Foreign heat treatment technology and heat treatment equipment
1. Current status of heat treatment technology
(1) Promote the application of high-pressure air quenching
Foreign heat treatment manufacturers attach great importance to cooling during the heat treatment process. 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 injection into the cooling chamber, and the computer controls the flow rate and flow rate changes to achieve the cooling rate within a specific time, so as to achieve the required cooling curve during the heat treatment process and ensure the heat treatment quality of the parts. In the past, the quenching gas used in the gas quenching method was nitrogen, helium, etc., but now it is strongly sprayed with air to cool the workpiece at a very rapid rate. After quenching, the surface has only a very thin oxide film, which is off-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 today. It has fast carburizing speed, excellent carbide structure, small quenching cracking and deformation, energy saving and carburizing agent raw materials, carburized parts The surface quality is good and it is conducive to environmental protection.
(2) The heat treatment equipment adopts oil cooling
Fan cooling, heat exchanger cooling, quenching oil tank cooling and other cooling devices all adopt oil-sealed self-cooling, completely replacing the water-cooled circulation system, and the entire heat treatment furnace does not require any cooling water. For example, hot air circulation 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 heats up, the proportion of heated oil is small. , Naturally float upward, causing natural circulation of oil. In the case of a small oil tank with oil storage and natural heat dissipation, the hot oil is added to the circulation after being cooled, so as to completely replace water cooling without fuel consumption and no power. The water in the plate heat exchanger of the quenching oil tank is replaced with cooling oil. The cooling oil is heated by the heat exchange of the hot oil. The change in the specific gravity of the oil causes the cooling oil to circulate by itself. A heat sink is added to the oil tank on the top of the furnace to match the fan. 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
The German company Ipsen 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 and natural gas heating. Gas heating technology and equipment are very mature in Europe. Natural gas burners have a standard series, which are manufactured and supplied by professional burner factories. The inner tube of the gas radiant tube is replaced from stainless steel to ceramic to extend the service life and increase the power. 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 of size, and the other is the change of part geometry. The heat treatment technology is different, the deformation of the part size and geometry and the method of preventing deformation are also different.
In the process of heat treatment heating austenitization, the longer the holding time and the higher the temperature, the more carbon is dissolved into austenite and the greater the expansion during martensite transformation. During cooling, martensite expands the most, followed by upper bainite, and the volume changes of lower bainite and troostite are small. When tempering at low temperature, martensite shrinks, and the shrinkage is proportional to the supersaturated carbon content. When heated at room temperature -200°C, part of the retained austenite will be transformed into martensite and will expand. However, due to the decomposition of martensite near 200°C, the expansion has 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 occur during heat treatment heating and quenching. The heating speed is too fast, the parts are too large compared to the heating furnace, and the temperature of each part of the parts is different, which will cause thermal deformation. During heat preservation, the residual stress of processing will be released and deform, and the weight of the part will also cause deformation. When 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 at the heart. Therefore, the previously transformed surface plastically deforms the untransformed core. If there is segregation of alloy components in the material, or the surface is decarburized, the phase transformation stress will be more uneven and the parts will be more likely to deform. In addition, if the thickness of the parts is uneven, the cooling rate will be different.
In the heat treatment of forgings, the ways to place the parts to reduce deformation are as follows: one is to hang vertically as much as possible, the other is to place it vertically at the bottom of the furnace, and the third is to use two points of horizontal support. The fulcrum positions are at one-third and four of the full length. Among them, the fourth is to lay flat on the heat-resistant steel tooling.
In 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, surface pressure of the medium, using additives, and stirring. The higher the viscosity of the quenching oil and the higher the temperature, the smaller the elliptical deformation. In a static state, the deformation is small.
The spring factory can effectively reduce the deformation in the following ways: ①Salt bath quenching; ②High temperature oil quenching; ③QSQ method; ④Reduced pressure quenching; ⑤One trough three-stage quenching. Salt-bath quenching is similar to high-temperature oil quenching, both of which are quenched at the martensite transformation temperature to increase the uniformity of the martensite transformation. QSQ is double liquid quenching. Decompression quenching is to reduce the liquid surface pressure of the quenching medium, thereby extending the vapor film stage, the cooling rate of the high temperature zone is reduced, and the cooling rate of each part of the part is uniform. The structure of one-tank 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 out of the furnace, and maintained in the atmosphere to make the overall temperature of the parts uniform, and then oil-cooled to transform the martensite Evenly, the irregularity of deformation is greatly improved.