1. Current status of heat treatment technology
(1) Promote the application of high-pressure gas-cooled quenching
Foreign heat treatment manufacturers attach great importance to cooling during heat treatment. According to the technical and technological requirements of the product, slow cooling, oil quenching, and one-time gas quenching can be performed. Rapid atmosphere circulation cooling uses high-pressure gas injected into the cooling chamber, and the change of flow rate and flow is controlled by the computer to achieve the cooling rate within a specific time, so as to achieve the required cooling curve in the heat treatment process and ensure the heat treatment quality of the parts. The quenching gases that used to be cooled by gas quenching method include nitrogen, helium, etc., but now they are strongly sprayed with air to cool the workpiece at an extremely fast speed. After quenching, there is only a very thin oxide film on the surface, which is gray-white, and the color of the parts is still beautiful. , and save a lot of nitrogen and inert gas, so that the cost of heat treatment is further reduced.
The combination of vacuum low pressure carburizing and high pressure gas quenching is an advanced carburizing and quenching process. The surface quality is good, and it is conducive to environmental protection and so on.
(2) The heat treatment equipment adopts oil cooling
All the cooling devices such as fan cooling, heat exchanger cooling, quenching oil tank cooling, etc., all use oil-sealed self-cooling, which completely replaces the water-cooling circulation system, and the entire heat treatment furnace does not need any cooling water. For example, hot air circulating fan cooling: change the inlet and outlet pipes of the original water cooling jacket to oil pipes, and place a small oil tank with a diameter of 102mm near the fan. The oil cooling system is fully enclosed. When the fan bearing has heat, the proportion of heated oil is small. , naturally float upward, causing the oil to circulate naturally. In the case of small fuel tank oil storage and natural heat dissipation, the hot oil is cooled and then added to the circulation to completely replace water cooling without fuel consumption and power. The water in the quenching oil tank plate heat exchanger is replaced with cooling oil. The cooling oil is heated by the heat exchange of the hot oil. The change of the oil specific gravity causes the self-circulation of the cooling oil. It can achieve the effect of full oil cooling and save a lot of cooling water.
(3) Hydrogen probe is used on the nitriding furnace
Ipsen Company in Germany has applied hydrogen probes and corresponding technologies to measure and control the nitrogen potential in the nitriding furnace to adjust and control the atmosphere of the nitriding furnace and realize the modernization of the nitriding furnace.
(4) Gas radiant tube
At present, most of the heat treatment equipment in Europe has adopted gas radiant tubes, which are heated by natural gas. Gas heating technology and equipment are very mature in Europe, and there are standard series of natural gas burners, which are manufactured and supplied by professional burner factories. Natural gas heating improves energy utilization and reduces production costs.
2. Heat treatment deformation and prevention methods
There are two types of heat treatment deformation: one is the change in size, and the other is the change in the geometry of the part. Depending on the heat treatment technology, the deformation and anti-deformation methods of the size and geometry of the parts are also different.
During the austenitization process of heat treatment, the longer the holding time and the higher the temperature, the more carbon dissolves into the austenite, and the greater the expansion occurs when the martensite is transformed. During cooling, martensite expands the most, followed by upper bainite, and the volume changes of lower bainite and troostite are small. During low temperature tempering, martensite shrinks, and the amount of shrinkage is proportional to the supersaturated carbon content. When heated at room temperature -200 ℃, part of the retained austenite will transform into martensite and expand. However, this expansion is caused by the decomposition of martensite around 200°C, so there is little change in performance.
In conventional heat treatment, the main reason for the shape change of parts is the thermal stress and phase transformation stress that occurs during heat treatment heating and quenching. Heating too fast, parts that are too large relative to the furnace, and different parts of the part at different temperatures can cause thermal deformation. During heat preservation, the residual stress of processing will be released and deformed, and the self-weight of the parts will also cause deformation. During cooling, due to the different cooling rates of different parts of the part, thermal stress will be formed and the part will be deformed. Even if the cooling rate is the same, the cooling is always fast on the surface and slow on the core. Thus, the pre-transformed surface plastically deforms the untransformed core. If there is segregation of alloy components in the material, or surface decarburization, the phase transformation stress is more uneven, and it is more likely to cause part deformation. In addition, if the thickness of the part is not uniform, the cooling rate will also be different.
In the heat treatment of forgings, the way of placing the parts to reduce deformation is to hang them vertically as much as possible; Between one part, the fourth is placed flat on the heat-resistant steel tooling.
During the cooling process of parts, the type of quenching medium, cooling performance, hardenability, etc. are related to deformation. The change of cooling performance can be adjusted by changing the viscosity, temperature, liquid surface pressure of the medium, using additives, stirring, etc. The higher the viscosity of the quenching oil and the higher the temperature, the smaller the elliptical deformation. In the resting state, the deformation is small.
The following methods can effectively reduce the deformation: ① Salt bath quenching; ② High temperature oil quenching; ③ QSQ method; ④ Decompression quenching; ⑤ One tank three-stage quenching. Salt bath quenching is similar to high temperature oil quenching, both are quenched at the martensitic transformation temperature, which increases the uniformity of martensitic transformation. QSQ is a two-liquid quench. Decompression quenching is to reduce the liquid surface pressure of the quenching medium, thereby prolonging the vapor film stage, and the cooling rate in the high temperature area decreases, so that the cooling rate of each part of the part is uniform. The structure of one-slot three-stage quenching is simple. First, the parts are oil-cooled from the quenching temperature to a temperature slightly higher than the Ms point, then released from the furnace and kept in the atmosphere to make the overall temperature of the parts uniform, and then oil-cooled to make martensitic transformation. Evenly carried out, the irregularity of deformation is greatly improved.