[Ben Krasnow] Discusses the Heat Treatment of Steel


For home metallurgy, there are two sources of information for the heat treatment and tempering of steel. The first source is academic publications that include theoretical information, while the second includes the home-spun wisdom of blacksmiths who learn through trial and error. [Ben Krasnow] put up a great video that tries to bridge that gap with some great background information with empirical observations to back up his claims.

For investigating the hardness of steel, a few definitions are in order. The first is stiffness, or the ability of a material to ‘spring back’ after being flexed. The second is strength, specifically yield strength, which is the amount of strain a material can withstand before being permanently deformed.

[Ben] did all these experiments with a 1/8″ W1 steel drill rod. As it came from McMaster, this rod could handle a bit of force before becoming permanently bent, and in terms of stiffness was much better than a piece of coat hanger wire [Ben] had lying around. After taking a piece of this drill rod, heating it up to a cherry red and quenching it in water, [Ben] successfully heat treated this steel to a full hardness. After putting it on his testing jig, this full hardness steel didn’t deform at all, it simply broke.

Full hardness steel is basically useless as a structural material, so [Ben] tried his hand at tempering pieces of his drill rod. By putting a few pieces in a kiln at the requisite temperature, [Ben] was able to temper his drill rods to be stronger than the stock material, but not as terribly brittle as a full hard rod.

18 thoughts on “[Ben Krasnow] Discusses the Heat Treatment of Steel

    1. Stiffness is NOT also known as “toughness”.
      Toughness is roughly the amount of energy a material can absorb before it fractures.

      Stiffness is also known as a modulus (slope or derivative) of a stress vs strain curve for a material. Materials that are very stiff do not deform as much for a given load.

      For example: quenched and tempered tool steel would be both tough and stiff.
      Fired Ceramic would be very stiff but not very tough.
      Car Tires(the rubber) are very tough but not stiff.

  1. “steel” is a material, that, if you dive into the information about it, you can keep reading. I make knives myself from steel that I order. If you read about a certain steels’ properties, you can see that they add all kinds of different metals or materials to make it harder, or more flexible, look at e.g. carbon steel (http://en.wikipedia.org/wiki/Carbon_steel).
    Each type of steel has its own properties, and results in a slightly different kind of tempering. For me the kind of steel a supplier delivers, usually has a tech-sheet that gives me the information I need for tempering.

    Ben perfectly explains here the difference between hard and flexible, thanks!

  2. One thing to know about a steel alloy when deciding which one to use for a specific application is the temperature at which it begins to soften, which is very much different from melting temperature. That’s important both for processing the metal and for if it’s intended to be used in high temperatures.

    Steel (and most other metals and alloys) aren’t like ice which goes almost directly from solid to liquid. They’re solid (but with decreasing strength in tension and compression) up to their softening temperature at which they’ll begin to bend under their own weight. If loaded with other weight, the temperature at which bending begins will be lower.

    The melting temperature is just that, the temperature at which a metal is hot enough to flow as a liquid. Doesn’t matter how it’s loaded with weight or stress, melting temp is melting temp.

    Just a bit of scientific data certain sorts of people ignore about a specific event around 13 years ago – and the fact can’t penetrate most of their hard heads.

    1. The chart he shows is a Time-Temperature-Transformation chart. For plain carbon steel. Steel Alloys will have a very different graph, some with multiple “phase” transformation zones. These are chrystaline phases-body centered cubic, face centered cubic. Sometimes they are called “alpha”, “beta”, & “gamma” phases.
      As for the explanation of “softening”-remember that time as well as temperature factors.
      Looking at the chart you can tell that in some regions a lower temp for a longer time will result in the same transformation as a higher temp for shorter time. Quenching “freezes” the crystals in this phase. Too many hard crystals = brittle steel. Heat and transform some of the crystals and it’s less brittle, more flexible.
      Also take into account the differences in fracture mechanics between brittle and ductile materials. Ductile materials fail gradually once they hit the yield point. Brittle materials fail at the instant they reach yield.

    2. The temperature at which it “softens” isn’t really what you should be concerned about. It’s the drop in strength as temperature rises, typically given as the rupture strength at some time. This is typically 1% creep at 10,000 or 100,000 hours. You have to compare that stress limit to what the item will see at that temperature. (This is a generality; actual limits depend on your specific application.)

  3. Steel was the first material we truly understood at the nanostructure. Each of the phases of steel is a uniqe micro and nano-composite of iron, iron/carbon mixtures and cementite (Fe3C ceramic-like material). Some of them like pearlite are truly beautiful examples of heirarchical structure.

    Much of the early advances in electron microscopy and atomic-scale materials science were funded by big steel (US Steel mostly). I believe the largest TEM in history (a 3.5 MeV monster the size of a warehouse) was a US Steel machine.

  4. I can’t let this go… I just can’t.

    Firstly, let me say that a little reading wouldn’t have killed the author. You know, probably.

    What’s being referred to as stiffness in the article is really elasticity, and in fact the definition is even stated “…or the ability of a material to ‘spring back’ after being flexed”. Hardness is related, but is typically used to indicate how material will wear, although you can get other information depending on the hardness test conducted.

    Yield strength is also mentioned, but nothing about the two primary modes of material response. When loads create stress (loads and stress are not the same thing) below the yield strength, the material will behave elastically, as stated in the article (although there probably will be some strain hardening). Once the stress goes above the yield strength, the material will undergo plastic deformation. Plastic deformation means the material will not resume it’s original shape and has had significant strain hardening. The material will continue to deform under load in the plastic region until it’s ultimate strength is reached.

    Plastic deformation is essentially cold working the material, changing the crystalline structure through outside physical forces. The best example you can put your hands on right now is a metal paperclip. When you take an end of the paperclip and start bending it back and forth (actually you can demonstrate elastic and plastic deformation too), you are creating stress above the yield strength and causing the metal to undergo plastic deformation. As you keep bending it back and forth, you are strain hardening the metal until it’s “brittle” enough to not be able to deform under stress, and it breaks.

    The stress-strain relation is called Young’s modulus or elastic modulus. Also, as you can imagine, all of this is related to fracture mechanics.

    Alright, enough rambling….

  5. Brian,
    Stiffness is defined as the slope of the stress-strain curve. It is not affected by strength (and therefore heat treatment) in any way. Any kind of strengthening (heat treatment, cold working, etc.) only move the plastic yield point of the material further up the stress-strain curve, they do not alter its slope.

    To everyone else, hardness is related to strength in the sense that hardness can be thought of as localized strength. The harder the material, the stronger it is, and vice-versa.

    Source: I’m a mechanical engineer.

  6. And I am surprised nobody has injected the shot peening process. I too am a mechanical engineer and when I need general data on material I pick up a Ryerson cat.
    “configure for success, test for failure”

    1. vreinel,
      You make an excellent point about shot peening. For those that are unfamiliar with the term, it involves cold-working the outer surface of a material by blasting it with steel shot (or some other very hard substance). This makes the outer surface of the material harder (and therefore stronger) but leaves the inside material more ductile, and therefore tough (ability to resist brittle fracture). This is a form of “case hardening”. It is especially useful for materials that are subject to cyclic loading, which often leads to failure far below the material yield strength.

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