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20CrMo gear vacuum heat treatment process

Gears are key components in mechanical equipment, and they are widely used in industrial fields such as automobiles, aircraft, tanks, and ships. It has the advantages of accurate transmission, compact structure and long service life. Due to the complex working conditions of the gear, it is required that the gear not only has excellent wear resistance but also has high resistance to contact fatigue and bending fatigue.

Gears are generally subject to alternating loads or even impact loads, with high contact stress and easy wear on the tooth surface. Therefore, the requirements for gears are that the surface is hard and wear-resistant, the core is strong and tough, has high fatigue resistance, and the surface does not crack, sink, or pit. In order to meet these performance requirements, normalizing, Vacuum heat treatment process of vacuum carburizing, vacuum quenching and low temperature tempering.

Normalizing is a preparatory heat treatment, the main purpose is to improve the hardness of steel and facilitate the cutting of billets. The purpose of vacuum carburizing is to increase the carbon content of the surface material of the workpiece in order to obtain high surface hardness in the subsequent vacuum quenching process while maintaining the toughness of the core. The purpose of vacuum quenching is to make the supercooled austenite undergo martensite or Bainite transformation to obtain martensite or bainite structure, and then combined with vacuum tempering at different temperatures to greatly improve the strength, hardness, wear resistance, fatigue strength and toughness of the steel. The purpose of vacuum tempering is to reduce Or eliminate the internal stress in vacuum quenched steel parts, or reduce its hardness and strength to improve its ductility or toughness and improve the dimensional stability of parts.

Through the analysis of vacuum heat treatment of gears, the problems that need to be paid attention to in the process of vacuum heat treatment are more clearly defined. The purpose of being able to correctly determine the heating temperature, time, holding time, and cooling method is to fully utilize the potential of the metal material through the correct vacuum heat treatment process.

1 Gear service conditions, failure modes and performance requirements

1.1 Service conditions and failure modes

When the gear is working, all the torque acts on the tooth roots of the meshing teeth, making it bear a large bending stress and alternating stress, and it acts on each tooth periodically, making it bear the complex alternating stress effect. In the process of meshing and transmitting torque, the speed of gears varies widely, and the surface of gears bears large contact stress and strong friction at high speeds. Gears sometimes work under overload and strong impact conditions, causing gears to have many different failure modes as follows:

(1) Contact fatigue: shallow pitting and deep peeling appear on the tooth surface. It mainly occurs after running at high speed for a long time.

(2) Bending fatigue: partial or complete tooth fracture. It mainly occurs under the conditions of overload and impact.

(3) Wear: scratches and wear caused by the sliding of the tooth surfaces against each other. It mainly occurs when the load is too large and the speed is low. Additionally, impact wear occurs on the gearbox gear ends.

1.2 Performance requirements

The surface is required to be wear-resistant, the hardness of the heart tooth is 58-63HRC; the hardness of the tooth root is 33-45HRC; the remaining mechanical properties are: tensile strength σb≥1000MPa; yield strength σs≥490 MPa elongation δ5 (%): ≥11 ; Reduction of area ψ (%): ≥ 45; Impact energy Akv (J): ≥ 55; Impact toughness value αkv (J/cm2): ≥ 69 (7)

2 Process flow of 20CrMo

2.1 Blanking – rough forging – normalizing – processing tooth shape – vacuum carburizing – pre-cooling and quenching – low temperature tempering – shot peening – gear grinding

20CrMo steel hot forging process specification chart:

After forging, a tooth blank with a maximum outer diameter of about 88mm and a height of 26mm will be obtained, and slow cooling is adopted. The figure below is a 20CrMo gear parts diagram.

2.2 Design of vacuum heat treatment process for 20CrMo

Vacuum carburizing treatment of 20CrMo:

Commonly used carburizing methods are: solid carburizing, liquid carburizing, gas carburizing. Gas carburizing is the most commonly used, and the original state of the carburizing agent used can be gas or liquid such as kerosene. But in the chemical heat treatment furnace are gaseous. It is required that the osmotic agent used can be easily decomposed into active atoms, economical, easy to control, non-polluting, and the osmotic layer has good performance. Due to the low carbon content, high plasticity and toughness, and poor hardness and wear resistance, gas surface carburizing heat treatment can be used to obtain lath-shaped martensite structure on the surface layer, that is, the high hardness can reach more than 58HRC, but the core The part is still relatively high plasticity and toughness, hardness is 33-45HRC, yield strength σs: ≥490 MPa elongation δ5 (%): ≥11.

2.3 Vacuum quenching treatment

The surface obtains high hardness, strength, wear resistance and contact fatigue resistance, and the core still has high strength and toughness. It is required that the hardness of the heart tooth after vacuum quenching is 58-63HRC; the hardness of the tooth root is 33-45HRC; the remaining mechanical properties are: tensile strength σb≥1000MPa; yield strength σs: ≥490 MPa elongation δ5 (%): ≥11 .

General process curve of heat treatment:

Vacuum furnace selection: RVGQ series vacuum heat treatment furnace produced by SIMUWU company is a high-quality product for 20CrMo gear vacuum heat treatment process. Good temperature control accuracy and temperature control uniformity ensure the effective vacuum heat treatment process. SIMUWU company focuses on the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.