Part 1: Metallurgical Characterization of the HAZ in A516-70 and Evaluation of Fracture Toughness Specimens and the Effect of Shallow Cracks in Welments and the Development of CTOD Criteria

An extensive study was conducted on A516 grade 70 steel to investigate the effect of shallow cracks in weldment HAZs. This program was a cooperative effort with the University of Kansas sponsored by PVRC Committee on Failure Modes.

Charpy V-notch (CVN) and crack-tip opening displacement (CTOD) tests were utilized to characterize the fracture toughness behavior of the heat-affected zone (HAZ) of A516-70 SMAW weldments. Initial tests were conducted on thermally simulated HAZ specimens. Specimens for CVN and CTOD tests were also extracted from weldments fabricated utilizing different weld procedures to confirm the results obtained from the thermally simulated HAZ specimens.

The test results are explained on the basis of microstructural features in the HAZ and fractographic examination. Optical light microscopy (OLM) and scanning electron microscopy (SEM) were used for these evaluations. A computer-assisted imaging system was also utilized and it proved to be a powerful tool for fracture surface analysis. It was possible to predict toughness of actual weldment HAZ specimens by determining the area fractions corresponding to ductile tearing and cleavage. In addition, a linear correlation between the CTOD and/or CVN-absorbed energy and depth of the ductile zone and the areal percentage encompassed by the brittle region was found.

Part 2: Comparison of the CTOD Fracture Toughness of Simulated and Weldment HAZ Regions in A516 Steel with Deep and Shallow Cracks

An experimental fracture toughness study was performed on A516 grade 70 steel to compare crack tip opening displacement (CTOD) values of laboratory specimens with simulated and actual weld microstructures. This study was undertaken as a joint project with the University of Tennessee. Specimens with simulated microstructures from the coarse, fine or intercritical zones were produced in a Gleeble thermal simulator with thermal cycles that represented a 39.4 Kj/cm (100 Kj/in) and 11.0 Kj/cm (28 Kj/in) heat input. Because the thermally simulated specimens were limited to 12.5 X 12.5 mm (0.5 X 0.5 inch) cross-section, the CTOD tests at the University of Kansas were performed on these same size three-point bend specimens. The effect of crack depth to specimen width, a/W, ratio on CTOD toughness was studied for a/W ratios of 0.2 and 0.5. Experimental testing also included Charpy V-Notch (CVN) impact tests at equivalent CTOD temperatures and tension tests of each of the simulated microstructures with 39.4 Kj/cm (100 Kj/in) heat input.