Defects of gear heat treatment and its influence

Whether carburizing quenching, carbonitriding quenching, induction heating quenching or overall heating quenching, the possible heat treatment defects during gear quenching and cooling mainly include:

(1) The hardness in quenched state is insufficient, the hardness is uneven, and the quenching and hardening depth is not enough.

(2) The core hardness is too high after quenching.

(3) Quenching deformation out of tolerance.

(4) Quenching cracking.

Such quality problems on site are often related to gear material, pretreatment, quenching heating and quenching cooling. Various defects produced during heat treatment of gears will have potential or obvious adverse effects on their quality and service life. The effects of several main defects are briefly described.

1.Uneven hardness and insufficient hardening depth

Too high or too low hardness value will have an adverse impact on the gear. High hardness will lead to low plasticity and toughness, which is easy to cause gear peeling and tooth breakage; Low hardness will reduce fatigue strength and wear resistance, resulting in excessive wear and failure.

The hardness of carburized and quenched hard tooth surface gear of vehicle transmission gear such as automobile and tractor should be 58 ~ 62Hrc. At this time, the fatigue strength value is also very high. The phenomenon of local uneven hardness sometimes occurs in the heat treatment of gears, which is also a reason for the early failure of gears.

Generally speaking, with the increase of carburizing thickness, the fatigue strength and bending strength also increase, but when the effective carburizing hardened layer depth exceeds 1.1mm, the strength begins to decrease. Therefore, the actual situation such as bearing load should be considered and the appropriate carburized effective hardening layer should be selected.

When the carbon concentration distribution of carburized layer is the same, a deeper quenched hardened layer can often be obtained by using quenching oil with higher cooling rate at low temperature. Therefore, the use of quenching oil with fast cooling speed can correspondingly shorten the carburizing time of the gear and still obtain the required effective hardened layer depth.

2.The core hardness is too high after quenching

Appropriate core strength and toughness can not only ensure the overall strength of the teeth and the anti peeling ability of the infiltration layer, but also have sufficient impact resistance to prevent the brittle fracture of the teeth.

In the case of quenching, the core strength and toughness of carburized steel mainly depend on the carbon content of steel. The research shows that the fatigue resistance of vehicle carburized gear is the highest when the core hardness is between 35 ~ 42hrc (tensile strength R M is 1100 ~ 1400MPa), and when the impact absorption energy is not less than 60j, the brittle fracture of gear can not occur under normal conditions. From the mechanical properties of carbon steel and low alloy steel with different carbon content after quenching and tempering at 160 ~ 180 ℃, it can be seen that the carbon content of steel is greater than 0.3%, and the reduction of plasticity and toughness increases. The high core hardness after quenching may be related to the high cooling rate of the selected quenching medium or the high low temperature cooling rate of the medium. One of the solutions is to change the quenching oil to meet the requirements; The second method is to contact the quenching medium supplier and add appropriate additives to reduce the medium and low temperature cooling rate of the existing quenching oil; The third method is to use steel with lower hardenability.

3.Quenching deformation

In the process of heat treatment, due to the uneven action of thermal stress and structural stress, the gear will eventually cause slight or obvious deformation, reduce the gear accuracy, and the existence of internal stress is easy to induce cracks. In recent years, the research on quenching deformation at home and abroad summarizes the causes of deformation as insufficient and uneven quenching cooling speed, and puts forward the countermeasures to improve the cooling speed and try to realize uniform cooling. For example, the internal spline deformation of gear is often due to insufficient high-temperature cooling rate of the selected quenching oil or too long vapor film stage of the oil. Generally, the deformation of internal spline can be solved by increasing the high-temperature cooling rate of oil and increasing the cooling rate of oil in the whole cooling process. For small and medium-sized gears, especially for precision car gears, selecting and using isothermal step quenching oil is an essential measure to control deformation.

4.Quenching cracking

The cracks not only destroy the continuity of the gear, but also most of the crack ends are very sharp, which is easy to cause stress concentration, promote the early failure of the vehicle gear under low stress and cause accidents. The causes of quenching cracks are the result of stress concentration caused by the separate or combined action of thermal stress and structural stress, but the specific causes are relatively complex, which can be divided into three categories:

(1) The cracks generated during forging and machining of materials further expand during quenching.

(2) The defects of raw materials (composition segregation and uneven carbide, more inclusions and structural defects, etc.) promote the generation and development of cracks during quenching.

(3) Improper heat treatment process or operation mainly occurs in induction heating quenching. For medium carbon steel, water-soluble quenching medium and PAG medium can obtain high and uniform quenching hardness and deep and stable quenching layer, and the risk of quenching crack is very small. Cracks (even micro cracks) appear in gear quenching, which reduces its mechanical properties, especially the impact on fatigue strength is more obvious, and it is easy to break teeth in use.


The macroscopic mechanical properties of gear materials depend on their microstructure. Under the same tensile strength, improving the microstructure has an obvious effect on improving the fatigue strength of gear materials.

When quenching and tempering high strength steel, tempered martensite structure has the highest fatigue performance. The content of non martensite structure in quenched steel has a great influence on the fatigue strength. Even if the steel contains 5% non martensite structure, the fatigue strength will be reduced by 10%. With the increase of undissolved ferrite, the fatigue strength decreases obviously, and the shape of undissolved ferrite also seriously affects the fatigue strength and impact toughness. In addition, the grain size in the structure and the decarburization phenomenon in surface quenching will also affect the fatigue life of gear materials.

Scroll to Top