Most Important Factors that Affect Hardenability of Steel

11 October 2018

Undergoing heat treatment, a steel workpiece warms until it’s almost red-hot. Engineers monitor the process, compare the actions taking place in the furnace to specially formulated graphs, and the metal exhibits a hardenability-biased phase change. The newly transformed martensite is ultra-hard, and that microcrystalline state is validated by the quench operation and tempering work. Just to add substance to that image, though, what key factors impact hardenability treatment work?

Quenching Rates 

Working on steel, the quench rate must be adjusted so that it falls in line with the fingerprint-like heat treatment graphs mentioned in the opening paragraph. Consistent quench operations yield fully phase transitioned steel workpieces. The martensite phase is reached throughout the material form, no pearlite or bainite or other less desirable grains are produced, so the part gains uniform hardenability.

Quenching Medium 

Traditionally, water is the medium of choice here, but the agitated fluid can introduce undesirable intragranular effects. Substituting oil, hot water, or gas quenching, the linearly processed phase change proceeds at a more predictable rate. Consequently, the hardenability scale is easier to manage when the quench medium has been intelligently selected according to the steel alloy’s innate material characteristics.

Workpiece Geometry and Composition 

Reducing the critical cooling rate, as required to make a steel part fully martensite, there are other hardenability factors to weigh. The geometry of the part, its mass and shape, will impact the martensite transformation process. Alloying elements and profile-dictated temperature management strategies are utilized if the material composition or product profile impacts the martensite quenching (steel hardenability) stage.

Grain Size and Carbon Content 

Among the important process influencing factors, the grain size of the stably formed austenite can profoundly hamper martensite phase modification action. There are grain nucleation effects and carbon diffusion processes taking place invisibly inside the heated steel component, and it’s the larger, coarser grains that facilitate steel hardenability. To accommodate this requirement, heat treatment engineers actively participate in the material sourcing stage. Communicating with the steel mill, coarse-grained alloys are selected as hardenability-accessible materials.

If nothing else, we’ve illustrated the number of important factors that can and often do impact steel hardenability in a heat treatment facility. The tempering medium and the way it’s wielded obviously influence the work. Back further in the process, the grain size of the selected alloy and its carbon content also directly impact the job. Finally, ultra-hard as the steel is after it’s been heat treated and quenched, that part is now brittle. On towards the tempering stage, the part receives its final material toughening operation, the tempering work.

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