Effect Of Morphology And Material Coupling Elements On Tensile Properties Of Low Carbon Steel

With the acceleration of China’s railway system, antenna mounting brackets of thetrains suffer from the increased powerful stress and strain due to the rapid starting orsudden braking, and their service environment tends to be more intricate and harsh. Inthis condition, higher strength and toughness of the mounting bracket is needed to meetthe requirements of the accelerated trains. Currently, materials and component structureused in the mounting brackets follow the design standards in2007before the6thspeedup. Ferrite/pearlite low carbon steel is widely used in mechanical engineering andhas always been employed to produce the mounting brackets for its superior plasticityand good weldability. However, a major shortcoming of low carbon steel is its poorstrength and toughness performance, which places a limit on its service life in theaccelerated trains. On the other hand, replacing the materials or redesigning thecomponent structure not only leads to a series of changes in the production process butalso to the scrap of the present mounting bracket, which would cause huge waste ofresources and economic losses. Therefore, how to improve the strength and toughnessof low carbon steels but without changing the present materials and componentstructure has imposed restrictions on China’s railway transportation and has also been akey issue in the engineering field.The phylogeny of science indicated that many great progresses of science andtechnology came from inspirations of the adaptability of biology to the naturalenvironment. In the same way, strengthening and toughening technique of low carbonsteels has been enlightened by the unsurpassable advanced performance of biologicalmaterials. In this paper, inspired by some tree leaf and insect elytrum with excellentcombination of strength and toughness, biomimetic coupling model for low carbonsteels has been designed to improve the service life of mechanical components. S355steel, which is wildly used in the antenna bracket of High-speed EMU, was chosen asexperimental material and the biomimetic coupling model was applied in it by lasertechnique. Though systematic study on the tensile properties of the laser processed biomimetic samples, we discussed the influences of morphology and material couplingelements on the strengthening and toughening effects as well as the mechanism behindthem. Based on this work, we expected to provide valuable information for thestrengthening and toughening technique of low carbon steels.Results indicate that the laser biomimetic coupling method can improve thestrength and ductility of S355steel simultaneously. After the treatment, laser affectedarea with certain morphology makes up the biomimetic units. Microstructures of theunits are significantly refined than the matrix and the dislocation density increases by anorder of magnitude. Remelted zone is mainly composed of fine martensitic lath with itslath width ranging between300nm and600nm. Compared with that of the matrix, theaverage microhardness of Remelted Zone is increased by125%. Heat affected zoneconsists of martensite and incompletely dissolved ferrite, and its microhardness isincreased by46%in contrast to that of the matrix. Strengthening effect of thebiomimetic coupling treatment is attributed to the microstructure changes in the unitsand the stress transition from the substrate to the units. Because of the stress transfer inbiomimetic specimens, the units would carry higher tensile stress, and thus stressdistributed in the substrate is much lower than that in the untreated specimens with thesame external loading. Therefore, higher external tensile load will be applied when thesubstrate of the biomimetic specimens reaches its yield point and failure point. Thiscontributes significantly to the enhancement of strength, leading to the higher yieldstrength and tensile strength of the biomimetic specimens. Toughening effect of thebiomimetic coupling treatment rests on that the units have a beneficial effect onredistributing the shear stress throughout the specimen and resisting the initiation andgrowth of the necking. Due to this toughening effect, uniform deformation process isprolonged and the regions away from the neck could perform a larger strain, thusenhancing the elongation of biomimetic samples.Morphological coupling element has an important impact on the tensile propertiesof biomimetic samples. Compared comprehensively on the strength and ductility of thesamples, biomimetic specimens with gridding-shaped units exhibit the most desirable strengthening and toughening effects, generating a superior increment relative to theuntreated specimen in yield strength, tensile strength and elongation by14.4%,13.1%and11.1%, respectively. Meanwhile, it is found that the strengthening and tougheningeffects increase with the reduction of unit distance. Among all the samples investigatedin the present study, gridding units with lateral distance of3mm and longitudinaldistance of5mm contribute to the most integrated improvement in the tensile propertiesof biomimetic specimens, producing a development relative to the original material inyield strength, tensile strength and elongation by16.3%,15.4%and19.8%, respectively.And further decreasing the unit distance would result in a sharp deterioration of theductility. The tensile properties of striation biomimetic specimens are dependentcritically on the orientation of striation unit. When the units are aligned along the loaddirection (unit angle=0°), the biomimetic sample exhibits the greatest improvement instrength. As the unit angle rises, the strengthening effect is decreased while thetoughening effect increases. The maximum elongation is achieved in the sample with itsunits aligned at a90°angle to the load.Martensite content, chemical compositions and the thickness of the matrix have agreat influence on the strengthening and toughening effects of biomimetic couplingtreatment and thus determine the tensile property increments of biomimetic samples.With the increase of martensite content, stress transition between the substrate and unitsis diminished, resulting in the decrease of strength increment in biomimetic samples.However, toughening effect increases with the martensite content, and the elongationincrement of biomimetic samples enhances continuously until the martensite contentreaches90%volume fraction. Both the strength and ductility of S355steel,45#steeland H13steel are improved after the biomimetic coupling treatment. The hardness andstrength of the units increase with the carbon and alloy content of the matrix, andtherefore the strengthening effect of biomimetic specimen increases as well. But on theother hand, the strengthening effect depends to a great extent on the properties ofinterfacial bond between the unit and substrate. The incompletely dissolved pearlite(45#steel) and especially the coarse particles (H13steel) in the heat affected zone impair the toughening effect, and the elongation of their corresponding biomimeticspecimens is relatively lower than that of S355steel. Study on the tensile properties ofthe biomimetic specimens with different thickness indicates that the mathematicalmodel and volume fraction of the unit could be applied to calculate the yield strength ofbiomimetic coupling processed S355steel, and the calculation errors are less than10Mpa when the thickness of samples is below3mm. It is also found that17.6%could beapproximately considered as the threshold value of the units’ volume fraction when thethickness of samples varies but the other processing parameters are fixed. Under thisthreshold value, desirable strengthening and toughening effect could be obtained afterthe biomimetic coupling treatment.Laser wire process was applied to fabricate alloying units which possess differentmicrostructures and materials with the matrix of low carbon steels. The alloying unitsbreak the limit of strength increment in laser remelting processed samples and furtherimprove the strengthening effect of biomimetic coupling treatment. However, theelongation of laser wire processed samples is decreased and lower than their untreatedcounterpart. This is because that though the interface between alloying unit andsubstrate has considerable hardness and strength, its deformability is reduced by thecolumnar grains and could not meet the requirement of biomimetic specimens. It is alsonoticed that localized interfacial debonding occurs during the tensile deformationprocess, which impairs the toughening effect of biomimetic coupling treatment. In thiscondition, electropulsing treatment on the laser wire processed samples was attemptedto improve their ductility. After the electropulsing treatment, interfacial microstructuresand hardness distribution of the alloying units could be improved. And consequently thetoughening effect of laser wire processed samples is considerably enhanced after thetreatment. The electropulsing stimulation with220ms discharing duration generates asuperior increment of laser wire processed samples relative to the untreated specimen inyield strength, tensile strength and elongation by28.2%,25.9%and6.9%, respectively.The idea for improving the strength and ductility of low carbon steels bybiomimetic coupling method in this paper has provided an original research thought for solving strengthening and toughening problems of mechanical components which existwidely in the field of mechanical engineering. The studies on the tensile properties oflow carbon steels processed by laser biomimetic coupling treatment have academicsignificance and applicable value in engineering. Meanwhile, the application ofbiomimetic coupling technique can not only improve the strength and ductility of lowcarbon steels simultaneously, but also remain the present materials and originalcomponent structure of mechanical components. And therefore, the development of thebiomimetic coupling technique is of profound societal and remarkable economic value.