Mechanism Of Bainitc Phase Transformation In The HAZs Of Low-Alloyed Structural Steels

16MnDR and 15MnMoVN, as one kind of typical Low alloy structural steel, are widely used in manufacturing. During welding, transformation of Bainite and Martansite takes place in the zone of heat affected zone (HAZ). In the past, people investigate the transformation structure using heat simulation and metallographic or electron scanning. Today, with the development of computer technology, a new type of computer-aided system for testing and analyzing weld thermal cycle is presented in this paper. By the system, the following work can be fulfilled: collecting thermal cycle data in welding HAZ , analyzing the curves, forecasting the microstructure and transformation of HAZ under the constraint/or non-constraint and pre-heating/or non-pre-heating conditions. The Hardware part of the system consists of thermocouples, a magnification filter circuit, an A/D interface board and a microcomputer. The system is an integration of temperature data filtering, data collecting, data storing, data processing and data analyzing. The results show the consistency between the features found in weld cooling by the and metallographic structures, and the difference of structure transformation progress and the final results of HAZ between constrained stress and non-constrained stress: the micro structural properties of the former is better than that of the later and the transformation temperature of the former is obviously higher that of the later. By analyzing the microstructure, the results show that. It has been found that constraining strain, preheating and the peak temperature value have the influence on transform in the progress of cooling in the near weld zone of 15MnMoVN and 16MnDR steel by improving the microstructure of the M-A structure and eventually increasing the toughness or by changing the temperature of Bainite transformation. Comparing with the weld structure of constraint and non-constraint, it is found that the mechanical properties of the former are better than that of the later. The system is applicable to study the microstructures and mechanical properties of HAZ. It may also be helpful for establishing the welding CCT curves for new materials and optimize the welding procedure.