Influence of steel family (type) on machinability

Introduction

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

This family includes the 304 (1.4301) and 316 (1.4401) 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 steels using feeds, speeds and depth of cut parameters for more conventional steels can result in excessive tool wear or breakage. These steels can be particularly difficult to machine in the cold worked condition (ie in the form of cold drawn bar stock). 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 steels 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 steels 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 steels.
In their annealed condition these steels 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 steels.
It is often a combination of these effects that make these steels appear difficult to machine.

Ferritic stainless steels

This family includes grades 430 (1.4016) and 1.4003.
The structure of these grades is similar to low alloy ferritic or low carbon martensitic steels.
Like the austenitic 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 steels are less problematic to machine compared to the austenitic steels. Due to their higher alloy content and hence higher tensile strength, these steels are however more difficult to machine than mild steels. Unlike the austenitic steels, they are ferro-magnetic and so suitable (strong) magnetic clamping devices can be considered for machining.

Duplex stainless steels

This family includes grade 2205 (1.4462)
The structure of these steels is a mixture of ferrite and austenite. They have higher tensile strengths 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 steels.

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 steels

This family 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 steels, 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 steels.
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 steels

These are covered in the article Free machining stainless steels grades

Comparison of stainless steel family machinabilities

The BSSA Stainless 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.

Steel type Machinability Index
Free-machining low alloy steel 1.0
Austenitic stainless steel -
Free-machining - 303 (1.4305) 0.85
Standard - 304 (1.4301) 0.52
Ferritic stainless steel -
Free-machining - 430F 0.90
Standard - 430 (1.4016) 0.60
Duplex stainless steel -
Standard - 2205 (1.4462) 0.50
Martensitic stainless steel -
Free-machining - 416 (1.4029, 1.4005) 0.95
Standard low carbon - 410 ( 1.4006) 0.50
Standard high carbon - 440C (1.4125) 0.40

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