Microstructures And Properties Of Fe-C-B-Cr-Nb Hardfacing Layers

With the rapid production pace of modern industry and increasing efficiency, theproblem of wear failures on mechanical equipment, components and parts is getting worseand worse. It brings huge economic losses to the enterprises. Through the surfacing repairof the worn parts, the damaged parts can be used again, which reduces the number ofreplacement of them, improves the utilization rate of the machines, reduces the productioncosts, and creates the economic benefits for the enterprises. Thereby it is significant tostudy wear-resistant materials and improve their wear resistances.In this paper, a series of homemade flux cored wires were used to prepareFe-C-B-Cr-Nb surfacing layers on Q235steel with arc surfacing welding method. And theinfluence of alloying elements of B, C and Nb on the microstructures and properties ofFe-C-B-Cr-Nb based hardfacing metals was studied with OM, SEM, XRD, macro hardnesstesting, abrasive wear testing, impact toughness testing methods.The phase compositions of Fe-C-B-Cr-Nb based hardfacing metals include austenite, asmall amount of martensite and （Fe,Cr）7（C,B）3in the experiment. Element B can reduce thecarbon contents of the eutectic region and make the eutectic point of Fe-C-Cr based alloysmove to left. The amount and volume of the first precipitates increase in the surfacingmetals with increasing the added amounts of CaB₆and B₄C. The element Nb has the effectof refining grains in the crystallization process. There will be a part of NbC appearing inthe hardfacing metals while some ferroniobium powders are added into the flux cored wires.The number of the first precipitates increases with increasing the added amounts offerroniobium, but their volume uniformly reduces.The macro hardness can be improved approximate linearly with increasing the addedamounts of CaB₆, B₄C and ferroniobium in the flux cored wires. The average macrohardness is53.0HRC without adding CaB₆, and it arrives to58.8HRC with the highestamount of CaB₆addition. Of course, the average macro hardness is from54.0HRC to57.8HRC when the Nb contents in hardfacing metals are increased from0to1.93%. The major wear mechanism of the hardfacing metals is micro cutting and regionalfracture peeling during the abrasive wear process. The abrasive resistance of surfacingmetals is firstly improved and then brings down with adding CaB₆powders. It arrives to thebest with3.0%addition of CaB₆. And the relative wear-resisting coefficient is3.3times ashigh as that of the Q235base metal. If continuously increasing the CaB₆addition, theexcessive amounts of the primary C-B compounds precipitate in surfacing metals. They arehard and brittle and lose protection from the matrix. They will be easily peeled off and thenthe abrasive resistance goes down. The abrasive resistance of surfacing metals is firstlyimproved with adding B₄C and ferroniobium powders. When the B₄C and ferroniobiumadditions in flux-cored wires both go to6.0%, the surfacing metals have the best abrasiveresistance. The corresponding relative wear-resisting coefficients are4.3and3.46times ashigh as that of the Q235base metal.The fracture pattern of the impact test specimen of the surfacing metals is mainlybrittle by SEM analyses. The bonding strength between the hard phase precipitates andeutectic matrixes is much lower in the hardfacing metals. And the stress concentration ordislocation pile-up will be easily resulted in on the edges or corners of these hard phases.This can lead to increasing the initiation points of cleavage and reduce toughness of thematerials. When C and B contents are lower in the hardfacing metals, the major fracturemechanism of the fractographes of hardfacing metal impact specimen is cleavage fracturebecause of less hard phase precipitates. On the contrary, when C and B contents areincreased the fracture mechanism is mainly intergranular because of more primaryprecipitates. Adding element Nb not only makes the volume of C-B compounds decrease,but also can refine the eutectic austenite grain sizes. Thus the impact toughness of thehardfacing metals is improved.