Question

Describe grain size refinement, solid solution strengthening, precipiation hardening, and work hardening. Why and when do you use each of these strengthening processes?

Answer 1

Grain refinement happens when recrystallization happens in metals.

When steels are heated above 673k & upto 973K recrystallization occurs

Recrystallization is a process by which deformed grains are replaced by a new set of defects-free grains that nucleate and grow until the original grains have been entirely

consumed.

Solid solution strengthening is a type of alloying that can be used to improve the strength of a pure metal.^[1] The technique works by adding atoms of one element (the alloying element) to the crystalline lattice of another element (the base metal), forming a solid solution.

Age Hardening or Precipitation Hardening is a heat treatment process by which the strength and hardness of some metal alloys are enhanced by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix.

This small particles of the new phase are called precipitates.

Work hardening also called cold working is one of the process of increasing the strength of the material. In order to understand the concept of strengthening one must know the basics of slip and dislocation movement. When we look at the micro-structural level, there are lots of imperfections in any material. There are planes within the crystal structure along which the imperfection moves and broadly that can be called as dislocation movement. In order to strengthen the material we have to hinder the movement of these dislocations.

In case of work hardening or cold working, we increase the amount of dislocation density within a material. This increase in dislocation density makes the movement along the planes very difficult thereby increasing the strength.

In cold working material is heated below the re-crystallization temperature and then mechanically deformed. for e.g. cold rolling. Cold working is employed at places where alloying is not required i.e. pure metal is required and inclusion of any other metal is not desired. For e.g. in the electric transmission cable we use cold worked aluminum which is reinforced with steel cables. Inclusion in aluminum can lower the electrical conductivity hence it is cold worked.

All of above use for increse strength dardness etc. of the alloys .