General principles of machining stainless steels

Introduction

The most common and hence most frequently machined stainless steels are the austenitic types, such as grades 304 (1.4301) and 316 (1.4401). These are characterised by their high work hardening rates and poor chip breaking properties during machining. This article covers the important issues that influence the successful machining of these steels.

Machine and tooling rigidity

When machining stainless steels it important to ensure that there is no dwell or rubbing caused by machine vibration or tool chatter. Machines must be 'substantial' and capable of making the deep cuts needed in machining austenitic stainless steel without slowing down the set feed or surface speeds. Small training or 'hobbies' lathes and milling machines intended for machining mild steel, brasses etc. are unlikely to be substantial enough for the successful machining of stainless steels.

Machines should not be prone to excessive vibration in the machine bed, drives and gear boxes or at the cutting tool or its mountings. Large overhangs of tool shank out of the tool box should be avoided. The distance between the cutting tip and toolbox support should be as short as practicable and the shank cross section as substantial as possible. This can also help in dissipating heat away from the cutting faces. Arbours for supporting barrel milling cutters should be stout as short as possible. The arbour supports should be as close as possible to the ends of the cutter to provide maximum support.

Some 'squealing' as the metal is being cut is not unusual, but can indicate that the tool may be wearing and need replacing.

Tool materials

Either high speed steel (HSS) (wrought or sintered) or cemented carbide tools can be used for machining stainless steels.

High speed steels

Either tungsten or molybdenum HSS can be used. These are particularly useful in machining operations involving high feed and low speed machining operations where there are variable cutting edge stresses induced from complex tool shapes.
The tungsten types (eg T15) can be useful for their good abrasion resistance and red hardness. The molybdenum HSS are more widely used, M42 being useful for applications such as milling cutters where a good combination of hardness and strength are required at lower cutting speeds. M42 has better hardness than grades like the more common M2, but may not be as tough however.
If the tools are prone to edge chipping, use a tougher grade, eg M2, M10 If tools are burning, use a higher red hardness grade, eg M42, T15 If the tools are wearing, use a more abrasion resistant grade, eg T15

Cemented carbides

Cemented carbides are normally used for machining stainless steels where higher speeds or higher feeds than those that can be produced using HSS are required. Either disposable insert or brazed-on tips (where lower cutting speeds can be tolerated) can be used and are composed of either tungsten carbides or a blend of tungsten and other metal carbides, including titanium, niobium, and chromium. The carbides are bonded with cobalt. The 'straight' tungsten carbides grades are used for machining austenitic and duplex stainless steels and the 'complex' carbides are used for machining martensitic and ferritic family grades.
Coated carbides have the additional benefit of improved wear resistance and resistance to breakage. Consequently they are capable of higher cutting speeds compared to un-coated carbide tools.
The wide range of carbide tools available usually means that machining trials are needed to get the optimum machining characteristics for specific situations.

Tool geometry and sharpness

It is essential to keep the cutting tools sharp when machining stainless steels. Careful grinding and honing of the tool faces to give accurate and sharp face angles is important. This helps optimise:

tool life
finish, accuracy and tolerances
productivity between regrinds

and reduce:

tool breakages
power requirements

Re-sharpening should be done as soon as the quality of the cut has deteriorated.
Machine grinding using properly dressed wheels, free from glazing, is preferable to hand grinding to get the necessary accuracy of tool geometry.

Correct tool geometry is important for minimising swarf build up on the tool faces.
Swarf build up can also result in increased machine power requirements and poor surface finish on the machined surfaces.
Tool relief angles must be flat. Concave relief faces can result in tool chipping or breakage due to the reduced support of the cutting edge.

Where possible the tool faces should incorporate chip curlers or breakers as austenitic stainless steels are prone to forming long spiralling turnings that can easily wrap around the tool and tool post. These can easily become entangled around the tooling and are difficult and time consuming to remove. In extreme cases the tool can become jammed by entangled turnings.

Lubrication and cooling

It is essential that cutting fluids are used when stainless steels are machined. This is due to the combination effects of the deep cuts and high feed rates needed to overcome the effects of work hardening, and the low thermal conductivity of the austenitic stainless steels, restricting the flow of heat away from the machined faces. Overheating stainless steel surfaces, characterised by the formation of heat tinting colours, during machining can impair corrosion resistance and so must be avoided. If formed pickling the surface can be used to restore corrosion resistance on the finished part. Overheating can also result in distortion that can be difficult to compensate for or correct.

The lubrication provided by cutting fluids also helps reduce tool wear and wash away the machining swarf.
Generally cooling is more important than lubrication with faster the cutting speeds and so high cutting fluid flow rates are normally used when machining stainless steels.

Either mineral oils or water soluble emulsifiable oils can be used. Minerial oils are more suited to severe machining operations with heavy loads at low speeds or where HSS tools are being used. Emulsifiable oils are used for machining at higher speeds with carbide tooling.

Mineral oils

Sulphurized, chlorinated or sulpho-chlorinated mineral oils can be used with additions of up to 10% fatty oils for machining non-free machining grades. Paraffin is used to dilute these oils, in oil/paraffin ratios between 1/5 for high speeds and light feed work to 1/1 for slower speed and heavier feed machining.
If excessive wear is being experienced, consider using greater dilutions. If the cutting edge is tending to burn, consider reducing the dilution.

Emulsifiable oils

These oils are diluted with water and provide better cooling than the paraffin diluted mineral oils. If extreme pressure (EP) emulsifiable oils are used, more sever machining operations can be supported. It is important that dilution is done by adding oil to water, not water to oil so that the correct form of emulsion, with the right lubrication and cooling properties, is formed.

After machining all traces of the cutting fluid should be removed from the surface so that the stainless steel surfaces can self-passivate. Under certain circumstances acid passivation should be considered.

Further information on the selection of cutting fluids for machining is available in the BSSA Training Note No9 'Machining Stainless Steels'

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