Microstructure and forming characteristics of spray-formed M42 high speed steel

M42 high speed steel(HSS),a high grade alloy,was produced using the spray forming technology.Optical metallography(OM),X-ray diffraction(XRD) and scanning electron microscope(SEM) were used to investigate the microstructures of the as-sprayed M42 HSS and the as-atomized powders.It is found that as-sprayed M42 HSS is composed of martensite,retained austenite and carbides.The grain size,as well as the morphology,size and distribution of the carbides have been greatly improved compared with those of the as-cast M42 steel(a traditional process).The size of the powder obtained at a high cold speed was ranged from 50 μm to 100 μm in diameter.Its representative microstructures include dendrite,dendritic fragments,a mixture of dendrites,equiaxed grains and dendritic fragments,and equiaxed grains.The rapid solidification in the spray forming is a key factor to cause the refinement of the M42 HSS.     

喷射成形含铌M3型高速钢的组织和耐磨性

采用喷射成形技术制备了M3型高速钢和以Nb替代V的M3型高速钢.利用扫描电镜、X射线衍射、差示扫描量热仪和金相显微镜研究了Nb对M3型高速钢组织的影响.喷射成形能有效消除宏观偏析,细化组织.以Nb代V,提高了MC型碳化物开始析出温度,大量MC相先于共晶反应析出,呈独立的近球形分布于晶界,同时其尺寸减小.由于消耗大量C,抑制了共晶反应,M₂C片层数量减少且厚度变薄,其在热变形过程中更易于分解,进一步增加了组织均匀性.低温低载荷时含铌的M3型高速钢抗磨损性能显著优于M3高速钢,温度升高到500℃时磨损机制逐渐以氧化磨损为主,两合金的抗磨损性能差距减小,主要原因是大量呈弥散球形分布的含铌MC型碳化物能有效提高高速钢的磨粒磨损抗性,而其对抗氧化性能并无明显作用.      

表层富立方相超细晶梯度硬质合金微观结构

通过富氮气氛烧结WC?10TiC?0.5VC?0.5Cr₂C₃?12Co和WC?12Co硬质合金,研究梯度结构和均匀结构硬质合金的微观结构及力学性能。利用扫描电子显微镜观察合金断面的微观形貌,使用X射线衍射仪和能谱仪分析合金物相组成,并对合金表面和芯部的硬度与断裂韧性进行测试。结果表明:与均匀结构的WC?12Co硬质合金不同,WC?10TiC?0.5VC?0.5Cr₂C₃?12Co梯度结构硬质合金的表层富含立方相TiCN,该表层的厚度大约为12 μm,表层下方是富钴粗晶过渡层。WC?10TiC?0.5VC?0.5Cr₂C₃?12Co硬质合金具有表面硬度高、芯部断裂韧性高、WC晶粒细小及分布均匀等优点。     

烧结方法对WC-Co硬质合金性能的影响

以原位还原碳化反应法制备的超细WC-Co复合粉为原料,分别采用放电等离子烧结、低压烧结和真空烧结工艺获得块体硬质合金,系统研究烧结方法对合金的显微组织、密度及力学性能的影响。结果表明:放电等离子烧结的合金中,主相为WC和Co,有少量η相(Co6W6C),低压烧结和真空烧结获得的合金中物相为WC和Co;所用3种不同的烧结方法均能获得细晶块体硬质合金,其中放电等离子烧结的晶粒最细为0.35μm;低压烧结合金具有优异的综合性能,HV30为15 121 MPa,断裂韧性为13.6 MPa.m1/2,横向断裂强度为4 210 MPa。     

电子束熔炼对高速钢碳化物的影响

 细小而弥散分布的碳化物是高速钢优良性能的保证,但复杂的合金成分导致铸态高速钢中碳化物粗大、偏析严重。为了探究电子束熔炼对铸态高速钢的影响,改善高速钢中的组织及碳化物状态,采用电子束熔炼技术制备M35高速钢,并对其成分、组织等进行了表征。结果表明, EBM-M35高速钢中平均枝晶间距为20 μm,碳化物尺寸细小,在组织中均匀分布,主要类型为MC和M₂C,且M₂C型碳化物由层片状向纤维状转变。对铸态EBM-M35高速钢热处理时发现,在1 180 ℃,保温30 min后碳化物断裂球化,达到细化碳化物、使碳化物在组织中弥散分布的效果,并且可以利用较低的热处理温度或较短的保温时间完成碳化物的优化。为铸态高速钢后续锻造、轧制等变形细化提供更优异的组织基础。     

Microstructural characterization of INCOLOY 903 weldments

The fusion zone and the heat-affected zone (HAZ) microstructures obtained during electron beam welding of thermomechanically processed INCOLOY 903 have been characterized by analytical electron microscopy. The microsegregation observed during solidification in the fusion zone indicates that while Fe and Co segregate in the gamma dendrites, Nb, Ti, and C are extensively rejected into the interdendritic liquid. Electron diffraction and energy dispersive X-ray microanalyses on secondary phases extracted from the fusion zone, from the HAZ microfissures, and from the HAZ grain boundaries on carbon replicas established the major secondary solidification constituent formed from the interdendritic liquid to be cubic niobium-rich MC carbides. Laves phase was observed only in trace amounts. Continuous sheets of the MC carbides were observed in the HAZ grain boundary microfissures, while the HAZ grain boundaries that resisted microfissuring were devoid of the continuous sheets of carbides and/or extensive fine carbide precipitation. The microstructure observed in the HAZ microfissures suggests grain boundary liquation and formation of low melting liquid films enriched in niobium and carbon. The liquid originates primarily from the constitutional liquation of primary MC carbides, MNP phosphides, and fine MC carbides already present on the grain boundaries of the base metal. The solidification pattern of the liquid films on the microfissures is observed to follow a similar pattern as that of the fusion zone. The observation of microfissuring on boundaries which show continuous and/or semicontinuous sheets of resolidified structure suggests that HAZ microfissuring occurs due to the presence of low melting liquid films. The existence of continuous and/or semicontinuous liquid films and the added presence of phosphorus in them appear to be one of the major causes for extensive HAZ microfissuring in thermomechanically processed INCOLOY 903.     

放电等离子烧结制备超细晶WC-3Co硬质合金的组织和性能

采用高能球磨制备纳米WC-3Co粉末,再通过放电等离子烧结(spark plasma sintering,SPS)制备超细晶WC-3Co硬质合金。研究SPS工艺参数对合金致密度、显微组织和力学性能的影响,并对SPS和热压工艺(hotpressing,HP)进行对比。结果表明:SPS可实现WC-3Co粉末的低温快速致密化。升高温度或提高压力都使得合金的致密度提高,同时导致WC晶粒长大。SPS较HP升温速率快且烧结时间更短,合金组织更加均匀,在1 300℃保温5 min、烧结压力为40 MPa的条件下所制备的合金具有最佳综合性能,其平均晶粒度为0.32μm,相对密度、硬度、抗弯强度、断裂韧性分别为99.3%、2257 HV30、1 906 MPa、10.36 MPa.m1/2。而在1 450℃、压力为50 MPa、保压5 min条件下,热压合金的致密度、硬度和断裂韧性分别为99.6%、2 264 HV30和11.01 MPa.m1/2,但抗弯强度只有1 301 MPa,平均晶粒度为0.47μm。     

气体雾化快凝W9高速钢粉末的组织特征

研究了颗粒粒度不同的气体雾化W9Mo3Cr4V高速钢粉末的组织、形貌及结构.结果表明:粉末由铁素体和奥氏体基体和分布在周围的MC及M2C型碳化物组成,基体组织为等轴晶及树枝晶,碳化物在空间上呈连续网状或树枝状分布;粉末中相的含量与粉末粒度相关,随高速钢粉末粒度减小,组织中的铁素体和MC型碳化物含量增加,奥氏体和M2C型碳化物含量减少.     

实际初始WC颗粒尺寸选择的LSW理论分析

超粗硬质合金是一类重要的矿用硬质合金.本文作者通过对WC平均晶粒尺寸为7～8 μm粗晶硬质合金的烧结实验,并用LSW动力学理论分析液相量相对较少的超粗硬质合金(＞5 μm)的晶粒生长机制.分析发现,可以用Co对WC晶粒的覆盖度替代LSW动力学中的原子动力学转移系数.经过修正的LSW动力学方程可以用来指导超粗WC(＞5 μm)实际初始颗粒平均粒度的选择.修正后的LSW动力方程形式为-2ax1--2ax0=0.125(C0γV2m/R)(t/T).     

Solid solution decomposition mechanisms in as-cast and microcrystalline Al-Sc alloys: IV. Effect of the decomposition of a solid solution on the mechanical properties of the alloys

The effect of the decomposition of the solid solution in Al-X wt % Mg-0.22 wt % Sc-0.15 wt % Zr (X = 0, 1.5, 4.5 wt %) alloys on their mechanical properties in the as-cast and microcrystalline (MC) states is studied. The contribution of the particles that precipitate during the decomposition to the mechanical properties of the as-cast and MC alloys is determined. For this purpose, the dependences of the volume fraction and the size of the particles precipitating during the decomposition on the annealing time and temperature and the magnesium content in the as-cast and MC alloys are calculated. An additional contribution to the hardening of the MC alloys is shown to be related to the internal stress fields created by the defects that accumulate at the disperse particles distributed in grain boundaries during grain-boundary migration. The calculated macroelasticity limits in the as-cast and MC alloys are compared with the experimental data.     

WC–Co硬质合金摩擦磨损行为的分子动力学模拟

利用分子动力学模拟研究了WC–Co硬质合金在不同条件下的摩擦过程,分析了晶粒尺寸、摩擦载荷和滑动速率等因素对硬质合金摩擦磨损行为的影响,从原子尺度揭示了硬质合金发生摩擦磨损的微观机制。结果表明,随晶粒尺寸增大,相比于晶粒转动,Co相和WC中的位错滑移逐渐在摩擦引起的塑性变形机制中起主导作用。摩擦载荷增大会导致易变形的Co粘结相被挤出表面而首先去除,通过减小晶粒尺寸可以抑制Co相的挤出–磨损机制,进而提高硬质合金的抗滑动磨损性能。滑动速率升高会降低磨损速率,主要原因是在高速滑动过程中,亚表层各相中位错的形核扩展缺乏持续的驱动应力,位错密度较低,WC不易发生断裂,Co相被挤出表面造成的磨损程度明显减轻。     

高氮不锈轴承钢碳化物分布与高温断裂机制

 通过高温拉伸试验研究高氮不锈轴承钢高温断裂行为,探究了170 ℃和470 ℃回火态钢中碳化物分布特征,分析了高温拉伸断裂及组织演变和碳化物分布规律。研究发现,回火温度从170 ℃升高至470 ℃,高氮钢中大于0.8 μm的碳化物明显增加,高氮钢中M₂₃C₆强化增量提高了2.59 MPa,固溶强化增量下降了118.82 MPa,470 ℃回火态钢的室温抗拉强度降低、拉伸断口表现为准解理和少量撕裂韧窝;拉伸温度升高至300 ℃,试样断口表现为等轴型韧窝特征,170 ℃和470 ℃回火态试样起裂源断裂碳化物尺寸分别为2.8~3.6 μm和5.5~6.7 μm;450 ℃拉伸断口表现为塑孔韧窝特征,170 ℃和470 ℃回火态试样起裂源断裂碳化物尺寸分别为2.7~3.4 μm和5.8~6.4 μm。拉伸温度从300 ℃提高至450 ℃,钢的固溶强化和位错强化作用减弱,金属原子间结合能下降,碳化物与基体不连续应力分布加剧变形不协调性,碳化物承担较高应力而发生断裂。单纯热作用下钢中0.5~0.8 μm尺寸碳化物数量比例增加;在热力耦合作用下,钢中应力所导致的位错增殖为碳元素扩散提供通道,钢中碳化物在晶界和位错线上形核析出0.2~0.8 μm碳化物。裂纹沿着与拉伸方向45°角的最大剪力方向快速扩展而断裂,最终形成锯齿状的断口,小尺寸碳化物增多阻碍位错滑移导致塑性降低;钢中大尺寸碳化物不均匀分布在碳化物间形成大变形塑孔而增加钢的塑性。     

从硬质合金微观结构和性能对湿磨工艺的敏感性评价WC粉末的内在质量

以XLWC25型WC粉末为原料,采用滚动湿磨和搅拌湿磨工艺经过不同的湿磨时间制备5组WC-6Co硬质合金。从合金微观组织结构参数(平均晶粒度、邻接度、晶粒分布)和力学性能对湿磨工艺的敏感度方面,探讨了硬质合金关键原材料WC粉末内在质量的评定方法。结果表明,合金微观组织结构和力学性能对湿磨工艺变化的敏感度较低,在一定程度上反映了XLWC25型WC粉末具有较好的内在质量。采用搅拌湿磨工艺,湿磨6¹0 h,WC-6Co合金的微观组织结构均匀性和综合性能可达到最佳状态,优于滚动湿磨58 h制备的参比合金的综合性能。     

Effect of rare earth elements on the thermal cracking resistance of high speed steel rolls

The effect of rare earth elements on the thermal cracking resistance of high speed steel (HSS) rolls was investigated. Laser rapid heating was used for thermal fatigue experiments. Thermal cracks and microstructure were observed using metalloscopy and scanning electron microscopy. The results showed that thermal cracks initiated from the interface between the matrix and eutectic carbides (including M6C and M7C3 type carbides),and propagated along the interface between the two phases. MC type carbides enriched with vanadium could prevent the propagation of thermal cracks. The presence of rare earth elements decreased the quantity of big eutectic carbides,and proportionally increased spherical and rod-shaped MC type carbide content. HSS0 (0.00% RE) had approximately three times the thermal cracking density of HSS3 (0.12 wt.% RE). Rare earth elements were shown to significantly improve the microstructure and thermal cracking resistance of HSS rolls.     

Effects of Alloying Elements on Microstructure,Hardness, Wear Resistance,and Surface Roughness of Centrifugally Cast High-Speed Steel Rolls

A study was made of the effects of carbon, tungsten, molybdenum, and vanadium on the wear resistance and surface roughness of five high-speed steel (HSS) rolls manufactured by the centrifugal casting method. High-temperature wear tests were conducted on these rolls to experimentally simulate the wear process during hot rolling. The HSS rolls contained a large amount (up to 25 vol pct) of carbides, such as MC, M₂C, and M₇C₃ carbides formed in the tempered martensite matrix. The matrix consisted mainly of tempered lath martensite when the carbon content in the matrix was small, and contained a considerable amount of tempered plate martensite when the carbon content increased. The high-temperature wear test results indicated that the wear resistance and surface roughness of the rolls were enhanced when the amount of hard MC carbides formed inside solidification cells increased and their distribution was homogeneous. The best wear resistance and surface roughness were obtained from a roll in which a large amount of MC carbides were homogeneously distributed in the tempered lath martensite matrix. The appropriate contents of the carbon equivalent, tungsten equivalent, and vanadium were 2.0 to 2.3, 9 to 10, and 5 to 6 pct, respectively.     

Effects of cobalt additions on WC grain growth

Abstract

Inhomogeneity in the particle size of the tungsten carbide raw material can result in abnormal WC grain growth in WC–Co cemented carbides. For the preparation of ultrafine tungsten carbide powders and ultrafine cemented carbides, abnormal WC grain growth is the most troublesome issue. This paper deals with the effects of cobalt additions on WC grain growth during the carburisation process of nano- and coarse tungsten powders and the sintering process of ultrafine tungsten carbide powders. For the preparation of tungsten carbide powders, it was shown that through the incorporation of 0·035 wt-%Co into W+C mixtures, a dramatic change in WC grain morphology took place for coarse tungsten raw material, while for nanotungsten raw material, a pronounced WC grain growth took place. Plate-like truncated trigonal and hexagonal WC grains were formed during the carburisation process of coarse tungsten raw material containing 0·035 wt-%Co. For the sintering of ultrafine tungsten carbide powders containing 0·3 wt-%Co, an anisotropic and abnormal WC grain growth took place. The mechanisms for WC grain growth were discussed, and suggestions were made for the quality improvement of nano- and ultrafine tungsten carbide powders and ultrafine cemented carbides.     

硬质合金添加陶瓷颗粒3Y-PSZ的实验研究

为研究3Y—PSZ颗粒在外力作用下t→m相变增韧增强金属陶瓷材料的可行性，采用热等静压真空烧结工艺制备不同含量3Y—PSZ的WC-20％Co硬质合金。试验结果表明：3Y—PSZ在WC-20％Co基体中呈球形，均匀分布在Co相和WC相中，添加了3Y—PSZ的WC-20％Co的硬质合金抗弯强度和冲击韧性明显提高，耐磨性能有明显改善。     

水冷铜模与砂模铸造M2钢显微组织对比

采用实验室25 kg高频真空感应炉熔炼M2钢,并用水冷铜模和砂模均浇铸为横截面100 mm×50 mm的M2钢铸锭,研究冷却速度对M2钢二次枝晶间距、渗透率、碳化物和晶粒尺寸及分布的影响.研究结果表明:M2钢凝固过程中,快的冷却速度能有效减小二次枝晶间距、渗透率、晶粒和网状碳化物的尺寸,同时可以改善晶粒和网状碳化物的分布和均匀性;砂模和水冷铜模M2钢铸锭的平均二次枝晶间距分别为42.5μm和21.6μm,平均冷却速度为1.06 K·s-1和12.50 K·s-1,平均渗透率分别为0.13μm2和0.035μm2.快的冷却速度能有效减轻中心碳偏析程度,砂模和水冷铜模模铸的M2钢铸锭中心碳化物面积分数分别为0.46和0.30,且其较各自的平均值分别增大38.7%和2.2%;水冷铜模铸锭平均晶粒尺寸（43.1μm）较砂模铸锭的平均晶粒尺寸（72.6μm）减小约40.7%,铸锭中心晶粒尺寸减小43.2%,且水冷铜模铸锭的晶粒尺寸较砂模铸锭均匀.文中获得了M2钢凝固过程中晶粒尺寸与冷却速度的关系式.      

GH2132合金混晶条带组织分析及其对力学性能影响

 混晶条带是影响GH2132合金组织及性能稳定性的重要因素,因此针对合金冷拉棒材出现的混晶条带缺陷进行了试验研究。利用金相、EPMA、EBSD和TEM手段,结合热力学平衡相图与硬度测试,揭示了混晶条带的主要成因,并分析了其内部微观组织状态及混晶组织对显微硬度的影响。结果表明,混晶条带组织中细晶区晶粒尺寸普遍小于10 μm,粗晶区晶粒最大可超过60 μm,而元素偏析与冷拉变形是造成晶粒大小差异并形成混晶条带的原因。合金铸态组织中Ti、Mo、C、B元素均表现出正偏析,其中C、Ti元素的偏析程度较高,能够在枝晶间析出MC与M₃B₂相,并最终遗传至奥氏体晶界;晶界溶质富集不仅起到钉扎作用,阻止再结晶过程晶粒长大,还能够通过溶质拖拽作用降低晶界的迁移速率而阻碍晶界变形,最终溶质富集区域形成细晶区、贫化区域形成粗晶区,并沿合金棒材组织冷拉方向表现出混晶条带。另外,在冷拉变形过程中,由于晶体取向不一,在受同样拉拔力条件下晶粒变形的实际应变并不一致,这也会加剧混晶现象。微观组织观察显示冷变形后细晶区与粗晶区相比形变更为均匀,组织内平均位错密度更高,混晶区域内存在退火孪晶与形变孪晶,但细晶区孪晶密度更高。最终混晶区存在的晶界数量不同、位错密度变化、孪晶分布的梯度组织,导致细晶区显微硬度明显高于粗晶区的硬度梯度。