It happens that you are not satisfied with the quality of the metal tool you have bought. If you need to increase the hardness or plasticity of the material you can perform such a technological operation as annealing. After detailed studying and preparation, it is possible even at home.
In some cases, we perform the heat treatment of steel not only to increase the hardness but also to reduce it. When the hardness parameter is low the cutting surface of the tool can be jammed in the process of work. On the contrary, if this parameter is too high the metal crumbles due to mechanical stress.
To solve this problem read the article to find out what tools you need to perform annealing of steel at home without professional equipment. Remember that not all types of steel are annealed, for example mild steel. However, you can easily increase the hardness parameter of carbon and steel alloys on your own.
The annealing process is divided into two stages. This first is a heat treatment of metal and the second is cooling. Different metals and steel categories have different structures and that’s why the heat treatment must be performed accordingly.
Two main characteristics of metal can be improved in the process of heat treatment. They are the hardness and plasticity. Follow all the recommendations to perform the process correctly and to avoid mistakes.
When black and blue spots appear on the surface of the metal it means something goes wrong. This way you can spoil the tool. The bright red color of the metal surface will tell you that everything is fine.
The process of heat treatment is not as simple as it looks from first sight. Always pay attention to the structure of the material and its characteristics. Without preliminary analysis and preparation, you are likely to deform the metal piece.
To avoid undesirable results and the deformation of metal which happens very often with mild steel check the category of metal in advance. You can find it on the Internet or from the provider. If you don’t know the information, heat treatment is not recommended in this case.
To heat treat the metal you have to reach very high temperatures. Several heat sources are available for home use. You can choose a special heat treating oven, a torch, or an open fire, but the choice will always depend on the temperature, which must be reached for annealing the definite category of steel.
Note that the heat of the open fire can be not enough in some cases. Always check the melting point and the tempering temperatures of the material before starting the procedure of heat treatment to receive the best results. In the opposite case, you risk deforming the tool.
A heat treating oven
A special heat treating oven is the best for full and reliable annealing. If the oven is computerized you set the required program and leave it. Return after a definite time to get the part, when it’s already been cooled down properly.
It is comfortable in use and suitable for pieces of different thicknesses. Of course, one doesn’t usually have such an oven at home and you have to know the grade of steel to use it. An average temperature from 1450 to 1650 F can be set but to anneal the steel fully you have to know it exactly.
As you reached the desired level of temperature let the piece soak – keep the metal for some time at this temperature. For every inch of diameter let 1 hour to soak. Use this golden rule when the metal piece is not even, always consider the thicker end as the indicator. Turn off the equipment and don’t open the door for several hours to cool the piece slowly.
Annealing with a torch is the easiest and the fastest method, especially for small parts. It’s a common tool and that’s why it’s more accessible to most craftsmen. The grade of steel is not important in this method, as the color of the metal shows the right temperature. For annealing reach orange-red, for heat-treating cherry-red is enough.
Special skills are needed to use a torch and it’s difficult to cope with different thicknesses. This tool takes second place after the oven, which is more reliable and consistent. Keep away the flame from a thin section not to melt it, instead heat the thicker end, let the heat spread through the whole piece.
An open fire
An open fire can be used as a heat source. For annealing, a fire with lots of hot coals is required. Prepare blacksmith tongs to take out the heated steel from the fire. Consider what cooling method is suitable for you and prepare everything beforehand.
When you finish all preparations and have enough hot coals in the fire you can start the heat treatment process and insert the steel piece inside. You can evaluate the temperature of coals by their color. They have to be bright red with a white surface.
Monitor the color of the flame as well. Crimson is perfect, the white color indicates that the temperature is too high and you risk to overheat and even burn the steel. Keep a close eye on the color of the heated steel. Never allow black and blue spots on the surface of the piece, which indicates that the metal is already too soft and plastic.
Pay special attention to elongated tools for example drills or nails. Descend them into the coolant vertically with the end of a bigger diameter. Never put them sideways, as the thinner part will deform and flatten because of the high temperature and fast contact with your insulator.
3 stages of annealing
There are 3 stages of annealing, which change with the increase in temperature. The first is recovery when the material softens and is relieved from inner stresses at quite low temperatures. The recrystallization stage follows when new grains without strain are formed to replace the deformed ones. The final stage is grain growth, when the steel microstructure begins to coarsen.
The cooling process
When you choose a cooling mode think of the material and the results you want to achieve. If you have a need to harden only a definite part for example the cutting edges of the tool don’t temper the entire piece. Carry out the cooling pointwise as well under the cold water flow.
The technological peculiarities can require rapid, gradual or multi-stage cooling. Instant cooling mode uses a single type of coolant. Carbonaceous or alloyed steels are hardened that way. An ordinary bucket, barrel or a bathtub will suit as a container, depending on the dimensions of the metal piece you are hardening.
The method with two coolants when you have to harden the steel of other categories or when you plan not only to harden but also to temper the steel. After heating the metal piece to the required temperature cool it in a container with water. later put it in the second container with mineral or synthetic oil. Never use the oily coolant first because it will ignite from the temperature of the heated metal piece.
It is recommended to think and prepare a coolant before heat treatment. Take any container, fill it with water, but keep in mind that the metal piece must fit completely into it. If you harden consumables made of steel you can use an ordinary cut plastic bottle. For an ax or similar working tools you will need a container with a bigger volume.
Slow cooling options for annealing steel
There are lots of recommendations when water and oil are used for cooling steel. But if you want to receive reliable and safe annealing results think of the following options of slow cooling.
For slow cooling, you can use dry sand, which keeps the temperature of the annealed piece longer. The sand for play or construction will do but it must be pure. Never apply wet sand as it will lead to negative annealing results and can be dangerous for you.
It is very convenient for use and storage – just roll it up and return to the shelf. Several types will do the job: blankets for wood stoves, chimneys, or a roll of insulating ceramic fiber, so you can choose the most available.
Salt solution (brine)
Among typical cooling methods, there is a salty solution or brine. Compared to water, it does the job quicker. In ordinary water, the surface of a metal piece is covered with steam bubbles. The brine gives better and quicker cooling results due to reduced bubble formation. This method is mostly used in production.
Useful tips and precautions
Many working tools do not feature the high quality of steel. Sometimes it’s not possible to harden the steel in the manufacturing process or, on the contrary, this parameter can be overdosed. Everybody knows about drills. If they are not hardened enough they bend. If overhardened, they crumble.
After the heat treatment in home conditions remember some peculiarities about cooling all elongated objects. You have to place them vertically into the container with the chosen coolant. Always immerse such items into the coolant with the thicker end. If the thinner part goes first it is likely to suffer from the compression and will deform.
Before you start to work with a recently purchased tool, you can check the quality of steel at home. Take an ordinary soldering iron. Draw along the steel surface of the tool with the tip of the soldering iron to find any defects.
When the tip of your iron sticks to the steel surface it means that the metal was not hardened enough. Most likely such a tool will deform if you use it often. The opposite situation is when the iron tip glides smoothly on the surface and practically bounces off it. It means the steel is overhardened and heat treatment is required.
Small parts are sometimes close to impossible to cool slowly unless a special heat treating oven is used. There is one more option for home use. Find a bigger piece of metal or two, heat it together with the small piece which is being annealed.
For cooling in the insulator attach the bigger hot parts in close contact with the smaller one. This allows steady slow cooling for the best annealing results. The blocks keep the steel part hot for a long time. The required decrease in temperature is 70 F an hour. The cooldown continues up to 500 F.
Remember that annealing is not the same as tempering. In the first case, the material becomes soft and malleable. In the second one, internal stresses are relieved and its brittleness is reduced. Higher temperatures are used for annealing about 1500 F, whereas 500 F will be enough for tempering.
Normalizing also differs from annealing. Annealing is carried out through a slow cooling process at controlled temperatures while normalizing cooling is performed fast in the open air. The steel becomes partially hard with reduced inner stress and uniform grain structure.
Metal heat treatment can improve the quality of tools or consumables. When annealing steel at home, it is important to remember the basic recommendations not to deform the material and get desirable results.