Heat resistant super alloys (HRSA) turning

​A super alloy is an alloy that exhibits excellent mechanical strength and resistance to creep (tendency for solids to slowly move or deform under stress) at high temperatures; good surface stability; and corrosion and oxidation resistance.

Heat resistant super alloys (HRSA) can be divided into four material groups; nickel-based, iron-based, cobalt-based and titanium alloys. Titanium can be pure or with alpha and beta structures.

The machinability of both HRSA and titanium is poor, especially in aged conditions, which impose particular demands on the cutting tools.

It emphasizes the importance of sharp edges to prevent the formation of so-called white layers, with different hardnesses and residual stresses. 

For inconel
Grades commonly used in finishing are 1105, 1115 and for roughing ceramic 6060 as 670 or S05F. The recommended geometries are -SF, -SGF, SM and -SMR.

For titanium
Grades to be used are H13A, H10A and 1115. The recommended geometries are -SF, -SGF, -SM and -SMR.

 

A common wear criterion in both titanium and HRSA is notch wear. By choosing a small entering angle or round inserts, feed and tool life can be increased considerably.

 S-style SNMG

​Notch wear can never be eliminated, but it can be minimized through good planning and by following some general rules:

  • Use round inserts.
  • Use the smallest possible entering angle.
  • Use the correct relationship between the insert diameter and depth of cut (see figure).
  • When using ramping or multiple passes, the depth of cut should never be less than 0.25 mm (0.0098 inch); otherwise there is risk of chipping
 

​In order to minimize notch wear, the best results are obtained by using a depth of cut that is a maximum of 15% of the diameter of a round insert, or 15% of the nose radius of a non-round insert.

Larger depths of cut can be used, but never greater than 25% of the insert diameter.

If these types of large depths of cut are being used, the workpiece must be free of forging scale/hard skin.

Pre-chamfering is recommended when using ceramics.

Pre-chamfering minimizes the risk of burr when the insert exits the cut. It also has a positive effect on the insert when it enters. To avoid notch wear when chamfering, use a direction feed of 90° to the chamfer that is being produced.

Coolant should always be applied when turning HRSA or titanium alloys regardless of whether carbide or ceramic inserts are used. The coolant volume should be high and well directed.