Why is Stainless Steel Difficult to Process? | MachineMFG

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Why is Stainless Steel Difficult to Process?

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As we all know, the machinability of stainless steel is much worse than that of medium carbon steel.

Taking the cutting machinability of ordinary No. 45 steel as 100%, the relative cutting machinability of austenitic stainless steel 1Cr18Ni9Ti is 40%; that of ferritic stainless steel 1Cr28 is 48%; and that of martensitic stainless steel 2Cr13 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 most 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 of σb, the yield limit σs increases.

The σs of annealed austenitic stainless steel does not exceed 30%-45% of σb, but it reaches 85%-95% after work hardening.

The depth of the work-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 higher than the original.

Because stainless steel has high ductility, lattice distortion occurs during plastic deformation, leading to a significant strengthening effect.

Additionally, austenite is not very stable, and under cutting stress, some austenite will transform into martensite.

Furthermore, compound impurities tend to decompose into a dispersed distribution under cutting heat, leading to the formation of a work-hardened layer during cutting.

The work hardening phenomenon produced by the previous feed or process seriously affects the smooth progress of subsequent processes.

(2) High cutting forces:

Stainless steel has a large plastic deformation during cutting, especially for austenitic stainless steel (which has an elongation rate more than 1.5 times that of No. 45 steel), which increases the cutting forces.

Additionally, the severe work hardening and high thermal strength of stainless steel further increase the cutting resistance, making it difficult for the chips to curl and break.

Therefore, cutting stainless steel requires high cutting forces, with the unit cutting force for turning 1Cr18Ni9Ti 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 that of No. 45 steel.

As a result, a large amount of cutting heat is concentrated at the cutting zone and the interface between the tool and the 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 bonding:

Stainless steel has high plasticity and toughness, resulting in continuous chips during threading, which not only affects the 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 exacerbates tool wear and tearing of the already machined surface, particularly with low-carbon martensitic stainless steel.

(5) Tool wear:

The affinity between the tool and chips during cutting of stainless steel leads to adhesion and diffusion wear, causing crescent-shaped recesses on the front cutting surface of the tool and minor flaking and notching on the cutting edge.

Additionally, 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 linear expansion coefficient of stainless steel is approximately 1.5 times that of carbon steel, making the workpiece susceptible to thermal deformation under cutting temperatures and difficult to control in terms of dimensional accuracy.

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