Choosing carbide inserts for stainless steel

Why stainless steel is hard on inserts

• Work hardening. Especially in austenitic and duplex grades, the cut can harden the surface it leaves behind. A light or rubbing pass makes the next pass meet harder material, which adds to notch wear at the depth-of-cut line.
• Built-up edge. Stainless steel, austenitic grades in particular, tends to stick to the cutting edge. When the built-up material tears away, it can take small pieces of the edge with it.
• Heat. Low thermal conductivity, especially in austenitic grades, raises cutting-zone temperatures, increasing the risk of crater wear and plastic deformation.
• Chips. Chips are tough and long, especially in austenitic and duplex grades, so chip control is harder than in carbon steel.

Not all stainless steels are the same

Most of the advice below is for austenitic and duplex grades, the ISO M materials. For ferritic, martensitic and precipitation-hardening grades, choose the insert by the alloy, its condition and its hardness.

Choosing the insert

Geometry

In austenitic and duplex grades, use sharp edges and a positive rake face: they cut cleanly instead of pushing the material, which can reduce work hardening and built-up edge. For general turning, a negative CNMG or WNMG insert with a chipbreaker made for stainless steel gives strength and two-sided economy. For finishing, small parts or low-power machines, a positive insert such as CCMT or DCMT cuts with lower forces.

Grade and coating

For austenitic and duplex stainless steel, choose a grade the maker recommends for ISO M (yellow), tough enough for the stickiness and the forces. For ferritic and martensitic grades, makers usually point to ISO P grades, chosen for the alloy and its hardness. Both CVD and PVD grades are made for stainless steel, with overlapping uses: PVD grades are often preferred for interrupted cuts, or where chip hammering or chip jamming damages the edge, and CVD grades for stable, continuous turning. Treat this as a starting point and follow the maker’s grade chart.

Nose radius

A larger nose radius gives a stronger edge but raises the cutting forces that push the tool away from the part. A common starting rule is a nose radius no larger than the depth of cut, especially where vibration is a concern. For finishing at small depths, use a small radius.

Entering angle

Notch wear forms at the depth-of-cut line, where the edge meets the work-hardened surface. Round inserts or a smaller entering angle spread the load along more of the edge and can reduce notch wear, especially in duplex stainless steel. They also raise radial forces and the risk of vibration, so they need a stable setup.

Cutting conditions

• Cut below the work-hardened layer left by the previous pass, and keep the engagement steady within a pass.
• Avoid light passes that only skim the surface, and don’t let the tool dwell in the cut.
• Use coolant on austenitic and duplex stainless steel, for tool life and for chip control.
• Cutting speeds are usually lower than in carbon steel. Start from the cutting data for the grade you use and adjust from the wear you see.

Reading the wear

Look at the worn edge before you change anything. Each pattern points to different causes and fixes:

• Notch at the depth-of-cut line: often work hardening, with adhesion, scale or a heavily loaded edge playing a part. Changing the depth of cut slightly between passes moves the notch point; a smaller entering angle or a round insert can also help.
• Material welded to the edge: built-up edge. A sharper, positive edge, coolant and adjusted cutting data, often a higher speed, can reduce it.
• Deformed or sagging edge: too much heat at the edge. Lower the speed or feed, or use a more heat-resistant grade.
• Chipping on interrupted cuts: impact, vibration, a weak edge or thermal cracking. Try a tougher grade (often a PVD one), a stronger edge and a more stable setup.

Asking for a quotation

Tell us the stainless steel grade, the operation, the insert you use now and what goes wrong with it. We match the chipbreaker, grade and coating to it. Browse the ISO insert sizes we make, or read about ISO material groups and choosing an insert shape.

Questions

What is the best insert for turning stainless steel?

There is no single best insert. For austenitic grades such as 304 and 316, a common starting point is a sharp, positive chipbreaker in a grade the maker recommends for ISO M, a nose radius suited to the depth of cut, and coolant delivered to the cutting zone. Negative CNMG or WNMG inserts with a stainless chipbreaker suit general turning; positive CCMT or DCMT inserts suit light cuts and small parts.

Why do my inserts wear fast in stainless steel?

Common causes include adhesion and built-up edge, work hardening, too much heat, unsuitable cutting data and an unstable setup. Look at the wear pattern before changing the process: a notch at the depth-of-cut line, material welded to the edge, a deformed edge and chipping point to different fixes.

PVD or CVD coating for stainless steel?

Both are used, and both families include grades for overlapping conditions. PVD grades are often useful for interrupted cuts, or where chip hammering or chip jamming damages the edge; CVD grades often suit stable, continuous turning.

Is 304 harder to machine than 430?

Generally, yes. Annealed ferritic stainless steel such as 430 machines much like low-alloyed steel; austenitic grades such as 304 and 316 are harder to machine, and duplex grades such as 2205 harder still. Hardened martensitic grades such as 410 or 420 are a separate case and need tooling chosen for their hardness.