Heat Treatment 101

September 25, 2020 10:39 am

Heat treatment is utilized in both pre- and post-production phases and is an essential part of the machining process. So, what is heat treatment? It’s the process of controlled heating and cooling procedures that alter the structural properties of metals. Why is this necessary in machining? Because the physical and chemical property alterations transform the metal to meet certain specifications and performance requirements.

There are several heat treatment methods and each one creates different results for different materials. To put it simply, the heat-treating process either prepares your metal before machining or completes your machining process.

There is a lot of jargon related to heat treatment methods. You can review some common terminology here.

Benefits of Heat Treatment

The physical and chemical properties of metal are completely altered during heat treatment, and since different methods change different properties, some metals must undergo a variety of heat treatment processes before ever making it to the machine. Some methods alter the strength of the metal, making it softer or harder, and some further develop the heat conductivity of the metal.

The softening of metal through heat treatments makes a part more flexible and relieves material stress that often makes machining difficult and susceptible to issues.

Parts are often permanently placed in environments that alter or wear them. Heat treatments increase wear resistance which can prevent deformations by improving brittleness. That way the metal does not succumb to those environmental influences.

The benefits do change depending on the material, however one of the most common reasons metal is heat treated is to develop maximum strength, durability, and hardness, conditioning the part for longevity.

Different Heat Treatment Methods

These are the most common heat treatment methods for machining. Note that each method of heat treatment has its own specific subcategories, but in this article, we’ll just stick to the basics.

Hardening is the most common heat treatment method. The name implies exactly what it does, which is to harden metals to increase their toughness and durability. In some cases, only the surface needs to be treated and this is called case hardening. Case hardening is done after the part has been machined as a final step to produce that hardened outer layer.

During the hardening process, the metal is heated to a specified temperature where it remains for a given amount of time. It then proceeds through a rapid cooling process. This is done by submerging the part or material into either cool oil, water, or brine. It is not uncommon for the metal to then be tempered (details below) to relieve the material of internal stresses.

Annealing is the process of heating the metals to extremely high temperatures and then controlling the cooling process at a very slow rate. Most commonly used with aluminum, copper, steel, and brass, this process softens the metal to make it more workable by enhancing machinability, ductility, and toughness. This is especially practical when preparing metal that will be cold machined and susceptible to cracks and fractures.

Normalizing is the heat treatment method commonly applied on ferrous metals. Normalizing is used to improve grain size, thus improving machinability and relieving stress. When a metal goes through normalizing, it is heated to a certain degree above its upper critical temperature (or transformation temperature) and held at this temperature to refine the microstructure and enhance the mechanical properties. The temperature depends on the metal. The metal is then cooled in room temperature air.

Tempering is usually done as a second step to alleviate stresses that built up within the metal during the previous heat treatments. Most commonly used on iron-based alloys, this process involves heating metal at a lower temperature (below the critical point) in order to produce the desired properties. Reducing the excess hardness and brittleness makes the material machine ready. Keep in the mind that the rate of cooling is not important during the tempering process.

Recommended Reads

Heat treatment encompasses way more than what we covered, so we recommend that you develop your knowledge based on the most commonly used methods within your shop. Here are a few books that will help you get started.

Heat Treater’s Guide: Practices and Procedures for Nonferrous Alloys

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This book mainly focuses on nonferrous alloys and reviews the most widely used heat treatment methods for such type of metals. With best practices and procedures, this is an authoritative, how-to style text for heat treating information.

Heat Treater’s Guide: Practices and Procedures for Irons and Steel

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This text includes information regarding metallurgy, cast irons, and ultra-high-strength steels. With a focus on best practices and procedures when heat treating iron and steel, this text includes plenty of examples, charts and graphics.

The Heat Treatment and Annealing of Aluminum and Its Alloys

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This text is a complete evaluation of aluminum, its alloys and their plasticity (the ability of a solid metal to change its structural properties). The book covers the exact chemical transformations that occur within an aluminum alloy during heat treatment.

Heat Treatment (Crowood Metalworking Guides)

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A relevant resource for beginner machinists, model engineers or tool makers, this guide features step-by-step photography to guide you through the most common heat treatment methods.

Recrystallization and Related Annealing Phenomena

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This text focuses on the research of recrystallization and grain growth, which explain the process that occurs within metal when subjected to high heats. Although targeted more toward metallurgists, this book can be beneficial to machinists who are interested in diving deep into what occurs inside the metals as they are being heated.

Hardening, Tempering and Heat Treatment

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This comprehensive guide will provide you with an in-depth knowledge of the different heat treatment methods specific to steel and explain what tools are required for each approach.

 

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1 Comment

  • Mike says:

    Thanks for this nice synopsis on metal treating. Many of your readers have most likely watched a TV show where contestants produce forged knives and other bladed weapons. Never do any of the contestants temper their work They heat it hoping to reach conversion temperature but never is anything tempered. This is a fatal mistake. Tempering sets the hardness and retards the reoccurrence of austenitic grain structure. This lack of tempering allows hardness to fade. As to telling when a heated item has reached conversion temperature, a magnet that is no longer attracted to the heated part is a pretty good indication that conversion temperature has been achieved. This ancient blacksmith method seems to elude these knife makers.

    Additionally they quench everything in oil. There are many quenchants and many specifications for different quenches dependent on the metal being quenched. For each metal there are quenchants that cool too fast and some too slow and sometimes only still air works best.

    No temper and using the wrong quenchant leads to premature part failure

    The article didn’t mention the use of cryogenic processing to improve the results of heat treatment. This evolving technology increases the life of tooling in multiples of normal life. Few people seem to know about this or they are only familiar with this great technology as practiced in years past using ineffective techniques that produced poor or no results. Cryogenics is now a reliable and cost effective way to make tooling last much longer.

    Thanks for your fine technical journal. The education you provide is marvelous.

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