Part 1. Characteristics of CNC Turning 304 Stainless Steel
Austenitic 304 stainless steel is a very common stainless steel with corrosion resistance, heat resistance, low temperature strength and overall mechanical properties. It is widely used in food equipment, chemical equipment and nuclear industry equipment.
Austenitic 304 stainless steel has a relative machinability Kr of about 0.4 and is a relatively difficult material to machine. It has high cutting forces, large machining hardening, high cutting area and high local temperature. Therefore, the following matters are required when CNC Turning 304 Stainless Steel.
1, Cutting force is large
Austenitic 304 stainless steel with low hardness ≤ Cr, Ni, Mn and other elements = 5, with 187 HbS and good plasticity (elongation after fracture) ≥ 40%, area ψ reduction ≥ 60%). The plasticity change in cutting is large and strength is maintained even at high temperatures (usually the strength of steel decreases significantly at increasing cutting temperatures). Under previous cutting conditions, the unit cutting force of austenitic 304 stainless steel is 2450mpa, more than 25% higher than 45 steel.
2、Hard work hardening
Austenitic 304 stainless steel is accompanied by significant plasticity deformation during processing, the material lattice produces serious deformation; at the same time, due to the stability of the austenitic organization defects, austenite part of the martensite, austenite impurities in the cutting process through the heating decomposition, the surface formation of hardened layer, process hardening phenomenon is very obvious, will be +B curing to 1500 MPa after the depth of the curing layer of 0.1 to 0.3 mm.
3, the local temperature of the cutting area is high
Austenitic 304 stainless steel requires a large cutting force, difficult to chip, so the operation carried out through the blade separation is also large. In the previous conditions, stainless steel cutting than mild steel is about 50% higher, generating more cutting heat. Poor thermal conductivity of austenitic stainless steel. Austenitic 304 stainless steel has a thermal conductivity of 0. 321.5 w/m.k, is one-third of the thermal conductivity of 45 steel. Therefore, the temperature of the cutting area is higher (in general, the heat generated by the insert during cutting accounts for more than 70% of the cutting heat). Make a lot of cutting heat concentrated in the cutting area and cutting tool surface, the heat transferred to the tool up to 20% (only 9% when cutting off the general carbon steel), in the same cutting conditions, austenitic 304 stainless steel cutting temperature than 45 steel 200 ~ 300 ° C higher.
4, the tool is easy to sticky and wear
Due to the high temperature strength and high work hardening of austenitic stainless steel, cutting load, austenitic stainless steel and tools and inserts affinity due to cutting in the austenitic stainless steel and tools affinity and increased significantly, resulting in jointing and diffusion and other phenomena. The result of tool adhesion and wear. In particular, hard inclusions are formed by a small piece of carbide, promoting tool wear and leading to edge collapse, greatly reducing tool life and affecting the surface quality of machined parts.
Part 2. Selecting a reasonable turning process
Due to the poor machinability of AISI 304 austenitic stainless steel, it is necessary to select a suitable turning in order to improve productivity and machining quality, including a reasonable selection of cutting tool materials, tool shape parameters, cutting parameters, and cooling materials.
Tool material
The correct choice of tool material is essential to ensure efficient cnc turning 304 stainless steel. The reduction in turning performance of austenitic 304 stainless steel shows that the selected cutting tool has high strength and toughness characteristics. At the same time, it has excellent wear resistance and almost no affinity with stainless steel. At present, carbide and high-speed steel are still the most commonly used cutting tool materials.
1、Cemented carbide
As the cutting force of hard-to-cut materials is large, the contact between the chip and the arc surface is short, so the cutting force is mainly concentrated near the edge, and the edge collapse is easy to occur. Therefore, yg carbide tools for machining can be selected. The toughness, wear resistance, red hardness and thermal conductivity of yg carbide are excellent. It is suitable for machining austenitic stainless steel. You can also choose YG 8 N tools. By adding nb, the cutting performance is 1~2 times higher than yg 8, and it works well in rough machining and semi-precision machining.
2、High-speed steel
High-speed steel tools, according to the size, shape and structure of the stainless steel workpiece turning, can effectively avoid hard tools easily damaged. Traditional high-speed steel tools (such as W 18 CR 4 V) in terms of durability does not meet the current processing conditions, you can use new high-speed steel tools with excellent cutting performance, such as high-speed steel (W 6 Mo 5 Cr 4 V 2 Al) and nitrogen-containing high-speed steel (W 12 Mo 3 Cr 4 V 3 N).
Tool shape parameters
Reasonable determination of the geometric parameters of the selected tool is an important factor in effectively improving the durability and machining results of tools made of austenitic 304 stainless steel. In general, the tool must have a large front and rear angle and sharp cutting edge.
1、Cutting parameters
AISI 304 stainless steel is usually difficult to cut materials, should be a reasonable choice of cutting parameters. Cutting parameters on the machining hardening, cutting force, heat and processing efficiency has a greater impact. Cutting speed has a great impact on the cutting temperature and tool durability. The second is the feed rate F. The reverse feed rate AP has the greatest impact.
2、Cutting oil
Due to the lack of cutting performance of austenitic 304 stainless steel, the selected cutting fluid has good cooling, lubrication and permeability (i.e., anti-bonding properties). In addition, emulsifiers and sulfide oils contain extreme pressure additives, such as S and Cl, must be selected as far as possible.
Emulsions have good cooling properties and are mainly used for rough turning of stainless steel. Vulcanized oils have some cooling and lubricating properties and low cost. It can be used for semi-finishing and finishing of stainless steel. Adding extreme pressure and oil additives to the cutting fluid can significantly improve the lubricating performance. Usually used for stainless steel machining finishing. Cutting fluids consisting of a mixture of carbon tetrachloride, kerosene, and oleic acid greatly improve the penetration of cooling lubricants and are particularly suitable for finishing aisi 304 austenitic stainless steel. Through the large cutting heat of austenitic stainless steel, spray cooling, high-pressure cooling and other methods can be carried out, which can improve the cooling effect.
