Today, machining of superalloys is becoming increasingly important. The structural materials currently used in industry have increasingly better properties, in particular increased mechanical strength, greater hardness, increased abrasion resistance, corrosion resistance. Heat and incandescence resistance is also becoming important. Increased demand by component manufacturers for increased mechanical properties of components is causing them to adapt their production capacity in the field of chip processing. Moreover alloy steels, heat-hardened tool steels, stainless steels, bleached cast irons and nickel- and cobalt-based super alloys become the material of choice for the component. These materials belong to the group of hard-to-machine materials.
Classification of difficult-to-machine materials.
Heat-resistant and refractory materials are classified according to their chemical composition and properties into cutting material application groups. This is precisely defined in the ISO 513:2012 standard. In addition, the Polish PN-EN 10095:2002 standard defines the properties and chemical compositions of heat-resistant steels and nickel alloys. PN-EN 10302:2009, on the other hand, defines the properties and chemical composition for heat-resistant steels and nickel and cobalt alloys. As no specific criteria for the division of steel into heat-resistant and refractory have been developed so far, it is customary to classify steel into a specific group according to the following definitions:
- Heat resistance is the property that determines a material’s resistance to the oxidising and corrosive effects of agents such as gases at high (above 500 degr C) temperatures. One of the main elements that increase this property is chromium. Its admixture of approximately 5 per cent increases heat resistance to 650 degr C.
- Incandescence, on the other hand, is the resistance of materials to various types of deformation under prolonged mechanical stress at temperatures higher than 500 degr C. The main admixtures that increase heat resistance are chromium, nickel, molybdenum and tungsten.
Applications of superalloys.
Hard-to-machine superalloys of steel, cast steel or nickel alloys are now widely used by various industries. They can be used in the manufacture of furnaces for industry. Also in the manufacture of chemical apparatus, boilers, for the production of engine components, turbines, disks, shields, rings, rotors, blades, etc. Superalloys have also been successfully used in the manufacture of implants, shafts and rings. That is, wherever the finished component will be exposed to prolonged high temperatures under mechanical load.
Machining of nickel- and cobalt-based superalloys.
The machining of alloyed metals, particularly those based on cobalt and nickel, is a very demanding process. During the process, we may encounter, above all, rapid wear of the cutting tool cutting edge. Regardless of the tool manufacturer, we are often faced with abrasive wear. That is to say, the basic wear and tear caused by the action of very hard particles contained in the workpiece material on the cutting edge. Adhesive and diffusive wear are also common
In rarer cases, we can include chemical-abrasive wear. In order to extend the life of the cutting tool, thereby reducing the cost of producing a workpiece, we use a suitable coolant. This is applied under high pressure and in the correct quantity. During the milling process, this quantity can be as high as approximately 50 l/min. An additional difficulty during a process such as machining superalloys is that the process is divided into phases. In the first stage, roughing is carried out, followed by profiling and, in the last stage of the process, finishing. In addition to theoretical knowledge, experience becomes necessary to achieve the desired production result (quality/performance).