Steam Oxidation Resistance Comparison of Several Silicon Carbide Refractories at Elevated Temperatures

The oxidation tests of different SiC refractories including Si3N4-SiC bricks,Si3N4-Si2N2O-SiC bricks from China(SNO-1),Si3N4-Si2N2O-SiC bricks from overseas(SNO-2),SiAlON-SiC bricks,oxides bonded SiC bricks,self-bonded SiC bricks as well as SiC castables were carried out in the steam atmosphere(1 000 ℃,32 kg·m-3·h-1)for different durations(100,200,300,400 and 500 h).The mass,the volume,the bulk density and the apparent porosity before and after the oxidation were tested.The XRD and SEM analyses were conducted.The results indicate that:(1)under the steam atmosphere condition(1 000 ℃,32 kg·m-3·h-1),as the oxidation time increases from 0 to 500 h,the volume and the mass of the silicon carbide refractories increase,while the bulk density decreases;in terms of the apparent porosity,oxides bonded SiC bricks and SiC castables present an increasing trend,Si3N4-SiC bricks,SiAlON-SiC bricks and self-bonded SiC bricks present an increasing trend first and then a decreasing trend,and Si3N4-Si2N2O-SiC bricks present a decreasing trend or a trend of decreasing first and then increasing;(2)as for Si3N4-Si2N2O-SiC bricks,SNO-1 and SNO-2 have basically the same chemical and phase composition,SNO-2 has the lower mass change rate than SNO-1 during oxidation from 200-500 h,which indicates that SNO-2 has the better steam oxidation resistance than SNO-1.

Micro-analysis of high-temperature oxidation-resistance of a new kind of heat-resistant grid plate in grate-kiln

To further improve the oxidation-resistance of materials and reduce the cost of grid plates in grate-kiln, a new kind of heat-resistant grid plate was developed. The microstructure of this grid plate with a life more than 18 months was studied by XRD, SEM and EDS techniques. The results show that high hardness, high intensity and good impact property make the new kind of heat-resistant grid plate and its oxide film have a higher resistance to deformation and abrasion at 900-1000℃ Besides, small grain size is beneficial to form a complete protective oxide film. The oxide film composed of SiO2 layer, Cr2O3 layer and Fe2O3 layer is rather thin and bonds closely with the backing. The forming of the chemical stable nickel-rich layer increases the density of Cr2O3 layer.

Research on Steam Oxidation Resistance of Si3N4-bonded SiC Refractories

The steam oxidation of Si3N4-bonded SiC was determined at 1000℃for 50,100,150,200,250 and 300 h,respectively,according to ASTM C863-2000.The evolution of the phase composition and the microstructure as well as their relationship was investigated by XRD and SEM.The results show that the oxidation rate of Si3N4-bonded SiC is periodic.The presence of nitrogen element can impede the crystallization of SiO2 glass;the local enrichment of CaO impurities is unfavorable for the existence of fibrous SiO2.SiO2 mainly exists as cristobalite when the CaO/SiO2 ratio reaches a suitable level,but gradually transforms to quartz along with the oxidation time when the SiO2 content increases,or the CaO/SiO2 ratio decreases,due to the insufficient mineralization of CaO.The crystallization of SiO2 glass,especially the formation of quartz is the key factor leading to the volume expansion and structural stress.When the cracks extend and reach the surface,the degradation of the material accelerates.

Steam Oxidation Resistance of Silicon Carbide Castables at Elevated Temperatures

Silicon carbide castables of different SiC contents(86%and 71%,by mass)were prepared using white fused corundum,silicon carbide particles and fines,activated alumina powder,silica fume and pure calcium aluminate cement as main starting materials,heat treating at 1000℃ for 3 h,and oxidizing in steam atmosphere at 1000℃ for different durations(100,200,300,400 and 500 h).The mass and volume before and after oxidation,the bulk density,the apparent porosity and the cold compressive strength were tested.The phase composition and the microstructure before and after oxidation were analyzed by XRD and SEM.The results indicate that:(1)within 300 h of oxidation duration,silicon carbide shows an increasing oxidation rate;however,the oxidation rate is low during 300-500 h of oxidation duration;2)the oxidation rate of the specimen with 71%SiC is slightly higher than the one with 86%SiC;3)with the increasing oxidation degree of silicon carbide,the apparent porosity of the specimens tends to increase,followed by the declining bulk density and cold compressive strength.

Steam Oxidation Resistance of Nitride Bonded Silicon Carbide Refractories for Waste Incinerators at Elevated Temperatures

Steam oxidation resistance of Si3N4 and Si2N2O as well as SiAlON bonded SiC refractories at 900℃was tested according to ASTM-C863.Phase composition and microstructure before and after oxidation were analyzed by XRD and SEM.The results show that Si3N4 and Si2N2O bonded SiC refractory presents better steam oxidation resistance than SiAlON bonded SiC.For Si3N4 and Si2N2O bonded SiC,the oxidation speed is higher with more pronounced volume expansion in the early 100 h;afterwards,the volume expansion slows down gradually and starts to level off after 300 h.It is considered that the high silica glass phase formed during the oxidation covers Si3N4 and Si2N2O,and SiC as a protective layer and fills the open pores.But for SiAlON bonded SiC,the volume expands gradually and constantly with the increasing oxidation duration even after 500 h,due to the continuous formation of mullite transformed from oxidation products and Al2O3 in SiAlON.

High-temperature oxidation behavior of heat resistant stainless steel 1.4828

The kinetic curves of the high-temperature oxidation of austenitic heat resistant stainless steel 1. 4828 at 1 050 ℃ were measured using a weighing method. It is shown that the oxidation curves at 1 050 ℃ followed the parabolic line law, and after 250 h of oxidation, the mass gain was about 80 g/m2. The surface morphology and structure of the oxide layers were studied by scanning electron microscopy and X-ray diffraction. A complicated oxide layer obtained at 1 050 ℃ was mainly composed, from inner to outer, of （FeSi） 3 04, Cr2 03, Fe2 03, and spinel oxides FeCr204 and NiMn204.

Oxidation resistance of powder metallurgy Ti-45Al-10Nb alloy at high temperature

TiAl alloy with high Nb content,nominally Ti-45Al-10Nb,was prepared by powder metallurgy,and the oxidation resistance at 850,900,and 950℃was investigated.The high-temperature oxidation-resistance mechanism and oxidation dynamics were discussed following the oxide skin morphology and microstructural evolution analysis.The oxide skin structures were similar for 850 and 900℃,with TiO_(2)+Al_(2)O_(3)mixture covering TiO_(2)with dispersed Nb_(2)O_(5).At 950℃,the oxide skin was divided into four sublayers,from the outside to the parent metal:loose TiO_(2)+Al_(2)O_(3),dense Al_(2)O_(3),dense TiO_(2)+Nb_(2)O_(5),and TiO_(2)matrix with dispersed Nb_(2)O_(5).The Nb layer suppressed the outward diffusion of Ti atoms,hindering the growth of TiO_(2),and simultaneously promote the formation of a continuous Al_(2)O_(3)protective layer,providing the alloy with long-term high-temperature oxidation resistance.

In-situ Synthesis and Oxidation Resistance of Sialon/SiC Composite Ceramics Applied as Solar Absorber

Sialon/SiC composites were synthesized in situ from SiC,α-Si_(3)N_(4),AlN,calcined bauxite,quartz and Y_(2)O_(3) via layered buried sintering at different temperatures (1 540-1 640 ℃).The results showed that the O’-sialon/SiC sample with 60 wt% silicon carbide sintered at 1 600 ℃ exhibited excellent mechanical properties,with apparent porosity of 16.01%,bulk density of 2.06 g·cm^(-3),bending strength of 52.63 MPa,and thermal expansion coefficient of 5.83×10-6 ℃^(-1).The oxide film formed on the surface was linked closely to O’-sialon,so the oxide film was not easily broken.After 100 h oxidization,the sample surface was smoother and denser,with oxidation weight gain rate 23.6 mg/cm^(2) and oxidation rate constant 2.0 mg^(2)·cm^(-4)·h^(-1).Therefore,the sample had the excellent high-temperature oxidation resistance.It was confirmed that the in-situ sialon/SiC composites could be a promising candidate for solar absorber owing to its high-temperature oxidation resistance.

Corrosion evolution of high-temperature formed oxide film on pure Sn solder substrate

The evolution of morphology, composition, thickness and corrosion resistance of the oxide film on pure Sn solder substrate submitted to high-temperature aging in 150 °C dry atmosphere was investigated. The results indicate that high-temperature aging accelerates the dehydration of Sn(OH)_(4)in the pre-existing native oxide film to form SnO_(2)and facilitates the oxidation of fresh Sn substrate, resulting in the gradual increase in oxide film thickness and surface roughness with prolonging aging time. However, the corrosion resistance of the film initially is enhanced and then deteriorated with an extending aging time. Besides, the formation and evolution mechanisms of the oxide film with aging time were discussed.

Effect of La_2O_3 on microstructure and high-temperature wear property of hot-press sintering FeAl intermetallic compound

FeAl intermetallic compound with different contents of rare earth oxide La2O3 addition was prepared by hot pressing the mechanically alloyed powders.Effect of La2O3 on microstructure and high-temperature wear property of the sintered FeAl samples was investigated in this paper.The results showed that 1 wt.% La2O3 addition could refine the microstructure and increase the density of the FeAl intermetallic compound,and correspondingly improved the high-temperature wear resistance.SEM and EDS analyses of the wo...

Experimental analysis of HR-120 alloy used for high-temperature fans

High-temperature circulating fans blow 800 ℃ HN protective atmosphere to the surface of the strip as a jet heating power source or suspending source,playing a core role in metal strip continuous annealing furnaces. In this study,several high-temperature alloys were experimentally compared based on their high-temperature mechanical properties,high-temperature creep resistance,and high-temperature oxidation resistance. The results indicate that though the price of HR-120 alloy is only half of Inconel 601,it is suitable for the manufacture of fans that can withstand a high temperature of 800 ℃ given its good creep resistance,high-temperature oxidation resistance,and price advantage.

Oxidation Behaviors of Different Grades of Ferritic Heat Resistant Steels in High-Temperature Steam and Flue Gas Environments

For steam tubes used in thermal power plant,the inner and outer walls were operated in high-temperature steam and flue gas environments respectively.In this study,structure,microstructure and chemical composition of oxide films on inner and outer walls of exservice low Cr ferritic steel G102 tube and exservice high Cr ferritic steel T91 tube were analyzed.The oxide film was composed of outer oxide layer,inner oxide layer and internal oxidation zone.The outer oxide layer on the original surface of tube had a porous structure containing Fe oxides formed by diffusion and oxidation of Fe.More specially,the outer oxide layer formed in flue gas environment would mix with coal combustion products during the growth process.The inner oxide layer below the original surface of tube was made of Fe–Cr spinel.The internal oxidation zone was believed to be the precursor stage of inner oxide layer.The formation of internal oxidation zone was due to O diffusing along grain boundaries to form oxide.There were Fe–Cr–Si oxides discontinuously distributed along grain boundaries in the internal oxidation zone of G102,while there were Fe–Cr oxides continuously distributed along grain boundaries in that of T91.

锆合金表面高温抗氧化涂层的研究进展

锆合金因其热中子吸收截面小、热膨胀系数低,以及与UO2良好的相容性而成为当前核反应堆中主要的构件材料。然而,在高温蒸汽氧化环境中,锆合金会快速氧化失效,并产生大量氢气,从而引发氢爆炸。为了提高核反应堆的安全性,对锆合金表面进行强化形成高温抗氧化防护涂层,是解决这一难题的有效途径。本文介绍了锆合金表面高温氧化行为,重点综述了高温抗氧化涂层(包括金属涂层、陶瓷涂层以及复合涂层)的氧化行为和失效机理,对比分析了不同锆合金表面涂层高温氧化性能。另外,还对锆合金表面高温抗氧化涂层的多元素成分设计、制备方法和梯度结构设计的发展方向进行了展望。

桑葚果粉添加量对馒头品质及抗氧化能力的影响

研究不同比例的桑葚果粉替代面粉对馒头品质及其抗氧化性的影响,以期制备高营养价值的馒头,扩大桑葚的利用途径。通过测定不同添加量的桑葚粉对发酵面团及馒头的色差、质构、微观结构、抗氧化效果等指标的影响,分析面团特性与馒头品质之间的相关性。结果表明,随着桑葚果粉添加量增多,面团弹性、色泽L*值、b*值、w值显著降低,而a*值升高。面筋蛋白结构的改变导致桑葚馒头的比容显著降低,硬度值增加,馒头品质下降。馒头的水分含量、弹性和咀嚼性呈先增高后降低趋势。对面团与馒头测定结果进行相关性分析,发现两者存在显著(P<0.05)或极显著(P<0.01)的相关性。此外,随着添加0%~5%的桑葚果粉,抗氧化性显著增强,桑葚馒头的1,1-二苯基-2-三硝基苯肼(DPPH)自由基清除能力从39.05%提高到76.28%,提高了37.23%;还原能力提高了282.5%,2,2-联氮-二(3-乙基-苯并噻唑-6-磺酸)二铵盐(ABTS)清除能力提高了35.94%。综上,桑葚果粉的添加会对面团的加工性能和馒头品质产生影响,桑葚馒头与质构指标存在相关性,在未进行配方优化时,桑葚果粉添加比例为4%时馒头具有较好品质。

不同处理方式对牛蒡根多糖提取率及抗氧化活性的影响

为了研究不同处理方式对牛蒡根理化性质及抗氧化活性的影响,为牛蒡根多糖提取、产品开发提供新思路,本研究以超微粉碎、蒸汽爆破和双螺杆挤压熟化处理牛蒡根后,提取牛蒡根多糖,测定牛蒡根多糖提取液的抗氧化活性。结果表明,超微粉碎处理后牛蒡根多糖提取率最高,为36.03%。蒸汽爆破处理后牛蒡根多糖提取液DPPH自由基清除能力和羟基自由基清除能力最强,其中较未处理的牛蒡根提多糖取液DPPH自由基清除能力提高15.96%。螺杆挤压熟化处理后牛蒡根多糖提取液还原力最强,较未处理的牛蒡根多糖提取液提高15.60%。蒸汽爆破处理后菊糖的提取率最高,为34.94%。

蒸汽爆破对芦笋副产物功能性成分及体外抗氧化性的影响

以芦笋加工过程副产物为原材料,探讨蒸汽爆破对芦笋加工副产物功能性成分及抗氧化性的影响。通过预处理分别得到汽爆压强为0.5 MPa、1.0 MPa、1.5 MPa、2.0 MPa的四种产物,并进行黄酮、多酚、多糖等成分及羟基自由基等体外抗氧化性的影响。结果表明,汽爆压强为1.0 MPa时黄酮、多酚的含量最高,分别为0.975 mg/g、8.23 mg/g,汽爆压强为1.5 MPa时多糖得率最高,达8.19 mg/g,汽爆压强为1.5 MPa时羟基自由基和DPPH自由基的清除率达到最大,分别为79.1%、77.9%。

High-Temperature Oxidation Behavior of a Ni-Cr-W-Al Alloy

A study was conducted to examine the isothermal oxidation behavior of a new Ni-Cr-W-AI alloy in air at 1250℃ with different time. Oxidation kinetics was determined from weight-change measurements. The microstructure and composition of the oxide scale were investigated by means of scanning electron microcopy and X-ray diffraction. The results showed that the oxide scales of the alloy were a compact and continuous outer Cr2O3 and NiCr2O4 layer and an inner Al2O3 layer that was in dendrite shape. Oxides scales with good adherence were formed on the surface of the alloy, which made the alloy perform excellent high-temperature oxidation resistance.

High-temperature oxidation behavior of SiBN fibers in air

SiBN fibers are one of the most admirable microwave-transparent reinforced materials for high Mach number aircrafts.Currently,the detailed high-temperature oxidation behavior of SiBN fibers has not been studied yet.In this work,we studied the high-temperature oxidation behavior of SiBN fibers with different boron contents at the temperature range of 1000-1400℃in air.SiBN fibers started to be oxidized at 1100℃,with Si_(3)N_(4) and BN phase oxidized to SiO2 and B_(2)O_(3),respectively.Due to the gasification and the escape of molten B_(2)O_(3) at high temperatures,amorphous SiO_(2) could be remained at the fiber surface.As the fiber further oxidized,the molten B_(2)O_(3) at the inside may infiltrate into the fiber interior to react with Si_(3)N_(4),causing the precipitation of hexagonal boron nitride(h-BN)nanoparticles and the formation of SiO_(2)/BN layer.Finally,complex oxidation layers with two distinct concentric sublayers accompanied with two transition sublayers could be formed after the oxidizing treatment.

Enhanced oxidation resistance of MoTaTiCrAl high entropy alloys by removal of Al

Refractory high-entropy alloys(HEAs) exhibit remarkable mechanical properties desired for high-temperature applications. However, their oxidation resistance at high temperatures received less attention. Recent work indicates that MoTaTiCrAl alloy exhibits excellent oxidation resistance,but the effects of Al and Cr remain unclear. In this work, we demonstrate that the addition of Al is unnecessary for preventing oxidation of the MoTaTiCrAl alloy and the removal of Al leads to a more oxidation resistant MoTaTiCr medium entropy alloy. Structural and chemical analyses indicate that the excellent oxidation resistance of MoTaTiCr is mainly associated with the formation of continuous CrTaO4 oxide layer.The results indicate that complex oxides can be adopted as effective candidates for enhancement of oxidation resistance,in addition to typical strategy of forming Al2O3, Cr2O3 or SiO2 barrier layer.

Microstructural regulation,oxidation resistance,and mechanical properties of C_(f)/SiC/SiHfBOC composites prepared by chemical vapor infiltration with precursor infiltration pyrolysis

To further improve the oxidation resistance of polymer derived ceramic(PDC)composites in harsh environments,Cf/SiC/SiHfBOC composites were prepared by chemical vapor infiltration(CVI)and precursor impregnation pyrolysis(PIP)methods.The weight retention change,mechanical properties,and microstructure of C/SiC/SiHfBOC before and after oxidation in air were studied in details.Microscopic analyses showed that only the interface between the ceramics and fibers was oxidized to some extent,and hafnium had been enriched on the composite surface after oxidizing at different temperature.The main oxidation products of Cf/SiC/SiHfBOC composites were Hf0_(2)and HfSi04 after oxidation at 1500℃for 60 min.Moreover,the weight retention ratio and compressive strength of the Cf/SiC/SiHfBOC composites are 83.97%and 23.88±3.11 MPa,respectively.It indicates that the Cf/SiC/SiHfBOC composites should be promising to be used for a short time in the oxidation environment at 1500℃.