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Microstructure of welded joint is bound up with its mechanical properties, in particular the weld metal zone, it occurs strong reaction of chemical metallurgy at high temperature, the course makes oxygen, nitroger and other impurities increase and carbon, manganese, molybdenum and other useful alloying elements reduce due to strong buring and evaporation. In addition, the welding materials will be deposited in weld metal so that the structure and properties of weld metal zone are different from base metal and HAZ, these all make it has great attention. Therefore, the detection and analysis of phase transformation process in weld metal have great significance.Using the temperature testing system, under two different heat input, through controlling welded cooling conditions, the root layer’s thermal cycle of weld metal for 10CrMo910 heat resistant steel was measured. Through SW-CCT diagram of thermal simulation, combining with metallographic observation and Vickers hardness test, transformation course was studied. The main conclusions can be drawn as follow:(1) Through the process of heating and cooling of pure aluminium together with theory checking of peak temperature of thermal cycle, the results showed that the welding thermal cycle test is accurate and reliable, it can provide indemnity for follow-up test.(2) The root layer of weld seam zone of 10CrMo910 steel in five different cooling condition under two different heat input were measured. It showed that the parameters of thermal cycle under different input and the same cooling rat are different, low heat input is low, high heat input is high; heating rat and peak temperature under the same heat input and different cooling condition is similar, but cooling is greater different,ω800-300℃below 5℃/s under alleviating cooling and above 70℃/s under saline cooling.(3) The results of structure observation and hardness test of room temperature showed that the structure and hardness values of same cooling rat and different heat input is little different, the structure and hardness values of same heat input and different cooling method is great different, the hardness values of alleviating cooling is about 200HV, saline cooling is about 360HV, the cooling rat is the main influencing factor of structure and properties as the chemical composition is certain.(4) From the results of thermal simulation, AC3 of 10CrMo910 is 930℃, AC1 is 760℃ and Ms is 490℃. When the cooling rat in the range of above 17.5℃/s, the structure at room temperature including M; when the cooling rat in the range of 4.7～17.5℃/s, the structure at room temperature is P+B; when the cooling rat is 0.69℃/s, the structure at room temperature is B+F.(5) According to SW-CCT diagram, together with microstructure and hardness values, course of phase transformation are: slow cooling, structure change from A to F and A to B, transformation temperature of B between 540 and 400℃; air cooling, from A to P and B, transformation temperature of P between 690 and 570℃; oil cooling, from A to M, transformation temperature is begin with 490℃.
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