|Time-Temp-Transition (T-T-T) diagram for iron-manganese-carbon alloy|
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.