Fe-Cr-C系硬面合金及其硬质相的研究进展

针对工件表面的磨损破坏,常通过熔覆、喷涂等手段,在失效位置得到高耐磨性的硬面合金层来进行修复强化。该方法不仅经济方便,还可有效提高工件服役寿命。在表面修复工艺中,硬面层的成分选用极为重要。Fe-Cr-C系硬面合金即一类典型的Fe基表面修复材料,目前正广泛应用于各类工矿耐磨部件的表面修复及强化中,与其他成分的硬面合金相比,它具有几大显著优势:(1)成本低廉；(2)强度、韧度、耐磨性优异且较平衡；(3)性能可调节范围广,能满足于多种磨损工况的修复强化。 传统的Fe-Cr-C系硬面合金主要依靠其凝固时产生的M₃C、M₂₃C₆、M₇C₃、高碳马氏体等几种高硬度物相来获得一定的耐磨性。然而在实际磨损工况中,通常会出现硬度更高的SiC、Al₂O₃等磨料,且近年来,随着各种高新技术的竞相出现,大量机械设备规格的转型升级成为大势所趋,这使得各类耐磨部件需要满足于更为苛刻的服役条件。因此,Fe-Cr-C系硬面合金的耐磨性有待进一步改善。 针对这一问题,目前国内外的研究焦点多集中于合金微观组织调控,尤其是硬质相的引入及其尺寸形态改善等,且取得了一系列可观的成果。在硬质相引入方面,探索出各有优缺点的两种引入手段——原位合成法与外界加入法。其中,原位合成法一方面可在熔池反应中得到高热力学稳定性的陶瓷硬质相,另一方面也可在一定程度上强化合金组织。然而,由于熔池高温停留时间短,某些高熔点硬质相生成效率较低。虽外界加入法可有效解决这一问题,但是也需注意硬质相溶解烧损、硬质相与基体界面稳定性差等现象；在硬质相形态控制方面,不少学者探索出合金成分、熔覆制备工艺对硬质相含量、尺寸形态、生长方向的影响。对于合金成分,调整Cr和C的质量比(以下均简称为Cr/C值)或增加C含量可提高碳化物的体积分数,适量合金元素的添加也可通过异质形核作用细化硬质相；在熔覆工艺方面,提高焊后冷却速率可抑制合金凝固初期C原子的扩散,使初生M₇C₃碳化物呈细小及高密度形态,控制焊后热梯度方向也可使M₇C₃垂直于堆焊面生长。此外,在焊接熔池中适当引入磁场也可诱导液态金属的一次枝晶臂分离,增加碳化物的形核质点,从而起到细化组织的作用。 基于近年来的最新研究成果,本文归纳了Fe-Cr-C系硬面合金中硬质相调控的研究进展。首先介绍了合金的凝固行为与组织结构,然后着重综述了硬质相的引入方式、形态控制手段,最后对Fe-Cr-C系硬面合金未来可能的发展趋势提出见解,并围绕硬质相拟定了潜在的研究方向,以期为进一步改善Fe-Cr-C系硬面合金的耐磨性提供参考。

A Brief Survey on the Fe-Cr-C Hard Facing Alloys and Its Hard Phases

Hard facing alloy layers with high wear-resistance can repair and strengthen the surface of worn workpiece, which is prepared at failure position through cladding and spraying technology usually. This method is not only economical and convenient, but also can improve the service life of workpiece effectively. In surface repairing process, it is extremely important for selecting the composition of hard facing alloy. Fe-Cr-C hard facing alloy, is one of the typical Fe-based surface-repair materials, which has been widely used in the surface repairing and strengthening of diverse engineering and mining wear-parts. Compared to other hard facing alloy with different composition, Fe-Cr-C exhibit several notable advantages including low cost, excellent and balanced strength, toughness, wear-resistance, wide adjustable range of performance, which can satisfy the surface repairing and strengthening of a variety of wear conditions. Regarding the traditional Fe-Cr-C hard facing alloys, the acquisition of certain wear resistance relies on the formation of several high-hardness phases like M₃C, M₂₃C₆, M₇C₃, and high carbon martensite during solidification. However, in actual wear conditions, there are abrasives like SiC and Al₂O₃ which show higher hardness than the alloys. More importantly, with the emergence of a variety of new and high technologies in recent years, the transformation and upgrading of a large number of mechanical equipment specifications have become an irresistible trend, which makes various wear-resistant parts to confront more severe service conditions. Therefore, the wear resistance of Fe-Cr-C hard facing alloys needs to be further improved. In order to solve this problem, most of the researches at home and abroad focus on the control of alloy microstructure, especially the introduction of hard phases and the improvement of their size and morphology. A series of considerable research achievements have been obtained. In the aspect of hard phase introduction, two methods, in-situ synthesis and external addition, are proposed. Through the method of in-situ synthesis, not only can high thermodynamically stable ceramic hard phases be obtained in the molten pool reaction, but the microstructure of alloys can also be strengthened. However, due to the short residence time in molten pool with high temperature, some hard phases with high melting point are difficult to produce. This problem can be effectively solved by the method of external addition, nevertheless, it is necessary to pay attention to the dissolution of hard phases and poor stability of the interface between hard phases and matrix. In the aspect of morphological control of hard phases, the effects of alloy composition and cladding technologies on the volume fraction, size morphology and growth direction of hard phases are explored by many scho-lars. For the alloy composition, adjusting the Cr/C value or increasing the C content contribute to the volume fraction improvement of carbides, and the addition of appropriate amounts of alloying elements can also refine the hard phase by heterogeneous nucleation. In terms of cladding process, improving the cooling rate after welding can inhibit the diffusion of C atoms in the early stage of solidification, which endows the primary M₇C₃ carbides with a fine and highly-dense appearance. Controlling the direction of thermal gradient after welding can also make M₇C₃ grow perpendicular to the surface of hard facing alloys. Besides, introducing a magnetic field into molten pool can also induce the separation of primary dendritic arms in liquid metal to increase the number of carbide nucleation cores, thereby playing a role in refinement. Based on the latest research progress, this article offers a retrospection of the research efforts with respect to the regulation of hard phases in Fe-Cr-C hard facing alloys. The solidification behavior and microstructure of Fe-Cr-C alloys are presented firstly. Then the introduction of hard phase and the means of morphology regulation are reviewed with emphasis. Finally, the development trends of Fe-Cr-C hard facing alloys are proposed and the potential research directions about the hard phases are drawn up in order to provide references for further improving the wear resistance of Fe-Cr-C hard facing alloys.