rods, nuclear fuel rods, coolant tubes, and other components.
Control rods are parts used to control the fission reaction in nuclear reactors, and are rod-shaped and several meters long. Fuel rods are the components that cause fission reactions in the reactor and generate thermal energy, and are usually cylindrical. A coolant tube is a tubular component used to cool the heat generated in the reactor.
2. Medical Devices
These include parts for artificial joints, surgical instruments, cardiac pacemakers, dental implants, etc.
3. Oil and Gas Industry
These include equipment parts for offshore oil drilling rigs, pipelines, and oil refinery plants (e.g., valves and piping).
4. Automotive Industry
Used in high performance engine parts, suspension and body parts (e.g., crankshafts, camshafts, etc.). The automotive crankshaft is one of the components that converts the vertical motion of the piston into rotational motion in an internal combustion engine. A camshaft in an automobile is one of the components that control the timing of valve opening and closing in an internal combustion engine.
5. Electronics Industry
High-precision measuring instruments and parts for semiconductor manufacturing equipment are examples. (e.g., sensors and gas distribution valves).
Gas distribution valves are used in semiconductor manufacturing equipment to supply high-purity gases through piping. Sensors are used in high-precision measuring instruments and semiconductor manufacturing equipment to measure parameters such as temperature, pressure, and flow rate.
Properties of Precipitation Hardening Stainless Steel
The main properties of precipitation hardening stainless steel are as follows:
1. Strength
Precipitation hardening stainless steel is a material with high strength. This is because precipitation hardening treatment forms fine precipitates, which increases the area of grain boundaries and strength by making crystal grains finer. The distribution of fine precipitates at the grain boundary also has the effect of strengthening the grain boundary, which also improves fatigue life.
Grain boundaries tend to have lower strength because they contain defects and amorphous regions inside the material. However, as in precipitation hardening stainless steel, the distribution of fine precipitates at the grain boundaries improves the strength of the grain boundary by consolidating the grain boundary and preventing slippage and migration at the grain boundary.
2. Corrosion Resistance
Precipitation hardening stainless steel is a corrosion resistant material. The reason is the formation of an oxide film due to the high chromium content and resistance to corrosion due to alloying elements. Alloying elements such as chromium, nickel, and manganese protect the steel surface, thereby preventing corrosion. Therefore, precipitation hardening stainless steel is characterized by the same resistance to corrosion as general stainless steel.
3. Hardenability
Precipitation hardening stainless steels are generally materials with excellent hardenability. This is because precipitation hardening stainless steels can be heat treated to form precipitation hardened products.
Precipitation hardening stainless steel is a solution treated (austenitized) at high temperatures and then quenched to form precipitation hardened products. Solution treatment changes the microstructure of the steel and improves hardness and strength.
4. Heat Resistance
Precipitation hardening stainless steel has excellent heat resistance. This is because the high chromium content and alloying elements form an oxide film that protects the steel surface, and the fine precipitates formed by the precipitation hardening treatment maintain the strength of the steel even under high temperatures. In addition, precipitates distributed within the grain boundaries increase the strength of the grain boundaries, which helps maintain the strength of the steel even at high temperatures.
5. Fatigue Resistance
Precipitation hardening stainless steel is a steel material with high strength and excellent corrosion resistance. The reasons are due to “high strength,” “excellent corrosion resistance,” “dense crystal structure,” and “excellent heat treatment performance,” as follows:
The high strength results in less deformation due to fatigue, extending fatigue life. Stainless steel generally has high corrosion resistance, which can suppress fatigue cracking. Slowing down the cooling rate results in a dense crystalline structure, which suppresses fatigue cracking and improves fatigue strength. In addition, heat treatment improves fatigue strength due to the formation of fine precipitates.
Other Information on Precipitation Hardening Stainless Steel
1. Weldability
Precipitation hardening stainless steels are generally weldable, but it should be noted that burning and welding defects may occur. Therefore, quenching treatment and preheating are necessary as well as appropriate welding conditions. It should also be noted that welding may cause grain boundary embrittlement due to the presence of fine precipitates near the grain boundary.
2. Magnetism
SUS630 and SUS631 are magnetic materials. In the solution heat-treated state, SUS630 and SUS631 are non-magnetic. However, after precipitation hardening treatment, SUS630 and SUS631 will exhibit strong magnetism. The reason is the formation of minute magnetic regions in the material.
The precipitation hardening treatment forms precipitates in the material, and the magnetism is caused by the presence of small magnetic regions in the precipitates. These regions are a part of the material and each region is minute, but as the number of regions increases, the material as a whole becomes magnetic.
3. Difficult to Fabricate
Because of its high hardness and strength, precipitation hardening stainless steel may be more difficult to process than ordinary stainless steel. However, by selecting appropriate processing methods and conditions, machinability can be improved.