High Temperature Oxidation of Micro-alloyed Steel and Its Scale Adhesion

The present work investigated the high temperature oxidation behaviour of the micro-alloyed steel and the adhesion of thermal oxide scale to its steel substrate. Oxidation testing was conducted at 815 °C in oxygen without and with 17.9% v/v water vapour. The oxidation kinetics in the two atmospheres were parabolic with similar rate constants, i.e. 1.13 × 10^?9 and 1.17 × 10^?9 g² cm^?4 s^?1 for the sample oxidised in oxygen without and with water vapour, respectively. The XRD peaks for wustite, magnetite, Ti-doped magnetite and titanium carbide were detected for the sample oxidised in oxygen. For the sample oxidised in the humidified atmosphere, Ti-doped magnetite was dominantly observed, additionally with titanium carbide. A tensile testing machine equipped with an optical lens was used to monitor scale failure during straining. For the sample oxidised for 1 min, the strain initiating the first spallation of the steel oxidised in the humidified oxygen was 1.74 ± 0.14%. This strain was higher than the strain initiating the first spallation of the steel oxidised in oxygen which was 1.00 ± 0.04%, indicating the improved adhesion of scale formed in the atmosphere containing water vapour. Mechanisms of water vapour effect on scale adhesion are discussed.     

Influence of Polishing-Induced Surface Hardening on the Adhesion of Oxide Scales Grown on a Ferritic Stainless Steel

The influence of surface preparation on the stress and adhesion of oxide scales formed on the ferritic stainless steel AISI 441 was studied. Steel coupons were surface-finished to different degrees of surface roughness from 400-grit SiC through to 1-micron diamond, and were also electropolished to remove the work hardened surface. Initial metal roughness was measured by optical profilometry. Oxidation was carried out at 800 °C under synthetic air for 100 h. Oxide residual stress was derived from the Raman shift of the main chromia line, and adhesion of oxide scales was quantitatively obtained using forced spallation by tensile straining. The results show that surface hardening is the most influential factor on adhesion, with the high dislocation-containing mirror-polished samples exhibiting the lowest adhesion energy (~4 J m^?2), and the electropolished samples with non-mechanically affected surface exhibiting the highest adhesion energy (17 J m^?2). Recrystallisation of the subsurface zone during heating to the oxidation temperature is thought to be the most influential factor reducing scale adhesion.     

Effects of Process Parameters on Mechanical Adhesion of Thermal Oxide Scales on Hot-Rolled Low Carbon Steels

The present work investigated the effects of process parameters in a hot-rolling line, finishing and coiling temperatures, on mechanical adhesion of scale on low carbon steel substrate using a tensile test. Modification of our previous model to quantify mechanical adhesion energy was proposed for a system consisting of a cracked scale on a metallic substrate by introducing a distribution function of stress in scale. When a linear distribution was assumed, the quantified mechanical adhesion energy lay in the range of 40–890 J m^?2. Higher finishing temperature had a prominent role on increasing final scale thickness and weakening scale adhesion. For scale with similar thickness, the mechanical adhesion energy was lowered for the sample subjected to higher temperature gradient between finishing and coiling temperatures. This was considered to be from the increased water vapour in atmosphere due to the higher amount of water used to cool down the steel strip. The mechanical adhesion test was further conducted to attest this assumption. It was found that humidified atmosphere during oxidation weakened the scale adhesion to low carbon steel substrate measured at room temperature.     

Mechanical Properties of Oxide Scales on Mild Steel at 800 to 1000°C

In the hot-rolling process of steels the oxide scales play a key role with regard to surface quality of the sheet. Therefore, a quantitative knowledge of the mechanical properties of oxide scales at rolling temperature can provide a significant improvement of sheet quality. In the present paper the mechanical properties of the oxide scales formed on mild steel were investigated in 4-point bend tests at 800, 900, and 1000°C in dry air, humid air (7–19.5 vol.% H₂O) and laboratory air at different deformation rates. It turns out that the environment has a significant influence on scale thickness and structure as well as on adhesion of the oxide scales. The mechanical measurements show plastic-creep deformation of the oxide scales. Water vapor did not have any significant effect on the creep properties of the oxide scales. In the measurements the secondary-creep-stress values of the oxide were determined as a function of the strain rate and plotted into an Ashby map for FeO. This plot can serve for an extrapolation of the data for even higher strain rates as relevant for the hot-rolling process.     

Moisture-Induced Delayed Alumina Scale Spallation on a Ni(Pt)Al Coating

Delayed interfacial scale failure takes place after cooling for samples of a Ni(Pt)Al-coated CMSX4 single crystal superalloy, cycled at 1150 °C for up to 2000 h. One sample exhibited premature coating grain boundary wrinkling, alumina scale spallation to bare metal, and a final weight loss of 3.3 mg/cm². Spallation under ambient conditions was monitored with time after cooldown and was found to continue for 24 h. This produced up to 0.05 mg/cm² additional loss for each hold, accumulating 0.7 mg/cm² (20% of the total) over the course of the test. After test termination, water immersion produced an additional 0.15 mg/cm² loss (a duplicate sample produced much less wrinkling and time dependent spalling, maintaining a net weight gain). The results are consistent with the general phenomena of moisture-induced delayed spallation (MIDS) of mature, distressed alumina scales formed on oxidation resistant M-Al alloys. Relative ambient humidity is discussed as the factor controlling adsorbed moisture, reaction with the substrate, and hydrogen effects on interface strength.     

Spallation Behaviour of Alumina Scale Formed on FeCrAlY Alloy After Isothermal Oxidation

The spallation behaviour of alumina scales grown on FeCrAlY alloy was investigated. Substrates with different thicknesses were oxidized at 1200 °C for 25 h and cooled at various cooling rates. Generally, the scale formed on a thicker substrate or with a faster cooling rate exhibits a larger compressive stress. However, the failure behaviour of alumina scales is more complicated than expected for a compressed film. Specifically, (i) the extent of the spallation is not proportional to the residual stress in the oxide; (ii) the spallation does not occur immediately after cooling, but requires a period of incubation. This indicates that the residual stress is not the sole reason for the failure of scales. It was found that the carbide forms at the oxide–metal interface after cooling, which acts in conjunction with the residual stress to control the spallation of oxides. In addition, the mechanics analysis suggests that the microscopic roughness at the interface is another important factor.     

Anin situ investigation of the tensile failure of oxide scales

A small four-point-bend jig has been used in a scanning electron microscope (SEM) to monitor the tensile fracture processes in iron and nickel oxide scales in situ. The scales were 3–40 m thick, the strain rate was 4×10^–5 sec^–1 and acoustic emission (AE) was used to correlate signals with specific cracking events. The technique provided detailed information of the failure processes, and several micrographs were taken as the crack pattern developed during testing. Failure started with short random through-scale cracks. These cracks soon formed a regular pattern. Spallation only occurred at much higher strains and resulted in very energetic AE signals. These signals were used together with the SEM observations to determine the strain to cause spallation. Measurements of the crack spacing as a function of strain showed that plastic stress relaxation by interfacial slip and/or substrate yielding processes affected cracking. Thus, the oxide remained attached to the substrate longer than would be expected from purely elastic behavior. An analysis based on the observed crack spacing at the onset of spallation gave ratios of 0.8–1.9 and 0.9 between tensile and interfacial strength for iron and nickel oxides scales, respectively.     

Wear Behavior of an Ultra-High-Strength Eutectoid Steel

Wear behavior of an ultra-high-strength AISI 1080 steel developed through incomplete austenitization-based combined cyclic heat treatment is investigated in comparison with annealed and conventional hardened and tempered conditions against an alumina disk (sliding speed = 1 m s^?1) using a pin-on-disk tribometer at a load range of 7.35-14.7 N. On a gross scale, the mechanism of surface damage involves adhesive wear coupled with abrasive wear (microcutting effects in particular) at lower loads. At higher loads, mainly the abrasive wear (both microcutting and microploughing mechanisms) and evolution of adherent oxide are observed. Besides, microhardness of matrix increases with load indicating substantial strain hardening during wear test. The rate of overall wear is found to increase with load. As-received annealed steel with the lowest initial hardness suffers from severe abrasive wear, thereby exhibiting the highest wear loss. Such a severe wear loss is not observed in conventional hardened and tempered and combined cyclic heat treatment conditions. Combined cyclic heat-treated steel exhibits the greatest wear resistance (lowest wear loss) due to its initial high hardness and evolution of hard abrasion-resistant tribolayer during wear test at higher load.     

Effect of Dynamic Change in Strain Rate on Mechanical and Stress Corrosion Cracking Behavior of a Mild Steel

The current work analyzes the effect of the dynamic change in strain rate during tensile loading of a mild steel on its mechanical and stress corrosion behavior in 3.5 wt.% NaCl solution. The sample experiences high strain rate (10^?2 s^?1) up to 10, 15 and 20% of total deformation and then very low strain rate of 10^?6 s^?1 till fracture without any unloading in between. The behavioral characteristics of the steel under these circumstances are found to be different from that exhibited during complete loading till fracture both at high and slow strain rates separately. Total strain increases with the increase in the strain at which change in strain rate happens, and this is attributed to the generation of large number of dislocations at higher strain rate and subsequently release of dislocation at low strain rate during change over due to more time available for dynamic recovery. This observation is common for both in air and corrosive environment. One unique observation in this study is the higher total strain and lower strength observed during dynamic change in strain rate in the corrosive environment compared to that in air, which is attributed to the hydrogen-induced plasticity mechanism.     

Investigation on formability enhancement of 5A06 aluminium sheet by impact hydroforming

High strain rate (HSR) forming has been found to be able to enhance the formability of sheet metals like electro-magnetic forming. Impact hydroforming (IHF) is proposed, in which the sheet is formed with high-pressure pulse combining hydroforming and HSR forming. An IHF bulge test setup was designed, 5A06 aluminium sheet was tested with strain rate of 2 × 10³ s^?1 showing remarkable thickness strain increase compared with quasi-static condition. A new IHF equipment is designed, the IHF process was verified effective with the equipment, complicated aluminium aircraft sheet part with high drawing ratio was formed that cannot be formed with quasi-static hydroforming.     

Synergistic influence of alloy grain size and Si content on the oxidation behavior of 9Cr−1Mo steel

The synergistic influence of prior-austenite grain size and silicon content of 9Cr–1Mo steel on the resistance to scale spallation has been studied in air at 773 K (for 500 hr) and 973 K (12 hr). Two steels, irrespective of their grain size and Si content, did not show spallation during oxidation at 773 K. Spallation occurred at 973 K, and fine-grain steels exhibited less spallation resistance than coarse-grain ones (in low-as well as high-Si steels). Among the four possible combinations of grain size ans Si content, the steel with low Si and fine grains showed least resistance to spallation, while the steel with high Si and coarse grains showed the best resistance. Spallation was found to initiate in the areas adjoining the oxide ridges formed at the alloy grain boundaries. Oxide scales at the ridges and within the grains were analyzed by scanning electron microscopy (SEM/EDX) and secondary-ion mass spectrometry (SIMS). These analyses suggest depletion of silicon from the areas adjoining grain boundaries, resulting in thicker scaling that triggers spallation in such areas. For similar grain-size materials, the necessary thickness for spallation was attained earlier with low-Si steel rather than in high-Si steel.     

The Effects of Specimen Geometry on the Plastic Deformation of AA 2219-T8 Aluminum Alloy Under Dynamic Impact Loading

The effects of specimen geometry on shear strain localization in AA 2219-T8 aluminum alloy under dynamic impact loading were investigated. The alloy was machined into cylindrical, cuboidal and conical (frustum) test specimens. Both deformed and transformed adiabatic shear bands developed in the alloy during the impact loading. The critical strain rate for formation of the deformed band was determined to be 2500 s^?1 irrespective of the specimen geometry. The critical strain rate required for formation of transformed band is higher than 3000 s^?1 depending on the specimen geometry. The critical strain rate for formation of transformed bands is lowest (3000 s^?1) in the Ø5 mm × 5 mm cylindrical specimens and highest (> 6000 s^?1) in the conical specimens. The cylindrical specimens showed the greatest tendency to form transformed bands, whereas the conical specimen showed the least tendency. The shape of the shear bands on the impacted plane was also observed to be dependent on the specimen geometry. Whereas the shear bands on the compression plane of the conical specimens formed elongated cycles, two elliptical shaped shear bands facing each other were observed on the cylindrical specimens. Two parallel shear bands were observed on the compression planes of the cuboidal specimens. The dynamic stress–strain curves vary slightly with the specimen geometry. The cuboidal specimens exhibit higher tendency for strain hardening and higher maximum flow stress than the other specimens. The microstructure evolution leading to the formation of transformed bands is also discussed in this paper.     

Nanocrystalline Nickel Deposition on the Titanium Alloys Using High Solution Flow Velocity

An innovative process has been developed for electroplating of nickel on titanium surface using fast solution flow technique. Nickel was directly deposited on a titanium alloy without using any pre-treatment process. Level of adhesion was determined using quantitative peel test and characterization of the deposition was performed by scanning electron microscopy. Results showed that the rate of nickel deposition at 60 °C was higher than that of the rate of nickel deposition at 40 °C. Moreover, Watts solution provided higher rate of nickel deposition compared to the sulfate-based nickel solution. The rate of deposition increased with increasing the solution flow velocity from 1.5 to 3 m/s and raising current density from 0.4 × 10⁴ to 1.6 × 10⁴ A/m² for both solution baths. Adhesion test indicated good level of adhesion between the deposited nickel and titanium surface. The bonding toughness increased to 4 J/m² for 1.2 × 10⁴ A/m² as a result of higher deposition rate. However, the mechanism responsible for the coating process was discussed in detail.     

Mechanism of adhesion of alumina on MCrAlY alloys

X-ray diffraction has been used to measure stains/ stresses generated in oxide films formed under isothermal conditions at 1150–1225° C and cooled to room temperature. High compressive strains, of the order of 1%, were measured in alumina films formed on FeCrAlY. However, little or no strain was measured in oxide films on NiCrAlY and NiCoCrAlY samples, even when there was no scale spallation. Auger Electron Spectroscopy (AES) experiments have also been conducted to evaluate the role of segregation on scale adhesion. Our studies suggest that the adhesion mechanism might depend on the alloy composition. On iron-based alloys, the scale spallation might be prevented by mechanisms that involve strong bonding at the interface. On the other hand, the scale spallation on nickel-based alloys might be prevented by a mechanism that relieves stresses. Yttrium segregation might help in this process.     

The Microstructure Evolution of Dual-Phase Pipeline Steel with Plastic Deformation at Different Strain Rates

Tensile properties of the high-deformability dual-phase ferrite-bainite X70 pipeline steel have been investigated at room temperature under the strain rates of 2.5 × 10^?5, 1.25 × 10^?4, 2.5 × 10^?3, and 1.25 × 10^?2 s^?1. The microstructures at different amount of plastic deformation were examined by using scanning and transmission electron microscopy. Generally, the ductility of typical body-centered cubic steels is reduced when its stain rate increases. However, we observed a different ductility dependence on strain rates in the dual-phase X70 pipeline steel. The uniform elongation (UEL%) and elongation to fracture (EL%) at the strain rate of 2.5 × 10^?3 s^?1 increase about 54 and 74%, respectively, compared to those at 2.5 × 10^?5 s^?1. The UEL% and EL% reach to their maximum at the strain rate of 2.5 × 10^?3 s^?1. This phenomenon was explained by the observed grain structures and dislocation configurations. Whether or not the ductility can be enhanced with increasing strain rates depends on the competition between the homogenization of plastic deformation among the microconstituents (ultra-fine ferrite grains, relatively coarse ferrite grains as well as bainite) and the progress of cracks formed as a consequence of localized inconsistent plastic deformation.     

High-Temperature Oxidation and Pickling Behaviour of HSLA Steels

The high-temperature oxidation behaviour of three boron-containing HSLA steels was studied at 800 and 900 °C in laboratory still-air conditions. At both temperatures, the oxidation rate of the specimens was in accordance with the parabolic rate law and was sensitive to the silicon content when they were oxidized at 800 °C. The chemical composition of the oxide scale was also influenced by the silicon content of the steel. The mechanical behaviour of the oxide scales formed at the metal–oxide interface was also different at each temperature. Finally, the pickling rate and surface quality of the steels depended on the chemical composition of the oxides that formed.     

应变速率对X80管线钢在库尔勒土壤环境中应力腐蚀开裂的影响

利用慢应变速率拉伸试验(SSRT)、动电位扫描及扫描电镜(SEM)技术研究了X80管线钢在库尔勒模拟溶液中应力腐蚀开裂(SCC)行为.结果表明,随着应变速率的增加,X80管线钢在模拟溶液中的腐蚀速率先增大后减小.当应变速率为5×10-7/s时,试样腐蚀较为缓慢,此过程电化学腐蚀起决定性作用;当应变速率为5×10-6/s时,试样的腐蚀情况最为严重,此时力学作用占主导地位.     

Effect of La2O3 on resistance to high-temperature oxidation of laser clad ferrite-based alloy coatings

Ferrite-based alloy powders with different contents of La₂O₃ were laser-clad on AISI 1115 steel substrates. The oxidation kinetic of the coatings was studied by testing the weight gain. Cyclic oxidation tests were performed to determine the eventual weight loss due to spallation. The morphologies and phase structures of the coatings were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The experimental results showed that Cr₂O₃ was formed on all coatings after oxidation. The coatings with La₂O₃ exhibited excellent high-temperature oxidation behavior including low oxidation rates and high resistance to spallation. In addition, the effect of La₂O₃ was discussed. This included the change of transport mechanism in the oxide scales, the increase in the nucleation rate of the oxide, the release of stresses in the oxide scales by refining the oxide grains and the improvement of the adhesion of the oxide scales to the coatings by changing the scale growth mechanisms.     

Effects of the Presence of Water Vapour on the Oxidation Behaviour of Low Carbon–Low Silicon Steel in 1 %O2–N2 at 1073 K

The effect of water vapour additions in the range of 2.5–24.8 % on the oxidation behaviour of a low carbon and low silicon steel in 1 %O₂–N₂ at 1073 K (800 °C) was examined. It was found that the characteristic of steel oxidation was completely changed by addition of water vapour in the atmosphere. First, the kinetics was changed from non-parabolic to parabolic. Second, the scale formed in 1 %O₂–N₂ for 30 min or longer was easy to spall upon cooling whereas the scales formed in the water-vapour containing atmospheres did not spall easily. Third, additions of 2.5–10 % of water vapour in 1 %O₂–N₂ resulted in the formation of numerous depressed locations in the scale, but the scale-steel adherence in the areas surrounding the depressed locations was very much strengthened. Finally, additions of 17.2 and 24.8 % of water vapour in the atmosphere prevented both scale spallation upon cooling and formation of depressed areas in the scale. The mechanisms of forming various scale structures and the roles of water vapour additions under different conditions were discussed.     

Dynamic Recrystallization during Hot Deformation of 304 Austenitic Stainless Steel

The kinetics of dynamic recrystallization (DRX) during hot compression of 304 austenitic stainless steel was studied over the temperature range of 900-1200 °C and strain rate range of 0.002-0.1 s^?1. The initiation and evolution of DRX were investigated using the process variables derived from flow curves. By the regression analysis for conventional hyperbolic sine equation, the activation energy for DRX was determined as Q = 475 kJ mol^?1. The temperature and strain rate domain where DRX occurred were identified from the strain rate sensitivity contour map. The critical stress (and strain) for the initiation of DRX was determined from the inflection point on the work hardening rate (θ = dσ/dε) versus flow stress (σ) curve. The saturation stress of the dynamic recovery (DRV) curve was calculated from the θ-σ plot at the same condition at which DRX occurred. Progress of fraction recrystallization was determined from the difference between the generated DRV curve and the experimental DRX curve. In addition, the microstructural evolution at different strain levels during DRX was characterized and compared with the calculated fraction recrystallization.