Machinability is the ease or toughness in which a material can be shaped with a cutting tool. A low carbon steel is softer and easier to machine than a harder alloyed steel. The machinability of the material is based upon its hardness. Hardness is an important property that must be taken into account when considering speeds and feeds; however, stainless steel is a material group that has rules of its own.
Stainless steel is an alloyed steel with the primary alloying element being chromium. Chromium forms the oxide on the skin of the material, which increases the material's ability to resist corrosion. Stainless steels also employ other types of alloying elements, such as nickel and carbon. The purpose of the alloying elements vary, but they are typically used to bring about a certain characteristic in the material. The required characteristic may be strength, grain structure, or toughness. Whatever the case, all of the different types of stainless steels fall into three broad categories: ferritic, martensitic, and austenitic.
Ferritic stainless steels are magnetic and contain chromium but no nickel. The most common ferritic stainless steels have low carbon contents. Ferritic would be considered to have a good machinability rating. There are also free machining types of ferritic stainless steel available. The free machining additive is sulfur. Ferritic stainless steel falls into the 400 grade in the AISI numbering system, along with martensitic stainless steel.
Martensitic stainless steels are also magnetic and can be hardened. These steels contain chromium and most, but not all, contain no nickel. Martensitic stainless steels are excellent in mild environments, but do not hold up well in corrosive environments. Martensitic, in the higher carbon content ranges, can be heat treated very easily and therefore must be treated like a medium or high carbon steel during machining. For the most part, the martensitic grades of stainless are considered to have a good machinability rating with the exception of the two types that contain nickel.
Austenitic is the most common type of stainless steel. Austenitic stainless steel falls into the 300 grade in the AISI numbering system. It is the most corrosion-resistant of the three groups. The other two groups of stainless were generally only alloyed with chromium, while austenitic stainless uses nickel along with chromium to give it superior corrosion-resistant qualities. Some types of stainless also use molybdenum as an alloying element.
Austenitic stainless steels are characterized by a high-work hardening rate and low thermal conductivity. They are more difficult to machine than other alloy steels, and tend to bond to the cutting tool, causing smearing or a built-up edge. The high-work hardening effect can result in extreme high hardness conditions on machines' surfaces. Unlike carbon steel, where the heat is carried away with the chip, austenitic stainless has little thermal conductivity and the heat stays at the cutting edge. Austenitic stainless steel is a very tough material. The chips, when machining this type of stainless, become long and stringy because of the material's unwillingness to break.
When turning austenitic stainless keep these general points in mind:
- Make sure that the machine and the setup are as rigid as possible.
- Keep tool overhang to a minimum.
- Make sure lathe spindle bearings are in good shape.
- Boring bars should be as short and as stout as possible.
- Use a tool geometry that employs a sufficiently large positive rake and plenty of clearance.
- For roughing cuts, use larger cutting depths and greater feed rates in combination with lower cutting speeds. Don’t raise the cutting speed and lower the feed rate and the depth of cut. Follow the cutting tool manufacturer's specifications.
- Leave sufficient amount for finishing. Allow the tool to get under the work-hardened skin of the material.
- Never allow the tool to dwell while in contact with the workpiece material. This will cause heat buildup and work hardening.
- Use coolant in high volume where possible. If you can't use coolant in high volumes, don’t use any. Coolant in non-sufficient amounts will cause the insert to thermal crack and fracture.
- If the cutting edge gets dull, change it. A dull cutting edge will cause heat buildup and work hardening.
- Try to employ a large lead angle on the cutting tool.
- If possible, use a coated insert. Ceramic works best, but any coating is better than nothing.
