The Basics of Heat Treating: A Comprehensive Guide
What is heat treating?
Heat treating is a process used to alter the physical and sometimes chemical properties of a material through heating and cooling. It is commonly used in the manufacturing of metal products, but can also be applied to other materials. The goal of heat treating is to improve the material's strength, hardness, ductility, and other properties, making it more suitable for its intended use. There are several different heat treating techniques, each with its own unique benefits and applications.
Metallic materials consist of grains or crystallites at the atomic level. The size or nature of the grain composition is one of the most effective factors that controls the behavior of the metal. Heat treatment manipulates these crystals by controlling the rate of diffusion or cooling. Through proper application, this changes the mechanical behavior of the metal you are working with such as making it more hard or elastic depending upon the desired use of the tool.
The different types of heat treating processes.
There are several different types of heat treating processes, each with its own unique benefits and applications. The usual processes are annealing, normalizing, quenching, tempering, and case hardening. Annealing is a process used to soften a material and improve its ductility, while normalizing is used to improve the material's strength and toughness. Quenching involves rapidly cooling a material to increase its hardness, while tempering is used to reduce the material's brittleness and improve its toughness. Another less common heat treatment process is case hardening and is a process used to create a hard outer layer on a material while maintaining a softer, more ductile core.
Annealing is the process of controlling the heat dissipation of a metallic project in a manner that allows the heat to dissipate slowly. This allows the metal grain to grow and makes the metal softer. Annealing has three main stages.
- Recovery stage.
- In this stage, the metal is heated. This relieves stresses that may be existing in the metal crystalline structure.
- Heating the metal to just below critical temperature causes new grains to grow stress free.
- Grain Growth
- Cooling the materials slowly (slower than air) causes new grains to develop. This makes the material more malleable.
Why Anneal? Annealing is useful for not only relieving stress that may have been accrued during the forging process, it is also useful for shaping and refining the profile or finish of the knife more easily since the material is in a more relaxed state.
Normalizing and Annealing are similar in process however; the principle difference between the two is that normalizing is done to improve the metals mechanical properties while annealing is used to soften the metal to make it easier to work.
The process of normalizing may also be refered to as thermal cycling and is the process of bringing the metal up to specific temperatures, soaking if needed depending upon the type of metal and thickness and then allowing the metal to air cool.
This process changes the size of the crystalline grain structure and helps make it more uniform throughout the steel as well as improves its strength and hardness.
Quenching is the process of rapidly cooling a metal to adjust the mechanical properties of its original state. This is done by heating the material to a certain temperature (depending upon the metal used this will vary) and then rapidly cooling it in water, oil, air or other fluids.
This process allows the metal to bypass undesirable phase transformations and can reduce the grain size of materials increasing the hardness of the material. This process can make your project excessively hard and brittle due to an overabundance of martensite and will need to be followed up with a tempering treatment to alieviate some hardness.
Tempering is usually performed after hardening a metal to alleviate stress and hardness the metal may have acquired during previous heat treat operations. This process involves heating the metal up to the recommended temperature, which varies according to the steel employed, and soaking it at the prescribed temperature for a period.
For example, if working with 1095, the recommended tempering range is between 400-500 Fahrenheit and is recommended to be held at this temperature (soaked) for 2 hours. Allow the steel to air cool and then repeat the process.
This allows some of the hardness to be reduced from the steel and will make the material less brittle.
The benefits of heat treating.
Heat treating offers a wide range of benefits for materials and is a required process for bladesmiths. By altering the material's microstructure through heating and cooling, heat treating can improve its mechanical properties, such as strength, hardness, and toughness. This makes the material more durable and resistant to wear and tear. Heat treating can also improve the material's resistance to corrosion and other forms of degradation. Additionally, heat treating can be used to create specific surface properties, such as a hard outer layer or a smooth finish. Overall, heat treating is an essential process for many industries aside from bladesmithing, including aerospace, automotive, and manufacturing.
Factors that affect the heat treating process.
The heat treating process is affected by a variety of factors, including the type of material being treated, the desired properties of the final product, the heating and cooling rates, and the temperature and duration of each stage of the process. Other factors that can impact the heat treating process include the size and shape of the material, the type of furnace or equipment being used, and the atmosphere or environment in which the process is taking place. Understanding these factors is crucial for achieving the desired results and ensuring the quality and consistency of the final product. For example if you are working with a large project and are unable to get the heat even prior to quench you may end up with softer spots in your blade.
When heat-treating in a forge you will need to get your blade to critical temperature ideally using dispersed heat. You can accomplish this a couple of ways but one of the more common is to insert your blade into a steel tube or piece of angle iron covering your blade while in the forge. This will keep you from getting hot spots in your blade that can affect the process.
If you have a thermo-coupler for your forge you can get a lot more precise in your measurements of temperature but if you do not there are a couple of ways to generally see how hot your steel is getting. First, you can use a magnet. Steel will use its magnetism right around 1420 degrees Fahrenheit. Depending on the steel you are working with you will need to go above that temperature but that is a decent benchmark. Another way is to use table salt, which melts at 1475 degrees.
Either way you will be guess quite a lot when it comes to doing your heat treat with a forge. That does not mean it will not or that it cannot be done well but you will likely never get the exact science down as you can with a heat treat kiln.
With a heat treat kiln, you can dial in the exact temperature that you want for the steel you are working with. Say, for example, you are using a steel that needs to be soaked at 1500 degrees for 10 minutes; you can do that easily with a Kiln. Most of them even have timers built in to make your job easier! Try getting that exact in a forge.
In addition to having the heat dialed in at a scientific level, you have the benefit of being able to do multiple blades at once. Look at the Jen-Ken Vertical Air Bath 16 for example. You can easily fit five or 6 blades in there simultaneously and work your thermal cycles down. That can save you a lot of money in propane or coal depending on what you are working with.
Technological advances are also making the job of getting an exact heat treat on your knives a lot easier. Be it the cool to touch technology of hot shot or the innovation in controls from Jen-Ken Kilns and others, kilns are making it safer, more exacting and easier than ever before to produce high quality steels that you and your customers can depend on.
As with many things in making blades, there is, no one size fits all correct answer. You can heat treat with a fire pit if you are determined enough. Ancient cultures did not have the Hot Shot kilns or the Jen-Kens they did what they needed to do with a bed of coals and likely some poor kid pumping bellows
Choosing the Right Heat Treat Oven for Knife making
If you are tired of getting inconsistent results from forge heat-treating and are looking to make the next step in getting consistent results and upgrading your quality then this guide is for you. There are multitudes of factors that go into your decision such as:
- How large do you need it to be
- 120v or 240v
- Side or top opening
- Heat range
- Hobbyist or professional
How large of a heat treat oven do I need?
I have heard it said and it is true that you will always want to go larger than what you think you need. How many times have you had to wedge a blade in because of an odd shape or lengths? If you are like me, more times than I had certainly planned on. That being said you would want to examine what you will be doing in the future as well as currently when thinking about your investment. Ovens like the Hot Shot 18" Deep Knife Oven allow for more blade room in a horizontal placement. The Jen-Ken vertical Air bath 30 allows up to 22" of blade in a vertical placement however. The Jen-Ken gets the hat tip in this category due to its 11x30 interior. The expanded interior of the Jen Ken Kiln vertical air bath 30 allows multiple blades to be batch treated which can save a lot of time versus doing the heat treatment for blades individually.
What voltage should I get?
Here there is no clear answer. If you have the capability or can get the wiring done for it, 240v would probably be your best bet. Why? Well, it is going to burn energy heating anyway and if you are investing into a heat treat oven then you are probably moving past the hobbyist level and turning this into a business of sorts. Time is always money and the less time you have to spend the more opportunity you will have to make money. There are no real clear winners for this question, as you will find this option as well as 110v configurations pretty standard among all heat treat ovens.
Side or top opening for my heat-treat oven?
Ultimately, this comes down to personal preference. Jen Ken is the only kiln that I currently know of that has a vertical orientation. This design allows for 360 degrees of uniformed heating and can lead to reduced warping but there is the potential for an unpleasant side effect you will want to keep in mind. That unwelcomed 1900-degree blast of heat to the face when you crank this thing open but the good thing is you could roast a quick hot dog while your heat treating your blades! Due to heat rising the side opening may be a better option for some folks. Unless of course you are in the mood for a quick snack. Another thing to consider is space allocation for the kiln configuration. If you have, a plethora of floor space a Jen Ken Vertical Air Bath 30 is not going to be a problem for you but if you are clamoring for floor space, a shelf-able heat treat oven may be a better solution.
No matter what heat treat oven you go with you will need them to go up to a minimum 2200 degrees. The more of a range of temperature that the heat treat oven has the more flexibility you will have as you go through your journey in knife making. For example, if you work with just 1084 right now which requires a quench temperature of 1500 but, in the future, you expand into working with CPM S110V stainless you will need to soak at 2150. With such an investment you will want to make sure, you are getting something that will serve you well in the future as well as the present.
Something to consider when looking into knife making ovens, or kilns if you prefer, is that they use electricity versus propane or coal. If you are still working with forge heating for your heat treat you are undoubtedly aware that when you are thermal cycling you are burning a significant amount of fuel to get as close to the perfect temps for the steel you are working with. If you are only making one or two knifes a year, heat treat kilns are probably not a good investment for you. If you are making many knives, plan to move your knife making hobby into a knife making business or just want to make your quality as on point as possible a heat treat oven is a good investment that will pay off over time. If you cannot afford the cost upfront, there are financing options available.
Hobbyist or Professional
No matter what the level of your knife making skills are a knife-making oven will help to ensure that you are consistently working at temperatures that you can count. This of course helps you make knives at a consistent level of quality, which leads to happier customers, and a product that you are always proud to put out into the world.
No matter what you decide, we are here to help you make products with soul. Remember to have fun, be safe, and always Forge Ahead!
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