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.
Old Smithy sheds are viewed as our heat treatment grandfathers. They didn’t install massive furnaces, but there was always that flickering red flame. The hot coals burned bright in primitive townships. Then, at day’s end, there would be that rattling hiss as a forged part was quenched in water. And water is still a quenching mainstay, as employed in contemporary heat treatment. Only, it’s been joined by other quenching solutions.
The Available Quenching Mediums
Rapidly immersed in oil or water, that old rattling hiss is still heard. The high-temperature energy is quickly sucked away from the hot metal workpiece, so its new crystalline form is maintained, even though the workpiece is now at room temperature. But there are other mediums used in quenching, not just water, not only oil. There are caustic soda quenchants and brine pools. On another heat leeching line, single-phase liquids are replaced by a quenchant gas. Oils, molten salt baths, even air cooling, the possibilities are many. The question is, now that the quenchant candidates have been singled out, what benefits do they have to offer?
Rapid Cooling Consequences
There are times when fast quenching is undesirable. Water is out of the picture after realizing this fact. The workpiece should cool quickly, and those mechanical properties should be locked inside the part, but the operation should accomplish these twin feats without causing any deformation. Basically, a workpiece that cools too quickly could warp and fracture. Let’s pull the part away from the water pool, then. Let’s put it in oil, which is a deformation-diminishing quench medium. Alternatively, a molten salt bath will uniformly regulate the alloy’s cooling curve so that there’s no chance of structural distortion.
Considering the Supplementary Benefits
Right off the bat, air quenched parts allow different rates of hardenability. For gas and salt bath quenchants, the cool gaseous mixes function like any other energy discharging mechanism. But, and this is an important point, the chemical bases add secondary features to the mix. Cyanide baths, for example, cool and quench hot workpieces while simultaneously adding a chemical processing feature to the process. Cyanide baths quench and case-harden alloy parts. Incidentally, since molten salt baths fully immerse treated parts, they’re able to reduce undesirable side effects, including process oxidization.
At the end of the day, heat treatment curves are complex. Controlling the upward curve, furnace heat creates a thermal profile. Once the process hold time is complete, the part begins its downward thermal curve, which is controlled by oil or water or salts, or by air or one of a dozen other heat regulating quenchants. Thanks to those thermally regulated quenches, the workpiece doesn’t distort, doesn’t warp, and it doesn’t fracture.