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Light hole vacuum carburizing gear for heavy duty gearbox

For some commercial vehicle companies supporting gearbox products, most of the self-made parts, especially gearbox gears, need to be vacuum heat treated and vacuum carburized and quenched. The processing process of some light hole gears is: blanking→forging→normalizing→rough turning→finish turning→hobbing→shaving→vacuum carburizing and quenching→sand blasting→(powerful shot blasting)→fine grinding→cleaning →Assembly. When the heat treatment equipment is abnormal or shut down due to failure, the products in the furnace often have low hardness, shallow carburized layer, and surface oxidation after the fault is eliminated; The secondary quenching deformation will lead to the scrapping of the subsequent fine grinding process without grinding amount, so it is necessary to formulate a reasonable rework process for this kind of situation.

Under normal circumstances, after an abnormal or faulty shutdown of the equipment, the gears in the furnace will inevitably be oxidized to varying degrees due to the long shutdown, exhaust, cooling, fault repair time and the inability to achieve absolute sealing of the atmosphere in the furnace. Some parts were scrapped due to serious oxidation.

By tracking the damage inspection of parts after multiple reworks (including samples and workpieces), the data analysis of the surface oxidation and carbon-deficient layer depth data of parts in the faulty equipment after rework has not been completed.

The results show that there are different degrees of carbon depletion after rework of the unfinished parts in the faulty furnace. The existence of the oxidized carbon-deficient layer will lead to the loss of the size of the part, and in severe cases, oxidation peeling will occur, so that the size of the part exceeds the lower limit of the process requirement and has to be scrapped.

According to the analysis principle of the fishbone diagram, the terminal factors are analyzed: In terms of process execution and operator operation, the operator union sets the process parameters such as time and temperature 100% according to the process requirements, and operates strictly in accordance with the operating procedures; periodic carburizing The equipment for secondary quenching and rework in the furnace is in normal operation; there is little difference in the materials of the same batch of parts; the size of the parts is measured with a verified inner diameter micrometer, and the measurement data is reliable; increasing the number of times of quenching will increase the internal stress of the parts, which will cause the inner holes of the parts to expand. big.

After a comprehensive analysis, the most important reason affecting the size tolerance of the inner hole of the part is the increase in the number of quenching in the rework process, which leads to an increase in the internal stress of the part, which causes the inner hole of the part to expand, and partially or completely loses the reserved grinding allowance.

In order to verify the conclusion of the factor fishbone diagram, vacuum quenching is added to the parts that have been measured in the previous status investigation, and then the grinding allowance of the inner hole is measured to obtain the grinding allowance data of the inner hole size of the part after three times of quenching. It can be seen from this that the grinding allowance is further reduced after the third quenching, and the average allowance is only 0.05 mm, which cannot meet the grinding requirements, and some parts have no grinding allowance. The structure formed during vacuum quenching of parts is mainly high-carbon martensite, and its microscopic morphology belongs to a supersaturated solid solution in which carbon dissolves in a body-centered cubic lattice. Its change law determines the deformation law of the part is that the grain volume increases, and the increase in the volume of countless microscopic crystals will increase the overall volume of the part, and the internal structure will change from austenite to martensite many times after many times of vacuum quenching. The result of the transformation process is that the number of crystal grains in the structure increases, which eventually leads to the gradual expansion of the inner hole of the part. According to the fishbone diagram analysis and subsequent conclusion verification, in order to eliminate the internal stress and the volume increase caused by the transformation of martensite, the vacuum annealing and slow cooling process is added before the final vacuum quenching process, so that the once quenched martensite undergoes martensite Tentenite-austenite-pearlite transformation. This method can effectively avoid the increase of internal stress caused by continuous quenching, and can analyze the carbon part melted into martensite during primary quenching, so as to partially restore the inner hole wear amount reduced during primary quenching. Then perform secondary quenching and rework to improve the success rate of parts rework and reduce the occurrence of waste. According to the improved rework process, follow-up measurement verifies the effect of increasing the vacuum annealing slow cooling and vacuum carburizing + vacuum quenching process on the inner hole size.

The results are as follows. After annealing and slow cooling, the inner hole of the part shrinks regularly, and the grinding allowance increases by 0.2~0.3 mm, basically returning to the state reserved before thermal processing.

After the improved vacuum annealing and slow cooling and vacuum carburizing + vacuum quenching rework, the inner hole of the part still shows the law of swelling. However, due to the increase of the inner hole deformation allowance during vacuum annealing and slow cooling, the final average grinding allowance is about 0.2mm, which can meet the processing requirements of grinding the inner hole allowance of gears.

(1) When the vacuum heat treatment equipment is shut down due to abnormal conditions or malfunctions, if the unqualified gearbox light hole gear is reworked, if the process of direct supplementary carburizing and quenching is followed, the inner hole of the gear will be deformed due to secondary quenching, which will lead to fine grinding in the subsequent process When there is no amount of wear and scrapped. The main reasons that affect the size of the inner hole of the part are: the number of times of vacuum quenching in the rework process increases, resulting in an increase in the internal stress of the part, causing the part to deform and the inner hole to expand; During the process of microstructure transformation, the number of grains in the microstructure increases, and the volume of grains increases, resulting in the gradual expansion of the inner hole of the part, and finally part or all of the reserved grinding allowance is lost.

(2) Using the analysis and verification of fishbone diagram, in order to eliminate the internal stress and the volume increase caused by the transformation of martensite, the heating, vacuum annealing and slow cooling process is added before the final vacuum quenching process, and the primary quenched martensite is The transformation of martensite-austenite-pearlite avoids the increase of internal stress caused by continuous quenching; at the same time, the carbon part melted into martensite during the first quenching is separated out, so that the reduced inner hole grinding amount during the first quenching is partially recovered.

(3) The improved rework process adds vacuum annealing and slow cooling and vacuum carburizing + vacuum quenching, and the inner hole of the part still shows the law of swelling, but due to the increase of the inner hole deformation allowance during vacuum annealing and slow cooling, the final average grinding The allowance can meet the processing requirements of the inner hole allowance of the grinding gear.