| It is believed that the three major failure forms of ferrous material are: abrasion, corruption, fracture, the world expends more than 350 million tons ferrous material every year by abrasion only. Because wear abrasion is a comparatively complicated system, it is determined by both work circumstance and material property. If the work circumstance is different, so is the abrasion mechanism. Therefore, to accomplish our aims at applications, with no omnipotent wear-resistance materials at all, we have to select the proper wear-resistance materials under a given work circumstance. Under different work circumstance, materials have different failure forms: some are resulted from abrasion of the whole body; others are from local abrasion, for example, the failures of crusher's hammerhead and grab's teeth are resulted from local wear abrasion. Under this circumstance, to guarantee the safety of the task process above, the ideal wear-resistance materials should be high wear-resistance at local abrasion failure position, while high obdurability at no rubbing abrasion position. However, there is a pair of contradiction between wear-resistance and obdurability: high rigidity can increase wear-resistance, but lower the obdurability and depress the safety; contrarily, if we strengthen the obdurability, hardness may be depressed and lower the wear-resistance. So simplex traditional material cannot resolve this contradiction. One of the best ways to resolve the problem is the selection of composites, and steel matrix composites rinforced by particulates are the most effective method in solving rubbing abrasion problems. There are some defects in the technology of ex situ particulates: ①the size of the particulate is influenced by the starting materials, and they have large size and always is 1~2μm or tens of microns; ②particulates are prone to be contaminated because of the absorbed gas and impurities at the surface, so there is poor wettability between particulate and matrix. Thus, it is difficult to control the interface reactions between them; ③these particulate will be oxidated, burned up, decomposed and so on when they are added into high temperature melts; and ④some other problems may happen when they are added, for example, gas and lard may be entangled into the melts. To overcome the inherent problems which are associated with conventional processed MMCs, new in situ processing techniques have been developed in recent years, which is able to solve the problems of the ex situ. However, all the progressed composites are monolithic composites, it makes the composites some disadvantages in the application, such as: (1) the toughness of the composites is decreased due to their monolithic reinforcement; (2) the composites cost much, because there are reinforced with ceramic particulate in everywhere, even through some need not reinforced; (3) for the ceramic particulates are high melting temperature and have been formed before processing, while its volume percentage is little it has not difference in melt castablity, but the reinforced of the composites is little. When high, it has the poor castability and makes the composites quality poor. In order to overcome the defects of the monolithic reinforced composites, the locally reinforced composites have been developed in this research. Compared to monolithic reinforcement, the most obvious advantages of local reinforcement are primarily: (1) it optimizes the performance of composites. There has a very well reinforcement in the local region that need reinforced, and other region in matrix has a super strength and toughness, as makes composites a good performance under some circumstance and a longer using time; (2) the high volume fraction (10%~60%) of reinforcement distributed only in the local region, which can not realized in the monolithic composite (usually with a reinforcement <10%~15%) fabricated by conventional casting route due to the poor castability, indicates that the local reinforced composite is a promising type of material for processing the large, complex and high volume fraction locally reinforced MMCs; (3) it reduces the processing cost and saves a lot of valuable reinforcement particulates; and (4) its technology is easy , it can universally applied. 1. According to the thermodynamic calculations of Fe-Ti-C-Al system, the theory evidence of system reaction of forming TiC particulate was provided. (1) It is suggested through thermodynamic analysis that TiC is much more stable than TiAl3, Fe2Ti, Fe3C and Al4C3. Generally, TiC could be formed in the liquid steel with the temperature of 1600℃.(2) The Gibbs energy ( ? G) of formation of TiC, TiAl3, Fe2Ti, Fe3C and Al4C3 during 1400~1600℃decreased with the increaseing of Al content; the increase of Al content can promote the formation of TiC; the increase of Ti content has no significant effect on ?G of TiC formation, but can decrease ?G of TiAl3 and Fe2Ti formation. Moreover, the increase of Ti conten can improve ?G of Fe3C formations; the C can promote the decrease of ?G of Fe3C, TiC and Al4C3 formations, but has no significant effect on formation of TiAl3, and superfluous C can promote the decomposition of Fe2Ti. 2. After pretreatment, the performs consisting of Fe, Ti, C and Al were placed in the mold. Then the SHS reaction was induced with the liquid steel of high temperature, and the solidification of liquid metal and the formation of in situ TiC in the steel liquid were accomplished simultaneously. It can avoid the poor wettability of ex situ particulates, the easily polluted interface, the poor fluidity of the molten metal with the addition of particulate and the difficult mould filling effectively, and TiC particulate with a large volume fraction locally reinforced cast steel matrix and cast 45Mn2 steel matrix composites were prepared successfully. A strong interfacial bonding of the reinforced region and matrix in the TiC particulate locally reinforced 45Mn2 steel matrix composites were achieved; furthermore, the emergence of transition region promote the improvement of the interfacial strength; the products in the particulate reinforced region consisting of TiCp and matrix steel. The sizes of TiC particulates changed from 2 to 5 μm, and most of particulates exhibit spherical except a small amount of polygonal particulates. 3. The effects of SHS reaction dynamical factors of Al-Ti-C and Fe-Ti-C-Al systems in the melt on the size and distribution of in situ TiCp were investigated. (1) Effect of SHS reaction dynamical factor of Al-Ti-C system in the melt on the size and distribution of in situ TiCp. a) When Al contents increased in the preforms, the size of TiC particulate decreased, and the distribution of TiC particulate was more uniform. At the same time, the size of TiC particulate increased with the increasing of the size of Al powder. These results were in accordance with the law of SHS reaction out of melts but under the protective atmosphere. b) When the size of Ti powder increased from 38 μm to 75μm, the size of TiC particulate increased slightly from 1~3 μm to 3~5 μm. c) The sizes of C powder had hardly effects on the size of TiC particulate.TiCp were uniformly distributed with the increasing of the sizes of C powder. These results were in accordance with the law of SHS reaction out of melts but under the protective atmosphere. (2) Effect of SHS reaction dynamical factor of Fe-Ti-C-Al system in the melt on the size and distribution of in situ TiCp. a) The compact density of green preforms as a function of uniaxial pressures were obtained, which provided reliable data for the following experiments. When the compact density of green perform was 55-65% theoretical density, the ability of infiltration of molten steel into the prefom was the best, and the smallest microporosity of the locally reinforced region was obtained (~ 3% of the volumetric ratio). b) In the Fe-Ti-C-Al system, the effects of the sizes of Ti-Fe and C powders and Al contents on the hardness of locally reinforced region with in situ TiCp in the as-cast steel matrix (45Mn2) composite was: the hardness hardly changed with the increasing of the size of Ti-Fe powder, and increased slightly with the increasing of the size of C powder. In addition, the hardness decreased with the increasing of Al contents. c) In the Fe-Ti-C-Al system, the morphologies of TiCp in the locally reinforced region were almost spherical, and the size of TiCp increased with the increasing of Al contents. When the Al content was 30%, the maximum size of TiCp was obtained, which maybe related with the exothermic reaction to form FeAl in the SHS reaction of Fe-Ti-C-Al system. 4. The following path is the transition from the starting powder (Fe-Ti-C-Al) to TiC particulates via SHS in steel melt: the preform in the mould was quickly heated by the steel melt and the Al powder in the preform was firstly molten and the molten Al reacted with Fe to form FeAl, and the exothermic reaction promoted the molten Al reacted with Ti in the preform to form TiAlx, which triggered the two reactions (TiAlx+C→TiC+xAl and Ti+C→TiC) occurred to form TiC particulates. 5. The heat treatment can change the shape of TiC particulates of locally reinforced region from spherality to polygon, which can increase the hardness in locally reinforced region and matrix region and the maximum hardness value is 66.9 and 53.7 HRC, respectively. Furthermore, the bonding between locally reinforced region and matrix region is strong. In addition,the optimal heat treatment parameters are water quenched at 805 ℃and tempered at 200 ℃. 6. The wear resistance disciplinarian of reinforced region of TiCp being synthesized via SHS reaction in Fe-Ti-C-Al system as follows: (1) When matrix steel is Mn steel by casting and reinforced region of TiCp containing 10wt.% Al content, the micro-hardness is the highest and wear resistance is the best, nevertheless the Al content increases, the wear resistance decreases. The relatively wear resistances compared to Mn steel matrix, when wear resistances test was carried out with a pin-on-disk tester under applied loads of 5N performed on SiC abrasive paper of 600 grit, and 20N performed on SiC abrasive paper of 240 grit, are 4.97 and 3.07, respectively. The main invalidation form of Mn steel matrix wear is plough. The hard SiC particles cut in the soft Mn steel matrix under the applied loads, and as the applied load increases, the furrow gets relatively broader and deeper. The wear invalidation mechanism of reinforced region of TiCp is: TiCp restrains SiC particles from cutting in Mn steel matrix and reducing the degree of ploughing, so the furrow becomes relatively narrower and shallower. (2) When matrix steel is 45Mn2, the wear resistances of reinforced region of TiCp and 45Mn2 steel matrix after heat treatment; when wear resistances test was carried out with a pin-on-disk tester under applied loads of 15N and 35N performed on SiC abrasive paper of 400 grit, are 2.00 and 2.34, respectively. The wear invalidation mechanism of TiCp locally reinforced 45Mn2 steel matrix composite is: after heat treatment, TiC particulate in reinforced region are more steadily inlayed martensite, and therefore, with the the applied loads increasing, the wear resistances compared to steel matrix improved. So heat treatment is an efficient method of increasing the wear resistance of reinforced region. In conclusion, an excellent combination of the SHS technology and the traditional casting technology was achieved in this paper. The SHS reaction in the preform was ignited by the steel melt and the SHS product was penetrated and diluted by the steel melt, which successfully solve the problem such as the densification technology, porosity and high volume fraction of reinforcement particulates of SHS. Therefore, the application range of SHS technology was enlarged and a new path with independent knowledge property right for fabricating particulate locally reinforced steel matrix composite was developed. |
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