Machining Stainless Steel
FABRICATION - MACHINING
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 steels, slightly different techniques are required when machining stainless steels. The relative machinability of Columbus Stainless steels in the annealed condition compared with carbon steels (100) is: ferritic grades - 70 and austenitic grades - 50. This difference is due to stainless steels being tough rather than hard with a tendency to seize and gall.
Columbus ferritic grades are usually supplied in the annealed condition and due to their toughness their machining characteristics are more similar to low alloy carbon steels rather than mild steels. 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.
Columbus austenitic 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 strengths. This increased ductility tends to produce stringy chips during machining and due to rapid work hardening can lead to problems. Heavier feeds and slower speeds are employed to reduce tool build up and minimise work hardening. Where possible it is recommended that cutting tools with chip breakers or curlers be used, especially for the high alloy grades such as types 309 and 310 where exceptionally tough and stringy chips are produced. As conductivity is even lower than for ferritic grades, heat removal is of greater importance.
When machining stainless steels 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.
Tool geometries are similar for both austenitic and ferritic grades and the summarized below for HSS:
Cutting edge angel ± 135°, point angel ± 138°, lip relief angle varies from 16° for 3mm diameter to 8° for 25mm diameter.
Rake angle 3° to 8°, margin width 0.125 to 0.35mm, relief angles primary 4° to 6°, secondary double primary chamfer angle 30° to 35°, chamfer relief angle 4° to 5°, helix angle ± 7°, lead angle ± 2°.
Straight fluted for > 12mm holes, spiral fluted for smaller holes hook/rake angle 15° to 20°, back relief angle 10°, chamfer angle/length plug taps 9° to 10° (3.5 to 4.5 threads) taper taps 4° to 5°, (8 to 10 threads).
Rake angle 20° to 30°, throat angle 20° to 25°, face angle 1.5°.
Back rake angle ferritics 5°, austenitic 0°, side rake angle ferritics 8°, austenitics 10° to 15°, end relief, side relief and end cutting edge angles all 5°.
Back rake angle 4° to 10°, end relief angles 7° to 10°, side relief and clearance angles 1° to 5°.
Back rake angle 6° to 10°, end relief angle 7° to 10°, side relief angle 2° to 3°, end cutting edge angle 10° to 15° for small diameters and shallow cuts, decreasing to 5° for larger diameters.
Sulphurised, chlorinated or sulphur-chlorinated mineral oils and emulsifiable oils are used. In cases of high pressure the latter must contain sulphonated or chlorinated additions.
After machining it is essential to remove the cutting fluid and degrease the workpiece. This is usually done with conventional degreasing agents or solvents. In situations where the workpiece has been subjected to excessive heating or where maximum corrosion resistance is required, it may be necessary to passivate and/or pickle and passivate. If this is required refer to the section on post Fabrication Treatment.