How Quenching Can Affect a Metal’s Microstructure

19 March 2019

What do visitors want to see when they stop by a heat treatment facility? Watching from behind a thick glass plate, they enjoy the sight of an orange-hot workpiece as it is processed in a furnace. Next on their agenda, they want to experience the explosive sizzle that’s discharged as that workpiece is quenched in oil or water. Quietly attentive, they watch the part as it is immersed and quenched treated.

Employing Quenching Microscopy

What if the engineering team could employ some kind of a before-and-after system? They look at a “Before” sample under the microscope, then swap it out for a piece of “After” material, a section that’s just been quenched. Under an optical microscope, the carbon content has clearly dissolved in the “After” quench cross-section, and now the lattice microstructure has assumed a body-centred tetragonal form, which the process engineer recognizes as martensite. The product-hardened crystal lattice contains the dissolved carbon. Clearly, the fine-grain microstructure can be attributed to the effects of liquid quenching.

A Comparative Microstructure Study 

Meanwhile, the “Before” treatment cross-section falls below the lens of the same optical microscope. It’s a fairly hard workpiece, but there are clear grain boundaries observable under the scope, and the carbon isn’t crystalized. In truth, there are all kinds of coarse grains in there because the part’s microstructure hasn’t been homogenized yet. As a comparison, martensite-quenched steel exhibits a needle-like crystal structure, which spreads uniformly throughout the alloy material. Sometimes, however, residual stresses can be seen in the grain. The microstructure is finely produced and the soluble carbon is locked inside that grain by the sudden cooling. Of concern, though, the grain boundaries appear deformed, and they’re stretched.

Analysing For Unforeseen Effects

Such residual stresses can deform workpieces or introduce dimensional incongruities. Likewise, if the quench operation is somehow interrupted, the grain boundaries grow fuzzy. That’s something of a problem, as fatigue resistance problems propagate when the crystal matrix doesn’t form properly. Plainly speaking, then, a heat treatment factory desires a post-processed product that features the desired microstructure. Whether that structure is martensite-loaded or austempered, it must be uniformly applied and its intergranular characteristics have to take form predictably. If this isn’t the case, things will quickly “go south,” as the more verbose engineers like to say.

Remember, the metal’s microstructure is like a giant alloy crystal after it leaves the quenching pool. It’s a mechanically hard metal, but it’s also a relatively brittle workpiece, and it’ll stay that way until it’s tempered. Like a flawless diamond, the quenching phase has to keep that metal microstructure uniform and fully intact so that the brittle material doesn’t fracture.

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