Factors Influencing the Microstructural and Mechanical Properties of ULCB SteelWeldments

摘要

The U.S. Navy has maintained a continuous research, development and certificationprogram in High Strength Low Alloy (HSLA) and Ultra Low Carbon Bainitic (ULCB) steels as a replacement for the HY steels used in ship construction. The major benefit of the HSLA and ULCB steels stems from the relative ease of weldability as compared with the HY steels which require large amounts of preheating to prevent weld metal crack formation. This study focused on ULCB steel weldment mechanical properties which were shown to be largely dependent on the heat input and to a lesser extent the base plate composition, cover gas and nonmetallic inclusion composition and content. The heat input rate and subsequent cooling rate for the ULCB/ULCB steel weldments determined the degree of constitutional supercooling indicating a high heat input (large constitutional supercooling) resulted in a tough, finely grained macrostructure while the lower heat input (small constitutional supercooling) generated a weak, columnar/dendritic macrostructure. The utilization of HY-130 base metal resulted in increased carbon pickup in the fusion zone and consequently poorer mechanical properties. The use of a reactive cover gas markedly increased the inclusion density and promoted the formation of carbides producing the poorest overall mechanical properties. The majority of the nonmetallic inclusions identified were spherical, complex aluminum silicates or manganese aluminum silicates. The presence of inclusions did not appear to be dominant in the nucleation of acicular ferrite or in controlling the microstructural development.