'n Ondersoek na die segregasie van fosfor en ander onsuiwerhede in 3Cr12 vlekvrye staal

[摘要] English: One of the main reasons for temper embrittlement in steel is the segregationof impurities like P to the grain boundaries. Segregation can be defined asthe diffusion of atoms from the bulk to the surface and grain boundaries insuch a way that the total Gibbs free energy is minimized. This means thatsegregation can take place against the concentration gradient, from a lowconcentration in the bulk to a high surface concentration. The chemicalpotential gradient is the driving force behind segregation.The aim of this study is to investigate the segregation behaviour of P andother impurities like S and Sn in 3Cr12 steel.A background theory is founded by using:(i) The semi infinite solutions to the Fick equations(ii) t½ and modified t½ models(iii) the modified Darken model.One of the advantages of the Darken model is that it supported bothsegregation kinetics and equilibrium behaviour. The multi componentmodel for ternary alloys could be expanded to quaternary alloysystems in this study.Segregation kinetics as well as the equilibrium was described bymaking use of constant and linear temperature heating.Auger electron spectroscopy was used to investigate the S, P, Cr, N,and Sn segregation behaviour in a Fe matrix. A personal computerwas used to control the Auger spectrometer as well as the constant andlinear heating runs.Three commercial 3Cr12 samples was investigated during the study.They were numbered according to their P contend as 26P for thesample with 0.026wt% P, 32P for 0.032wt% P and 62P for the samplecontaining 0.062wt% P. The constant temperature runs indicate thatSn competes with Cr, N and Pin sample 26P. A definite correlation isvisible between Cr and N in sample 32P while Sn and S compete withP in sample 62P.The constant and linear heating Darken simulation model was used to give aqualitative description of the experimental segregation behaviour. Thebehaviour of two segregating species were simulated in a Fe matrix, fromwhich the influence of the segregation parameters could be demonstrated,namely.If the surface concentration of species 1 is higher than that of species 2during segregation kinetics, it can be said that the diffusion coefficient ofspecies 1 is higher than that of species 2. If the surface concentration ofspecies 1 is less than that of species 2, then the diffusion coefficient ofspecies 1 is less than that of species 2.If the surface concentration of species 1 is less than that of species 2 atequilibrium, then the segregation energy of species 1 is less than that ofspecies 2. If the equilibrium surface concentrations are equal, thesegregation energies are equal. When the surface concentration of species 1is higher than that of species 2, then the segregation energy of species 1 ishigher than that of species 2.It is possible to sort the segregation parameters in order of magnitude fromthe results of the experimental work and the constant and linear heatingsimulations. The diffusion coefficients of the species could be arranged fromhigh to low (DN > DP > DSn = DS). The segregation energies of samples26P and 32P could be arranged in the same order, namely?GS < ?GS n< ?GP

[发布机构] University of the Free State