Coarse annealing

In cast pieces with damaged surfaces in the area of the crystal layer, the temperature at which the rupture of the cast pieces can occur is low. For this reason, the cooling rate must be reduced with decreasing temperature in such a way that the reduction of the thermal conductivity is compensated or, better, overcompensated. Through over compensation, a part of the permanent compressive stress at the surface of the blank is already established during the coarse annealing. Thus, the probability of rupture is reduced. 

The temperature is reduced to about 1400 °C for casting, and the useful content of the crucible is cast into one or more preheated steel moulds. In a coarse annealing process, the temperature is reduced slowly to room temperature. In the case of the castings for AXAF this took about four months. 

For the above reason, strict requirements are placed on the initial coarse annealing ... 

Coarse annealing may be aceomplished, particularly in a laboratory setting, by following these guidelines ... 

On tempering or annealing martensite, bainite or even pearlite at even higher temperatures (about 970K) a structure consisting of coarse cementite spheroids (readily visible in a light microscope) in a ferrite matrix is obtained. This is the most stable of all ferrite/cementite aggregates, and it is also one of the softest. 

The first series of experiments was devoted to determination of the effect scale on the slit temperature. In the second series of experiments we used the slits formed by massive plates of gold-silver alloy and pure palladium, varying considerably their microstructure. Namely, they were initially highly cold-hardened samples and later on the ones annealed at the recrystallisation temperature. In other words, we deal with either fine-grained or coarse-grained metal surfaces. 

High-purity UC and UCj can be prepared by arc melting the elements. Annealing below 1700°C, preferably under pressure, is required - to form UjCj. Because of the high reactivity of the powdered carbides, all studies should be done with material that is as coarse as possible. The finely divided carbides are pyrophoric and aquire any oxygen in an otherwise inert atmosphere at RT. 

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