Austenite Dendrites Research Articles

Growth temperatures and the limits of coupled growth in unidirectional solidification of Fe-C eutectic alloys

Growth temperature has been measured as a function of applied growth velocityV for grey (austenite-flake graphite) eutectic, austenite dendrites and white (austenite-cementite) eutectic in Fe-4.28 wt pct C at a temperature gradientG of ∼7 K/mm. Grey eutectic (0.4 to 65 µm/s) required an undercoolingΔTfor growth equal toK1V1/2 withK1 as 3.4 ± 0.1 Ks1/2/µm1/2, giving values ofΔT nearly an order of magnitude larger than predicted for growth at the extremum, as shown previously by Toloui and Hellawell for the related Al-Si eutectic. Austenite dendrites growing together with the grey eutectic atV between 11 and 65 µ m/s exhibited a range of tip temperatures giving rise to average undercoolings of magnitude [GD/V +K2Vn] withD as diffusion coefficient of carbon in the melt andK2 as 0.73 Ksn µm−n withn as 0.46 predicted by Ivantsov’s theory for growth of an austenite needle. White eutectic displaced both grey eutectic and austenite dendrites atV of 100 µm/s. Except for the persistence of grey eutectic rather than white eutectic in the presence of austenite dendrites (11 <V < 65 µm/s), these observed growth transitions are consistent with the derived relationships governing growth temperatures on the basis of competitive growth,i.e. that the growth form with the highest growth temperature at a particularV should prevail. The results are then applied to derive the limiting conditions for growth of grey and white eutectics as a function of composition in Fe-C (i.e. their coupled zones). The significance of observed dependences onV of volume fraction and spacing of austenite dendrites is discussed.

A composite material with a matrix of high-chromium cast iron

1. Saturation of the matrix with tungsten and carbon during interaction of the matrix with grains of tungsten carbide eliminates the substantial difference in the wear resistance of hardening particles and the matrix of the composite material and secures the tungsten carbide grains in the material. 2. The tungsten diffusing from tungsten carbide alloys both components of the eutectic structure of high-chromium cast iron; no tungsten is found in dendrites of primary austenite. 3. Electroslag hard facing with use of high-chromium cast iron as the binder for grains of tungsten carbide makes it possible to obtain a composite material with high wear resistance and fairly high strength characteristics.

Thermodynamics of microsegregation and influence of elements on structure of unalloyed and alloyed cast iron

Graphitizing elements in cast iron such as Si, AI, Cu, and Ni are subject to so-called ‘negative’ microsegregation during crystallization of primary austenite dendrites in hypoeutectic cast irons, while chill-forming elements such as Mn and Cr are subject in the same conditions to ‘positive’ microsegregation. This general rule is developed quantitatively and the theory of micro segregation of the components of cast iron is linked to the theory of graphitization. On this basis a new aspect of the thermodynamics of graphitic and non-graphitic iron alloys of the cast-iron type was worked out, using methods of geometrical thermodynamics developed from those proposed by Hillert in the early 1950s. A simple equation has been obtained which gives a relationship between the activity of components of ternary systems and the slope of the tie-line in the corresponding two-phase region of the phase diagram. The paper also includes the results of computations in the Fe-C-Si and Fe-C-Al systems. The results are analysed and compared with those obtained on the basis of relationships between the slope of eutectic lines and the angle at the summit of eutectic tie-line triangles.

Silicon microsegregation and first stage graphitization in white cast irons

The distribution of silicon in a series of as-cast white irons containing up to 3.25 pct Si and from 2.39 to 3.94 pct C was studied using electron probe microanalysis. The silicon content of the proeutectic austenite dendrites is a linear function of the silicon content of the alloy. The periphery of these dendrites is richer in silicon than the core due to the rejection of the element from the cementite formed during eutectic solidification and the enrichment is most pronounced at slow rates of solidification. It was also firmly established that silicon is present in the eutectic cementite, the level increasing with solidification rate and with the silicon content of the alloy. The nonuniform distribution of silicon persisted for a large part of a graphitization anneal. The effects of the silicon in the cementite and at the austenite-cementite interface on the nucleation of graphite during first stage graphitization are discussed in terms of the thermodynamics of graphitization.

亜共晶，過共晶鋳鉄の凝固過程に及ぼすMn, Sの影響

The solidification process of hypo-and hyper-eutectic Fe-C alloy was investigated by thermal analysis and quenching method. The flake graphite separated from the eutectic austenite in hypo-eutectic iron, while in hyper-eutectic alloy the primary graphite separated directly from the melt and was surrounded by small austenite dendrites followed by the eutectic solidification around them. Addition of manganese to these alloy made plate-like cementite in the melt at eutectic or at primary solidification which neucleates the ledeburite colonies, and the ledeburite structure was normal and the solidification temperature was not so much supercooled. On the contrary, addition of sulphur changed the flake graphite to undercooled structure containing small amount of nodular graphite or cluster-type mesh-like graphite and then produced the inverse-chilled or massive ledeburite structure. The temperature of austenite-graphite eutectic solidification was lowered by increasing the sulphur content and that of the formation of plate-like cementite in eutectic or hyper-eutectic melt was lowered considerably. By these investigations it was considered that manganese stabilized the cementite and made its formation more easy than that of the eutectic austenite or the primary graphite, but sulphur interrupted the formation of eutectic austenite or primary graphite and supercooled the melt resulting in the formation of some abnormal graphites or massive ledeburite.

白銑の黒鉛化に関する一実験

Specimens were prepared as 4.1% C, 0.3 % di ferro-alloys by chill-casting. These were heated of various temperatures and for various hours, and their microscopic structures were examined. Cementite precipitated specklike or granular in dendritic austenite during annealing at above A1 temperature, and they coalesced with increased annealing time. At last, in austenite some coalesced large cementite grained in austenite which are difficult to graphitze like the eutectic cementite in chill-castings. While cooling the graphitized specimens, the primary precipitated cementite will be swiftly graphitized even by furnace-cooljng. So, it may be considered that there are some differences between coalesced or eutectic cementite and primary precipitated cementite.

鋳鉄の研究（第4報）急冷せる鋳鉄の凝固過程について

To study the process of freezing of cast iron, Fe-C alloys were melted and freezed rapidly, then examined their microstructures. When hypo-eutectic Fe-C alloys were freezed rapidly, there appeared in their microstructures austenite as primary, cementite as secondary, and then ledeburite. In the case of hyper-eutectic Fe-C alloys, cementite appeared as primary, in their microstructures but austenite was never found. But in hyper-eutectic cast iron, which contained sufficient quantity of Si or which was cooled slowly in proper manner dendritic austenite appeared among austenite-graphite eutectic. To study these peculiar structures, eleven binary eutectic alloy systems were examined. In the four systems of Al-Si,Al-Fe, Sn-Bi, and Cd-Bi, the same peculiar structures were obtained by rapid freezing. To explain such peculiar structures, the velocity of crystallization of each component was discussed, and the solubility of Fe in cementite was also discussed.