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.