Metal Fatigue: Causes and Repairs Using Heat Treatment17 August 2018
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.
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