What is Meant by Normalising Process in Heat Treatment?06 December 2017
Engineering terms usually help out by subtly suggesting their meanings. If someone talks about a quenching operation, then the process becomes self-evident. But that’s not always the case, certainly not when we talk about the normalizing process. It’s a generic term, after all, one that has a dozen different meanings. In heat treatment terminology, however, there’s only one possible meaning. Let’s talk about stress relief.
What is The Normalizing Process?
Steel is generally normalized by inserting a workpiece into an extremely hot furnace. That controlled thermal envelope heats the steel component until it reaches the material’s stated critical transformation range. Hovering just above this transformation point, the grain of the steel alters. It acquires a more uniform structure. After a predetermined hold period, a length of time that maintains the thermal load, the component is allowed to cool in the open air.
What Does Normalizing Mean?
What’s it all about, this normalizing operation? The process has been briefly described, but we’re still left to wonder over the reasoning behind the process. Well, the clues are held in the resulting uniform carbide size and even grain distribution. Cold worked steel has a form of memory. Those ‘memory cells’ form inside the alloy part’s microcrystalline structure. Stress is the hardest memory to eliminate. The normalizing procedure wipes out that memory. It relieves material stress, restores ductility, and generally strengthens that steely backbone.
The Technical Side
As the soaking temperature is maintained, somewhere between 810°C and 930°C, the grain refinement phase is triggered. The normal air cooling stage further enhances the smaller metal grains and distributes the newly transformed cementite structure. Like a magnet wiping the patterns on an old reel-to-reel tape deck, the normalizing process erases work stress until the structural base of the part is ductile but still hard. Used to improve component workability, this technique is inexpensive, especially when compared with the annealing method that carries out a similar role inside a heat treatment facility.
Loaded with pearlite, with a refined grain type that’s rich in ferrite and cementite, the ferrous part is freed of mechanical irregularities, of work hardened stress, and made ductile. Cooled in the open air or in a nitrogen atmosphere, the consistent metal grain is transformed until it reaches a state of refined uniformity. Listen for other descriptive labels when this work is undertaken, like the heat and hold process. And, just as a by-the-way, do remember that the ferrous composition and thickness of the subject workpiece will affect the results of a normalizing treatment.
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