Influences of the conditions of weld thermal cycle and postweld heat treatment on the notch toughness, tensile properties and crack sensitivity in the HAZ of two high strength (80 kg/mm² type and 100 kg/ mm² type) martensitic stainless steels containing Ni, Mo, Nb and Cu were investigated by means of welding thermal cycle simulating method and the slit type cracking test. The results of these tests are sum-marizsed as follows.
(1) In the case of 80 kg/mm² type steel (base metal A), the impact value steeply falls when thermal cycle above 1, 300°C peak temperature is experienced, and recovery of the impact value due to the postheating is little even with postheating at 650°C for 2 hours.
(2) In case of the 100 kg, /mm² type (base metal B; precipitation hardening) steel, the impact value does not almost decrease even though peak temperature of the thermal cycle is changed within the range of Ac1 point-1, 300°C. However, postweld aged at 500°C for 4 hours after exposure to the tehrmal cycle, the impact value drops remarkably when the peak temperature of the thermal cycle exceeds 950°C.
(3) In the multipass weld joint tests on two test steels which was carried out under optimum welding procedure and postweld haet treatment as shown in Table 4, satisfactory notch toughness and tensile properties were obtained.
(4) In the slit type cracking test, the HAZ cracking can be perfectly prevented when the preheating temperature was higher than 200°C for base metal A and 100°C for base metal B, and holding temperature after welding was at 100°C for each kinds of the test steels and then postheated at 600°C for 10 minutes.

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When tensile tests on mild steel are carried out at various temperatures, the serrated yielding and flow are observed at a certain testing temperature range, specially 100-300°C, and the consequent em-brittlement occurs. This phenomenon is generally known as a blue brittleness or a dynamic strain aging and is related to testing temperature, solute atom contents (carbon and/or nitrogen contents), strain rate and so forth.
On the other hand, for base metal of QT-type high strength low alloy steels in which solute atoms such as carbon and nitrogen are fixed as carbide, nitride or carbo-nitride by heat treatment, such embrit-tlement seems to be not apt to occur comparatively. But, even if QT-type steels, there is a possibility of dynamic strain aging in heat-affected zone (HAZ) because of the above carbide, nitride or carbo-nitride dissolving by welding heats, and the consequent embrittlernent.
This study was investigated about the ductility loss due to dynamic strain aging in HAZ of a 80 kg/mm² class high strength low alloy steel.
It is found by this study that dynamic strain aging and, thus, the ductility loss can occur in HAZ even if QT-type steels. The degree of ductility loss is increased in the order of upper bainite, lower bainite and martensite structure.
These results can be qualitatively interpreted by considering as interaction between dislocations and solute atoms, specifically nitrogen atoms rather than carbon atoms.
Transformed HAZ, which is heated above 1200°C and has martensite structure and, thus, a high content of free nitrogen atoms, is sensitive to ductility loss due to dynamic strain aging as well as the deterioration of grain coarsened HAZ toughness.

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A new plasma arc hardfacing process has been developed for the overlay weld with excellent wear resistance.
In this process, the powder mixture, transition metal carbide NbC or VC with austenitic stainless steel SUS304L, is used as hardfacing material, and the overlay weld consisted of matrix alloy and unmelted carbide particles dispersed uniformly is formed.
The main results obtained are summarized as follows:
1) In the bead-on-plate welding with oscillation of 10 mm width, the overlay weld having good shape and low penetration is obtained by powder feeding rate of 10-15 g/min, welding current of 100-120 A and travelling speed of 3-6 cm/min.
2) Mean hardness of the overlay weld is adjustable from Hv 400 to 1000 according to the carbide content in powder mixture fed.
3) In spite of high hardness, the weld crack sensitivity of this process is less than conventional hardfacing materials such as Co-Cr-W alloy.
4) Abrasive and adhesive wear tests show that the wear resistance of this weld metal is excellent in comparison with other hardfacing materials and wear resistant materials.

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In the author's previous report, a new cracking parameter PH was developed so that the conditions leading to hydrogen-induced cracking in the welds of high strength steels could be analyzed and predicted using the standard data of the JIS oblique-Y (Tekken-type) cracking test. In this report, it is established that the effects of weld heat input and intensity of restraint on hydrogen-induced cracking can be defined by a more generalized PH formula.
The pertinence of PH analysis and prediction have been verified sufficiently with expermiental data obtained from the H-slit type cracking tests of thick plates with a single-bevel groove, the site-weldability tests of pipeline steels, proposed by Stout for the API and the heel crack tests in tab fillet welding.
It is also shown that the effect of diffusible hydrogen content is much smaller under mild restraint than under severe restraint. The intensity of restraint in any case should be converted in the P_H theory into an equivalent intensity in the JIS oblique-Y test, considering heat input, eccentricity and groove shape.

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This investigation has been made of the influence of precipitated and retained austenite on impact toughness in the synthetic HAZ of 5.5% Ni cryogenic steels. Specimens were heated and cooled through the selected thermal cycles by induction heating and then the instrumental Charpy test was employed for the determination of crack-initiation and crack-propagation energy.
From the result, the sufficient requirment is that the ductile fracture with plastic deformation occurs, that is, the temperature of impact test should be above the ductile-to-brittle transition temperature (DB-TT) in order to present a beneficial effect of retained austenite in HAZ on impact toughness. So the beneficial effect range of retained austenite on toughness in synthetic HAZ of 5.5% Ni steel represents the schematical diagram, lst cycle peak temperature vs. DBTT. The instrumented Charpy test reveals that the austenite phase produced by wled thermal cycles takes part in the energy of crack-propagation rather than crack-initiation.

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In the case of underwater wet welding, weld crack often occures in heat affected zone.
The purpose of the study was to decrease or prevent this crack by using Ni, Fe-Ni and stainless steel electrodes.
SS41 steel plate of 12 mm thickness, SM50C steel plate of 12 mm thickness and 80 kg/mm² grade high strength steel plate (WT80C) of 16 mm thickness were used as base metals.
The results were summarized as follows,
(1) Using Ni, Fe-Ni, D309 and D310 electrode in welding SM50C and WT80C were effective to prevent under bead crack and crack free weld was able to be obtained. Furthermore there were not found any craking in weld metals of SS41, SM50C and WT80C steel.
(2) There were found underbead crack and weld metal crack in welded parts of SS41, SM50C and WT80C steel with D430 electrode.
(3) The microscopic segregation of Ni and Cr in weld metals of SS41, SM50C and WT80C steel with austenitic stainless steel electrodes were a little.

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This investigation is carried out in order to establish an estimation procedure of fatigue strength under tensile shearing load of multi-spot welded joints arranged in perpendicular to the load direction, by using maximum load vs. number of repeated cycles curve (abbreviated to L-N curve, hereafter) of single-spot welded joint which has the same nugget diameter with those of multi-spot welded joints. In general, it has been known that static tensile shearing strength of multi-spot welded joints becomes nearly equal to the product of the spot number of the joint by the tensile shearing strength of single-spot welded joint which has the same specimen width with the spot space of multi-spot welded joints (abbreviated such a single-spot welded joint to the equivalent single-spot welded specimen), no matter how the mounting of multi-spot wleded specimen on the testing machine is made without sufficient cares. On the other hand, it is usual that fatigue strength of multi-spot welded joints becomes considerably lower than the product of the spot number of the joint by the fatigue strength of equivalent single-spot welded specimen, even if the mounting of multi-spot welded specimen on the fatigue testing machine is made with sufficient cares. (Here, we define that the fatigue life of multi-spot welded joints under a constant maximum load is the number of cycles when a first fatigue crack with the same length with the nugget diameter becomes visible around a spot weld of multi-spot welds.) It is made clear by the previous paper that this is mainly because of unequality of load bearing ratio of each spot weld in multi-spot welds. Moreover, it is recognized in our laboratory that when a special device is used for load bearing ratio of each spot weld in multi-spot welds to become equal, fatigue strength of multi-spot welded joints becomes nearly equal to the product of the spot number of the joint by the fatigue strength of equivalent single-spot welded specimen.
In practices of spot welded structures, load bearing ratio of each spot weld in multi-spot welds is scarcely equal. Moreover, depended on the unintentional variations of nugget diameter and spot space, unequality of load bearing ratio of each spot weld in multi-spot welds will become larger.
The purpose of this investigation is to find out a relationship between fatigue strengths of spot welded structures in practices as mentioned above and that of equivalent single-spot welded specimen. For this purpose, a hypothesis which is able to connect fatigue strength of multi-spot welded joints and that of equivalent single-spot welded specimen was set up, and experiments for the verification of the hypothesis were carried out. Materials of specimens used in these experiments are mild steel, high strength steel and commercially pure aluminum, respectively, of 0.8 mm thickness.

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The investigation on the automatic control system was made to detect the change in the groove width and the welding start and stop positions, to control the weld line tracking, and to set up the optimum welding conditions corresponding to the change in the groove width, for the purpose of the automatic control of the horizontal narrow gap welding.
In this report, it is described as the principle and the algorithm of direct arc observing method which progress the reliability of weld line tracking, and as the synthetical control algorithm to realize the full automatic control.
The results obtained are summarized as follows:
(1) The weld line and the wire position can be detected stably within the accuracy of 0.5 (mm) with the direct arc observing method.
(2) The welding device and the algorithm which control it synthetically were manufactured as a trial.
(3) As the experimental result of automatic control, the control device and the algorithm were certified to be used practically.

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Effect of REM addition on the toughness in the weld metal of high purity 30Cr-2Mo steel was investigated. Welding was carried out by TIG method using deliberately contaminated argon with various ratio of nitrogen or air as shielding gas in order to vary the toughness of weld metals. The toughness of weld metals was evaluatedd using sub-size Charpy V-notch specimens with thickness of 3 mm. The main results obtained in this study are as follows;
1) The toughness of the weld metal is significantly improved by addition of REM. Effective content of REM ranges from about 50 ppm to 200 ppm when nitrogen content in shielding gas being less than 1%.
2) The quantity of absorbed nitrogen decreases in the REM bearing weld metal, which yields the improvement of toughness in it.
3) It was deduced that the decrease of nitrogen in the REM bearing weld metal was attributed to the decrease of its absorption rate due to the formation of REM oxide film or absorbed layer of REM on the surface of the molten pool.

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Kinetics of the formation of austenite in Type 329J₁ duplex stainless steel at constant temperature was investigated as the first step to analyze the amount of the austenite in HAZ of this steel. Special attention has been paid to obtain quantitative data on the formation of austenite in the range of 950-1150°C, because in this range, Widmanstätten austenite which occupies the greater part of the amount of austenite in HAZ after welding precipitates from ferrite grain boundary. The volume fraction of austenite was measured metallographically using an area-analyzer.
As the rseult, it was elucidated that the formation of austenite in the range of 950-1150°C could be expressed by the Austin-Rickett's type equation with time exponent 'n' of 1. On the other hand, from the consideration of the activation energy for the growth of austenite, it was deduced that the formation of austenite was controlled by diffusion of nitrogen or carbon to austenite from ferrite matrix.

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In submerged arc welding, the direction of arc column and the position of molten metal in the crater have been examined using a X-ray image intensifier TV system in order to regulate weld bead shape by controlling welding parameters.
Arcs are classified into the open and buried arcs depending on the position of electrode tip to the plate surface. The arc at high welding current and high travel speed, is buried in the plate and is generated forward between the front of crater and the electrode tip, which results in a knife like shape of electrode tip. The arc at high welding current and low travel speed, is buried deeply in the plate and strikes radially between the weld pool and the electrode tip melted like a pencil. On the other hand, the open arc with low welding current is generated between the weld pool and the electrode tip over the plate surface. The direction of the arc column and the shape of the electrode tip at the open arc change in accordance with the position of molten metal in the crater.

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The molten pool surface profile in arc welding has been analyzed numerically by a finite difference method.
In the first part of this report, the numerical method has been discussed and it is made clear that an iterative method of finite difference is useful to solve the non-linear partial differential equation which describes the molten pool surface profile.
In the second part, the iterative method has been applied to some typical problems in arc welding and satisfactory results have been obtained.
From these numerical analyses, it is concluded that the method, used in this work, is quite available for further investigation of molten pool phenomena in arc welding.

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