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Commercial flake graphite cast iron substrate was coated with titanium powder by low pressure plasma spraying and was irradiated with a CO2 laser to produce the wear resistant composite layer. The macro and microstructural changes of an alloyed layer with the traveling speeds of laser beam, the precipitate morphology of TiC particulate and the hardness profile of the alloyed layer was examined. From the results, it was possible to composite TiC particulate on the surface layer by direct reaction between carbon existed in the cast iron matrix and titanium with thermal sprayed coating by remelting and alloying them using laser irradiation. The cooling rate of the laser remelted cast iron substrate without a titanium coating was about 1 × 104 K/s to 1 × 105 K/s in the order under the condition of this study. The microstructure of the alloyed layer consisted of three zones; the TiC particulate precipitate zone (MHV 400–500), the mixed zone of TiC particulate + ledeburite (MHV 650–900) and the ledeburite zone (MHV 500–700). TiC particulates were precipitated as a typical dendritic morphology. The secondary TiC dendrite arms were grown to a polygonized shape and were necking. Then the separated arms became cubic crystal of TiC at the slowly solidified zone. In the rapidly solidified zone near the fusion boundary, however the fine granular TiC particulates were grouped like grapes.

In order to modify the continuous network of eutectic cementite normally found in low alloy white cast irons into a dispersive distribution, strategies of controlling the morphology of eutectic cementite by additives are discussed. Qualitative arguments are presented and applied to the development of a complex modifier REAINTi. With the addition of this modifier to low carbon white cast irons nearly one fifth of the original eutectic cementite network can be modified into blocky particles in the plane of polish, and one third into isolated irregular ones. The experimental data shows that Ti(CN) may act as an effective nucleant or a grain refiner for eutectic cementite. The modification effects of rare earth elements are attributed to their abilities to purify the melt, promote divorced eutectic growth and refine the microstructure. Aluminium contributes to the modification by its purification effect.

Pure Fe-3.5 wt% C alloy was surface melted using a cw CO 2 laser and the microstructure of the laser tracks was investigated by scanning and transmission electron microscopy. Structure of the melted zones consisted of dendrites of partially transformed primary austenite and of very fine lamellar ledeburite. The secondary dendrite arm spacings were indicative of a cooling rate of ∼10 5 K s -1 , in good accord with calculations based on the model of a moving Gaussian beam. Using methods of quantitative metallography the volume fraction of dendrites within the melted zone and the volume fractions of the carbide and ferrite phases in the decomposed ledeburite were estimated. These data were combined with the results of a quantitative X-ray diffraction phase analysis (see Part II) and compared with the equilibrium phase diagram. It was found that the volume fraction of dendrites was near the equilibrium value while the volume content of cementite in the rapidly solidified structures was considerably higher than predicted from the equilibrium phase diagram.

The precipitation of cementite under the influence of an externally applied stress, during the tempering of martensite in steels, is investigated using transmission electron microscopy. The stress appears to favour the development of particular crystallographic variants of cementite in any given plate of martensite. Hence, a Widmanstatten array of cementite particles in a normally tempered sample changes to an array consisting of just one variant in stress-tempered samples. The results are discussed in the context of the mechanism of carbide precipitation during the lower bainite reaction.

Pearlitic grey cast iron was surface melted using a 500 W CW CO2 laser at travel speeds 0.5–100 cm s−1. Detailed structural analysis of the laser modified layer was performed by optical and scanning electron microscopy (SEM), and microhardness depth profiles were measured. Temperature distribution was calculated from a three dimensional moving point source model, taking only the heat transfer into account. From the structural details observed in the austenitized zone some conclusions on the mechanism and kinetics of the pearlite austenite transformation at high heating rates were drawn. The melted zone consisted of primary austenite and ledeburite. At lower scanning speeds the structure was dendritic, at higher scanning speeds transition to dendritic-cellular structure was observed. From the secondary dendrite arm spacings the cooling rate during solidification was estimated as a function of the depth. Some discrepancies were found between our measurements and the literature data as well as predictions by the simple model neglecting convection in the melt.

The microstructural changes produced by plasma arc local-melting method in the fusion region or deposited metal, fusion boundary region and heat-affected zone (HAZ) of flake graphite cast iron were investigated. Cylindrical base metal specimens were locally melted at fixed time intervals under a stationary plasma torch using argon plasma gas and Ar + 10% H2 shielding gas. The welds were produced autogenously and with filler metals. The cooling rate in the fusion region was recorded. Evaluation of the fusion boundary area included metallurgical analysis, microhardness and electron probe microanalysis. In the absence of filler metal, the structure of fusion region where completely melted was ledeburite. In the centre of the fusion region the structure appears to be that of hypoeutectic white cast iron, and in the fusion boundary of this region the structure appears to be fined ledeburite. In the fusion boundary region where supercooled phenomena can occur, fine martensite precipitates appear along the fusion line for small heat input and secondary graphite is seen for large heat input. The HAZ is composed of white martensitic, dark martensitic and martensitic-fine pearlitic layers for small heat input, and of finely laminated pearlitic and finely laminated pearlitic-ferritic layers for large heat input. Using filler metal, the structure of the deposited metal is found to be that of a nickel austenitic matrix precipitating tiny graphite nodules or slim graphite for nickel and Ni-Fe filler metals, but to be a pearlitic matrix for iron filler metal. In the fusion boundary region, nickel-martensite, eutectic or slim graphite and fine nickel-martensite are precipitated from the nickel system filler metal, and the columnar structure of ferrite and pearlite is obtained from iron filler metal. The HAZ is composed of thin ledeburitic, martensitic, martensitic-fine pearlitic and finely laminated pearlitic layers for the nickel filler metal, of ledeburitic and finely laminated pearlitic layers for the Ni-Fe filler metal, and of thick ledeburitic, eutectic graphite-crystallized and finely laminated pearlitic layers for iron filler metal. The hardness of the ledeburitic layer and the nickel-martensitic portion is very high where the liquid existed. The diffusion of nickel from the deposited metal into the HAZ can occur at least until the fused base metal of HAZ.

The microstructural changes in the fusion boundary area of nodular cast iron (NCI) were investigated by plasma arc local-melting with and without a filler metal. The NCI cylinder specimen was locally melted by the static plasma torch for a very short time, adopting the non-keyhole method. The microstructures produced in the fusion region, fusion boundary region and heat-affected zone (HAZ) were examined metallographically. In the absence of filler metal, the fusion boundary area is composed of ledeburite in the fusion region, and martensitic, troostitic—ferritic layers for the short arc-time and sorbitic, sorbitic—ferritic layers for long arc times in HAZ. Although the formation of white iron in the fusion region may be caused by a relatively high cooling rate, the major cause is due to supercooling effects produced by the decrease of magnesium content in the molten iron directly under the plasma arc. Using nickel and Ni—Fe filler metals, Nimartensite appears in the deposited fusion boundary, and the HAZ region is composed of ledeburitic and martensitic or sorbitic layers. The ledeburitic layer thickness varies with the melting point of the filler metal. The diffusion of nickel from the deposit metal to the HAZ occurs at least until the fused base metal in the HAZ.