ABSTRACT
A large amount of work has recently been done on the prediction of the microstructure of steel weld deposits, making it possible now to estimate the as-welded microstructure as a function of thermal history and chemical composition. This work is part of a complementary project aimed at obtaining quantitative and widely applicable relationships between weld microstructures and properties. The thesis begins with a literature review covering the major features of the development of microstructure in low-alloy steel welds, and the recent work on the modelling of this microstructure. A variety of factors influence the relation between microstructure and mechanical properties. The microstructure and properties of a weld are influenced strongly by the mode of solidification, whether this involves the formation of 8-ferrite or austenite as the primary phase, and the solidification stage determines the extent of chemical segregation and growth processes within the weld pool. Experimental work has been carried out to determine the cooling rates at the solid-liquid interface encountered in weld pools as a function of welding conditions. The critical carbon composition for low-alloy steel welds above which solidification will occur as austenite has also been established for the manual-metal-arc process. Thermodynamic models have been employed and developed
to allow the various phase transformations experienced by low-alloy steels during equilibrium solidification to be calculated for any reasonable combination of alloying elements. Calculations for the partition coefficients of solute elements during solidification are also presented. This work should provide a basis for the calculation of time-temperature-transformation diagrams for the solidification process. Detailed models are presented to allow the quantitative prediction of weld metal yield strength, tensile strength, flow stress, strain hardening characteristics, elongation, and reduction of area for a given microstructure and composition. The model for tensile strength is further developed to allow strength to be calculated as a function of temperature. The wide scatter in toughness results often associated \vith weld metals is shown to be explicable in terms of the inhomogeneity of the microstructure. Any attempt at modelling the toughness of welds requires a knowledge of the inclusion distribution. Work on experimental welds has shown that the inclusions in a weld deposit are not uniformly distributed, but segregate to the boundaries of the first phase to solidify. The implications of this work are particularly serious for welds solidifying as austenite, since the inclusions are then located away from the centres of the grains where they cannot act as intragranular nucleants for acicular ferrite. In a separate chapter, fresh evidence that the acicular ferrite phase in welds is bainitic is presented. In summary, the thesis presents work which has successfully modelled some of the important mechanical properties of welds, and work which has laid the foundations for further research aimed at obtaining quantitative microstructureproperty relationships.
Download PDF File of Chapter I
Chapter II
Chapter III
Chapter IV
Chapter V
Chapter VI
Chapter VII
Chapter VIII
Chapter IX
Chapter X
A large amount of work has recently been done on the prediction of the microstructure of steel weld deposits, making it possible now to estimate the as-welded microstructure as a function of thermal history and chemical composition. This work is part of a complementary project aimed at obtaining quantitative and widely applicable relationships between weld microstructures and properties. The thesis begins with a literature review covering the major features of the development of microstructure in low-alloy steel welds, and the recent work on the modelling of this microstructure. A variety of factors influence the relation between microstructure and mechanical properties. The microstructure and properties of a weld are influenced strongly by the mode of solidification, whether this involves the formation of 8-ferrite or austenite as the primary phase, and the solidification stage determines the extent of chemical segregation and growth processes within the weld pool. Experimental work has been carried out to determine the cooling rates at the solid-liquid interface encountered in weld pools as a function of welding conditions. The critical carbon composition for low-alloy steel welds above which solidification will occur as austenite has also been established for the manual-metal-arc process. Thermodynamic models have been employed and developed
to allow the various phase transformations experienced by low-alloy steels during equilibrium solidification to be calculated for any reasonable combination of alloying elements. Calculations for the partition coefficients of solute elements during solidification are also presented. This work should provide a basis for the calculation of time-temperature-transformation diagrams for the solidification process. Detailed models are presented to allow the quantitative prediction of weld metal yield strength, tensile strength, flow stress, strain hardening characteristics, elongation, and reduction of area for a given microstructure and composition. The model for tensile strength is further developed to allow strength to be calculated as a function of temperature. The wide scatter in toughness results often associated \vith weld metals is shown to be explicable in terms of the inhomogeneity of the microstructure. Any attempt at modelling the toughness of welds requires a knowledge of the inclusion distribution. Work on experimental welds has shown that the inclusions in a weld deposit are not uniformly distributed, but segregate to the boundaries of the first phase to solidify. The implications of this work are particularly serious for welds solidifying as austenite, since the inclusions are then located away from the centres of the grains where they cannot act as intragranular nucleants for acicular ferrite. In a separate chapter, fresh evidence that the acicular ferrite phase in welds is bainitic is presented. In summary, the thesis presents work which has successfully modelled some of the important mechanical properties of welds, and work which has laid the foundations for further research aimed at obtaining quantitative microstructureproperty relationships.
Download PDF File of Chapter I
Chapter II
Chapter III
Chapter IV
Chapter V
Chapter VI
Chapter VII
Chapter VIII
Chapter IX
Chapter X
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