Low-carbon steel weld with a high density of oxide inclusions prepared using a experimental metal-cored wire has been examined to study the effect of inclusion size on the formation of acicular ferrite, and to understand the role of inclusion in the nucleation of ferrite lath. Depending on the ferrite morphology associated with inclusions, a total of 282 inclusions observed under TEM could be classified into two groups, i.e. the non-nucleant and the nucleant. Experimental results showed that the group of inclusions acted as nucleant were appreciably larger in size compared with those of non-nucleant resulting in the increased probability of nucleation with the increase of inclusion size, even though the chemical and structural natures appeared to be the same. The group of nucleant-inclusion was further divided into two types depending on the degree of nucleation, which was evaluated by the number of ferrite lath nucleated. Statistical analysis performed on inclusion size indicated that the larger the inclusion size is the more ferrite laths could be nucleated. Those laths nucleated from a large single inclusion have grown in many different radial directions and mostly had a different crystallographic orientation from those of adjacent ferrite laths. As a result of this study, it is demonstrated that larger inclusions are indeed more potent nucleation sites when compared with those of smaller size. Thus it could be concluded that the provision of the inclusion surface as for the inert surface for the heterogeneous nucleation of acicular ferrite lath would be the principal role of inclusions playing in the weld metal of low alloy steels. Other possible mechanisms were also considered, but they were unlikely to be operated in the present weld metal system.

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Low-carbon weld metals with various amounts of Mn and Ni addition were made using metal-cored wires and Ar-2%O 2 shielding gas, and their mechanical properties were evaluated. The objective of the research, aimed to develop welding consumables with better resistance to cold cracking, was to determine the optimum composition ranges of Mn and Ni, in the presence of carbon content less than 0.02 %. The hardness of weld metals were found to increase linearly with Mn and Ni, which was attributed mainly to solid solution strengthening and in part to formation of hard phases. Varying Ni content influenced Charpy impact energy, the extent of which depended on Mn content. For a low-Mn composition, Ni addition increased hardness without sacrificing impact toughness whereas for a high Mn composition, Ni deteriorated the impact toughness seriously and caused intergranular fracture. The fracture path followed columnar grain boundaries that are identical to prior austenite grain boundaries since no d-ferrite phase formed during solidification. Accordingly, these boundaries without having ferrite phase were susceptible to cracking under dynamic loading. Based on hardness and impact resistance, the optimum levels of Mn and Ni were suggested to be 0.5-1 % and 4-5 %, respectively.

The gas metal arc (GMA) welding is one of the most widely-used processes in metal joining process that involves the melting and solidification of the joined materials. To solve this problem, we have carried out the sequential experiment based on a Taguchi method and identified the various problems that result from the robotic GMA welding process to characterize the GMA welding process and establish guidelines for the most effective joint design. Also using multiple regression analysis with the help of a standard statistical package program, SPSS, on an IBMcompatible PC, three empirical models (linear, interaction, quadratic model) have been developed for off-line control which studies the influence of welding parameters on bead width and compares their influences on the bead width to check which process parameter is most affecting. These models developed have been employed for the prediction of optimal welding parameters and assisted in the generation of process control algorithms.

KEY WORDS: hydrogen induced cold cracking (HICC); HSLA-100 steel; weld metal; susceptible microstructure; microhardness.is not relevant to that of single pass weld. Accordingly, the condition of three major factors, i.e. susceptible microstructure, diffusible hydrogen and residual stress, contributing to cold cracks in a multi-pass weldment is far different from those in single pass one. Several studies, mostly in Japan, have been performed on the risk of weld metal cracking in the multipass weld. [13][14][15] In Refs. (13) and (14), investigators have deposited weld beads into a highly restraint Vgroove joint with varying the weld metal strength, the hydrogen content and the height of weld metal. The preheat temperature necessary to prevent weld metal cracking was determined as a function of those variables and then proposed the following empirical equation, Eq. (1) (1) where Rm is the weld metal tensile strength, [H] is the weld metal diffusible hydrogen content, h w is the weld metal height, and A, B, C and D are constants. Unlike the various equations currently available for predicting the preheating temperature necessary to prevent HAZ cracking, 16,17) above equation does not contain a term like carbon equivalent but is showing a strong linear relationship with tensile strength of weld metal (Rm). If this is the case the multipass weld might have different resistance to HICC from location to location as it has a large variation in microhardness throughout the weldment, especially in high strength weld metals. Previously Moon et al. 18) measured the microhardness for multipass ultra-low carbon weld metal deposited in HSLA-100 steel and reported a variation of microhardness in the range between 260-320 Hv. The lowest hardness was recorded in the localized band that located in the outer portions of HAZ of weld bead. As this band was revealed in bright contrast in their optical micrographs they called it as 'white band'. The maximum hardness, on the other hand, was recorded in the midway between the fusion boundaries and white band and thus the distance between the maximum and minimum hardness was less than 1 mm. It means that the most susceptible microstructure is located just adjacent to the least susceptible one. Under such a situation, it would be very interesting to figure out the effect of local variation in microhardness on crack susceptibility and its propagation. This is the first purpose of this investigation.Traditionally, as described above, the microhardness has been regarded as a quantitative measure to evaluate the microstructural susceptibility to cold cracking not only in HAZ but also in the weld metal.11) In high strength steels, however, it has been well documented that hydrogen embrittlement is strongly affected by microstructure.19) For weld metal, it was also reported that higher hardness was not always corresponding to higher susceptibility and showed a combination of high strength and less susceptibility could be achieved through microstructural modification. 20,21) In other words, th...