Study of Residual Stresses and Microcracks in High Strength Steel Weldments; An Investigation of Microcrack Formation and Stressed Growth Using Laser Microscopy.

摘要

The confocal scanning laser microscope (CSLM) allows viewing welds with a resolution to 0.25 microns. The image processor of the CSLM provides effectively infinite depth of field by superposing the in-focus parts of images. Slopes of more than 30 degrees do degrade the image, however. 4-point bend tests of bead-on-plate welds on 1 1 mm thick mild steel, HY-100, HSLA- 100, and HY-130 were performed to strains of 0 to 0.02 parallel to the weld bead. A longitudinal, single-pass weld bead of E7014 was manually applied to one specimen of each material with shielded metal arc welding. Displacement-load and moment-curvature results were as expected. Images from gold-shadowed replicas studied using the CSLM were recorded both on video tape and photographically. Initial microcracks 1 200 microns long were observed in the weld metal. No microcracks were observed in the HAZ of any material. Slag and oxide layers sometimes interfered with observing the weld metal and adjacent baseplate. Under monotonic loading, cracks widened with little growth and Widmanstatten structures appeared unchanged. Crack-like features, 1.5 microns long were present on the weld and on the HAZ. These features are probably oxide cracks and did not appear to grow when stressed.