Hydrogen diffusion in steels is temperature-dependent, and accordingly if incubation period to failure in sulfide corrosion environment were affected by hydrogen diffusion, the incubation period would be also temperature-dependent. In this investigation the temperature dependence of sulfide corrosion cracking in weldable high strength steels, especially of 80 kg/mm2 level, has been studied. Both constant strain method and constant load method were used in this investigation and the time to failure after loading was measured in the temperature range 0°C to 70°C. The results obtained are as follows. 1). The susceptibility to sulfide corrosion cracking in high strength steels is temperature-dependent and the incubation period to failure is minimum at about 35°C. 2). At temperatures lower than 35°C the relation between the reciprocal of testing temperature and the logarithm of ratio of incubation period to testing temperature is represented by a straight line with a positive incline. Activation energy of cracking calculated from the incline is stress-dependent and activation energy of cracking at no load is estimated at about 27, 000 cal/mol by extrapolating the relation between applied stress and activation energy. This value is markedly high comparing with activation energy of hydrogen diffusion in steel, i. e., about 9, 000 cal/mol. 3). The susceptibility to sulfide corrosion cracking in high strength steels is anisotropic: the susceptibility in thickness direction is considerably higher than that in roll direction. It is supposed that the above results from the existence of thin and lamellar non-metallic inclusions extended in roll direction. Segregation of Mn, S, and small amount of Cr and C are recognized in the inclusions 4). The incubation period to failure remarkably depends on applied load in roll direction, while in the thickness direction applied load dependence of the incubation period is scarcely recognized.
Formation of spot defect is often observed on the surface of weld bead during the Tungsten inert gas welding of 18-8 stainless steel thin sheet. We investigated into the cause of this spot and discussed its formation mechanism from a viewpoint of non-metallic inclusion in the parent and weld metal. Results are summarized as follows: (1) The amount of inclusion in parent metal expressed as a cleanliness evaluated by point counting method has no effect on the occurrence of spot. (2) The inclusions in parent metal, which have high melting point and do not dissolve in molten pool, form the spot on the surface of weld bead. (3) On the other hand, the inclusions in parent metal, which have a large size, of more than 8-10 μ in mean dia., though with a low melting point, form the spot on the surface of weld bead. From these experimental evidences, we conclude that the cause of spot formation is related to the inclusions in parent metal, having a high melting point and/or a large size. And we illustrate the mechanism of spot formation: The inclusions in parent metal at first float from melt in molten pool, and then agglomerate on the surface of molten pool as a floater; when the floater grows to some extent, it is fixed at the end of the molten pool with tear drop type, and in situ the floater solidifies as a spot defect.
Using the calculated result of the ratio R of transient temperature rise to the quasi-stationary state temperature in a thin plate due to a moving point heat source, some considerations on the transient temperature variation of the points near arc starting or stopping in the mother plate are made. According to literature (1) the transient temperature of any point on the circle whose center is the present arc position and whose radius is equal to the distance of arc travel from the starting point, is just half the quasistationary state value. It is remarkable that the above mentioned relation holds true independently of thermal constant k and arc traveling velocity. Tables 1, 2 show some calculated results.
In the first stage of the research on stress corrosion cracking under dynamic load, we tried to study the phenomenon under pulsating trapezoid load (Fig.4). The tested material is AISI 304 stainless steel and the corrosion medium is a boiling solution of 42 percent MgCl2 (B.P.: 154°C). The apparatus used in this report is a constant tensile loading type and we can change the load time and the no-load time, arbitrarily. The results obtained from this experiment are summarized as follows: 1) The damage due to stress corrosion cracking under pulsating trapezoid load is not equal to the accumulated value of static damage. This damage is not only influenced by the dynamic effect due to the pulsating load but also by the length of no-load time. 2) We can presume the propagation time under the following assumption: the total sum of the damages due to 1) load time, 2) no-load time and 3) dynamic effect is equal to the damage under static load. 3) In the case of the ratio of load time to no-load time being constant, the crack propagation time is prolonged by the prolongation of the loading period.
The effects of carbon, manganese, phosphor, sulphur and nickel on the hot cracking and micro segregation of weld metal were examined in submerged arc welding process using the electron probe microanalyser. Based on the results of this study, the following conclusions are drawn: 1) Segregation of sulphur concentration is larger in weld metal containing more than 0.1% C than one containing less than 0.1% C, and manganese behaves in the same manner as sulphur. 2) Micro segregation or hot cracking of weld metal containing nickel is strongly influenced by the phase solidified as primary crystal in Fe-Ni diagram. Moreover newly solidified phase by peritectic reaction: δ+Liquid→γ promotes segregation of sulphur and phosphor concentration. 3) Prime cause of hot cracking in weld metal is sulphur segregation, and the effect of phosphor can be considered as far less than that of sulphur.