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As-solidified structure of an ingot is composed of the chill, columnar and equiaxed zones. The whole solidified structure is strongly affected by the chill crystals. Some initial solidification grains have been observed on the ingot surface and thought to be traces of the nucleation point. The aim of this study is, therefore, to develop the experiment technique to make one ‘grain’ and to crystallographically investigate the initial solidification grain using EBSD analysis. In order to start solidification at a very specified position, a small metallic protrusion was installed on an insulating plate. Al-6 wt%Si alloy was melted at 800 °C and was poured on the metallic protrusion. In this study, the amount of protrusion was varied to investigate the growth mechanism of the initial solidification grain. The longitudinal cross section of the specimen was observed by an optical microscope, a scanning electron microscope. The starting position of solidification was the area that was on the metallic protrusion. In this initial solidification grain, it was difficult to observe the dendritic structure. The shape of this grain was about hemispherical. The grain area seemed to increase with increasing the amount of protrusion. The results of EBSD analysis showed that almost all initial solidification grains were composed by several crystals. The reason of this is that the nucleation frequency may increase with the amount of protrusion. The dendrite grew radially from the initial solidification grain continuously. The crystallographic structure was also continuous on the boundary of the initial solidification grain.

Many crystals nucleate on the mold surface when the molten alloy is poured in a mold cavity. Because the crystallographic orientations of these crystals are random, the solidified structure near the mold surface is very complex. The ghost lines, which are sometimes thick and the angle between them is not 90 degrees, are often observed in this region. However, if the crystallographic structure of this alloy is cubic, such as bcc or fcc, the ghost lines are very regular. In order to understand the geometry of ghost lines, Al-20 mass%Cu alloys were unidirectionally solidified with constant growth velocity. The solidified structures on the obliquely crossed section were observed. The ghost lines were quite regular and parallel to each other in a solidification grain. The angles and the ratio of the width of ghost lines were measured and crystallographic orientations were estimated using these parameters, based on the solid analytical geometry. EBSD analysis were also performed on the area, where the ghost lines were characterized, and the precise crystallographic orientations were decided. The comparison between both analytical values indicated that the differences between them are within 10 degrees and it can be safely concluded that the estimation for crystallographic orientation using ghost lines agreed well with the EBSD analysis.

Creep rupture tests near the tempering temperature were performed, and the creep behavior at high temperatures and the structures of fracture specimens were investigated. Three kinds of F82H test specimens were used: IEA-heat, mod.3, and BA07. The time-to-rupture of the BA07 specimens was the longest under all the test conditions. This was because the minimum creep rates of BA07 were smallest, and a large quantity of fine precipitates of MX from the ESR treatment were considered to be effective in providing creep resistance. Although mod.3 specimens showed a high creep resistance under high stress, the time-to-rupture of mod.3 and IEA-heat were almost the same at low stress. This was because the fine tempered martensitic structure was weakened by being subjected to a high temperature for a long period. Therefore, it is considered that a large quantity of fine MX precipitates are effective for creep resistance near the tempering temperature.

Hot tearing is the one of the biggest problems when casting aluminium alloys. Although there has been much research work in this field, it is still not clear what causes hot tearing and how it propagates. Furthermore, the effect of solidified structure on the hot tearing has not been clear. Therefore, this study has been carried out to correlate the solidified structure and hot tearing. Al-2.0 wt% Cu alloy was used for test alloy. To change the solidified structure, some amount of refiner was added to the molten alloy. A permanent mould, which has been developed by us, can form hot tearing intentionally in the center region of a solidified shell. The cross section of a solidified shell was metallographically investigated and solidified structure and hot tearing were characterized. The length of hot tear decreased with increasing the amount of refiner. This may indicate that it is difficult for hot tear with fine equiaxed grains to propagate. In addition to this, the residual liquid around the hot tear was moved to the cracking due to negative pressure and hot tearing was partially healed.

It is reported actual volume fraction of the primary phase in alloys is larger than the equilibrium value. Larger volume fraction of the primary phase may cause shrinkage cavities and surface or internal cracks. Although control of the solidified structure is important for the quality of cast products, this problem has not been fully elucidated. Taking these results into account, this study has been carried out in order to comprehend the phenomenon of larger volume fraction of primary phase. The Sn-Pb alloy has been used as a test alloy to examine the relation between supercooling for nucleation and the volume fraction of primary phase. Actually, the volume fraction of the primary phase in Sn-Pb alloy is larger than that of the lever rule. It was also observed that the volume fraction of β-Sn decreases with decreasing the supercooling in the early stage of solidification. In the final stage of solidification, however, the effect of supercooling on volume fraction of primary phase is small. Futhermore, when the supercooling was low, the volume fraction of primary phase slowly increased.

Surface quality as well as internal quality of cast products of aluminum alloys are strongly affected by the process of initial solidification. Control of solidified structure in this region is therefore quite important. In order to understand the growth of solidified grain, crystallographic characterization has been performed using EBSD (Electron Backscattered Diffraction) in this study. Al-6 mass%Si alloy was cast at 750°C on the chill plate. Longitudinal cross section of solidified shell was analyzed. In the region of initial solidification, many small crystals nucleated on the mold surface. The crystallographic orientations of these grains were random. It is normally found that an unfavorable grain was eliminated by a favorable grain. However, occasionally, we have found that an unfavorable grain enlarged its size. In this case, dendrite, the growth direction of which was far from the heat flow direction, gradually changed its crystallographic orientation from unfavorable one to favorable one. The grain enlarged its size by multiplication of dendrite arms. Crystallographic orientation of dendrite changed little by little when it branched. This kind of phenomena may take place in unsteady condition, such as initial solidification region.

Knowing the volume fraction of a minor phase in a composite material is important for characterizing its mechanical properties. Assuming that a rod-like phase with a constant diameter is aligned but dispersed randomly in a matrix, a mathematical model for estimating the volume fraction of a rod-like phase has been developed. This model predicts how many rods are required for an accurate estimation of volume fraction. One of the effective features of this model is that the real dimensions do not need to be considered due to the similarity of their shape. According to this model, it is sufficient to measure the area that includes at least 20 rods to estimate an accurate value for volume fraction. This prediction was confirmed by measuring a unidirectionally solidified Sn-Cu eutectic alloy system. [doi:10.2320/matertrans.M2014424]

An unusual microstructure consisting of both Sn-Ag3Sn and Sn-Cu6Sn5 binary eutectic structures is observed in actual solder balls. In this study, the solidification process of the Sn-Ag3Sn binary eutectic structure after the growth of primary b-Sn in an undercooled Sn-2.8Ag-0.3Cu alloy was investigated by using thermal analysis and interruption tests to understand the formation of the unusual microstructure. First, fine Ag-enriched liquid zones formed around b-Sn after the growth of primary b-Sn. The Ag-enriched zones then gradually enlarged with the accumulation of Ag from the remnant liquid with a decrease in temperature. This indicated that the liquid-phase separation occurred in the remnant liquid after the nucleation of b-Sn. Eventually, when the temperature of the specimen decreased to approximately the binary eutectic temperature, eutectic Ag3Sn nucleated in the Ag-enriched zones. From interruption tests, we determined the precursor of the Sn-Ag3Sn binary eutectic structure before the beginning of Sn-Ag3Sn binary eutectic solidification. This finding corresponds to the precursor of the Sn-Cu6Sn5 binary eutectic structure observed in the Sn-1.0Ag-0.5Cu alloy.

In this study, the mechanism of equalization for λ1 has been investigated using succinonitrile-1.0 mass% water alloy. Approximately 15 columnar dendrites have been observed and the maximum and the minimum value of λ1 (λ1Max and λ1Min, respectively) have been characterized. It is found that λ1Max decreased and λ1Min increased with time, i.e., the tip of primary arm grow to the region of wide from region of narrow of dendrite arm spacing. And also, Local primary arm spacings are gradually equalized due to propagation of position of primary trunk. The time necessary for the equalization for λ1 is found to be longer than that of the process for adjusting dendrite tip radius and the change in λ1 due to branching or over growth. This may suggest that solutal filed in the interdendritic region plays and important role for equalization for λ1.

In order to investigate the mechanism for equalization of primary dendrite arm spacing, in-situ observation of solutal field using succinonitrile-H2O-dye system has been carried out. The brightness, which corresponds to the concentration of dye, has been characterized by an image analyzer along the centerline of interdendritic region. When the local primary dendrite spacings were uneven, the brightness along the centerline of interdendritic region was low in case of local primary dendrite arm spacing was large. After growth with rather long distance, the local primary dendrite arm spacings became even and the brightness along the centerline of interdendritic region became uniform. The solutal profile along the interdendritic region qualitatively agreed with the calculation results of newly developed model. A dendrite trunk is prone to grow toward the wide-spacing-region because the solute concentration of this side is lower than that of the other side. This leads to equal primary spacing.

A modified 9Cr–1Mo steel was cooled to 200 °C from the normalizing temperature and then directly heated to the tempering temperature. It was found that the time to rupture at 650–700 °C for the steel heat-treated at 200 °C increased three times over than that of the modified 9Cr–1Mo steel conventionally normalized and tempered. The microstructure of the improved steel was tempered martensite and the size of martensite blocks was larger than for the conventional treatment. The hardness of the improved steel was adequately recovered after tempering. Aging tests showed that the particle sizes of Cr23C6 and VN type carbonitride in the improved steel were finer in the conventional steel. The above-mentioned heat treatment was applied to the reduced activation martensitic steel F-82H and the improvement was confirmed.

Oxide dispersion strengthened (ODS) steels show a high anisotropy in their creep behavior because of the δ-ferrite grain elongated in the hot-rolled direction and the characteristic formation of creep cavities. In this work, the relationship between the δ-ferrite grain and the growth of creep cavities in 8Cr-ODS steels was investigated. The samples of two ODS steels with different δ-ferrite volume fractions were machined parallel and perpendicular to the hot-rolled direction. Creep rupture tests and interrupted tests were performed at 700 °C and about 197 MPa. Cavities formed in the martensite along δ-ferrite grains during creep deformation. The area fraction of the cavities of all specimens increased in proportion to the cube root of test time. When the volume fraction of δ-ferrite was high and δ-ferrite grains elongated parallel to the load direction, δ-ferrite then obstructed the propagation of cracks. However, when the volume fraction of δ-ferrite was low and δ-ferrite grains elongated perpendicular to the load direction, δ-ferrite grains had little effect on crack propagation.

In Sn-Ag-Cu solder balls, unusual microstructures consisting of both Sn-Ag3Sn and Sn-Cu6Sn5 binary eutectic stratures are sometimes observed. However, the formation mechanism of these unusual microstructures is still unclear. Therefore, in this study, the solidification process has been investigated to clarify the nucleation and growth of binary and ternary eutectic structures in Sn-1.0Ag-0.5Cu alloy by using thermal analysis and interruption tests.Cu-enriched zone was observed around β-Sn in the liquid before the nucleation of Sn-Cu6Sn5 binary eutectic structure. Sn-Cu6Sn5 binary eutectic structure formed in these regions. Moreover, some Ag-enriched zones were observed around the Sn-Cu6Sn5 binary eutectic structure in the liquid before the initiation of the ternary eutectic solidification. The unusual Sn-Ag3Sn binary eutectic structure formed in these regions just after the temperature reached the ternary eutectic point. The ternary eutectic structures were classified into three types depending upon the fluctuation of the remaining liquid composition at the ternary eutectic temperature.

A modified 9Cr-1Mo steel was cooled to a temperature between the Ms point and room temperature from the normalizing temperature and then directly heated to the tempering temperature. It was found that the time to rupture at 650 and 700°C of the steel heat-treated by the new heat treatment increased 2–3 times longer than that of the steel conventionally normalized and tempered. The microstructure of the improved steel was tempered martensite and the size of martensite blocks was larger than for the conventional heat treatment. The hardness of the improved steel was fully recovered after tempering. This strengthening is not caused by fresh martensite, but caused by the refining of M23C6 and V(C,N) particles in addition to the increase in the martesite block size, where M denotes metallic elements.

The tin-zinc binary alloy is one of the candidates for lead-free solder and plating. In order to investigate the evolution of solidified structure of the tin-zinc binary eutectic alloy, the unidirectional solidification technique has been applied using a Bridgman type furnace. The primary β-tin is obtained as dendritic structure. On the other hand, the primary zinc is obtained as plate crystal because of high anisotropy in surface tension. Coupled-zone is found to be extended to tin-rich side and to be rather symmetric. The lamella spacing in coupled zone agrees with Jackson-Hunt model. Furthermore, the broken-lamellar to fibrous transition has been observed at high growth velocity.

Hot tear is the one of the biggest problems of cast products. There have been many researches in order to understand the formation mechanism of hot tear. However, it is still uncertain when a hot tear initiates and how it propagates. Thus, the preliminary research has been carried out to observe the vibration of permanent mold which correspond to the initiation or propagation of hot tear. Al-2.0 wt% Cu alloy was used for test alloy. The signals of acceleration sensor were recorded at an interval of 5 ms. After casting, the vibration, which lasted scores of milliseconds, was observed intermittently. This may indicate that hot tear initiates or propagates intermittently and the strain due to contraction of solidification or cooling is relaxed by hot tear.

Horizontal centrifugal casting is widely used for production of water pipes, rolls and so on. Advantage of this process is to press molten metal against the permanent mold due to centrifugal force. Thanks to this, one can obtain fine structure and sound castings1). On the other hand, macroscopic segregation on tangential direction sometimes forms. The formation mechanism of macroscopic segregation has not well understood. This is because the solidification process itself is still uncertain since it is very complex2-4). Thus, in order to understand the solidification process in horizontal centrifugal casting an in-situ observation has been made in this study. Transparent organic substance has been used to simulate the metal solidification.

In international thermonuclear experimental reactor (ITER), reduced activation ferritic/martensitic steels will be used for plasma-facing materials. However, it is necessary to raise the temperature of operation in order to elevate efficiency of electric power generation by using the material which is more excellent in strength at elevated temperature. Oxide dispersion strengthened (ODS) steels are promising candidate for high temperature materials of a nuclear fusion reactor. There are many reports that ODS steels show very high creep strength, but there are few reports on creep deformation mechanism. In this work, creep deformation behavior of 8 wt% Cr ODS steel was investigated. This ODS steel had high density of fine dispersed Y2Ti2O7 particles and -ferrite grains elongated along the hot-rolling direction. The creep curve showed a low creep strain rate until specimen ruptured. Vickers hardness of the gauge part of specimens in interrupted creep tests decreased with increasing the loading time. However, that of the grip part did not change significantly. Accordingly, although dynamic recovery occurred in the ODS steel, it had not affected the creep deformation rate.

Solidified structure near the chill zone is complex, since the growth directions of dendrites are not in accordance with the observed plane. Many lines, which are called ghost lines, can be observed in the solidified structure. The ghost lines have been analyzed applying the solid analytical geometry. Dendrite of cubic metals has been simplified with rod as a dendrite trunk and four plates with the same thickness. In order to characterize the solidified pattern, angles of ghost lines (η and ξ) are chosen and calculated using θ, φ and β, which are the important parameters to indicate the dendrite orientation in the space. The calculated results have been confirmed by the experimental results solidified unidirectionally of Al–20mass%Cu alloy. It has been found that the spacing or thickness of ghost lines may be the useful parameters to characterize the spatial array of dendrites with respect to the plane for observation.

It has been found that 3D-CAD is a powerful tool for analyzing the apparent pattern of solidification structure. Using 3D-CAD, the apparent patterns of model dendrites have been analyzed in this study. The effect of distribution of dendrites and the morphology of secondary dendrite arms on the apparent pattern have been investigated. As far as secondary dendrite arms form, it has been found that it can be recognized the ghost lines and it is possible to measure the characteristic parameters of ghost lines, η, ξ and γ, which are defined previously. Therefore, we can analyze the spatial relation of dendrite using these characteristic parameters of ghost lines. On the other hand, when the solidified structures are constituted by cells, which have no secondary arms, ellipses stand in a row on the cross section. Therefore, one could recognize the apparent pattern. However, it is not possible to analyze the spatial relation of the cell, since the ghost lines cannot be specified because of the distribution of cells.

Solidified structure near the ingot surface is complex and it is difficult to analyze the crystallographic orientation by the solidified structure. The reason of complexity is that the growth direction of dendrite does not agree with the plane for observation. Thus thick lines called ghost lines are often observed. The ghost lines have been characterized in this study.Simplifying the shape of dendrite, the ratio of widths of ghost lines (=γ) has been formulated and the change of γ in orientation of dendrite have been analyzed. It has been found that the angles of ghost lines (η and ξ) as well as γ are the important factors to characterize the solidified structure. In order to determine the spatial orientation of dendrite (θ, φ and β) from η, ξ and γ, the computer program had been developed. Applying this computation method, the solidified structure has been compared with the cross sections of dendrite model. The cross sections obtained by 3D-CAD agreed quite well to the real solidified pattern.

In the chill zone of the alloy casting, many fine grains nucleate on the surface and many dendrites grow their preferred growth direction. Therefore, it is rare that a “typical” dendritic structure can be observed near the chill zone. Usually, the solidified structure exhibits a quite complex pattern. In this study, the observed shape of dendrite will be investigated. Using Al–20mass%Cu alloy, unidirectional solidification with constant temperature gradient and growth velocity was performed. Then the solidified structure was observed on a cross section, which was intentionally inclined from the heat flow direction. The spatial relationship between dendrite and plane for observation has been defined using three angles, θ, φ and β. The solidified patterns were analyzed as a function of these angles. 3D-CAD model for solidified pattern has been developed assuming that secondary dendrite arms form plates. Solidified patterns agree quite well with 3D-CAD model.

Mold flux is widely used in continuous casting of steel. The heat transfer control in the mold is one of the important roles of mold flux. The mild cooling due to crystallization of mold flux is established for middle carbon steel, which has a high tendency for longitudinal cracking. However, the mechanism of crystallization has not well understood. Therefore, the kinetics of crystallization via glassy state has been analyzed in this study, using a commercial mold flux, which easily crystallizes.Quenched mold flux was prepared and heat treated at some temperatures for various periods. After heat-treatment, the mold flux was analyzed by XRD to confirm the phase, glass or crystalline. The time for crystallization as a function of heat-treatment temperature was experimentally determined. Then the apparent activation energy for crystallization was estimated using Arrhenius plot. The calculated value is 193 kJ/mol and this may correspond to the activation energy for diffusion of some molecules or ions in a glassy state of mold flux.

The Sn-Cu Alloy is one of the candidate alloys for lead-free solder. It is important to understand the solidified structure of this alloy, but such reports are few. In order to make in-situ observation during solidification of this alloy, a laser confocal microscope has been used. Three alloys, hypo-eutectic, eutectic and hyper-eutectic alloys have been prepared. In the case of hypo-eutectic alloy, primary β-Sn dendrite and interdendritic eutectic were observed. A large recalescence was observed when the primary β-Sn nucleated in thermal histories. In the case of eutectic alloy, only eutectic structure was observed by the laser microscope. However, primary β-Sn dendrites were observed on the cross section. There was a large recalescence during solidification. This corresponded to the nucleation of β-Sn. In the case of hyper-eutectic alloy, fibrous primary Cu6Sn5 grew and then β-Sn dendrite grew as halo. This followed by the formation of eutectic. It was observed that a large recalescence after growth of primary Cu6Sn5 precedes the nucleation of β-Sn as halo.

Control of nucleation on the surface of solidified shell is of importance to establish the whole structure of continuously-cast products. In the conventional continuous casting of steel, the nucleation of steel takes place on the molten state of mold flux. Lab-scale dipping tests have been carried out, where the solidification satisfied the above-mentioned condition. Changing the material of chill block and mold flux, the cooling rates of initial solidification have been varied systematically. The detailed observations of the surfaces and cross section of solidified shells have been made.The grain size was affected by cooling rate and decreased monotonously with increasing cooling rate. Many small protrusions have been found on the surface of the solidified shell. They resembled discs as Biloni et al. named. The density of discs was affected by cooling rate and increased with increasing cooling rate. The effective nucleation ratio was defined as the ratio between the number of grains and that of discs. The effective nucleation ratio obtained was a few percent and indicated small dependence on cooling rate in the range of this study.

In continuous casting of steel, mold fluxes are used to prevent surface defects, such as longitudinal cracks. The crystallization of mold flux promotes mild cooling of the steel shell, but crystallization behavior has been still uncertain. Therefore, this study has been carried out to analyze the crystallization processes of mold flux. Mold flux used in this study has high tendency for crystallization. Quenched specimens were heat-treated in various conditions and characterized. Glassy specimen heat-treated in the electric furnace over 550°C for 180 min crystallized. These crystalline were confirmed to be cuspidine by XRD analysis. The number of grains decreased and the average grain size increased with rise of temperature of heat treatment. The growth velocity also increased with rise of temperature.In-situ observation by the laser microscope revealed that the glassy specimen crystallized in a moment over 600°C and the surface of the specimen turned to be rough. This may lead to mild cooling in the mold.

Precipitation behavior of 8%Cr–2%W martensitic steel during tempering at 740 °C was studied. Both M23C6 and TaC were observed at an early stage of tempering by TEM and XRD. M23C6 is coarse, about several 10’s nm. On the other hand, TaC is very fine, about 10 nm, when tempering up to 10 h. Particle size of TaC is gradually increases, however the size remains below 30 nm even after tempering for 1000 h. This suggests that TaC particles grow very slowly under the actual service conditions. After the saturation of precipitation of TaC, the time exponent for coarsening of TaC particles is about 0.1, the value of which is much smaller than the theoretical value, 1/3 or 1/5. This is re-confirmed by the X-ray integral breadth for (1 1 1) TaC peak. The reason for the small time exponent is discussed in terms of the interaction between TaC and dislocations during tempering.

In order to clarify the stability of precipitates in 8%Cr-2%W-0.09%Ta steel at high temperatures, tempering and precipitation behavior of the steel has been examined precisely. Tempering was performed mainly at 740°C for 0.5-1 000 h. Hardness peak was found in a tempering curve at 650-740°C for 3.75-8 h. Discontinuous changes in X-ray diffraction intensity of the matrix, apparent lattice strain and apparent crystalline particle size, which were estimated from integral breadth, the amount of extracted residue, and the Cr and Ta content in the residue were observed at 740°C for 2-12 h tempering. These discontinuous changes correspond to the rotation of preferred orientation or the generation of subgrains accompanying the dissolution and the subsequent re-precipitation of M 23 C 6 and TaC. The re-precipitation of fine M 23 C 6 particles is responsible for the hardness peak. Three kinds of TaC with different chemical composition and different size are found. The amount and the size of the coarse and medium TaC particles are affected by the discontinuous changes in microstructure. Fine particles of TaC grow very slowly, which is caused by the continuous dissolution of TaC due to the annihilation of precipitation site, dislocations, and re-precipitation on dislocations. Particle sizes of TaC and M 23 C 6 estimated using X-ray diffraction peak coincide with those observed in transmission electron microscopy.

Creep testing was carried out at around 650°C for a martensitic 8Cr-2WVTa steel (F82H), which is a candidate alloy for the first wall of the fusion reactors of the Tokamak type. Rupture strength of the double tempered steel (F82HD) is lightly higher than that of simply tempered steel (F82HS). On the other hand, creep rate of F82HD is obviously smaller than that of F82HS in acceleration creep, though creep strain of F82HD in transition creep, where creep rate decreases with increasing strain, is larger than that of F82HS. Hardness of the crept F82HD decreases with increasing creep strain, which corresponded with the transmission electron microscopy (TEM) observation. On the contrary, X-ray diffraction and electron back-scattered diffraction pattern measurements show that fine sub-grains are created during transition creep. The creep curves were analyzed using an exponential type creep equation and the apparent activation energy, the activation volume and the pre-exponential factor were calculated as a function of creep strain. Then, these parameters were converted into two parameters, i.e. equivalent obstacle spacing (EOS) and mobile dislocation density parameter (MDDP). While EOS decreases with increasing creep strain, MDDP increases with increasing strain during transition creep. The decrease in EOS and the increase in either EOS or MDDP are rate-controlling factors in transition and acceleration creep, respectively. On the other hand, in case of F82HS, EOS increases and MDDP decreases during transition creep. In this case, the decrease in MDDP controls the creep rate during transition creep of F82HS. It is concluded that both EOS and MDDP are representative parameters of the change in substructure during creep.

Seed crystal for equiaxed grain may be suffered from the temperature change in the liquid phase. Seed crystal may grow and/or melt depending on the thermal field. In order to analyze the growth and melting of a solid particle, in-situ observation using succinonitrile-water alloys with the newly constructed experimental equipment have been performed. In the central region of the ring heater, a small spherical solid particle is held for a while. When the voltage for the ring heater was abruptly changed, the solid particle grew or melted. The morphological change was recorded and analyzed.In case of growth, it indicated that the solid/liquid interface is dendritic. After stopping growth, the solid phase became round if the solute content is low. On the other hand, if the solute content is high, solid/liquid interface remained dendritic.In case of melting, the solid/liquid interface is complex. If the solid phase is round after growth, it melted remaining round. If the solid/liquid interface was dendritic after growth, dendrite arms became slender and the tip of dendrite became pointed. Some secondary dendrite arms may have been detached.It has been found that the melting velocity is lower than the growth velocity. Because of the difference in solid/liquid interfacial morphology and because of the difference in the traveling velocity of solid/liquid interface, the kinetics of growth and melting are different. One of the important reasons is the difference in operating point for growth and melting. In case of growth, the operating point is dendrite tip. On the other hand, in case of melting, it is rather wide area, which is inside of the envelope of the tips of the solid phase.

Metallurgical observation of 9%Cr–1%Mo–0.2%V steel tempered at 750°C for a maximum of 100 h has been made. Accompanying the recovery of martensitic structure, discontinuous changes in hardness, intensity of an X-ray diffraction peak of the matrix, the lattice strain calculated from the integral width of an X-ray peak, the amount of extracted residue and the size of M23C6 were observed when tempering time is around 10 h. These changes are caused by the annihilation of dislocations and the coalescence of martensite lath followed by the formation of subgrains in the later stage of tempering, which is supported by electron back scattered pattern (EBSP) measurements. The ultra fine grains of the order of 0.1 μm were confirmed around martensite lath and block by EBSP, which is obviously correlated with the discontinuous changes in the hardness and the lattice strain. Precipitation of M23C6 on excess dislocations of martensite induces a larger amount of precipitates as compared with the thermal equilibrium, which causes the dissolution of M23C6 during consequent tempering. The dissolution of M23C6 results in slower growth rate as compared with the Ostwald ripening. The observed over-all time exponent is 1/16.