Machinability is the term used to denote the machining performance of a material by a cutting tool. Due to their difference in properties when compared with carbon steel, slightly different techniques are required when machining stainless steel. The relative machinability of stainless steel in the annealed condition compared with carbon steel (100) is: ferritic grades - 70 and austenitic grades - 50. This difference is due to stainless steel being tough rather than hard with a tendency to seize and gall.

Ferritic stainless steel grades are usually supplied in the annealed condition and due to their toughness their machining characteristics are more similar to low alloy carbon steel rather than mild steel. Due to the difference in conductivity, care must be taken to ensure adequate removal heat from the workpiece and the tool. Overheating can result in blunting of the tool and localized burning of the workpiece surface.

Austenitic stainless steel grades are also normally supplied in the annealed condition and of more importance than their increased hardness over carbon steel, is the large difference between proof and tensile strength. This increased ductility tends to produce stringy chips during machining and due to rapid work hardening can lead to problems. Heavier feed and slower speeds are employed to reduce tool build up and minimise work hardening. Where possible it is recommended that cutting tool with chip breaker or curler be used, especially for the high alloy grades such as types 309 and 310 where exceptionally tough and stringy chip are produced. As conductivity is even lower than for ferritic grade, heat removal is of greater importance.

When machining stainless steel note must be taken of the following:

1. The machining equipment must be sturdy and rigid with up to 50% more power than equipment used for mild steels.

2. Machine tools and the workpiece must be firmly held to prevent vibration and chatter.

3. Cutting tools, either high speed steel or carbide must be kept sharp at all times, sharpening at regular intervals being preferable to sharpening when blunt.

4. Good lubricants should be used, especially for heavy cuts at relatively slow speeds. Thinning with paraffin is recommended for higher speed finishing cuts to keep the workpiece and tools as cool as possible.

5. The depth of cut must be such as to prevent the tool from riding in the workpiece. This is particularly important with austenitic grades to avoid work hardening and burnishing.

6. The largest possible tool must be used in order to dissipate heat.

7. Interrupted cutting must be avoided where possible as a greater degree of work hardening occurs as the tool enters the workpiece. The prime rule should be "get in and get out" with all tooling.

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