Study On Carbon Diffusion Behaviors Of Medium And Low Carbon Steels By Resistance And Internal Friction Methods

It is very important to study the diffusion behaviors of carbon atoms in the process of microstructure regulation of the new generation of high-strength steel.Although the development of visual characterization technology for microstructure of steel is rapid,the high sensitivity dynamic in-situ detection of carbon atom diffusion is still a challenge.Using the continuous evolution of physical parameters such as resistivity and internal friction in steel during different heat treatment,it is a very effective means to study the dynamic law of carbon diffusion and related physical mechanism.In this paper,the effects of carbon segregation and precipitation kinetics on the evolution of microstructure of medium carbon hot working die steel during tempering and serving,and the mechanism of martensite decomposition and carbide precipitation of low carbon dual phase steel during tempering are studied by in-situ resistance and internal friction methods.The following main results and conclusions are obtained:（1）Aiming at the demand of analysis of carbon atom diffusion behavior in steel,a metal resistance high-temperature characteristic measurement system based on micro-control unit was developed to achieve precise measurement of dynamic resistance in different heat treatment processes.The precision of resistance measurement is 1μΩand the accuracy of temperature control is±1℃.The key mechanical and electronic components of LMA-1 internal friction mechanical spectrometer are improved.Improved the center alignment of the system’s pendulum mechanism,reducing the background internal friction of the instrument from 5×10^-4to 2.5×10^-4.A swing bar fast braking system based on resistance-capacitance（RC）differential circuit is designed,which shortens the single test time to 1/3 of the original,and improves the data acquisition ability of the system.The development of metal resistance high temperature characteristic instrument and the improvement of internal friction mechanical spectrometer provide a technical basis for precise and rapid characterization of carbon diffusion behaviors.（2）The continuous diffusion behaviors of carbon atoms and the accompanying microstructure changes of quenched hot work die steels during heating process were studied by resistance and internal friction methods,the results show that:During80～125℃,the resistance of quenched samples decreases and the hardness reaches its peak value,and the carbon concentration near the dislocation line of the sampling micro-area was increased from 2.5at.%to 4.5at.%after 100 ℃ isothermal.This is mainly due to the diffusion of carbon atoms to dislocations,and the activation energy of diffusion is 83 k J/mol.During 140～250 ℃,the resistance deviation from linearity indecreases slightly,which is attributed to the nucleation ofεcarbide;When the tempering temperature rises above 300 ℃,the increase of resistance value slowing down with the increase of temperature is attributed to the precipitation of carbides.Combined with TEM analysis,the precipitation difference of carbides and alloy carbides can be distinguished by the different evolution trend of resistance,and the effect of carbide precipitation behavior on tempering hardness.Internal friction analysis of tempered medium carbon hot working die steel found that:the Snoek peaks appear near 40 ℃ which strength is 2×10^-5 and activation energy is0.71～0.75ev,indicates that some dislocations recover and disappear after high temperature tempering,and then the interstitial carbon atoms segregated near the dislocations break away from the dislocation binding and relax.The internal friction peak and hardness peak near 120 ℃ is caused by the segregation of interstitial carbon atoms under the heat and force condition.（3）Using resistance and internal friction methods,combined with microstructure analysis,the carbon atom diffusion behaviors of ferrite-martensite dual-phase steel in the martensite decomposition process are studied.The results show that the decomposition process of martensite consists of two processes:the pre-precipitation of carbon atoms from the lattice and the subsequent formation of carbides with alloy elements.1)At 430 ℃,the internal friction peak of transformation is related to the precipitation of saturated interstitial carbon atoms in martensite,and the modulus changes abruptly at the same time;The peak temperature of internal friction does not change with frequency,and the peak intensity has no strain amplitude effect.The activation energy of carbon atoms diffusing from martensite calculated by resistance method is 1179.46 k J/mol;2)The internal friction peak caused by precipitation of Cr₇C₃ is near 450 ℃,and the activation energy of precipitation is 299.68 k J/mol calculated by resistance method.The phenomenon of secondary hardening of carbides is reflected in the tensile properties.（4）For medium carbon steels,the resistivity is very sensitive to the diffusion behavior of carbon atoms during tempering.Through the resistance temperature evolution coefficient,it can accurately characterize a series of continuous transformation processes such as carbon atoms moving from interstitial position to dislocation,interface enrichment and nucleation,and precipitation from the matrix,and explain the diffusion kinetics from the perspective of carbon atom diffusion activation energy.For low carbon steel,the occupation or migration of carbon atoms can be described quantitatively or semi quantitatively through the corresponding physical model of typical Snoek peak or SKK（Snoek-Kê-K（?）ster）peak.Therefore,the combination of resistance and internal friction technology,together with microscopic characterization technology,can help material researcher more accurately and deeply grasp the diffusion behavior and internal mechanism of carbon atoms in steel.