Kinetics And Eutectics

Eutectic: A eutectic or eutectic mixture is a mixture of two or more phases at a composition that has the lowest melting point, and where the phases simultaneously crystallize from molten solution at this temperature. The proper ratios of phases to obtain a eutectic are identified by the eutectic point on a phase diagram.

Eutectoid:When the solution above the transformation point is solid, rather than liquid, an analogous eutectoid transformation can occur. For instance, in the iron-carbon system, the austenite phase can undergo a eutectoid transformation to produce ferrite and cementite (iron carbide), often in lamellar structures such as pearlite and bainite.

Since low-alloy steels are basically of the oil- or air hardening types it

The problems associated with welding stainless steels depend upon whether the steels are of the ferritic or austenitic types. Ferritic high chromium steels suffer from grain growth in the heat affected zone and also the formation of martensite if the steel contains sufficient carbon and is permitted to cool at a rate which will give rise to air-hardening.

c) Weld pool solidification

Most knowledge of weld pool solidification is derived from the extrapolation of the knowledge of freezing of castings, ingots, and single crystals at lower thermal gradients and slower growth rates. Therefore, parameters important in determining microstructures in casting, such as growth rate (R), temperature gradient (G), undercooling (δT), and alloy composition determine the development of microstructures in welds as well. However, microstructure development in the weld zone is more complicated because of physical processes that occur due to the interaction of the heat source with the metal during welding, including re-melting, heat and fluid flow, vaporization, dissolution

This is a mixture of parent metal, electrode, filler metal; the ratio depending upon the welding process used, the type of joint plate thickness. Weld metal zone is a cast metal of particular composition of mixture that has cooled; its microstructure reflects the cooling rate in the weld. Depending upon the chemical composition a martensitic structure in the weld indicates a very high cooling rate; fine pearlite and coarse pearlite shows comparatively slower cooling rates. From the molten weld pool the first metal solidifies epitaxially upon the solid grains of the un-melted base metal. Depending upon the composition and solidification rates the weld solidifies in a cellular or dendritic growth mode. Both modes cause segregation of alloying elements and consequently the weld metal is less homogeneous on the micro level than the base metal and therefore cannot be expected to have the same properties as wrought parent metal unless the filler metal in the as deposited condition has same properties as that of parent