Creep-rupture strength of a martensitic chromium steel after annealing between 750 and 950 °C

The influence of various temperature transients on the tirne-to-rupture of a martensitic chromium steel was investigated in tests in which the transient temperature and transient time, cooling rate, and test temperature were varied systematically. Above a temperature of approximately 800 °C an austenitic structure is produced which can be transformed back into martensite by rapid cooling to a temperature below approximately 200 °C. The time-to-rupture of this martensite may be reduced by a factor of five compared to the initial condition. At annealing temperatures below 780 °C (A_c1b) this loss in time-to-rupture is either less pronounced or non-existent. If austenite produced above some 800 °C is cooled slowly, pearlite of very low creep-rupture strength is generated, or the austenite remains in existence above 300 °C and, depending on the test temperature, also may have an unsatisfactory creep-rupture strength.     

Growth of recrystallised grains in 1.8% Si steel in temperature range 650 to 850°C

Three different types of grain growth were observed: uniform growth producing a regular microstructure; fast individual grain growth and finally a fast secondary recrystallisation initiated by the decarburisation of steel. Uniform growth follows parabolic kinetics. Above appr. 800°C a lower rate of growth is observed than it would be expected by extrapolation from lower temperature. Individual grain growth occurs in areas depleted of cementite and austenite and follows also parabolic kinetics. Nuclea for this type of growth are produced by the coalescence of smaller grains with similar space orientation. An apparent activation energy of 26 kJ/mole was determined for the process of coalescence, while for the uniform growth of recrystallised grains an apparent activation energy of 171 kJ/mole was established. Experimental data show that the texture rich with grains of orientation near to (111) in strips plane results from the greater density of such grains in recrystallised steel and the resulting faster formation of near (111) nuclea of sufficient size suitable for fast growth.     

Contribution to the investigation of microsegregation processes in 13% Cr 4% Ni steel during long-term annealing at 400°C

The present paper deals with the study of the development of microsegregation processes in the 13% Cr 4% Ni martensiteaustenite stainless steel during long-term annealing. The long-term annealing of 13% Cr 4% Ni steel at 400°C is accompanied by the decrease in notch impact toughness values, which is associated with an increasing tendency to the occurrence of the brittle failures. The conditions for the transition from the transcrystalline brittle failure to the intercrystalline brittle failure depend on the initial heat treatment affecting the achieved microstructure of investigated steel. The higher frequency occurrence of intercrystalline failure on the fracture surfaces of notch impact toughness specimens is accompanied by an enrichment of the prior austenite grain boundaries by phosphorus and nitrogen. At the same time the enrichment of intercrystalline fracture surfaces by nickel, or chromium was also observed. The application of an additional intercritical annealing after quenching accelerates the formation of intercrystalline failure towards shorter times during the isothermal annealing at 400°C.     

Phase equilibria in the system Na2CO3-CaCO3-CO2 at 750 to 900°C

Phase diagram of the binary system Na₂CO₃-CaCO₃. Determination of solid state phase equilibria by thermal analysis at 750 to 900°C and bar.     

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.     

Speciation of the Fe(II)–Fe(III)–H2SO4–H2O system at 25 and 50 °C

This work presents theoretical and experimental results on the speciation of the Fe(II)–Fe(III)–H₂SO₄–H₂O system in concentrated solutions (up to 2.2 m H₂SO₄ and 1.3 m Fe). The aim was to study the chemical equilibria of iron at 25 and 50 °C in synthetic aqueous sulphuric acid solutions that contain dissolved ferric and ferrous ion species. Raman spectroscopy, volumetric titration and conductivity measurements have been carried out in order to study the presence of specific ions and to characterize the ionic equilibrium. A thermochemical equilibrium model incorporating an extended Debye–Hückel relationship was used to calculate the activities of ionic species in solution. Model calculations were compared with experimental results. Model simulations indicate that anions, cations and neutral complexes coexisted in the studied system, where the dominant species were HSO₄⁻, H⁺, Fe²⁺ and FeH(SO₄)₂⁰. This indicated that these solutions showed a high buffer capacity due to the existence of bisulphate ions (HSO₄⁻), which presented the highest concentration. A decrease in the concentration of H⁺ and Fe³⁺ took place with increasing temperature due to the formation of complex species. Standard equilibrium constants for the formation of FeH(SO₄)₂⁰ were obtained in this work: log K_f⁰ = 8.1 ± 0.3 at 25 °C and 10.0 ± 0.3 at 50 °C.     

Physical properties in the temperature range from –180 to 400°C of nickel alloy steels for low-temperature applications

Nickel, which is the second neighbour to iron in the periodic table of elements, has a significant effect on most of the physical properties of the steels studied here, with the nickel content of the steels varying between 3 and 9%. Density and coercive force increase as the nickel content in the steel increases. By contrast, thermal expansion, modulus of elasticity and shear modulus decrease with increasing nickel content. Because of large scatter in the data, it is not possible to notice any significant influence of nickel content on Poisson's ratio and bulk modulus. Specific heat is also found not to vary with the nickel content of the steel. Electrical resistivity and thermal resistivity values increase, as expected, with increasing nickel content. The results of thermal conductivity measurements on 12 Ni 19 and X 8 Ni 9 have led to the development of modified Smith-Palmer relationships, which allow the calculation of the thermal conductivity values for the present set of steels and other steels with similar compositions.     

Elevated-pH bioleaching of a low-grade ultramafic nickel sulphide ore in stirred-tank reactors at 5 to 45 °C

This study is a continuation of previous work designed to assess the effect of elevated-pH bioleaching on a low-grade ultramafic nickel sulphide ore from Manitoba, Canada. The ore contains 21% magnesium and 0.3% nickel. Nickel is the only significant metal value, and is present primarily as pentlandite. A substantial fraction of the magnesium is present as lizardite, making processing of the ore difficult with conventional pyro- and biohydrometallurgical techniques. This work has two objectives: to maximize nickel extraction, and to minimize magnesium mobilization. Five-week stirred-tank bioleaching experiments were conducted with finely ground ore (− 147 µm) at three pH levels (3, 4 and 5) and five temperatures (5, 15, 22.5, 30, and 45 °C). The initial rate of nickel extraction from pentlandite was observed to be inversely correlated to acidity at all temperatures, while the final extraction of nickel after 5 weeks was determined to be moderately correlated to acidity at high temperatures and negatively correlated to acidity at low temperatures. The advantage of elevated-pH bioleaching was most evident at 5 °C, in which the final extraction of nickel at pH 5 was approximately 250% greater than at pH 3. Electron probe X-ray microanalysis of the post-leach residues revealed that the un-reacted lizardite was enriched with nickel during experiments conducted at pH 5, and that the extent of enrichment was a strong function of temperature. The undesirable extraction of magnesium exhibited a strong negative pH–temperature interaction and the consumption of sulphuric acid directly tracked the extraction of magnesium over all experimental conditions. Bioleaching at elevated pH substantially increased the ratio of nickel to magnesium in the leachate, and resulted in a substantial reduction in sulphuric acid consumption.     

The constitutive behaviour of a Nb-V HSLA steel in the temperature range of 900 to 975 °C

Isothermal true-stress true-strain flow curves of a Nb-V HSLA steel (0.13% C-1.55% Mn-0.028% Nb-0.059% V) are presented in this paper. Constitutive relations, valid at temperatures from 900 to 975 °C and at rates of strain from 0.005 to 5 s^?1, are derived. It is found that an Arrhenius-type equation, with a digital data bank storing its constants for different levels of normalized strains, represents the material's resistance to deformation adequately.     

The kinetics of carburization of γ-iron in CO-He and CO-H2 atmospheres at 920°C

Investigation of the carburization of thin iron foils in CO-He or CO-H₂ gas mixtures at 920°C in a flow apparatus by a resistance method. Discussion of the kinetics, mechanism of the chemical surface reaction steps in the carburization. Influence of the carbon diffusion in the solid iron on the kinetics of the carburization of thick samples.     

Microstructural Evolution and Change in Hardness of S30432 Heat‐resistant Steel during Creep at 650 °C

The microstructural evolution of S30432 heat‐resistant steel during creep at 650 °C and its effect on the change in hardness was investigated. The change of hardness during creep of S30432 at 650 °C can be divided into three stages. These are related to the precipitation and coarsening of ε‐Cu and M₂₃C₆ carbides, decrease in the number of twins and increase in grain size. The precipitation of ε‐Cu dominantly contributes to the significant hardening at stage I, and the coarsening of ε‐Cu is the key factor to decrease the hardness at stage II. At stage III, the hardness hardly changes since the microstructure of S30432 tends to be stable in the long‐term creep range.     

δ-γ phase equilibria in iron-rich iron-chromium-nickel alloys at temperatures between 1200 and 1350 °C

The δ-γ phase equilibria of iron-rich iron-chromium-nickel alloys with 23 to 38% Cr, 7 to 22% Ni are established at temperatures between 1200 and 1350 °C. The effect of chemical compositions and annealing temperatures on equilibrium δ ferrite fraction is discussed.     

Engineering approach to modelling the multiaxial creep and damage behaviour of compact tension geometry specimens of a 12 Cr steel at 550° C

Theoretical creep equations including tertiary creep were determined by fitting uniaxial constant load creep test results. Additionally, a new strain/time hardening rule was established. This theoretical approach was implemented in a finite‐element routine, and the load line displacement of a Cs20‐specimen of a 12 Cr steel under constant load was calculated as a function of time. The results were compared to experimental findings and correlated with creep damage by cavitation.