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Vacuum Hardening of Tool Steel

There are two types of classification of tool steel in engineering, one can be divided into: carbon tool steel and alloy tool steel according to the composition; the other can be divided into: cutting tool steel, die steel and measuring tool steel according to the use. In addition, it can also be divided into three categories according to the vacuum hardening cooling medium used: water hardened steel, oil hardened steel and air hardened steel. There are many classification methods for tool steel, among which the commonly used classification method is to classify according to the purpose.

1 Properties of tool steel

In addition to their own special properties, various tool steels have many common requirements in terms of performance and process performance. For example, high hardness and high wear resistance are one of the most important performances of tool steel. The reasonable combination of wear resistance and toughness is the key to the performance of tool steel. The alloying and vacuum heat treatment processes of tool steel are optimized around this main line.

2 Composition of vacuum hardened tool steel

Higher carbon (0.65-1.55%) ensures high-carbon martensite after vacuum hardening, thereby obtaining high hardness and cutting resistance, and forming a sufficient number of carbides to ensure high wear resistance.

The content of alloying elements mainly makes the steel have high hardness and high wear resistance carbide forming elements, such as: Cr, W, Mo, V, etc. For cutting tools with high cutting speed, more W, Mo, V, Co, etc. need to be added to improve the red hardness of the steel.

Tool steel has very strict requirements on the purity of steel, and the mass fraction of S and P is generally limited to below 0.02-0.03%.

3 Vacuum hardening process of tool steel

The cooling speed of vacuum hardening is fast, and it is necessary to use appropriate hardening media such as water, oil, gas, etc. Among them, oil cooling has a wide range of applications; air cooling has a small cooling rate and is more expensive, and has certain applications; water cooling has a faster cooling rate, but due to the characteristics of water cooling, it is only suitable for low-carbon and low-alloy steels.

From the perspective of improving the brightness of vacuum hardened steel, it is desirable to maintain a lower pressure, but too low pressure will reduce the cooling capacity of hardening oil. In order to meet the requirements of both, it is generally used to heat under low pressure first, and then pass a high-purity protective atmosphere (inert gas or nitrogen) before vacuum hardening, increase the pressure, and then perform vacuum hardening.

The purpose of tool steel alloying is to change the carbide type, improve harden ability, and improve tempering stability. The vacuum heat treatment process is designed to reduce the vacuum hardening stress as much as possible, reduce the tendency of deformation and cracking, and stabilize the structure. For large workpieces, the focus of the entire vacuum heat treatment process is to reduce deformation and cracking and take a series of measures. In terms of process measures, preheating and precooling are often used for general workpieces, and isothermal, grading, double-liquid hardening and other methods are commonly used for vacuum hardening, and vacuum tempering needs to be carried out in time. For the alloying and vacuum heat treatment process design of tool steel, it is necessary to focus on the performance indicators of wear resistance and toughness to optimize.

4 Vacuum hardening furnace equipment selection: RVGQ vacuum hardening furnace produced by SIMUWU company is a high-quality product for vacuum hardening process. Good temperature control accuracy and temperature control uniformity ensure the effective vacuum hardening process. The equipment is easy to manage and maintain, greatly reducing maintenance costs.