In Situ Evaluation of Tensile Properties of Heat-Affected Zones from Welded Steel Pipes

摘要

Currently, measuring the local tensile strength in inhomogeneous materials is not standardised, nor accepted techniques are available despite such technique would be beneficial in a variety of technological applications. Thus, this work introduces an innovative method for assessing stressstrain properties at a sub-millimeter scale and illustrates the potential of the technique by evaluating the strength of a sub-region in the HAZ from welded steels pipes. The method employs a fully instrumented stage inside a scanning electron microscope that stretches small tensile specimens (2.0 mm x 0.5 mm cross-section, 12.5 mm gage length) while registering detailed images of the deformed region. The specimens, cut from full-scale welds, include in their gage length weld metal, base metal and HAZ and have an 85 mu m period grid of evaporated lead on their surface to visualise the deformation. Upon straining, local strain is determined by correlating sequential images of the specimen surface with an open source code for particle image velocimetry. The calculated local strain within the HAZ and the load values recorded during testing are converted into a local stressstrain response. The results for two different heat inputs agree with usual, but indirect and less accurate assessments procedures, including local hardness measurements and notched bar testing.