In this study, spheroidal graphite cast iron (FCD400) and mild steel (SS400) were welded by the tungsten inert gas (TIG) welding method using inoculant coating cast iron welding rods. After welding, microstructure, Vickers hardness and quantitative analysis of the welding zone were investigated. The weld metal were classified into three zones, ledeburite; mottled; and graphite structure. The structure changed depending on the cooling rate and carbon equivalent (CE value). The graphite structure formed more easily at smaller cooling speeds at the same CE value. On the other hand, the graphite structure formed more easily at larger CE values by the CE at the same cooling rate. As the CE value increased, transition from ledeburite to graphite solidification occurred in the weld metal.

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  The weldability of cast iron is extremely poorer than that of steel, because of chill formation and hardening in the welded zone. Although methods to prevent chill formation have been developed, few studies have been conducted on the generation mechanism and factors influencing chill formation. In this study, the effects of various welding conditions on chill generation were investigated, the chill generation mechanism was established, and countermeasures were examined. The test results show that chill occurs in the weld bond because of two reasons: (i) the base metal does not mix properly with the filler metal and (ii) the cooling rate for the weld is far more rapid than that of usual casting in the mold. The layer of chill could therefore not be prevented with only filler metal improvement. Chill can be prevented by increasing the silicon in the base metal in order to promote graphite formation and by preheating the weld area in order to slow the cooling rate.

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  Pinhole generation during arc welding of cast irons was investigated. Flake graphite cast irons and spheroidal graphite cast irons with various contents of oxygen were prepared by spontaneous oxidation up to two years or by accelerated oxidation at elevated temperatures. Bead on pass tests were carried out for these base metals by tungsten inert gas arc welding with three different filler alloys at the constant arc condition. Then the pinholes were detected quantitatively. It was confirmed that the pinhole defects were inclined to appear in flake graphite cast irons rather than in spheroidal graphite cast irons. This tendency was regardless of carbon contents. The mechanism of pinhole generation in the fusion zones of cast irons was proposed as follows: the presence of SiO₂ and its reduction by graphite at the high temperature of welding in the molten pool produce carbon monoxide gas bubbles. Cast irons containing chromium showed little pinholes after welding, due to the oxidation resistance of dense formation of chromium oxide.

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  In this study, spheroidal graphite cast iron (FCD) and stainless steel (SUS) were welded by metal active gas (MAG) welding method using three kinds of Fe-Ni-Cr types welding wires with different nickel and chromium equivalents. After welding, the test specimens were heat-treated at 1023K, 1073K and 1123K in air atmosphere. After heat-treatment, the welding zone of each specimen was investigated by microstructure observation, Vickers hardness measurement, scanning electron microscopic observation and X-ray analysis. The results proved that the ledeburite layer at the bonding interface between FCD and weld metals was broken down into ferrite and graphite by heating at 1073K and 1123K. Consequently, the Vickers hardness of the bonded zone decreased from 800-900HV to 300-400HV. But the Vickers hardness of weld metals near the FCD side increased by heating at 1123K. This is because chromium carbide precipitated at the bonding interface between FCD and weld metals and also at the grain boundary of weld metals when a welding wire containing chromium was applied. The amount of chromium carbide increased through a rise in heating temperature, and oppositely decreased through a rise in nickel equivalent in welding wires.

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  The authors have developed a technique to envelope a ceramic bar insert with cast iron without preheating the bar by covering the bar with a cast iron cover, fixing it in a sand mold and then pouring molten metal. This technique allows the heat energy of the molten metal to be utilized to produce cast iron products which are added with functions of ceramic materials. The enveloping materials in the cast iron products produced in our laboratory were evaluated on the basis of presence/absence of cracks in the ceramic insert and presence/absence of voids in the cast iron-ceramic interface. However, to use an enveloping material in an actual mechanical part, it is first necessary to evaluate its strength quantitatively, because it is necessary to prevent such problems as the disjoining of the ceramic material from the cast iron. In this study, the strength of cast iron with ceramic insert was evaluated by conducting a punching test on specimens taken from products having a ceramic insert by slicing the products perpendicular to their long axes in parts of the products with the inserts. Moreover, computer simulations were performed to estimate the thermal stress and thermal shock generated during the process of enveloping the ceramic insert of each product, and the results were analyzed in relation to the measured results. The analysis showed that the shear strength (as measured through punching testing) increases as the amount of molten metal increases and that the development of cracks in the ceramic insert increases the strength of the enveloping material. The analysis also showed that enveloping the ceramic insert directly without using a cast iron cover after preheating the insert increases the shear strength.

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  The purpose of this research was to produce a cermet particle dispersion-hardening layer near the surface of cast material by the powder inserting process. Since cermet is difficult to insert, hard metal, which can be inserted easily by several molten alloys, was blended with cermet particles at various rates as a binder. Insert of hard powder consisting of a mixture of cermet powder and hard metal powder was attempted by four kinds of base materials; gray cast iron, 27Cr white cast iron, martensitic cast iron, and low manganese cast steel. While structural observation of the inserted samples was performed near the inserted layer, the mechanical properties of the layer were estimated by the macroscopic hardness at the bottom of the inserted layer. In the base material except low manganese cast steel, the insert layer was formed when hard metal powder over 30mass% was added to cermet powder as a binder. In these inserted layers, the hard metal particles promote the inserting of cermet particles by dissolving into the molten alloy, sintering each other and distributing into the parent phase as WC by losing the binding force. Moreover, the Brinell hardness of the inserted layer increased 1.3 to 1.8 times compared with each parent material.

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  The mechanism and conditions for the bonding of WC-Co hard alloy and cast iron in the cast-in insertion process were examined. WC-Co alloy samples and cast iron samples were heated together in a crucible by an electric furnace and an infrared oven. Samples were heated to 1448K～1623K and were maintained at this temperature for a certain period of time. The cast iron sample melted in the crucible and contacted with the solid WC-Co alloy sample. The experiments simulate the conditions at the interface of the WC-Co alloy and molten cast iron in the cast-in insertion process. The WC-Co alloy samples were bonded with the cast iron samples by the contacting of the molten cast iron. An alloyed layer, consisting of WC particles and cast iron structure, was formed at the interface, indicating melting of the WC-Co alloy. Mutual diffusion occurs between the molten cast iron and the WC-Co alloy. The Co binder of the WC-Co alloy was alloyed with the components of cast iron, Fe, Si, C, causing a drop in the melting point of the Co binder. The WC-Co alloy melted partially by the contacting of the molten cast iron for less than 100s. Temperatures higher than 1523K promotes melting of the WC-Co alloy, forming a thick alloyed layer. Rich Si content also advances melting of the Co binder at lower temperatures. Ductile cast iron is a suitable material for bonding with WC-Co alloy by the cast-in insertion process, because of its good formability, strength, and rich Si content.

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  This report describes a study on the hot dip aluminum coating (aluminizing) of as-cast iron surfaces for developing oxidation and corrosion resistant materials. Fe-Al intermetallic compound layers such as Fe₂Al₅ or FeAl_xSi (x=3～4.5) were formed between the aluminum surface and cast iron by aluminizing. It was clarified that substantial heat capacity of the molten aluminum bath was required to prevent fall in temperature of the Fe-Al interface and to accelerate reaction. In addition, it was found that including Mg in the molten aluminum was more effective for accelerating the reaction between molten aluminum and cast iron surface. After heating tests at the temperature of 1073K in an oxidation atmosphere, the Fe-Al intermetallic compounds increased to a large proportion of the plated layers, and the surface aluminum layers were reduced in contrast.

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  Torch brazing of dissimilar metals, such as flake graphite cast iron (FC), spheroidal graphite cast iron (FCD), structural steel (SS) and tough pitch copper (TPC) was investigated using electroless plating Ni-P alloy film (11mass%P) increased in thickness from 5 to 20μm as filler metal. In the present investigation, torch brazing was carried out at 1200K in air atmosphere without flux. The characteristics of FC/TPC joint with 20μm-thick film were also investigated by means of bending test and observation of microstructure under the following bonding conditions, decreased P content in film from 11 to 3mass% and increased heating time from 90 to 300s. EPMA and optical microscope were employed to clarify the characterization at the bond interface. When specimen was coated with 20μm-thick film contained 11mass%P, no voids and cracks were observed at the bond interface, suggesting the possibility of torch brazing of industrial materials at higher melting point than eutectic temperature of the Ni-P system (1143K). Bending test results and observation of microstructure suggest that the 8mass%P-film is suitable for the torch brazing of cast iron and different metals in the present investigation.

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