(1) where, vTrs; 500/0 fracture appearance of transition temperature. weld metal and inclusions. The amount of AF increases in proportion to the carbon equivalent (CEQ) of weld metal27) as shown in Fig. 9. In other words, it is necessary to ensure appropriate hardenability to obtain good toughness. Harrison considers that supercooling is important for AF formation.11) However, whenheat becomes large, supercoolinput increases and T 800/500 ing decreases, which results in a decrease in AF and an increase in GBF as shown in Fig. 10.27) It is also reported that the AF width has a relation with toughness.28) The size of AF increases as shown in Fig. 11 Fig. 12(a).Secondary phases, such as martensite-austenite constituent (MA) and pearlite (P), increase with increasing carbon content.29) Austenite grain size decreases with carbon content (Fig. 12(b)), which seemingly promotes intergranular transformation. (1995). No. 10 ferrite with the second phase increases instead of GBF (Fig. 14).lo) However, toughness increases in proportion to nickel content regardless of the change in microstructurei0,34) ( Fig. 17(a) control the transformation temperature range by adding molybdenum and titanium at the same time so that AF is easily formed30) (Fig. 15).Ti:As is apparent in Figs. 8 (Fig. 16) boron.12,19,36,40-43) An exarnple is shown in Fig. 20.Critical oxygen content for transition from UB to AF is affected by welding method, cooling rate, and alloying elements, and it ranges widely from 150ppml9) to 200-250ppm30,36) and to 450ppm.41) A decrease in oxygen content lowers the start temperature of transformation,

High toughness are requested in the weld metals for offshore steel structures and steel line-pipes used at low ambient temperatures. Micro-alloying of titanium and boron effectively improves the toughness of low-alloyed weld metals with tensile strength ranging between 490 and 590 N/mm2. It is well known that refined intragranular ferrite or acicular ferrite nucleates on titanium containing oxides. However, there have been few reports on the chemical composition at local positions of these effective inclusions and their crystal structures.Two types of submerged arc weld metals were used ; one is a silicon-and-manganese type weld metal with a ferrite with aligned second phase and the other is a titanium type one with the acicular ferrite. The mechanical and metallurgical examination included the microscopic observation, Charpy impact tests of the welds and the characterization of oxides in weld metals with X-ray diffractions and analytical electron microscope.The following facts were clarified from the above investigations. The oxides in the titanium bearing weld metal are crystallized in a form of (Mn, Ti) (Al, Ti)2O4 with angularly rugged surfaces, while the oxides of a Si-Mn type are amorphous with smooth spherical shape. Titanium as low as 0.005 wt% in a weld metal satisfactorily crystallized oxides if titanium is included in oxides with aluminum and manganese.

Developments in some difficult repair welding technologies in Japan during the past decade are reviewed. The topics covered include the repair welding of bridges in service, the temper bead method which makes it possible to omit post-weld heat treatment (PWHT) of repaired pressure vessels, the seal welding of a reactor vessel in which stress corrosion cracks were detected, low heat input repair welding of neutron irradiated stainless steel and nickel based alloys, the prevention of solidification cracking in repair welding of aged heat resistant cast steels, the development of welding materials for the mending of single crystal nickel based superalloy turbine blades, underwater repair welding of nuclear reactors, the reduction of residual stresses in repair welding, and an ultrasonic testing method for nickel based weld metals. The local PWHT of creep resistant ferritic steel tubes is also reviewed.

Large heat-input welding is increasingly employed because of its high productivity. For this welding, titanium and boron (Ti-B) bearing welding materials are suitable in acquiring high toughness at low temperatures. However, their properties at high temperatures have not been known. Preferable ductility at high temperatures is required when they are applied to the welding of fire-resistant steels. The present study examines the mechanical properties of Ti-B bearing welding materials for fire-resistant steels in large heat-input welding.Submerged arc welding with two electrodes in tandem was conducted with the heat-input of 13 kJ/mm. The mechanical and metallurgical investigation were v-notch Charpy tests, weld metal tensile tests, chemical analyses, and metallographic observation on ruptured surfaces.It was found that the addition of titanium and/or boron lowers the high temperature elongation. This reduction was caused by grain boundary fracture at the prior-austenite grain boundaries of the weld metal with titanium and boron. However, the high temperature properties were recovered in the weld metal with titanium as low as 30 ppm and nil boron, and this weld metal fractured in an intragranular mode with enough toughness.

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