A Study On Development Of Detecting And Analyzing System For Ms Temperature In The HAZs Of Carbon Structural Steels

It is usually difficult to find out the exact phase transformation temperatures in the heat affected zone (HAZ) of carbon and alloyed steels. The only alternative and referential way to predict the phase transformation temperature is to make use of the continuous cooling transformation (CCT) diagrams, which are measured dilatometer curves that have not taken the effect of practical welding conditions into account. In order to improve the quality of welding joints, it is essential to investigate the phase transformation behavior of the weld HAZ under real welding conditions. For this purpose, a testing and analytical system for measuring welding thermal cycles is developed The main hardware of system is made up of thermo-couples, a temperature converting unit, a data acquisition card and a microcomputer. Using the system, the data of welding thermal cycles of in the HAZs of 45 steel and 40Cr steels were collected in real time, upon which several important welding thermal recycle parameters including peak temperature value Tm and cooling time length t 8/5 were calculated. Further analysis shows that there is a temperature plateau point at which the temperature-time curve has the largest absolute quadratic differential value. It is concluded that the temperature plateau point is the martensitic transformation starting point according to thermodynamic analysis, CCT diagram referencing and metallographic observation.The paper further compares and analyzes the microstructure and hardness changes in the HAZs of the two steels welded under different welding conditions,thus it offer important according and information for making the best welding processing and improving quality of the welding products,as well as it supply Offer one new-type research approach which can predict HAZ organization and martensitic transformation. The above-mentioned research results have theoretic significance as well as practical application values in several aspects, such as in understanding the mechanism of microstructure formation in the heat-affected zone, providing the alternative method for making up CCT diagram of new materials, and optimizing weld procedures, etc.