Heat-Affected Zone Fracture Toughness of 420–500-MPa Yield Strength Steels: Effects of Chemical Composition and Welding Conditions

During the last five years, high-strength steels with yield strengths in the range 420 to 500 MPa have attracted considerable interest within the offshore industry, primarily due to the potential for weight saving and reduction in volume of weld metal through the use of reduced section thicknesses. With respect to chemical composition these steels are developed following much the same philosophoy as for the modern normalized structural steels. Due to the increased stress level in these higher strength steels, it is anticipated that brittle fracture initiation occurring in the coarse-gained HAZ will be more critical for these steels than for the lower strength normalized grades. The objective of this paper is to present the results from several experimental investigations carried out at VERITEC during the last five years to study the factors affecting the crack tip opening displacement (CTOD) fracture toughness of the heat-affected zone (HAZ) in structural steels in the yield strength range 420–500 MPa. Typical CTOD fracture toughnesses of the HAZ in normalized 350-MPa yield strength steels used in offshore structures are also presented for comparison. The results of the investigations confirm that the same chemical compositional factors which are known to influence the HAZ fracture toughness of normalized steels are also important for the 420–500-MPa yield strength steels. It is demonstrated that the width of the HAZ is important for the initiation of brittle fracture of pop-in and that this width must exceed a certain minimum value for such events to occur. For different welding conditions, i.e., welding currents/travel speeds, the width of the coarse-grained HAZ changes considerably along the periphery of the weld bead. Thus, one may obtain widely differing results depending on the exact fatigue pre-crack placement. In addition to the weld cooling time between 800 and 500° C, Δt_8/5 and the retention time above 1000° C during the weld thermal cycle, RT1000, the welding process and consumables also exert a strong influence on the CTOD values of the coarse-grained HAZ.