Analysis of Material Selection and Heat Treatment Process for Gear Parts

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Heat treatment of gear parts is a common heat treatment. The main function of gear parts is to transmit torque, adjust speed, and change the direction of movement. A pair of mutually meshing gears transmit power through the contact and sliding of the tooth surface, so the tooth root is subjected to a large alternating bending stress, and the tooth surface is subjected to a large contact stress and has strong friction and wear. When shifting, starting, and meshing is poor, it bears a certain impact load. Common failure modes include broken gear teeth, wear on the tooth surface, peeling off the tooth surface, pitting on the tooth surface, and overload fracture.

1. Mechanical Zerformance Requirements of Gears

1). High bending fatigue strength.

2). The tooth surface should have high contact fatigue strength, high hardness and wear resistance.

3). The gear core should have good comprehensive mechanical properties or good strength and toughness.

2. Common Materials and Heat Treatment of Gear Parts

1). Medium carbon steel and medium carbon alloy steel. Generally, low- and medium-speed transmission gears with low load-bearing capacity are often selected from 40, 45, 40Cr, 40MnB steel, etc., which can be quenched and tempered or normalized (when the requirements are not high) to meet the performance requirements (better comprehensive mechanical properties). After surface quenching and low temperature tempering, the surface hardness can reach 52～58HRC, and it has high wear resistance. This type of gear cannot withstand large shock loads.

2). Low-carbon steel and low-carbon alloy steel. Heavy-duty high-speed gears that can withstand large impact loads generally choose 20Cr, 20CrMnTi, 20MnVB, 18Cr2Ni4WA steel, etc., and 20 steel can also be selected for those with low individual requirements. After carburizing, quenching, and low-temperature tempering, the tooth surface can obtain high hardness (58-62HRC) and wear resistance, and the core has high strength and toughness.

3). For gear blanks with large diameters (φ>400～600mm) and complex shapes, which are difficult to forge, cast steel, such as ZG270-500, ZG310-570, etc. can be used.

4). Gray cast iron, such as HT200, HT250, HT300, etc., can be used for some less important gears with light load, low speed, no impact, and low precision requirements. Gray cast iron is mostly used for open transmission; in closed transmission, nodular cast iron can be used instead of cast steel to make gears, such as QT600-3, QT500-7, etc.

5). For some light-loaded gears working in corrosive media in the instrument, non-ferrous metals such as brass, aluminum bronze, tin bronze, silicon bronze and so on can be used; The pinion gear that works under can be made of engineering plastics such as nylon, ABS, polyoxymethylene, etc.

Take the machine tool gear as an example. The gear has a low load bearing capacity, works relatively smoothly, and has little impact. It can be made of medium carbon steel. Because the gear is thicker, considering the hardenability problem, 40Cr steel is more appropriate. The process route is:

Cutting → forging → normalizing → rough machining → quenching and tempering → finishing → gear induction hardening + low temperature tempering → fine grinding.

Normalizing is a pre-heat treatment, which can eliminate the residual stress caused by forging, adjust the hardness to facilitate machining, refine the grain, and improve the structure; quenching and tempering can make the gear obtain higher comprehensive mechanical properties, improve the strength and toughness of the core, and make Gears can withstand greater alternating bending stress and a certain impact; induction hardening can improve the hardness, wear resistance and contact fatigue strength of the tooth surface; low temperature tempering can eliminate quenching stress, prevent grinding cracks, and improve impact resistance ability.

The task of automobile gears is to transmit the power of the engine to the rear wheels, and bear heavy loads and large impacts. The working conditions are more complicated than machine tool gears, and the tooth surfaces are required to have high hardness (60～62HRC) and wear resistance. With high strength and excellent toughness, considering the requirements of carburizing processability and hardenability, 20CrMnTi steel can be selected. The process route is:

Cutting → Forging → Normalizing → Roughing, Semi-finishing → Carburizing → Quenching + Low Temperature Tempering → Shot Peening → Correction of Spline Hole → Fine Grinding of Gears.

Carburizing is to increase the carbon content of the surface layer. Quenching is to obtain high-carbon martensite and carbides on the surface layer, with high hardness (58-62HRC), wear resistance and fatigue strength; low-carbon martensite is obtained in the core, and the hardness can reach 33-48HRC, with high The toughness. Low-temperature tempering eliminates quenching stress, improves impact resistance, and prevents grinding cracks. Shot peening can increase the compressive stress of the carburized layer and help improve the fatigue strength.

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