Alloying And Sintering Mechanisms Of Powder Metallurgy Chromium-Containing Low Alloying Steels

To meet the higher requirements for the manufacturing process and performance of powder metallurgy（PM）structural components due to the transformation of automotive technology,there is an urgent need to develop economical,environmental friendly and high-performance PM low-alloy steels.As a tranditional alloying element in solid steel promoting excellent hardenability,as well as low cost and good recyleability,the application of Cr in PM low-alloy steels is an important area of research.In this the7sis,Fe-Cr（≤3 wt.%）-（Mo）-C were prepared by powder metallurgy methodolgy.The diffusion behaviour of Cr and its influence on the diffusion of C were analysed from a thermodynamic point of view.The evolution of its hardenability and microstructures in sintered steels were studied.The reduction mechanism and process control of the oxides formed on powder particles surface during sintering were discussed.The predictive models considering consolidation parameters and chemical compositions of the investigated materials were designed by artificial neural network（ANN）.The fatigue strength and fracture mechanism were illustrated in view of the porosity and the phases constitution.The main research results of the paper are as follows:According to the thermodynamic calculation,the alloy system Fe-3Cr-0.5Mo has an obvious effect on the slowing diffusion of C atoms,which indicates that the material is more likely to acquire a non-equilibrium martensitic structure during cooling.As the results of CCT curves and hardenability tests,the cooling rates of bainite and martensite starting transformation for the material Fe-1.8Cr-2Ni-0.6C are 0.8 K/s and4 K/s,while Fe-3Cr-0.5Mo-0.5C are 0.1 K/s and 1.5 K/s.Studies on surface oxides of water-atomized Fe-3Cr-0.5Mo showed that the oxides in the presence of spherical particulates with around 200-350 nm diameter in average were rich in Cr.The critical oxygen partial pressures as low as₂=4×10^-18 atm at 1120℃and₂=1×10^-15 at1250℃ensured a reducing effect during sintering.The residual oxides along prior particle boundaries have a detrimental effect on the tensile properties of sintered steels.However,the fine oxides entrapped in the interior of particles have a limited effect on the tensile properties.The potentials of oxygen and carbon in a continuous belt furnace were monitored and analyzed using a self-desinged thermal measuring unit consisting of thermocouple,oxygen probe,and carbon monoxide sensor.The improved atmosphere characterized 4.7×10^-24 bar oxygen partial pressure and carbon potential 0.6-0.8%C was achieved,in comparision with the orginal conditions of 1.1×10^-22bar and 0.2%C.Finally,the super properties in tensile strength and surface hardness of Cr-containing low alloy steel reached 920 MPa and 397 HV₁₀ respectively.An ANN,in the form of a nonlinear multi-layered perceptron,was designed to predict the patterns correlating alloying elements（of Cu,Ni,and C）and consolidation parameters（sintering temperature and cooling rate）to tensile properties of sinter-hardened Fe-Cr-Ni（Cu）-C alloy steel.The excellent agreement of data modeling by the ANN with the experimental data was demonstrated,with the MSE and R² being 0.01 and0.89,respectively.Moreover,the impairment of tensile strength of Fe-Cr-Ni（Cu）-C predicted by the model was the results of plate martensite formed in the conditions of higher alloying content（0.6～1.0 wt.%）,a higher cooling rate（>3℃/s）,or more diffusion of Ni（Cu）at high sintering temperature（1250℃）.The plane bending fatigue strength_A of the material Fe-3Cr-0.5Mo-0.5C prepared by sintering hardening process,can reach 380 MPa.Different from diffusion bonding alloy steel,the fatigue fracture of prealloyed steel with a homogeneous mono-phase of martensite is orignated from pores and propagates along sintering necks,resulting in an inter-particle fracture.A modified Murakami model considering micro-scale defect and micro-hardness is effective to predict the fatigue performance of the alloy steels with heterogeneous microstructure and composed of complex phases constitution.