Causes of Steel Warping During Heat Treatment and How to Prevent Them from Happening

04 July 2019

Ideally, heat treatment work affects the microcrystalline structure of a steel part. In the furnace, then cooled gradually or quenched suddenly, the physical properties of the steel alters. The processed component becomes harder. It also gains a more corrosion-resistant structure. What the high-tensile alloy should never experience is any amount of steel warping. That, unfortunately, is what happens when steel is exposed to different thermal envelopes.

Analysing Unwanted Furnace Defects

It’s difficult to stop dimensional alterations when steel workpieces enter a furnace. The alloy crystals soak up thermal energy. If this material was entirely ferrous, the likelihood of a parts warping event taking place would be manageable. But remember, steel is an alloy. There’s carbon in the iron, plus a number of alloying materials. As one would expect, those materials all transform at slightly different temperatures, hence the workpiece warping effect in the furnace. To control this phenomenon, heat treatment engineers apply uniformly controlled thermal gradients. A normalizing operation which homogenizes the alloy grains will further reduce this effect.

Stopping In-Solution Quench Warping

Still super-heated, the carbon diffusion cycle has taken place. Next, the steel component is off to be hardened. Carefully, the glowing steel is lifted on an air quench rack or other such workpiece transportation apparatus, and then it’s lowered into the oil or water. The correct medium stops part’s deformation when excess energies burn away above the liquid bath. The rack is stopping the hot component from sagging, and its controlled entrance, right into the centre of the cooling liquid, prevents medium transition warping. Replaced recently, there are no contaminants in the pool. If the liquid was contaminated, a quench operation could proceed non-uniformly.

Steel warping issues can occur when a heat treatment process proceeds without incident. The part was exposed to a uniform thermal envelope and there were no unexpected heat gradients. Even the quench operation concluded as expected, again without any incident. But was the steel part holding stress? Perhaps it was cold worked or machined aggressively. Exposed to heat and quenching, the stress is released in an uncontrollable way, so the steel deforms. Normalizing and/or annealing operations can be utilized when applying a stress reduction process.

Human error is the last factor to counter. For instance, someone has lifted a batch of steel pieces into a top-tier furnace. That expensive piece of equipment will discharge a uniform thermal envelope. However, because the parts have been stacked in an offhand manner, the thermal energies in that furnace can’t reach all the way around each steel item. Convection currents and radiated thermal energies are obstructed, so the parts warp. Stack those items correctly.

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