Elevated temperature mechanical properties of line pipe steels

Abstract

The effects of deformation temperature near the dynamic strain aging (DSA) regime on the tensile properties of X70 and X52 line pipe steels were evaluated. Testing temperatures of interest were in the range of 25 to 350 °C, at engineering strain rates in the range of approximately 10 4 to 10 3 s 1. The X70 steel had a quasi-polygonal ferrite microstructure with austenite, martensite-austenite, and carbide microconstituents present. The X52 steel was a control material (anticipated to exhibit elevated temperature behaviors comparable to observations in the DSA literature) with a ferrite-pearlite microstructure. Elevated temperature tensile testing was used to strain material and measure the mechanical properties of the X52 and X70 steels. Further mechanical and microstructural characterization was performed on the as-received and deformed line pipe steels, or in situ during elevated temperature deformation. Digital image correlation (DIC) was used in situ to characterize localized deformation behaviors during testing of the line pipe steels. Strain rate jump tests (cycling of strain rates during deformation) were performed to measure the temperature dependence on the instantaneous strain rate sensitivities. Stress relaxation tests were used to measure the internal/effective stress components of the flow stress and apparent activation volumes. Torsional pendulum experiments were performed to measure internal friction carbon Snoek peak heights related to the free interstitial concentrations in bcc ferrite. Microstructural changes after tensile deformation were assessed at various length scales using scanning and transmission electron microscopy. Finally, high sensitivity phase analyses were performed using synchrotron wide-angle x-ray scattering. Both pipeline steels exhibited evidence of DSA under similar testing conditions. The DSA temperature regime for the X70 steel was slightly smaller than the DSA regime for the X52 steel. Manifestations of DSA during tensile testing (i.e. mechanical behavior within the DSA regime) include: serrated flow curves, stable localized deformation (in the form of propagating and nucleating deformation bands), increasing flow strength as a function of temperature, reduced ductility relative to room temperature, high strain hardening rates, high dislocation densities relative to room temperature, and negative strain rate sensitivities. There was a direct relationship between the flow curve morphologies and localized deformation band behaviors. Deformation band propagation was associated with relatively low frequency load drops in the flow curve and was typically observed at relatively low temperatures within the DSA regime. Deformation band nucleation was associated with relatively high serration frequencies and was typically observed at relatively high temperatures within the DSA regime. The critical strain required for serrated flow was greatest at the upper and lower boundaries for serrated flow. Negative strain rate sensitivities associated with samples tested within the DSA regime influenced necking behaviors in the pipeline steel tensile samples. Necking in samples that exhibited negative jump test strain rate sensitivity m-values was more concentrated (i.e. influenced a smaller portion of the gauge length) compared to samples with positive strain rate sensitivities. Necking initiated from previously propagating type deformation bands within the dynamic strain aging regime, and concentrated at a slightly higher rate than samples that exhibited uniform plastic deformation. At high temperatures outside the DSA regime, the X70 steel exhibited tensile behavior that deviated from the X52 that was a result of strain assisted tempering of the quasi-polygonal ferrite microstructure (referred to as strain assisted bainitic tempering, due to the similarities in phase changes present between the two microstructures during tempering). Evidence of tempering of the quasi-polygonal ferrite microstructure was the decomposition of austenite observed with wide-angle x-ray scattering and scanning electron microscopy and the dissolution of transition carbides. Precipitation of cementite, which is associated with low temperature tempering, was theorized to contribute to the relatively high temperature mechanical behavior. Strain assisted bainitic tempering caused a shift in the flow stress maximum to higher temperatures (i.e. precipitation strengthening caused an increase in the flow stress), reduced ductility, increased strain hardening rates, negative overall strain rate sensitivities, increased interstitial carbon concentrations in the ferrite, increased apparent activation volumes, and increased dislocation densities. Austenite decomposition (which was used as the primary indicator of tempering in the quasi-polygonal ferrite microstructure) was highly strain dependent at temperatures below approximately 300 °C. However, increased temperatures reduced the strain dependence on austenite decomposition (and tempering), therefore reducing the strain dependence on carbide precipitation and coarsening (i.e. tempering occurred during isothermal heating, prior to deformation). A theoretical X70 alloy (based on the steel used in the present study) was designed for optimized elevated temperature mechanical properties. It was theorized that maximum strengthening from strain assisted bainitic tempering could be achieved by retarding the coarsening of cementite precipitates in the quasi-polygonal ferrite grains. Additional carbide forming elements were added to the designed X70 alloy to retard cementite growth during tempering.