Numerical Study of Solid Iron Carburization in a Carbon Monoxide Atmosphere

The diffusion step during the high temperature carburization of a spherical iron particle in a carbon monoxide atmosphere is mathematically modeled and numerically simulated in this work. The continuity equation is numerically solved using a forward finite-differences scheme, and a temperature range of 1,486-1,664 K is considered. An integration time during which the solid iron particle is not yet melted is assumed. The programming code was verified using experimental results previously reported by other authors. Radial carbon concentration is determined for several temperatures, and the average carbon concentration in the particle is obtained by numerical integration.     

铜在碳钢中扩散及其对碳钢耐腐蚀性的影响

用水溶液电沉积法在碳钢表面电镀铜并进行高温扩散退火,用Den-Broeder法计算铜在碳钢中的扩散系数,研究了铜在碳钢中的扩散行为及其对碳钢耐腐蚀性的影响。结果表明,铜在碳钢中的扩散主要沿晶界进行,铜的扩散抑制了热处理过程中碳钢晶粒的长大。铜在碳钢中的扩散系数为1.11×10-16～3.03×10-11 cm2/s,扩散系数随着退火温度的提高而升高,随着铜浓度的提高而降低。铜在碳钢高温奥氏体区中扩散所需的激活能为126～167 kJ/mol,在高于低温铁素体+奥氏体混合区中激活能为90～108 kJ/mol。通过铜在碳钢中的扩散制备的Cu-Fe梯度材料,具有优良的耐腐蚀性。     

The influence of silicon on carbon redistribution in steel weldments

Carbon redistribution was measured in ST1/ST2 Fe-2.5Si-0.8C/Fe-0.32Si-0.49C steel weldments in the temperature range 500–1000 °C. At the temperatures where austenite exists, carbon diffuses from ST1 into ST2; when ferrite is present, the diffusion flow reverses from ST2 into ST1. This effect is attributed to the degree of the silicon influence on the graphite precipitation and carbon activity in ST1 and ST2 steels. The opposite signs of the activity gradients in austenite and ferrite cause the reversal of the carbon diffusion when the annealing temperature is changed from the austenite to the ferrite temperature region. The carbon diffusion coefficients D_C and the thermodynamic interaction coefficients ε_C^Si in austenite have been assessed from the experimental data for ST1 and ST2 steels.     

The influence of diffusion of carbon in ferrite as well as in austenite on a model of reaustenitization from ferrite/cementite mixtures in Fe-C steels

The kinetics of formation of austenite from ferrite and cementite mixtures has been modelled by assuming the local equilibrium at the planar phase interfaces. The exact solutions to the diffusion equations governing the volume diffusion of carbon in austenite and ferrite are presented. The concurrent motions of the two interfaces are calculated via solving a set of transcendental equations derived from the flux balance conditions. At low isothermal transformation temperatures, it is found that the time required for reaustenitization is slightly greater than the time previously calculated with no diffusion of carbon in ferrite.     

Study of the Adsorption Space of Modified Clinoptilolites

Carbon dioxide （CO2） adsorption is an important adsorbent characterization method and a significant industrial process. In separation and recovery technology, the adsorption of the CO2 is important to reduce the concentration of this gas considered as one of the greenhouse gases. Natural zeolites, particularly clinoptilolite, are widely applied as adsorbents. In this regard, in the present research, the structure, composition and morphology of modified with hexafluorosilicate （HFSi） and orthophosphoric acid （H3PO4） clinoptilolites were investigated by characterizations and measurements made with, X-ray diffraction （XRD）, thermogravimetric analysis （TGA）, scanning electron microscopy-energy dispersive X-ray analysis （SEM-EDAX） and gravimetric adsorption. Additionally, the surface Chemistry of the modified clinoptilolites was analyzed by applying diffuse reflectance fourier transform infrared spectrometry （DRIFTS）. Further, the interaction of CO2 within the adsorption space of these modified clinoptilolites and a synthetic ZSM-5 zeolite was studied with the help of adsorption measurements. After all, an appropriate theoretical methodology for the analysis of the XRD and adsorption data was applied. The calculated cell parameters of the tested are similar to those reported for a typical clinoptilolite of： a = 17.662 A, b = 17.911A, c = 7.407 ~ and fl = 116.40 The resolution of the TGA derivative profiles indicated the presence of two steps for water release, one of them represents the loss of majority of the water present in the micropores. This was evidenced as a broad peak centered at about 50℃ for the CSW-HFSi-0.1, but at 100 ℃ for the samples CSW-HFSi-0.4. The SEM micrographs corresponding to the modified clinoptilolites, was evidenced that the CSW zeolite shows secondary particles exhibiting diameters from 3 to 40 μm, formed by primary clinoptilolite crystallites showing a crystallite size, φ = 40 nm. The EDAX elemental analysis it can be demonstrated that the exchange process replaced about 85% of the charge compensating ions. The DRIFT spectra of the modified clinoptilolites, specifically, CSW-HFSi-0.1, show a narrow band at about： 3,740 cm-1 corresponding to terminal silanol groups （Si-OH） and a band 3,600-3,650 cm1 resulting from extra-framework AI-OH. With the precision of the measured micropore volumes related to the excellent fitting of the adsorption data by the D-R isotherm equation, it can be affirm that carbon adsorption took only place in the micropore region. The isosteric heat of adsorption calculated for the modified clinoptilolites was greater than those values reported of ZSM-5 zeolite, particle packing silica, dealuminated Y zeolite （DAY） Cd, Zn and Ni-nitroprussides and Cu-nitroprusside and a Ni-MOF. With the obtained result it can be concluded that the modified clinoptilolites with HFSi showed a quality as adsorbent comparable to commercial synthetic zeolites.     

Formation of austenite from a ferrite-pearlite microstructure during intercritical annealing

The formation of austenite from a coarse grained ferrite-pearlite microstructure during intercritical annealing was studied. According to the variation of the microhardness values in ferrite and austenite with intercritical annealing time, austenite formation was classified into four stages: (a) austenite growth into pearlite at a slower rate than the dissolution rate of pearlite, (b) subsequent growth of austenite into pearlite and formation of thin film type austenite at ferrite grain boundaries, (c) growth of austenite into ferrite, (d) equilibrium of ferrite and austenite. In particular, plate-like austenite was observed. The experimental results indicated that the growth process of this type austenite was controlled by carbon diffusion in austenite.     

A Tool for Interrogation of Macromolecular Structure

Our program BABELPDB allows browsing and interrogating the native and derived structural features of biomacromolecules using data obtained from the Protein Data Bank （PDB）. Major features of BABELPDB are： （1） convert from PDB to other formats, （2） add or remove H-atoms, （3） strip the crystallization water molecules and （4） separate the ct-carbons （Cα）. The co-ordinates obtained with BABELPDB permit characterizing the presence of H-bonds. The algorithm for detecting H-bonds is implemented in our program TOPO for the theoretical simulation of the molecular shape, An example is given to illustrate the capabilities of the software： the calculation of the fractal dimension of the lysozyme molecule with （1.908） and without （1.920） H-atoms. The figures compare well with reference calculations performed with our version of program GEPOL and results from Pfeifer et al. For proteins, Cα-skeleton extracted with BABELPDB allows drawing the ribbon image, which determines their secondary structure.     

A Mixed-control Mechanism Model of Proeutectoid Ferrite Growth under Non-equilibrium Interface Condition in Fe-C Alloys

By combining the α/γ interface migration and the carbon diffusion at the interface in Fe-C alloys, a mathematical model is constructed to describe the mixed-control mechanism for proeutectoid ferrite formation from austenite. In this model, the α/γ interface is treated as non-equilibrium interface, i.e., the carbon concentration of austenite at γ/α interface is obtained through theoretical calculation, instead of that assumed as the local equilibrium concentration.For isothermal precipitation of ferrite in Fe-C alloys, the calculated results show that the rate of interface migration decreases monotonically during the whole process, while the rate of carbon diffusion from γ/α interface into austenite increases to a peak value and then decreases. The process of ferrite growth may be considered as composed of three stages: the period of rapid growth, slow growth and finishing stage. The results also show that the carbon concentration of austenite at γ/α interface could not reach the thermodynamic equilibrium value even at the last stage of ferrite growth.     

Laser beam hardening of carbon and low alloyed steels: discussion of increased quantity of retained austenite

It was shown that mainly three effects are responsible for the increased quantity of retained austenite in carbon and low alloyed steels after laser transformation hardening. At low peak temperatures a high dislocation density appears in the initial austenite phase during the reversed polymorphic transformation which is preserved till the martensite transformation begins and retards this reaction. At high peak temperatures this is more complete carbides dissolution than after standard furnace hardening. Both effects are overlapped by a third one. This is the carbon up-hill diffusion resulting in surface enrichment of carbon and a decrease of the martensite start temperature.     

淬回并回火后钢的奥氏体形成动力学的研究

研究了淬火回火态30CrMnSiA钢重新加热时奧氏体形成动力学。建立了回火钢奥氐体等温形成的动力学方程和T～1nτ等温曲线。计算了碳的扩散激活能。表明,奥氏体形成过程具有明显扩散相变的特征。     

Carburising behaviour of low carbon steel with nanocrystalline surface layer induced by ball milling

Abstract

Ball milling, as a surface nanocrystallisation method, was employed to investigate the influence of severe plastic deformation on the carburisation treatment performed on low carbon steel. The results indicated an enhancement in the carburisation efficiency as a result of surface milling. This enhancement was attributed to the formation of a nanocrystalline layer in the surface of the treated samples. It was found that the main reasons for the accelerated kinetics of the carburisation process would be the considerable amounts of non-equilibrium defects and the finer austenite grains in the early and later stages of the treatment respectively, which facilitate the carbon diffusion.     

Study of Carbon-poor Regions of Austenite during Bainitic Incubation Period by SAM

The variation of carbon composition in undercooled austenite during bainitic incubation period has been investigated in 40CrMnSiMoVA by scanning auger microprobe (SAM). The results show that the outstanding diffusion and redistribution of carbon occur in isothermally reacted specimens. This leads to the formation of carbon-poor regions (CPR) near austenite grain boundaries and in austenite grains. The carbon content in CPR exceeds the equilibrium carbon composition of ferrite. Furthermore, it is indicated that the existence of CPR is thermodynamically possible.     

The kinetics of low-pressure carburizing of alloy steels

In the article the author analyzed the kinetics of low-pressure carburizing of alloy steels, based on steel 16MnCr5 and CSB 50NIL. It was found that an active radical-carbon layer (carbon deposit) deposits on the surface of austenite grains or austenite and carbides during boost steps. This layer mediates in moving the carbon deeper into the austenite grains. During the diffusion steps the layer deposited on the carbide surfaces transfers into the austenite grains. Both this layer and the one previously deposited on the austenite grains surface now undergo another catalytic decomposition into atomic carbon and hydrogen, other types of radicals and fine-crystalline graphite. The resulting carbon atoms are absorbed by surface austenite grains and next diffuse deeper into the austenite grains, and, alternatively, into the carbide surface, making the carbides grow and contributing to the increase in the carburized layer thickness.     

The kinetics of low-pressure carburizing of alloy steels

In the article the author analyzed the kinetics of low-pressure carburizing of alloy steels, based on steel 16MnCr5 and CSB 50NIL. It was found that an active radical-carbon layer (carbon deposit) deposits on the surface of austenite grains or austenite and carbides during boost steps. This layer mediates in moving the carbon deeper into the austenite grains. During the diffusion steps the layer deposited on the carbide surfaces transfers into the austenite grains. Both this layer and the one previously deposited on the austenite grains surface now undergo another catalytic decomposition into atomic carbon and hydrogen, other types of radicals and fine-crystalline graphite. The resulting carbon atoms are absorbed by surface austenite grains and next diffuse deeper into the austenite grains, and, alternatively, into the carbide surface, making the carbides grow and contributing to the increase in the carburized layer thickness.     

Austenite films in bainitic microstructures

Abstract

Bainitic microstructures in which fine platelets of ferrite are intimately mixed with films of austenite are known to exhibit good combinations of strength and toughness. It isfound that the thickness of these austenite films can be estimated by assuming that the carbon diffusion field around an existing plate of ferrite prevents the close approach of another parallel plate. This is because the regions of austenite with the highest carbon concentration are unable to transform to bainite.

MST/3052     

Influence of post-weld heat treatments on microstructure and mechanical properties of gas tungsten arc maraging steel weldments

Abstract

The response to post-weld heat treatment of an 18%Ni (250 grade) gas tungsten arc weld metal has been investigated. The post-weld heat treatments are (a) direct aging at 480°C/3 h/air cooling, (b) solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling and (c) homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling. Metallographic characterisation of fusion zone revealed pronounced segregation of titanium and molybdenum along the interdendritic and intercellular boundaries. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Solutionised treatment at 815°C does not remove the segregation. Homogenisation treatment (1150°C/1 h/air cooling) succeeded in making the composition become homogenised. Mechanical properties including tensile, hardness and impact toughness were evaluated. Tensile test results showed that directly aged weldments exhibited lower strength but higher ductility than the other cases; this was attributed to the presence of reverted austenite. Homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling resulted in optimum tensile properties. A substantial increase in fusion zone toughness was observed after homogenisation+solutionising+aged condition due to a decrease in the content of austenite content compared to the directly aged condition. The reduction in microsegregation by diffusion of alloying elements from cell boundaries to the cell during homogenisation treatment is responsible for the decrease in austenite content.     

A study of the solid–liquid interface in cobalt base alloy (Stellite) coatings deposited by fusion welding (TIG)

Microstructural features present at the interface between a weld deposited Stellite 6 hard facing and an austenitic stainless steel substrate are described. Elemental X-ray maps indicate that diffusion of carbon from the liquid Stellite to the austenitic stainless steel takes place along grain boundaries resulting in the formation of chromium carbide “arms” that penetrate along the austenite grain boundaries in the interfacial region.     

Alloy design of ductile phosphoric iron: Ideas from archaeometallurgy

Alloy design criteria to produce ductile phosphoric irons have been proposed based on a detailed microstructural study of ancient Indian irons. The alloy design aims at avoiding phosphorus segregation to the grain boundaries by (a) soaking the phosphoric iron at high temperatures within the ferrite + austenite region to precipitate austenite allotriomorphs, (b) utilizing a critical amount of carbon to segregate to grain boundaries, and (c) precipitation of some of the phosphorus in solid solution in the ferrite matrix as fine coherent phosphide precipitates.     

Low Temperature IR Study of Shape Transition in Carbon-, Silicon-and Germanium Tetrachlorides

The IR data in 20-296 K temperatures interval for carbon-, silicon- and germanium tetrachlorides are given. It is shown that the spectral picture of bands, appearing in higher frequencies region relatively to element-chlorine stretching, considerably changes at the temperature variation and therefore can be assigned to the vibrations of different molecular shapes. The interpretation of the obtained results based on the suggested model of transformation in the structure of tetrachlorides. This transformation leads to the arising of an active chlorine atom in molecules of element-tetrachlorides.     

Bainite transformation during continuous cooling of low carbon microalloyed steel

Abstract

The evolution of the final microstructure for a low carbon Nb–Ti microalloyed plate steel was studied during a simulation of thermomechanical processing for hot rolling following by accelerated cooling. The effects of austenite deformation below the non-recrystallisation temperature T _NR, cooling rate, and interrupt temperature on the formation of conventional (intergranular) bainite (CB), acicular ferrite (intragranular) (AF), and martensite–austenite (MA) constituents were determined. With increases in austenite deformation and cooling rate, and decrease in the interrupt temperature, the final microstructure changed from a mixture of CB+MA through CB+AF+MA to a dual phase AF+MA.