Design Of WC Coating With High Hardness, Strength And Toughness And Its Application In Remanufacturing Of Construction Machinery

Because of the severe working conditions such as heavy-load, high-abrasion andimpact, surface coating for remanufacture of construction machinery parts need ahigher hardness and toughness. Therefore, based on the analysis of failuremechanism of construction machinery shaft parts, a design idea of―multi-scale WCbased cermet composite coating‖was proposed, and a new high velocity oxy-fuel(HVOF) sprayed bimodal WC-(nanoWC-Co) coating of high hardness, strength andtoughness was developed and successfully applied in remanufacturing of shaft partsin construction machinery.Firstly, we analyzed dismantling, cleaning and testing methods of grader andloader, especially focused on the failure mechanism of transmission shaft parts, andfound that abrasive wear and contact fatigue were the main reasons for the failure ofthe parts. Furthermore, a surface fatigue layer with thickness of~50microns wasfound in scrap parts. Consequently, this fatigue layer has to be removed before thescrap parts are remanufactured by HVOF technique. In order to obtain high-qualitywear-resistant coating, effects of process parameters and powder structure on thepreparation of HVOF-sprayed WC-12Co coating were investigated. The optimizedprocess parameters were obtained which were propane gas pressure of0.65MPa,propane flow rate of30L/min, oxygen pressure of0.7MPa, oxygen flow of245L/min and spraying distance of270mm. Furthermore, four kinds of WC-12Copowders with different WC particle size (micron, submicron, bimodal and nanometer)and one kind of WC-Co-Al were designed as feedstock used to study the influence ofpowder structure on the properties of coatings sprayed under the optimizing HOVFparameters. The results showed that decarburization behavior of WC particlesaggravated during deposition process with the decrease of the WC particle size, andthe corresponding coating hardness increased, but the toughness decreased. Duringspraying Al reacted with oxygen and Al2O3phase was generated, which inhibited thedecarburization of WC and improved the mechanical properties of the coatings.Nanostructured WC-12Co coating showed the highest hardness (1380HV0.1) in thefive kinds of coating, but the worst fracture toughness (5.79MPa·m1/2), bimodalWC-(nanoWC-Co) coatings shows the best toughness which was10.76MPa·m1/2,and also a superior hardness of1291HV0.1. The results of nanoindentation test showed that the existence of nanometer WC particles increased the hardness andelastic modulus of the matrix in bimodal WC-(nanoWC-Co) coatings. Study on thefriction and wear behavior of five kinds of WC coatings showed that bimodalWC-(nanoWC-Co) coatings exhibited the most excellent wear esistance in the fivekinds of coatingswhen dry sliding against a GCr15ring. Through analysis of wearmorphology and wear debris, the main wear mechanism of WC coatings was found.That was adhesion and micro-cutting of the binder phase by GCr15ring and weardebris, spalling of WC particles and local delamination caused by fatigue crackpropagation, and accompanied by the formation and detachment of oxide film.Finally, this bimodal WC-(nanoWC-Co) coating has been successfully applied onremanufacturing of construction machinery shaft parts such as double row sprocketshaft. The remanufacturing coating had a compact microstructure with the porositylower than1%and excellent mechanical properties with a microhardness of1256HV0.1, the indentation fracture toughness of10.5MPa·m1/2, bonding strength morethan60MPa and wear mass loss less than1/20of GCr15bearing steel under thecondition of dry sliding wear. Through the establishment of evaluation model ofwaste product remanufacturing, the re-manufacturability of double row sprocketshaft remanufactured by this method was evaluated and the result was very satisfied.