Determinação dos diagramas de aquecimento e resfriamento do aço API 5L X70Q através do calor latente de transformação de fase

Abstract

The heat imposed by the welding is responsible for metallurgical changes in the material, and as a result determines the mechanical properties and susceptibility to cracks. The named heat-affected zone (HAZ) deserves special attention, because it represents a steep gradient of microstructures and grain sizes, becoming a region where cracks usually start and propagate more easily. Unfortunately, the behavior of a HAZ cannot be predicted by NDT. Therefore, to know the effect of welding energy on the HAZ formation is a necessity, in order to mitigate the material deterioration in this region. One means of studying HAZ is through the Continuous Cooling Diagram (CCT) of the material, which is obtained after physical simulation. Commercial equipment, such as the Gleeble® and high-speed dilatometer, make it possible to study this region, but they require high capital investment. The general objective of this work was to present and evaluate a different approach for HAZ physical simulation, based on samples heated by Joule effect and natural cooling controlled by systematic variations of the sample geometry. The determination of the starting and end points of transformation, both in the heating (novel for welding) and in the cooling, were made by differential analysis between the predicted thermal cycles without transformation and the thermal cycles altered by the latent heat of transformation. The specific objective was to study techniques able to mitigate the effect of physical simulator characteristics, such as the relation between heating rate and cooling (as in real welding) and intrinsic errors, such as the deviation between actual and overshoot value of the peak temperature. For this evaluation, a HSLA (high stress low alloy) API X70Q steel was chosen, imposing thermal cycles with cooling rates varying from 3 to 415 °C/s. Metallurgical characterization and microhardness were made on the sample cross sections and a heating transformation and a CCT diagrams were raised and compared with the ones of similar steels for validation. The results demonstrated that this approach works efficiently. As it uses cooling control based on the sample geometry, it creates an intrinsic relationship between heating and cooling rates, similarly to actual welding. It is also concluded that problems like overshoot can be eliminated. After validation with the HSLA, it was found that the starting and finishing temperatures of the transformations during the heating decrease for lower heating rates. It was also possible, through the CCT diagram, to find ranges of welding energies that characterize the material with microstructures and hardness with different levels (high, low and without risk) of tenacity/susceptibility to cracks.