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.

ABSTRACT: Toughness and microstructure of HSLA welds reheated by successive welds was studied. Toughness changes can be classified into four categories by attained peak temperature. The causes of embrittlement are increasing of GBF, MAC, and effective grain size and the main governing factors depends on the peak temperature. The low C, Ni-TiB type weld metal shows higher toughness in all reheated regions. 1. INTRODUCTION As energy development advances into deeper seas and Arctic areas, the toughness requirements for weldments has increased in severity from -40·C to -60 or -80·C.l) Especially through-notch CTOD properties, which are dominated by local embrittle zone2), are required for offshore structures. In order to get high CTOD properties, all regions of the weld metal in the weldments should be toughened. Further, fabricators demand large heat-input welding for efficiency. It is well known that the most preferable microstructure for toughness is the acicular ferrite (AF) in the as-welded region and there are many reports on the condition of AF formation3,4, S). As the TiB bearing system gives the AF under wider welding conditions and the weld metals provide excellent CTOD properties, many kinds of welding materials have been developed for offshore structures, etc. based on the Ti-B bearing system6, 7,8). But toughness of weld metal including the region reheated by successive welds with large heat-input is not always high compared with the aswelded region as shown in Fig.I. Then as shown in Fig.2 a brittle fracture initiated from the reheated region in the CTOD specimens. Therefore the toughness of reheated region should be increased to improve the CTOD properties in the joint weld metal. However the attained peak temperature is changed continuously and the most embrittle parts are not always specified in the reheated region. There are few reports on the toughness of reheated weld metal.

ABSTRACT: While recently developed platform steels have excellent low temperature toughness properties they may be nevertheless, susceptible to cleavage fracture initiation at focally brittle zones (LBZ" s) developed in coarse grain heat affected zones (CGHAZ" s) of welds. Current offshore material specifications such as EEMUA and EPRCO require prequalification of plate in regard to LBZ such that a COD notchtip intersecting 15 percent or more of the CGHAZ in a weld sample meets lower bound toughness values. In the present work the significance of the 15 percent intersection requirement has been assessed using a roll bonded sandwich technique with a 0.45 percent carbon steel plate bonded between outer plates of offshore steel. It was observed that at this percentage intersection cleavage fracture, initiated as microvoid cleavage, readily propagated into the off shore plate resulting in crack opening displacement toughness values of less than O.lmm at -30°C. The significance of the pre-qualification requirement and the roll bonding technique are discussed. 1. INTRODUCTION Owing to section size, offshore platforms are fabricated using multi-pass welds, with the heat affected zone (HAZ) adjacent to the welds comprising a complex array of individual zones where the HAZ of one weld is influenced by the heat of subsequent passes. Multi-pass weldments as a whole have improved toughness as compared with single run welds due to the refining and tempering effect subsequent weld runs have on previous deposits. However" there are small, zones generated in multi-pass welds that suffer poor toughness, properties, similar to those experienced in single weld HAZ" s. Of particular importance are coarse grained heat affected zones (CGHAZ) from a previous run that constitute tile inter critical- heat affected zone (ICHAZ) or CGHAZ as shown in Figure: I. The ICHAZ is that region exposed to temperatures between AC 3 and AC 3 of approximately 850°C.

ABSTRACT: The demand of high strength steel for ship and offshore structure application has been changed from conventionally heat treated steel to TMCP steel, because the latter provides the improved strength - toughness balance and weldability. Enhanced property of TMCP steel comes from the effective conditioning of austenite with controlled rolling and pertinent selection of microstructure by accelerated cooling process. This new thermo-mechanical control process, named PILAC(POSCO In-Line Accelerated Cooling) has been developed to produce the high qual1ty structural steel. The production conditions of thermo-mechanical control process were studied to obtain the appropriate combination of strength and toughness of base metal. The effects of carbon to manganese ratio and Ti addition on the heat affected zone toughness were evaluated to optimize the chemical composition of steel. Based on these studies, the 50kg/mm2 tensile strength grade steels for ship and offshore structure were produced with PILAC process. These steels showed good heat affected zone toughness, even in heat input of 18kJ/mm, and good weld :rack resistance. Introduction Recently, the demand of structural steel has moved from mild steel to high strength steel and the consumed quantity of high strength steel has continuously increased. During last five years, the amount of high strength steel used in Korean industry have continuously increased as shown in Fig. I. From this figure, it could be found that there is a big jump in consumption rate of high strength steel from 36% in 1986 to 50-55% in 1989. Recent increase in consumption rate of high strength steel is largely dependent on a change in the demand of steel user and a quality renovation of high strength steel through TMCP technology. In ship building industries, as a measure of energy saving, the use of high strength steel is increased to reduce the total weight of ship.