As we all know, the machinability of stainless steel is much worse than that of medium carbon steel.
Considering the cutting machinability of common No. 45 steel as 100%, the relative cutting machinability of 1Cr18Ni9Ti austenitic stainless steel is 40%; that of 1Cr28 ferritic stainless steel is 48%; and that of 2Cr13 martensitic stainless steel is 55%.
Among them, the cutting machinability of austenitic and austenitic-ferritic stainless steels is the worst.
Stainless steel has the following characteristics during the cutting process:
(1) Severe work hardening:
In stainless steel, the phenomenon of work hardening is more prominent in austenitic and austenitic-ferritic stainless steels.
For example, the strength σb of austenitic stainless steel after hardening reaches 1470-1960 MPa, and with the increase in σb, the yield strength σs increases.
The σs of annealed austenitic stainless steel does not exceed 30%-45% of σb, but reaches 85%-95% after hardening.
The depth of the hardened layer can reach one-third or more of the cutting depth, and the hardness of the hardened layer is 1.4-2.2 times that of the original.
Because stainless steel has high ductility, network distortion occurs during plastic deformation, leading to a significant strengthening effect.
Furthermore, austenite is not very stable, and under shear stress, some austenite will transform into martensite.
Furthermore, the composite impurities tend to decompose in a dispersed distribution under the cutting heat, leading to the formation of a hardened layer during cutting.
The hardening phenomenon produced by the previous feed or process seriously affects the smooth running of subsequent processes.
(2) High cutting forces:
Stainless steel exhibits large plastic deformation during cutting, especially austenitic stainless steel (which has an elongation rate greater than 1.5 times that of No. 45 steel), which increases cutting forces.
Furthermore, the severe work hardening and high thermal resistance of stainless steel further increase cutting resistance, making chip rolling and breaking difficult.
Therefore, cutting stainless steel requires high cutting forces, with the unit cutting force for 1Cr18Ni9Ti turning being 2450 MPa, which is 25% higher than that of No. 45 steel.
(3) High cutting temperatures:
Plastic deformation and friction between the tool and the workpiece during cutting generate a lot of cutting heat, while the thermal conductivity of stainless steel is only about 1/2 to 1/4 of that of No. 45 steel.
As a result, a large amount of cutting heat is concentrated in the cutting zone and the interface between the tool and chips, with poor heat dissipation conditions.
Under the same conditions, the cutting temperature of 1Cr18Ni9Ti is about 200°C higher than that of No. 45 steel.
(4) Difficult chip breaking and easy gluing:
Stainless steel has high plasticity and toughness, resulting in continuous chips during threading, which not only affects smooth operation, but also creates adhesion and chip nests under high temperature and pressure due to the strong affinity between stainless steel and other metals .
This aggravates tool wear on the already machined surface, especially with low carbon martensitic stainless steel.
(5) Tool wear:
The affinity between the tool and the chips when cutting stainless steel leads to adhesion and diffusion wear, causing crescent-shaped indentations on the front cutting surface of the tool and small flaking and nicks on the cutting edge.
Furthermore, the high hardness of carbide particles (such as TiC) in stainless steel causes direct contact and friction between the particles and the tool, leading to tool abrasion, work hardening, and accelerated wear during threading.
(6) Large linear expansion coefficient:
The coefficient of linear expansion of stainless steel is approximately 1.5 times greater than that of carbon steel, making the part susceptible to thermal deformation under cutting temperatures and difficult to control in terms of dimensional accuracy.
