Mechanical behaviour of an austempered ductile iron

Austempering of a ferrite-pearlitic grade of ductile iron was carried out to assess the potential use of the material for crank shaft application reported. A commercial material was austempered at 340°C to realize the properties. The austempered ductile iron gave good strength although the ductility values were lower. The material developed had complete ausferritric structure free of pearlite. The various phase constitution and phase transformation associated with the treatment and during mechanical deformation was examined. Using XRD analysis the volume fraction of the austenite in the matrix was estimated. The various aspects of processing a commercial cast iron during ausetmpering, the phase transformation, microstructural evolution have been examined along with the property of the material. The mechanical behaviour of the material and the scope for further improvement is discussed.     

Mechanical properties and strain-strengthening of sintered iron

Conclusions The particle size of the starting powder, like the grain size of sintered specimens, does not have a pronounced effect on the mechanical properties of sintered iron because the effective length or a slip plane is determined by the size of the smaller fragments bounded by the pores in the particles and interfaces. The strength and ductility characteristics of sintered iron in a wide range of porosity variation are satisfactorily described by the expression obtained in [3]. The strain-strengthening of sintered porous iron, like that of polycrystalline nonporous iron, has a parabolic character, with the exponent of strainstrengthening being independent of porosity and the coefficient of strain-strengthening decreasing with, increasing porosity according to the same law as the limit of proportionality.Translated from Poroshkovaya Metallurgiya, No. 7(259), pp. 68–72, July, 1984.     

中碳钢高温力学和冶金行为

采用Gleeble 1500热模拟机对CSP生产的SS400、Q235B和Q345B钢的热塑性进行了研究.结果发现,所研究的钢存在两个低塑性区,即凝固脆性温区(T_m~1 310℃)和低温脆性温区(850~725℃).试样断口金相和成分分析表明:产生凝固脆性温区的原因主要是高温下枝晶间有害元素S、P和O富集形成液膜;产生低温脆性温区的原因主要是奥氏体晶界出现铁素体薄膜以及细小AlN析出造成连铸坯的塑性降低.根据研究结果,提出了改善钢的热塑性防止铸坯裂纹的工艺建议.     

Mechanical behavior of a bulk nanostructured iron alloy

Bulk, fully dense materials were prepared from Fe-10Cu with grain diameters between 45 nm and 1.7 μm. The materials were prepared by ball milling of powders in a glove box, followed by hot isostatic pressing (hipping) or powder forging. Larger grain sizes were obtained by thermal treatment of the consolidated powders. The bulk materials were relatively clean, with oxygen levels below 1500 wpm and other contaminants less than 0.1 at. pct. The mechanical behavior of these materials was unique. At temperatures from 77 to 470 K, the first and only mechanism of plastic deformation was intense shear banding, which was accompanied by a perfectly plastic stress-strain response (absence of strain hardening). There was a large tension-compression asymmetry in the strength, and the shear bands did not occur on the plane of maximum shear stress or the plane of zero extension. This behavior, while unusual for metals, has been observed in amorphous polymers and metallic glasses. On the other hand, the fine-grained Fe-10Cu materials behaved like coarse-grained iron in some respects, particularly by obeying the Hall-Petch equation with constants reasonably close to those of pure iron and by exhibiting low-temperature mechanical behavior which was very similar to that of steels. Transmission electron microscopy (TEM) studies found highly elongated grains within shear bands, indicating that shear banding occurred by a dislocation-based mechanism, at least at grain sizes above 100 nm. Similarities and differences between the fine-grained Fe-10Cu and metals, polymers, metallic glasses, radiation-damaged metals, and quench-damaged metals are discussed. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

Mechanical and physical properties of high-velocity oxy-fuel-sprayed iron aluminide coatings

Tensile tests and thermal-expansion measurements were performed on free-standing, high-velocity oxy-fuel (HVOF)-sprayed Fe₃Al coatings produced at spray-particle velocities of 390, 560, and 620 m/s. To examine the relationship between properties and spray conditions, the microstructures of the coatings were characterized in terms of the fractions of unmelted particles, porosity, and oxide inclusions, as well as the dislocation density assessed by X-ray diffraction (XRD) line-broadening analysis. Residual coating stresses were determined as a function of coating thickness using curvature measurements. The tensile behavior was entirely brittle at room temperature; fracture strengths increased with spray-particle velocity; and the increase in fracture-strength results from decreasing fractions of microstructural defects and better interparticle bonding. The mean thermal-expansion coefficients for the coatings were lower than those for an equivalent wrought material; the differences were attributed to a 7 to 15 vol pct fraction of oxide inclusions. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Characterisation studies on swelling behaviour of iron ore pellets

At JSW Steel Limited (JSWSL), pellets form the major part of the iron-bearing feed to corex and blast furnace. JSWSL produces low-basicity pellets ((CaO/SiO₂) – 0.40 to 0.50). The quality of the pellet is affected by the raw material chemistry (gangue content), flux proportion and their subsequent heat treatment to produce the fired pellets. The raw material silica, limestone addition, i.e. basicity – CaO/SiO₂ of pellet decides the mode, temperature and the amount of melt formed. The properties of the pellets are, therefore, largely governed by the form and degree of bonding achieved between ore particles and also by the stability of these bonding phases during the reduction of iron oxides. In the present study, laboratory pelletisation experiments have been carried out to know the effects of silica and basicity on the microstructure and swelling behaviour of pellets during reduction. Phase analysis was carried out using image analyser, and chemical analysis of oxide and slag phases was carried out using SEM–EDS. From the laboratory studies, it was observed that the swelling index of the pellets decreased with an increase in silica content due to the decrease in porosity. The presence of higher silica in pellet hinders the reduction step of haematite to magnetite at lower temperatures. Pellets with basicity range 0 to 0.1 exhibited lower swelling index due to the formation of high melting point fayalite phase and also at this basicity range the structure is held together by the seam-like compounds between Fe₂O₃ and SiO₂ primarily at high silica content. Higher swelling index was observed at the basicity range 0.3 to 0.7 due to the presence of low melting point calcium olivines (1115°C) between fayalite (FeSiO₄) and dicalcium silicate (Ca₂SiO₄). Low melting point slag phase enhances the swelling index of the pellets. Swelling index of the pellets considerably dropped between the basicity range 0.9 to 1.1 due to the formation of calcium ferrite phases with a close pore structure.     

Mechanical behaviour of thermomechanically produced ultrafine grained dual-phase steels

Dual-phase (DP) steels are an excellent alternative in the production of automotive parts that require high mechanical resistance, high impact strength and elevated elongation. These materials are produced using low alloy steels as a basis, reducing costs and resulting in a combination of martensite and ferrite structures with ultrafine grain size. These characteristics are achieved through strict control of rolling conditions, strain rate, cooling rate and coiling temperature. This work presents the results of tension testing of two types of DP steels, along with microstructural characterisation, in order to understand the effect of the advanced thermomechanical controlled rolling processes on the formation of the microstructure and resulting mechanical properties.

Les aciers biphasés sont une excellente substitution pour la production de composantes automobiles qui requièrent une résistance mécanique élevée, une haute résistance à l’impact et une grande élongation. Ces matériaux sont produits en utilisant des aciers faiblement alliés comme base, réduisant les coûts et engendrant des combinaisons de structures martensitiques et ferritiques à granularité ultrafine. On obtient ces caractéristiques grâce à un contrôle strict des conditions de laminage, de la vitesse de déformation, de la vitesse de refroidissement et de la température d’enroulement. Cet article présente les résultats des essais de traction de deux nuances d’acier biphasé, de même que la caractérisation de la microstructure, afin de comprendre l’effet des procédés avancés de laminage thermomécanique contrôlé sur la formation de la microstructure et les propriétés mécaniques qui en déroulent.     

Mechanical processing of iron powders in reactive and nonreactive gas atmospheres

Mechanical alloying, usually the blending together of elemental metal powders, has been used successfully to produce nonequilibrium microstructures. In this Bureau of Mines study, the effect of mechanical processing on powders in reactive (nitrogen) and nonreactive (argon) gas atmospheres was investigated and compared to results for similar powders that had nitrogen diffused into them. Infusion of nitrogen into powder by mechanical processing resulted in a nitrogen concentration exceeding 1 wt pct, 20 times the as-cast solubility in pure iron. Even at these high concentration levels, no nitrides formed. The nitrogen remained in solution even after consolidation by hot isostatic pressing (“hipping”). The presence of nitrogen retarded grain growth during consolidation.     

Mechanical properties of iron processed by severe plastic deformation

In the present study, the mechanical properties of Fe processed via severe plastic deformation (equal-channel angular pressing (ECAP)) at room temperature were investigated for the first time. The grain size of annealed Fe, with an initial grain size of about 200 μm, was reduced drastically during ECAP. After eight passes, the grain size reaches 200 to 400 nm, as documented by means of transmission electron microscopy (TEM). The value of microhardness during pressing increases 3 times over that of the starting material after the first pass and increases slightly during subsequent pressing for higher-purity Fe. Examination of the value of microhardness after eight passes as a function of post-ECAP annealing temperature shows a transition from recovery to recrystallization, an observation that resembles the behavior reported for heavily deformed metals and alloys. The tensile and compression behaviors were examined. In tension, a drop in the engineering stress-engineering strain curve beyond maximum load was observed both in the annealed Fe and the ECAP Fe. This drop is related to the neck deformation. The fracture surface, examined by scanning electron microscopy (SEM), shows vein patterns, which is different from the dimples found on the fracture surface of annealed Fe. In compression, an initial strain-hardening region followed by a no-strain-hardening region was observed in the ECAP Fe. The yield strength in tension of the ECAP Fe was observed to be higher than that in compression. The strengthening mechanisms and softening behavior are discussed.     

Mechanical properties of fully densified injection-molded carbonyl iron powder

The mechanical properties of carbonyl iron powder shaped by injection molding techniques are affected by the grain size, sintered density, and carbon content. Control of the carbon level depends on several factors, including the binder composition, debinding approach, and sintering conditions (atmosphere, temperature, time, and furnace design). Sintered compacts were densified by containerless hot isostatic pressing, giving smaller grain sizes and superior properties than were possible by pressureless sintering at a high temperature. A quick hot isostatic pressing route (gas forging) with a peak pressure higher than 500 MPa for 1 minute helps retain carbon and results in excellent properties due to a high final density and small grain size. This approach resulted in a final strength of 732 MPa with extensive ductility (23 pct reduction of area) for injection-molded carbonyl iron.     

Role of iron oxy-hydroxides on settling behaviour of red mud

Nine bauxite samples from Brazil and one from Ghana were submitted to alkaline digestion at 145°C. All initial samples and their red muds were chemically analysed and characterised by X-ray diffraction and Mössbauer spectroscopy at 298 and 77 K. The settling rates of all red muds were also measured. It was observed that the presence of goethite in the bauxite has a profound effect on the settling rate of the red mud produced in the Bayer process. The goethite possess high Al-for-Fe substitution and/or nanometric crystallites, and decreases the settling rate. The dissolution of this goethite during the caustic digestion at 145°C was also observed, leading to the formation of hematite. Finally, the hematite in the red muds presents crystals with larger sizes in comparison with those in the original bauxites.

On a soumis à la digestion alcaline à 145°C, neuf échantillons de bauxite du Brésil et un échantillon du Ghana. On a analysé chimiquement tous les échantillons initiaux et leurs boues rouges et on les a caractérisés par diffraction des rayons x et par spectroscopie Mössbauer à 298 K et à 77 K. On a également mesuré la vitesse de sédimentation de toutes les boues rouges. On a observé que la présence de goethite dans la bauxite avait un effet important sur la vitesse de sédimentation de la boue rouge produite par le procédé Bayer. La goethite possède une substitution élevée d’Al pour le Fe et/ou des cristallites nanométriques, et elle diminue la vitesse de sédimentation. On a également observé la dissolution de cette goethite lors de la digestion caustique à 145°C, qui menait à la formation d’hématite. Finalement, l’hématite des boues rouges présente des cristaux de plus grandes tailles en comparaison à ceux des bauxites originales.     

Contribution to phosphorus behaviour in steelmaking with sponge iron

Experiments were conducted in a 70 t UHP electric arc furnace to study the phosphorus behaviour in steelmaking with sponge iron. The proportion of sponge iron in the metallic feed was varied from 52 to 95.4 wt.%. The results are used to show the effect of the sponge iron proportion in the charge on the phosphorus content of the bath. They are also utilized to illustrate the effects of the composition, temperature and theoretical optical basicity of the slag on the phosphate capacity and to investigate the kinetics of dephosphorization. It is found that both initial and final phosphorus contents of the bath decrease almost linearly with increasing sponge iron proportion in the charge. Using regression analysis, linear equations were obtained which show the change of the phosphate capacity of a slag, at a given temperature, with the total lime content as well as with the theoretical optical basicity. Based on these equations, a functional relationship was derived which shows the effect of simultaneous variation of total lime content and temperature of a slag on its phosphate capacity. A similar equation is given for the influence of simultaneous change of theoretical optical basicity and temperature on the phosphate capacity of a slag. The effect of the compositions of the slag and metal on the phosphorus distribution ratio is expressed by an equation. The kinetics of dephosphorization are investigated, assuming that the mass transfer of phosphorus in the metal is the rate controlling step. In the present investigation the activation energy of dephosphorization is equal to 38 kJ/mol and the mass transfer coefficient of phosphorus at 1600°C is 0.0135 cm/s.     

The morphology and substructure of Ti-Cu martensite

The martensites in Ti-Cu alloys containing up to 8 wt pct Cu have been examined using transmission electron microscopy techniques. The martensite has a massive morphology in the alloys which contain 4 pct Cu or less, whereas the alloys containing 6 and 8 pct Cu exhibit acicular martensite. Experimental evidence is presented to show that the lattice invariant deformation in the massive martensite occurs by internal slip with a Burgers vector and these results are discussed in terms of recent calculations. The transition from the massive to acicular martensite morphology is also discussed. Formerly Pre-Doctoral Research Associate, University of Washington, Seattle, Wash.     

Reduction behaviour of powder mixtures of iron oxide and carbon in reactive atmospheres

Mixtures of fines of iron ore and carbon were kept in hot zone of furnace for various durations under flowing argon or mixture of hydrogen and carbon dioxide. Experiments were conducted at temperatures of 1150, 1250 and 1300 K. Other variables were inlet H₂/CO₂ ratio both oxidising and reducing to wustite, Fe₂O₃/C ratio in sample and sample size. 2 types of carbon were selected, viz. graphite and activated char. The degree of reduction of oxide (F) was determined by subsequent hydrogen reduction of reaction products. F vs. time data did not follow any set pattern due to complex mechanism and kinetics. Equilibrium calculations predict that, inlet H₂+CO₂ mixture which undergoes water gas shift reaction inside the furnace, reduces to wustite at a H₂/CO₂ ratio of 3. As expected, generally it yielded highest value of F. However at 1300 K, even the gas with H₂/CO₂ ratio of 1.5/1, although oxidising to wustite, gave comparably high value of F, presumably due to enhanced gasification rate at higher temperature. Although reactivity of graphite was much lower than that of activated char, they exhibited comparable extents of reduction.