Morphology and locality of sulfide inclusions in HAZ of both HY80 and SM50B steels were observed, and the relation between sulfide inclusions and susceptibility to hydrogen-assisted cracking was examined. The sulfide inclusions in HAZ of HY80 steel heated to 1450°C were located in the form of array in a rolling direction. The morphology and locality of dotlike sulfide inclusions in HY80 steel HAZ were quite different from those in SM50B steel HAZ heated to 1450°C. Consequently, it was known that the morphology and locality characteristic of sulfide inclusions seen in HY80 steel HAZ have a close velationship to the high susceptibility to hydrogen-assisted cracking of HY80 steel HAZ.
Partial melting was pointed out as a main factor affecting the morphology and the distribution of dissolved globular sulfide inclusions, which have a close relation to hydrogen-assisted cracking susceptibility in HAZ. In conclusion, some metallurgical factors and mechanism in connection with hydrogenassisted cracking in weld heat-affected zone of high strength steel were summarily considered.
Pure iron was welded in controlled arc atmosphere. Effects of the welding conditions and the partial pressure of carbon dioxide in Ar-CO2 welding atmosphere on the oxygen content of iron weld metal were systematically studied. The results are summarized as follows: 1. The oxygen content of iron weld metal decreases with increasing welding current and increases with increasing arc voltage. 2. The oxygen content of iron weld metal increases with the partial pressure of carbon dioxide in Ar-CO2 welding atmosphere. The oxygen content of iron weld metal in Ar-CO2 welding atmosphere is smaller than that in Ar-O2 welding atmosphere. 3. The carbon content of iron weld metal increases with the partial pressure of carbon dioxide in Ar-CO2 welding atmosphere. The carbon content of iron weld metal is about 0.027 wt.% under the welding current of 250 A in carbon dioxide welding atmosphere. 4. The high partial pressure of carbon dioxide, blow holes are observed in iron weld metal. 5. Behaviour of oxygen absorption into iron weld metal is discussed by using equilibrium data.
Effects of sulphur contents in base metals (0.008-0.026%S) on hot cracking susceptibility of welds for CO2-O2 gas shielded arc welding of high carbon steel (0.65-0.70%C) have been investigated with angle expanding type cracking test (gas mixture ratio, 0, 20, 40%) and 15 ton class tensile restraint cracking test (CO2+20%O2). The results obtained are as follows: (1) In case of angle expanding type cracking test, remarkable variation of hot cracking susceptibility of weld metals are not recognized by different sulphur contents. (2) In case of 15 ton class tensile restraint cracking tests, hot cracking susceptibility of welds (mainly heat affected zone) are affected by sulphur contents in base metal and the relation between hot cracking susceptibility of welds and sulphur contents in base metals have been clarified with increasing tensile stress during testing.
Mainly mechanical properties of high carbon steel (0.65-0.70%C, 0.008-0.026%S) welds for narrow gap (gap; 10 mm) butt welding in the flat position (build up process; intermittent, continuous) and vertical position (continuous) have been investigated by CO2+20%O2 gas shielded arc welding. The results obtained are as follows: (1) In case of tensile tests of welded joints by butt welding in the flat position (intermittent), excellent tensile properties with break down in weld metal and base metal are obtained for 0.008%S base metal. Tensile properties decrease as sulphur contents increase from 0.014%S to 0.026%S and Fractured position are almost in heat affected zone. For different base metals, tensile properties of welded joints by continuous welding are lower than by intermittent welding and fractured position by continuous welding are almost in heat affected zone. In case of vertical welding, excellent tensile properties with break down in base metal are obtained for different base metals. (2) In case of rotating bending fatigue tests of welded joints by butt welding in the flat position and vertical welding, excellent fatigue strength are obtained for 0.008%S, 0.020%S base metals. In case of welded joints in the flat position for 0.026% S base metal, low fatigue strength are obtained and fractured origin exists at intercrystalline crack in heat affected zone for continuous welding.
A mechanism and some factors to cause a reheat cracking in HAZ of high strength steel weldment were examined and discussed, using a T-1 type of steel. The following results were obtained in this study. (1) The reheat cracking in HAZ can be induced by both a segregation of impurities and a precipitation of carbides of secondary hardening elements at grain boundaries. In particular, the effect of a grain boundary precipitation is predominant in case of T-1 type of steel HAZ. (2) It is likely that the influence of carbides precipitated at grain boundaries on the reheat cracking susceptibility appears through a decohesion at grain boundary and a lowering of a deformability or a ductility at grain boundary because of an intense interaction between dislocations and carbides. (3) The effects of a segregation of impurities and a precipitation of carbides at grain boundary on the reheat cracking phenomenon were able to be represented schematically.
The present work is the initial part of a broadly based investigation of the fatigue crack propagation in a Type 304 stainless steel welded joint. In particular, attention is focussed to the fatigue crack growth behavior of the cross-bond type crack, the front of which is inhomogeneous and consists of two regions; one is the weld metal region and the other is the base metal region. Although effect of residual stress on fatigue crack growth rate was quite significant, that of microstructure of the weld metal was also found to be significant: The growth rate of a weld metal specimen was larger than that of a base metal specimen when the growth rate was plotted against effective stress intensity range. The effect of residual stress on fatigue crack growth rate can be found even after the solution treatment (1050°C for 1.5 hr). Hence, it may be conservative to consider that after usual PWHT, residual stress which can have an effect on fatigue crack growth rate is still remaining. For the cross-bond type specimens, it was found that the cracks in the two regions did not interact each other and propagated independently in each region: The fatigue crack in the weld metal region propagated faster than that in the base metal region.
Fracture performance of explosive clad steel composed of SUS304 stainless steel and HT80 high strength steel is investigated. To clarify the effects of composite structures and metallurgical properties on the fracture strength and fracture strain, tensile tests are carried out by using homogeneous and heterogeneous specimens which are extracted from the clad plate and have artificial surface flaws. The critical value of stress for the macroscopic unstable fracture initiation of the heterogeneous specimen with surface flaw in SUS304 steel is larger than that of the homogeneous SUS304 steel specimen over all temperature range, and also larger than that of the homogeneous HT80 steel specimen at very low temperature. The fracture strain of the heterogeneous specimen with surface flaw in SUS304 steel lies between that of the homogeneous SUS304 steel specimen and that of the homogeneous HT80 steel specimen over all temperature range. The critical crack mouth opening displacement of the heterogeneous specimen with surface flaw in SUS304 steel is lower than that of the homogeneous SUS304 steel specimen.
The JIS-y root cracking specimen and H-slit restraint cracking specimen have been widely used in Japan in investigating cold cracking of steel weldments. While the former is small but possesses constant restraint, the latter is large and expensive. In this report, the Controlled Restraint type Slit cracking specimen (CRS specimen) with slits at both edges of test welding line of JIS specimen is proposed, and the cold cracking tests are conducted after investigation of relation between slit length, restraint intensity, and stress. When these test data are arranged using restraint stress and residual diffusible hydrogen content of weld metal, a distinct line is found as the critical curve for cold cracking. Moreover the results are applied to actual weld procedures.
The brittle fracture strength of fillet welded joints was investigated through fracture mechanics techniques. Three point bend specimens of 50 mm thickness were extracted from the butt joint with partial penetration of mild steel. The critical crack opening displacement at fracture, δc, was evaluated and converted to the fracture toughness value in the linear fracture mechanics, Kc, by the use of the equation, Kc=√EσYδc, where E and σY are Young's modulus and yield stress, respectively. Brittle fracture tests on various types of fillet welded joints of 50 mm thickness were performed at low temperature. The stress intensity factors at the root of these fillet welded joints where combined modes of deformation appear were analyzed with the aid of the finite element method. It was found that the fracture strength of the fillet welded joint can be evaluated on the basis of maximum tangential stress criterion.
Fatigue life prediction of welded joints under plane bending load and its application to fatigue behaviour analysis of welded tubular joints were performed on the basis of the results stated in the 1st Report. The results are summarized as follows. (1). The fatigue crack initiation life Nc at the weld toe could be evaluated using strain-controlled fatigue concepts. (2). The fatigue failure life Nf of welded joints under plane bending could be estimated using edge crack model with an initial crack size (a/T)i=0.0125. (3). The fatigue life Nc and Nf in welded tubular T-and X-joints also could be evaluated from the above procedures by using hot-spot stress range Δσa in stead of Δσb.
This investigation, as the preliminary stage of investigations for static and fatigue strengths of spot welded joints under especial circumstances: various temperature and/or corrosion circumstances, treats the temperature dependency of spot welded joint strengths. Specimen materials used are mild steel sheets and high strength steel sheets (45 kg class and 80 kg class) of 0.8 mm thickness. Tests were carried out under temperature circumstances of three kinds of -20°C, room temperature and 100°C. Main conclusions obtained in this investigation are as follows: (1) Tensile shearing strengths of spot welded joints decrease with the increase of temperature for all steels and this dependency on temperature is alike as that of tensile strength of mother sheet. (2) Fatigue strengths for spot welded joint decrease with the increase of temperature for all steels, but the degree of the decrease is a little in comparison with that of mother sheet. (3) Fatigue strengths of spot welded joints in steels with different tensile strengths are almost the same, under various temperature circumstances in this investigation. (4) Temperature dependency of fatigue strengths of spot welded joints has a rather strong correlation with one of tensile shearing strength of spot welded joint and in addition also has a correlation with one of tensile strength of mother sheet. (5) The fatigue cracks at 100°C for mild steel initiate in the position far away from the nugget and propagate in outer direction from the nugget, which is different from usual initiation positions and propagation paths.
In this paper, the cross-sectional shape of I-groove weld beads in DC electrode negative and in DC electrode positive (shielding gas composition: 30%Ar+70%CO2) MAG welding were studied, and the penetration angle (θ) of the side wall, the contact angle (α) of beads and the wetting height (h) were measured. Special attention was focussed on clarifying the cause of undercut by direct observation of the flow of molten metal using high speed photography. The results are as follows: 1) Mechanism of origination of under-cut Dry under-cut The arc moves around in I-groove rapidly and gouges the side walls of the base metal above the molten pool, exposing fresh solid surface. Moreover, owing to the mild flow of molten metal on the side walls, the solid surface exposed by the arc is not covered by molten metal, and under-cut of toe originates. Wet under-cut The arc ignites towards the groove bottom mainly, and gouges the side walls of the base metal below the surface of the molten pool. Moreover, the flow of molten metal on the side walls is strong and melts the side walls above the molten pool, covering the lower portion of the side walls already gouged by the arc. But on account of the side walls being melted too high, the molten metal on the side wall flows down and under-cut of toe originates. 2) Cross-sectional shape of beads In cases where under-cut does not originate, (α+θ) becomes, with a little dispersion, a constant value unrelated to welding conditions and h becomes higher with increasing θ. The bead surface shape has been discussed using surface tensional theory applied to the two dimensional model.
The dynamic deformation occurring on the welded plate during arc spot welding was investigated. As a result, it was observed that the welded plate deformed to the plus side of the plate (welding arc side) at about the same time as arc ignition and the deformation reached a maximum soon after. When the deformation reached the maximum, it reversed itself, reached, zero, and then proceeded towards a maximum at the minus side (back side) of the plate. In terms of arc time, in mild steel (plate thickness<8mm), the deformation toward the plus side reached its maximum value in less than 50% of arc time and returned to zero before the arc was extinguished (i.e. within the arc time). However, these characteristics changed with the material welded, as has been shown by similar experiments on stainless steel and aluminium alloy plate.
Automatic gouging by air carbon-arc method has been widely used as a step in welding work and in consequence the demand for all-position operation is mounting. The automatic gouging devices are classified into two types from the standpoint of electrode feed speed control: one type utilizing arc voltage for the control purpose and the other utilizing constant feed rate. The latter is one recently developed and is found appropriate for operating in all positions because of its light weight and easy operation. In this report, gouging in flat position of steel plate was tried, using the above constant electrode feed type device as the first step to application of this method to circumferential pipe welds. Then the effects of various gouging conditions (characteristics of electric power source, gouging speed, electrode feed speed, electrode diameter and shape of cross section, and torch angle) on arc voltage, gouging current, arc stability (continuity), heat input, groove shape, amount of removed metal and melting efficiency were studied. The main results are as follows: 1) The stability of arc largely depends on the characteristics of electric power source and the amount of fed electrode per unit gouge length (Wr). 2) The heat input (H) and removed metal per unit gouge length (Wm), ratio Wm/H and that of groove depth to groove width (d/w), an index of groove shape, are affected by gouging speed (Vg) and Wr. The ratio d/w is also affected by torch angle and the shape of electrode section. 3) If Vr and Vg are varied under constant ratio Vr/Vg as in practical use, Wm and melting efficiency (η, ratio of heat content of molten metal to heat input) are held almost constant except for smaller values of Vg (and Vr), Vr being the feed speed of electrode. 4) Linear regression analyses are made of data obtained under constant Vg, showing that there is close relation between Wr and Wm as well as between ratio d/w and η.