TY - BOOK

T1 - Residual stress evolution in low-alloyed seamless steel tubes

T2 - A simulation based approach on three different length scales

AU - Leitner, Silvia

N1 - no embargo

PY - 2020

Y1 - 2020

N2 - Residual stresses in a component can alter its mechanical performance and environmental resistance against corrosion or hydrogen embrittlement. To continuously reduce component weight and still increase the mechanical performance by the use of steels with higher yield strengths in corrosive environments, the residual stress formation during manufacturing must be monitored and understood. Due to steel's complex microstructure, residual stresses form on different characteristic length scales and not only macroscopic scale and require thus a multi-scale investigation. The residual stress formation in low-alloyed, seamless, sour gas resistant steel tubes on different length scales was investigated using a model based simulation approach. Starting from the macroscopic scale a model for calculating the temperature, phase and residual stress evolution as a function of radial position within the tube during cooling was created. The heat treatment model was validated with thermal measurements, the phase evolution model was verifed with microhardness measurements over the tube's cross section and the residual stress evolution using high-energy x-ray diffraction measurements. In this thus validated macroscopic model, the cooling boundary conditions were varied in order to and potential low-residual stress cooling strategies for mostly martensite or mostly bainite microstructures. The experimentally validated continuum model was employed to calculate the effects of local chemical enrichment and depletion in segregation lines on residual stresses, i.e. the mesoscopic stresses. The martensite transformation start temperature is delayed due to chemical enrichment of C, Mo and Cr and the concomitant changes in thermal expansion, volume expansion and elastic properties were derived from calculations and literature. The mesoscopic residual stresses are calculated by means of a submodel and as a function of composition. On the microscopic scale the residual stresses around precipitates are investigated. This is owing to the recent trend of using precipitates as hydrogen traps and that some have been already proven to be beneficial in some cases. Thus a relative comparison of occurring precipitates was sought, for low alloyed steels. This was done using an analytic, thermo-elastic comparison of average matrix stress and average inclusion stress based on literature data. The inclusions are ranked by their residual stress formation potential. For a single inclusion the crystallographic relations, interface properties and creep dissipation are considered to calculate the energetically favored shape and orientation of Mo2C in bcc Fe. The anisotropic interface energies, calculated with frst principle methods, suggest that the experimentally observed elongated shape of Mo2C in bcc Fe is caused by interface energy minimization. With these developed methods, residual stress in a low- and microalloyed sour gas resistant steel tubes are calculated and their evolution is resolved on three different length scales. The macroscopic model allows a detailed understanding of how the cooling process and residual stress formation interact. Based on the developed models, low-residual stress cooling strategies are provided. It is benefcial to reduce surface temperature gradients as far as possible and use spray water cooling on both the inner and the outer surface. To account the steel's complex microstructure, the residual stresses caused by interdendritic segregation and precipitate formation are discussed. Stresses caused by the chemical inhomogeneities resulting from segregation, act over a wider characteristic length scale and are comparatively low compared to stresses due to thermal misfts of precipitates.

AB - Residual stresses in a component can alter its mechanical performance and environmental resistance against corrosion or hydrogen embrittlement. To continuously reduce component weight and still increase the mechanical performance by the use of steels with higher yield strengths in corrosive environments, the residual stress formation during manufacturing must be monitored and understood. Due to steel's complex microstructure, residual stresses form on different characteristic length scales and not only macroscopic scale and require thus a multi-scale investigation. The residual stress formation in low-alloyed, seamless, sour gas resistant steel tubes on different length scales was investigated using a model based simulation approach. Starting from the macroscopic scale a model for calculating the temperature, phase and residual stress evolution as a function of radial position within the tube during cooling was created. The heat treatment model was validated with thermal measurements, the phase evolution model was verifed with microhardness measurements over the tube's cross section and the residual stress evolution using high-energy x-ray diffraction measurements. In this thus validated macroscopic model, the cooling boundary conditions were varied in order to and potential low-residual stress cooling strategies for mostly martensite or mostly bainite microstructures. The experimentally validated continuum model was employed to calculate the effects of local chemical enrichment and depletion in segregation lines on residual stresses, i.e. the mesoscopic stresses. The martensite transformation start temperature is delayed due to chemical enrichment of C, Mo and Cr and the concomitant changes in thermal expansion, volume expansion and elastic properties were derived from calculations and literature. The mesoscopic residual stresses are calculated by means of a submodel and as a function of composition. On the microscopic scale the residual stresses around precipitates are investigated. This is owing to the recent trend of using precipitates as hydrogen traps and that some have been already proven to be beneficial in some cases. Thus a relative comparison of occurring precipitates was sought, for low alloyed steels. This was done using an analytic, thermo-elastic comparison of average matrix stress and average inclusion stress based on literature data. The inclusions are ranked by their residual stress formation potential. For a single inclusion the crystallographic relations, interface properties and creep dissipation are considered to calculate the energetically favored shape and orientation of Mo2C in bcc Fe. The anisotropic interface energies, calculated with frst principle methods, suggest that the experimentally observed elongated shape of Mo2C in bcc Fe is caused by interface energy minimization. With these developed methods, residual stress in a low- and microalloyed sour gas resistant steel tubes are calculated and their evolution is resolved on three different length scales. The macroscopic model allows a detailed understanding of how the cooling process and residual stress formation interact. Based on the developed models, low-residual stress cooling strategies are provided. It is benefcial to reduce surface temperature gradients as far as possible and use spray water cooling on both the inner and the outer surface. To account the steel's complex microstructure, the residual stresses caused by interdendritic segregation and precipitate formation are discussed. Stresses caused by the chemical inhomogeneities resulting from segregation, act over a wider characteristic length scale and are comparatively low compared to stresses due to thermal misfts of precipitates.

KW - residual stresses

KW - low-alloyed

KW - steel

KW - tube

KW - simulataion

KW - finite element

KW - phase transformation

KW - Eigenspannungen

KW - niedrig-legiert

KW - Stahl

KW - Rohr

KW - Simulation

KW - Finite Elemente

KW - Phasenumwandlung

KW - Wärmebehandlung

M3 - Doctoral Thesis

ER -