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Languages: English
Types: Doctoral thesis

A series of carbon manganese steels containing varying amounts of carbon, vanadium and nitrogen was investigated in relation to the solubility of VC and VN in austenite, the grain coarsening characteristics of austenite, the tempering of martensite and other structures, the transformation during continuous cooling, the effect of vanadium addition and increasing nitrogen content on the thermo-mechanical processing of austenite, and the transformation of various morphologies of austenite to ferrite. The sites for preferential nucleation and growth of ferrite were identified and the effect of ferrite grain size inhomogeneity was investigated with a view to minimising it.

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The C/N ratio in the V(CN) precipitates was largely controlled by C/N ratio in the steel and it was also influenced by the austenitising treatment. As expected, the solubility of VN was\ud less than that of VC.

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A systematic investigation of austenitising time and temperature on the grain coarsening characteristics was carried out showing the effects of vanadium, carbon and nitrogen. It was tentatively suggested that C-C and N-N clustering in the vanadium free steels controlled the grain growth whereas in the presence of vanadium, it was shown that VN and VC pinned the austenite grain boundaries and restricted grain growth. However coarsening or solution of VC and VN allowed the grain bondaries to migrate and grain coarsening occurred. The grain coarsening temperature was controlled predominantly by VN, whilst the VC dissolved frequently below the grain coarsening temperature.

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In the as quenched martensite, increasing nitrogen progressively increased the as quenched hardness, and the hardness also greatly increased with increasing carbon and vanadium added together. Examining the precipitation strengthening in tempered martensite showed that in the absence of vanadium, martensite softened progressively with increasing temperature and time. Vanadium additions increased the hardness level during low temperature tempering and at higher tempering temperature introduced secondary hardening. The intensity of secondary hardening increased with increasing vanadium, whereas austenitising temperature had little or no effect. The softening after the secondary hardening was faster after austenitising at the higher temperature and when recrystallisation occurred at the highest tempering temperatures, the hardness was lower due to coarse recrystallised ferrite.

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Isothermal transformation studies showed that vanadium additions raised the Ar3 temperature and accelerated ferrite nucleation, whilst the growth of ferrite was delayed due to the format ion of V(CN) interphase and general precipitation pinning, of the transformation front. Increasing nitrogen content in the V-steel increased the incubation period for ferrite nucleation and increasingly reduced the ferrite growth by increasing V(CN) precipitation pinning of the transformation front.

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Transformation during continuous cooling was examined in relation to the effect of vanadium, carbon and nitrogen together with the effect of austenitising temperature. Increasing austenitising temperature increased the austenite grain size, and it then became apparent that increasing vanadium, carbon and nitrogen increased the hardenability and raised the hardness level of the jominy curve for the non-martensitic products. This was particularly the case for the higher austenitising temperature, but at lower austenitising temperatures the effects were much less. A small increase in hardness was observed at some distance from the quenched end of the jominy specimen, which distance increased with increasing austenitising temperature. Tempering at 550°C caused small increases in hardness which developed into maxima, and occurred at shorter distances from the quenched end. Increasing tempering temperature and time caused the peak to be overaged and the general hardness level was increased. This increase in general hardness was attributed to the dislocation density and supersaturation of the transformation products, which caused hardening during tempering by precipitation of V(CN) particles.

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The effects of vanadium addition and increasing nitrogen, together with the deformation temperature and amount of deformation, were examined in relation to the austenite morphologies and subsequent ferrite nucleation and growth. Vanadium additions, increased ferrite nucleation by refining the austenite grain size and increased the volume fraction of ferrite at a given transformation time by providing additional nucleation sites both at the V(CN) precipitates, and at the austenite/ferrite interfaces. In the absence of apparent retained deformation, and at low nitrogen contents, nucleation of ferrite was delayed but the ferrite grew rapidly due to insufficient V(CN) to pin the ferrite/austenite interface. Increasing nitrogen however retarded both ferrite nucleation and growth, and reasons for this have been discussed. The growth of ferrite during isothermal holding was dictated by the nucleation sites such as grain boundaries, deformation bands and intra-granular nucleation sites. Both grain boundary and deformation band nucleated ferrite formed small ferrite grain sizes due to the numerous nuclei and their early impigement, whereas intragranularly nucleated ferrite grew rapidly and formed large ferrite grains due to insufficient precipitation to pin the transformation front. This type of structure led to duplex ferrite grain sizes and inhomogeneous type structures. Continuously cooled specimems verified these effects.

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