Characterization Of Thermal Damage In 300M Steel Using Barkhausen Noise And Fatigue Property

Steel parts always get thermal damage in the process of machining, heat treatment, superficial treatment and service, which can be characterized as change of residual stress and distribution of hardness in the surface of parts. Thermal damage is thought to be the direct cause of fatigue failure, so it is important to detect and characterize it. The disadvantage of conventional techniques and shortcoming of Barkhuasen Noise technique in the understanding about physical essence are studied as the basis of this study. The subject of dissertation is to characterize thermal damage in 300M steel using Barkhausen Noise, simulate different degrees of thermal damage by laser burn, study the relativity between level of Barkhausen Noise and stress, metallographic structure and other influencing factor by means of four-point bending and retempering, discuss the hypostasis of Barkhausen Noise characterization of thermal damage, evaluate residual stress of thermal damage by this method. Meanwhere, the relation between Barkhausen Noise characterization of thermal damage and fatigue properties, the variety rule of Barkhausen Noise in the process of fatigue are studied in this paper. The main efforts and achievements made in this study are as follows:This paper firstly presents the hypostasis of Barkhausen Noise characterization for thermal damage is residual stress. In the magnetizing direction where response of singal is excellent, Barkhausen signal increases with increasing tensile stress, decreases with increasing compressive stress. A formula to evaluate residual stress by BNs is developed: σ=σ₀+147.06 (BNs—1) . The predicted values are consistent with results of X ray diffraction stress testing.The influence rule about quantity and direction of stress to BN level is explained by model of magnetostrictive respone, which shows the main influence factor for movement of magnetic domain walls is magnetostrictive respone, and BN level reflects the intensity of magnetic domain walls' jump-style movements.The change of BN level after short time tempering is not notable for uncompleted deformation of structure, which shows that, for the thermal damage induced by short time heating in actual condition, structure have no influence on change of BN level.Fatigue properties of samples decrease with increasing BNr sharply. The influence of thermal damage on fatigue properties is strong in low stress level, and weak in high stress level. And the influence goes to stabilization when BNr is higher than 2.5.Fine fatigue samples' fracture surface are flat and only have one crack resource each, but samples with a burn are step fracture and have more than one crack resources which always lie in the area of burn. This result shows that the area of burn, where crack resources often present, is weaker than the other region, and burned samples have a different microcosmic cracksin mechanism from normal samples, have low lifes from a macro prespective.