3. Special steels
page 16
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Photo of grinding
Picture 3.14 Grinding a free machining steel component.

Free machining steels
Modern cars contain many steel components with complex shapes. These components are machined into shape using hard, sharp shaping tools. Special free machining steels have been developed that are more readily machined and which prolong the life of the shaping tool.
Interrupting the smooth running
Problems are caused during the shaping of steel components by the tool striking alumina (aluminium oxide) inclusions within the steel. The alumina inclusions are abrasive. They slow down the machining process and damage the cutting edge of the shaping tool.
Microstructure of rail steel
New improved machining steels (IM®) have been developed in which calcium is added to the molten steel before casting. This converts the alumina inclusions into calcium aluminate, often encapsulated by manganese sulphate. Instead of being abrasive, these inclusions deform when struck. This has the effect of lubricating the machining tool tip which significantly improves machining speed and tool life.

The modification of inclusions in this way is sometimes called inclusion engineering.

Question 3-5.
a) Why do alumina inclusions slow down the machining process and damage the cutting edge of shaping tools?

b) What is the solution to this problem?


Summary                   Close
  • machine tools need to be hard and consistent
  • alumina inclusions can disrupt the process
  • new machining steels have been developed without the alumina inclusions