The fatigue crack propagation behaviour which propagated on the electron beam weld bead was investigated and its estimation was done by elasto plastic analysis. The fatigue crack propagation rate was increased monotoniously when the crack was remote from the weld bead. The crack coming near the bead, the bead was found to reduce the increment of its rate. On the fusion line, the rate became very small. After the crack came into the bead, the rate was increased again monotoniously. As the crack was coming near the bead, the fatigue crack path was beginning to get inclined to base metal. The weld bead was also found to influence the fatigue crack propagation direction. The fatigue crack propagation behaviour estimated by elasto plastic analysis was in good agreement with the behaviour which was observed in fatigue test.
The second part dealed with the stress intensity factor (KI) and the stress concentration factor (Kt) at the place which cold cracking frequently occured. The finite element and the body force methods were used to obtain exactly the values of KI and Kt which related the distribution or the value of local stresses at the neighbourhood of crack or notch. The results are summarized as following: (1) The exact values of KI and Kt in the various types of groove and in the implant specimen were obtained. (2) The equation was derived that related the value of KI and Kt with the factors involved in the types of groove and in the implant specimen.
The fracture toughness in a simulated weld heat-affected zone (HAZ) of the high tensile strength steel (HT80) were evaluated through JIC values using a Charpy-size three point bending specimen at a temperature range between 18°C and -150°C.The effect of the microstructure on crack initiation was investigated by a fractographic method using the scanning electron microscope (SEM). In the tempreature range from about -50°C to 18°C, for the base metal and HAZ whose cooling time from 800 to 500°C (Tc) was 17 sec, the JIC values increased with decreasing test temperature. The SEM examination showed that the area fraction of martensite-austenite constituent (M-A) in the microstructures mentioned above was less than 4 %. The fracture occurred by stable crack extension after the formation of the stretched zone at the precrack tip. On the otherr hand, the JIC values decreased with decreasing test temperature on HAZ microstructures whose Tc were 89 and 223 sec. In these microstructures, which had more than 8 % of area fraction of M-A, cleavage fracture occurred immediately after the formation of the stretched zone. JIC values in the microstructures which included more than 8% of area fraction of M-A were nearly a quarter of the one in the microstructures whose M-A were less than 4%, at about -50°C. The critical stretched zone width (SZWC) and JIC were found to be related as follows: JIC=α⋅σySZWC where -σy is the yield stress of steel and α is an experimental constant which was found to be 8.5.
A new method was proposed to estimate a susceptivility of weld metal to solidification cracking and various advantages in this method were made clear in this investigation. The results obtained are as follows; (1) The minimum strain to form solidification cracking is obtained by using this method. (2) The minimum strain was termed "TSSC (Threshold Strain for Solidification Cracking) value", and TSSC value is obtained by a following equation after performing a Trans-Varestraint Test. TSSC=ε1 sin2 θ, where ε1 is augmented strain and θ is the angle between the welding direction and the normal to the solidiocation crack. (3) The following raltion exists between TSSC and CST value under the heat input per unit thickness of 19 kJ/cm2 and the welding speed of 20 cm/min. TSSC=0.0807 (ln(CST))2+0.888 In (CST)+2.51 (4) The effect of the welding conditions on the susceptivility of the weld solidification cracking are able to be examined by using this method.
In the field welding with the ordinary CO2 arc welding torch, the limit of wind speed which can be obtained satisfactory welds is considered to about 2 m/sec. Several investigators have examined to increased effect of protective gas shield, but welds obtained with these torchs have such drawbacks as low reliability of mechanical properties and necessity of large flow-rate shielding gas. Authors therefore carried out to fundamental study of automatic CO2 arc welding in field using three type wire brush nozzles which were obtained good results to applied under water wet MIG/CO2 Welding. Observation of condition make raid white smoke wind into the wire brush nozzles and the mechanical tests of welds which are obtained under a artificial condition were carried out and both results were evaluated to lead the conclusions. (1) The observation of white smoke wind was severe than the mechanical testings of welds to know the effect of the wind. (2) In CO2 arc welding without the wire brush nozzles, satisfactory welds were not obtained over wind velocity of about 3 m/sec under a flow rate of 25 1/min shielding CO2 gas. (3) With a medium mesh type wire brush nozzle, satisfactory results were obtained under the same flow rate of shielding gas, even at a wind velocity of about 10 m/sec.
Many previous studies have reported that various properties of stainless steel weld zone depend sensitively on the micro-structure. Generally the micro-structure is locally different in the weld zone. Therefore, in the present study the micro-structure distribution of SUS304L stainless steel weld zone by TIG arc welding was investigated mainly with a transmission electron microscope. In this paper, the observations of the weld metal are reported. The results are as follows: (1) The ferrite content in the weld metal was higher at the surface region and lower at the fusion boundary region, while both Cr and Ni contents were nearly about constant all over the weld metal. (2) The ferrite structure was acicular or Widmanstätten-like only in the surface layer and vermicular in the other region. The ferrite shape was roundish in the upper region and rectangular in the central region. The ferrite phases formed networks, or were connected each other by sub-boundaries. (3) Cr and Ni contents, and Vickers hardness changed according to the ferrite structure distribution in the weld metal.
It has been already reported that the locally expanded fusion zone in electron beam welding of steel, which often accompanies with vertical cracks, closely relates with the focal point of electron beam. Theree fore the properties of the electron beam were measured by an electrical method and the AB test, and relation between these results and the shape of fusion zone were investigated. The results are summarized as follows; (1) Beam power density distribution is of trapezoid mode at the lens side from the focal point obtained by the AB test and gaussian mode at the other side. (2) Width of the neck portion of penetration is mainly predominated by the beam diameter. (3) The locally expanded fusion zone is formed at large ratio of the width at the surface to the neck portion of the fusion zone under large penetration depth. (4) Observations using a high speed camera indicate that the locally expanded fusion zone is caused by the secondary melting process due to the stagnation of the molten metal flow. (5) For prevention of the locally expanded fusion zone, sideway beam oscillation of small amplitude is usefull.
Time variation of temperature rise of any point P which holds a distance ζ from the flashing surface is calculated in non dimensional form from the fundamental heat conduction equation, assuming that the temperature of the flashing surface is held at the fusion temperature θf, the room temperature being equal to zero. Assumption A. The burn off per specimen is assumed to be expressed as B=1/n⋅αi⋅tn where αi and n are constant, n being n≥1. The base quantities of distance and time for non dimensional expression are choiced as shown in equ. (12) so that for t=τ0, the burn off becomes B=z0. See Fig. 2. It must be remarked that z0 and τ0 are bound through n and thermal diffusivity k as shown in equ. (17). The calculated results (Fig. 4, 5) show that the non dimensional temperature ψ=θ/θf for any given point η (=ζ/zo) takes a maximum value ψm at a certain time τm (=tm/τo) and that the tempera-ture ψ holds nearly constant value of ψ m for considerable long time, though the above mentioned tendency decreases as n increases from 1. In Fig. 6 the calculated relations between the maximum temperature ψm for η and the corresponding time τm are plotted. It is remarkable that for case of n=2, 3 the time τm for a given value of η takes smaller (nearly minimum) value compared to the cases of n=1.5, 6. In Fig. 7 the relations between ψ m and η are plotted for n=1.5-6 as well as for n= 1, ∞. In case of n= 1, the curve takes the well known simple exponential form of equ. (21). Case when the temperature of flashing surface is assumed to rise linearly. with time from zero to θf in time of t=0 to is also calculated. Assumption B. Curves (2) and (3) in Fig. 8 show the case of n=2 for to=0.5 τo and 1.0 τo. Curves (1) are for to=0, which are same as Fig. 4, and we see the degree of decrease of Ψ m and the delay of τm from the figure. The calculated results of relation of Ψ m and η in case of n=2 are compared with the experimental data executed in Rensselaer Polytechnic Institute, for which Dr. Suzuki et al. have proposed in Welding Journal (34 June 271-s 1955) the experimental formula of Ψm=ε-0.92ηs. See Fig. 9. It is remarkable that our calculated results coincide fairly well with the experimental results of the Institute and the pro-posed formula, though the experimental conditions may not coincide with our assumptions. Several comments on the theoretical considerations in the above mentioned literature are described in our paper.