Q&A on Key Knowledge of Heat Treatment

　　Q&A on Key Knowledge of Heat Treatment (Part 1)

 

　　1. Why is it necessary to perform stabilization treatment on measuring instruments? What is the typical stabilization treatment process for conventional measuring instruments?

 

　　Processing can reduce the positive anisotropy of M, making it more stable and promoting the aging of A’ during transformation. It also helps to relieve residual stresses after quenching and deep cryogenic treatment, thereby providing excellent dimensional stability.

 

　　2. What are the two methods for ultrafine processing of bearings, and what are their purposes?

 

　　① Purpose of forging and quenching pretreatment: This process can reduce the residual K content to 7.11% while maintaining an A’ content of 11.9% to 12.1%, and achieve an A grain size of 9 to 10 grades. ② Dual refinement treatment for bearings: After this treatment, the original grain size can be refined by 1.5 to 2.0 grades, and the carbide particle size will be less than 0.6 μm. This is conducive to obtaining a fine needle-like M microstructure after quenching, and can also enhance toughness, wear resistance, and fatigue strength.

 

　　3. What factors should be considered when developing a heat treatment heating process?

 

　　① Advanced Process Technology: Fully adopt new process methods, advanced heat treatment technologies, and novel process materials. ② Reliable, Rational, and Feasible Process: The selected processes must be highly reliable and stable. ③ Economic Efficiency of the Process: The process should make efficient use of energy resources; energy-saving process equipment should fully leverage existing facilities, and auxiliary tooling methods should be employed to meet the process requirements of different parts. ④ Process Safety: The process must be safe and reliable, with necessary safety precautions in place. ⑤ As much as possible, adopt highly mechanized and automated process equipment. This not only boosts labor productivity but also facilitates better control over the process, ensuring the reliable quality of heat treatment.

 

　　4. What is spheroidizing annealing? What are its process characteristics?

 

　　Answer: The so-called spheroidizing annealing of steel is an annealing process designed to spheroidize the carbides in the steel. 1. In conventional spheroidizing annealing, the heating temperature for spheroidizing the carbides is AC1 plus 20–30°C. The holding time depends on the time required for the workpiece to be fully heated through, but it should not be excessively long. As for the cooling rate, it is typically 10–20°C per hour within the furnace, and the workpiece is removed from the furnace and air-cooled once the temperature drops below 550°C. 2. Isothermal spheroidizing annealing is mainly used for high-carbon tool steels and alloy tool steels. This process offers thorough and easily controllable spheroidization, has a relatively short cycle time, and is particularly suitable for large-sized parts. The heating temperature is AC1 plus 20–30°C, and the holding time depends on the time required for the workpiece to be fully heated through. The holding temperature is AC1 plus 20–30°C, and the isothermal holding time depends on the TTT curve.

 

　　5. Why does hypoeutectoid steel, after normalizing, achieve higher strength and hardness than after annealing?

 

　　Answer: Both annealing and normalizing produce pearlitic microstructures. However, when comparing normalizing with annealing, the pearlite formed during normalizing is obtained under a greater degree of supercooling. Consequently, for hypoeutectoid steels, less proeutectoid ferrite precipitates, resulting in a greater amount of pearlite and a smaller interlamellar spacing of the pearlite lamellae. Moreover, due to the lower transformation temperature, the nucleation rate of pearlite is higher, leading to smaller pearlite colony sizes. Because of these microstructural differences, the mechanical properties also differ. Compared to annealing, normalizing results in higher strength and hardness, while the ductility remains similar.

 

　　6. What is martensitic step quenching? What is bainitic isothermal quenching?

 

　　Answer: Step quenching involves cooling a workpiece directly from the quenching temperature to a temperature slightly above the Ms point, holding it at that temperature for an appropriate period, and then allowing it to air cool to obtain a martensitic microstructure. Typically, the cooling temperature is around 200°C (above the Ms point of the material). This method is suitable for carbon steels and alloy steels with small effective dimensions and relatively complex shapes. In some cases, step quenching below the Ms point is also employed, with a step-quenching temperature ranging from 130°C to 160°C. This approach is particularly suited for workpieces with low hardenability and larger dimensions.

 

　　A quenching process in which steel or steel components are heated to austenitize them, then rapidly cooled to the bainitic transformation temperature range (260–400°C) and held isothermally, allowing the austenite to transform into bainite. This is a common type of quenching process.

 

　　7. What is shot peening? What are its effects on the surface morphology and properties of materials?

 

　　Answer: By using high-speed jets of fine particles that impact the surface of the workpiece at room temperature, the surface material undergoes elastic-plastic deformation at its recrystallization temperature, resulting in significant residual compressive stress. This process enhances the surface strength, fatigue strength, and resistance to stress corrosion of the workpiece. The impact causes plastic flow and work hardening on the workpiece surface, significantly increasing the surface hardness of the material. At the same time, it reduces surface roughness and maintains residual compressive stress on the workpiece surface, thereby substantially improving the material’s fatigue strength, fatigue life, and resistance to stress corrosion.

 

　　8. What are the commonly used quenching methods? Discuss the principles for selecting a quenching method.

 

　　Answer: ① Single-medium quenching: Carbon steel workpieces with simple shapes are water-cooled, while alloy steels and alloy tool steels are oil-cooled; ② Two-medium quenching: Suitable for workpieces with complex shapes that are prone to deformation; ③ Pre-cooling quenching is used for tool and die steels to reduce deformation and cracking; ④ Step quenching is employed for tool steels to minimize deformation and cracking; ⑤ Isothermal quenching is used for alloy steel workpieces that require minimal deformation and high toughness.

 

　　9. What is the inherent grain size of steel? What measures should be taken when heating steel to obtain fine austenite grains?

 

　　The nominal grain size refers to the grain size obtained by holding the material at approximately 930°C for a sufficient duration according to the standard test method.

Grain size measured (3–8 hours later).

 

　　10. How does Widmanstätten structure form in low-alloy steels? What are its microstructural characteristics? What impact does the Widmanstätten structure have on the properties of steel? How can we prevent the formation of Widmanstätten structure during heat treatment?

 

　　For carbon steels or low-carbon alloy steels with a carbon content Wc below 0.6%, under conditions of relatively coarse austenite grains and a certain cooling rate, the proeutectoid ferrite precipitates in the form of lamellar or coarse feather-like structures—this microstructure is known as Widmanstätten structure.