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The effect of hardness and grain size of steels and their welds on fracture stress and sensitivity of Liquid Metal Embrittlement Cracking in molten Zinc (LMEC) was investigated. The experiment was conducted using commercial weldable structural steels and Sustained Load Test (SLT) was conducted in molten Zinc with heating range of 430°C-470°C.The results obtained are as follows.(1) The LMEC behavior of steels and welds in molten Zinc is of thermal activated process, and fractures are intergranular (IG) type which corresponds to actual LMEC in hot dip galvanizing.(2) Estimation of fracture stress of steels and welds in molten Zinc is obtained by measuring Vicker's hardness and grain size, and these estimation values are correspond with test results.(3) The sensitivity of LMEC is affected by hardness of steels and welds.

The liquid metal embrittlement cracking (LMEC) of heat affected zone (HAZ) of steel in hot dip galvanizing was investigated. The experiment was conducted using synthetic HAZ of JIS G 3444 STK55 and Sustained Load Test (SLT) at 370°C-470°C heating range in liquid (molten) and solid Zinc.The results obtained are as follows.(1) LMEC behavior is of thermal activated process and LMEC sensitivity is affected by hardness of HAZ.(2) The coarse grain region in HAZ showed the highest LMEC sensitivity.(3) The interfacial energy of steel is reduced by the existence of Zinc.(4) The reaction rate process of LMEC of steel is controlled by grain boundary diffusion of Zinc.(5) The nucleation and the propagation of LMEC in steel are likely to have a close relation to the reduction of interfacial energy due to the grain boundary diffusion of Zinc.

This study deals with the estimation of occurrence and preventive condition of the Liquid Metal Embrittlement cracking (LMEC) due to hot dip galvanizing using Y-Groove Crack ing Test pieces.The results obtained are as follows, (1) Using Y-Groove Cracing Test piece is appropriate for the estimation test of the LMEC in welded steel structures.(2) The critical condition for LMEC occurrence is indicated by the relation between the intensity of restraint and the maximum hardness in HAZ (Hv max.).(3) The LMEC preventive condition for welded steel structures to be galvanized are obtained by the Hv max. under the condition that welded joints have the most severerity value of the intensity of restraint in each type of structure. For example, the LMEC preventive condition of weldment in transmission tubular tower structures is assumed that Hv max. keeps less than 260.