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Quench ageing behaviour of duplex cast steel with nano-scale epsilon-Cu particles

Dyja D., Stradomski Z.

Journal of Achievements in Materials and Manufacturing Engineering

2007

Vol. 20, nr 1-2

435-438

Purpose: The aim of the study was to examine the effectiveness and usefulness of the quench ageing on the service properties of massive duplex cast steel. The mechanism of precipitation of a epsilon-Cu phase and its effect on the mechanical properties of the cast steel were investigated. Design/methodology/approach: The ferrite substructure was examined on a JOEL JEM HREM. The analysis of chemical composition of selected micro-regions was carried out using a scanning electron microscope. The verification of the metallographic examination results was done using the Thermo-Calc program. Findings: Quench ageing causes an increase in hardness and drop in impact resistance, which results from the formation of the epsilon-Cu phase. The ageing parameters have a substantial influence on the ferrite substructure and the degree of coherence, dispersion and amount of the epsilon-Cu phase. The ageing treatment at 480 degrees centigrade causes, in addition to the precipitation of the epsilon-Cu phase coherent with matrix, the formation of an alpha'-Cr phase and an alpha-Fe phase. Practical implications: Duplex cast steels are becoming an irreplaceable material in the elements of equipment exposed to the simultaneous action of corrosive and errosive environment. In the case of massive elements, such as pump (rotors and guide vanes) and pipeline elements etc., which are operated in corrosive environments of water suspensions of solids of different type and gradation, the effect of ageing will be much lower than in small laboratory specimens, which is associated with the presence of a large amount of the incoherent epsilon-Cu phase in the cast steel after the solution heat treatment. Originality/value: The lower limit of ageing temperature (480 degrees centigrade) coincide with the temperature of an undesirable spinodal decomposition of the ferrite, which is partially responsible for the slight increase in hardness and drop in plastic properties. This is the indication that the lower temperature of ageing duplex cast steels with copper should not be lower than 500 degrees centigrade.

Microstructural evolution in a duplex cast steel after quench ageing process

Dyja D., Stradomski Z.

Archives of Materials Science and Engineering

2007

Vol. 28, nr 9

557-564

Purpose: The effectiveness and usefulness of the quench ageing on the service properties of massive duplex cast steel was presented in this work. The mechanism of precipitation of a ε-Cu phase and its effect on the mechanical properties of the cast steel were investigated. Design/methodology/approach: The microscopic analysis of the cast steel was performed on a Zeiss Axiovert 25 optical microscope. The substructure of ferrite was examined on a JOEL JEM 3010 high-resolution transmission electron microscope. The analysis of chemical composition of selected micro-regions was carried out using a JEOL JSM 5400 scanning microscope equipped with an EDS microanalyzer. The verification of the metallographic examination results was done using the Thermo-Calc program. Findings: The formation of the ε-Cu phase during quench ageing causes an increase in hardness and drop in impact resistance. The ageing parameters have a substantial influence on the ferrite substructure and the degree of coherence, dispersion and amount of the ε-Cu phase. The ageing treatment at 480oC causes the precipitation of the ε-Cu phase coherent with the matrix. This temperature of quench ageing produce also the formation of a α'-Cr phase and an α -Fe phase. Practical implications: Duplex cast steels are becoming an irreplaceable material in the elements of equipment exposed to the simultaneous action of corrosive and erosive environment. In the case of massive elements like pumps and pipeline elements, the effect of quench ageing is much lower which is associated with the presence of a large amount of the incoherent ε-Cu phase in the cast steel after the solution heat treatment. Originality/value: The lower temperature of quench ageing duplex cast steels with copper addition should not be lower than 500°C because of the temperature of an undesirable spinodal decomposition of the ferrite in 480°C which is partially responsible for the slight increase in hardness and a drastic drop in plastic properties.