What is Warping in Heat Treatment and What are the Reasons Why It Happens?07 September 2018
Heat treated metals are undeniably hard and rigid. Still, their dimensional parameters can and often do change when their microcrystalline matrices are exposed to massive quantities of thermal energy. The crystal structure, tortured by thermal stresses, starts to distort. The alloy piece is warping. Then, quenched in oil or water, it deforms again. To prevent this unacceptable side effect, we need to know what’s going on here.
Non-Uniform Heating Issues
One section of the heat treated part is hot, the adjacent section is cooler, and the furthest corner is cooler still. There’s a thermal gradient stressing the alloy. The thermal energy is breaking the crystal bonds and reshaping them at that super-heated section, but the bonds within the cooler zone are breaching more reluctantly. That’s stress, and it’s going to cause component distortion.
Non-Uniform Cooling Issues
At the opposite end of the thermal spectrum, the heat gradient is inverted. This time around, the cooling medium isn’t uniformly dropping the part’s temperature. The oil or water is contaminated. The quality of the quenching fluid is dubious, and it needs replacing. Alternatively, the cradling mechanism is flawed, abnormal thermal gradients are propagating, or the part possesses intricate geometry.
Asymmetric Component Profiles
In this heat treatment scenario, thin sectional pieces are quickly soaking up the thermal load. Elsewhere, on that same part, the thicker sections are also absorbing the heat, but there’s more mass to the thicker sections, so they take longer to reach the same temperature. Again, the heat curve isn’t uniform throughout the alloy, so it experiences unbalanced stresses. Oftentimes, product designers split larger, more complex components so that these contrasting geometrical profiles can be separated.
Mulling Over the Sidereal Factors
The issues encountered here are serious, but many of these costly processing errors can be solved. For starters, the water or oil medium must be replaced regularly. Cradling mechanisms and furnace flames/elements require maintenance, too. If the warping continues, it’s time to check the product stacking pattern. Loaded in stacks, in buckets and on belts, too, the stacking order can seriously impact heat distribution gradients, which then go on to produce internal stresses within alloy-reinforced workpieces.
Attend to the stacking stage with all due diligence, for this straightforward loading phase could just be responsible for the dimensional distortions. Left in the care of a seasoned engineer, the errors should soon disappear, because this heat treatment expert knows that the upward side of the heat curve isn’t always the culprit. Sometimes, even when the heat gradient is linear, it’s the cooling side of things that causes the warping.
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