The Impact Toughness And Mechanical Properties And Fatigue Crack Propagation Rate Research Of High-strength Steels

In the history of the development of material,strength is always one of the most mechanical properties.However,the improvement of strength often leads to the loss of toughness.Some components with high strength in the service process often do not reflect good fatigue performance.Because there often exists a tradeoff relationship between strength and toughness in materials,therefore,for engineering materials,how to choose the material of the best match in strength and toughness in the service process which has good security service performance becomes to be a problem of engineering.In order to improve the mechanical performance of engineering material or component in the service process.There aspects should be taken into consideration: To clear the physical meaning of strength and toughness of materials;2)To study the influence of microstructure on the match of strength and toughness;3)To optimized the match of strength and toughness,and to achieve fatigue performance improved.There are many test methods to characterize strength and toughness of the materials.For example,it can be possible to characterize the strength of the materials through the most simple room temperature tensile test.And toughness of materials can be tested by the plane strain fracture toughness testing or other methods.Charpy notch impact test is a method to characterize the toughness of metal materials,which can reveal the mechanical behavior of metal materials under the impact load and brittleness of metal materials at low temperature.However,the change process of energy in impact fracture for the material is unclear so that impact toughness can not behalf the characteristic of material itself.In this paper,a series of Charpy U-notch impact testing has been carried out at first on a maraging steel with four different strengths to investigate the relationship between size of shear lips and specimens.And the size of shear lips was measured and studied with the help of stereoscope.Then the relationship between energy of normal fracture and shear fracture in impact fracture was discussed in this study.In addition,the mechanical properties of leaf spring of FAS3550 with different heat treatment process was investigated in this paper,including hardness test,and room temperature tensile test and plane strain fracture toughness.And the influence of heat treatment process on strength and toughness was analyzed.At last,fatigue crack propagation of FAS3550 under different heat treatment processes and Benz were measured and compared in this paper.The factors which influence the fatigue crack growth rate curves were analyzed and the influence of heat treatment on fatigue crack propagation and the influence of microstructure and fracture mechanism on mechanical properties were discussed in this paper.In conclusion,the following conclusions can be achieved.1.The impact absorbed work was mainly made up of the normal fracture work and shear fracture work.The shear fracture work was consumed away during the process of the formation and development of shear lips.And normal fracture work was consumed away mainly through the propagation of the I type crack.The size of shear lips will increase as the increasing of the height of the specimen.However when the height of specimens reach to a certain value,the size of shear lips will finally approach to a steady value due to energy balance criterion.At this time,the size of shear lips will satisfy t=3?c/8Ut.Furthermore,the expression was verified well in other three kinds of steel of different strength and a kind of titanium alloy.2.The microstructure of FAS3550 under different heat treatment is tempered troostite and a little carbide.And the banded structure was founded under the macroscopic observation.The strength and toughness of those FAS3550 with different heat treatments show a tradeoff relationship and all of them have a same yield ratio which is approach to 0.9.Besides,when the FAS3550 was tempered at 470 ? and quenched at 870 ?,it showed the best comprehensive mechanical performance.3.There are different levels of “jump” in the fatigue crack propagation curves of the domestic leaf spring of FAS3550 with different heat treatment process.Fracture morphology of fatigue crack propagation specimens is mainly made up of cleavage and tearing.The change of the fracture area of cleavage and tearing leads to the deflection in fatigue crack propagation curves.When the heat treatment process is that tempering temperature is 470? and quenching temperature is 870? and 910? respectively,the fatigue crack propagation rate of FAS3550 is close to its of Benz.