Steel with high hardness obtained by quenching metal is commonly known as quenched steel.
Its structure is generally martensite, and its hardness can be between 50 ~ 65hrc.
Quenched steel accounts for a large proportion of difficult to cut materials.
Its traditional machining process is basically grinding, but the grinding efficiency is low.
After quenching, some thin-walled and complex parts are ground, and the workpiece is very easy to deform, affecting the machining accuracy and quality.
At present, due to the emergence and development of high hardness tool materials, the use of turning, milling, boring and other tools to replace the traditional grinding process has become a trend of cutting quenched steel at home and abroad.
1. Characteristics of quenching steel cutting
The machining characteristics of quenched steel are shown in table 7-4.
Table 7-4 machining characteristics of quenched steel
|High hardness and strengthLow plasticity (almost no plasticity)||When the hardness of hardened steel reaches 50~60HRC, its strength can reach Rm=2100~2600MPa. According to the grade of machinability, the hardness and strength of quenched steel belong to the most advanced materials that are difficult to cut, that is, the materials that are difficult to cut.|
|Large cutting forceHigh cutting temperature||During machining, due to the high strength and hardness of quenched steel, the unit cutting force is up to 4500MPa (both axial force and radial force are large), the workpiece is prone to vibration and the tool loss is large.|
|Less outgoing heat||Generally, the thermal conductivity of quenched steel is 1/7 of that of 45 steel. It is easy to generate serrated chips and generate adiabatic shear. The cutting heat is not easy to be taken away by the chips. The high temperature during cutting aggravates the tool wear.|
|The cutting edge is easily broken and worn||Quenched steel is brittle, the tool chip contact length is short, and the cutting force and cutting heat are concentrated near the cutting edge of the tool, which is easy to cause the cutting edge damage and rapid wear.|
2. Specific measures for cutting quenched steel
The specific measures for cutting quenched steel are shown in table 7-5.
Table 7-5 specific measures for cutting quenched steel
|Selection of tool materials||The selection of tool materials should first consider the high hardness and heat resistance of the tool, and also take into account the strength, wear resistance and thermal conductivity.The grade of cemented carbide with TaC and NbC ultrafine grains shall be preferred;Ceramic materials shall be hot pressed composite ceramics and hot pressed silicon nitride ceramics, and their brands such as AG2, LT35, AT6, HS73, ST4 and HDM3;Cubic boron nitride (composite sheet) is the best choice for finishing and semi finishing of quenched steel because of its higher hardness, heat resistance and good strength than the above two kinds of materials.|
|Selection of tool geometric angle||1. The front angle is generally zero or negative, γ。=-15 ° -0 °;|
2. The back angle is usually used to reduce the friction with the back face of the tool α。=- 8°~10°;
3. The main deflection angle should be selected Kt= 30°～45;4. The blade angle should be λs=-5 ° ~-109.
|Selection of Cutting Parameter||1. When cemented carbide tools are used, vc=30~60 m/min;|
2. When ceramic cutter is used, vc=60~120m/mim;
3. When cubic boron nitride composite blade is used, e=100~150mmin;
4. The feed rate is generally 0.05~0.3mm/r;
5. The back cutting amount is generally ap=0.05~2mm.
|Other||When ceramic cutting tools and cubic boron nitride cutting tools are used, generally cutting fluid can not be used. If cubic boron nitride cutting tools use cutting fluid, water-based cutting fluid should not be used.|