Stainless Steel Heat Treating for Beginners
Getting the results you want as an amateur metalworker is about more than shaping and machining your metal. Whether you like to make knives or other products, heat treating is a must to improve properties such as strength, hardness, and flexibility.
But heat treating can be complex. That’s particularly true with stainless steel, a popular choice for knife makers and other hobbyist artisans.
So we’ve put together this guide to take you through the basics of heat treating stainless steel for knife making or other pursuits. We’ll cover what you need to know about the different stainless steel types and the right heat treatments for your products.
Whether you’re just starting out or looking to improve your skills, this guide will give you the knowledge to make the cut with your stainless steel heat treatments.
Stainless Steel Definition
Stainless steels are alloys—mixtures of two or metals and a nonmetal. Unlike your basic 01 tool steels and other carbon steels, which are high in carbon, “stainless” steel means an alloy steel that’s mostly iron and at least 10.5% chromium. In stainless steel production, the chromium forms an oxide barrier that provides strong corrosion resistance.
Other elements such as nickel, molybdenum, and manganese are present in some stainless steels to improve their properties.
Why Heat Treat Stainless Steel
Corrosion resistance is a key attribute of stainless steel. Other properties, less so. And some of these other properties may be important to what you’re making.
Take ductility, for example. It refers to the ability to stretch or bend under stress without breaking or cracking. Ductility is a valuable property for knives. You can't do much with a broken blade. And you can improve ductility by heat treating.
Other treatments will depend on the type of stainless steel (which we'll get to in just a moment) and the needs of the specific application. But the most common reasons for heat treating these metals are hardening, stress relieving (to offset manufacturing stresses) and improving abrasion resistance.
Types of Stainless Steel
There are lots of types of stainless steel, each with different mechanical properties, hardness levels, etc. You probably don't need or want to memorize them all. But the main types are divided into five categories based on their microstructure and alloying elements.
- Most common type of stainless steel
- Name comes from the mineral austenite, a solid solution of iron and carbon
- Composed of at least 50% iron, 18% chromium, and 8% nickel
- Excellent formability, weldability, and corrosion resistance
- Why heat treat: Relieve stress or improve ductility after cold forming or welding; cannot be hardened
- Examples: 304 and 316 stainless steel
- Name comes from the mineral martensite, a type of iron-carbon phase that forms when certain steels are cooled rapidly.
- Martensitic microstructure provides high strength and hardness, but also makes it brittle and less corrosion-resistant
- Composed of at least 50% iron and 11-17% chromium
- Why heat treat: Increase strength and hardness
- Examples: 410 and 420 stainless steel
- Name comes from the mineral ferrite, a type of iron-carbon phase that forms when certain steels are cooled slowly
- Ferritic microstructure provides good formability, but also makes it less corrosion-resistant
- Composed of at least 50% iron and 10-27% chromium
- Why heat treat: Improve hardness, relieve stress after cold forming or welding
- Examples: 430 stainless steel
- Combination of austenitic and ferritic
- High corrosion resistance and higher strength than austenitic
- Contains 21-23% chromium and 4-6% nickel
- Why heat treat: Solution annealing (see below) to restore duplex structure after welding
- Examples: 2205 and 2507 stainless steel
- Also known as PH stainless steel
- Composed of at least 17% chromium, 4-7% nickel, and other alloying elements such as copper, niobium, and molybdenum
- Why heat treat: Increase strength and hardness, and achieve high corrosion resistance
- Examples: 17-4PH, 15-5PH, and 13-8Mo
Common Heat Treatments for Stainless Steel
As you can see, heat treatment processes vary between the types of stainless steel and what you’re using it for. If you’re making a stainless steel knife, for example, here are a few heat treatments you may need.
Heat the material to a specific temperature, hold it at that temperature for a certain period, then slowly cool it. This process softens the steel and improves its ductility and machinability. This can be beneficial if you will be doing a lot of forging or shaping.
Heat the steel to a temperature high enough to dissolve any detrimental precipitates in the microstructure. Then quench it (cool it fast, unlike regular annealing) to lock the dissolved atoms in the solid state. This process improves corrosion resistance, strength, and ductility. Solution annealing restores corrosion resistance after welding or other fabrication processes.
Heat metal to a temperature above its critical range, followed by rapid cooling. This process changes the microstructure of the metal from austenite to martensitic stainless steel. The hardness and strength of the metal are increased, but the brittleness can make it less suitable for some applications. To overcome this, temper it (see below), reducing brittleness and improving toughness.
For some stainless steels that you can’t harden via standard methods, use precipitation hardening. This process involves heating the metal to a specific temperature for a certain time, followed by cooling. The result forms intermetallic compounds that increase strength and hardness without increasing brittleness.
After hardening, steel is brittle. Temper to improve toughness. Heat the steel to a lower temperature than the hardening process and then allow it to cool slowly.
Heat the metal up to a level below its critical temperature limit (sometimes higher than in tempering) and hold it there for a time. Use it to improve the stability and dimensional accuracy of a finished product.
Best Practices & Considerations for Stainless Steel Heat Treating
Heat treating is tricky stuff. Here are some things to keep in mind to help you develop these skills.
- Understand the type of stainless steel you’re working with. Different types of stainless steel have different properties and respond differently to heat treatment. Make sure you know the composition and characteristics so you can choose the appropriate heat treatment.
- Use accurate temperature measurement. Precise temperature is critical to the success of any heat treat recipe. Invest in an accurate thermocouple or pyrometer.
- Use appropriate heating and cooling methods. Stainless steel should be heated and cooled in a controlled manner to prevent cracking and warping. Use a heat treating oven, or a heat-treating salt bath or oil bath for heating and quenching.
- Follow the heat treatment process to a T. Do the steps in their specific order. Stick to the recommended time and temperature parameters.
- Be patient. Heat treatment takes some time, so don’t rush it.
- Test and verify. Before and after heat treatment, run appropriate metallurgical tests like hardness and microstructure analysis.
- Safety first. We’re talking about some very high temperatures. Always use proper protective gear and make sure you are working in a well-ventilated area. Choose an oven that adds a layer of protection, like Hot Shot Ovens with Cool-Touch technology.
Getting the Most out of Stainless Steel
Hobbyists, craftspeople, and manufacturers across many industries value stainless steels, with good reason. But heat treating is essential to realizing the full advantages of stainless steel.
We hope this article gives you the knowhow to ensure the best results for your knives or whatever else you enjoy making. Remember to approach heat treating with great care. Good luck advancing your craft!