You work in a company that manufactures steel springs for ca

You work in a company that manufactures steel springs for car suspensions. These are made from quench and tempered 1095 steel and require the hardness to be above 55 Rc and the unnotched impact energy to be greater than 70J. Inadvertently, the springs were painted the wrong colour. In order to remove the paint, a worker heated the springs with a gas torch until they glowed orange and then cooled them in a bucket of water before repainting them. A significant number of these springs were failing in service. Explain what has happened to the material and why it is failing Can the properties of the material be restored to the correct values and if so, how. If the springs were instead made from precipitation hardened aluminium, what would change in properties would occur (explain the changes from a microstructural point of view).

Solution

a) The heat treatment of the spring using the gas torch has induced a strain in the material which has resulted in an intrinsic plastic deformation. The thermal strain is due to the formation of Martensite (BCT) structure.

b) The material can be restored to the original working condition by heat treating it to austenite temperature followed by CCT profile to create a FCC structure of the steel.

c) The springs made of aluminum hardened by precepitation tranforms the crystalline structure of aluminum to a strain resistant FCC structure due to incereased aging process. The aging process reduces the BCT martensite structure due to rapid cooling and transfoms it to a plastic FCC structure.