ABSTRACT: The local corrosion of high-strength steel of 440N/mm 2 class manufactured by the thermo-mechanical control process was examined. INTRODUCTION The local corrosion of welded joints of icebreakers resulting from damage of paint and cathodic protection by ice collision is a problem to be discussed [2] [l] [5]. The steels for icebreakers are required to have (i) high resistance to local corrosion in seawater, (ii) high toughness at low temperature and (iii) good weldability. In this paper, the local corrosion of high strength steel of Y.P. 440 N/mm 2 class manufactured by a thermo-mechanical control process (TMCP) is studied. And the properties of the steel, and welding materials developed on the basis of the results of the local corrosion resistance are also discussed. 2. LOCAL CORROSION OF THE TMCP STEELS 2-1. Test Specimens 2-1-1. Tested Steels Table I shows the chemical compositions of the steels tested. PI and CT are high-strength steels having yield strength of 440 N/mrn2 class produced by the TMCP. Both belong to the Cu-Ni series. A small amount of Nb was added to improve the notch toughness of the base metal, and a similar amount of Ti was added to improve the notch toughness of the weld heat affected zone (HAZ) [6]. CF and TB are the reference steels. Both belong to the Si-Mn series; CF is low Mn steel and TB is low C steel. Photo. 1 shows the basemetal microstructures. PI and CT have fine proeutectoid ferrite (F) and bainite (Bu) microstructures. CF and TB have ferrite and pearlite (F+P) microstructures. 2-1-2. Test Welding Materials Table 2 lists the test welding materials. N100, N100Cu, N10, N13, ASl5 and NUSN were used for shielded metal are welding (SHAW) and YE-NB55E, IN-NB55E and YDN-NB55L for submerged are welding (SAW).

ABSTRACT: The higher strength steels often show HEAT-AFFECTED ZONE (HAZ) softening. In the previous study, it was shown that CTOD toughness is much more important than strength matching to get higher welded joint performance in a stress concentrated region on the basis of the FEM analysis and wide plate test with through-thickness notch. This paper describes the further test results of the surface notched wide plate test with and without a stress concentrator. In conclusion, it shows that HAZ undermatching does not influence the deformability at the fracture of a welded joint in offshore structures. Moreover, it was shown that the local strain based CTOD approach can be applied to assess the fracture behavior of strength in an unevenly matched joint. INTRODUCTION Offshore structures have been successfully used in cold and harsh environments such as the North Sea. This is partly due to the fact that high fracture toughness, especially CTOD toughness has been demanded in offshore structural steels as well as in welding consumables based on the fracture mechanics evaluation. Stringent CTOD testing is now carried out for weld HAZ of the steel. As offshore structures are going to be installed deeper and deeper in the sea, structures will become bigger and bigger. In order to save the weight of the topside structures from an economical viewpoint, there has been a trend toward using higher strength steels. Ih higher strength steels manufactured by either the thermo-mechanical control process (TMCP) or the quenching and tempering process (Q&T), HAZ softening (HAZ undermatching of strength) may occur, unlike normalized steel in which strength in the HAZ region is usually higher than the base plate, although the level and the width of HAZ softening depends on the chemistry of the base plate and the welding condition, especially heat input.

ABSTRACT: Influence of the local brittle zone on HAZ CTOD was discussed. From weld HAZ CTOD tests and simulated HAZ CTOD tests, it was clarified that the local brittle zone of a multi-pass weld HAZ is intercritically reheated coarse-grained zone, where high-carbon martensitic island was formed. Factors influencing the formation and decomposition of high-carbon martensitic island (M*) in the intercritically reheated coarsegrained HAZ was investigated. Based on a simple estimation model of volume fraction of the M*, it became possible to estimate quantitatively the influencing factors on the formation and decomposition of the M*. It was clarified that combination of steel with low carbon, low microa110y chemical composition and suitable welding condition, such as application of preheating and thin weld bead placement, is important for reducing or eliminating the H*. 1. INTRODUCTION Since low CTOD (Crack-Tip Opening Displacement) value was obtained in a HAZ (Heat-Affected Zone) CTOD test of offshore structural steel in the early 1980s, extensive studies have been made to clarify the cause of the low CTOD value. It has been made clear that the low value was caused by the LBZ (Local Brittle Zone) in the HAZ. Factors influencing the microscopic toughness of the LBZ and measures to prevent low CTOD value have been studied (OMAE, 1988),(OMAE, 1989) and new steels which have high HAZ CTOD value have been developed (e.g. Chijiiwa et aI, 1988). For improving the LBZ toughness, selection of chemical composition which has low susceptibility to cleavage fracture is primarily important. In addition, selection of a suitable welding condition (heat input, preheating and bead placement etc.) is also important. For this purpose, it is of urgent necessity to elucidate these factors quantitatively. From this standpoint, the present paper will discuss the factors controlling the LBZ toughness.