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Vacuum oil quenching

Vacuum oil quenching is the main process of vacuum heat treatment at present. The technical difficulty encountered during the development of vacuum oil quenching technology was the problem of carburization in vacuum oil quenching. The key technical issues such as vacuum quenching oil and vacuum oil quenching surface carburization were solved. In addition to its application in the heat treatment of tools and molds, it has been successfully used in Precision heat treatment of important structural parts.

Carbonation problem

The surface of steel is activated when it is heated to high temperature in a vacuum. During vacuum quenching, the activated carbon atoms formed by the thermal decomposition of the quenching oil penetrate into the oil quenching process, forming a carburized layer on the surface of the workpiece. This carburization phenomenon becomes more serious as the quenching temperature rises, and has nothing to do with the quenching heating and holding time. In order to prevent this phenomenon, good quality vacuum quenching oil should be selected and the heat treatment process should be improved. Vacuum quenching oil requires low saturated vapor pressure, good chemical stability, less carbon residue and impurities, and low acid value. The key is to ensure the stability of long-term use, ensure that the workpiece is bright after quenching, and has appropriate quenching and cooling performance. In the vacuum quenching process, in order to prevent the phenomenon of carburization, it is necessary to select an appropriate heating vacuum degree, and adopt the gas-inflated and then oil-introduced method or the gas-oil two-stage cooling method.

Carbon pollution phenomenon

Carbon contamination (carburization) is a very difficult problem when using vacuum oil quenching furnace solution treatment. During the operation of the vacuum oil quenching furnace, the oil attached to the forks and baskets of the vacuum oil quenching furnace is brought into the heating chamber to become carburizing agent during the operation. Austenitic stainless steel, martensitic precipitation hardening stainless steel, high-temperature alloys, and precision elastic alloys are mostly low-carbon or even ultra-low-carbon alloys. They contain a lot of chromium and are thin strip products, which provide excellent conditions for carburization. Under certain conditions, the heating process can quickly increase carbon on the substrate, which is called carbon pollution. Constant elastic alloys, austenitic stainless steel, precipitation hardening stainless steel, and high-temperature alloys are all low-carbon or even ultra-low-carbon alloys. Adding carbon not only increases the cold work hardening rate, but also causes deterioration of corrosion resistance. Adverse consequences such as changes in the alloy’s strengthening mechanism may greatly affect normal production and even product quality and performance.

Solution

Scrub clean the oil attached to the material forks and baskets of the vacuum oil quenching furnace; wrap it and then solid solution heat it to prevent the possibility of contact between carbon and alloy, but you will get twice the result with half the effort. The best way is to switch to other furnaces, such as austenitic stainless steel, precipitation hardening stainless steel, and high-temperature alloys. The cooling speed of solid solution treatment is not fast. Vacuum air quenching furnace can be used. Generally, high-purity liquid argon and oxygen are used for air cooling. The content should be less than 2PPm.

Oil surface pressure selection

Under low pressure, the cooling capacity of vacuum quenching oil decreases, and for some steels, the purpose of quenching may not be achieved. For this reason, before quenching, high-purity neutral or inert gas is filled into the quenching chamber to create a certain pressure on the oil surface, so that the steel can be fully quenched and a bright surface can be obtained. The lowest liquid surface pressure that can obtain the same quenching hardness as at atmospheric pressure is called the critical quenching pressure. The critical quenching pressure is related to the characteristics of the vacuum quenching oil and the hardenability of the steel.

For steel with poor hardenability, the method of inflating first and then adding oil should be used for quenching. The oil surface pressure should be higher than its critical quenching pressure. Generally, the adjustment is around 5×10⁴Pa, but it should not be lower than 1×10⁴Pa. For steel with medium hardenability, it should also be quenched by inflating first and then adding oil. The oil surface pressure can be adjusted to about 1×10⁴Pa, but should not be lower than 5×103Pa. Steel with good hardenability can be quenched with or without inflating; if inflated, it can be inflated first and then filled with oil, or it can be filled with oil first and then inflated; when inflated first, the inflation pressure can be 2.5×10⁴Pa Selected within the range of ~5×10⁴Pa. In order to prevent and reduce oil vapor from entering and contaminating the heating chamber, a higher inflation pressure should be selected. The gas used should be high purity neutral or inert gas. The post-inflation method can be used to fill gas with lower purity, and the inflation pressure can also be lower. Quenching is performed in a non-inflated manner, that is, the workpiece is not inflated before and after entering the oil.

In addition, the quenching and cooling capacity of vacuum oil quenching is slightly lower than that of ordinary oil quenching, and the hardenability of vacuum oil quenching is generally about 75% of that of ordinary oil quenching.