Like making things with metal? Whether you create knives, machinery parts or decorative artwork in your shop, you're going to want to heat-treat your products.
Heat treating is a crucial step in finishing a metal product for quality and durability. It involves heating (as you probably guessed) and cooling metal to improve properties including hardness, softness, strength and wear resistance.
Complex science is at work in heat-treat processes. But you don’t need a PhD to gain a working knowledge of what it’s all about. In this article, we’ll explore the basics of heat treating for metalworking hobbyists.
What Is Heat Treating?
Heat treating is a process that alters physical, chemical, or mechanical properties. There are several types of heat treatments (see below). But the essence remains the same: Heat a part or product at a specified rate up to a certain temperature. Hold at that temp. Then cool at a specified rate.
Depending on the variables in these steps, the result gives your product the properties called for by the application. In the case of knives, heat treating can improve strength and durability and help keep the blade from wearing down too quickly.
Just How Hot Is a Heat Treatment?
Well, how hot do you think you need a metal to change its microstructure? You might be dealing with temperatures from a few hundred degrees F to 2,250F!
Benefits of Heat Treatment
It might seem like a simple thing, heating up and cooling metal. But heat treating fundamentally alters physical and chemical properties. Potential benefits include:
- Hardening: Creating cutting tools and other products that can withstand impacts on the job.
- Softening: Improving the formability of metal products, such as automotive parts.
- Strengthening: Reducing brittleness to make metal less prone to cracking.
- Improved wear resistance: Making metal more durable for stressful application environments – automotive parts, for example.
- Increased corrosion resistance: Reducing the likelihood of rust and other forms of deterioration.
Types of Heat Treating
So, how many different ways can you heat and cool a metal product? You might be surprised at the variety of types of heat treating. Most fall into four categories:
Flame or electromagnetic induction hardening are some of the most common heat treatment processes. The hardening process involves heating a metal without melting, then rapidly cooling. The process transforms molecular structure to be more stable and crystalline. How long you hold the product at the target temperature depends on the desired hardness level, the thickness of the metal, or the depth of the required hardening. For example, you may only need to case harden the surface of a part.
Either way, you lock in the hardening of your metal by quenching – submerging the part into cool oil, water or brine to bring the temperature down fast.
Unlike hardening, annealing softens the metal to make it easier to machine and less likely to crack. The process also involves subjecting the metal to high heat. But unlike hardening, you cool the metal slowly.
Annealing is a common process for aluminum, copper, steel, and brass.
Normalizing is another way to make metal more machinable. In this case, it helps improve the grain size in ferrous (iron-containing) metals.
Again, you heat the metal up to a specified temperature and hold it there for a designated duration. But this time you let it cool at room temperature. The result refines the microstructure and improves mechanical properties.
Once your material has been quenched, as in the hardening process, it may be too hard and brittle to be useful.
To reduce these internal stresses, tempering involves reheating the material to a temperature below its critical temperature.
Stress relieving is like tempering. But it's more to improve the stability and dimensional accuracy of s finished product.
In this case, you heat the metal back up to a level below its critical temperature limit (sometimes higher than in tempering) and hold it there for a time. Think of it like giving your metal a nice, hot, stress-relieving bath.
3 Key Steps of the Heat Treating Process
You might have noticed a pattern in those types of heat treatments. Each of them follows a similar process. It usually goes something like this:
Heat the material to just below its critical, microstructure-altering temperature. You're literally getting warmed up.
This is where the real metallurgic magic happens. Heat the material above its upper transformation temperature, transforming the microstructure into a metal phase called austenite.
3. Quenching or Cooling
Dunk the material in a liquid or gas for rapid cooling, called quenching. This step is key to prevent the microstructure transformation from being reversed. As described above, other heat treating processes call for slower cooling.
Which Metals Are Well Suited to Heat Treating?
Heat treatments are good for many different metals for many different reasons.
Steel is the most common subject of heat treating. Iron-carbon alloy steels come in many varieties, from low-carbon, high-chromium stainless to basic carbon steel to higher-carbon tool steel like 01 tool steel. Each type responds differently to heat treatments. Ubiquitous can be heat treated to improve strength, toughness, and wear resistance.
Aluminum alloys are lightweight, corrosion-resistant metals used in aerospace, automotive, and construction. Aluminum heat treating increases strength and hardness.
Copper and brass are both highly conductive metals used in electrical applications. They can be heat treated to improve their strength and hardness, as well as to change their color.
Titanium. Strong and lightweight, titanium is popular in aerospace, medical implants, and sporting goods. Heat treating titanium can increase its strength and toughness.
Common Applications of Heat Treating
Let’s take a quick look at a few examples of heat treating.
Hardening and Tempering Tool Steel
Heat-treated steel is common in a variety of applications, including construction, automotive, and aerospace.
After you harden steel through heat treatment, it may be too brittle for some applications. To increase its ductility and toughness, temper it. This process is important for hand tools, knives, and automotive parts.
Aluminum heat treating makes it more suitable for use in structural components and other load-bearing applications.
Annealing Copper and Brass
Annealing copper and brass increases the ductility of the metal, making it easier to work with and less prone to cracking.
Key Considerations for Heat Treating Success
Heat treating is a complex process that varies by application and material. With the right equipment and the support of experts, it’s possible for novices to learn to do their own heat treatments. A few things to keep in mind for quality control:
- The recipe. Tight control over temperature and time at temperature are essential to get the desired microstructure. Here's a good rundown of common heat treat recipes.
- Quenching media. The method used to cool the material, such as oil or water, has a significant impact on the final properties of the material.
- Cleanliness. Metalworking can be messy business. Clean your material thoroughly before heat treatment to remove any contaminants that could affect the final properties.
Additional Heat Treating Resources
Each different type of metal will have a slightly different heat treatment process. Tracking down all the various recipes is a challenge for newcomers, so we put together a list of further resources and guides for you to dive into:
- Heat Treating 410 Stainless Steel
- Heat Treating 416 Stainless Steel
- Heat Treating 440C Stainless Steel
- Heat Treating 4140 Steel
- Heat Treating 52100 Steel
- How to Heat Treat D2 Steel
- Heat Treating O1 Tool Steel
- Heat Treating 17-4 Steel
Saving Time in Heat Treatment
So, given the quality concerns, should you consider doing your own heat treating for knives or other metal products you make? A commercial heat treater will often have long lead times. And then there's the cost.
While there's a learning curve and an investment in equipment for heat treating, doing your own can save you time and money in the long run. Want to go for it? Hot Shot Ovens provides a range of fast-heating heat treating ovens with excellent temperature uniformity. All with Cool-to-Touch Technology for safe placement anywhere in your shop.
If you decide to try your own heat treating, feel free to contact our experts for help getting started.