Marcal made an early summary of experiences from welding simulation , Chihoski sought a theory to explain why welds cracked under certain conditions but not others he explained that the weld was divided into longitudinal and transverse strips and then computed the thermal expansion and contraction in each of these strips due to the temperature field for edge and butt weld joints and concluded intense compression biaxial stress fields exists near welds , Chihoski developed a Moiré fringing technique to measure displacements during edge and Butt weld it is considered that chihoskis work is one of the most important in computational welding mechanics Reviews of Karlsson, Smith , Radaj and Goldak include references to simulations performed up to 1992 . But Finite Element Analysis (FEA) methods gained a wide acceptance only over the last decade The Moiré fringing technique of Chihoski is today one of the most powerful means of assessing FEA of stress and strain in welds. The use of finite difference methods is a transition between analytical and finite element methods. The main advantage of the finite difference method is that it is simple and easily understandable physically but the finite element method has achieved considerable progress and has powerful techniques for solving thermal-mechanical manufacturing process such as welding. Pilipenko A in his thesis Computer simulation of residual stress and distortion of thick plates in multi electrode submerged arc welding showed development of an experimental, numerical and analytical approach to the analysis of weldability

1.The first stage included a solution of the transient nonlinear 3D thermal problem, in which model of the welding process is the driving heat source of the appropriate power. The thermal properties of the materials are functions of the temperature. After laying the first joint weld, at the next stage of a solution the members modelling the second joint weld were taken into account. The obtained earlier solution is transferred to a new model and the all process repeats. As a result, the variation of temperature fields during welding is obtained, and the time of the joint weld cooling up to an ambient temperature was found. 2. The second stage included the solution of a series of the quasistatic nonlinear problems of the plasticity theory with account of thermal fields obtained at the previous stage. For simplification of a problem the transferring of metal in to a fluid phase during welding was not considered. For accounting plastic strains, the bilinear model of kinematical hardening was chosen. As a result, the plastic strain localization areas were found out, and it was stated that the contact pressure field is characterized by asymmetrical distribution on the contact surface and stabilizes with time.. As a result, the variation of temperature fields during welding is obtained, and the time of the joint weld cooling up to an ambient temperature was found. The basic theory of heat flow that was developed by Fourier and applied to moving heat sources by Rosenthal and Rykalin in the late 1930s is still the most popular analytical method for calculating the thermal history of welds but Rosenthal's point or line heat source models

are subject to a serious temperatures errors in or near the fusion zone and heat affected zone The infinite temperature at the heat source assumed in this model and the temperature sensitivity of the material thermal properties increases the error as the heat source is approached. To overcome these models researchers have used FEM to analyze heat flow in welds. Pavelic suggested that the heat source should be distributed and proposed a Gaussian distribution of flux deposited on the surface of the work piece FIG 4.1 below represents a circle surface heat source and a hemispherical volume source, both with Gaussian normal distribution. Pavelic’s disc model is combined with FEM analysis to achieve significantly better temperature distributions in the fusion and heat affected zones than those computed with the Rosenthal Model.

Paley and Westby used a constant power density distribution in the fusion zone with a finite difference analysis and showed the heat source should be distributed throughout the molten zone to show the effect of digging arc. Finally a 3 dimensional double ellipsoid model was proposed that showed a better heat distribution.

The conservation of energy is the fundamental principle in thermal analysis Therefore in heat transfer theory energy is considered and stress, strain and displacement are ignored .The temperature field in the neighborhood of weld pool can be obtained by specifying the heat input all weld heat source models based solely on the heat equation, they specify either the source term Q, or choose a boundary of the domain near the weld pool and prescribe the temperature or prescribe the flux q on that Boundary as a function of time. Heat conduction is assumed to be governed by Fourier’s law for the process is given by Heat required for melting the metal and creating the joint comes either from arc, flame or friction, in case of arc welding this is controlled by current, voltage and wire speed, welding speed etc. In order to model the welding heat source the knowledge of properties such as thermal conductivity, specific heat , latent heat , youngs modulus, poisons’ ratio are required , if the weld heat source model is intended to predict hot cracking accurate data near the weld pool is essential .Or if the weld heat source is intended to predict the distortion in steel accurate data up to temperatures of 800 degree centigrade is important.

overall mechanism is that austenite stabilizers depress the A1 temperature, while ferrite stabilizers have the opposite effect, the primary aim of adding alloying elements to steels is to increase the hardenability, that is, to delay the time required for the decomposition into ferrite and pearlite. This allows slower cooling rates to produce fully martensitic structure. Naturally, the chemical composition then influences the material properties indirectly through the microstructure formations in welds exert a strong influence on the final material properties including hardness, wear resistance and toughness. Hence, the behavior of a material on the macroscopic scale and, in its entirety, the integrity of the welded joint, depends largely on its microstructure.

The prediction of the formation of the microstructure during welding, and of other solidification processes for that matter, can be a determining factor in the material’s cost of a structure and a decisive factor for the structure's performance. Nowadays, the