Mechanism Of Martensitic Phase Transformation In The HAZs Of Constrained Welding Joints

A computer-aided system for testing and analyzing welding thermal cycle is presented in this paper. Its hardware part consists of a microcomputer, thermocouples, a temperature converting unit and a A/D card. To calculate and analyze the parameters of welding thermal cycles, the system is programmed using C++ Builder 7.0 language and Labview language. The system is an integration of data collecting, data processing and data analyzing. It has the advantages of high precision, easy to use and high ratio of quality to price.By using the system, the following work is carried out: collecting thermal cycle data in weld HAZ of 45# and 40Cr steels, analyzing the temperature-time curves, forecasting the microstructure, hardness and phase transformation of HAZ under the constraint/or non-constraint conditions.By analyzing the second-order differential algorithm, a remarkable inflexion can be found on the temperature-time curve in the range of 800-300 ℃, and this is judged to be the starting point of martensitic phase transformation by observing the microstructure of weld joints of 45# and 40Cr steels.It has been found that constraining degree has significant influence on transform behavior near the weld zone of 45# and 40Cr steels by improving the microstructure and eventually increasing the toughness. Comparing the microstructures in constraint and non-constraint weld joints, it is found that the mechanical properties of the former are better than that of the later and the martensitic transformation temperature of the former is obviously higher that of the later.The system may be helpful for establishing the welding CCT diagrams for metallic material because temperature points of phase transformation can be easily obtained in the thermal cycle curves of HAZ by using the system.