Tempering is an important heat treatment to affect the performance of stainless steel. Though stainless steel is already known for its strength, corrosion resistance, and long life, those qualities will be heightened — or fine-tuned — by tempering. The effects of tempering on the microstructure and properties of stainless steel are key information for engineers, fabricators and users of high performance metal parts alike. Some metals get hard but brittle when in a specific environment where tempering improves the toughness to withstand the brittle trend.
The Basics of Stainless Steel
Stainless steel is a type of steel that contains 10.5% chromium and other alloys, including nickel. With this chemical composition, it is strong and durable.
Steel is naturally strong because it forms martensite during cooling. Martensite has a higher carbon content, making it strong, but also making it more difficult to alter its atomic structure. Stainless steel can be made more ductile through tempering, which involves rapidly cooling the metal to prevent reactions caused by slow cooling.
What is Tempering
Tempering is a heat treatment process for iron-based alloys (especially steel) aimed at improving their mechanical properties. This process is critical for reducing the brittleness that occurs during hardening, enhancing the metal’s toughness and ductility. The process involves reheating the quenched 304 metal plate to a temperature below its critical point, followed by controlled cooling.
What are the Benefits of Tempering
Improving toughness and hardness: This is one of the common benefits of tempering, but the tempering process offers even more advantages.
Assisting manufacturing processes: It can alter the physical properties of stainless steel composite plates, facilitating other manufacturing processes such as cold forging, welding, and machining.
Reducing stress: It helps alleviate stress, making carbon steel composite plates easier to weld or machine.
Enhancing overall performance: While increasing strength, it makes nickel steel composite plates more flexible and ductile, while also increasing hardness and introducing wear resistance to the surface or the entire metal.
Overcoming environmental effects: Some metals become brittle and prone to fracture in specific environments, and tempering helps overcome this brittleness and enhance toughness.
Enhancement of electromagnetic properties: It can improve the electromagnetic properties of copper-steel composites plates, it has increased the probability of metal being compatible with other materials.
Tempering Process Steps
Preparation and initial heating: Preparation and preliminary heating The tempering begins with the preparation of the metal and the regulation of its hardness, followed by its initial heating. The first step in this hardening is to create a hard but excessive (breakage) martensitic structure. The metal should then be cleaned of any surface contaminants that would interfere with the heating.
Initial Heating Stage: The steel is heated to a homogeneous level at a temperature lower than its critical point, with a value chosen according to the final mechanical properties required. Heating should be even so that new stresses are not introduced into the material by thermal gradients.
Soaking and holding stage: The metal composite plate needs to be maintained at the tempering temperature for a period of time for the carbon atoms to diffuse and the internal stress to be relieved. The time this holding period takes depends on the kind of steel and the required specific properties.
Cooling technique: After the holding stage, the metal must be cooled. The cooling rate has a significant impact on the final properties of the metal.
Controlled cooling: The cooling process can be carried out in various ways, depending on the desired properties. Slow cooling in air or a furnace allows more time for stress relaxation and phase transformation, thereby enhancing toughness; faster cooling retains some of the hardness from the quenching state while reducing brittleness.
Types of Tempering
Low-temperature tempering: Conducted at temperatures a little above the lower critical temperature (150-300°C), in order to retain maximum hardness but slightly increasing toughness (used for cutting tools and wear-resistant parts).
Medium-temperature tempering: In the range of 300 and 500°C, carbides continued to precipitate and initiate growth, the hardness of hardness drops significantly but the toughness and ductility increased. It is ideal for applications where a balance of hardness and toughness is desirable like gears.
High-temperature tempering: Reheating the material to temperatures between 500°C and 700°C, this process maximizes toughness at the expense of some hardness, making it suitable for components that must withstand high impact and deformation, such as structural components and automotive parts.
When is Tempering Necessary
After quenching: Tempering is commonly applied after quenching operations. When carbon steel is heated and rapidly quenched, the material becomes hard and brittle. Tempering helps restore its ductility.
After welding: Metals that have undergone welding, such as 316 metal composite plates, require stress relief and the removal of undesirable mechanical properties.
After work hardening: Processing operations such as bending, drilling, rolling, and punching can cause work hardening in materials, which have high residual stresses. Tempering can alleviate these stresses.
Effect of Tempering Time on the Stainless Steel
Taking 5Cr15MoV MSS as an example, when it is tempered from 480°C to 600°C, its hardness and corrosion resistance decrease as the tempering temperature and time increase.
The tempering process is roughly divided into four stages based on tempering time: when tempering does not exceed 0.5 minutes, the hardness of the metal material slightly increases, and the corrosion resistance shows no significant change compared to the quenched sample; when tempering for 0.5 to 2 minutes, carbides begin to precipitate, and both hardness and corrosion resistance rapidly decrease; when tempering for 2 to 16 minutes, corrosion resistance improves as Cr atoms diffuse from the matrix to replenish Cr-depleted regions; when tempering exceeds 16 minutes, hardness and corrosion resistance continue to decrease.
Common Applications of Hardened Steel
Tempered steel is used in many industries and applications, including: building structures and bridges, manufacturing durable storage tanks, producing cutting edges for saws and drill bits, manufacturing gears, and as linings for chutes, dump trucks, and trailers, as well as deflector plates.
Conclusion
Tempering plays a critical role in fine-tuning the mechanical and structural properties of stainless steel. From improving toughness and reducing brittleness to enhancing corrosion resistance and facilitating downstream processing, the benefits of tempering are extensive and application-specific. A clear understanding of tempering conditions—especially time and temperature—is essential for achieving the desired balance of hardness, ductility, and durability in stainless steel-based applications.
