Heat treatment quality problems

Most of the heat treatment process is a periodic operation, which is a collective operation. Therefore, there are unclear work boundaries in the heat treatment process. For example, the quenching of a batch of parts may be completed in two shifts, and the quenching and tempering are often completed in two shifts. Quality problems often occur in the production process. After a problem occurs, analyzing the problem and finding the cause is not only time-consuming, laborious, and sometimes even unable to find the real cause.
Shanghai Baiheng Instrument combined the communication with users over the years, and summarized the following points:

1. Low hardness of carburized and quenched gears:

A batch of more than 800 gears that were carburized and quenched in the Unicase drip gas carburizing and nitriding furnace (Japan) requires a surface hardness of 58-63HRC after carburizing and quenching, and the surface hardness of the parts is 52-56HRC during sampling. Whether this is a carburizing problem or a quenching problem, whether quenching is a heating problem or a cooling problem, it is difficult to draw conclusions at the moment. Due to the urgent production task of this batch of gears, the author took 3 of the tested gears and bundled them with iron wire, reheated in a salt bath furnace, and quenched and cooled in an oil bath. After about 30-40 minutes, the final quenching hardness was 63-65HRC. . After reheating and quenching this batch of gears, the hardness of random inspection is all qualified. This method of cutting the mess with a sharp knife may not necessarily be able to find the real cause of the problem, but it has solved the urgent production problems.

2. After a batch of 14mm x 240mm 40Cr bars were quenched and tempered, almost all cracks were found after about a week (when in use). The shape of the cracks was a single longitudinal crack, and most of the cracks penetrated both ends of the bar.

Based on this, it is judged that the crack is a quenching crack, but the operator on duty does not admit it. Checking the operation records, it can only be found that the batch of bars is quenched in the second shift and tempered in the third shift, while the process parameters such as the parts materials, quenching temperature and cooling medium are not recorded. The author took a bar and 45 steel joints and heated it in a salt bath furnace, and then quenched and cooled in salt water. After cooling, the bar cracked and the shape of the crack was the same as the above crack. In the face of facts, the operator admitted that the batch of bars was mistakenly treated as 45 steel for quenching.

3. Uneven annealing hardness of box-type resistance furnace:

The shaft bar material of the vane pump produced by a company is 38Cr MoAl A. The process route is: annealing → band saw cutting → rough turning → quenching and tempering → fine turning → grinding → nitriding. When cutting the band saw, it is often found that the hardness on a bar is uneven, the local hardness is high, the cutting efficiency is low, and the saw blade wears quickly. After analysis, it is due to the length of the bar or the front of the bar when the furnace is installed. 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 installed to ensure uniform heating temperature of the furnace parts.

4. Cast iron quenching should control trace alloying elements:

Cast iron has poor thermal conductivity and oil cooling is generally used for quenching and cooling. The matrix of cast iron is the same as steel, and it is also composed of pearlite and ferrite. Cast iron has a high carbon content. Although the increase in carbon content can increase the hardenability, the increase is not large after all. Therefore, improving the hardenability of cast iron parts depends on the effect of the trace alloying elements in the cast iron and controlling the content of alloying elements to ensure the quality of heat treatment and quenching.

The vane pump stator produced by a certain company is made of wear-resistant alloy cast iron and requires a heat treatment hardness of 50 to 56 HRC. Since the content of alloying elements such as Cr, Mo, Mn, and Sn in the castings is not well controlled, the hardness after heat treatment and quenching is uneven and the hardness is low from time to time. It has been suggested that the low hardness after quenching is due to the low proportion of pearlite in the as-cast matrix structure of the casting, and it is required to increase the normalizing process before quenching. The test shows that the hardness of the casting is still low after being normalized and then quenched. In fact, under the same casting conditions, the proportion of pearlite in the as-cast matrix structure of the casting is related to the content of trace alloy elements.