Machining of Nickel base super alloys

Machining of Nickel alloys should be done carefully by using sharp tools with positive rake angles. Adequate feed rate and depth of cut are essential and tools should be controlled to prevent rubbing. Even in the supreme conditions, stress can occur that may cause distortion of the work. For the highest dimensional stability, it is recommended to rough the part to size, stress relieve it and then finish it to size. Stress relieving has nominal influence on shapes, however may influence mechanical properties.

Category of alloys

Group A: Alloys comprise of 95% or more nickel. They have average mechanical strength and high hardness. They are hardened by cold processing. The alloys are gummy in the annealed and hot processed condition and cold processed material is preferred for the supreme machinability and smooth finish.

Group B: Comprises of standard nickel-copper alloys. They have higher strength and nominally lower hardness than those in group A. They are only hardened by cold processing. Cold drawn and stress relieved material provides the supreme machinability and smooth finish.

Group C: Comprises of solid solution nickel-chromium-iron alloys that are similar to the austenitic stainless steels. They are only hardened by cold processing and are machined readily in the cold-drawn or cold-drawn and stress relieved condition. These alloys are Inconel 600 wire, 601, Incoloy 800, 825 and Monel K500.

Group D: Comprises mainly of the age hardenable alloys. Group D1 comprises of alloys in the unaged condition. Group D2 comprises of alloys of group D1 in the aged condition, and many other alloys in aged and unaged conditions.

Cutting Fluid

Any cutting fluid can be used in machining nickel alloys. They respond well to general sulfurized mineral oil, sulfur offer enhanced lubricity and anti-weld properties. If the temperature of oil and work material is sufficient during machining to result in brown sulfur staining of the material. The stain can be removed with a cleaning solution of the sodium cyanide. It should be performed before heat processing including welding due to exposure to high temperature the staining may cause intergranular surface corrosion. To prevent intergranular corrosion, the components should be immersed in cleaning solution for sufficiently long time to prevent the stain. High speed machining operations create high temperatures that preclude the use of a sulfurized oil because of sulfur embrittlement of carbide tools.