What Is Hot Working?
Hot working process metals are plastically deformed above their re-crystallization temperature. Being above the re-crystallization temperature allows the material to re-crystallize during formation. This is important because re-crystallization keeps the materials from strain hardening which ultimately keeps the yield strength and hardness low and ductility high. The ultimate goal of metal hot working is to produce metal components of the required structural orientation and geometrical shape.
During hot metal working, distorted grain structure and strain hardening produced by deformations are rapidly eliminated due to the formation of new strain-temperature of the hot working of metal is set 50oC below its melting point.
Application of Hot Working
- Hot rolling
- Forging
- Extrusion
- Hot drawing
- Altering the form of iron and steel without fracture and use of excessive force.
What You Need To know About Hot Working
- Hot working is the process of plastically deforming a metal above the metal’s re-crystallization temperature.
- Hot working is carried out above the re-crystallization temperature and below the melting point. Hence the deformation of metal and recovery take place simultaneously.
- If hot metal working process is properly done, it does not affect tensile strength, hardness, corrosion and fatigue resistance of the metal.
- Hot working refines metal grains resulting in the improved mechanical properties.
- Hot working is most preferred where heavy deformation is required.
- In hot working, no internal and residual stresses build up in the metal.
- Heavy oxidation occurs during hot working therefore pickling is required to remove oxide.
- Hot working requires less energy for plastic deformation because at higher temperature, metal become more ductile and soft.
- There is chance of embrittlement by oxygen in hot working hence metal working is done at inert atmosphere for reactive metals.
- The uniformity of the metal is very high after hot working.
- Deformation of the metal and its recovery happens simultaneously in hot working.
- Due to higher deformation temperatures, the stress required for deformation is much less.
- Due to oxidation and scaling, poor surface finish is obtained.
- Crystallization takes place.
What Is Cold Working?
Cold working or work hardening is the process of strengthening a metal by plastic deformation at temperatures below the re-crystallization temperature, usually the ambient temperature. In most cases, such cold forming is done at room temperature. The major cold-working operations can be classified basically as squeezing, bending, shearing and drawing.
When metals such as steel, aluminum and copper are cold worked, permanent defects change their crystalline makeup. These defects reduce the ability of crystals to move within the metal structure and the metal becomes more resistant to further deformation.
The resulting metal product has improved tensile strength and hardness but less ductility (the ability to change shape without losing strength or breaking). The major cold-working methods can be classified as squeezing or rolling, bending, shearing and drawing.
The possible uses of cold working are extremely varied including large flat sheets, complex folded shapes, metal tubes, screw heads and threads, riveted joints and much more.
What You Need To Know About Cold Working
- Cold working or work hardening is the process of strengthening a metal by plastic deformation at temperatures below the re-crystallization temperature.
- Cold working is carried out below the re-crystallization temperature. As such, there is no appreciable recovery.
- Cold working process improves ultimate tensile strength, yield and fatigue strength but reduces the corrosion resistance of the metal.
- Most of the cold working processes decrease mechanical properties of metal like elongation, reduction of area and impact values.
- Cold working is preferred where work hardening is required.
- In cold working, there is internal and residual stresses build up in the metal.
- Cold working does not require pickling because no oxidation of metal takes place.
- Cold working requires more energy for plastic deformation.
- Embrittlement does not occur in cold working due to no reaction with oxygen at lower temperature.
- The uniformity of the metal is low after cold working.
- There is no appreciable metal recovery that happens in cold working.
- The stress required to cause deformation is much higher.
- Cold worked parts carry better surface finish.
- Crystallization does not take place.
Also Read: Difference Between Open-Die Forging And Closed-Die Forging
Difference Between Hot Working And Cold Working Process
| BASIS OF COMPARISON | HOT WORKING | COLD WORKING |
| Description | Hot working is the process of plastically deforming a metal above the metal’s re-crystallization temperature. | Cold working or work hardening is the process of strengthening a metal by plastic deformation at temperatures below the re-crystallization temperature. |
| How It Is Done | Hot working is carried out above the re-crystallization temperature and below the melting point. Hence the deformation of metal and recovery take place simultaneously. | Cold working is carried out below the re-crystallization temperature. As such, there is no appreciable recovery. |
| Tensile Strength | If hot metal working process is properly done, it does not affect tensile strength, hardness, corrosion and fatigue resistance of the metal. | Cold working process improves ultimate tensile strength, yield and fatigue strength but reduces the corrosion resistance of the metal. |
| Mechanical Properties | Hot working refines metal grains resulting in the improved mechanical properties. | Most of the cold working processes decrease mechanical properties of metal like elongation, reduction of area and impact values. |
| Preference | Hot working is most preferred where heavy deformation is required. | Cold working is preferred where work hardening is required. |
| Internal And Residual Stresses | In hot working, no internal and residual stresses build up in the metal. | In cold working, there is internal and residual stresses build up in the metal. |
| Pickling | Heavy oxidation occurs during hot working therefore pickling is required to remove oxide. | Cold working does not require pickling because no oxidation of metal takes place. |
| Energy | Hot working requires less energy for plastic deformation because at higher temperature, metal become more ductile and soft. | Cold working requires more energy for plastic deformation. |
| Embrittlement | There is chance of embrittlement by oxygen in hot working hence metal working is done at inert atmosphere for reactive metals. | Embrittlement does not occur in cold working due to no reaction with oxygen at lower temperature. |
| Uniformity | The uniformity of the metal is very high after hot working. | The uniformity of the metal is low after cold working. |
| Metal Recovery | Deformation of the metal and its recovery happens simultaneously in hot working. | There is no appreciable metal recovery that happens in cold working. |
| Stress Required For Deformation | Due to higher deformation temperatures, the stress required for deformation is much less. | The stress required to cause deformation is much higher. |
| Surface Finish | Due to oxidation and scaling, poor surface finish is obtained. | Cold worked parts carry better surface finish. |
| Crystallization | Crystallization takes place. | Crystallization does not take place. |
Also Read: Difference Between Forging And Casting
Hot Working
Advantages Of Hot Working
- Pores may reduce in size or close completely during deforming.
- Metals regain softness and ductility.
- Hot working is a fast, reliable and economical process.
- The metal size and shape can be easily changed.
- It is applicable for mass production work.
- Stresses and other defects can be minimized.
Disadvantages Hot Working
- It is difficult to attain dimensional accuracy due to uneven shrinkage of metal
- Handling and maintaining of hot metal is difficult and troublesome.
- Some metals cannot be hot worked because of their brittleness at higher temperature.
- Hot working does not get a required surface finish.
- Due to high temperatures, metal life gets reduced.
- The process may form scaling and oxidations.
Cold Working
Advantages Of Cold Working
- Smooth surface finish can be easily produced.
- Accurate dimensions of parts can be maintained.
- Strength and hardness of the metal are increased.
- Dimensional accuracy can be maintained.
- Directional properties can be imparted into the metal.
- Strength, fatigue and wear properties are improved through strain hardening.
Disadvantages Of Cold Working
- Heavier and more powerful equipment is required.
- Undesired residual stresses may be produced.
- Higher forces are required to initiate and complete the deformation process.
- Metal surfaces must be clean and scale-free.
- Components of cold working have poor resistance to shocks and vibrations.
Also Read: Difference Between Coining And Embossing Metal Sheet Operations