Dissimilar material pipe joints are used frequently as a conduit which passes gas liquid, and also for the purpose of a reduction in product weigth and material cost. In the production of such joints, friction welding is most effective process because it is possible to make the thin compound layer at the weld interface by the operation of high axial pressure. However the process has the characteristic that the formation of burr is indispensable. In the case of pipe joints, the inside and the outside burrs are formed, especially the inside burr obstructs the flow of a fluid.
In this study, 6061 aluminum alloy pipe and S45C carbon steel pipe were friction welded and the effects of thickness ratio on joint strength and burr shape were investigated. The main results obtained are as follows.
As to the tensile strength of pipe joints composed of various thickness ratio welded under the constant welding condition, the joint strength of pipe of large thickness ratio was low so that increasing the thickness ratio of weld pipe required more large forging pressure in order to obtain the same as the joint strength and the burn-off quantity of pipe of small thickness ratio. The average joint efficiency of weld joints seemed to be excellent was 57.3%. The burr progressed complicatedly as the welding time elapsed, that is, the outer burr spread to the radial direction along the aluminum alloy following by curl back of the tip of burr, on the other hand, the inner burr grew to the thrust direction along the carbon steel rather than spreading to the radial direction. An experimental equatuin which estimates the hollow diameter d_i from the outside burr diameter d_o was introduced as follows.
d_i=0.058d_o²-2.998d_o+47.656.
Also, the estimation of hollow diameter was more easy using the neural network which was composed of 3 input units of outer diameter of pipe, outer burr height and burn-off quantity or 4 input units of outer diameter of pipe, outside burr heigth, burn-off quantity and width of outer burr.

View full abstract

Two empirical relations by Barsom and Hagiwara between fracture toughness and impact toughness were examined, based on the assumption for the fracture occurrence condition that the local flow stress at crack tip attains to the microscopical cleavage fracture stress σ_F. It was shown that;
(1) The stain rate sensitivity of fracture toughness is controlled not only by that of the yield strength, but also by the fracture mode characteristic to the microstructure.
(2) It was suggested that σ_F depends on temperature in such a high temprature range that cleavage fracture is possible only in the dynamic loading condition.

View full abstract

The extent of cleavage process zone and its effect have been studied using mild-steel materials and large power-generation-type rotor materials having different levels of strength, and the cleavage crack initiation sites in the temperature region under ductile-to-brittle transition temperature were examined. In particular, a detailed examination was performed on the distance from the tip of a fatigue pre-crack to the cleavage crack initiation site, and on the distance from the tip of the ductile crack to the cleavage crack initiation site for the case of a progressing fibrous ductile crack. Also the relstionships between the distance (X) and the value of fracture toughness were examined and the criteria for cleavage fracture as proposed in the past were studied. The following results were obtained.
(1) The distance X is distributed in a random manner between 100 and 1000μm in the case of SM400C steel and between 80 to 600μm in the case of NiCrMoV steel, regardless of whether the fibrous ductile crack extension (Δa) exists or not.
(2) The distribution range of the distance X increases with the increasing temperature. And the maximum value of the distance X goes toward the larger value as the temperature increases.
(3) Since no correlation could be observed between the distance X and the value of Kc(J), the site of crack initiation does not necessarily originate at the point of maximum stress. An explanation could therefore not be given using the conventional criteria for cleavage fracture.
(4) The initiation of cleavage fracture is governed by the relative relationships held by the maximum weak point in the toughness distribution with the R curve and with the cleavage process zone of the crack tip. If the maximum weak point happens to lie within the cleavage process zone of the tip of the fatigue pre-crack, the cleavage fracture will occur on the blunting line before the occurrence of fibrous ductile crack growth. On the other hand, if the maximum weak point exists outside this cleavage process zone, the ductile crack will progress while searching for the maximum weak point, and the cleavage fracture will occur at the intersection of the R curve and the toughness-distribution curve.

View full abstract

An aluminium alloy spherical tank for an LNG carrier is introduced as an example of the pressure vessel type tank.
On the first LNG carrier completed in 1958, stiffened prismatic tanks of aluminium alloy were installed. After her success LNG trade started. In the 1960's membrane tanks were developed. Stainless steel and invar were used for them. These tanks were welcomed by the shipbuilders around 1970.
In the midst of the industrial situation, a spherical tank was developed. The LNG load was supported by the membrane stress of the unstiffened tank skin. After an LNG carrier of this type was firstly completed in 1973, she proved to be most reliable and economical. Since then a tank competing with this did not appear.

View full abstract