Common Heat Treatment Defects and Remedies27 August 2018
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.
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.
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