How Does Hardening and Tempering Improve the Strength of Materials?

28 April 2017

Material strength isn’t a straightforward property, not when we’re talking about structurally-capable alloys. Yes, a hardened workpiece is stiff, but it could also be brittle. In reality, strength is an amalgamated property, something that combines hardness and material tempering. In using metal solidification technology as our starting point, we’ll explain the importance of hardening, then we’ll introduce tempering, a process that counters the hardening work by adding ductility to the alloy amalgamation regime.

Determining Central Precepts 

The purpose of the hardening stage is to ensure it won’t deform, no matter how heavy the applied load. If that load compresses the metal part won’t collapse in upon itself, won’t fracture, nor warp. Instead, it stands resolute, with its original shape locked in place. In essence, the mechanical backbone of the processed metal is stiffened. But wait, a structure that’s imbued with this lone property could crack and crumble, unless there’s a ductility feature in place, that is. Workpiece tempering assumes this role, for this important low-temperature work phase is purpose-built as a material brittleness alleviator.

Heat Treatment Balancing and Counterbalancing 

If a super-heated environment raises the transformative temperature of a selected alloy past its specified critical transformation threshold then is cooled rapidly, the workpiece hardens. Technically speaking, all pearlite content has been converted into martensite, an allotrope whose needle-like microcrystalline structure is extremely hard. The problem here is the brittleness of the martensite, an issue that can introduce stress and material deformation, although these effects are mitigated somewhat by the length of time the part was held at its transformative temperature. Tempering is required to counteract the strengthening (hardening) phase, a stage that can add material weakness (brittleness). The workpiece has been hardened and rapidly cooled, via a quenching station, so now the tempering process enters the heat treatment line as a counterbalance. Again, heat is the brittleness mitigating agent, but this is a reduced thermal load, a heat source that increases material strength by adhering to a low temperature. Intelligently managed in this manner, the tempering temperature reduces alloy brittleness while augmenting the workpiece’s strength and overall ductility.

We’ve described quite a few hardening and tempering methods over the months. They create tough metal parts and superior finishes, parts that are as material strong as they are corrosion-resistant. Still, what we’re stressing today is an overall strategy, a meeting of different heat treatment principles and processes. Designed to produce a desired hardness-to-strength ration, the hardening and tempering equipment use time and fiery temperatures to imbue a chosen part with a requisite material strength rating.

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