Why does stainless steel need secondary quenching?

Stainless Steel Spring Secondary Quenching: Principles, Purposes, and Effects

Stainless steel, an indispensable material in modern industry, plays a significant role in numerous fields due to its excellent corrosion resistance and mechanical properties. However, to further enhance the performance of stainless steel, particularly its hardness, wear resistance, and toughness, the quenching process is crucial. In certain specific scenarios, stainless steel even requires a secondary quenching treatment. So, why does stainless steel need secondary quenching? This article will provide a detailed explanation from the aspects of stainless steel’s microstructure, quenching principles, and the purposes and effects of secondary quenching.

I. Microstructure of Stainless Steel and Quenching Principles

Stainless steel is primarily composed of elements such as iron (Fe), chromium (Cr), and nickel (Ni). Its microstructure undergoes significant changes during heat treatment. Quenching, as an important heat treatment process, works by rapidly cooling the stainless steel material, causing changes in its internal microstructure, thereby achieving the goal of enhancing hardness and strength.

During the quenching process, stainless steel is heated to the austenitizing temperature, at which point its internal grains gradually grow and distribute evenly. Austenite is a high-temperature stable phase with a face-centered cubic structure, exhibiting good plasticity and toughness. However, when austenite is rapidly cooled, due to a sufficiently fast cooling rate, austenite cannot transform into stable ferrite or pearlite, but rather transforms into hard phases such as martensite or lower bainite. These hard phases have higher hardness and strength but relatively lower toughness.

II. Limitations of Single Quenching of Stainless Steel

Although single quenching can significantly enhance the hardness and strength of stainless steel, it also has some limitations. Firstly, single quenching often leaves a relatively large amount of retained austenite in the stainless steel. Retained austenite is an unstable phase that may transform during subsequent use, leading to instability in material properties. Secondly, the grains in stainless steel after single quenching may be relatively coarse, which can affect the material’s toughness and fatigue resistance. Additionally, although the hardness and strength of stainless steel are improved after single quenching, they may not meet the high performance requirements of certain specific applications.

III. Purposes and Effects of Secondary Quenching

To address the limitations of single quenching and improve the comprehensive performance of stainless steel, the secondary quenching process has emerged. Secondary quenching is a process of reheating and rapidly cooling the material on the basis of single quenching. Its purposes and effects are mainly reflected in the following aspects:

• Grain Refinement:
  • Through reheating and rapid cooling during secondary quenching, the grains in stainless steel can be further refined. Grain refinement not only improves the material’s hardness and strength but also enhances its toughness, enabling the material to better resist deformation under external forces.
• Reduction of Retained Austenite:
  • The retained austenite present in stainless steel after single quenching further transforms into hard phases such as martensite or bainite during secondary quenching. This reduces the content of retained austenite and improves the material’s stability. Simultaneously, due to the transformation of retained austenite, the material’s hardness and strength are further enhanced.
• Adjustment of Microstructure and Properties:
  • Secondary quenching can also adjust the microstructure and properties of stainless steel according to specific needs. For example, by controlling the heating temperature and cooling rate of secondary quenching, different morphologies and distributions of martensite structures can be obtained, thereby meeting the material performance requirements of different applications.
• Improvement of Wear Resistance and Corrosion Resistance:
  • The surface hardness and density of stainless steel are increased after secondary quenching, which helps improve its wear resistance and corrosion resistance. During friction and wear, a harder surface can more effectively resist external wear and prolong service life. At the same time, a dense surface structure also helps improve the material’s corrosion resistance.
• Elimination of Internal Stress:
  • Internal stress generated during the quenching process is an important factor affecting material performance and service life. Secondary quenching can release and balance internal stress through reheating and cooling, thereby improving the material’s stability and reliability.

IV. Process Parameters and Precautions for Secondary Quenching

The process parameters of secondary quenching include heating temperature, holding time, cooling rate, etc. The selection of these parameters needs to be adjusted according to the specific material and application. Generally, the heating temperature should be higher than the single quenching temperature to ensure sufficient transformation of retained austenite. The holding time should be determined according to the material’s thickness and the heating equipment’s performance. The cooling rate needs to be selected according to the required microstructure and properties.

When performing secondary quenching, the following points also need to be noted:

• Avoid Overheating and Burning:
  • Excessive heating temperature or holding time may cause material overheating or even burning, which can severely damage the material’s performance. Therefore, temperature and holding time need to be strictly controlled during heating.
• Select Appropriate Cooling Media:
  • The selection of cooling media has a significant impact on the quenching effect. Different cooling media have different cooling rates and cooling characteristics, and need to be selected according to specific needs. At the same time, factors such as the temperature and flow rate of the cooling media also need to be noted.
• Control Quenching Deformation:
  • Due to the effects of thermal stress and structural stress during the quenching process, the material may deform. Therefore, appropriate pre-treatment and fixture design of the material are required before quenching to control quenching deformation.
• Subsequent Tempering Treatment:
  • Stainless steel after quenching often has high hardness and brittleness, and subsequent tempering treatment is needed to stabilize the microstructure and reduce brittleness. The selection of tempering temperature and time needs to be adjusted according to the specific material and application.

V. Application Examples of Secondary Quenching in Stainless Steel

The secondary quenching process has achieved significant effects in the application of stainless steel. For example, when manufacturing high-precision molds, tools, and other parts that require high hardness and wear resistance, the secondary quenching process can be used to improve the material’s hardness and wear resistance. In addition, in fields such as aerospace and automobile manufacturing, stainless steel parts often need to work in extreme environments, with high requirements for material performance. The secondary quenching process can improve the comprehensive performance of these parts and meet usage requirements.

VI. Conclusion

The main reason why stainless steel needs secondary quenching is to address the limitations of single quenching and improve the comprehensive performance of the material. Secondary quenching can refine grains, reduce retained austenite, adjust microstructure and properties, improve wear resistance and corrosion resistance, and eliminate internal stress. When performing secondary quenching, it is necessary to pay attention to controlling process parameters and avoiding common problems. With the continuous development and progress of heat treatment technology, the secondary quenching process will play a more important role in the application of stainless steel.