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

Abstract

English: One of the main reasons for temper embrittlement in steel is the segregation of impurities like P to the grain boundaries. Segregation can be defined as the diffusion of atoms from the bulk to the surface and grain boundaries in such a way that the total Gibbs free energy is minimized. This means that segregation can take place against the concentration gradient, from a low concentration in the bulk to a high surface concentration. The chemical potential gradient is the driving force behind segregation. The aim of this study is to investigate the segregation behaviour of P and other 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 both segregation kinetics and equilibrium behaviour. The multi component model for ternary alloys could be expanded to quaternary alloy systems in this study. Segregation kinetics as well as the equilibrium was described by making 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 computer was used to control the Auger spectrometer as well as the constant and linear heating runs. Three commercial 3Cr12 samples was investigated during the study. They were numbered according to their P contend as 26P for the sample with 0.026wt% P, 32P for 0.032wt% P and 62P for the sample containing 0.062wt% P. The constant temperature runs indicate that Sn competes with Cr, N and Pin sample 26P. A definite correlation is visible between Cr and N in sample 32P while Sn and S compete with P in sample 62P. The constant and linear heating Darken simulation model was used to give a qualitative description of the experimental segregation behaviour. The behaviour of two segregating species were simulated in a Fe matrix, from which the influence of the segregation parameters could be demonstrated, namely. If the surface concentration of species 1 is higher than that of species 2 during segregation kinetics, it can be said that the diffusion coefficient of species 1 is higher than that of species 2. If the surface concentration of species 1 is less than that of species 2, then the diffusion coefficient of species 1 is less than that of species 2. If the surface concentration of species 1 is less than that of species 2 at equilibrium, then the segregation energy of species 1 is less than that of species 2. If the equilibrium surface concentrations are equal, the segregation energies are equal. When the surface concentration of species 1 is higher than that of species 2, then the segregation energy of species 1 is higher than that of species 2. It is possible to sort the segregation parameters in order of magnitude from the results of the experimental work and the constant and linear heating simulations. The diffusion coefficients of the species could be arranged from high to low (DN > DP > DSn = DS). The segregation energies of samples 26P and 32P could be arranged in the same order, namely ?GS < ?GS n< ?GP <?GN while that of 62P is ?GS < ?GN< ?GP< ?GSn. In consideration of the bulk concentrations of the samples it is evident that 62P has the highest P concentration. Therefore it’s diffusion coefficient DP is the highest, the segregation rate is higher and it has a higher maximum P enrichment. From the comparison of the P profiles of the different samples it is evident that the maximum surface concentration shift to higher temperatures and that the temperature interval for a certain P concentration increase with an increase in P bulk concentration.

Afrikaans: Die segregasie van onsuiwerhede, soos P, na die korrelgrense in stale, word as een van die hoof oorsake van temperverbrokkeling gesien. Die definisie van segregasie is die diffusie van atome na oppervlakke en korrelgrense op só 'n wyse dat die totale Gibbs vrye-energie van die kristal geminimeer word, wat beteken dat segregasie nie altyd van 'n hoë na 'n lae konsentrasie plaasvind nie, maar soms juis van laer bulkkonsentrasie na 'n hoër oppervlakkonsentrasie. Die dryfkrag van segregasie is die chemiese potensiaalgradiënt. Die doel van die studie is om die segregasie gedrag van P en ander onsuiwerhede soos byvoorbeeld S en Sn in 3Cr12 staal te ondersoek. In die teorie gedeelte is die agtergrond gevorm met behulp van: (i) Die semi-oneindige oplossing van die Fick vergelykings. (ii) t½ en aangepaste t½ modelle. (iii) Gemodifiseerde Darken model. Een van die voordele van die gemodifiseerde Darken model is dat dit die segregasie kinetika sowel as ewewig beskryf. Daar is daarin geslaag om die segregasie model wat vir multikomponent sisteme tot en met ternêre sisteme bestaan, in hierdie studie uit te brei na kwaternêre sisteme. In die studie is van konstante en lineêre temperatuur verhitting gebruik om segregasie kinetika sowel as ewewig te beskryf. Augerelektronspektroskopie is gebruik in die ondersoek na die segregasie gedrag van S, P, Cr, N en Sn in 'n Fe matriks. Die Augerspektrometer is met behulp van 'n persoonlike rekenaar beheer. Dieselfde rekenaar is gebruik in die beheer van die linieêre en konstante verhittings lopies. Drie komersiële 3Cr12 monsters is ondersoek wat onderskeidelik 0.026, 0.032 en 0.062 gew% P bevat. Die monsters is na aanleiding van hul P konsentrasie genommer as 0.026 gew% - 26P,0.032 gew%- 32P en 0.062 gew% - 62P. Die konstante temperatuur lopies het getoon dat Sn kompeteer met Cr, N en P in monster 26P. In monster 32P is 'n duidelike verband tussen Cr en N merkbaar terwyl Sn en S met P kompeteer in monster 62P. In 'n poging om die eksperimentele gedrag kwalitatief te bespreek is konstante en linieêre verhitting Darken model simulasies van 'n ternêre sisteem gebruik. Twee segregerende spesies se gedrag is in 'n Fe matriks gesimuleer. Uit die simulasies kon die invloed wat die verandering in die segregasie parameters op die profiele het, beskryf word, naamlik: Wanneer die oppervlakkonsentrasie van spesie 1 groter is as die oppervlakkonsentrasie van spesie 2 tydens segregasie kinetika, dan is die diffusie koeffisiënt van spesie 1 groter as dié van spesie 2. As die oppervlakkonsentrasie van spesie 1 kleiner as die van spesie 2 tydens die segregasie kinetika is, is die diffusie koeffisiënt van spesie 1 kleiner as dié van spesie 2 Gestel die oppervlakkonsentrasie van spesie 1 is kleiner as die van spesie 2 tydens ewewig, dan is die segregasie energie van spesie 1 kleiner as die van spesie 2. As die oppervlakkonsentrasies van die spesies gelyk is by ewewig, dan is die segregasie energie van die spesies gelyk. Wanneer die oppervlakkonsentrasie van spesie 1 groter is as die van spesie 2, by ewewig, is die segregasie energie van spesie 1 groter as die van spesie 2. Uit die bevindings van die linieêre en konstante verhitting simulasies en eksperimentele resultate is die segregasie parameters in volgorde van grootte gerangskik, nl: Die diffusie koeffisiënte van al drie monsters se volgorde, van groot na klein is: DN > DP > DSn = DS. Monsters 26P en 32P se segregasie energieë kon in dieselfde volgorde geskryf word naamlik ?GS < ?GSn< ?GP <?GN. Die volgorde van segregasie energie vir monster 62P is ?GS < ?GN< ?GP <?GSn. Na aanleiding van die bulkkonsentrasies van die monsters is dit duidelik dat monster 62P die grootste P inhoud bevat wat ‘n groter diffusie koeffisiënt DP, hoër P segregasie tempo en ‘n hoër maksimum P bedekking tot gevolg het. Uit die vergelyking tussen die verskillende monsters se P profiele is dit ook duidelik dat die maksimum bedekking verskuif na hoër temperature en dat die temperatuur interval vir ‘n sekere P konsentrasie op die oppervlak vergroot met ‘n verhoging in P bulkkonsentrasie.