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.     

Stress Generation and Adhesion in Growing Oxide Scales

The experimental evidence for stress generation in growing oxide scales is briefly summarised, and the origin of these stresses is discussed. The limited experimental data related to oxide adhesion is discussed, and it is concluded that the adhesion between metal and oxide probably involves chemical interaction, and that the effects of impurities collecting at the interface are not easy to predict. It is emphasised however that the problem of adhesion at the metall oxide interface is complicated by the dynamic nature of the situation, so that it is necessary to postulate a growth mechanism which will allow contact between the oxide and the metal to be maintained. Finally, the problem of plastic flow in the oxide is considered, and it is suggested that some of the evidence which has been adduced to support the idea of large plastic flow is less than entirely convincing. Some of the phenomena may possibly be interpreted in terms of “stress-directed growth”.     

The Effect of Temperature on the Formation of Oxide Scales Regarding Commercial Superheater Steels

This study addresses the surface changes of three commercial steels (a low alloy ferritic 10CrMo9-10 steel, a Nb-stabilized austenitic AISI347 steel, and a high alloy austenitic Sanicro 28 steel) by comparing the oxide scale thicknesses, chemical compositions, and surface morphologies of samples after pre-oxidation at 200, 500 and 700 °C with different exposure times (5 and 24 h) under humid or dry conditions. With all three steels, the oxide scale thickness increased as functions of temperature and exposure time, the effect of temperature being more prominent than the effect of exposure time. The presence of water resulted in thicker oxide scales at the studied low alloy ferritic steel, whereas in the two austenitic steels, the presence of water increased chromium diffusion to the oxide scale rather than the scale thickness. The oxide layers characterized and analyzed in this paper will be further studied in terms of their abilities to resist corrosion by exposing them under corrosive conditions. The results regarding the corrosion resistance of the steels will be published in a sequel paper.     

Investigation of Adhesion and Fracture Toughness of Thermally Grown Oxide Scales by Interface Indentation Test

HIGH TEMPERATURE ALLOYS rely on theformation of a dense layer against oxidation.Howeverthe crack,spallation and detachment of oxide layeunder growth stress and thermal stress fromtemperature difference cause the failure of theprotective layer.Therefore,high temperature materialsare required forming a good oxide scale with highstrength and high interface bonding strength.Anappropriate measuring method and evaluation fooxide/metal interfacial adhesion is of great importanceto understand …     

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.     

Effect of Heat Treatment on Phase Transformation Texture and Mechanical Properties in Hot-Rolled High-Strength Steels

Metal Science and Heat Treatment - The effect of heat treatment on the phase transformation texture and mechanical properties of hot-rolled high-strength automotive steels with different chemical...     

Characterization of Oxide Scales Formed on Plain Carbon Steels in Dry and Wet Atmospheres and Their Eutectoid Transformation from FeO in Inert Atmosphere

Thermal gravimetric analysis (TGA) was used to investigate plain carbon steel exposed to dry air and wet air containing 4 vol% water vapor at 750 °C to simulate the formation of tertiary scale during hot-strip rolling. After the exposure to oxidation condition, the specimens were isothermally held at temperature in the range of 300–500 °C in inert argon (Ar) atmosphere for 10–240 min, and the isothermal phase transformation behavior of FeO was investigated. The morphology, element distribution, and phase composition of oxide scales after isothermal transformation were characterized by scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction, respectively. The experimental results indicate that wet air accelerated the growth rate of the oxide scale. Moreover, the time–temperature-transformation (TTT) diagrams of oxide scales grown in dry and wet air conditions were constructed to elucidate the isothermal phase transformation behavior of the FeO layer. The rate of isothermal phase transformation in the scale formed in wet air was significantly delayed compared to that in dry air, which caused the C-type TTT diagram to shift to the right. Two mechanisms elucidating the effects of initial oxidation atmosphere on the subsequent isothermal phase transformation behavior of FeO during inert atmosphere annealing were proposed.     

温度对油管钢CO2腐蚀产物膜的影响

利用静态高压釜对二种油管钢N80、P110和J55进行了CO2腐蚀试验,通过扫描电镜(SEM)对比分析了腐蚀产物膜厚度和表面晶粒尺寸随温度的变化。结果表明,在试验条件下,三种钢的腐蚀产物膜厚度随温度变化情况相近,最大值均在120℃,最小值对应温度则不同;表面晶粒尺寸出现了一峰一谷,峰值均在120℃,谷值对应温度却小同。     

Modeling of Residual Stress in Oxide Scales

The magnitude of the residual stress in an oxidescale, and how this varies with temperature, is of majorimportance in understanding the failure mechanisms ofoxide scales. This stress encompasses both growth stresses introduced at the oxidationtemperature and thermal-expansion-mismatch stressesinduced on heating and cooling, as well as anyexternally applied stresses or stress relaxation whichtakes place in the scale/substrate system. Althoughsome of these components are reasonably well understood(e.g., thermal stresses), growth stresses and therelaxation of the total scale stress by creep orfracture processes are much less well understood. Inthis study a model has been developed to predict stressgeneration and relaxation in oxide scales as a functionof time and temperature for both isothermal exposure and cooling to room temperature. The modeldetermines growth stress and thermal-stress generationin the scale and how this is balanced by stresses in thesubstrate. The substrate stresses are then allowed to relax by creep and the scale stressesrecalculated. This model accurately predicts theroom-temperature scale stresses for a range ofscale/alloy systems. The model can be used to show howthe scale stress depends on oxidation temperature,cooling rate, substrate, and scale thickness. The modelpredictions are discussed in light of experimentalobservations for alumina scales on FeCrAlY.     

Investigation of the Mechanical Properties of Oxide Scales on Nickel and TiAl

In this paper, a modified four-point bending test is described, which allows a quantitative determination of the mechanical properties of oxide scales on metallic substrates at temperatures from ambient to 900°C. Scale failure is detected by acoustic emission. By combining deformation data with the physical defect-structure values for the scales, fracture toughnesses have been evaluated for the cases of through-scale cracking and delamination/spalling. The test was applied to NiO scales on pure nickel and to the oxide scales on an intermetallic TiAl alloy at room temperature and 900°C, respectively. The fracture-toughness values determined in these investigations correspond to the order of magnitude expected from the rare literature data. The test itself can also be applied to a variety of brittle coatings on more-or-less ductile substrates.     

The Effect of Adhesion and Tensile Properties on the Formability of Laminated Steels

Laminated steel has been implemented in vehicle structures by several automotive manufacturers to reduce in-cabin noise. This study provides an understanding of how the adhesion between the steel skin and the viscoelastic polymer core affects laminate formability. Material properties, including peel strength, shear strength, and tensile strength were determined. The presence of the viscoelastic core was found to slightly reduce tensile properties of the laminate compared to the skin sheet. Forming limit diagrams were also determined. These indicated that the viscoelastic core properties can significantly affect formability of laminated steel compared to that of solid steel sheet. In general, the formability of laminated steel was found to be similar to or less than that of the much thinner skin sheet material, which indicates that its formability should be less than that of solid steel of the same gauge. This article was presented at Materials Science & Technology 2007, Automotive and Ground Vehicles symposium held September 16-20, 2007, in Detroit, MI.     

Amounts and Distribution of Phases in Sulfide Plus Oxide Scales on Iron

Pure iron was exposed at 800°C to flowing, catalyzed-gas mixtures of N₂/CO₂/CO/SO₂ adjusted to control the partial pressures of SO₂, S₂ and O₂. The equilibrium gas compositions were such that iron oxide was thermodynamically stable with respect to sulfide. The reaction product scale was invariably a mixture of oxide plus sulfide, and grew according to parabolic kinetics at high P_SO2 values and by linear kinetics in dilute gases. In both cases the reactant gas species was SO₂, not molecular oxygen or sulfur. The relative amounts of sulfide and oxide corresponded to stoichiometric reaction of SO₂ at high P_SO2 values, but not in dilute gases. At low P_SO2 values, the relationship between scale-sulfide volume fraction and P_SO2 corresponded to two independent scale-SO₂ reactions leading to oxide and sulfide growth. The two-phase mixture was lamellar, with platelets oriented approximately parallel to the mass-transfer direction. An inverse relationship between lamellar spacing and linear scaling rate is interpreted as evidence of a cooperative (cellular) growth mechanism.     

氧化温度对SS400钢氧化皮结构及耐蚀性能的影响

以宝钢SS400热轧钢为原料,控制加热温度,在钢表面制备氧化皮.采用SEM分析了氧化皮表面、截面形貌;X射线衍射仪分析氧化皮的相组成;进行室内干湿周期浸润腐蚀试验,对氧化皮试样在NaHSO3(0.01 mol/L)溶液中进行了耐蚀性测试.结果表明,分别在600、700、800℃氧化180 min的过程基本遵循抛物线规律.经过600℃加热保温30 min,所形成的氧化皮厚度小于10μm,700℃时厚度为15μm左右,800℃达到50μm.600、700、800℃制备的氧化皮均含Fe3O4、Fe2O3相和Fe颗粒.同时,800℃制备的氧化皮中保留有少量FeO相.不同温度所制备氧化皮试样耐蚀性能依次为700、600、800℃.     

Characterization of High-Temperature Oxide Films on Stainless Steels by Electrochemical-Impedance Spectroscopy

Oxide films formed on three stainless steels(UNS S30403; S44600; S30815) in air at 800°C werecharacterized by electrochemical-impedance spectroscopy(EIS). The film evolution vs. oxidation time wasinvestigated from 1 to 1000 hr. A three-electrodeelectrochemical cell and 0.1 MNa₂SO₄ solution were employed forEIS measurements. The spectra were interpreted in termsof a two-layer model of the films, where the capacitance and resistance obtained can berelated to the thickness (or roughness) anddefectiveness of the films. The results reveal that theoxide on S30403 grows and becomes defective, the oxideon S44600 thickens rapidly and retains its protectiveability for a relatively long time, and the oxide onS30815 remains thin and resistive. The two-layer modelis supported by surface characterization with SEM/EDS and in-depth profile of the oxide filmsobtained through glow discharge optical emissionspectroscopy (GDOES).     

Potentiometric Estimation of the Protective Ability of Oxide Films on Pearlitic Steels

A new method of estimating the protective properties of oxide films on pearlitic steels consists in potentiometrically titrating and determining the relative quantity of potassium molybdate required to a specimen (with or without an oxide coating) to reach a potential of –200 mV against the reference silver-chloride electrode in a 0.1 N solution of sodium sulfate.     

Influence of Oxidation Temperature and Cr Content on the Adhesion and Microstructure of Scale on Low Cr Steels

Descalability of steels during hot rolling processes is essential in order to maintain the surface quality of steel products, because the remaining scale can be impressed into the steel, producing surface defects. Therefore, a quantitative evaluation of scale adhesion at high temperature is needed in order to find suitable heating conditions for hot rolling processes that promote the formation of scale that easily detach. Round bar shape specimens of low Cr steels were oxidized in a fuel combustion environment, and then compressed at high temperature to detach the scale. The area fraction of the remaining scale was evaluated by image analysis. Under a thick outer oxide scale, a thin continuous inner oxide layer, which consisted of Cr and Si oxides, strongly adhered to the substrate, resulting in poor descalability. On the other hand, if this inner scale changed into a discontinuous layer with a porous FeO layer beneath, the descalability improved. It was found that scales with greater descalability formed on steels containing less Cr and for oxidation at higher temperatures.     

化学成分对热轧带钢氧化铁皮生长速率的影响

通过氧化模拟实验,观测了试样加热到不同温度时的氧化铁皮起皮情况,绘制了氧化动力学曲线,研究了高温下钢板在不同硅、磷质量分数时对氧化铁皮生长速率的影响。实验结果表明,在1100 ℃且经30 s氧化后,除含硅较低的钢种C外,其它钢种的表面都出现了独立的小气泡;经120 s氧化后各钢种的表面都完全形成了氧化膜。在较高温度(＞1000 ℃)时增硅会促进氧化铁皮的起皮;w（P）≤0005％时,无论在任何温度下都抑制了氧化铁皮起皮。