Question

Which tool is used for rapid machining of hard metals?
1. Cemented Carbide
2. High Speed Steel
3. Stellites
4. None of these

Answer 1

Correct Answer - Option 1 : Cemented Carbide

Explanation:

Cutting tool material:​

• There are many types of cutting processes done in different conditions. In such conditions along with the general requirements of the cutting tool, they need some unique properties. To achieve this property the cutting tools are made up of different materials.
• The material chosen for a particular application depends on the material to be machined, type of machining, quantity, and quality of production.

​High-speed steel:

• This is high carbon steel with a significant amount of alloying elements, such as tungsten, molybdenum, chromium, etc. to improve hardenability, toughness, and wear resistance. It gives a higher metal removal rate. It loses its hardness at a moderate temperature of about 650°C.
• Therefore, a coolant should be used to increase tool life. It can use many times by re-sharpening. Some surface treatment is done on the HSS to improve its property.
• Cutting speed range - 30-50 m/min
• Temperature - 650°C
• Hardness – up to HRC 67

​Cemented Carbide:

• The cemented carbide cutting tool is produced by powder metallurgy technique. It consists of tungsten, tantalum, and titanium carbide with cobalt as a binder (when the binder is nickel or molybdenum, then it is called cermet).
• Cemented carbide tools are extremely hard; they can withstand very high-speed cutting operation. The carbide tool does not lose its hardness up to 1000° C. A high cobalt tool is used for a rough cut while a low cobalt tool is used for finishing operations.
• Cutting speed range - 60-200m/min
• Temperature - 1000°C
• Hardness – up to HRC 90

​Stellite:

• Stellite alloys are a range of cobalt-based alloys, with significant proportions of chromium (up to 33%) and tungsten (up to 18%).
• Some of the alloys also contain nickel or molybdenum. Most of them are fairly high carbon content when compared to carbon steel,
• though they contain less than 3% iron, and in the stellite alloys the carbon is primarily associated with the chromium to form hard chromium carbide particles which are dispersed in the cobalt-based matrix.

Cast iron is not used as cutting tool material because  

• Cast iron machining typically is not considered to be problematic. This premise is partly because of the material’s higher graphite content when compared to st​eel’s. 
• Graphite makes the chips brittle and short and possesses anti-friction properties that contribute to cutting-edge lubrication. And with its ability to absorb vibrations, graphite also improves machining stability.