Common Defects in Carburizing and Quenching

The heat treatment process is inseparable from three core issues: heating, insulation, and cooling. In detail, it includes heating temperature, heating rate, insulation time, cooling rate, and of course atmosphere problems. So once a problem occurs, we will habitually analyze the cause from these aspects.

Carburizing and quenching often test these indicators: product surface appearance, surface hardness, core hardness, carburized layer depth, (effective hardening layer depth, full hardening layer depth) metallographic structure, and deformation. Let's share our views on these indicators.

1 Appearance problem

1.1, Oxide scale: This is mainly due to equipment leakage, impure carrier gas, or water, and the cause needs to be found from the equipment and raw materials.

1.2, Another most troublesome problem is the color spot problem, which is also a challenging new requirement for heat treatment in modern times. The reasons are complicated and very profound.

2 Unqualified hardness

2.1, High hardness (not discussed)

2.2, Low hardness: There are two situations, one is unqualified carburizing. The reason may be that the carburized layer is too shallow and does not meet the requirements of the drawings (carburization is not penetrated) or the selected detection scale exceeds the existing carburized layer's tolerance range, and the carburized layer is penetrated.

Solution: Re-infiltration, detection scale according to. JBT 6050-2006 "General Rules for Hardness Inspection of Heat Treatment of Steel Parts" The depth of the carburized layer is actually a function of temperature, time, and carbon potential. From the above factors, you can consider increasing the heating temperature, extending the insulation time, and increasing the carburized carbon potential to deal with it. (Of course, the adjustment of each parameter should be fully combined with the requirements of its own equipment and products) It may also be because of the presence of non-horse tissue on the surface.

There is another situation where the hardness is low, that is, carburization is qualified, but quenching is unqualified, in layman's terms, it is not quenched. This situation is the most complicated. As the saying goes: three-tenths of heat treatment depends on heating, and seven-tenths depend on cooling. It also reflects the position of the cooling process in the heat treatment process.

2.3. Uneven hardness: uniform furnace temperature (affects carburizing uniformity), equipment structure, atmosphere circulation, furnace loading, (affects carburizing uniformity and quenching uniformity).

2.4. Unqualified core hardness.

High hardness: high quenching temperature, too good material hardenability, too fast carbon and alloy composition upper limit, medium cooling rate.

Low hardness: just the opposite.

3 Deep or shallow carburizing layer or effective hardening layer

As mentioned earlier, the depth of carburizing layer is a comprehensive function of temperature, time, and carbon concentration. To solve this problem, we must start from heating temperature, heating rate, insulation time, cooling rate, and control of carbon concentration gradient in carburizing layer.

The higher the temperature, the longer the time, the higher the carbon potential, the deeper the carburizing layer, and vice versa. But in fact, it is far more than that. To design a carburizing process, we must also consider equipment, furnace loading, oil characteristics, metallographic structure, material hardenability, carbon concentration gradient in carburizing layer and cooling rate. This can be analyzed by referring to the previous low hardness situation, and no further explanation will be given.

4 Metallographic structure

Martensite exceeds the standard: the raw material grains are coarse, or not normalized, and the carburizing temperature is too high.

Solution: Normalizing or multiple normalizing, (it is recommended that the normalizing temperature is 20-30℃ higher than the carburizing temperature). If conditions permit, consider reheating and quenching after carburizing and slow cooling.

Residual austenite exceeds the standard: the quenching temperature is too high, and the carbon content in austenite is too high (carbon potential is too high).

Solution: Full diffusion, if conditions permit, the quenching temperature can be reduced, high temperature tempering can be reheated and quenched, or deep cryogenic treatment can be performed.

Carbide exceeds the standard: the carbon content in austenite is too high (carbon potential is too high), and the cooling process is too slow to precipitate carbides.

Solution: Full diffusion, control the cooling rate, reduce the temperature difference between carburizing and quenching if conditions permit, and use low temperature or sub-temperature quenching as little as possible. If this process must be used, the furnace load must be controlled. Imagine this: the same equipment is used for carburizing at 920℃ and quenching at 820℃, with a furnace load of 1000kg and 600kg. Is the cooling speed the same? Which one takes longer? Which carbide level is higher?

5 Non-marine and internal oxidation

Internal oxidation: It is the reaction of alloy elements such as chromium, manganese and molybdenum in steel with the oxidizing atmosphere in the atmosphere (mainly oxygen, water, and carbon dioxide), which makes the alloy elements in the matrix depleted, resulting in a decrease in the hardenability of the material. Under the microscope, you can see the black reticular structure, which is actually the troostite structure obtained by the depletion of alloy elements in the matrix leading to a decrease in hardenability.

The solution is to find a way to increase the cooling speed of the medium, increase the quenching intensity, and reduce the oxidizing atmosphere in the furnace (to ensure the purity of carburizing raw and auxiliary materials, minimize the amount of balancing air, control the water content of balancing air, ensure that the equipment does not leak, and exhaust is sufficient). Conventional equipment is difficult to eliminate.

It is said that low-pressure vacuum carburizing equipment can completely eliminate it. Because I have not actually touched it, it is difficult to comment. In addition, strong shot peening can also reduce the level of internal oxidation. I have consulted some experts' opinions. Some of them believe that excessive ammonia in carbonitriding will also produce serious non-martensite. Some foreign machinery industries attach great importance to internal oxidation, especially the gear industry. In China, the depth is generally required to be no more than 0.02mm to be qualified.

Non-martensite: The carburized layer appears non-martensite martensitic structure on the surface after quenching due to carburization or quenching problems, such as ferrite, bainite, and of course, internal oxidation type troostite. Its generation mechanism is similar to internal oxidation, and the solution is similar.

6 Deformation problem

This is a system problem and the most troublesome problem for those engaged in heat treatment. It is guaranteed from several aspects of raw materials, process and cooling medium. The above content is only based on personal experience. If there is any inaccuracy, please correct it. Thank you.