Studying Phase and Structural Transformation Upon Formation of Weld Joint of Rail Steel. Report 3. Using Thermal Kinetic and Isothermal Diagrams of Austenite Decomposition while Selecting Optimum Modes of Electric Welding

Abstract

During contact flash welding of rails, the metal in the area of thermal impact is heated and continuously cooled. Accelerated heating and subsequent intensive cooling during pulse flash welding lead to the formation of quenching structures, which subsequently leads to cracking and to brittle destruction upon operation of rail weld joint. This work studies capabilities of contact heating after welding in order to eliminate formation of quenching structures in metal weld joint of R350LHT rail steel. Thermal cycles during welding and subsequent contact heating have been recorded. The regularity of structure formation of metal weld joint (including the area of thermal impact) was determined during pulse contact heating for R350LHT rail steel. It is demonstrated that the contact pulse heating allows to prevent the formation of quenching structures and decelerates cooling of weld joint. In the case of non-optimum modes, the contact pulse heating can result in reverse effect. It has been determined that upon significant contribution of heat by contact heating, the metal cooling rate exceeds critical value and the transformation runs according to diffusion free mechanism with formation of coarse grain martensite structure. The use of thermal kinetic and isothermal diagrams of austenite decomposition with known thermal welding cycles allows to restrict significantly the search ranges of optimum modes of electric welding of railroad rails and subsequent contact heating. The use of optimum modes of contact heating allows to obtain minimum length of areas of thermal impact without the formation of quenching structures in weld joint of railroad rails.