Phosphorus segregation in austenite in Fe–P–C,Fe–P–B and Fe–P–C–B alloys

The equilibrium grain boundary segregation of phosphorus was investigated in Fe–P–C, Fe–P–B and Fe–P–C–B alloys after austenitising at temperatures ranging from 825–1100 °C. The grain boundary concentrations were determined by Auger electron spectroscopy on intergranular fracture surfaces. Phosphorus, carbon and boron segregate to the austenite grain boundaries. The segregation of P in austenite occurs mainly in equilibrium, but some additional segregation takes place during quenching. Boron and, in a lesser degree, carbon were found to decrease the grain boundary concentration of phosphorus. The results can be explained by assuming equilibrium segregation and mutual displacement of these elements in austenite.     

Oxidation of Fe–18%Mn–0.6%C Steels in Air and a N2–CO2–O2 Mixed Gas Atmosphere at 1273–1473 K

Austenitic Fe–18 wt% Mn–0.6 wt% C steels were oxidized at 1273, 1373, and 1473 K for up to 2 h in either atmospheric air or an 85%N₂–10%CO₂–5%O₂ gas mixture. The alloys oxidized faster in air than in the mixed gas, but the morphology and composition of the oxide scale formed were similar in both atmospheres. The scales that consisted primarily of FeO, Fe₂O₃, and MnFe₂O₄ were highly susceptible to cracking and spallation due to the severe oxidation condition. Since Mn was consumed to form MnFe₂O₄, the original γ‐matrix changed to an α‐matrix in the subscale area, in which Mn‐rich internal oxide precipitates formed locally.     

Thermodynamics of Mn–Fe–C and Mn–Si–C system

The solubility of C in Mn melts with different contents of Si and Fe at 1400 and 1500°C was determined. With these data the interaction coefficients e^Si_C, e^Fe_C, e^C_C as well as γ^o_C were evaluated. The standard free energy of solution of C in liquid Mn based on 1 wt.% solution standard at 1400 and 1500°C were calculated, respectively. The solubility of C in liquid Mn formulated in relation to temperature was made as to the conformability of the present results with those given in the literature.     

Microstructures and Mechanical Properties of High‐Strength Fe‐Mn‐Al‐C Light‐Weight TRIPLEX Steels

High‐strength TRIPLEX light‐weight steels of the generic composition Fe‐xMn‐yAl‐zC contain 18 ‐ 28 % manganese, 9 ‐ 12 % aluminium, and 0.7 ‐ 1.2 % C (in mass %). The microstructure is composed of an austenitic γ‐Fe(Mn, Al, C) solid solution matrix possessing a fine dispersion of nano size κ‐carbides (Fe,Mn)₃ AlC_1‐x and α‐Fe(Al, Mn) ferrite of varying volume fractions. The calculated Gibbs free energy of the phase transformation γ_fcc → ?_hcp amounts to ΔG^γ→? = 1757 J/mol and the stacking fault energy was determined to Γ_SF = 110 mJ/m². This indicates that the austenite is very stable and no strain induced ?‐martensite will be formed. Mechanical twinning is almost inhibited during plastic deformation. The TRIPLEX steels exhibit low density of 6.5 to 7 g/cm³ and superior mechanical properties, such as high strength of 700 to 1100 MPa and total elongations up to 60 % and more. The specific energy absorption achieved at high strain rates of 10³ s^?1 is about 0.43 J/mm³. TEM investigations revealed clearly that homogeneous shear band formation accompanied by dislocation glide occurred in deformed tensile samples. The dominant deformation mechanism of these steels is shear band induced plasticity ‐SIP effect‐ sustained by the uniform arrangement of nano size κ‐carbides coherent to the austenitic matrix. The high flow stresses and tensile strengths are caused by effective solid solution hardening and superimposed dispersion strengthening.     

Multiscale Characterization of the Work Hardening Mechanisms in Fe–Mn Based TWIP Steels

When strained in tension, high‐manganese austenitic twinning induced plasticity (TWIP) steels achieve very high strength and elongation before necking. The main hypotheses available in the literature about the origin of their excellent work hardening include deformation twinning and dynamic strain ageing. In order to provide some answers, various experiments at different scales were conducted on Fe–Mn–C steels and the Fe–28 wt%Mn–3.5 wt%Al–2.8 wt%Si alloy. At a macroscopic scale, tensile tests were performed on all the studied grades. It was shown that, though the Fe–Mn–Al–Si based alloy retains very high elongation, the Fe–Mn–C steels properties are even more extraordinary. Tensile tests at different strain rates with the help of digital image correlation were also performed on the Fe–20 wt%Mn–1.2 wt%C steel to study the PLC effect occurring in this type of steel. It is suggested that supplementary hardening could come from reorientation of Mn–C pairs in the cores of the dislocations. At a microscopic scale, the Fe–20 wt%Mn–1.2 wt%C TWIP steel and the Fe–Mn–Al–Si grade were thoroughly investigated by means of in situ TEM analysis. In the Fe–Mn–C steel, the formed twins could also lead to a composite effect, since they contain plenty of sessile dislocations. In the Fe–Mn–Al–Si alloy, mechanical twins are thicker and contain fewer defects, leading to a lower work hardening than the other grade.     

Improving High‐Temperature Wear Resistance of Fe–Cr13–C Hardfacing Alloy by Nitrogen Alloying

Nitrogen alloying of Fe–Cr13–C hardfacing alloy produces marked precipitation strengthening to achieve an improvement in high‐temperature wear resistance. Two hardfacing alloys of Fe–Cr13–C (with and without nitrogen) are slid on carbon steel at high‐temperature of 600°C and high load of 600 N, and wear behaviors are studied systematically. It is found that abrasive wear occurrs on the surface of the hardfacing alloy due to abrasive action of crushed oxide particles coming from the surface of carbon steel on the high temperature. The wear resistance is determined by the size and distribution of precipitates. The results show that the hardfacing alloy can obtain a great increase in hardness and a marked decrease in wear depth of grooving due to the effect of carbonitirde precipitates. The high‐temperature wear resistance of the Fe–Cr13–C hardfacing alloy is improved by nitrogen alloying, and the wear mechanism is mainly plastic deformation with minimum depth of grooving caused by the oxide particles.     

Spinel deoxidation equilibria in the Fe—Mn—Al—O system – experiment and computation

The concentration and chemical potential of oxygen in liquid Fe—Mn alloys equilibrated with the spinel solid solution, (Fe, Mn)Al_2+2xO_4+3x, and α-Al₂O₃ have been determined at 1873 K as a function of manganese concentration. The composition of the spinel phase has been determined using electron probe microanalysis. The results are compared with data reported in the literature. The deoxidation equilibrium has been computed using data on free energy of solution of oxygen in liquid iron, free energies of formation of hercynite and galaxite, and interaction parameters reported in the literature. The activity-composition relationship in spinel solid solution was derived from a cation distribution model. The model is in excellent agreement with the experimental data on oxygen concentration and potential and the composition of the spinel phase.     

The Development of a Processing Window for the Continuous Galvanizing of a Mn–Cr–Si Martensitic Steel

Martensitic or complex phase steels are leading candidates for automotive impact management applications. However, achieving high strengths while obtaining high quality coatings via continuous galvanizing is a challenge due to cooling rate limitations of the processing equipment and selective oxidation of alloying elements such as Cr, Mn, and Si adversely affecting reactive wetting. The galvanizability of a Cr? Mn? Si steel with a target tensile strength above 1250 MPa was investigated within the context of the continuous galvanizing line. The continuous cooling transformation behavior of the candidate alloy was determined, from which intercritical and austenitic annealing thermal cycles were developed. The evolution of substrate surface chemistry and oxide morphology during these treatments and their subsequent effect on reactive wetting during galvanizing were characterized. The target strength of 1250 MPa was achieved and high quality coatings produced using both intercritical (75% γ) and austenitic (100% γ) annealing using a conventional 95%N₂–5%H₂, ?30°C dew point process atmosphere and 0.20 wt% dissolved (effective) Al bath, despite the presence of significant Mn and Cr oxides on the substrate surfaces. It is proposed that complete reactive wetting by the Zn(Al, Fe) bath was promoted by in situ aluminothermic reduction of the Mn and Cr‐oxides by the dissolved bath Al.     

The Thermodynamic Properties of Fe‐Mn‐C Melt at Reduced Pressure

The solubility of carbon in the Fe‐Mn‐C system was measured under reduced pressure of 1 Pa at 1573 K and 1673 K, respectively. The carbon solubility in terms of mole fraction in the iron based solution was found varying with manganese mole fraction in the solution and with temperature, as x_C= 0.1819 + 0.2531 x_Mn at 1573 K and x_C = 0.1981 + 0.2515x_Mn at 1673K. Manganese has a stronger influence on the carbon solubility at lower pressure than it has at higher pressure. The pressure was found to have an insignificant effect on the carbon solubility when manganese in the melt was absent. The activity interaction parameters between manganese and carbon in the alloy were determined from carbon solubilities at 1573 and 1673 K. Analysis of the experimental results showed that under vacuum conditions, the activity interaction parameter of manganese with carbon is higher than that at atmospheric pressure.     

Enhancement of Pearlite Nucleation by Deformation of Austenite Matrix in an Fe–12Mn–0.8C Alloy

The effect of plastic deformation of austenite on the nucleation of pearlite was investigated using an Fe–12Mn–0.8C (mass%) alloy. Slight warm deformation of austenite prior to pearlite transformation effectively accelerates the intragranular nucleation of pearlite although intragranular pearlite is hardly formed without deformation. Formation of pearlite at annealing twin boundaries is promoted in the warm‐rolled specimens. Additionally MnS particles are activated as intragranular nucleation sites of pearlite. Cold rolling of austenite introduces many deformation twins in the alloy used. Subsequent isothermal transformation heat treatments result in nucleation of pearlite at intersections of the deformation twins. It is concluded that incoherent portions introduced by deformation onto the annealing or deformation twin boundaries are effective nucleation sites in the pearlite transformation.     

Sulphidation studies of Fe—15Cr—4Al alloy in H2—H2S environment at 850°C

The sulphidation behaviour of Fe—15Cr—4Al alloy has been investigated under low sulphur vapour pressure of 10^?9 to 10^?3 atm in the temperature range of 700–1000°C. Distinct changes in sulphide scale morphology were observed with the increase of partial vapour pressure of sulphur (ps₂). Triplex and duplex sulphide layers were observed at higher and lower ps₂ respectively. Parabolic rate constants of sulphidation of this alloy are strongly dependent on ps₂ and activation energies of the order of 83,6 kJ/mole suggesting diffusion of iron as a rate controlling step.     

Effects of Carbon on the Corrosion Behaviour in Fe‐18Cr‐10Mn‐N‐C Stainless Steels

The effects of carbon on general and pitting corrosion behaviour of Fe‐18Cr‐10Mn‐(0.33～0.44)N‐(0～0.38)C alloys were investigated using potentiodynamic tests. Carbon made the nitrogen‐bearing alloys inert and thus promoted general corrosion resistance. These results were supported by experimental findings, such as elevated corrosion potential, reduced active dissolution rate, lowered passive potential and accelerated hydrogen evolution rate in sulphuric acid solution. The resistance to pitting corrosion in chloride solution was also enhanced by the addition of carbon, which was attributed to the improvement of the stability of the passive film. XPS analysis revealed that the cationic fraction of chromium in the passive film was increased and hence the protection ability of the film was improved by the carbon addition.     

Microstructural hot corrosion behaviour of austenitic Fe-29.7Mn-8.7Al-1.04C alloy in fused Na2SO4 and Na2SO4-NaCl mixture

A short-time hot corrosion test was performed on the austenitic Fe-29.7Mn-8.7Al-1.04C alloy in sodium sulphate at 900°C. The corrosion scales formed on the alloy investigated were studied by scanning electron microscopy and X-ray mapping techniques. The hot-corrosion scales morphology of the austenitic Fe-29.7Mn-8.7Al-1.04C alloy were characterized by the formation of Al₂S₃ and α-MnS sulphides at and beneath the internal alumina scale. No fluxing of the scales was observed. The effect of the addition of sodium chloride to the fused sodium sulphate on the hot corrosion scales morphology was also investigated.     

High Cycle Fatigue of (Fe,Ni, Co)3V type alloys

High cycle fatigue tests in vacuum have been performed on ordered (Fe, Co, Ni)₃V alloys between 25 °C and 850 °C. Heat-to-heat variations in fatigue properties of a Co-16.5 wtpct Fe-25 pct alloy, LRO-1, appeared to be due to differing quantities of grain boundary precipitates. Modification of this alloy with 0.4 pct Ti, to produce an alloy designated LRO-23, reduced the density of grain boundary precipitates and increased ductility, resulting in superior fatigue strength at high temperatures. The fatigue lives of LRO-1 and LRO-23 decreased rapidly above 650 °C, and increased intergranular failure was noted. The fatigue resistance of a cobalt-free alloy, Fe-29 pct Ni-22 pct V-0.4 pct Ti (LRO-37), was examined at 25 °C, 400 °C, and 600 °C; there was little evidence for intergranular fracture at any of these temperatures. Fatigue behavior of the LRO alloys is compared to that of conventional high temperature alloys.     

Oxygen and carbon sensing in Fe—O—C and Fe—O—C—Xn melts at elevated carbon contents

In the present study on solid electrolyte probes, the attempt was made to measure accurate and reproducible EMF values representing the extremely low oxygen activities in Fe—O—C melts at varying C contents. Various types of sensors were designed and successfully tested in laboratory experiments. Reliable oxygen activities were measured in Fe—O—C melts up to 4 wt. % C under pure CO gas, and f_o and f_c values were derived as a function of C content at 1 400 to 1 600°C. Further measurements were made in Fe—O—4 wt. % C—X_n melts at various contents of X_n (S_n = Si, Mn, Cr). Moreover, a solid oxide electrolyte probe with a CO gas channel to measure oxygen and carbon activities was developed and successfully tested in high-carbon iron melts.     

Segregation of phosphorus and boron in austenite in Fe–10%Mn–P–B-alloys

Equilibrium segregation of phosphorus at the grain boundaries of austenite has been studied in Fe–10%Mn–P–B-alloys. The samples were equilibrated at temperatures of 750–1100°C and analysed after rapid quenching using Auger-electron spectroscopy. The results show that boron markedly reduces segregation of phosphorus in austenite. Boron was also found to be segregated at the prior austenite grain boundaries. The intergranular boron concentration increases slightly with a rising austenitising temperature, but does not show any dependence on boron or phosphorus content of the alloy. The results can be explained by assuming segregation equilibria and mutual displacement between B and P in austenite, a value of ?97 kJ/mole for the free energy of boron segregation and ?47 kJ/mole for phosphorus segregation.     

Mechanical Properties and Microstructure Evolution During Deformation of Fe–Mn–C TWIP Steel

The mechanical and deformation microstructure properties of the Fe–Mn–C TWIP steel was investigated by means of tensile experiment, in situ scanning electron microscope (SEM) and transmission electron microscope (TEM).The results showed that the sample has excellent mechanical with tensile strength of the steel is about 1140 MPa and the yield strength is higher than 480 MPa, while the elongation is above 57%, the true stress–strain curve from tension tests exhibited repeated serrations and its strain‐hardening rate is constantly changing. It is found that there were different deformation mechanisms at different deformation stages result in the unique true stress–strain curve. Dislocation slip dominated the initial deformation and with the accumulation of deformation stress concentration reached the twin shear stress resulting in twin shear, which lead to TWIP effect. As the strain capacity increased continually, the parallel twins can no longer rotate and shear deformation occurred, which lead to the forming of shear bands. The intercoordination of slip deformation, twin deformation, and shear deformation mechanism make the TWIP steel show high strength and high plasticity.     

Effect of carbon on the solidification morphology of Fe—Mn—Al steels

An investigation into the effect of carbon content on the weld solidification morphologies for two heats of Fe—Mn—Al steels was conducted. Preliminary results set the stage for extended research into weldability properties of a potentially invaluable steel.     

高Al低密度钢的高温氧化行为

为观察Fe-Mn-Al-C系高Al低密度钢在高温下的抗氧化性能,采用氧化增重法测定了4种合金在950~1 100℃下的氧化动力学曲线。通过SEM观察氧化层形貌特征,利用线扫描能谱分析氧化后合金元素的变化,分析了28Mn-10Al合金氧化膜发生膨胀破裂的原因。结果表明,Al元素提高钢的抗氧化性,而Mn元素则降低Al的抗氧化性。抗氧化性较好的28Mn-12Al、20Mn-10Al合金表层未发现Fe的氧化,而氧化严重的28Mn-8Al合金氧化层Fe元素含量较高。28Mn-12Al合金抗氧化性最好,氧化膜的连续性及致密性优于其他3种合金,氧化物连续且晶粒细小,在氧化过程中具有一定的防护作用。     

Effects of Strip Casting and Annealing on Electrochemical Properties of LPCNi3.55Co0.75Mn0.4Al0.3 Hydrogen Storage Alloys

The effect of thickness (1～10 mm) of the ingots on the electrochemical properties of as-cast and annealed strip cast LPCNi3.55Co0.75Mn0.4Al0.3 hydrogen storage alloys was investigated. It is found that the 0.2 C discharge capacity of as-cast LPCNi3.55Co0.75Mn0.4Al0.3 alloy increases with the increase of the thickness of the ingots. As-cast alloy with the thickness of 10 nun shows better activation property, higher 1C discharge capacity and better cyclic stability than others.It is mainly contributed to its larger unit cell volume and less internal stress. Annealed LPCNi3.55Co0.75Mn0.4Al0.3 alloywith the thickness of 3 mm shows much better comprehensive electrochemical properties than as-cast one; The cyclic stability of the alloy with the thickness of 6 mm and the activation properties of the alloys with the thickness of 3～6 mm are improved after annealing. It is mainly owing to the great release of intemal stress and the decrease fo the segregation of Mn in the alloys.