Solving heat treatment quality problems some techniques

Most of the heat treatment process is a cycle operation, is a collective operation. Therefore, there is an unclear work boundary in the heat treatment process, such as a batch of parts quenching may be completed by two shifts, quenching and tempering is often completed by two shifts, and the quality of the operation personnel is uneven, the management measures are small enough to improve, often quality problems in the production process. After the occurrence of the problem, the analysis of the problem, find the cause, not only time-consuming, laborious, sometimes even can not find out the real cause.

The following author has summarized some ideas and methods of solving problems in the production site for many years to provide some useful references for readers.

1. Carburizing and quenching gear hardness is low

A batch of more than 800 gears carburized and hardened in (Japan) Unicase drip gas carburizing nitriding furnace require surface hardness of 58 ~ 63HRC after carburizing and quenching, while the surface hardness of parts is 52 ~ 56HRC during spot check. Is it carburizing or quenching? Whether quenching is a heating problem or a cooling problem is hard to conclude. Due to the urgent production task of these gears, the author took 3 pieces of tested gears and bound them with iron wire, reheated them in the salt bath furnace, and quenched and cooled them in the oil tank. After about 30 ~ 40min, the final quenching hardness was detected to be 63 ~ 65HRC. After the re-heating and quenching of these gears, the hardness of all qualified. This quick way to cut the gorse may not be able to find the real cause of the problem, but it solves the immediate problem of production.

2. Bar material quenching crack

There is a batch of 14mm×240mm 40Cr sids buried after conditioning, after about a week time (when using) was found to be almost all cracking, crack shape for longitudinal single crack, most cracks through the two ends of the bar. According to the judgment of the crack for the quenching crack, and the operator on duty did not recognize the account. Check the operation record, can only check that the batch bar material for the second shift quenching, three shifts tempering, and parts materials, quenching temperature and cooling medium and other process parameters are not recorded. The author took a bar material and 45 steel joint together in the salt bath furnace heating, and then in the brine quenching cooling, cooling about 20 ~ 30min, the bar material cracking, and the crack shape and the shape of the same crack. In the face of the facts, the operator admitted that the batch of bar material was mistaken for 45 steel for quenching.

3. Uneven annealing hardness of box-type resistance furnace

The pump shaft bar material of the vane pump produced by our company is 38CrMoAlA. The process line is: annealing → strip saw blanking → rough car → tempering → fine car → grinding → nitriding. When cutting the band saw, it is often found that the hardness on a bar material is not uniform, the local hardness is high, the cutting efficiency is low, the saw blade wear fast. According to the analysis, it is because the bar material is long or the bar material is near the front when loading furnace. There is no heating resistance wire at the furnace mouth of the box-type resistance furnace, and the heat loss is large. Therefore, for the general box-type resistance furnace, the parts should be 200 ~ 300mm away from the inside of the furnace mouth when the furnace is loaded, so as to ensure the heating temperature of the parts in the furnace is uniform.

4. Trace alloying elements should be controlled in iron quenching

Iron conducts heat. Poor performance, quenching cooling when the general oil cold. The matrix of cast iron is the same as that of steel, which is also composed of pearlite and ferrite. Cast iron carbon content is high, carbon content increase although can increase the hardenability, but after all the increase is not large. Therefore, improve the hardenability of cast iron: on the role of trace alloy elements in cast iron, control the content of alloy elements, in order to ensure the heat treatment quenching quality.

The blade pump stator produced by our company is made of wear resistant alloy cast iron, which requires hardness of 50 ~ 56HRC under thermo?? It has been suggested that the low hardness after quenching is due to the low proportion of pearlite in the cast matrix, which requires the normalizing process before quenching. The test shows that the hardness of castings is still low after quenching after normalizing. In fact, under the same casting conditions, the proportion of pearlite in the cast matrix is related to the content of trace alloying elements.

5. Conclusion

Many problems occurred in the process of processing, in fact, are due to the process control is not strict, production management confusion. Although this paper puts forward some ideas and methods to solve the problem, but the author thinks it is not the best way.

The author believes that only to strengthen the process control, formulate effective corrective and preventive measures, improve the level of production management, is the enterprise to reduce costs and increase economic benefits.

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