Monday, November 28, 2022

Heat-treating steel


Time-Temp-Transition (T-T-T) diagram for iron-manganese-carbon alloy
One of the miracles of the 20th century was the "science" of heat-treating alloys to produce a range of mechanical properties.

This was understood in a not-so-scientific way for hundreds of years. For example, Moorish sword makers in Toledo, Spain understood that if you ran a yellow-hot blade into the body of a strong slave it would become much stronger while if you got cheap and ran it through the body of a skinny slave it might not be so strong.

Spurious conclusions were drawn.

Progress was made when steel samples of uniform chemistry were tested under controlled conditions. After a specific heat-treatment, the mechanical properties were measured and samples were polished and acid etched to provide insight into the granular structure.

One of the vexing things about heat-treatment is that different grain-structures have different mass-densities due to dimensional changes. If those dimensional changes result and if it proceeds through the part unevenly, the part can warp.

Two results that are not very intuitive are Mar-tempering and the temperature-history to create fine pearlite.

Some bright young metallurgist, looking at the bull-nosed shape if the iron-manganese-carbon T-T-T chart realized that he could rapidly cool a part from a high temperature to about 300C in a molten salt-bath (typically a mix of sodium nitrate and potassium nitrite) and hold it in the bath until the temperature through the section had become constant. Then, he could pull it out of the bath and let it gently air-cool. Air cooling is so gentle that all portions of the part walk through the austenite-to-martensite transition simultaneously.

Toolmakers rejoiced. "Air-cooled" tool steel is expensive due to the exotic alloying elements. Less expensive alloys could be "air-cooled" with Mar-tempering protocols.


The thing about fine pearlite is that it makes small chips and can be machined relatively inexpensively with carbide tools. Pearlite is also ideal for later heat-treating because the iron carbide goes rapidly into solution.

High-strength, near-net-form parts can be cast of nodular iron. If the proportions of casting-to-sand are well tuned, and if there is a "buffer" in the process, then the cooling castings can "dwell" at about 600C long enough to become pearlite through the vast majority of the section, facilitating machining and later heat-treatment if required.


  1. Great post. Most of the Metallurgy I learned was during my Hot Rod days when I was learning about the individual components in an engine. Learned a LOT by reading and talking to machinists. I always thought the art of Specialty Steel Making had more than a touch of Alchemy to it....

  2. Heat treating can be difficult for sure. I spent a couple weeks at a blacksmithing school 48 years ago and we spent a lot of time on that. Getting knives, axes and other tools to have a good edge that could be sharpened and hold an edge and not be brittle can be a real challenge. ---ken

  3. Yep, hard to learn, and harder to do correctly...

    1. Yea, and hard to remember as time marches on. ---ken

  4. Technology of a sufficiently advanced level is indistinguishable from magic. Metallurgy would have fallen into that category two or three centuries ago. Many things we now take for granted on a daily basis would have been considered magic not very long ago. And such skills are easily lost if society were to crumble.

  5. ERJ, having more than a passing interest in Japanese swordsmanship, this is always of interest to me. How the Japanese smiths arrived at the combination of hard and soft steel via differential heating (the "hamon", or line just above the edge) is lost in the mists of time, but the fact that they did (and managed to keep the knowledge alive) remains a marvel to me. That they could consistently, for hundreds of years, produce blades of quality and strength using visual cues and experience alone is a marvel.

    And reading up on Damascus steel, something that can be lost in a single generation.