Milling Stainless Steel (ISO M): Work Hardening, Parameters and Tool Selection

Stainless steels form group M in the ISO 513 classification and behave differently from carbon steels in machining. This guide covers the milling behaviour of the main stainless families, why work hardening governs parameter selection, and parameter ranges that can be used on an application basis. The general material classification is described separately in the ISO material groups guide.

1. Different behaviours under ISO M

"Stainless" is not a single material. Under the same ISO M heading there are microstructures that differ markedly from one another, and each produces a different difficulty.

The difference between the groups is reflected directly in parameter selection. Ferritic and annealed martensitic grades behave close to ISO P, while austenitic and duplex grades call for a markedly lower cutting speed and a more rigid set-up. Since 304 and 316 are the grades most often encountered in the workshop, the weight of this guide is on austenitic behaviour.

2. The three physical causes of the difficulty

Low thermal conductivity

The thermal conductivity of 304 stainless is around 16 W/(m·K) at room temperature; in carbon steel this value lies in the 45–60 W/(m·K) range. In other words, stainless conducts roughly one third as much heat as carbon steel. The consequence is direct: most of the heat generated in the cutting zone cannot leave with the chip and accumulates at the cutting edge. As the cutting edge temperature rises, diffusion and plastic deformation wear accelerate. The wear mechanisms are covered in detail in the tool wear guide.

Adhesion tendency and built-up edge

The high ductility of the austenitic structure favours the chip welding onto the cutting edge under pressure. This welded accumulation is called built-up edge (BUE). Before it breaks away, the built-up edge changes the tool geometry; when it does break away, it tears off the coating with it and leaves micro-chipping on the edge. In milling, the reason a cutting edge is lost is frequently not direct wear but the edge chipping caused by built-up edge.

Work hardening

When austenitic stainless undergoes plastic deformation, a thin layer of increased hardness forms beneath the surface. When the next cutting tooth enters this layer, it meets higher forces and higher temperatures. If the layer is not thick enough, cutting proceeds normally; but if the tool is not cutting deep enough, every pass works in the zone hardened by the previous one and the process feeds itself.

Duplex and super duplex grades show stronger work hardening than austenitic, ferritic and martensitic stainless steels. While the tool wears, the machined surface continues to harden at the same time.

3. The constant and sufficient chip thickness rule

The most common mistake in stainless milling is reducing the feed in the face of a problem. This reflex works the wrong way round. As the feed per tooth decreases, the thickness of the chip formed comes close to falling below the edge hone radius of the cutting edge. At this point the edge ploughs and burnishes the material instead of cutting it. Ploughing both generates heat and thickens the hardened layer.

Sandvik Coromant's milling guide states this behaviour explicitly: a very low fz value increases wear because the cutting edge works in the work-hardened zone. The same reasoning is even more pronounced on duplex grades; the largest possible depth of cut is recommended in order to get below the hardened layer. The IMOA guideline on milling duplex stainless steels expresses this with a concrete limit: in climb milling the average chip thickness is required to be kept at least 0.1 mm, and as the entering angle rises from 45° to 90° the feed is required to be corrected by a factor between 1.0 and 0.7.

Three practical conclusions follow:

• The problem is not solved by reducing the feed. If there is vibration or edge chipping, rigidity, tool length and strategy are reviewed first.
• The feed is corrected as the radial engagement decreases. When the ae value falls below the cutter radius, the actual chip thickness drops below the nominal fz value. Without correction, ploughing begins. The calculation method is given in the chip thinning guide. The relation vf = fz × Z × n is used for converting between spindle speed and feed.
• Chip thickness is kept constant. In corners and radius transitions the engagement angle increases; if the feed is left constant, the force jumps. Generating a constant-engagement toolpath on the CAM side is preferred.

4. Cutting speed and feed ranges

In stainless steel the cutting speed has both a lower and an upper limit. Heat sets the upper limit, built-up edge the lower one. At very low speed, weld-related adhesion increases; at very high speed the cutting edge collapses thermally.

That the ranges differ this much between solid carbide and indexable tooling is not surprising: on indexable tools the cutting edge is thicker, the coating is thicker and the thermal load is carried for a shorter time. A solid carbide end mill, with its slender cross-section, cannot tolerate the heat.

On the feed per tooth side, the common reference gives a band that scales with diameter: from the order of 0.013 mm/tooth at 3.175 mm (1/8") diameter up to the order of 0.15 mm/tooth at 25.4 mm (1") diameter. These two extremes produce a starting rule of roughly fz ≈ 0.4–0.6% of the diameter. The values are for rough shoulder milling; in full slotting the lower limit is approached, and in a trochoidal strategy the upper limit is approached after the thinning correction.

5. Tool, coating and edge preparation

The three properties that decide tool selection for stainless steel are sharpness, edge toughness and resistance to adhesion.

Coating. PVD coatings are typically 1–4 µm thick and preserve the sharpness of the cutting edge. For this reason the general preference in stainless steel is on the PVD side. Coatings containing aluminium (the AlTiN / TiAlN family) stand out for their oxidation resistance and low adhesion tendency; these coatings also behave consistently in dry or limited-coolant work. A comparison of the coating families is detailed in the coating guide.

Edge preparation. Edge honing is a two-sided choice. An over-honed edge increases the ploughing effect and feeds work hardening; an over-sharp edge, on the other hand, chips in an interrupted cut. On a solid carbide end mill a light and even edge preparation on fine-grain carbide is preferred, and on indexable tools a reinforced edge geometry. Fine-grain carbide is widely used in this group because it provides toughness without spoiling edge sharpness.

Geometry. In rough shouldering and pocketing 4–5 flutes are common. In full slotting the flute count is kept low because chip evacuation is decisive. In finishing side-wall passes a helix angle above 40° and a flute count of 5 or more are used. In a trochoidal strategy the radial engagement is small, so a high metal removal rate can be reached with 5–7 flute tools; the method is described in the trochoidal milling guide.

6. Interrupted cutting, dwell and entry-exit

In stainless steel, the tool dwelling inside the cut produces damage directly. When the machine pauses for a second, the material at the contact point hardens and on the next pass the edge strikes that hard spot. For this reason:

• Dwell commands that zero the feed within the program have to be avoided.
• The tool must not be stopped without being retracted from the cut surface.
• Interrupted cutting is reduced as far as possible. Face milling paths that cross over holes, interrupted shoulder passes and paths that repeatedly take the tooth into air accelerate edge chipping.
• Entries are made with a radial ramp or an arc; helical interpolation is preferred over vertical plunging.

Rigidity is a separate heading. The high cutting force of stainless steel, combined with a long tool and weak clamping, turns into vibration, and vibration breaks the sharp edge. The tool has to be mounted at the shortest possible length in a holder whose runout has been measured.

7. Coolant: pressure and position

In stainless steel the function of the coolant is less to cool than to carry the chip away from the cutting zone and to reduce adhesion. What matters as much as the flow rate is the pressure and the jet hitting the correct point.

In high-pressure coolant applications the standard capability is of the order of 70/80 bar (1,015/1,160 psi), and this level is considered sufficient for most applications. On materials whose chips are difficult to break, such as super duplex and nickel-based alloys, it goes up to 150 bar (2,176 psi). Nozzle diameters are kept in the order of 1 mm so that the jet reaches the cutting zone without dispersing, and the total flow requirement is limited. Correctly directed high-pressure fluid forms a hydraulic wedge between the chip and the rake face and reduces the contact area; as a result chip control improves and the thermal load on the cutting edge decreases.

Coolant is not the right choice in every operation, however. In roughing with indexable face mills the cutting edge heats and cools on every revolution, so coolant accelerates the formation of thermal cracks; dry cutting is recommended under this condition. IMOA's duplex guideline likewise states that in indexable face milling, particularly in roughing, coolant is not to be used so that the chips can be thrown clear of the tool. In slotting, pocketing and deep shoulder work with solid carbide end mills, on the other hand, coolant is indispensable because the chip must be evacuated from the flute.

8. The additional difficulties of duplex and super duplex

Duplex stainless steels contain the austenite and ferrite phases in roughly equal proportions. This structure carries the yield strength clearly above that of the austenitic grades and raises the cutting force. Super duplex grades (for example S32750 and S32760; pitting resistance equivalent PREN in the 40–43 range) show the same behaviour at a more extreme point. For comparison, the PREN value of 316L is 24–25 and that of standard duplex 2205 (S31803 / S32205) is in the 33–36 range.

Three differences stand out in practice:

1. The cutting speed is reduced. In face milling with indexable carbide, the ranges given are 50–80 m/min for 2205 roughing and 80–110 m/min for finishing; 30–50 m/min for 2507 roughing and 50–70 m/min for finishing. These values are clearly below the 150–250 m/min band recommended for austenitic grades. The speed has a lower limit as well, though; an excessively low speed brings built-up edge back.
2. The depth of cut is increased. The deepest possible section is taken in order to get below the hardened layer of the previous pass.
3. Rigidity and cooling are raised. Even slight vibration breaks the cutting edge; high-pressure coolant is regarded as the rule rather than the exception in this group.

Chip control requires particular attention as well. Since high strength and high elongation occur together, the chip stays long, wraps around the part and spoils the surface. The chip has to be removed from the part continuously.

9. Parameter table with application examples

The values below are starting points selected from within the manufacturer ranges given above; they have to be corrected according to machine power, clamping rigidity and tool length.

The solid carbide tools used in stainless steel work are listed in the square end mills category.

Sources

• ISO 513:2012 — Classification and application of hard cutting materials for metal removal with defined cutting edges — Designation of the main groups and groups of application
• ISO 8688-2:1989 — Tool life testing in milling — Part 2: End milling (iso.org)
• ISO 15510:2014 — Stainless steels — Chemical composition
• Sandvik Coromant — How to do milling in different materials (sandvik.coromant.com)
• Sandvik Coromant — Machining with coolant / coolant aspects (sandvik.coromant.com)
• Harvey Performance Company — Slaying Stainless Steel: Machining Guide (harveyperformance.com)
• Nickel Institute — Stainless Steels for Machining, A Designers' Handbook Series No. 9011 (nickelinstitute.org)
• International Molybdenum Association (IMOA) — Practical Guidelines for the Fabrication of Duplex Stainless Steels, 3rd Edition (imoa.info)