碳氢协同还原制备纳米W粉的机理及其在制备纳米WC粉和超细晶WC-Co硬质合金中的应用

发布时间：2018-02-23 19:50

本文关键词： 碳氢协同还原纳米钨粉纳米碳化钨挥发-沉积阶段碳化　出处：《南昌大学》2016年博士论文　论文类型：学位论文

【摘要】：超细/纳米晶WC-Co硬质合金因兼具高硬度和高强度(即兼有高耐磨性和高韧性的“双高”性能)而成为硬质合金的发展方向,制备性能优良的纳米W粉和WC粉是生产超细/纳米晶WC-Co硬质合金的基础和关键。本文针对氧化钨氢还原过程中因挥发-沉积作用而导致的W粉颗粒快速长大和异常长大的现象,采用碳氢协同还原法制备纳米W粉,然后分别通过阶段碳化法和碳氢协同还原-碳化法制备纳米WC粉,并采用低压烧结制备超细晶WC-Co硬质合金。论文系统研究了纳米W粉、WC粉及其烧结体的制备工艺、性能和机理,具体内容如下:1.研究了碳氢协同还原过程中的物相和形貌演变以及还原工艺参数W粉性能的影响。结果表明:还原过程遵循分步还原规律,非晶前驱体依次转变为WO_(2.9)、WO_(2.72)和WO_2低价中间氧化物,最终还原为α-W,随着还原过程中发生晶型转变以及碳与水蒸气反应被消耗,还原产物变得疏松多孔。W粉平均粒径随前驱体配碳比升高而减小,当配碳比高于2.6时,残余碳含量会显著增加;随着还原温度升高,前驱体中的碳参与反应并被消耗,W粉颗粒的长大作用被削弱,还原W粉的平均粒径和残碳量随还原温度升高而减小。配碳比为2.6的前驱体经760 oC还原60 min后,得到平均粒径56 nm的球形W粉。2.研究了还原方式对W粉形貌、粒度和显微结构的影响,通过分析不同还原温度W粉的晶粒长大曲线,提出了碳氢协同还原制备纳米W粉的机理。结果表明:W粉晶粒的长大速率随还原温度升高而变慢,760 oC以上时,还原产生的水蒸气与碳反应生成CO和H_2,显著降低体系中p[H_2O]/p[H_2],抑制挥发性水合物WO_2(OH)_2的产生,W粉的主导长大方式也由挥发-沉积转变为原子扩散。还原方式会对W粉的粒径和形貌产生重要影响,碳氢协同还原W粉的还原长大机制以固相局部化学反应为主,所得W粉为均匀细小的球形颗粒,结构疏松、分散性良好;普通氢还原W粉的还原长大机制以挥发-沉积为主,所得W粉颗粒粗大,发育完全,呈现W本征晶体的多面体形貌。3.以碳氢协同还原纳米W粉和碳黑为原料,采用阶段碳化法制备纳米WC粉,研究了阶段碳化工艺(碳化温度和保温时间)对WC粉物相、形貌和粒径的影响。结果表明:WC粉的粒径取决于W粉的碳化速率和长大速率,高的碳化速率和低的长大速率有利于降低粒径;低温预碳化能够在W粉颗粒表面形成一定厚度的WC层,使颗粒间的接触状态由W/W接触变为WC/WC接触,抑制碳化初期因W粉颗粒烧结合并长大而导致的WC粒径增粗。碳氢协同还原纳米W粉阶段碳化的最佳工艺为:预碳化温度900 oC,保温时间60 min,二段碳化温度1150 oC,保温时间90 min,平均粒径56 nm的W粉经900 oC+1150 oC阶段碳化,得到平均粒径106 nm的WC粉。4.采用连续碳氢协同还原-碳化法制备纳米WC粉,研究了前驱体配碳比对WC粉碳含量的影响,还原、碳化温度对WC粉形貌和粒径的影响。结果表明:WC粉化合碳含量随前驱体配碳比升高而逐渐增加,当前驱体配碳比为3.6时,化合碳含量达到理论值6.12%,游离碳含量为0.06%;当配碳比高于3.6时,游离碳含量迅速升高。还原-碳化过程中由W向WC的转变具有结构遗传性,长大系数在1.4~1.6之间,WC粉的平均粒径随还原温度升高而降低;升高碳化温度会促进WC粉颗粒的晶界迁移,WC粉的平均粒径随碳化温度升高而增大。碳氢协同还原-碳化制备纳米WC粉的最佳工艺为:前驱体配碳比3.6,还原温度760~800 oC,碳化温度1100~1200 oC;所得WC粉为均匀细小的近球形颗粒,平均粒径87.3 nm。5.以制备的纳米WC粉为原料,采用低压烧结技术制备超细晶WC-Co硬质合金,研究了烧结工艺参数对WC-Co硬质合金显微组织和力学性能的影响。结果表明:随着烧结温度升高和保温时间延长,烧结体的致密度增加,平均晶粒尺寸增大,试样的硬度和抗弯强度也会随致密度上升而提高;若烧结温度过高或保温时间过长,则会使烧结体的晶粒发生异常长大,导致致密度降低,合金力学性能下降。WC-6Co烧结的最佳工艺参数为:烧结温度1360 oC,保温时间60 min,所得硬质合金样品的平均晶粒尺寸为305 nm,为超细晶硬质合金,洛氏硬度达到94.6 HRA,抗弯强度达到4450 MPa。
[Abstract]:Ultrafine nanocrystalline WC-Co cemented carbide with high hardness and high strength (i.e. both high wear resistance and high toughness of the "double high" performance) as the development direction of hard alloy, preparation of excellent performance of nano W powder and WC powder is the production of ultrafine / nano crystalline WC-Co hard alloy according to the foundation and the key. W particles caused by evaporation deposition of tungsten oxide by hydrogen reduction process of rapid growth and abnormal growth of the phenomenon, the hydrocarbon co reduction method for preparing nanometer W powder, and then through the stage of carbonization and hydrocarbon preparation of nano WC powder reduction and carbonization of collaborative, ultrafine grained WC-Co hard alloy by low pressure sintering. This dissertation has systematically studied the preparation process of nano W powder, WC powder and sintered body, performance and mechanism, the specific contents are as follows: 1. research of hydrocarbon evolution phase and morphology of the reduction process in cooperative and reduction process parameters of W powder The influence of the performance. The results showed that reduction process follows the fractional reduction of amorphous precursors are converted to WO_ (2.9), WO_ (2.72) and WO_2 low intermediate oxide, eventually reduced to a -W, with the consumption of crystal transformation and carbon and water vapor reaction in the reduction process, the reduction product becomes loose the porous.W powder average particle size with the precursor carbon ratio decreases when the carbon ratio is higher than 2.6, the residual carbon content significantly increased; with the increase of reduction temperature, precursor of carbon and consumed in the reaction, W particles grow up is weakened, the average particle size of W powder reduction and the carbon content decreases with the increase of reduction temperature. Carbon ratio of 2.6 precursor by 760 oC reduction after 60 min spherical W powder.2. average particle size of 56 nm on the morphology of W powder reduction method, effect of particle size and microstructure, through the analysis of different reduction temperature of W powder crystal Grain growth curve, and presents a collaborative mechanism of preparation of nano W powder reduction hydrocarbon. The results showed that the growth rate of W powder grain with the reduction temperature and slower, more than 760 oC, water vapor and carbon reduction reaction of H_2 and CO, significantly decreased p[H_2O]/p[H_2] in the system, WO_2 OH (volatile gas hydrate inhibition) _2, leading W powder by evaporation deposition method to grow up into atomic diffusion. Reduction on W powder particle size and morphology has an important impact on reduction of W powder by reduction of hydrocarbon synergistic growth mechanism with solid local chemical reaction, the W powder as spherical particles, uniform and fine loose structure, good dispersion of W powder; ordinary hydrogen reduction reduction mechanism to grow evaporation deposition, the W powder coarse particles, fully developed,.3. presents the W intrinsic polyhedral morphology of crystals by hydrocarbon synergetic reduction of nano W powder and carbon black as raw materials. Preparation of nanometer WC powder by carbonization of the stage, stage of carbonization process (carbonization temperature and holding time) on WC powder phase, influence the morphology and particle size. The results show that the carbonation rate of WC depends on the size of the powder W powder and growth rate, high rate and low growth rate of carbonation is conducive to to reduce the particle size of WC layer; low temperature carbonization can form a certain thickness on the surface of W particles, the contact state between particles by the W/W contact to WC/WC contact inhibition of W particles during the initial stage of carbonization by sintering the combined growth caused the particle size of WC. The optimum synergistic thickening hydrocarbon reduction of nano W powder phase carbonization for the pre carbonization temperature 900 oC, holding time of 60 min, two oC 1150 carbonization temperature, holding time of 90 min, the average particle size of 56 nm W 900 oC+1150 oC powder with WC powder.4. carbide phase, the average particle size of 106 nm with the continuous reduction of carbon nano - Preparation of hydrocarbon co legalization M WC powder, reduction effect, the carbon ratio of precursor powder of WC carbon content, effect of carbonization temperature on the morphology and particle size of WC powder. The results showed that the carbon content of WC powder combined with precursor carbon ratio increased, the body carbon ratio of 3.6, the combined carbon content the theoretical value of 6.12%, the free carbon content is 0.06%; when the carbon ratio is higher than 3.6, the free carbon content increased rapidly. Reduction and carbonization process from W to WC transformation with genetic structure, growth coefficient between 1.4~1.6, the average particle size of WC powder with the reduction temperature and reduce the temperature rise of carbon; will promote the migration of grain boundary of WC particles, the average particle size of WC powder with the carbonization temperature increasing. The best preparation technology - reduction hydrocarbon co carbonization of nano WC powder as precursor of carbon ratio 3.6, reduction temperature of 760~800 oC 1100~1200 oC, carbonization temperature; the WC near the ball into a uniform fine powder The shape of particles, the average particle size of 87.3 nm.5. in the preparation of nano WC powder as raw material, preparation of ultrafine WC-Co cemented carbide by low pressure sintering technology, studied the effects of sintering parameters on Microstructure and mechanical properties of hard alloy WC-Co. The results showed that with the increase of the sintering temperature and holding time, the density of sintering body increase, the average grain size increases, the hardness and bending strength will be increased with the density increasing; if the sintering temperature is too high or the holding time is too long, will make the grains grow abnormally, resulting in lower density, optimum parameters of mechanical properties of.WC-6Co sintered alloy decrease as sintering temperature of 1360 oC, holding time of 60 min, the average grain size of the hard alloy samples is 305 nm for ultrafine cemented carbide, Rockwell hardness of 94.6 HRA, the flexural strength reached 4450 MPa.

【学位授予单位】：南昌大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TF123.7;TF125.3

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