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For high volume process annealing, gas fired conveyor furnaces are often used. For large workpieces or high quantity parts, car-bottom furnaces are used so workers can easily move the parts in and out. Once the annealing process is successfully completed, workpieces are sometimes left in the oven so the parts cool in a controllable way.
Ferrous alloys are usually either "full annealed" or "process annealed". Full annealing requires very slow cooling rates, in order to form coarse pearlite. In process annealing, the cooling rate may be faster; up to, and including normalizing. The main goal of process annealing is to produce a uniform microstructure.
This allows quenching to start at a lower temperature, making the process much easier. High-speed steel also has added tungsten, which serves to raise kinetic barriers, which, among other effects, gives material properties (hardness and abrasion resistance) as though the workpiece had been cooled more rapidly than it really has. Even cooling ...
Once the workload is properly "cased", the metal is quenched using oil or high pressure gas (HPGQ). For HPGQ, nitrogen or, for faster quench helium, is commonly used. This process is also known as case hardening. Another low temperature application of vacuum furnaces is debinding, a process for the removal of binders.
The process was used throughout the ancient world, from Asia to Europe and Africa. Many different methods and cooling baths for quenching have been attempted during ancient times, from quenching in urine, blood, or metals like mercury or lead, but the process of tempering has remained relatively unchanged over the ages.
The term hardened steel is often used for a medium or high carbon steel that has been given heat treatment and then quenching followed by tempering. The quenching results in the formation of metastable martensite, the fraction of which is reduced to the desired amount during tempering. This is the most common state for finished articles such as ...
Differential tempering is obtained by quenching the sword uniformly, then tempering one part of it, such as the spine or the center portion of double edged blades. This is usually done with a torch or some other directed heat source. The heated portion of the metal is softened by this process, leaving the edge at the higher hardness. [4]
Annealing is a process of slowly cooling hot glass objects after they have been formed, to relieve residual internal stresses introduced during manufacture. Especially for smaller, simpler objects, annealing may be incidental to the process of manufacture, but in larger or more complex products it commonly demands a special process of annealing in a temperature-controlled kiln known as a lehr. [1]