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Many types of hard material are coated on the surface to improve their wear resistance. Addition of vanadium carbide to Co based alloys (stellite no. 21) as a hard material powder is one of the ways to improve the wear resistance characteristics of the surface layer. The plasma transfer arc (PTA) welding process was introduced as a coating technology for elevated temperature surface modification. This process has recently generated interest in the surface modification field owing to its operability, low initial cost of equipment, high deposition rate, and small dilution rate. Coated layers produced by PTA considerably improve the hardness and wear resistance of surface layers for elevated temperature applications. Vanadium carbide (VC) addition into stellite powder showed a significant improvement in wear resistance. However, alloys containing VC showed pronounced sensitivity to hot cracking under repeated heating and cooling environments. This study clarifies the cause of thermal fatigue cracking in Co based alloy deposits with VC powder additions. Cracks result from the difference in thermal expansion coefficient between the matrix and the carbides. Cracks initiate in the central part of the surface region and grow in a perpendicular direction towards the surface. The tendency for thermal fatigue crack initiation seems to increase with increasing carbide volume fraction and decrease as the volume fraction of the dendritic region decreases.

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Multipass self-shielded flux cored arc welding with different heat inputs (1.3-2.0 kJ/mm) was conducted to determine the effects of the heat input on the proportion of the reheated region, impact toughness, and diffusible hydrogen content in the weld IntrodcuctionSelf-shielded flux cored arc welding (FCAW-S) has advantages over semi-automatic welding processes by virtue of its self-shielding capability, which makes it especially suitable for site erection welding of structural steel, shipbuilding, and in constructing offshore oil production platforms (Keeler, 1981;Rodgers and Lochhead, 1987;Hesbrrok, 1993). Because no external shielding gas is used, strong nitride formers such as Al must be added in the weld metal (Killing, 1980;Kotecki and Narayanan, 2005). This induces a bainitic microstructure in the as-deposited weld metal and results in low impact toughness. As it is known that the reheating of the as-deposited weld metal by successive passes in multipass welding retransforms the brittle bainitic microstructure to the tough fine polygonal ferrite, research works (Dorling et al., 1978;Pisarski et al., 1987;Boniszewski, 1992) have been conducted to find out a suitable welding procedure to get a higher proportion of reheated region in the weld metal.Boniszewski (Boniszewski, 1992) reported that, when welded with a stringer bead technique, the proportion of reheated region becomes higher as travel speed becomes faster, i.e., heat input becomes lower. In contrast, Dorling and Rogerson (Dorling and Rogerson, 1982) reported that, when welded with a weaving technique, the proportion of reheated region becomes higher in higher heat input welding. This suggests that the effect of welding procedure, especially welding technique and heat input, on the proportion of the reheated region and thus the impact toughness of multipass FCAW-S weld metal is complicated.If low heat input welding is performed to get a higher proportion of reheated region, the possibility of hydrogen cracking in the weld metal should be concerned. Hydrogen cracking is one of the most serious welding problems and may result in expensive time consuming repair operations.Hydrogen cracking basically depends upon the following three factors: hydrogen content, residual stress, and hardness of the weld metal (Easterling, 1992). Among these three factors, low heat input welding especially influences following two factors: hydrogen content and hardness. Low heat input welding facilitates fast cooling of the weld metal, giving less time for the hydrogen to diffuse away from the weld metal, and easily hardening the weld metal. This implies that attention should be paid when low heat input welding is utilized to get a higher proportion of reheated region in the weld metal. However, studies on the effect of heat input on 540

Joining of grey cast iron by fusion welding has much difficulties for its extremely low ductility and low toughness because of the flake form of the graphite. And the brittle microstructure, i.e. ledeburite may be formed during fusion welding by its rapid cooling rates. By these kinds of welding problem, preheat and post heat treatment temperature must be increased to avoid weld crack or welding problems.In order to avoid these fusion welding problem, friction welding of cast iron was carried out for improving joint soundness, establishing friction welding variables.There is no factor for evaluating friction weldability in continuous drive type friction welding. In this point of view,this study proposed the parameters for calculating friction weld heat input. The results obtained are as follows ; 1. There was a close relationship between tensile strength and flash appearance of friction welded joint. 2. Tensile strength was decreased and flash was severely oxidized as increasing frictional heating time. 3. As increased forging pressure P 2 , flash had a large crack and tensile strength was decreased. 4. As powdered graphite by rotational frictional force induced flat surface and hindered plastic flow of metal ,tensile strength of welded joint was decreased. 5. Heat input for continuous drive type friction welding could be calculated by the factors of P 1 , P 2 and upset distance(δ).