Surface Decarburization of the Hypo-Eutectoid Carbon Steel C45 during Annealing in Steady Air at Temperatures T > A(C1)

The non-alloyed hypo-eutectoid steel C45 was annealed in a steady air atmosphere at T > A(C1) using three temperature ranges and annealing times t(a) = 1/2, 1 and 2 h. The oxidation and decarburization obeyed the parabolic law of growth with time and increased with higher temperatures and longer annealing times. The maximum visible thickness of the decarburized surface layer for all the temperature ranges can be predicted from the equations for the oxidation of iron and from the Van-Ostrand-Dewey equation (derived from Fick's 2nd law) for the distribution of carbon. In this relation, the equation that gives the lowest values for the diffusion coefficient of carbon in austenite is appropriate. It was found that the Van-Ostrand-Dewey equation is applicable at T > A(C1), despite the fact that it was derived for the decarburization of austenite at T > A(G). A comparison of the results showed that the depth of the decarburization with a carbon content of 0.91-0.98C(0) can be determined with optical metallography and that both the theoretical and measured values are, in general, mutually accurate in the regions of 93% to 98%. In practice, visible decarburization might not be present under some conditions while annealing in a steady air atmosphere. The reason for this is that the process of oxidation is faster than the process of decarburization. Considering the oxidation of steel under these conditions, annealing in a steady air atmosphere can be performed without any protective measures.