The stainless steels currently applied are mainly classified into martensitic stainless steels, ferritic stainless steels, and austenite stainless steels according to their structural conditions. Alloy steels containing greater than 11.7% chromium or greater than 8% nickel are often referred to as stainless steels. The addition of additional metallic elements (Cr and Ni) to the alloy steel grade changes the physical and chemical properties of the alloy. The improved corrosion resistance in both air and acidic solutions is less susceptible to oxidative rust and exhibits high strength at high temperatures (> 450 ° C). Therefore, it is widely used in the aviation, aerospace, chemical, petroleum, construction and food industries and in everyday life.

1. Main cutting characteristics of stainless steel

(1) Large cutting force

Among them, austenitic stainless steel is particularly prominent. The hardness of this material is not high, but grade 1Cr18Ni9Ti is taken as an example, the hardness is ≤187HBW, but the plasticity is very good (elongation after fracture δ = 40%, reduction in cross-sectional area Ψ = 60%). Therefore, the plastic deformation during the cutting process is large, and the cutting force is large. For the same cutting parameters, austenitic stainless steel consumes about 50% more energy than low carbon steel.

(2) Cnc Machining quenching

Among stainless steels, the work hardening phenomenon of austenitic and austenitic ferritic stainless steels is the most prominent. They are plastically large, and when plastically deformed, the lattice twists strongly. At the same time, austenite is less stable and some austenite transforms into martensite under the action of cutting forces. Further, the compound impurities are easily decomposed, dispersed and dispersed under the action of cutting heat, and as a result, a cured layer is formed in the cutting step. All of this makes the work hardening phenomenon more apparent.

(3) The tool sticks and easily wears

Stainless steel materials generate high temperatures during cutting and have a high affinity with tool materials.
It binds and diffuses the tool and material, and facilitates the formation of “knife tumors” that cause attachment and wear of the tool and reduce the useful life of the tool.

(4) The local temperature of the cutting part is high

This type of material requires a large cutting force and also consumes a large amount of power for separation and cutting, resulting in more cutting heat. The heat transferred to the tool can reach up to 20%, but only 9% when machining carbon steel. At the same time, the thermal conductivity of stainless steel is not good (the thermal conductivity of stainless steel is about one-third that of carbon steel). The local temperature of the cutting zone is high because a large amount of cutting heat is concentrated at the interface between the cutting zone and the contact area between the blade and the tip.

Tool material selection

According to the cutting characteristics of stainless steel, the tool material is required to have good heat resistance, high wear resistance, and low affinity with stainless steel. Tool materials currently in use are cemented carbide, high speed steel and coated tools.

(1) Carbide tool

In general, YG-based carbide tools are suitable for most difficult-to-cut materials. Especially when processing 1Cr18Ni9Ti austenitic stainless steel, it is best not to use YT type cemented carbide. YT type cemented carbide should be absolutely avoided. Since titanium (Ti) in stainless steel and Ti in YT-based cemented carbide have an affinity, cutting tends to remove Ti in the alloy, which intensifies tool wear. YG cemented carbide has good toughness, high wear resistance and hardness. Suitable for machining stainless steel materials. It can employ a larger front angle to make the cut lighter. It also has good thermal conductivity. At the same time, tips and tools are not easy to glue.

(2) High-speed steel tools

When the shape, size and structure of the workpiece is inconvenient to use the cemented carbide cutter, or the cemented carbide cutter is vulnerable to damage. Requires high performance high speed steel tools. Normal high speed steel (such as W18Cr4V) has a very short tool life and does not meet the requirements.
New high speed steel cutting tools can be used. Cobalt-containing high-speed steel (W 2 Mo 9 Cr 4 Vc 08), aluminum-containing cemented carbide high-speed steel (W 6 Mo 5 Cr 4 V 2 Al), nitrogen-containing high-speed steel (W 12 Mo 3 Cr 4 V 3) N) etc.

(3) New tool material coating tool

Chemical vapor deposition (CVD) or physical vapor deposition (PVD) methods are used. Cemented Carbide or Other Material A tool material obtained by substantially coating a thin layer of wear-resistant insoluble metal (or non-metal) compound in a tool. The coating of the covering tool prevents direct contact between the tip and the tool, reduces friction and reduces various mechanical thermal stresses. Covered tools can reduce cutting times, reduce costs, reduce the number of tool changes, improve machining accuracy, and extend tool life. Covering tools reduce or eliminate the use of cutting fluid.

Determining tool parameters

Reasonable choice of prop parameters is very important to improve tool life and workpiece quality. Larger rake angles should be used on the premise that the knife is strong enough and strives to avoid the cutting edge, which not only reduces cutting force and cutting temperature. , Also reduces the quenching depth. The front angle of various stainless steel turns is generally 12 ° -13 °. You should choose a larger clearance angle, assuming you have enough tools. It generally takes 10 ° -20 °. It is also possible to use negative chamfers on the main cutting edge to strengthen the head and improve the wear resistance of the tool. Due to the toughness and plasticity of stainless steel, it is difficult to break the insert during cutting. The method of forced deformation is mainly to rationalize the parameters and cutting amount of the chip breaker on the rake face. Based on a reasonable choice of cutting amount, we generally adopt the double edge tilt plus external tip flute method. The outer turning tool grinds the angle of both blades to make the surface of the insert a surface shape. The outer diagonal arc-shaped tip flute is sharpened on the cutter knife, so the tip bends along the guide flute into a pagoda-like coil or a short spiral coil. When machining stainless steel, chip adhesion to the cutter is strong, and stacked edges are likely to occur. Therefore, in order to reduce cutting resistance, it is necessary to reduce the surface roughness of the front and rear surfaces of the tool and the cutting edge. Reduces cutting adhesive force and extends tool life.

Cutting amount

• (1) Cutting speed v:Cutting speeds can be reduced to ensure a reasonable tool life. Generally, the cutting speed when cutting stainless steel is 40% -60% of normal carbon steel.
• (2) Feed rate f:To improve the quality of the machined surface, it is wise to use smaller values ​​for the feed.
• (3) Cut amount AP
• (4) Generally, ap = 2 to 5 mm can be selected for roughing, and ap = 0/2 to 0.5 mm can be selected for finishing.

Cutting fluid

Due to the poor machinability of stainless steel and the high temperature of the cutting area, the cutting fluid is required to have high cooling performance, lubrication performance, and permeability. High pressure spray or oil mist cooling can be adopted to enhance the cooling effect.

The commonly used cutting fluids are:

• (1) Sulfide oil: It has reliable cooling performance and lubrication performance, and has abundant resources and low cost.
• (2) Carbon tetrachloride + mineral oil or other oils:The addition of carbon tetrachloride to mineral oils or other oils greatly improves the permeability of the cutting fluid, especially for finishing stainless steel materials.

Example

Coarse 1Cr18Ni9Ti stainless steel shaft parts (60mm diameter) for CNC lathes. With a coated cutter, cutting speed n = 800r / min backing knife amount ap = 0.7-1mm, feed amount f = 0.2-0.3mm / r, cutting is light and chips are good. Tool wear is low and one blade can be machined continuously for 4 hours or more, saving tool change time. At the same time, I use used blades from finished cars for roughing. This saves the cost of the tool and can save energy by secondary use of the blade to achieve the same effect.

Conclusion

Based on the above, the basic principle of turning stainless steel is as follows.
Tool materials are made of tough, high-strength carbide tools or coated tools. The tool parameters should be as rational as possible and the tool should be as sharp as possible. Chips must be easy to roll and break, the amount of cutting must be adequate, and the amount of cutting fluid supplied must be sufficient. Appropriate measures should be used whenever possible. When using covering tools, it is reasonable to use rotating elements. Through multiple trials, the batch processing of covering tools is even better, and the processing efficiency of ordinary lathes is more than five times higher than before.