One of the most popular processes in surface engineering is coating the surface of the specimens since the surface of the industrial specimen is the most important part of them, many destruction mechanisms are usually begins from their surface. Therefore, cladding can be defined as a low cost method for coating by filler metal on a work piece in order to improve hardness and microstructure for this purpose, in this study, cladding of 304stainless steel and molybdenum synthesis from molybdenum trioxide and its effects on the microstructure and hardness of the resulting clad with and without the presence of acid boric and carbon is studies simultaneously. Due to this end, 304stainless steel samples are cladded by GTAW process with a constant current and speed of welding. At first, the powder mixture including MoO 3 , H 3 BO 3 and C mixed in a ball mill with a specified mole ratio. Then, pre-placed paste, cladded on the base metal. Then, placed in an oven with the temperature of 80?c for an hour, finally, the samples welded. Result shows that by increasing boric acid, because of its exothermic, the temperature of weld pool increases and more molybdenum synthesis is formed from molybdenum trioxide which causes in increasing the ratio of (Cr/Ni)eq. By increasing this ratio, the structure would be as lathy ferritic. While, by adding carbon, the ratio decreased and microstructure of clad change to vermicular ferrite. To identify the formed phases in clad surface XRD has been used. Based upon XRD of cladded samples, existence of MoC, FeMo, MoNi4 phases Show, that molybdenum synthesis is formed in arc. By using micro hardness test, hardness profile of samples provided which most hardness obtained in MoO 3 +C+1.5H 3 BO 3 sample. Also, hardness of 190 viker, in base metal increased to almost 340 viker in the clad. Furthermore, using macrostructure images of weld pool, it can be concluded that by increasing the amount of ferrite in clad as a result of its increasing thermal-conductivity, the ratio of “depth to with” of the welded clads have been decreased. Keywords: Synthesis, molybdnum, 304 stainless steel, lathy ferritic, GTAW
