Hot Metal
Hot metal can also be referred to as 'Heat Treatment' in schools and it is a way of making metals more suitable for processing or for the jobs which they have to do. For example, a piece of high carbon still being used to make a cold chisel must be annealed or softened so that it can be shaped and then hardened and tempered so that it can cut other metals. Effectively the process alters the properties of the material.
The most common types of heat treatment used in schools are:
- Annealing/Softened - Makes the metal as soft as possible to relieve internal stressing and to make it easier to shape.
- Brazing- Brazing is a way of bonding materials together, a filler metal is used and heated at melting point to combine the material.
- Soldering (Soft and Hard) Soldering is a type of brazing which works at lower temperatures. Soft soldering is used to make permanent joints and is the normal way of joining electronic circuit components. The solder flows into the heated metal along the joint, distributes itself by capillary action, and grips the two pieces together when it all cools.Hard solder works is used for stronger joints.
- Welding -Joins together metal pieces or parts by heating the surfaces to the point of melting and fixating them together for example by hammering or Forging.
Hardening and Tempering are also two other common processes used in schools.
Hardening increase the hardness and strength of the material. You would tend to harden a a material to create things like cutting tools, springs or hard wear resistant surfaces. Tempering removes the extreme hardness and brittleness from hardened steel and it makes it tougher so that it can be used. By increasing the tempering temperature it reduces hardness, but increases toughness and the final compromise between hardness and toughness depends on the purpose for which the material will be actually used for.
Below the table shows a useful tempering guideline which displays different colours, temperatures and uses for when you heat the material.
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_When you work with hot metal you generally go through three stages. The three common concepts and principles are as follow:
Although in terms of outcome each process is slightly different the above bullet points would the generic concepts that you would follow.
These three stages mentioned above also can be transferred and referred to when you apply the Annealing process. These are common principles and concepts that never change and relate to the specific type of material you use when annealing;
The main common concepts and principles with Heat treatment in schools are:
Below is an example of me using a Hot metal process.
- Heat the metal (Each metal will have a different temperature)
- Keep it at the specific temperature for the required amount of time
- Cool the metal in the correct way to enable the desired properties
Although in terms of outcome each process is slightly different the above bullet points would the generic concepts that you would follow.
These three stages mentioned above also can be transferred and referred to when you apply the Annealing process. These are common principles and concepts that never change and relate to the specific type of material you use when annealing;
- Mild steel - is heated to a bright red colour, then soaked for a short time, and left to cool slowly
- Tool steel - is heated a to bright red colour, soaked for a short time, and left to cool very slowly in hot ashes. The slower the metal cools the softer it will be.
- Cooper - is heated to cherry red or dark orange colour and quenched in water
- Gilding metal - is heated to a salmon pink colour and quenched in water
- Brass - is heated to a dull red colour and left to cool
- Aluminum- is covered in soap, heated gently until the soap turns black and then left to cool.
The main common concepts and principles with Heat treatment in schools are:
- In every process when heating the metal you always try to identify the colour of the metal when it is being heated. The colour will be the indicator as to when it has reached it's correct temperature. For example with brass it reaches a dull red
- The tip of the flame is always used for each process as it is the hottest and will heat the material faster
- Whatever process you use, you would heat around the material as well as focusing on the material itself to ensure all the material is heated effective. Generally the materials around it would be blocks and bricks
- Putting a sign to indicate that the bricks are hot due to recent use - A simple hot sign can avoid any nasty burns and injury for the next user
- The taps are colour coded with Red representing the Gas and Blue representing the Oxygen
- It would be good practice to turn off the gas and power after you complete a process as you then are not relying on the main power and gas proving system
- There are different types of flux's however they both facilitate the joint and make it stronger and it directs the solder to run wherever it goes
- The main reasoning behind the whole process in general is modify and manipulate the properties of materials as well as hardening and softening
- The higher the temperature the softer the metal the easier the manipulation
- For each process good practice in terms of Health and Safety would be displayed - Wearing appropriate clothing to protect yourself from injury
- The process of Brazing and Soft soldering use similar concepts yet there is a big difference in the temperatures used to heat the material
- Soft and Hard soldering again use similar concepts however Soft melts at 200°C and Hard solder's melting point is 625°C
- Generally speaking the processes collectively are not too complex
- You can virtually join any dissimilar metals
- The bond line can be very neat in appearing displaying a nice finish
- The joints are strong enough for most non-heavy duty use applications.
- Sometimes students may add to much water to make the flux
- The Flux used is dependent on the temperature that you are working with but sometimes this can be forgotten by pupils
- Some students will be careful when maneuvering material especially when it is hot and consequently this may lead to burns and injury - particularly a metal at black heat that might not appear to be hot
- That brazing and soft soldering are the same kind of process because they use the extremely similar methods yet different temperatures are required and students may find this confusing at first if not explain properly
- Sometimes clay bricks are used and they should not heated as they can disintegrate violently
- Ceramic chips, bricks and blocks remain hot for a considerable time after the heat, if a sign has been removed sometimes a user could forget about this and not leave any sign to the next user and as a result this could lead to injury
- Not every material has the same process
- Using a foreign piece of metal in a forge can cause bad fumes
- Some metals are not cooled down properly
- The quenching of hot metals can cause scalding if not handled appropriately
- Students may also think that hot metal work is a process used just for hardening and that metal can only be heated once and bent whilst hot
- Sometimes students do not apply the heat treatment correctly. For example if steel is heated until it glows red and is quenched and it could cause a piece of material to become hard, brittle which means it is likely to break or snap if put under great pressure. If steel is heated until it glows red and is quenched in clean water immediately, it becomes very hard but also brittle. This means it is likely to break or snap if put under great pressure
- In annealing even though mild and tool steel are heated to a bright red heat and soaked for a short time, mild steel is left to cool slowly and tool steel is left to cool very slowly in hot ashes, - as it makes it softer
- In Brazing if a student completes badly brazed joint it can look very similar to a good join and this will have low strength causing the piece to potentially snap in the long term.
Below is an example of me using a Hot metal process.
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