Effect of Oxide Scale Formation on the Behaviour of Cu in Steel during High Temperature Oxidation in O2‐N2 and H2O‐N2 atmospheres

Cu enrichment at the steel‐scale interface and its migration from there was investigated during the heating of steel cast at 1200°C under various oxidizing conditions. The behaviour of Cu enrichment was found to be largely dependent on the morphology of oxide scale formed during oxidation. At the early stage of oxidation, Cu‐rich phase formed and accumulated at the steel‐scale interface in both O₂‐N₂ and H₂O‐N₂ atmosphere. However, as the oxidation proceeded, the enrichment was vastly different for each oxidizing atmosphere. In the case of O₂‐N₂ oxidation, an oxide layer was formed initially at the steel surface, but a gap was developed soon after at the steel‐scale interface and grew in size, which practically separated the scale from the steel substrate. The scale layer formed under this condition was porous. The Cu‐rich phase initially formed at the interface seemed to migrate to the scale layer, leaving no Cu‐rich phase at the interface. In the case of H₂O‐N₂ oxidation, however, the scale layer formed was dense and tightly attached to the steel surface, and the Cu rich‐phase continued to accumulate at the interface. Regarding the behaviour of the Cu‐rich phase formed at the interface, it is proposed with experimental evidences that, when a gap forms at the steel‐scale interface, it is the vaporization of Cu in the Cu‐rich phase through the gap that brings Cu to the scale.     

Microstructure modification and surface hardening of 12% chromium steels by pulsed gas plasma flows

The changes in the microstructure and the surface hardening of chromium (12 wt % Cr) ferritic–martensitic steels in various initial states treated by high-temperature pulsed gas plasma flows (flux energy density Q = 17–78 J/cm², pulse duration τ_p = 15–20 μs) have been studied experimentally. Treatment of fuel-element pipes and monolithic specimens of 12% chromium steel under melting of near-surface layers is found to form a gradient structure–phase state with a submicrocrystalline (~130 nm) surface layer up to 10 μm thick. The parameters of the formed cellular submicrostructure and the modified layer thickness are found to weakly depend on the composition and the thermomechanical treatment of the steels. It is shown that treatment of fuel-element pipes made of 12% chromium steels by plasma flows leads to their surface hardening by 40–60% and by a factor of 1.7–1.9 upon surface liquid-phase alloying with aluminum and chromium irrespective of steel composition.     

碲处理对20CrMnTi齿轮钢中MnS夹杂物改性效果

 为了研究碲处理对钢中MnS夹杂物形貌的影响,利用SEM-EDS扫描电镜,研究了20CrMnTi钢中添加高纯碲粉后MnS夹杂物的改性效果。试验结果表明,碲处理使钢中夹杂物的平均长宽比由3.17降至1.83,球化效果较为明显;当碲硫比控制在0.33时,不同硫含量的钢中夹杂物形貌有明显差异,硫质量分数为0.21%的钢中,形成了MnS镶嵌在碲化物中的大型夹杂,而在硫质量分数为0.11%的钢中,形成了碲化物包裹MnS的复合夹杂;当碲硫比为3.21时,发现钢中出现了单独存在的高碲相,MnS外层的碲化物层也较厚,改性率仅为8.75%,这表明高碲硫比并不能提高硫化物改性的数量。     

Investigation of Corrosion Behavior of Borided Gear Steels

In this study, corrosion behaviors of GS18NiMoCr36 (GS 18) and GS32NiCrMo6.4 (GS 32) gear steels borided in Ekabor-II powder at the temperature of 950 °C for 2 and 6 h were investigated in a 6 % M HCI acid solution. The boride layer was characterized by optical microscopy, X-ray diffraction technique and the Micro-Vickers hardness tester. X-ray diffraction analysis of boride layers on the surface of the steels revealed the existence of FeB, Fe₂B, CrB and Cr₂B compounds. The thickness of the boride layer increases by increasing boriding time for gear steels. The hardness of the boride compounds formed on the surface of the steels GS 18 and GS 32 ranged from 1,728 to 1,905 HV_0,05 and 1,815 to 2,034 HV_0,05 respectively, whereas Vickers hardness values of the untreated steels GS 18 and GS 32 were 335 HV_0,05 and 411 HV_0,05, respectively. The corrosion resistance of borided gear steels is higher compared with that of unborided steels. The boride layer increased the corrosion resistances of gear steels 4–6-fold.     

Effect of phosphorous surface segregation on iron-zinc reaction kinetics during hot-dip galvanizing

Phosphorous was ion implanted on one surface of a large grain (10 to 20 mm) low-carbon steel sheet in order to study the effect of surface segregation on the formation of Fe-Zn phases during galvanizing. Both an Al-free and a 0.20 wt pct Al-Zn bath at 450 °C were used in this investigation. It was found that P surface segregation did not affect the kinetics of Fe-Zn phase growth for the total alloy layer or the individual Fe-Zn gamma, delta, and zeta phase alloy layers in the 0.00 wt pct Al-Zn baths. In the 0.20 wt pct Al-Zn bath, the Fe₂Al₅ inhibition layer formed with kinetics, showing linear growth on both the P-ion implanted and non-P-ion implanted surfaces. Fe-Zn phase growth only occurred after extended reaction times on both surfaces and was found to directly correspond to the location of substrate grain boundary sites. These results indicate that P surface segregation does not affect the growth of Fe-Zn phases or the Fe₂Al₅ inhibition layer. It was shown that in the 0.20 wt pct Al-Zn bath, substrate grain boundaries are the dominant steel substrate structural feature that controls the kinetics of Fe-Zn alloy phase growth.     

Oxide inclusions and tool wear in machining

The inclusions formed in experimental steels by calcium, aluminum, and silicon additions are characterized. The morphology, phase identity, elemental analyses, and semiquantitative wt pct of the inclusions in each steel are presented. A correlation of tool wear tests conducted on the experimental steels and the inclusion characteristics indicates that a major factor in tool flank wear is the high temperature hardness of the inclusion phases or inclusion abrasion. A steel containing glass-like inclusions produced minimal tool wear but when the inclusions were a crystalline silicate, tool wear increased. The most severe tool wear occurred when machining steels with CaO·6Al₂O₃, Al₂O₃, and/or AIN as the major inclusion phases. By judicious selection of the deoxidation practice, the formation of refractory type oxide inclusions may be prevented and inclusions that enhance machinability may be formed. GLORIA M. FAULRING, formerly Graduate Student, State University of N.Y. at Buffalo     

Maximum Copper and Tin Contents in LC‐steel Thin Strip ‐ Hot Shortness Model Calculations

For conventional casting processes low copper and tin contents have to be ensured in LC‐steel to avoid hot shortness. It is expected that higher cooling rates, e.g. in thin strip casting, permit higher copper and tin limits. Hot shortness occurs because of selective oxidation of the iron whereby the more noble copper is enriched at the steel‐oxide interface. A liquid metallic copper phase which wets the grain boundaries supports cracking during hot deformation. The enrichment of the liquid copper phase depends on the oxidation temperature: At low temperatures copper is solid, cannot wet the steel surface and is incorporated into the growing oxide layer. At mid temperatures (1083‐1177 °C) the copper phase is liquid, wets the grain boundaries of the steel surface and causes hot shortness. At high temperatures a liquid fayalitic slag is formed in the oxide layer if the steel contains silicon. The fayalitic phase occludes parts of the steel surface and removes copper from the steel surface; then hot shortness is reduced or even avoided. Other mechanisms to remove copper from the steel surface need the presence of Fe₃O₄ and Fe₂O₃ in the oxide layer. These iron oxides are not formed for short oxidation times where linear oxidation takes place. Diffusion of copper into the steel is too slow to reduce hot shortness if copper has an elevated concentration in the steel, e.g. 0.5 wt.‐%. Therefore, only the occlusion mechanism is of importance during linear oxidation. A model is established on the basis of these observations in order to predict an upper copper limit in dependence of the steel strip thickness (cooling behaviour) and the oxygen content in the cooling atmosphere (nitrogen‐oxygen mixture). The model is compared to experimental results from KIMAB which are presented in this issue. It is demonstrated that a copper layer thickness of 0.098 μm at the steel‐oxide interface is sufficient to cause cracks of a depth of more than 0.2 mm. For strip thicknesses below 5 mm a simple approximation can be used to predict the maximum copper content in LC‐steel to avoid hot shortness. For example, thin strip of a thickness of 2 mm will have no cracks (above 0.2 mm) even if 0.7 wt.‐% of copper is contained in the LC‐steel. For atmospheres with a reduced oxygen partial pressure even higher copper contents are possible. Tin is with short oxidation times not a problem concerning hot shortness, as shown by the KIMAB results. This may be explained by the much higher diffusivity of tin in iron compared to copper.     

Surface of high-chromium steel modified by an intense pulsed electron beam

The formation of nanostructural multiphase surface layers in high-chromium 12Х18Н10Т and 20Х13 stainless steel under the action of an intense pulsed electron beam in a SOLO system is studied. The Fe–Cr–C system is thermodynamically analyzed. Alloying Fe–Cr alloys with carbon considerably changes their structural and phase state and determines the regions of existence of the carbides M₂₃C₆, M₇C₃, M₃C₂, and M₃C with α and γ phases. The temperature field formed in the surface layer of the steel under the action of the electron beam is numerically calculated. When the energy density of the electron beam is 10 J/cm2, regardless of the pulse length of the electron beam (50–200 μs), the maximum temperature at the sample surface corresponding to the end of the pulse is less than the melting point of the steel. The structure and the mechanical and tribological properties of the surface layer of high-chromium 12Х18Н10Т and 20Х13 steel formed under the action of the intense pulsed electron beam are investigated. It is found that electron-beam treatment of the steel with melting and subsequent high-speed crystallization is accompanied by solution of the initial carbide particles of composition M₂₃C₆—specifically, (Cr, Fe)₂₃C₆—and hence saturation of the crystal lattice in the surface layer with carbon and chromium atoms. In addition, submicronic cells of dendritic crystallization are formed, and nanoparticles of titanium carbide and chromium carbide are deposited. Overall, electron-beam treatment improves the surface and tribological properties of the materials. For 12Х18Н10Т steel, the hardness of the surface layer is increased by a factor of 1.5 and the wear resistance by a factor of 1.5, while the frictional coefficient is decreased by a factor of 1.6. For 20Х13 steel, the microhardness is increased by a factor of 1.5 and the wear resistance by a factor of 3.2, while the frictional coefficient is decreased by a factor of 2.3.     

Effect of Al2O3–SiO2–MnO inclusions on precipitation of MnS in Si–Mn-killed 304 stainless steels

Laboratory experiments and thermodynamic calculation were conducted to investigate the precipitation of MnS inclusions in Si–Mn-killed 304 stainless steels with various Al and S concentrations. Three types of MnS-contained inclusions were detected: MnS phase dissolved in the MnO–SiO₂ inclusion, the Al₂O₃-rich core phase surrounded by a MnS out layer, and the individual MnS. In steel with less than 0.001% Al, the liquid SiO₂–MnO-rich inclusions can hardly influence the precipitation of MnS inclusions during the cooling process of 304 stainless steels. With the increase of Al in steel, more solid Al₂O₃-rich inclusions are formed, which can act as nucleation agents for MnS inclusions and dramatically promote the precipitation of MnS inclusions during the cooling process of Si–Mn-killed 304 stainless steels.     

The influence of oxide adherence on the hot-salt corrosion of mild steels

The corrosion of pure iron and some mild steels by sodium chloride has been investigated in the temperature range 600 to 900° using hot-stage microscopy, a vibration technique, and quantitatively by means of a vapor exposure test. It has been found that for corrosive attack to occur it is necessary for the sodium chloride to have access to the metal surface, and that such access may be prevented by the presence of an adherent, impervious, scale layer at the metal surface. Such a layer was found to be present on an EN2 steel examined but was not formed on any of the other materials tested. The EN2 steel contained 0.2 pct Ni, and 0.21 pct Si, and both have been shown to become concentrated in the scale at the scale-metal interface. This produced an inner adherent scale which was resistant to sodium chloride penetration and which gave the material an appreciably enhanced corrosion resistance.     

钛铌微合金化齿轮钢的奥氏体晶粒长大研究

 采用热模拟渗碳方法研究了Ti、Ti-Nb微合金化的20CrMnTi和20CrMnTiNb渗碳齿轮钢在930~1200℃的奥氏体晶粒长大规律。结果表明,添加0. 038%(质量分数,下同)的钛和0. 048%的铌的20CrMnTiNb钢中含有铌和钛的析出相,其粒子间距为0. 361μm;而含0. 054%的钛的20CrMnTi钢中仅含有较大尺寸的TiN析出相,粒子间距为0. 471μm,前者奥氏体晶粒粗化倾向明显低于后者。20CrMnTiNb钢经1000℃奥氏体化10h后奥氏体晶粒长大不明显,且无混晶现象,适合高温渗碳工艺。     

Effect of liquid phase on scale formation during high-temperature oxidation of AlSi-transformation-induced plasticity steel surfaces

The oxidation kinetics, and the structural evolution of the resulting surface scale, of cast transformation-induced plasticity (TRIP) steel (0.97 wt pct Al and 1.11 wt pct Si) has been investigated in the temperature range of 850 °C to 1250 °C under atmospheres with oxygen partial pressures close to 0.2 atm. Direct visualization using a high-temperature confocal scanning laser microscope (CSLM) showed that at 1050 °C and higher temperatures, a liquid oxide phase formed beneath the surface, penetrating and liquefying the scale that had formed initially. After a period of time, which was dependent on temperature, the liquid became fully crystallized. A microprobe analysis of the steel/scale interface indicated an Al₂O₃-SiO₂-FeO _n composition in the liquid oxide. This phase formed a network that penetrated the scale. The rest of the outer scale consisted primarily of Fe₂O₃, while Al-Si-rich oxides were observed close to the metal/scale interface. Thermogravimetric analysis indicated a parabolic growth rate below 1000 °C and a linear growth rate at 1000 °C. At higher temperatures, a parabolic rate dominated once again. The scale thickness appears to be limited by the time period during which the liquid oxide could contribute to rapid mass transfer, which resulted in the observed linear oxidation rate. As the upper temperature limit of the linear oxidation region is reached, the liquid oxide becomes enriched with FeO _n , decreasing the stability of the liquid phase. This leads to crystallization of solid Fe oxides at the surface or the formation of appreciable amounts of Al- and Si-rich oxides at the interface. These processes block access of the liquid oxide to the steel.     

高锰合金钢的SME、TRIP、TWIP效应

 由于锰的价格低廉以及在材料中的重要作用而成为钢铁工业常用的合金元素。锰含量高时,可使Fe Mn合金形成的奥氏体在较低温度下存在。加入Si、Al元素可对合金中奥氏体的稳定性产生不同程度的影响,从而使材料在承受外界载荷时呈现出不同的反应。研究表明：Si可降低奥氏体层错能,有利于A→ε M相变,从而使合金易产生形状记忆效应。加大变形量,由于大量的奥氏体转变为α′ M时体积膨胀,在使材料伸长率提高的同时,强度也得到提高(相变诱发塑性效应),因此可用作高性能结构件。Al和Mn是提高奥氏体层错能的合金元素。对于Al、Mn含量高的钢,在外力作用下则可通过孪生诱发塑性变形产生孪晶诱发塑性效应,因而材料在具有较高强度的前提下,还具有60%~80%的伸长率。     

Characterization of carbonitrided layers formed on stainless steel by conversion electron Mössbauer spectrometry

Austenitic stainless steel was carbonitrided by the tufftride process, and the hardened layers formed on the surface were investigated by conversion electron Mössbauer spectrometry (CEMS) and grazing angle X-ray diffractometry (GXRD). It was found that carbides such as M₇C₃ (M = Fe, Cr), chromium nitride (CrN),ε-nitride (M₂N, M = Fe, Cr), andε-carbonitride º_2+x (C,N), M = Fe, Ni} were precipitated on the outermost surface at the initial stages of carbonitriding. By the increase of treatment time up to 20 and 30 minutes,∈ M_2+x (C,N) became a main component, while M₇C₃ and CrN disappeared in the outermost surface. After 60 minutes, M₇C₃ and CrN were observed again, and theγ nitride, the oxide of iron and chromium (FeCr₂O₄), was formed on the outermost surface for the first time. Cross-sectional micrographs of surface layers using a scanning electron microscope (SEM) after etching the hardened layers with Marble reagent revealed the presence of black and white layers. The former layer mainly consisted of∈ M_2+x (C,N),∈ M₂N, CrN, and M₇C₃, and the latter layer did not contain nitrogen, although carbon was detected in both layers. The Vickers hardnesses of the black and white layers were HmV(0.l) 1000 to 1200 and HmV(0.l) 500 to 600, respectively. It was said that both layers were harder compared with HmV(0.1)200 of bulk. The white layer was far superior to the black one in the corrosion resistance proved by anodic polarization curve measurements in 5 vol pct H₂SO₄ solution. The white layer formed on carbonitrided stainless steel beneath the black layer has possibilities as an excellent corrosion and wear resistive layer.     

Synthesis and Characterization of an Fe/Co Ferrite Spinel Oxide Film Produced by Using N2/Steam Heat Treatment on Two Maraging Steels

An experimental procedure was developed to obtain an oxide layer formed mainly by spinel on maraging steels. It consists of different stages with specific conditions, such as atmospheres rich in nitrogen and water vapor, and different steps of temperatures and times. Tests were performed on grade 300 and 350 maraging steels. Oxide layer characterization was done using optical and electron microscopy, spectroscopy, X-ray diffraction, and nanoscratch tests in order to determine the adhesion force as well as to observe the main deformation mechanism induced under sliding tests. In both steels, oxide layers are formed by the spinel’s Fe₃O₄ and CoFe₂O₄ in amounts close to ca. 85 pct, whereas TiO₂ and MoO₃ represent the other 15 pct. No hematite was found. The low oxygen availability during the heat treatment was fundamental for avoiding hematite formation. A nickel-rich austenitic phase formed at the metal-oxide interface due the kinetics of the oxidation process of the cobalt, iron, and molybdenum. The particular conditions of the heat treatments induced the formation of a mixture of iron, nickel, and cobalt spinel ferrites, thereby contradicting previous studies that said that only magnetite would be formed. The sliding tests at the nanometric length scale highlight that the layer formed on maraging 300 grade presents a better adhesion than the other investigated material due to the fact that it requires more load in order to induce cracks located at the edge of the sliding track and, subsequently, the chipping of the formed layer.

Graphical abstract

     

铌对65SiCrV6弹簧钢氧化增重的影响

 铌对Si-Cr-V系弹簧钢强度和脱碳层特征的影响已受到较多关注,但铌对该系弹簧钢氧化增重影响的研究还较少。以65SiCrV6弹簧钢为研究对象,在其中添加质量分数约0.017%的铌(65SiCrV6Nb)。采用SEM+EDS、XRD、TEM、FactSage化学热力学软件、反应扩散理论和数理统计相结合的方法,从研究铌加入是否会对该弹簧钢在炉氧化增重和氧化铁皮物相组成等产生明显影响的角度,对铌加入是否会对该弹簧钢高压水除鳞难易度产生影响进行评价。结果表明,铌的加入提高了锻态65SiCrV6钢中的珠光体和未溶M(C,N)的相对含量,降低了铁素体的相对含量,并细化了组织。在氧气浓度为2%~7%(体积分数)、加热速度为8~20 ℃/min、保温温度为1 050~1 150 ℃和保温时间为60~90 min等工艺条件下,铌的加入使65SiCrV6钢的氧化增重明显提高,提高幅度为2.54%~27.82%且具有统计学意义。影响试验钢在炉氧化增重的主次效应依次为保温温度>保温时间>加热速度>氧气浓度,保温温度和保温时间对试验钢在炉氧化增重的影响为正相关,氧气浓度和加热速度对试验钢在炉氧化增重的影响为负相关。65SiCrV6钢在炉氧化增重达最小值的工艺为氧气浓度为7%、加热速度为14 ℃/min、保温温度为1 050 ℃和保温时间为60 min;65SiCrV6 Nb钢在炉氧化增重达最小值的工艺为氧气浓度为7%、加热速度为8 ℃/min、保温温度为1 050 ℃和保温时间为60 min。影响铌对65SiCrV6钢在炉氧化增重提高幅度的主次效应为保温时间>保温温度>加热速度>氧气浓度。铌的加入使65SiCrV6钢在炉氧化增重提高幅度最小的工艺为氧气浓度为2%、加热速度为8 ℃/min、保温温度为1 050 ℃和保温时间为75 min。由反应扩散控制的氧化固态相变是造成保温温度、保温时间、加热速度和氧气浓度对试验钢在炉氧化增重产生不同影响的主要原因。铌的加入未改变65SiCrV6钢表面氧化铁皮的3层结构特征,氧化铁皮由外向钢基体主要由Fe₂O₃、Fe₃O₄和FeO(或FeO+Fe₂SiO₄)等构成。铌的加入降低了65SiCrV6钢氧化铁皮中Fe₂SiO₄的相对含量,对高压水除鳞有利。     

Reaction of iron-based magnetically soft materials with glass enamels

We have studied the reaction of M45 and SK20 glass enamels with PZhR3 iron and an iron — phosphorus alloy containing 0.8 mass% phosphorus using the “sessile” drop method in the temperature range 1000– 1300°C. We have established that when the M45 glass enamel reacts with the iron, then oxides of alkali metals Mg and Na oxidize it to Fe₂O₃ . The iron in the contact zone of the iron — SK20 glass enamel system is oxidized by zinc oxide, while the iron oxide formed dissolves in the glass enamel. Adhesion of the SK20 glass enamel to the iron surface is significantly better than in the Fe - M45 system. The best adhesion properties are observed in the iron-phosphorus alloy — SK20 glass enamel system, due to the more vigorous reaction between the components.     

A Study of Surface Reactions on Steel Samples Using a Mass Spectrometer Probe

This study presents a new probe to analyse the gas composition near a metal surface. The probe uses a mass spectrometric detector and is incorporated in an experimental reactor for the thermal treatment and surface formation of steel sheet samples. Steel samples were exposed to various oxidizing and reducing gas flows at pressures between ambient and 3 bar. The reaction product H₂O was measured with the MS‐probe while reducing oxidized steel surfaces with H₂ as well as the consumption of H₂ as a function of the sample temperature. Similarly, the reaction products CO and H₂ were monitored during the oxidation of the bulk carbon with H₂O. The sample temperature was ramped linearly from ambient to 900 °C. From these measurements it was possible to evaluate the onset temperatures and the activation energies for the respective reactions. The in‐situ monitoring of the gas phase near the steel surface within the experimental reactor allows controlling the formation of a layer of iron as a result of the iron oxide reduction. Similarly, the decarburization of steel can be followed up by measuring the time course of the CO concentration. It is expected that the MS‐probe will become an efficient tool for the understanding and optimization of the annealing and formation processes during strip annealing before hot dipping.     

Fatigue of cold-work tool steels: Effect of heat treatment and carbide morphology on fatigue crack formation,life, and fracture surface observations

The fatigue properties of two types of cold-work tool steels tempered at various temperatures were evaluated. The microstructure and fracture surface morphology were correlated to the fatigue behavior. Cold-work tool steels using this study were a conventional tool steel (JIS SKD11; 1.4C-11Cr-0.8Mo-0.2V) and its modified steel (M-SKD11; 0.8C-8Cr-2Mo-0.5V). The fatigue strength of the M-SKD11 steel increased 20 pct over that of the SKD11 steel for any number of cycles. This is attributed to the refinement of primary M₇C₃ carbides. These M₇C₃ carbides fractured during fatigue and were found at the sites of fatigue crack initiation. Change in crack initiation behavior was confirmed by acoustic emission testing. The S-N curves of the steels are similar to those of most structural steels. However, the subsurface fatigue crack initiation was dominant at lower alternating stresses. This study points to a general approach of carbide refinement that can be used for the enhancement of fatigue properties.     

Continuous reduction of the oxide scale of hot-rolled steel strip in hydrogen

The continuous reduction in the oxide scale of hot-rolled steel strip in H₂–N₂ atmosphere was simulated in laboratory. Scale specimens were reduced in 20% H₂–N₂ or 50% H₂–N₂ atmosphere. The sample weight losses were measured after soaking at 550, 700 and 800°C. In both atmospheres, specimen reduced at 700°C showed the minimum weight loss after soaking for 240?s. At 700 and 800°C, higher hydrogen concentration accelerated the reaction in the beginning of soaking, but had little effect once the dense-reduced iron layer formed. While at 550°C, the reduced iron kept growing in porous structure and the weight loss rate increased significantly in higher H₂ concentration.