How Heat Treatment Can Contribute to Steel’s Strength and Weakness

25 June 2018

Counterintuitive consequences have been known to compromise heat treatment work. By way of illustration, steel strengthening operations should logically produce strong workpieces. However, there are times when these mathematically predictable results go out the window. Are there strange laws of probabilities in effect? Or is there a straightforward reason for heat treatment induced weakness? Uncertainty factors are not acceptable, not in heat treatment work.

Holding Up the Process for Scrutiny

Think of a heat treatment procedure as a two-stage operation. The component is heated, then it’s cooled. Like a sawtooth graph, thermal energy is the initial ramp, then the downward slope of the triangle is created by the cooling procedure. It should be obvious at this point in the proceedings that the workpiece is about to become weaker. It has to be transformed and softened so that it can be worked and homogenized. For that latter procedure, the microcrystalline structure of the ferrous alloy is redistributed and altered. Meanwhile, the apex of the triangle is imminent.

Peak Transformative Thresholds

The highest temperature in the furnace stops just short of allowing the steel particles to move freely. At approximately 720°C, the grain is about to transform. So where is the weakness introduced? Well, depending on the heat treatment technique, several zones on this graph can weaken the steel part. If the metal isn’t hot enough, carbon can’t be absorbed and diffused. Then there’s substandard stress relieving work, where the process uses heat to normalize internal stresses. Unaddressed, those stresses will turn a strong steel part into a fracture-prone one.

Tempering and Quenching Issues

Exposed to an advanced alloy toughening stage, the steel hardens. It’s brittle, but a tempering operation removes that weakness. Likewise, quench-hardened components can be brittle, and it’s the tempering phase that again removes this overly rigid substructure. Using the correct temperature bands and a corresponding cooling period, steel part hardness drops while its ductility quotient rises. However, certain low temperatures can actually diminish these effects and leave the part brittle. It’s the same with quenching and other cooling operations. Hypothetically, perhaps the downside ramp of the graph sees the quenching phase take place in an unsuitable medium. Water, as one example, can induce part’s expansion and contraction, a response that could weaken or even deform the steel part.

Wielded by seasoned exports, a heat treatment processes always yields predictably strengthened parts. However, if the equipment has a vacuum leak, if the process is quenched improperly, or if it’s heated unevenly, then alloy weakness is a likely upshot. That’s a lot of “ifs,” but they all fall before an expertly conducted heat treatment service.

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