Effect of surface layer depth on fatigue life of carburized steel and analysis of fracture process

摘要: Nowadays, it has been strongly required to extend the fatigue life of machine components and structures due economic. To meet this demand, various surface refining processes have become major interest because they can provide additional properties such as high strength, thermal barrier, corrosion wear resistance structural materials. One most commonly used techniques is carburizing. The depth carburized layer one important parameters determining endurance components. This work deals with dependence on depth. Introduction In materials science, progressive localized damage that occurs when a material subjected cyclic loading. nominal maximum stress values are less than ultimate tensile limit, may be below yield limit Fatigue repeated loading unloading. If loads above certain threshold, microscopic cracks will begin form at surface. Eventually crack reach critical size, structure suddenly fracture. By changing machinery parts possible increase considerably their load-capacity. ways obtain complex physical mechanical metallic us hardness, resistance, contact others chemical heat treatment method nitriding or carburizing [1-3]. Also, basic methods for increasing strength [2-5]. Positive effect explained by compressive residual stresses in Experimental technology Two types specimens different geometry were used. cylindrical torsion bending tests. flat samples tested only experiments carried final made stainless steel four depths obtained two temperetaures. temperatures caused concentrations C layers, means layers (roughly) same thickness differs hardness.The sampes depths. room temperature. Geometry both type shown Fig. 1. CSN 41 4220 (equivalent C15E EN 1008494). composition displayed Table Fig.1. Specimens geometry. Thickness plate specimen TS = 3 mm. 2122 element [%] Mn Si P S Cr Mo V Cu Ni According Standard: 10 204 0.16 1.21 0.21 0.012 0.029 0.9 0.23 0.09 spectrometric measurements 0.13 1.25 0.15 0.01 0.002 1.02 0.05 0.03 0.22 minimal σy,min= 588 MPa Su= 785 MPa. improving some Chemical kind hardering. Carburizing process which heated presence another ( often range 850 950 °C) liberates carbon decomposes. Depending amount time temperature, affected area vary content. Longer times higher lead greater diffusion into part well increased diffusion. When iron cooled rapidly quenching, content outer becomes hard via transformation from austenite martensite, while core remains soft tough ferritic pearlite microstructure. gas carburizing, commercially variant source carbon-rich furnace atmosphere produced either gaseous hydrocarbons, example, methane (CH4), propane (C3H3), butane (C4H10), vaporized hydrocarbon liquids. Parameters gas-carbunizing Table.2. Test test embedded activated inside pot was then tightly sealed clay cover prevent CO escaping unwanted entering during heating. temperature adjusted TC (860 940°C). typical tTC were: 3, 4, 5 6.5 hours 860 °C 1, 2.5, 3.5 h 4.5 980 °C. case application composed 23% CO, 36% H2, 40% N2, 2% CH4 (CO2 + H2O). role agents CH4. cooling tcl 2.5 860°C 940°C hours. After twice hardened, first 990°C again 780°C. cases hardening teh 35 min tcool min. As medium water solution 10% NaOH. tempered 180°C (1.5 hours). hardened its 2. 0.65 0.45 Fig.2. (a) microhardness depth, (b) grain concentration, (c) average size DA carbide graine (d) distribution across 2(a). show specimens. micro-hardness profile performed using 9.81 N Vickers hardness scale, effective defined distance where equal 500 HV. Fig.2 (b). shows relationship between density (on plane) particles. Fig.2(c) particles temeprature time. (d). stresse measured X-ray diffraction method. an strengthening decreasing d described known Hall–Petch equiton: k Y 1 0    , (1) σy stress, σ0 ‘friction stress’ factor depends measures relative contribution boundaries/interfaces. difference carbid (for treatments) not significant, but occurrence extremely large grains grow with, see 3. From 2 clear that, growth number all sizes particle approximately same. Most lamellar, lamellar ratio LV 0.25 0.40. Fig.3. Size (subscription FS samples). Sample Surface h[mm] Time tcl[h] [°C] Hardness [HV] [min] Tch1[°C] Tch2[°C] A 0.3 920 990 780 B 4 86