Influence of Stainless Steel on Machinability

Stainless steel are often regarded as 'difficult to machine' and classed a single group of steel, based on experience with the most common austenitic stainless steel types.

The machinability of the stainless steel ie austenitic, ferritic, duplex, martensitic and precipitation hardening is however different. It is important to consider these property differences when selecting machining parameters and conditions.

Austenitic stainless steel

Includes 304, 304L, 304H, 305 , 316/316L, 316Ti , 317L, 321/321H, 309S, 310S, 347/347H grades.

Although relatively soft in the annealed condition and with very good ductility, these grades undergo extensive work hardening during cold working, which includes all forms of machining. For this reason machining these steel using feeds, speeds and depth of cut parameters for more conventional steel can result in excessive tool wear or breakage. These steel can be particularly difficult to machine in the cold worked condition. Although cold drawn bar can have better surface finish and accuracy of tolerances than 'black' bar, the relative ease of machining of the fully annealed 'black' bar may make this a better overall choice for the machinist or engineer.

The high ductility of these steel also works against them in machining. Poor chip-breaking and a build up of metal at the cutting face can easily occur. The thermal conductivity of the austenitic stainless steel is low, compared to other steel types, so heat can easily build up at the cutting tool face. Distortion or poor tolerance control during machining can be affected by the high thermal expansion rates of these steel.

In their annealed condition these steel are not ferro-magnetic ie they are not attracted to a magnet. This means that magnetic clamping devices cannot be used during the machining of these steel.

It is often a combination of these effects that make these steels appear difficult to machine.

Ferritic stainless stainless

Includes grades 430 (1.4016) and 1.4003.

The structure of these grades is similar to low alloy ferritic or low carbon martensitic steel.

Like the austenitic stainless steel grades, they are relatively soft, but have much lower work hardening characteristics. Together with a lower ductility, higher thermal conductivity and lower thermal expansion coefficients, these steel are less problematic to machine compared to the austenitic steels. Due to their higher alloy content and hence higher tensile strength, these steel are however more difficult to machine than mild steel. Unlike the austenitic steel, they are ferro-magnetic and so suitable (strong) magnetic clamping devices can be considered for machining.

Duplex stainless steel

Includes grade S32101, S32205/S31803, S32304, S32750, S32760

The structure of these steel is a mixture of ferrite and austenite. They have higher tensile strength than either ferritic or austenitic steels, but other properties significant to machining are an approximate average of these constituent 'phases'. Although work hardening is not as significant as it is for the austenitic grades, the higher strength means higher machining forces (power) and lower speeds are needed. As with the austenitic stainless steels good cooling is needed during machining, but distortion should not be as big a problem as the thermal expansion coefficients are more like the ferritic steel.

Chip formation and breakage is similar to the ferritic steels and so does not give the specific problems of associated with the austenitic stainless steels of tool tip build up.

For reasons which are not yet clear, the lean duplex steel 1.4162 has much better machinability than that expected from its higher hardness. 

Martensitic stainless steel

Includes grades 17/4PH (1.4542), 520B (1.4594), 17/7PH (1.4568) and A286 (1.4980).

These steels are hardenable (strengthenable) by heat treatment and can be either martensitic, austenitic or "semi-austenitic" in structure. Generally the heat treated hardnesses are not as high as the martensitic family of stainless steel, but they have higher tensile strength and better impact toughness. The most common 17/4 PH martensitic type can be machined in either the solution treated (annealed) (condition A) or precipitation heat treated conditions, but the higher double ageing temperature treatment ie H1150-M gives highest cutting rates. In condition A the machining response of the martensitic 17/4PH is similar to the 304 type austenitic. The semi-austenitic 17/7PH type does not machine as easily. The austenitic A286 type machines with some difficulty and requires cutting rates slower than the non-hardenable highly alloyed austenitic stainless steel.

The modest ageing treatment temperatures used to strengthen these steels enables machining in the annealed condition to be followed by a single treatment that minimises scale formation and distortion. Where closer machined tolerances are required, machining in the finally heat treated condition should be considered.

Improved (free) machining steel

These are covered in the article Free machining stainless steel grades

Comparison of stainless steel family machinabilities

TStainless Steel Specialist Course training note No.9 'Machining Stainless Steels' shows the machinability of stainless steels in the annealed condition relative to a resulphurized / rephosphorized free-cutting carbon manganese low alloy steel, taken to be 1.0. These indices are only to rank the steel types as achievable speeds and feeds also depend on the machining method used, type tool, lubrication conditions etc.

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