The influence of HCl on the oxidation of iron at elevated temperatures

A study has been made of the effects of 0.1 to 1.0% HCl on the oxidation of iron in argon – 20% O₂ at 600 °C and 700 °C. The overall weight‐change kinetics included contributions from weight increases due to growth of oxide and chloride scale by solid‐state diffusion and to deposition of oxide following vaporization of FeCl₂ and subsequent reaction with oxygen, and weight losses due to complete removal of FeCl₂ vapour to the environment. The weight gains in HCl‐containing gases were less than that in the HCl‐free gas at 600 °C, but greater than that in the HCl‐free gas at 700 °C. The kinetics could be described by a modified Tedmon equation; it was found that much of the scale growth resulted from the chloride‐to‐oxide reaction following vaporization of FeCl₂ from local sites on the metal surface while the contributions from solid‐state diffusion were relatively small.     

Sheet metal forming at elevated temperatures

Due to the need to significantly reduce the part weights in automotive manufacturing, the use of lightweight materials becomes ever more important. Unfortunately, these materials are often associated with a limited cold formability. Due to this fact, production of large, complex sheet metal components using forming technology frequently entails increased expenditures. Moreover, processing high-strength materials requires correspondingly high processing forces and pressures. In order to find a solution to counter the disadvantages mentioned above, the use of elevated temperatures as a process parameter in forming operations represents a potential solution approach. The following paper will give an overview about research activities in the field of forming operations at elevated temperatures.     

Oxide scale adhesion and impurity segregation at the scale/metal interface

The chemistry at scale/metal interfaces was studied using scanning Auger microscopy after removal of the scale in ultra-high vacuum using an in situ scratching technique. Al₂O₃ and Cr₂O₃ scales formed between 900°C and 1100°C on Fe-18 wt.% Cr-5 wt.% Al and on Ni-25 wt.% Cr alloys, respectively, were investigated. The adhesion of these scales was determined qualitatively by way of micro-indentation and scratching on the surface oxide. All of the alumina scales fractured to the same degree to expose the metal surface, regardless of the oxidation temperature. The chromia-forming alloy on the other hand, developed more adherent scales at lower oxidation temperatures. About 20 at.% sulfur was found at the metal surface in all cases, and its presence was not only detected on interfacial voids, but also on areas where the scale was in contact with the alloy at temperature. Results from this study clearly demonstrated that sulfur as an alloying impurity does segregate to the scale/alloy interface. However, for alumina scales and chromia scales, the effect of this segregation on oxide adhesion is noticeably different.     

Tribological behaviour of borided bearing steels at elevated temperatures

Borided steels are known to exhibit excellent wear resistance at room temperature. However, the sliding wear behaviour of borided steels at high temperatures is not known. In the present study, AISI 440C and 52100 bearing steels which are extensively used in industry, were borided by pack method at 950 °C for 2 h. X-ray diffraction analysis of boride layers on the surface of steels revealed various peaks of FeB, Fe₂B and CrB. The thickness and hardness of boride layers on the 52100 and 440C steels were 56 ± 6 and 47 ± 4 μm and 1970 and 2160 HK, respectively. Dry sliding wear tests of these borided steels were performed against Si₃N₄ bearing ball at a constant sliding speed and load at elevated temperatures. The temperature changed between room temperature and 600 °C. These tests indicated that the wear rates of unborided and borided steels increase with temperature and borided 52100 and 440C steels exhibit considerably lower wear rate at all temperatures, compared with unborided steels. At temperature of 600 °C, borided 52100 and 440C steels have a wear resistance of about 3 and 2.5 times higher than that of unborided steels, respectively. Examination of the worn surface of borided steels showed that, worn surfaces were covered with a discontinuous compact layer especially above temperature of 300 °C.     

Oxidation behaviour of PACVD TiBN coating at elevated temperatures

The elevated-temperature oxidation behaviour of a TiBN coating on a plasma-nitrided hot-work tool steel (DIN 1.2367) by means of plasma-assisted chemical vapour deposition (PACVD) was investigated under the condition where a coated die would be preheated prior to being mounted on the press for aluminium extrusion. The TiBN coating was found to possess good resistance to oxidation up to 400 °C. Rapid oxidation started to occur at 450 °C. Radio frequency glow discharge optical emission spectroscopy (rf-GDOES) indicated that the oxidised layer was thickened from 100 nm to 1.0 μm, as the soaking time at 500 °C was prolonged from 2 to 16 h, which was attributed to the high temperature that promoted the penetration of oxygen into the coating. rf-GDOES also showed that boron initially in the coating vanished from the oxidised layer when the temperature was 450 °C or higher. X-ray diffractometry confirmed that the oxidised layer was composed mainly of TiO₂. SEM revealed that the TiO₂ layer was pulverised, leaving many microcracks and cavities, as a result of the losses of boron oxide and nitrogen. The rapid oxidation at above 450 °C was attributed to the pulverised TiO₂ layer that was unable to hinder the diffusion of oxygen into the coating. It is therefore recommended to apply a protective gas during the preheating of the TiBN-coated die for aluminium extrusion. Alternatively, an advanced TiBN coating with enhanced resistance to oxidation must be developed, which will be conducive to its application for aluminium extrusion dies.     

Nonstoichiometry and diffusivities in iron monosulphide at elevated temperatures

The equilibrium diagram of the iron monosulphide phase has been reviewed in the temperature range 670–1100°C. Thermodynamic properties of the system are derived as a function of composition. Data for the parabolic rate constant for the growth of iron monosulphide on iron are presented as a function of nonstoichiometry for relatively small values of y in Fe_1-yS in the range 750–902°C. From these data the interdiffusion coefficient and the iron self-diffusion coefficient have been derived as a function of the metal deficit in Fe_1-yS.The calculated self-diffusion coefficient seems to be in good agreement with iron tracer diffusion data from the literature. The results indicate that the self-diffusion coefficient is practically independent of composition for values of y less than 0·01 and 0·02 at 750°C and 902°C, respectively. The self-diffusion coefficient is, however, found to be proportional to the square of the metal deficit for 0·015 < y < 0·055 at 750°C and 818°C. Other data show that the iron tracer diffusion coefficient in Fe_1-yO is proportional to the square of the iron deficit within experimental error. The defect structure of Fe_1-yS is discussed in view of the results obtained, but no definite conclusions are given.     

The influence of ceria surface additions on manganese oxidation at high temperatures

The kinetics, scale composition, and growth mechanism of ceria-coated and blank specimens of manganese oxidation in air were examined. The scale growth obeys the parabolic rate law at 700°C for all specimens. Lower parabolic rate constants for coated specimens are attributed to the presence of a CeO₂ external scale. It constitutes a limiting factor of the oxygen activity at the gas-oxide interface. This lower-oxygen activity leads to a less-metal-deficient state of the scale. Due to this, the inner-MnO scale becomes more adherent to the substrate. Preheating at 700°C, in hydrogen (P_{O 2}=10^–24 atm), was performed in order to be placed in the MnO stability domain and try to introduce cerium in the manganese-oxide scale. This pretreatment promotes macroscopic bonding in the layer formed during subsequent oxidation in air. It ensures a better scale adherence. A new diffusional-transport mechanism in manganosite is proposed in accordance with all experimental observations of the literature and with the cerium-manganese-oxygen system studied in the present work. This model considers the high Mn³⁺ stability in octahedral sites of the MnO oxygen ion body. Low-oxygen partial pressure conditions permit the formation of an adherent inner-MnO scale on coated specimens. A CeO₂ scale formed above the MnO scale; MnO is present as a minor component in this scale and it is located mainly at the internal interface. The difficulties in forming the cerium-orthomanganite are attributed to the very high stability of MnO related to this wide range of nonstoichiometry and to the low manganese diffusivity through the cerium-containing scale.     

Shear strength and sliding at a metal–ceramic (aluminum–spinel) interface at ambient and elevated temperatures

We show that the shear strength of aluminum–spinel (Al–MgAl₂O₄) interfaces at room temperature, as measured by the periodic cracking method, increases by a factor of 2 when the interface is annealed at 625 °C for 1 h. The same method was used to measure the interfacial sliding resistance at elevated temperatures, following the work of Jobin et al. on copper–silica interfaces. A simplified analysis for studying diffusional sliding by the periodic cracking technique is presented. The crack spacing distributions at ambient temperature and at elevated temperature are quite different. While the ambient temperature spacing is bounded by a factor of 2 (as predicted), the distribution is much broader at elevated temperature. This discrepancy is tied to the time-dependent evolution of the interfacial tractions when the experiment is carried out at a high temperature.     

The oxidation behaviour of an AZ91D magnesium alloy at high temperatures

As-cast AZ91D magnesium alloy was exposed to air in the temperature range from 470 to 800 K for time intervals up to 10 h. Thermogravimetric measurements revealed three distinct stages of the reaction where an initial formation of protective oxide was followed by an incubation period with a subsequent transient to non-protective oxidation, at a rate either constant or sharply increasing over time. The approximate temperature and time frames for an onset of each stage were identified. A strong link was found between the oxidation kinetics and the scale morphology. The initial protective film, grown anisotropically over the microstructural features of the substrate, was transformed to oxide ridges. The non-protective oxidation was associated with a formation of oxide nodules and their further coalescence into a fine-grained scale of a loose structure. All the morphologies were comprised of a randomly oriented magnesium oxide MgO with traces of MgAl₂O₄ spinel. The oxidation mechanism represented a complex reaction where morphological and phase transformations within the alloy substrate were accompanied by magnesium sublimation/evaporation and subsequent condensation within the scale pores or cracks, and superimposed on the reaction with oxygen. Some implications for the high temperature processing of magnesium alloys are discussed.     

Estimate of the damage sustained by metal operating at elevated temperatures under stress

Conclusion Examination of the dislocation structure makes it possible to expose in a timely fashion the outset of the accelerated stage of creep in a metal that has been in service for a prolonged period — the formation of recrystallized volumes in the metal. Steam pipes with this structure should be replaced.Sverdlovénergoremont. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 60–61, October, 1985.     

The influence of silicon on the oxidation properties of iron

Five Fe—Si alloys containing between 0·06 and 5·3 w/o have been oxidized in a gaseous mixture of 20 w/o oxygen in argon. The oxidation temperatures were 500°, 625°, 800°, 900° and 1000°C. The reaction kinetics have been studied gravimetrically and the oxidized samples examined microscopically, with an electron microprobe and with X-ray methods. The detrimental effect of silicon on the oxidation rate of iron is most pronounced at high temperatures and silicon contents. For comparative purposes, the oxidation of two chromium-nickel alloyed steels has been studied at 800° and 1000°C.     

Effect of Ca addition on the oxidation resistance of AZ91 magnesium alloys at elevated temperatures

AZ91 magnesium alloys containing 0.27–5.22 wt.% Ca, were melted and cast to study the effects of Ca addition on oxidation resistance at elevated temperatures. An ignition temperature test showed that the ignition of AZ91 alloy occurred at about 350–450 °C below the melting point, whereas that of the Ca-containing AZ91 alloys did so at above 650 °C. Weight gain measurements indicated that the oxidation resistance of the AZ91 alloys improved with Ca addition. The oxidation rate was dependent on the oxidation temperature. In the temperature range of 300–400 °C, the oxidation rate increased linearly. By contrast, the weight of 5 wt.% Ca-containing AZ91 alloy increased slowly due to the formation of a protective oxide layer. The oxidized surfaces were analyzed with low-angle XRD, FE-SEM equipped with EDS and AES. Complex structures were found in the oxide layers of the Ca-containing alloys: the outer layer mainly consisted of CaO, which was of uniform thickness, and the inner layer was a mixture of CaO, MgO, and Al₂O₃. In contrast to the loose and porous MgO formed on the surface of AZ91, the compact and dense oxide layers acted as an effective barrier to the further oxidation of the Ca-containing AZ91 alloys.     

Unusual oxidation behaviour of Zr50Cu50 alloy at high temperatures

The oxidation of Zr₅₀Cu₅₀ alloy at 500-700 °C is characterized by preferential oxidation of zirconium, while the excess of copper is accumulated at the alloy-oxide interface forming the Zr₁₄Cu₅₁ phase. The strong reaction at 800 and 850 °C resulted in the total corrosion of the specimens in 21 and 15 h, respectively. The oxidation at elevated temperatures showed an anomalous decrease of the oxygen consumption rate in the temperature range 930-1000 °C, corresponding to the preferentially oriented crystallites of ZrO₂ in the oxide scale at 900 and 1000 °C. The oxide layer consists of ZrO₂ and CuO in the whole temperature interval of the oxidation. The reaction kinetics obeys a parabolic rate law. An activation energy of 92.0 ± 0.3 kJ/mol has been estimated.     

Flow behaviour and constitutive modelling of a ferritic stainless steel at elevated temperatures

The flow behaviour of a ferritic stainless steel (FSS) was investigated by a Gleeble 3500 thermal-mechanical test simulator over the temperature range of 900–1100 °C and strain rate range of 1–50 s^?1. Empirical and phenomenological constitutive models were established, and a comparative study was made on the predictability of them. The results indicate that the flow stress decreases with increasing the temperature and decreasing the strain rate. High strain rate may cause a drop in flow stress after a peak value due to the adiabatic heating. The Zener-Hollomon parameter depends linearly on the flow stress, and decreases with raising the temperature and reducing the strain rate. Significant deviations occur in the prediction of flow stress by the Johnson-Cook (JC) model, indicating that the JC model cannot accurately track the flow behaviour of the FSS during hot deformation. Both the multiple-linear and the Arrhenius-type models can track the flow behaviour very well under the whole hot working conditions, and have much higher accuracy in predicting the flow behaviour than that of the JC model. The multiple-linear model is recommended in the current work due to its simpler structure and less time needed for solving the equations relative to the Arrhenius-type model.     

The influence of borate coatings on the high-temperature oxidation of iron

Coatings of di-sodium tetraborate and of calcium metaborate have been found to protect iron against oxidation in the temperature range 750°–1050°C in oxygen. Protection is due to the compound iron boro-ferrite (4 FeO.Fe₂O₃.B₂O₃) that forms as a non-coherent, granular, blocking layer along the scale/metal interface, which effectively decreases the interfacial area for iron ion diffusion. Void formation, associated with the layer of complex oxide, may possibly enhance the blocking effect.     

Interfacial shear strength behaviour of Ti/SiC metal matrix composites at room and elevated temperature

Synchrotron X-ray strain measurements have been used to follow the distribution of elastic strain, and hence interfacial shear stress, along single SCS-6 and Sigma SM2156 Ti–6Al–4V matrix coated SiC monofilaments sandwiched between Ti–6Al–4V foils during single-fibre-fragmentation testing. The interfacial shear strength behaviours were characteristically different. For the SCS-6 system, the interfacial response was dominated by classical frictional sliding, initiating near the ends and progressing along most of the length of the fragments, in common with previous observations. By contrast, for the SM2156 fibre system, a significant threshold stress was evident which must be exceeded before sliding could occur. As a result, sliding was only found near the ends of the fibre fragments. Upon unloading, reverse frictional sliding was found to take place, initiating from the fibre ends at a shear stress somewhat lower than that for forward sliding. Finite element modelling suggests that this is due to a reduction in the radial fibre clamping stress upon unloading rather than a change in the friction coefficient. For both the systems, the frictional sliding strength fell approximately linearly with increasing temperature towards zero at ～600–700 °C, consistent with a Coulomb friction coefficient of ～0.4 and thermal clamping residual stresses that approach zero at these temperatures. By contrast, the threshold stress required to initiate sliding for SM2156 falls at a slower rate with increasing temperature, such that it would still be significant at these temperatures. Some evidence was found for a decrease in interfacial shear stress with decreasing fragment length.     

Kinetics and morphological development of the oxide scale on iron at high temperatures in oxygen at low pressure

The oxidation properties of iron have been investigated at temperatures in the range 800–1000° C in oxygen over the pressure range 0.3–760 Torr. A duplex scale consisting of wustite and magnetite was formed during the earliest intervals examined. Hematite grew on the magnetite surface after an induction period which decreased with increasing oxygen pressure; this oxide developed as whiskers and platelets at temperatures less than 860° C and as small grains at higher temperatures. Iron transport occurs through the scale and involves short-circuit diffusion in the hematite layer. The oxidation kinetics obeyed a parabolic law independent of oxygen pressure since multilayer scale growth was directly dependent on the lattice diffusivity of iron and the iron gradient established in the wustite layer.This work was sponsored by the National Research Council of Canada.     

Role of kinetic effects on metal partition at the alloy-scale interface during the oxidation of alloys

An alternative mass balance at the alloy-scale interface is proposed for the Wagner theory of binary alloy oxidation. A simple relation between the bulk composition of the alloy and the scale composition at the alloy-scale interface, , is derived in terms of the transport properties of the scale and the limit of application of the relation defined. There is good agreement between calculated and measured values of .Supported by a Science Research Council grant.     

Effect of Ca and Y additions on oxidation behavior of AZ91 alloy at elevated temperatures

In order to develop the ignition-proof magnesium alloy, the effect of alloying elements, Ca and Y, on the oxidation behavior of AZ91 magnesium alloy at elevated temperatures was investigated. The ignition-proof performance, oxide products and oxidation kinetics of Ca- and Y-containing AZ91 alloys were studied. The results indicate that the proper addition of Ca can increase the ignition point of AZ91 alloy greatly. However, the oxide film of Ca-bearing AZ91 alloy formed at elevated temperature is thick and brittle, which is prone to crack in melting and cooling process. In addition, the oxide film of AZ91-xCa alloy is incompact and cannot inhibit the diffusion of reaction particles. The oxide film of AZ91-xCa alloy turns to thin and plastic one after Y is added, and the density of the oxide film increases greatly due to the formation of composite oxide film composed of MgO, CaO and Y2O3.