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钨、钼、碳三元共渗等离子表面冶金低合金高速钢的研究
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摘要
高速钢是机械加工中经常使用的一种工具材料。近年来世界高速钢的产量一直保持在25万吨左右。我国的年产量达4万吨。高速钢成分复杂,合金元素含量多,工艺特殊,性能要求高,价格昂贵。在钢铁材料尤其是特殊钢中占有较重要的位置。由于高速钢有较高的硬度(HRC63~67)和在550~600℃的温度下可保持高的硬度(HRC60以上)、耐磨性和耐热性,因此被广泛的应用于机械加工的刀具、加工成型的模具以及一些要求高耐磨或高温工作的零部件上。
     高速钢的生产大多采用传统冶金方法,即铸锭—锻轧工艺。由于钢的合金含量高,化学成分复杂,铸锭尺寸大,冷却速度缓慢等缘故,在其凝固时不可避免地会产生粗大的莱氏体碳化物偏析组织。偏析的存在不仅给钢的锻、轧等热加工造成困难,而且还明显地损害了钢的各种性能,限制了高速钢中合金含量的增加,影响了高速钢的发展。
     在冶炼铸态高速钢这种严重的成分偏析、组织偏析、碳化物不均等缺陷中,前两种缺陷通过后续的热处理退火及淬火均可消除。但是存在着粗大共晶莱氏体组织的碳化物不均匀缺陷,其危害程度较前两种缺陷要大的多,且由于是在结晶过程中形成的共晶碳化物,即使加热到1300℃的高温,也难溶于奥氏体中,因而无法通过热处理改变其组织和形态。粗大的网络状和密集的条带状碳化物在很高的加热温度下不能充分溶解于奥氏体中,这就减小了奥氏体内的合金度,使得高速钢刃具回火后的硬度、热硬性和耐磨性大大降低。
     铸态组织中莱氏体共晶网络和碳化物不均匀性,解决方法只能是随后进行锻造加工。通过碎化共晶碳化物,改善组织不均匀状态。不均匀越严重所需的锻压比越大。但即使这样也难以完全消除。有实验指出,对一般合金钢给予3~4倍形变度,组织均匀可以满足使用需要。而对高速工具钢五倍以下的变形度,沿金属流变方向仍保留碳化物网状的特征,七倍时共晶网才破裂,十一倍时块状和网状的共晶改变为连续条带状,但宽度仍较大。变形度增大到三十倍时,碳化物才成为狭窄的线条,再增大变形度效果也不明显。但硬而脆的共晶碳化物偏析依然存在,未能得到较圆满解决。因此共晶碳化物偏析程度成为鉴定高速钢质量优劣的重要技术指标之一。
     根据统计资料表明,高速钢的主要元素(W、Mo)的资源在世界范围内勘定和可取的储量只够40~60年使用,加上潜藏的储量也只够100年。因此节约合金元素既有重要的战略意义,又可节约钢材成本。于是各国都在研制一些低合金高速钢。70年代以后,由于
    
    太原理工大学博_卜论文
    合金资源短缺日趋严重,低合金高性能高速钢的发展引起了全世界
    的普遍贡视。世界各国己相继发展了SW3S2(波兰),D950(瑞典),
    VaeoDyne(美国),301、Dlol、Dlo6(中国)等低合金高速钢。低合金
    高速钢的成分设计一般以高速钢的基体成分为基础,其W、M。合
    金元素总量只占高速钢的1/2一1/3。新型低合金高速钢的性能特
    点是抗弯强度和韧性一般高于传统高速钢,二次硬化和600℃以下
    的红硬性略高于或接近于传统高速钢,但600℃以上的红硬性和高
    温硬度低于传统高速钢,这是因为W、Mo含量大幅度降低引起的。
     近几年出现了利用双层辉光离子渗金属技术制备等离子复合渗
    表面冶金高速钢。其形成机理是通过在廉价的金属材料表面,渗入
    合金元素W、M。、Cr、V、Ti、Co等,然后再进行渗碳,使表面成分
    达到高速钢的基本要求。通过后续高温淬火和高温回火,使表面达
    到高速钢的性能。由于仅是在材料表面渗入合金元素,占整体材料
    的极少部分,从而大大节约了合金元素。该工艺技术的突出优点是:
    形成的表面高速钢层与基体是冶金结合,不存在着渗层与基体的剥
    落问题;渗层碳化物是在较低的温度下固态形成,属二次碳化物(相
    对于从结晶中析出的碳化物而言),较易溶入奥氏体中,可充分的发
    挥合金元素的作用;形成的碳化物均匀、细小、弥散、一般呈粒状
    分布,没有粗大共晶莱氏体组织。该工艺技术的发明,引起了各方
    面的极大关注。
     但是,该工艺技术由于采用先渗W、Mo,再固溶处理,最后渗
    碳的多步形成表面高速钢的方法。高温加热次数多,工件和卡具严
    重变形,且工艺复杂。还存在多次高温加热后粗大组织的遗传,使
    最后的淬火组织不正常,易出现过热或欠热现象。另外有些沿晶析
    出的粗大呈网状的碳化物或金属间化合物,造成渗层成分的不均匀,
    严重的影响了湘L械性能。
     本旬{究课题就是针对前期双层辉光离子渗金属技术形成的等离
    子复合渗表面冶金高速钢现存的问题,进行的技术改进研究。其基
    本思路是:在原进行离子渗W、M。仅通入惰性气体氢气的同时,通
    入碳一氢化合物气体甲烷,使气体中的碳原子和氢原子参与离子溅
    射、电离、源极和工件表面活化以及渗入过程。源极中被溅射出的
    W、Mo原子进入被渗金属材料表面的同时,气氛中的碳原子也同时
    被渗入金属材半}表而。使被渗金属材料表而一次直接形成具有既含
    源极,1,欲渗金属元索W、Mo,又有才C体l一卜元索碳的等离子表面冶金
    高速钢层。氢原子参与源极和工件表面的还原和活化过程。
     W、Mo、C共渗工艺还可以在形成等离子表面冶金高速钢层以
    后,直接淬火?
High speed steel (HSS) is a very important tool steel in machining industry. The global output of HSS is 1/4 million tons in recent years, and about 40,000 tons comes from China. The producing of HSS is a quite sophisticated process due to the addition of multiple alloy elements and their high contents, so it has always been expensive but very important alloy steel. HSS has high wear-resistance and heat-durability, its hardness reaches HRC63 ~ 67 after heat treatment and keeps at above HRC60 at 550-600.These features of HSS make it a very important material for producing cutting tools, dies and those parts requiring high wear performance or working at high temperature.
    Majority of the HSS is produced by the traditional metallurgy process, or ingot-casting and forging rolls. Due to the high alloy content, high dimension of ingot and low cooling rate, the coarse ledeburite eutectic segregation will be formed within the ingot inevitably. The segregation is harmful to the mechanical property of the steel and also makes the forging and rolling process more difficult..
    The severe composition and structure segregation could be eliminated by annealing and quenching process, but the elimination of the non-homogeneous distribution of the coarse ledeburite is impossible through any kind of heat treatment process, and it is more damaging to the property of the steel.. Since the ledeburite eutectic carbides are formed during the crystallization process, they are not soluble to the austenite even at high temperature of 1300C, resulting the lack of alloy elements soluble to the austenite, and hence the decrease of hardness, wear-resistance and heat-durability.
    The general solution to the problem caused by the ledeburite eutectic carbides is a forging operation to break up the carbides. High forging ratio is beneficial to reduce the segregation, but it is almost impossible to eliminate the non-homogeneous distribution through forging process. Investigation shows that for average low alloy steel, deformation of 3-4 times is enough to obtain uniformed microstructure which could meet the need for working condition. While for HSS, with deformation lower than 5 times, the netshaped carbides still exist along the direction of deformation, it starts to break up at 7 times of deformation, and the netshape turns to continuous wide strip. At 30 times of deformation, the wide strips change to narrow strips and remain almost unchanged at further higher deformation rate. The hard and brittle eutectic carbide always exists after very high rate of forging, so the extent of segregation of the carbides is a very important criterion to judge the quality of the HSS..
    According to statistics, the main alloying elements in HSS, tungsten and
    
    
    molybdenum, will be exhausted on earth in 40-60 years, even if considering the potential ore, the time span is no more than 100 years. Saving the valuable elements is very important and can also reduce the cost to produce the steel. Toward this end, HSS with lower alloy content are developing in different countries. Since 1970's, with the enlarged shortage in alloy elements reserve, high performance HSS with low alloy content has drawn more and more attention around the world. Until now, a series of products has been developed, such as SW3S2 in Poland, D950 in Sweden, VacoDyne in U. S., 301/D101/D106 in China, etc. The alloy content in low alloying HSS usually is close to the matrix alloy content in traditional HSS, and the whole alloy content is reduced to about 1/2-1/3 of traditional HSS. And to ensure comparable main properties, usually Si, N, Re are added. The main feature of the low alloying HSS compared with traditional HSS is: higher bending strength and toughness; a little higher or similar secondary hardening ability and red-hardness under 600C; lower red-hardness and high temperature hardness above 600C, a result of reduced W/Mo contents.
    To further save the alloy elements, surface metallurgy HSS has been developed in recent years. By applying surface alloying process on surface of cheap steels ,alloy
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