How Does Heat Treatment Affect Steel Grain Structure?

28 February 2019

As the temperature rises in a heat treatment furnace, strange transformations occur. In steel workpieces, the grain structure of the component changes size. Alternatively, new grains form. They undergo phase transformative processes. There’s carbon in the mix, too. As those phase changes take place, the alloy-strengthening element becomes more soluble. The carbon diffuses, so the workpiece hardens. Here, this list of phase states should clarify the matter somewhat.

Normalizing the Grain Size

Before moving onto the phase states and grains, let’s check out a straightforward example. Referred to as “Normalizing,” the heat treatment work holds a steel workpiece just above its critical transformative phase. At these austenitizing temperatures, the grain uniformly changes size. Held for a predetermined period at this temperature, the steel is then cooled at room temperature. Using between 750°C and 980°C (the temperature varies because of carbon content) of furnace heat, all of the steel ferrite is transformed into a harder, uniformly distributed pearlite structure.

Charting Grain Structure Changes

Body-centred ferritic steels can’t easily diffuse carbon. Infusing more thermal energy into the process, taking the temperature gradient up to that 750 to 980°C sweet spot, the grain structure transforms. Face-centred austenite allows the carbon to diffuse. Hard islands of cementite dissolve, and now the crucial moment has arrived. If the second half of the heat treatment process mirrors the first half, then the carbon will resurface while the steel assumes its ferritic microcrystalline structure once more. Obviously, that’s not the result we’re after. Taking control of that second stage, the cherry-hot steel is quenched in oil or water. Now, because of the sudden cooling, the carbon becomes locked inside the cubic grains.

Phase Transformations: Desirable Mechanical Properties

Produced after the high temperatures and quenching operations are complete, martensitic steels are super-hard but brittle until tempered. Austenitic processing changes the nickel-to-chromium ratio slightly so that the alloy gains a stronger corrosion resistance feature. They’re also non-magnetic. By the way, the electrical conductivity and magnetic parameters change when the grain structure is altered. Typically, however, the process targets mechanical hardenability, fatigue resistance, malleability and workability, and corrosion resistance, too. Magnetism and conductivity are important, too, of course, just not as process-integral as those mechano-chemical attributes.

At the end of the day, there are ingredients and heat treatment temperatures to manage. The ingredients are already inside a steel workpiece, or they’re added to a furnace’s atmosphere. Then, by managing temperature levels, hold times, and the quench/tempering work, we can create the differently shaped grain structures mentioned above. As those grains alter size and shape, the carbon in the steel becomes more soluble.

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