It can be seen from the analysis that the material of gear is the most fundamental cause of gear failure, and the gear material should have the following conditions: ① the tooth surface has enough hardness to obtain high resistance to pitting, gluing and plastic flow; ② it has enough bending fatigue strength under variable load and impact load; ③ it has good processing and heat treatment technology; ④ the price is lower. The most commonly used material is steel. There are many kinds of steel, and comprehensive properties suitable for work requirements can be obtained through various heat treatment methods.
Due to good working conditions (no strong impact, stable operation), most of the machine tool gears can be made of tempered steel (45, 40 Cr, etc.), which is a general rule after tempering (or normalizing) treatment + high-frequency quenching and low-temperature tempering. Special case: in case of high speed and large impact load (such as the important gear on vertical lathe), it is advisable to select 20 Cr MNTI (20 CR) and other carburizing steels for manufacturing, and then conduct carburizing + quenching + low temperature tempering treatment. However, due to heavy load and frequent impact, the transmission gear of agricultural tractor is required to be higher in wear resistance, fatigue strength, core strength and impact toughness than the gear of machine tool. Therefore, it is generally made of carburized steel. After carburizing + quenching + low temperature tempering treatment, the carbon content in the surface layer is greatly increased to ensure high hardness, high wear resistance and high contact fatigue resistance after quenching. Due to the improvement of hardenability of alloy elements, high strength and sufficient impact toughness can be obtained after quenching and tempering. But for the occasion of light load and small impact, such as the oil pump gear of agricultural tractor, the quenched and tempered steel can also be used (this is a special case).
There is a gearbox gear of agricultural vehicle, which is impacted during use, with heavy load. The surface of the gear is required to be wear-resistant, with a hardness of 58-63 HRC, a hardness of 33-45 HRC at the tooth core, and other mechanical properties: σ B ≥ 1000 MPa, σ – 1 ≥ 440 MPa, AK ≥ 95 J / cm2. The proper steel grades for the gear are selected from the given materials, the technological process is formulated, and the purpose and organization of each step of heat treatment are analyzed.
It can be seen from the meaning of the question that the gearbox gear of the agricultural vehicle has a heavy load when working, and each tooth is affected by the alternating bending moment, so it must have high strength and high fatigue strength. Gear is also impacted greatly, so high impact toughness is required. In order to prevent wear, the tooth surface should have high hardness and wear resistance (58-63 HRC). In addition to the larger bending moment of each tooth, the tooth surface also bears larger pressure. Therefore, it is not only required that the surface hardness of teeth is high and wear-resistant, but also that the core of teeth has certain strength and hardness (33-45 HRC). According to the above analysis, it can be seen that the working conditions of the gear of the agricultural vehicle are more stringent than that of the machine tool gear, so the requirements on wear resistance, fatigue strength, core strength and impact toughness are higher than that of the machine tool gear. Quenched and tempered steel 45 can not meet the use requirements (the surface hardness can only reach 50-56 HRC). 38 Cr moal is a nitriding steel with thin nitriding layer, which is suitable for the conditions of high speed, low pressure and no impact. Therefore, it is not suitable for the gears of agricultural vehicles. High hardness (58-63 HRC) and high wear resistance can be obtained on the surface of carburized 20 CrMnTi. Because the steel has good hardenability, the tooth core can obtain the structure of combination of strength and toughness, and has high impact toughness, so it can meet the use requirements. Therefore, the gearbox gear is made of 20 Cr MNTI steel.
The suitable steel grade for manufacturing the gear is 20CrMnTi. The processing route is: blanking → forging → normalizing (950-970 ℃ air cooling) → machining → carburizing (920-950 ℃ carburizing for 6-8 h) → precooling and quenching (precooling to 870-880 ℃ oil cooling) → low temperature tempering → shot peening → gear grinding.
The purpose and corresponding organization of each step of heat treatment are: ① normalizing. Refine and even the structure, improve the structure after forging, and improve its machinability. After normalizing, the tissue was F + s. Carburizing. High carbon is obtained on the surface to ensure high hardness and wear resistance after quenching. The corresponding structure at carburizing temperature is a + K. ③ Precooling quenching. The tooth surface has high hardness and wear resistance, and the corresponding structure is high carbon m + carbide + residual a; the tooth core has strong and tough combination, and the corresponding structure is low carbon m + F + t. ④ After low temperature tempering, the quenching stress is reduced and the structure is stable. The corresponding structure is: surface m-circuit + K + Ar, core m-circuit + F + t.