|HRSA materials are used in aircraft turbine applications because of their high strength at elevated temperatures. |
Ever since their introduction many years ago, heat resistant super alloys (HRSA) have been difficult to machine.
But with developments in machinery, equipment and above all in tool materials and new cutting processes, HRSA components now can be machined efficiently and economically.
This a priority for aerospace, energy and medical industries as competitive and environmental issues are increasingly important.
Sandvik Coromant (www.coromant.sandvik.com/us) suggests a machining strategy for heat resistant super alloys that includes optimization with the latest cutting tools and the latest in machining methods.
Because HRSA materials are metallurgically composed to have high strength at high temperatures, the stresses that are generated when machining also are high. The unique capability of these nickel, iron or cobalt-based super alloys to perform close to the melting point of their basic metal gives them varied but generally poor machinability.
About twice as much power is needed to machine HRSA materials as is needed for low-alloy steel, and the specific cutting force is 4,000 N/sq-m for HRSA compared with 2,500 N/ sq-m for steel.
Although these alloys are ductile, their fatigue resistance, hardness and toughness at high temperatures combine to develop a number of wear mechanisms for cutting tools. The edge of a cutting tool is exposed to considerable mechanical stress, strain and heat in machining these alloys.
High compressive and shearing forces attack the cutting edge while it is in its most vulnerable condition, as the chip-flow temperature approaches the temperature at which the cutting tool material begins to lose sufficient hardness.
Many HRSA materials also work harden readily. That gives rise to diffusion wear, and usually leads to heavy burr formation. Work hardening also can make subsequent operations more difficult.
This means that the cutting speed — the factor largely responsible for the amount of heat generated — is limited and has to be kept well below that of more common workpiece materials.
Plan for machining
The first consideration in planning to machine heat resistant super alloys is the state of the workpiece material.
Various types of machining – roughing, semi-finishing and finishing – are best suited to certain conditions. Heat treatment, solution treatment and aging all affect the workpiece in ways that influence machining and consequently, the different machining operations need to be carried out at the correct stages of manufacturing.
Whether the material is cast, forged or bar stock also affects HRSA machinability and applications.
Then, a number of crucial tool and method factors must be considered. For example: specific cutting forces for heat resistant super alloys can vary from about 3,500 N/sq-m in the annealed/ solution-treated condition, to about 4,150 N/sq-m in an aged condition, and hardness can range from 30 to 48 HRC.
Making tools last longer
The combination of high cutting forces with high cutting-edge temperatures results in a tendency to develop specific types of cutting-edge wear.
The primary ones are:
Notch wear, a mechanical type of wear in which the depth of cut sets the workpiece material line.
Plastic deformation of the cutting edge, a product of the combined high temperature and pressure and abrasive wear caused by the high hardness of the material.
Top--slice wear, which develops on ceramic inserts. This wear is characterized by layers of the top of the cutting edge being sliced off.
These destructive wear patterns have to be contained by a combination of having the right cutting tools and using the correct machining methods.
Cemented carbide inserts, especially fine-grained inserts, are suitable and broadly applicable along with SiAlON ceramics and, to some extent, whiskered ceramics.
The selection of an insert grade is not as important a consideration for roughing and finishing operations as it is for other workpiece materials. Instead, grade selection is a consideration as an optimization factor, and depends on the shape of the insert, the approach of the cutting edge and the type of operation, such as turning, profiling or grooving and recess machining, that is being done.