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What is cold - working in Manufacturing ?

The working of metals at temperatures below their re-crystallization temperature is defined as cold-working. Sometimes this working temperature is well above room conditions as in cold finishing of hot-rolled products. Most of the cold-working processes, however, are performed at room temperature. 

When materials is cold - worked the resulting change in material shape brings about marked changes in the grain structure. The effect of the cold-working on the structure of steel is to distort or elongate the grain in the direction of the flow of metal. Much greater pressures are needed for cold-working than for hot-working. The metal, being in a more rigid state, is not permanently deformed until stress exceeding the elastic limit is passed. Severe stress, known as residual stress is, therefore, set up inside the metal during cold-working. This effect of cold - working is reduced or minimized by heating the metal into the re-crystallization or annealing range, and finally the metal is brought to its original condition. This sequence of  repeated cold working and annealing is frequently called the cold work anneal cycle.

It is customary to produce cold-worked products like strip and wire in different tempers, depending upon the degree of  cold reduction following the last anneal. The cold-worked condition is described as the annealed (soft) temper, quarter-hard, half-hard, three-quarter hard, full-hard, and spring temper. Each temper condition, indicates a different percentage of  cold reduction following the annealing treatment.

Cold-working is employed chiefly as a finishing operation, following the shaping of  the metal by hot-working. It also controls the mechanical properties of steel in a large measure. It increases the tensile strength, yield strength and hardness of steel, but lowers its ductility. The increase in hardness resulting from cold-working is described as work-hardening. 

In general, cold-working produces the following effects : 

1. Internal stresses are set up which remain in the metal unless they are removed by proper heat - treatment. 

2. A distortion of  the grain structure is created.

3. Strength and hardness of the metal are increased but ductility is decreased.

4. Smooth surface finish is produced.

5. Accurate dimensions of parts can be maintained. 

The principal methods of  cold-working are as follows :

1. Rolling

2. Forging

3. Drawing

4. Bending

5. Spinning 

6. Extruding

7. Squeezing

8. Peening

9. Sizing, coining, hobbing.  


Cold Working in Manufacturing






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