You work in a company that manufactures steel springs for car suspensions. These

Question

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.

(a) Explain what has happened to the material and why it is failing

(b) Can the properties of the material be restored to the correct values and if so, how.

(c) 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). Note assume that the springs are not melted by the gas torch.

Explanation / Answer

1) The spring steel when heated close to m.p. (glowing orange) would revert back to the parent phase and then if quenched immediately, depending on the hot temp and the cooling time /cooling temp would either become martensitic or austenitic steel. One is brittle, the other ductile. Hence quite ulikely to have the original properties of spring steel

2) Th properties of the original material may be restored by repeating the heat and cooling cycle as the factory process intended

3) For precipitation hardened Aluminum,. the heating process would solutionise the preceiptates to some extent, they would dissolve and perhaps new phases would form depending on the phase diagram. A complex set of changes occur which may involve vacancies, change of parent lattice and other factors. Depending on the temperatures and times involved, there could be many changes. Restoring the original properties may not be that simple, since usually springs are made by work hardening also involving surface treatments together with heat and alloy treatments.

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