It is well known that pressure welds of carbon steels are often characterized by the original weld interface being formed with very fine ferrite grains and being less ductile than the parent metals. It is the purpose of this investigation to observe the microscopic structure of the area near pressure welds interface under the high temperature metallurgical microscope and clarify various phenomena taking place in pressure welds of carbon steels. Specimens of about 1×10×10 mm size for high temperature microscope were prepared from the core of pressure welds bonded at 700°C by closed butt welding in atmosphere. They were heated without pressure to 1200°C at a rate of 20°C/min in the high temperature microscope filled with 99.99% pure argon of l atom after polished. The following results were obtained. 1. Neighboring grains of the original weld interface were less coarsened than those of parent metals, and the original weld interface of medium carbon steels like S45C seemed to become the austenite grain boundaries. And such phenomenon was more remarkable in the grades of steels with richer carbon con-tent, while in low carbon steels like S10C, grain boundaries at 1200°C definitely were found to proceed across the original weld interface. 2. Pressure welds that were made by closed butt pressure welding in atmosphere had nearly the same structures as those in high vacuum. 3. Complete trans-interfacial austenite grain growth in S45C was obtained by reheating to 1200°C in the high temperature microscope after cooling to room temperature. 4. The greatest cause to fromation of austenite grain boundaries along the original weld interface seemed to be presence of microcavities on the original weld interface. 5. Formation of fine ferrite grains on the original weld interface of carbon steels was caused by occurrence of austenite grain boundary along the original weld interface during pressure welding.
In the previous papers, the diffusivity of hydrogen in steel could be explained comparatively well by using the Fick's second law, assuming that steel was homogeneous on the whole. In this report, using the steel specimens varied in structures, the effects of the structures on the diffusion coefficient of hydrogen were described. The experimental data which were obtained from the permeability tests were analyzed by using the Time Lag method, and from its analysis, the diffusion coefficients of hydrogen in some structures of steel were calculated. The results obtained are as follows; (1) The hydrogen diffusivity depends upon the structure of steel. The ferrite+pearlitc structure has a large diffusivity, and the martensite has a small one. This tendency of the hydrogen diffusivity well agrees with the results from the hydrogen occlusion test. (2) The solubility of hydrogen in each structure is respectively 40cc/100gFe (ferrite+pearlite), 26cc/100gFe(troostite) and 24cc/100gFe (martensite). These values are consistent with the results from the hydrogen occlusion test.
In the previous paper, it was shown that the weld crack sensitivity of Inconel 713C mainly depended upon aluminum content; that is, according to the results of a bead-on-plate test, decreasing the aluminum content from 6% to 4.45% prevented weld cracking in this alloy. To clarify the influence of aluminum on the weld cracking, the solution and precipitation behavior of γ'phase [Ni3(Al, Ti)] during rapid heating and cooling was investigated. The results obtained in this study are summarized as follows: (1) In the heat-affected zone of Inconel 713C, the region of γ' phase being considered to be solutioned, is recognized. This region becomes narrower with increasing aluminum content. (2) Transverse hardness on the bead of Inconel 713C becomes higher with increasing aluminum content. (3) γ' phase solution temeprature during rapid heating is elevated with increasing aluminum content. (4) γ' phase precipitation temperature during cooling after rapid heating becomes higher with increasing aluminum content, that is, the temperatures of 3.58, 4.45 and 6% Al alloy are 1000, 1060 and 1130°C, respectively.