Evaluation of tempering behaviour in modified 9Cr–1Mo steel by magnetic non-destructive techniques

Modified 9Cr–1Mo steels were normalized at 1050 °C/30 min and tempered over a wide range of temperatures to observe the effect of tempering temperature on the material properties. The material revealed mechanical softening when tempered at temperature less than the lower critical temperature (～810 °C: Ac₁). Beyond Ac₁ temperature the materials strength increased drastically due to the formation of fresh martensite. Magnetic Hysteresis Loop (MHL) and Magnetic Barkhausen Emissions (MBE) techniques were used to evaluate the magnetic properties of the materials with tempering. Magnetic softening was observed by tempering the material below the Ac₁ temperature where a decrease in coercivity and an increase in RMS voltage of the MBE were found. Tempering beyond Ac₁ magnetic hardening was observed by the increase in coercivity and the decrease in RMS voltage of MBE. Such results revealed that magnetic techniques could be a better tool for the evaluation of tempering of modified 9Cr–1Mo steel.     

The effects of Cr and Al concentrations on the oxidation behavior of oxide dispersion strengthened ferritic alloys

The oxidation of six oxide dispersion strengthened (ODS) ferritic alloys was investigated at 1050 °C in air up to 200 h. Al plays the dominant role in improving the oxidation resistance of the ODS alloys. Cr and Y are of importance in forming the stable Al₂O₃ scale. To produce the dense alumina layer with enhanced adherence to the metal substrate, the concentrations of Al and Cr should be larger than 2 and 14 wt.%, respectively.     

Mechanical alloying of lanthana-bearing nanostructured ferritic steels

A novel nanostructured ferritic steel powder with the nominal composition Fe–14Cr–1Ti–0.3Mo–0.5La₂O₃ (wt.%) was developed via high energy ball milling. La₂O₃ was added to this alloy instead of the traditionally used Y₂O₃. The effects of varying the ball milling parameters, such as milling time, steel ball size and ball to powder ratio, on the mechanical properties and microstructural characteristics of the as-milled powder were investigated. Nanocrystallites of a body-centered cubic ferritic solid solution matrix with a mean size of approximately 20 nm were observed by transmission electron microscopy. Nanoscale characterization of the as-milled powder by local electrode atom probe tomography revealed the formation of Cr–Ti–La–O-enriched nanoclusters during mechanical alloying. The Cr:Ti:La:O ratio is considered “non-stoichiometric”. The average size (radius) of the nanoclusters was about 1 nm, with number density of 3.7 × 10²⁴ m^?3. The mechanism for formation of nanoclusters in the as-milled powder is discussed. La₂O₃ appears to be a promising alternative rare earth oxide for future nanostructured ferritic steels.     

Influence of the dissolved oxygen content on corrosion of the ferritic–martensitic steel P92 in supercritical water

The corrosion of the ferritic–martensitic steel P92 exposed to supercritical water at 550 °C under 25 MPa with the dissolved oxygen contents of 100, 300 and 2000 ppb was investigated. The results indicated that the weight gain increased with the dissolved oxygen content. The oxide scale with a typical dual-layered structure including a Fe-rich outer magnetite layer and a Cr-rich inner layer was formed on all samples in spite of different dissolved oxygen. Finally, the possible explanations for the influence of the dissolved oxygen content on the weight gain and exfoliation of oxide scale were given.     

Microstructure and stress rupture property of titanium-containing 11Cr ferritic/martensitic steels

The microstructure and stress rupture behavior of 11Cr ferritic/martensitic steels with 0.02 wt.%Ti (low Ti) and 0.14 wt.%Ti (high Ti) have been studied. The steels are prepared by vacuum induction melting followed by hot forging and rolling into plates. The results show that titanium is easy to combine with oxygen and other elements to form complex inclusions. Large MX particles with 1–3 μm are found in the high titanium steel. Most of the large MX particles have a TiO₂ cored structure. After normalizing at 1100 °C for 1 h, cooled in air and tempering at 750 °C for 1 h, nano-sized MX precipitates distribute densely near martenstic lath boundaries in the high titanium steel. The large MX particles cannot be dissolved even at austenitizing temperature up to 1300 °C. Creep cracks nucleate at the interface between matrixes and the large MX particles or titanium-containing oxide inclusions.     

Corrosion behavior of an alumina forming austenitic steel exposed to supercritical carbon dioxide

Microstructure characterization of corrosion behavior of an alumina forming austenitic (AFA) steel exposed to supercritical carbon dioxide was conducted at 450–650 °C and 20 MPa. At low temperature and short exposure times, the oxidation kinetics were parabolic and the oxide scales were mainly composed of protective and continuous Al₂O₃ and (Cr, Mn)-rich oxide layers. As the temperature and exposure time increased, the AFA steel gradually suffered breakaway oxidation and its oxide scales showed a multilayer structure mainly composed of Fe₃O₄, (Cr, Fe)₃O₄, NiFe/FeCr₂O₄/Cr₂O₃/Al₂O₃, FeCr₂O₄/Al₂O₃, and NiFe/Cr₂O₃/Al₂O₃, in sequence. The corrosion mechanism based on the microstructure evolution is discussed in detail.     

A new 3D multiphase FE model for micro cutting ferritic–pearlitic carbon steels

A new three-dimensional multiphase finite element computation model is proposed for the simulation of micro drilling two-phase ferritic–pearlitic carbon steels in order to understand the cutting, ploughing, tribological and heat transfer mechanisms at the microscale. Based on the Split-Hopkinson-Pressure-Bar technique, a constitutive material law has been developed to model the thermo-mechanical material behaviour including the effect of the microstructure. Micro drilling tests using solid carbide twist drills with different diameters (d = 50 μm to 1 mm) were performed on ferrite–pearlite two-phase steel AISI 1045 for the verification of the developed 3D FE computation model regarding chip formation, feed force, and torque.     

Interfacial reaction between Co–Cr–Mo alloy and liquid Al

The interfacial reaction between Co–Cr–Mo alloy and liquid Al was investigated using immersion tests. Microstructure characterization indicated that the Co–Cr–Mo alloy was corroded by liquid Al homogeneously, with the formation of a (Co,Cr,Mo)₂Al₉ layer close to alloy matrix and “(Cr,Mo)₇Al₄₅ + Al” layer close to Al. Kinetics analysis showed that the corrosion of the Co–Cr–Mo alloy followed a linear relationship with the immersion duration. Compared with pure Co–liquid Al reaction system, the alloying of Cr and Mo changed the solid–liquid interface structure, but the corrosion of the solid metal was still dominated by the dissolution of an intermetallic layer.     

Design and synthesis of a novel class of inhibitors for mild steel corrosion in acidic and carbon dioxide-saturated saline media

p-(9-(2-Methylisoxazolidin-5-yl)nonyloxy)benzaldehyde I, prepared using a cycloaddition protocol, was elaborated into its cinnamaldehyde derivative II which upon quarternization with propargyl chloride afforded III bearing an interesting blend of structural traits suitable for imparting inhibition of mild steel corrosion. Novel compounds I–III showed efficient inhibition against mild steel corrosion in CO₂–0.5 M NaCl (40 °C, 1 atm; 120 °C, 10 bar), 1, 4, 7.7 M HCl, and 0.5 M H₂SO₄ at 60 °C as determined by gravimetry and electrochemical methods. The presence of carbonaceous surface and nitrogen, as revealed by XPS study, indicated the formation of a film covering the metal surface, which imparted corrosion inhibition.     

Oxidation of Fe–10Cr in O2 and in O2 + H2O environment at 600 °C: A microstructural investigation

Oxidation of Fe–10Cr in dry and wet O₂ was studied at 600 °C for up to 168 h. Oxide microstructure was investigated by STEM/EDX, FIB/SEM and TEM. Oxidation in dry O₂ gives a Cr-rich protective (Fe_1−xCr_x)₂O₃ scale. The same protective oxide initially forms in O₂ + H₂O environment, but after an incubation period scale breakdown is triggered by CrO₂(OH)₂ evaporation that depletes the substrate in Cr and converts (Fe_1−xCr_x)₂O₃ to FeCr spinel oxide. Internal oxidation occurs after breakaway. Alternating external and internal oxidation result in the inward-growing scale showing a characteristic banded morphology.     

Corrosion behaviour of Al–29at%Co alloy in aqueous NaCl

Microstructure and corrosion behaviour of a binary Al–29 at%Co alloy have been studied. The alloy was prepared by arc-melting of Al and Co in high purity Ar and rapidly solidified on a water-cooled Cu mould. The alloy chemical composition and microstructure were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Furthermore, the corrosion behaviour was studied by potentiodynamic polarization in aqueous NaCl (0.6 mol dm⁻³) at room temperature. The alloy was found to consist of three phases: hexagonal Al₅Co₂, Z-phase and AlCo (β). The corrosion resistance of different intermetallic phases is characterized. The results are compared to previously published results of Al–TM (TM = transition metal) alloys.     

Evolution of surface chemistry and morphology of oxide scale formed during initial stage oxidation of modified 9Cr–1Mo steel

Initial stage oxidation characteristics of the modified 9Cr–1Mo steel in ambient air at 650 °C have been investigated, for exposure times ranging from 5 to 500 h. Oxygen flux from the gas phase causes high initial oxidation rate, but the growth kinetics do not follow parabolic law. In “as-received” condition, binary oxides of Fe and Cr were found as native oxides. Upon oxidation, segregation of Mn resulted in the formation of MnCr₂O₄ along with FeCr₂O₄ and binary oxides of Fe, Cr and Mn. Thus, the initial oxide scale constitutes multiple oxides with delineated interface, unlikely to have a layered structure.     

Correlation of precipitate stability to increased creep resistance of Cr–Mo steel welds

An order of magnitude decrease (from 16.0 × 10^?4 to 4.1 × 10^?4% h^?1) in steady-state creep rate was observed in the fine-grained heat-affected zone (HAZ) of a Cr–Mo steel weld by the reduction of the pre-weld tempering temperature from 760 °C (HTT) to 650 °C (LTT). The microstructure during each stage of the manufacturing path, including pre-weld temper, thermal cycling and post-weld heat treatment, was characterized using a suite of characterization techniques. The techniques included simulated thermal cycling, dilatometry and electron microscopy, as well as time-resolved X-ray diffraction using Synchrotron radiation. Both LTT and HTT steels before welding contain M₂₃C₆ (M = Cr, Fe) and MX (M = Nb, V; X = C, N) precipitates in a tempered martensite matrix. During simulated HAZ thermal cycling with different peak temperatures, changes in M₂₃C₆ carbide characteristics were observed between the HTT and LLT conditions, while MX precipitates remained stable in both conditions. Simulated post-weld heat treatment samples show larger M₂₃C₆ in the HTT condition than in the LTT condition. The results provide a solution to extending the life of Cr–Mo steel welded structures used in power plants.     

On the effect of long-term creep on the microstructure of a 12% chromium tempered martensite ferritic steel

In the present study we investigate the evolution of the microstructure of a 12% Cr tempered martensite ferritic steel under conditions of long-term aging and creep (823 K, 120 MPa, t_R = 139,971 h). We show how subgrains coarsen, that the close correlation between carbides and subgrain boundaries loosens during long-term creep and that the frequency of small-angle boundaries increases. All these elementary deformation processes have been discussed in short-term creep studies. The present study shows that they also govern long-term creep. However, during long-term creep, precipitation and coarsening reactions occur that are not observed during short-term creep. Three types of particles (M₂₃C₆, VX and Laves-phase) were identified after long-term creep. M₂₃C₆ particles coarsen at constant volume fraction and establish their equilibrium concentration after 51,072 h; VX particles are stable; and the Laves-phase particles never reach thermodynamic equilibrium.     

Adsorption and inhibition effect of Ascorbyl palmitate on corrosion of carbon steel in ethanol blended gasoline containing water as a contaminant

The adsorption and inhibition effect of Ascorbyl palmitate (AP) on carbon steel in ethanol blended gasoline containing water as a contaminant (GE10 + 1%water) was studied by weight loss and electrochemical impedance spectroscopic (EIS) techniques. The results showed that the addition of ethanol and water to gasoline increase the corrosion rate of carbon steel. AP inhibits the corrosion of carbon steel in (GE10 + 1% water) solution to a remarkable extent. The adsorption of AP on the carbon steel surface was found to obey the Langmuir adsorption isotherm model. The values of activation energy (E_a) and various thermodynamic parameters were calculated and discussed.     

Glass coatings on stainless steels for high-temperature oxidation protection: Mechanisms

The oxidation behavior of a martensitic stainless steel with or without glass coating was investigated at 600–800 °C. The glass coating provided effective protection for the stainless steel against high-temperature oxidation. However, it follows different protection mechanisms depending on oxidation temperature. At 800 °C, glass coating acts as a barrier for oxygen diffusion, and oxidation of the glass coated steel follows linear law. At 700 or 600 °C, glass coating induces the formation of a (Cr, Fe)₂O₃/glass composite interlayer, through which the diffusion of Cr³⁺ or Fe³⁺ is dramatically limited. Oxidation follows parabolic law.     

Long-term monitoring of atmospheric corrosion at weathering steel bridges by an electrochemical impedance method

Weathering steel corrosion was monitored for one to two years under natural atmosphere by an electrochemical impedance technique. Two identical comb-shape weathering steel sheets embedded in epoxy resin were used as monitoring probe electrodes at two different bridges in Japan. Impedances at 10 kHz (Z_10kHz) and 10 mHz (Z_10mHz) were automatically measured every hour. Coupons (50 × 50 × 2 mm³) prepared from the same steel sheets were exposed together to measure the corrosion mass loss. The average (Z_10mHz)⁻¹ value for half to one year exposure correlated well with the average corrosion rate determined from the corrosion mass loss.     

Characterization of corrosion products formed on Ni 2.4 wt%–Cu 0.5 wt%–Cr 0.5 wt% weathering steel exposed in marine atmospheres

Weathering steel manufactured with high concentrations of copper (0.5 wt%), chromium (0.5 wt%) and nickel (2.4 wt%) was studied with the aim of furthering knowledge on corrosion product characterization and performance in marine environments. Specimens exposed for two years in a rural atmosphere and two marine environments were characterized by optical microscopy, SEM/EDS, XRD and Raman spectroscopy and corrosion rates measured. The main phases found were ferrihydrite, maghemite and goethite in the inner corrosion layer, and lepidocrocite in the outer layer. Cu and Ni were homogeneously distributed while Cr tended to be concentrated in the inner layer.     

Isothermal oxidation behavior of two-phase TiAl–Mn–Mo–C–Y alloys fabricated by different processes

The isothermal oxidation behavior of two-phase Ti–46.6Al–1.4Mn–2Mo–0.3C–0.3Y based alloys in synthetic air at 800 and 900 °C was investigated. The main emphasis was focused on the effect of microstructures obtained by different processes of extrusion, melting and hot rolling, i.e. fully lamellar, nearly lamellar and duplex. On the exposure for 350 h at 800 °C, the growth rate and the spallation behavior of oxide scales of the alloys depended strongly on the fabrication process. The extruded alloy with a fine fully lamellar microstructure showed an excellent oxidation resistance due to the lowest mass gain and strong adhesion of the scale to the substrate, and Y-addition was effective in improving oxidation resistance because of its oxygen-scavenging effect in EPM processing. However, the overall oxidation process was dominantly controlled by the beneficial effect of the Mo-addition in the melted alloy due to the doping effect and formation of Mo-rich Al-depletion layer close to the substrate. The rolled alloy with the duplex microstructure greatly accelerated the oxidation rate because of a high volume fraction of α₂-Ti₃Al. At 900 °C, the melted alloy showed an excellent oxidation resistance during 350 h exposure. The oxidation rate and the spallation resistance of oxide scales did not depend strongly on the microstructure for the extruded or the rolled alloy. The poor spallation resistance of the oxide scales in the extruded alloy was probably related to thicker Mn-containing scales. Neither Mo-rich layer nor Y-rich zone in the oxide scales in the rolled alloy was observed at two exposure temperatures as a result of fast oxidation rate of the scale.     

Atom probe tomographic study of L10 martensite in a Pt-modified NiCoCrAlYTa bond coating

The L1₀ martensite formed in a Pt-modified NiCoCrAlYTa bond coating has been investigated by atom probe tomography. It was found that obvious segregation of Co and Cr occurred in the micro-twins zone inside the martensite lath. Based upon the compositional analysis, it is known that Pt destabilizes the β phase and Co and Cr act as β stabilizers with respect to the β → L1₀ martensitic transformation. In addition, some α-Cr particles precipitated inside the martensite lath.