Modern steels like TMCP or HSLA steels are excellent structural material for many engineering applications. Exploitation of physical strengthening mechanisms enables lowering of carbon and alloying element's content and improves weldability remarkably. On the other hand demands for higher welding efficiency requires higher and higher heat inputs which, in turn, favours softer transformation products like upper or granular bainite in the weld HAZ. Grain growth and the M-A constituent present in that structure are the main reason of toughness deterioration in the weld area. Paper brings an analysis of the effect of grain growth and as welded microstructure on notch toughness of as welded and PWHT welds.

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Nowadays welding engineers use more frequently computer based methods for the analysis of weldability, like data bases, parametric equations, knowledge based systems and mathematical models. It is shown that the choice of an appropriate method and the effort spent for the model development is strongly dependent on the desired objectives. In the R&D field, mainly numerical models are applied because they are based on the underlying mechanisms and provide an increased understanding of the effect of the influencing parameters and their interactions. Beside their strong capabilities, there are however application fields where fundamental models are not so predestinated. The necessity for experimental data generation and verification will also be highlighted.

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The authors have been engaged in an extensive program of fundamental and systematic research intended to clarify the effects of alloying elements on the oxygen absorption during arc welding process. In this report, the outline of an oxygen absorption behavior in iron and steel weld metal during Gas-Metal-Arc welding process is described on the basis of the authors group's data. Initially the oxygen absorption behavior of the pure iron weld metal is thermodynamically analyzed. The similar investigations are carried out for Fe-Si, Fe-Mn, Fe-Al, Fe-Ti, Fe-Cr and Fe- Ni weld metat, and the results are analyzed. Finally, the oxygen absorption behavior of the mild steel is also described.

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As advanced steels are developed and used in welded structures, performance of weldments is one of prime issues as well as cost performance of welding. Demands for high-performance steel welds have been increasing in various application areas, not only in shipbuiding and offshore sructures but also in building construction and engineering works. Improvement in low-temperature toughness and stress-corrosion-cracking resistance of welds is critical to recent oil and gas exploitation and gathering in severe environments. Large heat-input welding and higher-strength welds are required with increasing the size of welded structures, while fire-resisting properties of steels and their welds are important for high-rise buildings.
The present paper reviews a recent progress of welding technology for low alloy steels in light of the above trend. Effect of alloying elements and microstructures on weld performances are outlined, and significance of controlling alloy and phase chemistries in the welds is discussed in order to meet the increasing demands for improved weld performances.

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The experimental results of y-groove weld cracking tests were compared with the prediction methods of minimum preheat temperatures to avoid cold cracking in the heat affected zones, which include the BS-5135, the AWS D1.1, the JSSC procedure and the CEN chart method. The effects of steel chemical compositions, plate thicknesses, diffusible hydrogen contents and ambient temperatures on cold cracking susceptibility were investigated in this study.
The CEN chart method can predict minimum preheat temperatures most precisely as long as homogeneous preheating is conducted and the ambient temperature is 20°C. When local preheating is conducted, cooling time down to 100°C, t₁₀₀, must be used. The minimum preheat temperture for local preheating is selected so that t₁₀₀ of local preheating is equal to or longer than that of the minimum preheat temperature for homogeneous preheating.
Ambient temperatures greatly affect cold cracking susceptibility more than expected from its effect on t₁₀₀. As ambient temperature becomes lower, minimum preheat temperature to avoid cold cracking becomes higher. However, all the previous prediction methods cannot estimate this effect. In this study, the effects of ambient temperatures on necessary preheat temperatures were converted into CEN increments, which were determined from the experimental results. Using these CEN increments, the CEN chart method can estimate the effect of ambient temperature.

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The susceptibility to hydrogen induced cracking under applied stress was examined for a large diameter high strength sour service line pipe steel produced by employing the thermo-mechanical control process, i.e. controlled rolling followed by accelerated cooling. The low carbon content aimed at high toughness and weldability resulted in softening in the grain-refined and inter-critical heat-affected zone by thermal cycles of longitudinal sub-merged arc welding. The cracking susceptibility of weldment was transversely evaluated by TM0177-90 method A and compared with the parent metal.
The parent metal indicated high resistance to cracking. Nonethelss, the weldment indicated the decrease in threshold stress, since the weldment ruptured preferably along the softened heat-affected zone. The cracking morphology in the softened heat-affected zone was recognized as stress oriented hydrogen induced cracking. Also performed was a large scale test to reveal the internal incipient cracks in the softened heat-affected zone. The observation confirmed that the initiation site was a small cluster of non-metallic inclusions which had probably no adverse effect on the resistance of the parent metal. He stress analysis of the weldment proved that the tension through thickness was generated at mid-thickness under the deformation constraint from surrounding harder parts. The enhanced tri-axiality of stress in company with the through thickness tension facilitates the onset of cracking caused by the hydrogen pressure mechanism. The crack morphology was consistently associated with the stress distribution.

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Effects of total N content on HAZ toughness, microstructure and precipitate behavior in Nb-V-Ti bearing C-Mn microalloyed steels has been investigated under non-equilibrium state such as welding thermal cycle.
The HAZ toughness is improved with increasing total N content and the lowest 100 J transition temperaturre concerning HAZ toughness was obtained at a steel with high total N content about 80 ppm. Improbement of HAZ toughness with increasing total N content is caused by refinement of microstructure and decreasing souluble Nb and Ti with increasing total N content. Complex precipitates that contain both Nb and Ti are observed in Nb-V-Ti bearing steels after a weld thermal cycle, and they are refined and their density tend to increase with increasing total N content. The amount of Nb and Ti as precipitates increase with increasing total N content. And then the complex precipitation behavior in HAZ could be quantitatively predicted by a thermodynamic model.
In Nb-V-Ti steels refinement of prior austenite grains occurs at higher total N content than in Ti bearing steels. It is caused by increasing the stability of precipitates through the formation of complex precipitates in Nb-V-Ti bearing steels.

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In order to examine HAZ properties of SQV-2A steel welded by the temper-bead technique, the effect of temper-bead thermal cycle on the toughness has been investigated for the simulated ICCGHAZ (intercritically reheated coarse-grained HAZ). The simulated ICCGHAZ specimen was produced by applying double thermal cycle consisting of first thermal cycle with a peak temperature of 1623 K and subsequent second thermal cycle with various peak temperatures T_p2 involving A_c1 and A_c3 temperatures. At T_p2 from 973 to 1003 K, serious embrittlement of the ICCGHAZ occurred which could be attributed to the formation of coarse necklace-like M-A constituent along the prior austenite grain boundary region and fine elongated M-A constituent within the prior austenite grain. When the cooling rate of the second thermal cycle was slow, serious embrittlement was also observed at T_p2 from 1073 to 1100 K, and this could be attributed to the formation of upper bainite in the reaustenitized area during the second thermal cycle. The temper-bead (third) thermal cycle with a peak temperature of 673 K could improve significantly the toughness of ICCGHAZ embrittled at T_p2 from 973 to 1003 K. However, the temper-bead thermal cycle was not effective in improving the toughness of the ICCGHAZ embrittled at T_p2 from 1073 to 1100 K in case of slow cooling rate.

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The effects of welding parameters, wire compositions and others on the gas pocket (pit, blowhole) generation have been studied regarding MAG welding of galvanized steel sheets, and the mechanism of the gas pocket generation also has been investigated.
As to the welding joint type and position, lap joint and vertical-down position make the number of generations large in comparison with others. The increase in welding current and speed or Ar content of shielding gas also furthers the gas pocket generation. In contrast, the application of metal-cored wire in CO₂ welding or pulsed current welding machine in MAG welding with Ar-CO₂ mixture gas reduces the generation. The increase in C, Ti and P content of the metal-cored wire is effective for the reduction of gas pocket generation. Through these investigations, special metal-cored wire for CO₂ welding of galvanized steel sheets has been developed, and the wire is currently used in car manufacturing, house construction and other fields.
Based on the above phenomena, zinc vapor seems to be the main cause of gas pocket generation. These phenomena can be mainly explained by the penetration depth which corresponds to the amount of zinc vapor, and by the stability of short-circuiting arc which influences the coming up and release of zinc vapor through the molten pool.

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The influence of titanium additions and interpass temperature on the microstructures and properties in low carbon-1.5Mn-3Ni-0.5Mo multiple pass steel weld metals produced using the shielded metal arc welding (SMAW) process was studied. Robust weld metals with high strength (>690 MPa) and toughness (100 J at -70°C) were produced when titanium concentrations of 180 to 400 ppm were added to a base chemical composition of low carbon-1.5Mn-3Ni-0.5Mo steels. High toughness was measured in weld metals containing either 30 to 90 ppm titanium or 180 to 400 ppm titanium while deteriorated weld metal toughness was observed when weld metal titanium concentrations were less than 10 ppm, 90 to 180 ppm, or more than 400 ppm. The microstructures of these low carbon weld metals were complex. The difference between the classification systems used for weld metals and base metals was addressed.

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The variations of toughness of reheated weld metals were studied in the Pcm range of 0.13 to 0.20 mass%, using welding thermal cycle simulator. The reheating temperature dependence of toughness is quite different between weld metal of low Pcm and that of high Pcm. There is more remarkable toughness degradation in the range of 900 to 1000°C than that of 1200 to 1350°C in reheating of weld metals with high Pcm. The toughness degradation in 900 to 1000°C is characterized by Ti-B bearing weld metal with high Pcm. The toughness degradation of high Pcm weld metal in 1200 to 1350°C is not more remarkable than that of low Pcm such as Si-Mn weld metal. By increasing the Pcm, the toughness degradation in 900 to 1000°C is more intensified, and that in 1200 to 1350°C is more recovered. The following quantitative evaluation on the toughness degradation is conducted by microstructural analysis using Petch's relationship. The toughness degradation in reheating at 1000°C which is corresponding to be 72°C in Charpy impact transition temperature comes from the component of grain coarsening, about 46°C, and that of MA constituents, about 26°C. On the other hand, the toughness degradation in reheating at 1350°C, 32°C in Charpy impact transition temperature comes from the component of grain coarsening, about 11°C, and that of MA constituents, about 21°C.

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High heat input electroslag welding (ESW) is widely used for the four side welding of 490 N/mm² class steel between thick flange or web plate and diaphragm without Post Weld Heat Treatment (PWHT). The notch toughness of ESW weld metal is generally lower in the central part (C zone) then in the peripheral part (R zone) of the weld metal. This report has treated from metallurgical investigation the reason why the notch toughness of C zone is inferior to R zone. Moreover the improvement of the notch toughness in C zone has been investigated with the change of chemical element in the weld metal. As a result the reduction of Si content in weld metal is beneficial to improve the notch toughness of C zone and equalizes the ductility of both zones.

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In an austenitic stainless steel weld metal solidified as primary ferritic single phase, subsequent ferrite-to-austenite transformation becomes massive when the postsolidification cooling is sufficiently rapid. The effect of solidification on the massive transformation was studied by analyzing microstructural changes during solidification and subsequent cooling. As a result of the analysis, crystallographic orientation relationship between the parent phase and the massive product phase, and the onset temperature of the massive transformation were clarified. A specific orientation relationship between the parent phase and the product phase by massive transformation was present. The onset temperature of the massive transformation was found to be below the T₀ and inside the two-phase region of ferrite and austenite.

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The properties of weld metal of 18Cr heat resistant austenitic steel containing high copper, niobium and nitrogen were investigated to obtain the matching filler metal for an austenitic stainless steel of 18Cr-9Ni-3Cu-Nb-N steel. This 18Cu-9Ni-3Cu-Nb-N steel tube has been registered as KA-SUS304HJ1TB in the Japanese official standard for fired power plant materials. This steel tube has a high temperature strength 1.4 times as high as the conventional 18Cr steel ASME TP347H. Welding material applicable to this steel is necessary in the fabrication. However, the weld metal consisting of the same chemistries as the base metal had a lower strength than that of the base metal. In this work, the effect of chemistries of molybdenum and nitrogen on the strength of weld metal was investigated. The addition of molybdenum up to 1 mass% was effective for improving the strength without a deterioration of the corrosion resistance. The nitrogen was also effective for improving the strength, but the nitrogen can be decreased by vaporizing during tungsten inert gas arc welding. For improving the solubility of nitrogen in the molten pool, the addition of manganese was effective. From these findings, the chemistry of welding material was determined as 18Cr-16Ni-3Cu-1Mo-3Mn-0.6Nb-0.2N and the tube weldment with that welding material was evaluated in terms of mechanical properties including creep rupture property. It was confirmed that the weldment had comparable properties to the base metal.

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Low temperature sensitization (LTS), which is a sensitization for intergranular corrosion at temperatures below the normal isothermal sensitization range owing to forming of chromium carbide nuclei at grain boundaries already, in the weld metal of Type308 stainless steel and its improvement by laser surface melting treatment were investigated in this study.
Three kinds of SUS308 stainless steels which were varied in carbon contents were used. TIG welding method was adopted to make the weld metals. The degree of sensitization in the sensitized weld metals was evaluated by the EPR test and the Strauss test.
Experimental results revealed that LTS in weld metals occurred at much shorter time than in HAZ, which was attributed to the accelerated precipitation of chromium carbides at δ/γ grain boundaries in the weld metals.
Surface melting treatment by a CO₂ laser generator was found to improve the corrosion resistance in sensitized weld metals. Especially on the conditions with high cooling rates, the corrosion resistance was markedly improved, because re-sensitization during cooling could be suppressed. Such effects were considered to be attributed to the dissolution of chromium carbides with this treatment, which results in annihilation of the chromium depleted zone adjacent to grain boundaries

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Study has been carried out aimed at predicting the cooling time from 1200 to 800°C (t_12/8) of arc welds in stainless steel using an existing heat flow model and direct measurement of the weld thermal cycle. The properties of welds in austenitic-ferritic duplex stainless steel depended on the delta (δ) ferrite content of the weld metal (WM) and the heat effected zone (HAZ). The t_12/8 was the major cooling parameter relating to the δ-ferrite content. It was shown that the travel speed and the distribution of the heat source had negligible effect on the value of t_12/8 in a practical welding situation. The proposed equation predicted t_12/8 with an error of approximately 15% relative to experimental measurement.

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Duplex stainless steel base material was welded using gas tungsten arc welding with an Ar-10%H₂ shielding gas and laboratory-made filler wires were employed to deposit duplex and fully ferritic weld metals having different nitrogen contents. Weld metal slow extension rate tensile (WM-SERT) testing was used to examine the hydrogen-induced cracking susceptibility and fractography of the weld metals. An increase in nitrogen content in fully ferritic stainless steel weld metal increased the density of precipitates and the hydrogen-induced cracking susceptibility. The facets on the quasi-cleavage fracture surfaces of broken WM-SERT test specimens were parallel to the {100} plane in ferrite. Scanning and transmission electron microscope observations revealed the crystallographic features and morphology of the precipitates. The precipitates were rod-like Cr₂N nitrides. Many of them had <100> directions and were parallel to the cleavage {100} plane in ferrite. An orientation relationship shown between Cr₂N precipitates and ferrite suggested that the axes of the Cr₂N precipitates were parallel to <001> direction in ferrite and that they were more coherent along their long faces than at their tips. As a result, the tip of these Cr₂N precipitates could act as sinks for hydrogen and may be preferential sites for initiation of hydrogen cracking; this could promote crack propagation on {001} cleavage planes in ferrite on which Cr₂N precipitates are located. Higher densities of Cr₂N precipitates were nucleated at solidification boundaries and at oxide inclusions in ferrite.

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The creep rupture strengths of welded joints of Modified 9Cr-1Mo steel and 12Cr steel tend to decrease as compared with those of base metals. Therefore, the softening behavior at the heat affected zone (HAZ) was examined by means of mainly welding thermal cycle simulation, and the improvement of the creep rupture strength of GTAW joint was investigated from the standpoint of precipitation strengthening and/or solid solution strengthening. Consequently, the maximum HAZ softening was found at the peak temperature around 900°C, which was closely associated with the lack of fine Nb and V carbonitrides coherent with the matirix in addition to the fine grained microstructure formed by recovery and polygonalization of the tempered martensite. When conducting creep rupture test for their welded joints, the fracture location shifted from the base metal or the weld metal into the softened HAZ with increasing the Larson Miller parameter.
As far as the improvement of the creep rupture strength of GTAW joints is concerned, the control of morphology of Nb and V carbonitrides in the base metal prior to welding by using the thermo-mechanical control process (TMCP) was not effective. On the contrary, the creep rupture properties tended to be improved by increasing W and Cu contents similar to the case of base metal.
In conclusion, since the effect of precipitation strengthening is reduced at softened HAZ, the decrease in the creep rupture strength of weldment to some extent must be taken into account, and it is of importance to improve the strength of weldment by increasing that of base metal resulting from mainly solid solution strengthening.

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A duplex stainless steel was diffusion-bonded in a vacuum using duplex stainless steel intermediate materials with different initial microstructures. The effects of bonding temperature, pressure, and microstructure of the intermediate material on the soundness of bonding interface region and bonding strength were examined. The number of voids at the bonding interface reduced with increases in bonding temperature and pressure, and with a decrease in average grain size of the intermediate material. When the intermediate material had a microduplex structure, void-free bonding interface was achieved even in a specimen bonded at a temperature as low as 1173K and at a pressure as low as 0.7 MPa. Tensile tests on the joints revealed that the bonding strength of void-free specimen was not less than the tensile strength of base material. A TEM study suggested that superplastic deformation of intermediate materials due to sliding at ferrite/austenite boundaries and ferrite/ferrite grain boundaries can contribute greatly to the bonding.

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This study investigated bonding and the crystallization behavior of Ni-base single crystal superalloy, CMSX-2, base material during transient liquid phase (TLP) bonding using MBF-80 insert metal. Joint strength was evaluated using tensile and creep rupture testing at elevated temperature. TLP-bonding of CMSX-2 was carried out at 1373-1548 K for 0-19.6 ks in vacuum and the (001) plane of each test specimen was always aligned perpendicular to the joint interface. The dissolution width at the bonding temperature increased when the bonding temperature and holding time increased. The eutectic width decreased linearly with the square root of holding time during isothermal solidification. Electron back scattering patterns of completed joints revealed that the bonded layer had single-crystallized during the TLP-bonding process and matched the crystallographic orientation of the bonded substrates. The elevated temperature tensile strength and creep rupture strength of the joints were the identical ones of the base metal.

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Continuous SiC fiber reinforced Ti-6Al-4V composites were joined to a Ti-6Al-4V plate by transient liquid phase (TLP) bonding process using Ti-Cu-Zr filler metal. Three processes, which were the dissolution of base metal, the isothermal solidification and the homogenization processes, were observed. The saturated dissolution width increased as the fiber volume fraction increased. Namely, the dissolution width was inversely proportional to the fiber volume fraction. Although the time for completion of the dissolution of bases metal and the isothermal solidification would be long with increasing fiber volume fraction, these two processes could be completed for a short time and so marked difference could not be observed. Homogenization process is important to obtain high strength joint. It was cleared that the process could approximate to one dimensional diffusion model.
Since the filler metal was melted during the bonding process, the liquid phase reacted with the SiC fiber. As a result, the brittle products such as TiC, ZrC and Ti₅Si₃ were formed at the interface between the fibers and the filler metal. However, these products did not affect the joint strength because they were formed only at the end of fibers in very small amounts.

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The influence of joining parameters (rotational speed, frictional time and pressure) on the notched tensile strength of dissimilar MMC/AISI 304 stainless steel friction joints is investigated. Frictional pressure and rotational speed have a statistically-significant effect on notched tensile strength values. The highest notched tensile strength properties occur in joints produced using a high frictional pressure (120 MPa) since extremely thin transition regions are produced at the joint interface and the area fraction of joint interface fracture in broken notched tensile test specimens is negligible.
The average particle diameter and inter-particle spacing decrease markedly in the region immediately adjacent to the bondline. It is suggested that the decrease in particle dimensions results from the combined effects of non-uniform plastic straining during friction welding and due to thermal shock and impaction of alumina particles on the contacting surface of the stainless steel substrate.

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