The heat treatment process is mostly a cycle operation and is a collective operation. Therefore, the heat treatment process has unclear working limits. For example, quenching of a batch of parts may be completed by two shifts. Quenching and tempering are often completed by two shifts. In addition, the quality of operators is uneven, and management measures are small enough to improve production. Quality problems often occur in the process. After a problem arises, analyzing the problem and finding the cause is not only time consuming, laborious, and sometimes even impossible to find out.
Below, the author summarizes some of the ideas and methods for solving problems in the production site over the years to provide readers with some useful references.
1. Carburized and quenched gears have low hardness
A group of more than 800 pieces of gears carburized and quenched in the (Uniase) drip-type gas carburizing nitriding furnace in Japan (Japan) require a surface hardness of 58-63 HRC after carburizing and quenching, and the surface hardness of the parts at the time of sampling is 52-56 HRC. This is a problem of carburizing or quenching. Quenching is a heating problem or a cooling problem. It is difficult to draw conclusions. Due to the urgent production task of this batch of gears, the author took 3 pieces of tested gears and tied it with wire, reheated in the salt bath, and quenched and cooled in the oil bath. After about 30-40 minutes, the final quenching hardness was 63-65HRC. . After the gears were reheated and quenched, all the bolting hardness was qualified. This kind of rapid tooling may not be able to find the real cause of the problem, but it solves the urgent need for production.
2. Bar quenching crack
After a batch of φ14mm×240mm 40Cr tantalum material was buried in the quenched and tempered place, it was discovered that almost all cracking occurred after about one week (when used). The crack shape was a single longitudinal crack, and most cracks cracked through both ends of the bar material. According to this, it is judged that the crack is a quench crack, and the operator on duty does not confess. Check the operation records, only to find that the batch of bar material for the second shift quenching, three shifts of tempering, and parts of the material, quenching temperature and cooling medium and other process parameters are not recorded. The author took a bar and 45 steel joints in the salt bath furnace heating, and then quenched in brine cooling, about 20 ~ 30min after cooling, the bar material cracking, and the crack shape and the same crack shape. In front of the facts, the operator admitted that it was mistaken to quench the batch of bar material as 45 steel.
3 box resistance furnace annealing hardness is not uniform
The shaft material of vane pump produced by our company is 38CrMoAlA. The process route is: Annealing → Band saw cutting → Roughing → Conditioning → Finishing → Grinding → Nitriding. When cutting with a band saw, it is often found that the hardness on one bar is uneven, the local hardness is high, the cutting efficiency is low, and the saw blade wears fast. According to the analysis, it is because the bar length or bar stock is on the front when the furnace is installed. There is no heating resistance wire at the mouth of the box type resistance furnace, and the heat loss is large. Therefore, for a general box type resistance furnace, the parts of the furnace should be 200 to 300 mm from the inside of the furnace mouth to ensure uniform heating temperature of the furnace components.
4. Cast iron quenching should control trace alloy elements
Cast iron heat conduction. Poor quality, quenching and cooling are generally used oil cooling. The base of cast iron is the same as that of steel, which is also composed of pearlite and ferrite. The high carbon content of cast iron, although the increase in carbon content can increase the hardenability, but it is not much. Therefore, to improve the hardenability of cast iron parts: rely on the role of trace alloy elements in cast iron, control the content of alloying elements in order to ensure the quality of heat treatment quenching.
The stator of vane pump produced by our company is made of wear-resistant alloy cast iron, and the hardness of hot spot is required to be 50~56HRC. Because the content of alloy elements such as Cr, Mo, Mn and Sn in the casting is not well controlled, the hardness after heat treatment and quenching is not uniform, and the hardness is Low occurrences and other phenomena occur from time to time. It has been suggested that the low hardness after quenching is due to the small proportion of pearlite in the as-cast matrix of the casting, requiring an increase in the normalizing step before quenching. Tests show that the hardness of the casting after quenching after normalizing is still low. In fact, under the same casting conditions, the proportion of pearlite in the casting matrix matrix is ​​related to the content of trace alloy elements.
5 Conclusion
Many of the problems that arise in the process of processing are actually due to the rigorous control of the process and the chaos of production management. Although the article puts forward some ideas and methods for solving problems, the author thinks repeatedly that it is not the best policy.
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