The authers have conducted the systematic investigation in order to develop a low nickel alloy steel for extremely low temperature service line pipe. This paper is to describe the results of the study on tandem submerged arc welding to get higher toughness of weld metal at low temperature. Low C-high Mn-(2.0-2.6)% Ni steel proved to be the best plate obtaing high toughness weld metal for use in the arctic region. A Mn-Mo-Ti-B wire was developed, and the nickel content necessary for toughness was introduced to the weld metal by dilution from the base metal. The solidification phase through peritectic reaction of Fe-Ni-C system which was detremental to the toughness of weld metal could be avoided by judicious combination of base metal and wire. Boron reduced proetuectoid ferrites and made them fine in the presence of titanium, and their optimum content in weld metal ranged from 0.02 to 0.03 % for titanium and was about 0.002 % for born.
Mechanical properties of 80 kg/mm2 class high strength steel plate and forged steel were investigated concerning the quality change in base metal and heat-affected zone caused by stress-relief heat treatment. Base metal of both the steel plate and the forged steel showed little change in the yield strength, the tensile strength and the Charpy V-notch impact energy, up to the Hollomon Jaffe Parameter, [P] value of 18.3 × 103 (620°C × 3 hrs.) But, there was a marked deterioration in the notch toughness of the fusion line after the stress-relieving and the 50% FATT (vTrs) was increased in temperature by approximately 30°C in the manually welded joint and approximately 50°C in the submerged-arc welded one. It became clear that the metallurgical change near the fusion line induced the deterioration in the notch tongnesss.
Dissolution and Deposit of Base Metal in Brazing to Dissimilar Materials and its Application (Part 1) In brazing of dissimilar metals, each base metal has the different equilibrium quantity for the dissolution into molten filler metal. As the boundary which attains to the equilibrium prior to another becomes "constitutional supercooling" by the solute from another base metal, the excessive solute metal deposits at a constant brazing temperature. On this report, the phenomenon is studied in copper-brazing of steel which contains each different quantity of carbon.
It is well known that epoxy resin coating improves fatigue strength of welded joint. The reason of this improvement has been explained by one of authors as, the protective effect of epoxy resin against oxygen and/or humidity in air, which decreased fatigue crack propagation rate. However, there is a report which attributes the fatigue strength improvement to the stress distribution of epoxy resion layer. This study was carried out to know whether the stress distribution of epoxy resin coating has beneficial effect ob the fatigue strength of welded joint. Mild steel and high strength welded joints which were coated by epoxy resin and glass fiber reinforced resin were tested under bending and axial load. The results obtained are as follows; 1) The fatigue strength of steel welded joint improves by epoxy and glass fiber reinforced resin coating under axial and bending loading conditions. 2) The reason of improving fatigue strength of steel welded joints by epoxy resin coating is caused by protective effect which was reported by one of the authors.
The embrittlement behavior of the welded joints of 1/2 Mo steels exposed to high pressure hydrogen at high temperature has been investigated. And, the effects of carbon content and microstructure on the hydrogen attack and comparison between Nelson diagram and the critical embrittling temperature have been discussed. The results are summarized as follows: (1) The hydrogen attack is easy to occur in the coarsed-grained heat-affected zone (HAZ), especially as welded, comparing with the base metal. (2) The critical embrittling temperature is lowered as increasing the carbon content, and the drop of the temperature is remarkable in the coarsed-grained HAZ as welded. (3) Nelson's curve can be applied to the base metal, but not be applied to the welded joint of the steel containing high carbon content, because even if in the safety range of Nelson diagram, it may be suffered by the hydrogen attack. In order to apply to the welded joint, carbon content of the steel should be less than 0.20% and the welded joint should be sufficiently post weld heat treated to be less than 225 in Vickers hardness of HAZ. (4) It is considered that the coarsed-grained HAZ is easy to become brittle by hydrogen attack because carbide dissolution and carbon supply to the grain boundary is more easy than in the base metal.
The effect of weld profile on the fracture toughness of bond has been investigated. The engineering criterion for the brittle fracture initiation under mixed mode (opening mode, I, and tearing mode, III), was studied on the basis of COD-concept. The following conclusions have been developed: (1) In the case where the part of bond has lower toughness as compared with that of weld metal and heat-affected zone, and where the embrittle zone is relatively wide as in the case of welded joint in large heat input, the critical COD, δc, of bond for the welded joint with V, Y or X type weld profile nearly equals to that with I, _??_ or K type ones. All fatigue notches in these test specimens are located perpendicularly to the surface of plate. (2) For a crack having its inclination between 65°and 90°to the surface of plate where the mixed mode condition prevails, the brittle fracture initiates when the COD corresponding to the opening mode, δI, reaches to the critical COD, δc, but the effect of COD corresponding to the tearing mode, δIII, can be hardly observed.
The relationship between the fracture appearance transition temperature vTrs in the V-notch Charpy test and some fracture toughnesses in the COD test has been investigated. The obtained results can be summarized as follows:- (1) The fracture toughnesses can be calculated from vTrs by developing the equation, which relates vTrs to the brittle fracture initiation temperature under the conditions of any crack length and any applied stress. (2) The brittle fracture initiation characteristics under any residual stress can be calculated using the values of residual stress and vTrs. This method can be applied to estimate the effects of the stress relief heat-treatment on the brittle fracture initiation. (3) When Kc is expressed by the following equation, Kc=Koexp(-k0/T) K0 and k0 increase with an increase in vTrs and plate thickness, and with a decrease in the strength of steel plates.