This paper describes the microstructure, deformation, and fracture of low-temperature-tempered (LTT) martensitic steels. The microstructural reasons for the ability of these steels to achieve ultrahigh strengths and the factors controlling ductility and toughness are described for low-carbon, medium-carbon, and high-carbon LTT martensitic steels. The key strengthening mechanism of LTT martensitic steels is the strain hardening provided by the transition carbide/dislocation substructure of the martensite crystals. In low- and medium-carbon steels, LTT microstructures fail by ductile fracture mechanisms, and ductility decreases as strain hardening rates increase with increasing carbon content. In high-carbon LTT steels, quench embrittlement associated with phosphorus segragation and cementite formation at austenite grain boundaries limits toughness and fatigue resistance. Approaches which permit the application of the high strengths of high-carbon LTT steels and minimize the effects of quench embrittlement are discussed.
The reduction of Australian ilmenite was carred out in a bubbling fluidized bed with pure hydrogen. It was found that the reduction did not proceed topochemically with respect to each grain due to its small size and porous characteristics, but the experimental data fitted the volume reaction model expressed by the following equation. –ln(1–x)=kvcgt kv=0.0299 exp[–61.0×103/(RT)] The reduction mechanism and rate constants obtained were applied to the mathematical model of a fluidized bed developed on the basis of two-phase theory for analyzing the rate limiting processes. Results of theoretical analysis showed that the mass balance of the reduction of ilmenite occurring in a fluidized bed operated in batches was described under the unsteady-state assumption. It was further found that both the mass interchange of the reactants between the two phases and the intrinsic chemical reaction played important roles as rate-limiting steps during the reduction of ilmenite.
In this paper, the slag-metal equilibrium experiments were performed to investigate the effect of TiO2 concentration in the slag and basicity (R=wt%CaO/wt%SiO2) on the sulphur distribution ratio (Ls) between CaO-SiO2-TiO215%Al2O3-8%MgO slag and carbon-saturated iron at 1773 K. Sulphide capacities (Cs) were calculated based on the sulphur distribution ratio. The sulphur distribution ratio and sulphide capacity of CaO-SiO2-TiO2-15%Al2O3-8%MgO stage increase with increase of basicity and with decrease TiO2 concentration in the slag. The relationship between sulphide capacity of CaO-SiO2-TiO2-15%Al2O3-8%MgO slag and optical basicity was also investigted. The experimental results are useful for smelting vanadium titaniferrous magnetite of blast furnace.
The permeability of the combustion-melting zone in the sintering bed of the Dwight-Lloyd process, which affects process efficiency in terms of productivity and product yield as well as quality control, was found to be dependent largely on the sinter cake load upon the combustion-melting zone during sintering. In order to control and reduce the sinter cake load, the magnetically-levitated sintering method was devised. As a result of sintering pot tests, the sintering time was shortened by a maximum of 30% without a decrease in product yield and coke combustion efficiency was improved despite an acceleration of sintering speed. In addition, the reduction degradation index, RDI, and reducibility, JIS-RI, were improved. The size distribution of product sinter tends to be sharp. Improvement in permeability were proportional to the degree of load reduction. CT analyses of sinter cake samples showed that the load on the combustion-melting zone during sintering accelerates ordinarily the plugging of stem pores with melt and disturbs gas flow through the sintering bed in the middle and bottom layer. The permeability improvements resulting from applying the magnetic levitation are attributed to the formation of a homogeneous network with a pore size appropriate for gas flow as well as increase in porosity of stem pores by about 8%.
Filtration tests were made on Al-deoxidized iron melts in a 50 kg vacuum induction melting and casting unit using commercial foam, multi-hole and loop filters. Samples from the solidified metal were prepared for chemical analysis and quantitative metallography. Al2O3 inclusions were produced by the addition of pure Al to the oxidized metal. Filtration efficiencies based on the content of chemically analysed total oxygen or oxide particle size distribution were evaluated. Filtration of pure iron melts using ceramic foam filters reaches efficiencies up to 90%. A minimum of total ixygen content around 20 to 30 ppm is attained independent from the initial oxygen content of the melt. The lowest levels are achieved when 25 ppi foam filters are applied. Filtration tests using 100 cpsi multi-hole filters for pure iron reveal filtration efficiencies of 80% and total removal of inclusions larger than 20 μm in diameter. Filtration efficiencies of 72 to 84% for pure iron were obtained using ceramic loop filters of different string diameters. From the evaluation of volume flow capacities and filter resistances it was evident that casting time is prolonged up to the threefold as compared to casting tests without integrated filters. Filtration of steel melts using ceramic foam, multi-hole or loop filters is an effective tool. High separation efficiencies are achieved, but filtration capacities of the porous filter media are limited in view of large steel volumes.
Electromagnetic centrifugal casting technique based on magnetohydrodynamic principles has been applied to improve the solidification structure and mechanical properties of a 25Cr-20Ni-Fe-C alloy. The experimental results show that the solidification structure can be controlled by electromagnetic centrifugal casting. As the magnetic flux density increases, the solidification macrostructure changes from coarse columnar grains to a mixed structure consisting of fine curved columnar and equiaxed grains, and finally, into fine equiaxed grains. The columnar-equiaxed transition is achieved through the fracture of growing dendrites, and the size of equiaxed grains is also affected by the magnetic flux density. The alloy with equiaxed grain structure produced by electromagnetic centrifugal casting shows both better tensile strength at room and higher temperatures, and longer stress-rupture life compared with the same alloy with columnar grain structure produced by conventional centrifugal casting under the same stress.
The planar flow casting (PFC) process for rapid solidification of metallic materials has been in commercial use to produce thin strips of glassy or microcrystalline materials. The conditions of melt puddle between nozzle and rotating wheel affect significantly the quality and dimensional uniformity of the downstream ribbon. A previous study had developed a mathematical model to analyze the shape of the puddle and the momentum transfer phenomena during the formation of puddle. The model was based on a computational fluid dynamics technique, called the SOLA-VOF scheme, which possesses the capability of treating transient fluid flow problems with evolution of free boundaries. The balance between the inertia effect and the surface tension effect has to be reached for the formation of a steady puddle. The previous model, however, had empolyed a rather simplified treatment for the free surface boundary conditions. In this study a more accurate treatment of the free surface boundary conditions is proposed. The results show that the modified treatment results in a wider puddle and a longer time is required for the steady puddle to be formed. With the model, we also evaluate the effects of different process variables on the puddle and compare the difference of the simplified treatments with the modified treatments for the free surface boundary conditions. The results reveal that the modified treatments of the free surface boundary conditions results in a more wider melt puddle near the substrate than the puddle near the nozzle. This is more consistent to the experimental observation.
The morphology of MnS inclusions in steel formed during solidification and the modification of this morphology by additions of alloying elements Al, Si, C and Ti have been investigated using optical and electron microscopy techniques. In sulfur-lean melts the morphology of the secondry MnS inclusion formed after the primary crystallization of the Fe phase during solidification can be classified under the following categories: i) Globular or droplet shaped MnS resulting from a monotectic reaction, ii) Rod like MnS formed through an eutectic reaction and iii) a fish-bone type MnS formed as a result of an irregular eutectic reaction. In sulfur-rich melts the primary crystallization phase is MnS and depending on the nature of additional elements present in the melt and the melt atmosphere, the morphology of these primary MnS inclusions can be classified as iv) spherical, v) dendritic or vi) angular. The formation mechanism of the different MnS morphologies is discussed on the basis of phase diagram information. It sis shown that the spherical morphology of the primary and secondary MnS inclusions is a result of metastable reactions, while the eutectic, dendritic and anglular morphologies are the products of stable reactions, with the high melting dispersed particles like TiN and Al2O3 acting as nucleants for the MnS crystals. It is also shown that the addition of C and Si increases the probability of stable reactions by increasing the temperature interval between the eutectic and the monotectic points.
Research has carried out on the magnetostriction in grain oriented silicon steel, which is known to be one of main causes for noise problem and be crucially influenced by stress condition on the steel in either manufacturing or application process. Generalization of the formulation for the influence of the stress on the magnetostriction has been investigated, which enables computing design to evaluate an occurrence of the magnetostriction under various practical stress conditions, proposing a predictive formulae for the occurrence. The deteriorating influence on the magnetostriction of compressive stress applied to grain oriented silicon steel was attributed mainly to the formation of 90° supplementary domains or triangular closure domains around grooves made on the surface. The supplementary domains or the triangular closure domains were found to occur under generalized condition that the strain elastic energy in easy magnetization axis , which is generally parallel to rolling direction in Goss orientation texture, is smaller than those in  and  (other easy magnetization axes). This result led to practical equation for the influence of the stress on the occurrence of the magnetostriction, having been substantiated by elaborate experimental results ranging from the effect of coating stree on the magnetostriction to the influence of residual internal tensile stress near scratched line on the surface. The obtained formulae were confirmed to be effective even for the magnetostriction under complicated state of superimposition of bi-axial stresses experimented by Moses et al.
The fracture toughness must be evaluated under the dynamic loading condition, when the materials are used for structures tha are desired to a higher safety and must be taken into account of the effect of dynamic loading. Moreover, in order to obtain the valid Jlc value, a criterion concerning both specimen thickness B and ligament width b0 must be satisfied, which is called as "valid condition" and is described as the following equation. B, b0 ≥ α(Jin/σfs), (α=25) However, this equation is presented for the static loading condition and there is no assurance to be able the apply for the case of dynamic loading one. Therefore, under static and dynamic loading conditions, the effects of both B and b0 on Jin value and the flow stress σfs are investigated and the valid conditions are discussed. The results are presented that the factor α of the above equation is reduced as 20 for b0 and is increased as 28 for B under dynamic loading condition.
The WG in High Strength Line Pipe Research Subcommittee of Steel Pipes and Tubes Committee of the Joint Research Society of ISIJ investigated the applicability of the Chevron Notched Drop Weight Tear Test to Japanese high toughness line pipes, and developed a new test method, The Slit Chevron Notched Drop Weight Tear Test. The effects of the slit width and the notch angle of the specimen was investigated in details, and the established method was verified by the burst tests on actual pipes. The fracture appearance transition temperature of the established test is a little safe side estimation to the brittle fracture of line pipes, and the impact energy of the test gives the prediction to the ductile crack propagation and arrest in natural gas transmission pipelines. It is expected that the developed method serves for the quality evaluation of high toughness line pipes against brittle fracture and ductile crack propagation and arrest.
Influence of superficially applied CeO2 coating on the high temperature oxidation behaviour of AISI 347 grade stainless steel under different rates of heating followed by isothermal holding at 1273 K in dry air is reported. The reactive oxide coating not only reduced the oxidation rate but also facilitated imporved oxide scale adherence to the alloy substrate. The coated steel could withstand a number of thermal cyclings without scale rupture and exhibited further reduction in rate in sequential exposures. Reduced reaction rate has been attributed mainly to a change over in the rate controlling process from outward migration of cations to the ingress of oxidant species. Improved scale adhesion is due to change in chemical composition and grain size of the resultant oxide scale promoted by the formation of silicon-oxide stringers into the base alloy favouring pegging mechanism. Scale growth mechanism has been substantiated by post oxidation analyses using SEM, EDS, EPMA and XRD which could illustrate the role of reactive oxide coating and the formation of various cmplex oxide phases along with NiNb2O6.
A process route comprising air induction melting (AIM) and electroslag remelting (ESR) using steel scrap and commercial aluminium is described for production of an Fe3Al based ordered intermetallic alloy. The ESR cast alloy exhibited room temperature mechanical properties superior to those of conventionally processed material using purer raw material as feed stock. Hot forging at 1273 K refines the microstructure but only a marginal improvement in mechanical properties is obtained indicating that the properties of as-cast ESR product are comparable to that of the wrought product, as is the case in steels, and that hot working is required only to achieve the desired component dimensions. To screen the sample from environmental moisture during testing, tensile testing was also carried out after coating the samples with oil. There is no significant improvement in mechanical properties after oil coating contrary to the results reported in literature. This may be attributed to significant levels of carbon present in the alloy used in this work.