Strengthening Mechanism And Impact Wear Behavior Of Aging Hardening Light-weight Ultra-high Manganese Steel

Conventional high manganese steel(Hadfield steel)with a single-phase austenite at room temperature has excellent strain hardening ability and impact resistance,thus,it is widely used as a wear material under impact load.However,due to the low yield strength and initial hardness,the plastic deformation occurs when the wear resistance cannot be fully exerted under low impact load,and as a result,the service life is reduced.In this paper,a kind of light-weight ultra-high manganese steel(Fe-31.6Mn-8.8Al-1.38C)was designed for wear condition under low impact load.It had the characteristics of low density,high yield strength,high initial hardness,good impact toughness and so on.The strengthening mechanism and wear mechanism of the experimental steel were analyzed through subsequent studies on phase transformations,compressive deformation and impact wear after aging.After water toughening treatment at 1050℃ for 1.5h,the microstructure of experimental steel exhibited a single-phase austenite.And nano-sized κ’-carbides precipitated evenly within the austenite matrix after aging,which improved the yield strength and initial hardness.The optimum comprehensive mechanical properties were obtained after aging at 550℃ for 2h,and the yield strength and initial hardness increased by 107.4%and 28.7%compared with the as-quenched specimen.Its tensile strength was 1041.7 MPa,yield strength was 1002.7 MPa,elongation after fracture was 17.6%,impact toughness(V-notch)was 62 J/cm2 and hardness was 268.5 HB.After aging at temperatures from 550℃ to 900 0C,the phase transformation sequence was described by κ’→nano-K’+β-Mn→submicron-κ’+β-Mn+α→nano-κ’.Moreover,four types of K-phase precipitates involving size,morphology and distribution were summarized,including intra-granular nano-κ’(<50nm)and submicron-κ’(>100nm),inter-granular κ*(～1μm),and lamellar κ among colonies of α+κ.After aging at 550℃,nano-sized κ’-carbides could promote the β-Mn phase precipitating along the grain boundaries without accompanied with α-precipitate.After long-term aging at 700℃ and 800℃,there was a large percentage of a in the austenite matrix and the P-Mn phase appeared through the decomposition of γ→α.The nano-hardness of matrix and precipitates at different aging temperatures was obtained by the nano-indentation test.The theoretical SFE was estimated about is 82.3 mJ/m2.Due to the glide plane softening effect,the deformation mode is mainly dislocation plane slip.With the increase of deformation,the main sequences of substructural evolution were described by planar dislocation arrays→ planar dislocation configurations(dipole bundle and Lomer-Cottrell locks)→ Taylor lattices→micro-bands.In this study,taking advantage of compression,the suppressed deformation twins under high SFE and a kind of polycrystalline structure were observed.By analyzing the theoretical critical twinning stress(σT),when the applied stress is larger than the σT,deformation twins appear.Dislocation tangles were dominant inside the polycrystalline structure and dislocation cell blocks were formed by cross slip.Also,the strengthening mechanism of multi-effect synergism is proposed:i)dislocation planar slip leading to slip band refinement and micro-bands formation,ii)deformation twins and iii)the polycrystalline structure.The appearance of these deformation substructures jointly restricted dislocation movement and promoted the uneven density of dislocation in the matrix,which enhanced the strain hardening.Under low impact load(0.5J),the experimental steel after aging had better wear resistance as well as lower wear percent(0.55%～0.57%).However,under high impact load(4J),its wear percent(0.48%)was higher than that of being water toughening treated(0.21%)due to the reduced strain hardening rate and impact toughness after aging.By observing the white layer in the worn subsurface,it was found that the average thickness of the white layer decreased with the aging time increasing.With the increase of the impact load,the dislocation density increased in the austenite matrix of light-weight ultra-high manganese steel,and complex planar dislocation substructures formed such as Taylor lattice and micro-band.However,with the increase of aging time under the same wear condition,the evolution of planar dislocation substructure from simple to complex was also observed,which indicated that short-range ordered structures(κ’-carbides)can enhance the glide plane softening effect.