The short time tensile test was conducted on the base metal, TIG weld metal, TIG weld were observed by means of a scanning electron microscope, and the microstructures by means of both optical and electron microscopes, so as to investigate the relation between the fracture mechanism and microstructure.The results obtained are summarized as follows: (1) The fracture mode of the base metal changed from dimple fracture to intergranular gradually.the fracture mode transition from interdendritic fracture to intergranular fracture, while the ductility of EBW joint increased gradually similar to that of the base metal.The fracture surfaces of EBW joint revealed that the fracture type was almost dimple fracture.(3) Both the dimple fracture of base metal and the interdendritic fracture of TIG weld metal and TIG weld joint were caused by the crack initiation at the precipitated M6C carbide and its propagation.
An experimentaL investigation on corrosion of steeL tubes in flowing naturaL seawater under heat-transfer conditioll in the 1l0nboiLing !'al/ge has been carried out using a unique Loop tyjJe test apparatus. The results obtained are summarized as foLLows : (I) Corrosion rate increases rapidly for carbon steels and decreases for stainless steels because of passivity with increase of flow rate of seawater. Two typicaL trends of diffusion controLLed corrosion under aqueous flow were confirmed under heat-transfer condition as weLL. (2) H eat-transfer has an effeet on corrosion rate of aLL of the materiaLs investigated except SUS316 and 329J , stainLess steeLs, though the relationship is not Linear. (3) As for SUS316 and 329J , stainless steels, heat-transfer e./Ject on corrosion rate is almost negLigible unde!' exposure to air. Under oxygen saturated and deaerated cOllditions, however, those steeLs show about the same trends as other materiaLs investigated. (4) P itting corrosion of a /JeCuLiar type occurs on the SUS 329J , two-/Jizased stainLess steeL. (5) The test method using the unique test apparatus has jnoved avaiLabLe .
The corrosion fatigue tests in city water were carried out on precipitation hardening stainless steel, SUS630, variously heat treated (H1075, H1025, and H900), which was manufactured by three different melting processes (air, double and triple melting process). The results obtained are as follows.The corrosion fatigue strength of all H1075 specimens manufactured by three melting processes decreased compared with fatigue strength in air. The triple melted H1075 specimen showed the highest value of corrosion fatigue strength, followed by the double melted and air melted specimens in the descending order. This result seemed to be connected with the maximum size of inclusions in the specimens.As for the triple melted specimens subjected to different heat treatments, the H1025 specimen showed the highest value of corrosion fatigue strength and the H900 and H1075 specimens were about the same.
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