Cutting characteristics of stainless steel

The machinability of stainless steel is much worse than that of medium carbon steel. Taking the machinability of ordinary No. 45 steel as 100%, the relative machinability of austenitic stainless steel 1Cr18Ni9Ti is 40%; ferritic stainless steel 1Cr28 is 48%; martensitic stainless steel 2Cr13 is 55%. Among them, the machinability of austenitic and austenitic + ferritic stainless steels is the worst. Stainless steel has the following characteristics in the cutting process:

(1) Serious work hardening: In stainless steel, the work hardening phenomenon of austenite and austenite + ferritic stainless steel is the most prominent. For example, the strength σb of austenitic stainless steel after hardening reaches 1470-1960  MPa, and with the increase of σb, the yield limit σs increases; the annealed austenitic stainless steel σs does not exceed 30%-45% of σb, and after work hardening Up to 85% to 95%. The depth of the work-hardened layer can reach 1/3 or more of the cutting depth; the hardness of the hardened layer is 1.4-2.2 times higher than the original one. Because of the high plasticity of stainless steel, the character is distorted during plastic deformation, and the strengthening coefficient is large; and the austenite is not stable enough, under the action of cutting stress, part of the austenite will transform into martensite; Under the action of heat, it is easy to decompose in a diffuse distribution, so that a hardened layer will be produced during cutting. The work-hardening phenomenon caused by the previous feed or the previous process seriously affects the smooth progress of the subsequent process.

(2) Large cutting force: Stainless steel has large plastic deformation during cutting, especially austenitic stainless steel (its elongation is more than 1.5 times that of No. 45 steel), which increases the cutting force. At the same time, the work hardening of stainless steel is serious, and the thermal strength is high, which further increases the cutting resistance, and the curling and breaking of chips are also difficult. Therefore, the cutting force of processing stainless steel is large. For example, the unit cutting force of turning 1Cr18Ni9Ti is 2450  MPa, which is 25% higher than that of No. 45 steel.

(3) High cutting temperature: the plastic deformation and the friction between the tool and the cutting tool are very large during cutting, and the cutting heat generated is much; in addition, the thermal conductivity of stainless steel is about 1/2 to 1/4 of that of No. 45 steel, and a large amount of cutting heat is generated. They are all concentrated on the interface between the cutting zone and the tool-chip contact, and the heat dissipation conditions are poor. Under the same conditions, the cutting temperature of 1Cr18Ni9Ti is about 200°C higher than that of No. 45 steel.

(4) Chips are not easy to break and easy to bond: stainless steel has great plasticity and toughness, and chips are continuous during turning, which not only affects the smooth progress of the operation but also squeezes the processed surface. Under high temperatures and high pressure, stainless steel has a strong affinity with other metals, which is prone to adhesion and build-up edge, which not only aggravates tool wear but also causes tearing to deteriorate the machined surface. This characteristic is more obvious in martensitic stainless steels with lower carbon content.

(5) The tool is easy to wear: the affinity effect in the process of cutting stainless steel causes bonding and diffusion between the knife and chips, so that the tool produces adhesive wear and diffusion wear, resulting in crescent craters on the rake face of the tool, and the cutting edge is still Minute peeling and gaps will be formed; in addition, the hardness of carbide (such as TiC) particles in stainless steel is very high. When cutting, it will directly contact and rub against the tool, scratch the tool, and work hardening will aggravate tool wear.

(6) Large linear expansion coefficient: The linear expansion coefficient of stainless steel is about 1.5 times that of carbon steel. Under the action of cutting temperature, the workpiece is prone to thermal deformation, and the dimensional accuracy is difficult to control.