The thin-walled sleeve is one of the key components of the coal mine excavator loader. It is subjected to a large impact load and friction during operation. The heat treatment quality and geometric dimensional accuracy of the thin-walled sleeve have an important influence on its service life.
Because the thin-walled sleeve is under the action of stress, the geometrical dimension is easily out of heat during the heat treatment, which increases the difficulty of heat treatment of the thin-walled sleeve. Through the analysis and test of the heat treatment process of the thin-walled sleeve, the heat treatment process plan and process parameters are reasonably formulated, thereby controlling the heat treatment deformation of the thin-walled sleeve.
First, the thin wall cover technical parameters
The thin wall sleeve of the coal mine type loader is shown in Figure 1. For different materials (see the attached table for chemical composition), the technical parameters are as follows:
(1) 42CrMo quenching treatment, surface hardness of 45 ~ 50HRC.
(2) 38CrMoAlA quenching and tempering treatment + nitriding treatment, core hardness 240 ~ 280H BW, nitride layer depth 0.2 ~ 0.4mm, surface hardness 970 ~ 1100HV.
Second, heat treatment process plan and test
1. Formulation of the process plan
(1) Pre-heat treatment The blank is prepared by quenching and tempering. The main purpose is to eliminate the forging stress, refine the grain, uniform structure, improve the cutting performance of the part, reduce the cutting stress of the part, and prepare for the next heat treatment process.
(2) Aging treatment In order to eliminate the cutting stress generated during the machining of the part, the stress relief aging treatment is added after the semi-finishing process and nitrogen gas is introduced to prevent oxidation, so as to prepare for the next heat treatment to reduce the deformation amount of the part.
(3) Heat treatment process 42C r M o The steel thin-walled sleeve is rapidly heated to 900 °C, and is heated at a high temperature and quenched at 860 °C. Reduces high temperature heating time and reduces oxidation, decarburization and deformation. The temperature of the cooling medium is ≥100 °C, and it is quenched in the cooling medium in a vertical and static manner, and tempered in time to eliminate the quenching stress.
The 38CrMoAlA steel thin-walled sleeve is nitrided by an ion nitriding furnace and uses a nitriding process to reduce part deformation.
2. Process plan test
The actual workpiece is selected according to the process plan for simulation test, and the test process is strictly controlled to ensure the quality of each process.
There are generally many metallurgical defects in the raw materials, and the formation of the defects is eliminated by repeatedly repeating the piercing and forming.
(1) Heat treatment process The quenching temperature of the hypoeutectic steel is generally (Ac3+30～70)°C, and the Acc3 temperature of 42CrMo steel is about 800 °C, so the quenching temperature is selected as 850 °C, oil cooling (oil temperature ≥ 80 ° C); tempering at 620 ° C, water cooling, metallurgical structure after quenching and tempering as shown in Figure 2.
The temperature of Ac3 of 38CrMoAlA steel is about 885 °C, so the quenching temperature is 920 °C, oil cooling (oil temperature ≥80 °C); tempering at 650 °C, water cooling, metallurgical structure after quenching and tempering is shown in Figure 3.
After the semi-finishing of the thin-walled sleeve, the aging treatment temperature is slightly lower than the quenching and tempering temperature, and 580 °C is selected, and the heating and cooling are slowly performed to avoid the stress generation principle, and the protective gas is introduced to prevent oxidation and decarburization.
The 42CrMo material thin wall sleeve adopts quenching +200 °C low temperature tempering process, and the 38CrMoAlA steel thin wall sleeve adopts ion nitriding process.
(2) Geometrical size control The external circle size of the 42CrMo steel thin-walled sleeve before quenching is controlled at φ99. 23~φ99. 25mm; the outer radius of the 38CrMoAlA steel thin-walled sleeve is controlled to be φ99.90~φ99.95mm.
Third, the process test inspection
(1) Geometric dimension variable 42CrMo steel thin-walled sleeve deformation (single-sided) ≤ 0.12mm; 38CrMoAl A steel thin-walled sleeve deformation (single-sided) ≤ 0.06mm.
(2) The hardness testing equipment is HR-150, HLN-11A, HV-150, 42CrMo steel thin wall sleeve outer circle, end face hardness is 47～49HRC; 38CrMoAlA steel thin wall sleeve nitride layer brittleness ≤2 grade, hardness is 987 ~1003HV.
(3) Metallographic examination The metallographic structure of 42CrMo steel thin-walled sleeve is needle-like tempered martensite, as shown in Fig. 4; the thin layer of 38CrMoAlA steel thin-walled casing has a small amount of vein nitride, as shown in Fig. 5.
Fourth, the conclusion
(1) For thin-walled forgings, quenching and tempering heat treatment should be used to better eliminate the stress generated by forging, and to refine the grains and improve the cutting performance.
(2) Thin wall sleeve nitriding treatment has small deformation and high surface hardness, so the high hardness thin wall sleeve heat treatment should be the nitriding treatment process.
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