Undergoing heat treatment, a steel workpiece warms until it’s almost red-hot. Engineers monitor the process, compare the actions taking place in the furnace to specially formulated graphs, and the metal exhibits a hardenability-biased phase change. The newly transformed martensite is ultra-hard, and that microcrystalline state is validated by the quench operation and tempering work. Just to add substance to that image, though, what key factors impact hardenability treatment work?
Quenching Rates
Working on steel, the quench rate must be adjusted so that it falls in line with the fingerprint-like heat treatment graphs mentioned in the opening paragraph. Consistent quench operations yield fully phase transitioned steel workpieces. The martensite phase is reached throughout the material form, no pearlite or bainite or other less desirable grains are produced, so the part gains uniform hardenability.
Quenching Medium
Traditionally, water is the medium of choice here, but the agitated fluid can introduce undesirable intragranular effects. Substituting oil, hot water, or gas quenching, the linearly processed phase change proceeds at a more predictable rate. Consequently, the hardenability scale is easier to manage when the quench medium has been intelligently selected according to the steel alloy’s innate material characteristics.
Workpiece Geometry and Composition
Reducing the critical cooling rate, as required to make a steel part fully martensite, there are other hardenability factors to weigh. The geometry of the part, its mass and shape, will impact the martensite transformation process. Alloying elements and profile-dictated temperature management strategies are utilized if the material composition or product profile impacts the martensite quenching (steel hardenability) stage.
Grain Size and Carbon Content
Among the important process influencing factors, the grain size of the stably formed austenite can profoundly hamper martensite phase modification action. There are grain nucleation effects and carbon diffusion processes taking place invisibly inside the heated steel component, and it’s the larger, coarser grains that facilitate steel hardenability. To accommodate this requirement, heat treatment engineers actively participate in the material sourcing stage. Communicating with the steel mill, coarse-grained alloys are selected as hardenability-accessible materials.
If nothing else, we’ve illustrated the number of important factors that can and often do impact steel hardenability in a heat treatment facility. The tempering medium and the way it’s wielded obviously influence the work. Back further in the process, the grain size of the selected alloy and its carbon content also directly impact the job. Finally, ultra-hard as the steel is after it’s been heat treated and quenched, that part is now brittle. On towards the tempering stage, the part receives its final material toughening operation, the tempering work.
Here’s a case study where a steel workpiece was hardened in a cutting-edge furnace. The process is locked in, and it has been exhaustively tested, so the repeatability factor is indisputable. The internal structure of the alloy workpiece has transformed. It’s mechanically harder, more durable and rigid than before the heat treatment action. However, at this juncture anyway, hardness isn’t enough, not if the part brittle.
Post-Quench Tempering
The heat treatment work has plainly created a hardened product, and the quenching phase has essentially set that phase change in stone. Sinking into the quench medium, the rapid cooling effect discharged a huge cloud of steam. Everything sizzled for one long minute until the hardened steel was returned to room temperature. In all honesty, that workpiece can’t just leave the facility, not yet, not until it’s tempered. Imagine the steel installed and put under load. It’s hard, but it shatters because brittleness is part of the hardening process. Therefore, right after the quenching station, the process turns to the tempering stage.
Injecting Tempering Strength
Thanks to semantic subtleties, the general populace has somehow mixed up certain words and their meanings. A hard material, for instance, resists abrasion and friction, but it can still fracture and even break apart. Tensile strength is the goal, then, with the tempering stage relieving the steel part’s quench-trapped brittleness state. The goal is not only to remove this mechanically undesirable feature, but to also add tensile strength, toughness, and an ingrained quantity of deformability.
A Stress Removing Technique
Alongside the annealing work, we temper steel as a stress elimination technique. The alloy becomes more stable and dimensionally consistent when the low-temperature tempering operation is properly conducted. Inside that crystalline structure, the carbon atoms of the steel alloy are on the move. They’re forming steel carbides and other deformation-facilitating particles inside the workpiece’s Martensite form. It’s still incredibly hard, but that feature has been offset by brittleness and stress relieving tensile strength. Pressed into service, that newly tempered steel part is as tough as it is hard, all thanks to the precision-managed application of a low-temp environment.
In olden times, Samurai swords were rolled and folded, then they were hardened by that same flame and quenched in water. But they weren’t taken into battle, not right then and there, not until they were tempered. To do otherwise would be to see the blade shatter as soon as the sword crossed paths with another weapon. No, the tempering stage, carried out by a seasoned blacksmith, made the sword tough. In heat treatment technology, the exact same principle applies, only its structural loads, not weapon edges that require the extra strength.
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.
Transforming metal workpieces with intense levels of thermal energy, heat treatment processing manipulates microstructure compositions until a desired mechanical or physical property is achieved. However, even the finest operation can go astray. A defect forms, it’s held over as the part cools, and the requisite heat toughening parameters are not realised. To stop such occurrences in their tracks, plant engineers trace and correct common heat treatment defects.
Decarburisation Weakness
Occurs when the carbon at the surface of a steel part reacts to the hot furnace atmosphere. Taking the form of a gaseous phase, the alloy-strengthening carbon atoms are lost. The maximum depth of decarburisation emerges as the process continues, with more carbon being diffused from the part’s interior makeup. To correct this issue, a protective atmosphere is employed as a migration prevention mechanism. If the part is already showing signs of decarburisation, the effect can be reversed by heat treating the workpiece, which typically means employing a carburisation cycle.
Minimising Dimensional Warping
This time around, the engineering team is going to get a workout. That’s because warping issues aren’t limited to a single causal factor. Indeed, work stress is a known culprit here, as is non-uniform heat treatment work. Even quenching stations deserve some of the blame, although it’s usually the lowering mechanism that causes part of the hot workpiece to enter the quench pool before the rest of the metal part. A normalising operation corrects already distorted parts. To stop this problem from happening, inspect the heating and cooling mechanisms to ensure they’re working uniformly.
Stopping Quench Cracking
Extreme thermal fields introduce invisible quantities of metallic stress. Complex geometrical profiles further complicate this issue. The metal part enters the quench pool, the aggressive thermal fields pull and tug at the geometry, and the heated alloy microstructure fractures. The cracks propagate along potential fracture lines. Tempering is utilised as a crack prevention process here, but it may be wise to switch to an alternative quenching medium. Additionally, use two-piece designs for those geometrically complex parts.
Some defects are caused by furnace flaws and conveyance errors. A leak in the furnace or a damaged induction element will obviously cause parts distortion. Elsewhere, a poorly programmed project is overheating the workpieces. A damaged seal is also causing oxidisation scale, so the maintenance program has fallen short. Maintenance related or machine fault, operator error or material-based flaw, the problems must be addressed before they spread to the entire batch. Fortunately, many of these defects can be reversed by a secondary heat treatment process. Otherwise, a definitive remedy, such as protective gas or an alternate quench medium, must be actioned.
Modern alloys are among the toughest materials known to man. A selected alloy, perhaps destined to become a key part in a massive construction project, can handle massive loads, yet still flex ever so slightly when a stiff breeze blow. However, despite being remarkably durable, even the hardest alloys become fatigued. Time is a primary ingredient here, but there are other forces at work.
Determining the Causal Factors
Loading and unloading effects cause metal parts to crack. The massive structure mentioned above is perhaps a crane this time, and the superstructure of this heavy lifter is experiencing intergranular cracking. Elsewhere, a roving eye has spotted cracks in the plating of a pressure vessel. A bowing and flexing effect is distorting the alloy lining. As a fluid changes state or expands, internal stresses and external material surfaces are in conflict. The stress is wrenching the metal and torturing its microcrystalline structure. Heat expansion and cooling, loading and unloading effects, these transient forces create unendurable stress, which then manifests as metal fatigue.
Heat Treatment Repairs
First of all, identify the stress factor. Cyclic stress, the example mentioned above, isn’t the only culprit. There are vibrational events, which propagate along metal surfaces until they find material weak spots. Corrosive chemicals weaken and even transform formerly hard steel parts into brittle shadows of themselves. So, what can be done to remedy metal fatigue? As ever, we turn to heat treatment technology. Think about what’s happening to the alloy workpiece. It’s too rigid, so it’s not stress-capable. By employing a heat treatment process, we restore ductility to the metal component. The cracks no longer propagate when the alloy is heated, quenched, and tempered. In fact, the fracture lines can reverse. A lengthy cooling period is typically required to achieve this fracture-negating goal, but that process requirement is a small sacrifice, considering the stress-relieving gains.
A Purpose-Designed Heat Treatment Solution
Normalization is chosen as the internal stress remover, with the process applying 900°C of material transformative thermal energy. Quenched and air-cooled, the broken grain structure within the formerly stressed metal part benefits from a newly imbued microcrystalline structure, which is uniform and free of crack-inducing tension.
To really solve this issue, reduce the cyclic energies that are impacting the alloy parts. Remove the vibrations by breaking the propagation paths. Install hoses, or simply eliminate the root cause. In the short-term, machining and penetration welding can help somewhat, but these tools don’t address the underlying issue. For a long-term answer, heat treatment energy sinks deep into a fatigued part to find and release all crack-eliciting stress.