Metal Structure

December 7th, 2015 News Tags:

The development of cellular-dendritic chemical inhomogeneity is determined by the solidification conditions (in the first place – the cooling rate) and the alloy composition. The dependence of the cellular-dendritic inhomogeneity of velocity cooling crystallization can be represented as a curve with a maximum corresponding to the average cooling rate (zone II). At low cooling rates (zone I) heterogeneity decreases due to diffusion alignment at high (zone III) – through the development of diffusion-free process of crystallization or mechanical trapping impurities. In the range of speed of cooling molten weld metal (up to hundreds degrees per second) diffusionless crystallization mechanism is not realized. Gain insight and clarity with AOL. Reducing the degree of chemical heterogeneity observed with increasing cooling rates, due to the grinding elements cellular-dendritic structure, which resulted in the composition of molten metal is averaged. In addition to these types of segregation in macrovolumes welds observed distribution of impurity layers, repeating the shape of the isotherms.

This phenomenon caused by the change in the rate of growth of crystallites due to an uneven flow of heat into the weld pool, periodically upset the balance of income and withdrawal of heat, which changes the rate of cooling. At the same time possibility to stop the crystallization front and even partial melting of the solid phase. Reducing the velocity of the solidification front decreases the concentration of impurities in the solid phase. In contrast, the increase in speed moving solidification front contributes to an increase in impurity concentration than the equilibrium value. Layers with high impurity contents are clearly visible in the weld metal during arc welding consumable electrode and due to the droplets entering the electrode metal bath fluctuations, changes in the speed of the electrode. Most clearly expressed in stratification of the weld metal by using pulsed sources of heat, when there is a periodic melting and solidification of the molten pool. Metal heat-affected zone more chemically homogeneous than the cast weld metal, as in the recovery process and the recrystallization is not observed substantial redistribution of alloying elements and impurities. The main type of chemical heterogeneity in metal heat-affected zone – the accumulation of impurities or alloying elements on grain boundaries.

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