Heat Resistant Superalloys for the leading industrial sectors

Heat resistant alloys are group of alloys used in the different industrial sectors-

Aerospace engine, stationary gas turbines, oil and gas and medical.

The characteristics that make them suitable materials are:

·         Retention of strength and hardness as high temperatures

·         Corrosion resistance

Heat resistant alloys are divided in three categories- nickel based, iron based and cobalt based alloys. The physical properties and machining behaviour of each varies significantly due to the chemical nature of the alloy and the precise metallurgical processing it receives during production. Whether the metal is annealed or aged is specifically effective on the after machining characteristics.

Nickel based alloys are commonly used and presently make about 50% of weight of advanced aircraft engines. Commonly used alloys in Heat engines are Inconel 718 wire, Inconel 625 and solution strengthened nickel alloys.

Iron based materials are developed from austenitic stainless steels. Some have low thermal expansion coefficient that make them suitable for shafts, rings and casings. But they lack in hot strength properties such as A286.

Cobalt based alloy show hot corrosion resistance at high temperature as compare to nickel based alloys. They are costlier and also more difficult to machine because of their extreme wearability. The application in turbines is limited to combustion parts in the hottest engine regions. Their main application is observed in surgical implants that use their inherent corrosion resistance.

With above wide range of materials in the generic name of heat resistant alloys, the machining behaviour widely varies even in the same class of alloys. Actually same material can have different machining recommendations.

Material processing condition

The form of heat treatment affects the hardness of the component and thus the wear mechanism. The development of the chip is a good sign of the hardness with hard materials it is easier to crack the chip.

Hardened materials have increased cutting temperatures and show a potential to notching of the cutting edge at the depth of cut. The combination of a low entering angle and a hard substrate with a heat barrier coating is needed. Softer materials machine in a similar way to the stainless steel family.

Insert grades with greater toughness and decreased hot hardness, resistance to high temperatures, are needed due to reduced cutting temperatures and increased chip hammering. The damage to areas outside the real cutting edge is caused by the chip breaking against the insert. Considering the size, shape and strength requirements of the component, different production methods for the empty material will be used. The production methods depends used depends on the machinability of the material and will alter the wear characteristics.

These raw material types directly affect the microstructure of an alloy and also its after machining behaviour.

Forged materials have a finer grain size as compared to in castings that enhances the strength and grain flow of the component. When machining forgings, decreasing the speed and increasing the feed normally gives the maximum feasible metal removal rate with good tool life.

The opposite rule is applied in castings and using low feeds ad higher speeds can be advantageous. Castings have poor machinability and tend to be most sensitive to notch wear and abrasive wear. They can be easily recognized due to their visibly mottled surface.