Effect of M-A constituent on toughness of weld-heat affected zone of high tensile steel was investigated by Charpy impact test. Main results obtained were as follows. (1) Both the coarsenig of structure and the M-A constituent were factors of loss of toughness. (2) M-A constituent in case of long cooling time exerted less influence on loss of toughness than that in case of short cooling time. (3) Ferrite-carbide structure formed by the decomposition of austenite in case of long cooling time exerted less influence on loss of toughness than M-A constituent in case of short cooling time, but that exerted almost the same influence as M-A constituent in case of long cooling time. (4) Harmful effect of M-A constituent on toughness was mitigated by tempering at low temperature.
Study of fatigue strength and the crack initiation and propagation on notched specimens and welded joints of high strength steel were carried out. On this experiments, difference of the high strength steel and the mild steel was checked. The main results are summarized as follows: (1) In the notched specimens, the HT specimens have larger notch sensitivity than mild steel specimens. However the fatigue strength of HT specimens has higher than that of mild steel specimens in the case of tip radius of notch is greater than 0.1 mm. (2) In the welded joint specimens, the HT specimens have somewhat higher fatigue strength than mild steel specimens on the following four sections of joints; base metal, heat affected zone, bond and weld metal. The crack propagation rates on the four sections of joints of HT specimens are shown as follows; Bond>Weld metal>HAZ>Base metal. In contrast, the crack propagation rates of mild steel specimens are almost the same on the four sections of joints.
In the previous report, the authors have not only clarified the formation of the bead configuration under various welding conditions in the horizontal narrow gap welding with the high-peak pulsed MIG welding but also showed the possibility of setting up correct welding conditions to have satisfactory bead configuration and to assure the maintenance of the constant bead height even in case of the change in groove width, and resultingly consideration was made to the input data for its automatic selection by the computer. This report refers to the algorithm and its features for automatic setting up the optimum welding conditions by the computer, and to the result of the automatic control experiment which evaluate the correctness of the algorithm. The main results obtained are as follows: 1) Even in case of the change in the groove width of a joint, cosideration is given to the algorithm to set up the optimum welding conditions to have satisfactory bead configuration and the bead of the constant height, and it was shown that the automatic setting-up and control making use of the micro computer is available. 2) Algorithm for control was established basing upon the method of (1) above. This has been performed chiefly by the serial search of the domain of the welding conditions in the three dimensional space of welding speed-welding current-groove width. 3) An automatic control system making use of the micro computer was manufactured as a trial, the automatic control experiment was carried out with the algorithm of (2) above, and this method was proved effective.
For the purpose of estimating susceptibility of steels to cold cracking in underwater wet welding, implant weldability tests (Test piece: 8 mmφ, 0.5 mm (40°, 0.10 mmR) helical V-notch) for underwater welds were conducted, while diffusible hydrogen evolved from weld metal was measured according to the method specified in JIS Z 3113-1975. The test welding (Bead welding in flat position) was carried out at a depth of 20 cm in fresh water by using a plasma arc as a source of heat. No filler metal was supplied. Theresultsaresummarizedasfollows: 1) The under water weld just after solidification was estimated to contain diffusible hydrogen of the level of some 9.4ml/100g of weld metal (Collecting medium: Glycerin). 2)For the lower critical stress (σcr)imp (below which cracking does not lead to complete rupture of the implant) to exceed the yield point σY of the base metal, it was required that cooling time from peak temperature to 100°C, S(P-100°C), during undcrwater welding was over 2-1) 23 sec for a JIS-SM41A steel (PCM specified in WES-3002A-1973: 0.21%, C: 0.15% and σY: 31 kgf/mm2), 2-2) 35 sec for SM41A steel (PcM: 0.28%, C: 0.24% and σY: 30kgf/mm2) and 2-3) 100 sec for a SM50B steel (PCM: 0.24%, C: 0.16% and σY: 38kgf/mm2). 3) At a fixed time of S(P-100°C), (σcr)imp's of steels under water were higher tha n those in air because of lower hardness in coarse-grained regions of underwater welds.
Weldability of sulphur free machining steel in friction welding was investigated. Base metal has a diameter of 12 mm. Results are summarized as follows: (1) A suitable welding condition for sound welds in tensile test is; Rotational speed N: 3620 rpm Heating pressure Ph: 3.0 kg/mm2 Upsetting pressure Pu: 8.0-12.0 kg/mm2 Heating time Th: 0.5 s. (2) In any case, increase of heating time lowers tensile strength of weld. (3) Even the welded joint, which is made under suitable condition, has poor bending ductility. Normalizing can improve ductility of weld. However, it makes the joint break at the weld in tensile test. (4) The weld made under suitable condition has impact value of 1.3 kg-m/cm2in Charpy test by using specimen with V notch. On the whole, impact value of the weld of free machining steel is much lower than that of base metal, 3.2 kg-m/cm2. (5) Fatigue strength of the weld is 29.0 kg/mm2, which is lower than that of base metal by 10 %. (6) When heating time is prolonged, softened area in the weld becomes wider, and it deteriorates mechanical properties of weld. (7) Mechanical properties of weld is affected by shape, size and orientation of sulphide at the heat-affected zone.
The effect of Mo and N on chloride pitting corrosion has been studied in the welds of commercial type and high N-containing (0.34 %N) austenitic stainless steel. The results are as follows: 1) Austenitic stainless steel weld metal is more susceptible to chloride pitting corrosion than base metal. This loss of pitting corrosion resistance is due to the dendritic micro-segregation of alloy elements such as Mo and Cr which are preferable effect on pitting corrosion. 2) The pitting corrosion resistance in weld metals increases with nitrogen content in both commercial type and high N-containing steel, independent of whether molybdenum coexists or not. 3) Increase of Mn which segregates in γ phase on solidification process improves pitting corrosion resistance by enhancing the solubility of nitrogen in γ phase of the weld metal.
Weldability of the HT80 steel was investigated by making the SH-CCT diagram, heat cycle simulating test and the slit type cracking test in order to make clear the influence of the welding procedure on the notch toughness, the hardness and the crack sensitivity in the weld heat-affected zone. The results of these test are summarized as follows. (1) The impact value (vE-15) of simulated HAZ steeply fails when the peak temperature of single heat cycle exceeds the ACI point (710°C). And the lowest impact value is indicated when the peak temperature is made above 900°C and transformed to upper bainite during contenuous cooling. (2) The impact value of simulated HAZ will be remarkably increased when cooling time of the heat cycle being shorter than the critical cooling time of upper bainite transformation and then postheated at 600°C, 3 hours with the cooling rate after postheating is more than 100°C/h. (3) It has been found that temper brittleness is occured in the range of 450-550°C. (4) In weld joint tests, the effect of cooling time on the impact value of HAZ agree well with the ten-dency in the results of the heat cycle test. (5) In the slit type cracking test, when the bainite structure of HAZ is made approximatly more than 45 % (preheating temp. is higher than 150°C), HAZ cracking can be prevented even if the diffusible hydrogen in weld metal is increased from 2 cc/ 100 g to 12.7 cc/ 100 g.
In a weld joint, the base metal and weld metal are changed in their microstructures and mechanical properties by heat effect. This study is going to develop new welding process, taking advantage of this heat effect, by which high tough joint can be produced, covering the theoretical analysis of heat conduction of weld bead, variation of the properties of heat affected under bead, algorithm of welding, and weld equipment. In this report, the properties of synthetically heat affected 3.5 % Ni weld metal and the applicability of 3.5 % Ni steel to the production by the new welding process are introductorily described. This new welding process is put to production welds of 3.5% Ni steel and 21/4 Cr-l Mo steel to get high quality joint.