When handling a metalwork project, metallurgists tend to deal with several physical properties. However, temperature is among the most prominent on that list.
When it comes to temperature treatments, two of the most prominent are hot working and cold working. Both can be applied to achieve similar objectives, but they also differ on many levels. This piece will examine the lines of convergence and divergence between hot working and cold working, helping metallurgists and manufacturers discern which might be better in different scenarios.
Temperature Treatments For Metals
Temperature treatments go a long way in optimizing the success of the overall metal forming process. These treatments generally involve taking metals and subjecting them to multiple heating and cooling cycles, all based on the metallurgist’s objectives and desired outcomes.
Generally, temperature treatments help to optimize several aspects of a metal’s characteristics. Most metallurgists focus on cold vs hot working, but these processes don’t necessarily tell the entire story. All in all, everyone wants to create a material that is reliable and can easily operate based on their requirements.
Of course, we can’t go into the hot vs cold working debate without also shedding some light on their umbrella term – metal forming. Used in multiple fields (from automotive to manufacturing to construction and more), metal forming is a process that involves modifying and changing a metal’s shape to meet a specific set of requirements.
The objective of metal forming is to create metal components that have specific geometrics and specifications, all based on what the manufacturer wants.
Cold Working vs Hot Working: Process Comparison
Now that we know what temperature treatments and metal forming help to achieve, let’s examine the main issue – how do these operations work?
Understanding Hot Working
In a nutshell, hot working requires an elevated temperate environment where material can be disassembled and shaped based on specifications. As expected, metallurgists and manufacturers heat the material, using the heightened temperatures to break it down and capitalize on its malleability.
In many hot working examples, metallurgists aim to capitalize on the material’s optimal plasticity to achieve different structures for it going forward. Hot working is done using different materials – from rollers and hammers.
Manufacturers have their pick of the litter when conducting hot working operations. Some of its most popular variants include:
- Hot Forging: In hot forging, metal ingots or billets are mostly used. As expected, the metal is heated to a point where its temperature rises above the regular crystallization temperature. Then, it is hammered and shaped into the desired shape.
- Hot Spinning: In hot spinning, a metal blank is rotated on a lathe and properly heated to an appropriate shape. The process is mostly used for designing symmetrical metal tools.
- Hot Rolling: As for hot rolling, the metal is essentially put through different rotating rolls. The process helps it to achieve shape modification – although other effects include a change in its mechanical properties and a refinement of its grain structure.
- Hot Drawing: With hot drawing, a heated metal is pulled through a die. This process essentially helps cut down on the metal’s diameter, thus allowing the metallurgist to change its shape. With benefits such as an improvement in mechanical properties and an optimization of a material’s finish, hot drawing is usually applied in developing rods, tubes, and even some metal wires.
- Hot Extrusion: Also mostly applied to metal billets, hot extrusion involves forcing a heated material through a die to achieve the desired results. The metal is heated to a temperature where it is ductile enough for proper extrusion, and this process is especially notable for producing items like tubes that need to have constant cross-sections.
For most scenarios, the hot working process tends to follow this sequence:
When you decide to begin with this operation, you want to start by heating your metal – or whatever material it is you’re working with. This temperature is usually above the material’s normal crystallization temperature, where the material shows the highest level of plasticity.
Metallurgists can heat the material using different tools – from furnaces to induction heaters and more.
After the initial heating step, metallurgists take the material and subject it to different deforming techniques to change its shape across the board. This deformation process is where a metallurgist can choose from the different types of hot working.
The shaping step comes next, and as its name suggests, it is where the manufacturer forms their desired structure.
You can shape a material differently – from bending to compression and stretching. In some hot working examples, metallurgists could even combine two or more of the shaping techniques. In a nutshell, the metallurgist’s objective is what really matters here.
While many manufacturers see this as an optional step, we at Tuolian Metal include it as well.
After shaping, the structure can be annealed. The objective here is to help relieve internal stress and also optimize its mechanical properties. Annealing involves a gradual change in temperature – from hot back to cool. All in all, achieving a more refined shape and cutting internal stress is what a metallurgist looks to achieve here.
Metallurgists can also add extra finishing steps – from grinding to machining and polishing – all based on what they hope to create. The objective of finishing is to ensure that the right mechanical properties are achieved, as expected.
Once again, keep in mind that these steps are subject to change. You can always reach out to us at Tuolian Metal to learn more about how we work.
Just as well, it is worth noting that you need to refine specific factors – including and especially deformation rate, temperature control, and more – to achieve the desired outcomes. Your technique, the material being developed, and other factors will help to provide clarity on this.
The Cold Working Procedure
Also known as cold forming or cold deformation, cold working is a process of manipulating the structure of metals as well. The difference of course, is that in cold forming, metals are held either at room temperature or at a point where they are below the metal’s crystallization temperature.
As expected, the process is used to achieve shape differences and implement changes to a material’s mechanical properties as well.
Cold working has several variants – in fact, pretty much every variant of hot working is available here. As long as you remember the temperature difference, you’re good to go. However, you should note the following two cold working processes that might not be present in the former:
- Cold Bending: Cold bending is a procedure that is usually applied to metal sheets, plates, and bars. Here, the material is deformed into a curved or angular shape at room temperature. It is usually employed in the creation of items such as pipes, tubes, and others that can be applied in construction.
- Coining: In situations where accurate and detailed patterns are required on the surface of the material, coining is employed. Here, the metal is placed between two dies, then a compressive force is applied that will leave its mark on the surface. The process is mostly used in producing items like medals and coins – all of which can be decorative or symbolic.
When metallurgists decide to implement cold working, they usually go through the following steps:
The first step of the cold working process is to ensure that the material is properly prepared and ready to be worked on. The preparation process involves steps such as machining, cleaning, and cutting the material to the desired shape.
The material, already prepared, is then subjected to different forces – from compression to bending and more – in order to deform it and produce a different shape. As expected, this is where metallurgists determine which of the cold rolling types to implement.
Due to the external forces being applied to the material, it is important for it to also undergo plastic deformation. In this process, the material’s physical properties essentially change, but it doesn’t reach the point of fracturing. The material is put through a die or some other tool and essentially forced to take that tool’s shape.
Among the many effects of cold working, you have to deal with the possible development of faults within your metal’s internal structure. And, over time, dislocation density could also raise. This leads to strain hardening – a phenomenon where the metal’s strength increases while its ductility also drops.
Also an optional step, intermediate annealing helps to get rid of any internal stress that builds up in the cold working process. This is especially true for the strain hardening process, where possible impurities and defects could be introduced to the material’s structure.
With annealing, the metal is heated to a specific point, then slowly cooled to provide additional relief.
Following it all, the material is made to go through a few finishing processes. Treatments like grinding and polishing are required to ensure that the material is able to achieve the desired specifications and build.
Cold Working vs Hot Working: Stress Buildup
There are several things that affect the overall outcome of a metal-forming process in the long run. One of those is stress buildup. Essentially, stress buildup describes the presence or otherwise of internal stress when a material goes through the usual metal-forming process.
When you’re considering the hot worked vs cold forged debate, you want to know how both processes affect the stress levels in your material.
Generally, stress buildup in hot working is quite low compared to cold working. This difference is because, in hot working, the material is essentially subjected to higher temperatures – a phenomenon that would lead to greater movement and dislocations, as well as a reduction in resistance to deformation.
Just as well, the increase in temperature that materials experience during hot working will ensure that they can easily overcome strain hardening, thus becoming more ductile and less prone to stress accumulation. All in all, the hot working process brings a lower propensity for material cracking.
To be fair, cold working will do a great deal to improve strength. But, a consequence of that will be the risk of internal stress – something that is little or non-existent in hot working. So, if excessive stress is applied during the cold working process, you might see the material become very prone to cracking.
That said, metallurgists can employ the annealing process to help with this. In cases where the material being produced is needed to have a lower stress buildup profile, annealing can be used to help slow things down significantly.
Cold Working vs Hot Working: Temperature
Of course, this is a bit of an obvious one – even the names say it; when working out in the cold vs hot working process, you know you’d be dealing with different temperature processes.
Hot working’s process requires very high temperatures. These temperature ranges vary based on the material and its physical properties – although, for metals, temperatures usually vary between 50% and 80% of the material’s melting point.
In the case of steel, you have a material with a recrystallization temperature of around 900°C to 1200°C (1650°F to 2200°F). So, a proper hot working process for the material will probably be conducted between 1200°C to 1300°C (2200°F to 2400°F).
As for cold working, you’re essentially going the opposite direction – a key difference in the cold work vs hot work comparison. The process operates usually at room temperature, where the material is kept very cold. The cold working process is usually done at a temperature that’s below 50% of the material’s melting point.
So, for steel, the cold working process will need to take place at a temperature that’s below the 1200°C to 1300°C (2200°F to 2400°F) range mentioned earlier.
Cold Working vs Hot Working: Product Recovery
Many people who look into hot vs cold working and how it works will also want to examine the performances of both processes on product recovery. Essentially, this comparison looks into both processes as well as their effects on a product’s microstructure and its ability to revert back to its original properties.
In hot working, the material is made to undergo a significant amount of deformation. However, despite this, the material ends up recovering significantly over time. Thanks to the heightened temperature, the material can easily recrystallize, meaning that there is a formation of new strain-free grains from the original material.
Thanks to this recrystallization process, deformation’s effects are eliminated, and the material’s original properties are properly reinstated. Besides this, processes like annealing and other heat treatments can also be applied to help the material regain its form once and for all.
Product recovery is also possible in cold working. However, it is worth noting that the process is more complex than what you’d have in hot working. Since the process is applied at room temperature, it will be more challenging to eliminate the effects of consequences such as strain hardening and component dislocations.
And, in cases where the material deformation from cold working is extensive, it will be almost impossible to reverse the changes made to the material’s mechanical properties.
Once again, this is where annealing comes in for cold working. Since annealing involves significant product heating, it opens the possibility of product recovery and recrystallization. And, when you consider the other benefits involved – internal stress relief, improved toughness, reduced dislocations, etc. – you’d find that it works pretty well.
So, in general, hot working provides a better opportunity for product recovery after its mechanical properties have been changed. However, while the process is much more difficult for cold working, the presence of annealing can help to ameliorate things significantly.
Cold Working vs Hot Working: Uniformity
Understanding the uniformity dynamic is another reason why you would want to make a significant distinction between working out in cold vs hot situations.
Generally, hot working provides a more uniform level of deformation across the material. This is especially true of metal, of course – once the metal gets heated considerably, it softens up, making it easier for the material to flow and for the stress to be evenly distributed.
Thus, when the material is being changed, it’s easier for a metallurgist to ensure that it is uniform and that stress is being distributed evenly. Regardless of the material’s surface area, hot working – if applied properly – can ensure optimal uniformity in the final shape.
The same can’t necessarily be said about cold working. With the temperature being significantly lower, the material itself displays low levels of malleability. With greater resistance to plastic deformation, the material can develop localization in strain distribution – especially when the material is being treated to form complex shapes.
It is also worth noting that certain cold processes – including cold drawing and cold rolling – can introduce uneven deformation patterns, thus causing variations in thickness, shape, and other characteristics.
Just as well, the level of uniformity seen in cold working can also be influenced by several factors – ranging from the material’s internal structure and even the specific process of cold rolling employed. If uniformity is a significant consideration, it is important to ensure that the process is handled by a skilled metallurgist.
At Tuolian Metal, you can rest assured of our ability to help you get the right levels of uniformity – even if you believe that cold working is the best way to go.
Hot Working vs. Cold Working: Advantages And Disadvantages
With all that’s been said, let’s take a quick, proper look at the advantages and disadvantages of hot working vs cold working steel and other materials
Advantages Of Hot Working
- Higher Material Ductility: Thanks to the heightened temperature involved in the hot working process, whatever you’re working with will become significantly more ductile and easily shaped. This makes it easier to manipulate complex materials without so much force.
- Less Force Required: As higher temperatures lead to a softer material, it’s much easier to shape it into the required form. You don’t need more force, and particle flow can be optimized.
- Grain Refinement: Hot working refines a material’s grain structure, thus optimizing its mechanical properties.
- Better Formability: With the ability to form and shape larger sections, hot working improves its overall formability.
Disadvantages Of Hot Working
- Oxidation Risk: With hot working requiring higher temperatures, there is always the possibility of exposure to air – leading to oxidation and scale formation. This eventually causes additional layers to grow on the material’s surface, which might need additional cleaning down the line.
- Limited Dimensional Accuracy: Since materials and particles flow freely during hot working, there is the risk of a reduction in accuracy while shaping the material. So, be sure to get a skilled metallurgist to handle this.
- Lower Surface Finish: Another consequence of higher temperatures is the possibility of having a reduced finish quality. Interactions between the material and other tools can cause irregularities, which could require additional treatment or surface finishing to achieve the desired outcome.
Advantages of Cold Working
- Optimal Hardness & Strength: Cold working can do a great deal to improve the hardness and strength of a material. The process leads to a higher resistance to deformation, which would eventually improve properties such as tensile strength and hardness.
- A Better Surface Finish: When looking at hot vs cold working, remember that the latter provides a much better and more precise surface finish. Since temperature drops, you can easily control how the material is deformed. Thus, you get a greater level of dimensional accuracy and surface quality across the board.
- Enhanced Product Elasticity: Cold worked materials actually show higher levels of elasticity. If the materials need to retain their shape after deformation, this property is highly underrated.
Disadvantages Of Cold Working
- Lower Ductility: A major consequence of the cold working process is the drop in ductility of the material. Just as well, the material also becomes more prone to cracking and developing other defects, which could affect its quality in the long run.
- Risk Of Material Fatigue: Cold worked materials can become even more susceptible to material failure. This is especially true in high-stress applications, where internal stress buildup eventually affects the material.
- Less Flexibility: Cold working doesn’t necessarily give enough space and accommodation for complex and more intricate materials to be formed. While annealing could help to solve this problem, it isn’t necessarily a sure solution.
Hot Working vs Cold Working: Effects On Crystal Structure
While we already went through the major differences between hot and cold working, we also need to examine the effects of cold working vs hot working on crystal structure.
As we all know, both processes cause deformation at different temperatures. Thus, crystal structure variations also exist that need to be considered. So, besides the hot working vs cold working advantages and disadvantages, below is a breakdown of what you need to know:
The heightened temperature at which hot working is being operated is its biggest peculiarity. This temperature makes it easy for atoms to move and dislocate within a material’s crystal lattice.
Due to this, several significant changes could include:
- Recrystallization: Hot working provides a stronger basis for recrystallization – the process where new strain-free grains are developed within the material’s surface. Recrystallization occurs as grain boundaries move and additional grains grow. Over time, this could cause a more uniform grain structure, thus eliminating all the effects of any deformation that happened before.
- Grain Growth: Hot working might also cause existing grains to expand – a direct consequence of increased atom mobility. But, you should know that a shorter exposure time to high temperatures might prevent this from happening.
- Recovery: In a nutshell, recovery is a partial annealing process that happens during the hot working stage. It involves dislocation removal and stress reduction within the material, thus helping to restore the crystal structure.
All in all, hot working can give you a more refined grain structure, while also ensuring recrystallization and eliminating any effects of deformation. The crystal structure becomes more homogenous as well, so you get higher refinement in the material’s mechanical properties.
The cold working process leads to a restriction of mobility in atoms, as well as dislocations within the crystal lattice. At the end of the day, the crystal structure gets the following effects:
- Strain Hardening: Cold working introduces significant plastic deformation, resulting in the accumulation of crystal structure dislocations. Over time, these dislocations will accumulate, thus creating barriers to the movement of other dislocations, increasing the material’s strength and hardness. This process is known as strain hardening.
- Deformation Twins: In some metals, the formation of deformation twins could also happen in cold working operations. Twins are regions with a mirror image atomic arrangement that contribute to the material’s overall strength and hardening.
- Reduced Grain Size: The cold working process also includes fragmentation of existing grains – possibly leading to a drop in overall grain size.
- Texture Development: In some cases, cold working could also lead to a more optimized crystallographic texture or orientation. An alignment of the deformation process and the crystal planes will cause the material to become anisotropic, with different mechanical properties in varying directions.
All in all, both hot working and cold working can have effects on a material’s crystal structure. However, the resulting effects will differ – again, due to the varying temperatures and deformation processes.
Hot and cold working are highly complex operations that require a significant amount of preparation. However, when done right, they have significant benefits for materials.
At Tuolian Metal, we provide the best hot and cold worked materials in the market today. And, we can also help with a consultation to help you understand how best to apply both operations. Need any further help? Contact us today!
Frequently Asked Questions
What is the best scenario for hot working?
Hot working is best suited for scenarios where ductility and formability are the priority. Also, metallurgists who desire complex structures and need enough flexibility will find it to be more ideal.
Which is more expensive? Hot working or cold working?
Generally, hot working tends to cost less than cold working. When comparing examples of hot working and cold working, you’d find that the former tend to use less energy than the latter. Just as well, the materials needed for hot working – whether it is the furnace or the heating system and more – also don’t cost as much as the specialized materials needed for cold working.
Besides all of this, hot working doesn’t require much surface preparation. And, considering that it generates far less waste than cold working, you’ll find it more accommodatable.
When should I choose cold working over hot working?
Cold working is a preferred option to hot working in situations where you need to optimize material strength and hardness. And, if you require better surface finishing and more accurate dimensioning, you’d find cold working to be more advisable.