Study On The Microstructure And Fatigue Properties Of B2Mould Steel Laser Cladding Layer

Stamping moulds usually occur forms of failures, such as corrosion, wear, fatigue andfracture; for the moulds are bearing alternating loads while at work. This paper aimed tosolve the problem of stamping moulds failure, especially the mismatch of the mould edgefatigue behavior and the mould overall life. The failure mould surface was repaired by lasercladding. In the experiment, Ni60A and Ni60A+WC cladding layers were cladded on the B2mould steel substrate, respectively. Effect of process parameters and powder compositionson the cladding layer microstructure and properties were analyzed.The cladding specimen included three parts: the cladding layer, the heat-affected zoneand the substrate. Ni60A cladding layer microstructure included two parts, the bottomcellular crystal and the upper dendrite crystal. The former was γ-(Fe,Ni),and the latter wasCr2Ni3, strengthening phases such as M23C6and CrB distributing among the dendrite. Themicrohardness value of the cladding layer was between570HV and580HV, while thesubstrate was between400HV and420HV. The furrows on the cladding layer surface wearscar were narrow and shallow, the amounts of pits were fewer, which indicated the wearproperty of the cladding layer was higher than the substrate. When load F=15KN, cyclestimes N=2×104, the sources of cracks occurred inside the pores of the cladding layer; whenN=3×104, the sources of cracks occurred at the hard phase of the cladding layer; whenN=4×104, cracks occurred on the fusion line between the cladding layer and the substrate;when N=5×104, fatigue cracks occurred inside the γ phase of the cladding layer.Effect of process parameters on the Ni60A cladding layer microstructure and propertieswere studied. The results revealed that with the pulse current increasing, the bottom cellularcrystal microstructure of the cladding layer became uniform and compact. When the currentwas higher than80A, a bright white striped area formed between the substrate and thecladding layer. With the pulse width increasing, the upper dendrite crystal of the cladding layer became thicker. The change of frequency had no significant effect on the cladding layermicrostructure. With the pulse current increasing, the cladding layer microhardnessdecreased；with the pulse width increasing, the microhardness change tendency fromcladding layer to substrate became apparent; frequency had no significant effect on thecladding layer microhardness. The dilution rate increased with pulse current, pulse width andfrequency.Effect of WC content on the Ni60A+WC cladding layer microstructure and propertieswere studied. When ω(WC)=10wt%, most of the WC melted in the cladding layer,promoting the forming of strengthening phases such as Cr23C6, Cr4Ni15W and Cr3Ni2Si.When ω(WC)=20wt%, the upper dendrite crystal disappeared and bulk crystals formed, andthe microstructure was refined. When ω(WC)=30wt%, the microstructure of the claddinglayer was massive eutectic with rich W, unmelted WC, carbides and borides forming in thelayer. The microhardness of Ni60A+WC increased with WC content, higher than the Ni60Acladding layer significantly; the highest microhardness value was1520HV. The wearproperty of Ni60A+WC cladding layer was improved due to the WC particles. Comparedwith the Ni60A cladding layer, the furrows on the surface of the Ni60A+WC cladding layerwear scar were narrow and shallow, the amounts of pits were fewer; when ω(WC)=35%,chunks of hard phase particles of the Ni60A+WC cladding layer the worn shed off from thesurface. When F=15KN, N=5×104, peeling cracks occurred at the surface of the Ni60A+WCcladding layer. When ω(WC)≤15wt%, the peeling thickness of the cladding layer was verysmall, and the fatigue properties of the cladding layer was excellent. When ω(WC)≥20wt%, the peeling thickness of the cladding layer increased with the WC content. Whenω(WC)≥30wt%, almost the entire cladding layer peeled from the substrate.