Approximate Soaking Time for Hardening, Annealing, and Normalizing Steel
18 September 2019Three primary heat treatment phases break down into multiple branches and subsets. First comes the heating, the stage that thermally alters a part’s microcrystalline structure. As the work comes to a climax, the cooling phase allows the material to return to room temperature. At least as important, an “in-between” phase holds the furnace temperature at a predetermined apex level until all of the material structure is completely transformed.
What Is Approximate Soak Time?
Heat treatment graphs aren’t node-based, they’re shaped by plateaus. Like little thermal mesas, the temperature reaches a set level, and then it’s held there for a preordained amount of time. Otherwise, if those heat shelves were removed, there’d be no time for a metal part to uniformly accept its newly ingrained crystalline structure. Of course, these timed intervals aren’t arbitrarily assigned, but they’re still regarded as approximations. Remember, workpiece dimensions vary. To carry out the approximation process, heat treatment engineers measure item thickness, then there’s the time that the part is held at the soak temperature. Furnace temperatures are key of course.
Hold Times for Different Heat Treatment Processes
To avoid trial-and-error work and impossibly intricate engineering equations, soak time tables can be consulted. However, as every furnace technician knows all too well, there are several different structures altering branches. For pure hardening work, approximate soak times are a function of a component’s density. The temperature in the furnace rises, the alloy diffusion effect accelerates, and then the quench phase rapidly cools and hardens the material. Moving over to an annealed component, the cooling period is extended. Annealed parts cool over many hours. Consequently, the soak time plateau also alters. As for normalized parts, the process requires higher temperatures to create stress-free parts, which are maybe not quite as ductile as an annealed workpiece. Again, the soak time has to alter to compensate for the changes in the heating and cooling profile.
Ultimately, a three-sided line graph takes form during a heat treatment operation. That’s something of a simplification, but it’s essentially the truth. A temperature line ramps up as a heat treatment furnace hardens a metal workpiece. Up top, closing in on an alloy’s core transformative temperature, the soak time holds the temperature for a predetermined period, until its entire mass transforms, then the notch-shaped graph drops. It cools to room temperature. Only, it’s not quite that simple. The angles of those lines, the rate at which the heat piles into a component will alter as different heat procedures are undertaken. With hardening, the work proceeds apace. That heat “notch” changes significantly when the job requires an annealing or normalizing service.
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