Annealing is usually done after cold working to reduce internal stresses caused by plastic deformation. These stresses can make the material more prone to cracking during later processing or use, and are a major cause of material failure. Annealing is the process of making something softer or more flexible by heating it to a high temperature and then letting it cool slowly. It causes the atoms of the metal to line up in a more uniform direction, which makes it softer.
Before we get into further mechanical working or finishing, let’s look at why annealing is necessary after cold working.
What is Cold Working?
Mechanical working or cold working is done to change the shape or dimensions of metal, without changing its temperature. This is achieved by applying force to the metal in some fashion, thus inducing plastic deformation. Cold working can be done by rolling, hammering, drawing, extruding etc.
How Cold Working insert residual stresses?
When a metal object is subjected to mechanical working, it changes its shape. But it does not change its temperature. For example, if a piece of steel is struck with a hammer, the shape will change but the temperature will remain unaffected (unless the steel becomes too hot from being handled badly).
In the case of work hardening, this means that the internal of the metal will be under compression and its surfaces will be under tension. These stresses are called elastic stresses and they result from deformation.
How increased temperature relieve Residual Stresses in Cold Worked Structure?
But what happens when we heat up a piece of cold-worked steel (say, in a furnace) to attain its normal temperature? The material will expand in all directions, including the surfaces. If the value of Young’s modulus is larger than that of the shear modulus, then these elastic stresses will be decreased and this phenomenon results in the reduction of internal stresses caused by cold working.
If you look at a piece of cold-worked steel, you would see it has a dull, greyish color. If you heat the steel to a certain temperature, this state will change and it will become shiny.
What is Stress Relieve Annealing?
Stress relief annealing is an after-treatment that aims to reduce internal stress within castings through annealing and slow cooling-down, as suggested by its name.
Stress Relieve Annealing is a process in which metal is heated to a temperature that’s low enough to avoid forming austenite, but high enough to fully melt the alloy. With steel, at least, it takes place at temperatures between 550 and 650 degrees Celsius, therefore well below the range of austenite formation.
How Stress Relieve Annealing reduces residual stresses and return structure to its original condition?
When the cast material is heated to the annealing temperature, its internal structure changes. The grains grow in size and become more uniform, while their boundaries become amorphous, thus greatly reducing the internal stress within the structure. So by relieving residual stresses, it can return to its original condition.
For this reason, this process must be carried out at the lowest possible temperature and for as long as possible. The annealing process requires a specific combination of time and temperature, depending on the composition of the material.
Want to read more about stress relieve annealing, check out articles on types of annealing.
Does the structural change dimensionally during Stress Relieve Annealing Process?
Stress Relieve annealing is carried out at lower temperatures i.e., lower than 600 C for Steel. Below this temperature, elongated grains transform into polygon shapes thereby lowering the stresses developed in the material. Since no grain growth takes place the dimensional change in the process is very small.
When Stress Relieve Annealing is employed?
Stress-relieving is employed on cold-worked material to reduce internal stresses created by cold working. Cold working is done with the aim of altering the shape or dimensions of metal without changing its temperature.
Following are some more points on stress relieve annealing:
1) It reduces internal stress developed in the cold working process i.e. which is caused by the transformation of grains within the metal.
2) It can be employed on castings or forgings to eliminate internal stress in them.
3) It helps in making the product more ductile and softer than before so that it can further undergo the working process at room temperature without causing any damage to its surface.
4) It can also be employed after welding, brazing, and soldering.