(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,

Solidification and subsequent transformation of austenitic stainless steel weld metals that solidified in the ferritic–austenitic mode were investigated from the viewpoint of crystallography. The formation mechanisms for the vermicular and lacy ferrite observed in the weld metals were clarified. The ferrite morphology is determined by both the crystallographic orientation relationship between ferrite and austenite established at the stage of ferrite nucleation and the relationship between the welding heat source direction and the preferential growth directions of ferrite and austenite. In particular, for the formation of continuous lacy ferrite, it is necessary that the ferrite continues to grow with the Kurdjumov–Sachs orientation relationship with austenite that is established at the stage of ferrite nucleation.

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.

The formation and stability of inclusions in low alloy steel welds were investigated using equilibrium calculations. Based on the results, the origin of inclusions effective in acicular ferrite production is discussed. Particular emphasis is placed on the effect of the Al/O ratio and titanium addition on inclusion formation since these two factors are experimentally critical to acicular ferrite production. Mullite (2SiO2.3Al2O3) is found to be formed in addition to Ti3O5 in the steel melt at 1800 K when the melt has a wt-%Al/wt-%O ratio of ∼0·6 optimum for acicular ferrite production. However, the mullite is unstable in the austenite and galaxite (MnO.Al2O3), having the spinel structure, becomes stable at the lower temperatures. Therefore, it is proposed that galaxite is responsible for the nucleation of acicular ferrite. The formation of the galaxite should be in the solid state after weld solidification, being associated with pre-existing mullite. Titanium additions are found to be beneficial to acicular ferrite production by decreasing the formation of ineffective glassy oxide.

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