This is the first Instructable in my series about heat treating carbon steels.
Before we get started, I'd like to touch on a few important points that I think will offer some context to what can be considered to be some pretty dense subject matter:
*In order to talk about heat treating, we must first know a little about metallurgy. This of course, is the branch of science and material engineering that studies the behavior of metals. As of the date of this Instructable, there are 84 metals on the periodic table of elements.
*Generally speaking, metals share several qualities: thermal & electrical conductance, malleability, ductility, and hardness. Except for mercury, they are solid at room temperature, and can be chemically combined with other metals. These combinations are called alloys and they exits in the hundreds, if not thousands. Alloys are typically categorized in groups by chemical make up / properties, and some of them can be quite exotic. Due to the large number of alloys, this series will only focus on a few commonly used one.
*There are a lot of confusing terms that are used in the fields of welding, blacksmithing, and metallurgy that overlap and sometimes carry slightly different meanings. So, for the sake of clarity, I'll be linking key terms to their definitions as they appear in the text.
*Lastly, some of the best technical universities on the planet have programs dedicated to the study of metallurgy. I do not hold a degree from any of these places, nor would it be realistic to expect the following series of Instructables to cover all there is to know about the topic. That said, twenty years working as a blacksmith and fabricator has given me plenty of hands on experience, and with that comes a confidence in my understanding of the fundamentals. This Instructables series was written with the DIY tool / knife-maker, tinkerer, or anyone who has an interest in upping their basic metalworking game in mind.
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Step 1: Getting Specific
Of all the groups of metals in existence, there is one in particular that is quite large. These are called ferrous metals, which means "containing iron". This group is important because it includes all of the steels, which are one of the most versatile and commonly used materials in our civilization.
Within the ferrous metals group, there exists another large, and widely used group called carbon steels. These are an alloy of iron and carbon typically in the 0.30–1.70% carbon by weight. Carbon steels are common, easy to obtain new and from scrap, are relatively easy to work with and function well as tools, knives and anything else that requires a hardness / toughness more than non-carbon steels can offer.
For these reasons, carbon steels will be the focus of this Instructables series. In particular, medium - high carbon steels will be the focus here, as they contain enough carbon to be heat treatable. Low-carbon or plain-carbon steels do not and are of no consequence in this context.
Step 2: So What Is Heat Treating Anyway?
The term heat treating is referring to the manipulation of the molecular structure of a metal in service to changing its mechanical properties. This can increase or decrease ductility, hardness, yield strength, or toughness without altering electrical and thermal conductivity.
Heat treating has four basic states:
These terms describe the transition from one state to another and each will be covered as its own Instructable in this series.
Step 3: How Does Heat Treating Work?
Above: (Top) Microscopic view of the different phases of carbon steel. (Bottom) The iron / carbon phase diagram, which shows temperatures at which phase change occurs.
Heat treating works by exposing carbon steels to a range of specific temperatures for a prescribed period.
Carbon steel's molecular structure is crystalline. Exposure to hot and cool temperatures will change the shape, or phase, of these crystals. Holding, or "soaking" the steel in these temperatures will further change the shape of the crystalline structure. The temperatures at which this process begins is called the steel's critical temperature. The range in between the critical temperature and the upper melting point is called the steel's transformation range.
There are a number of phases that the steel can potentially contain, and each exhibits its own set of properties. These properties can be beneficial in different ways to the material's application. The different phases are called: austenite, ferrite, cementite, martensite, banite, and pearlite (technically not a phase, but a mixture of ferrite and cementite.)
That's right! By controlling a carbon steel's exposure to specific temperatures, we can make that steel harder, softer, or tougher. This greatly increases the material's durability and makes carbon steels very suitable for industrial applications.
The following series will break down all the stages of heat treating and individually address the basic hows and whys of each process.
Stay Tuned for the next Instructable which will cover Annealing.