Microstructural Characterization of Quenched and Tempered Heat-treated P110 Casing Steels

  • Author / Creator
    Yang, Mingzhang
  • P110 is a quenched and tempered (Q&T) heat-treated high strength casing used in oil/gas mild sour well drilling. Critical properties influencing performance are strength and hydrogen embrittlement (HE) resistance, which are related to carbide morphology, steel composition, and Q&T conditions. This study investigated the effects of Cr and C content, as well as Q&T conditions on the type and morphology of carbides that form in P110 steel. Specifically, a series of Q&T heat treatments were conducted on as-rolled steels with two nominal compositions, 0.278C-0.77Cr and 0.255C-0.52Cr (wt%).
    The effects of austenitizing and quenching (A&Q) treatment (950°C for 15 to 90 min) on as-rolled P110 plates were studied using optical microscopy (OM), micro-hardness, x-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), electron microprobe analysis (EMPA), and scanning-transmission electron microscopy (TEM/STEM). Microsegregation of Cr and Mo was observed in the as-rolled steels with the former exhibiting a higher extent of microsegregation. A&Q at 950°C for 30 min caused a slight reduction in the extent of microsegregation, but the growth in prior austenite grain (PAG) size during this period was significant. Based on the optimum balance bewteen solutionizing and PAG growth, A&Q steels from 950°C for 15 min were selected as the starting materials for tempering studies. The A&Q steel (950°C-15 min) contained less 0.5 vol% retained austenite, and a fine PAG size (~ 11 µm), as well as fine (< 20 nm) and dispersed (Ti, Mo)C carbides formed during the hot-rolling process.
    Following A&Q treatment, the effects of tempering conditions on carbide morphology were studied by tempering the A&Q treated steels (950°C-15 min) between 550°C and 715°C for 15 min to 390 min. The carbide morphology (i.e., type, size, shape, volume fraction, and distribution) was studied using SEM and STEM. The relations among carbide morphology, steel composition, and tempering conditions were studied using Thermo-Calc simulations. The average Mo content in the metallic constituent of (Ti, Mo)C particles increased from 34 to 44 wt% after tempering at 715°C for 45 min, whereas their shapes and size distribution were unaffected by subsequent tempering heat treatments. Coarsening of M3C particles was accelerated by increasing tempering temperature (T), particularly at T > 650℃. It is noteworthy that after tempering at 650°C for 45 min, M3C particles in ferrite were enriched in Cr and approached equilibrium. Any increase in tempering T or t beyond this condition led to a significant increase in particle size and a decrease in number density. The increase in nominal Cr and C content did not result in a prominent difference in the size and volume fraction of tempered carbides. However, M23C6 carbides were only identified in tempered 0.77Cr steel as a result of the higher nominal Cr content.
    The effects of carbide morphology on tempered structure were studied. With increasing tempering temperature, coarsened carbides reduced the pinning action of lath boundaries, which enabled lath structure coarsening. The average lath width at temperatures of 600℃, 650℃, and 715℃ (45 min) corresponded to 329 ± 69 nm, 335 ± 89 nm, and 638 ± 90 nm, respectively, with the most substantial change in coarsening occurring between 650℃ and 715℃.
    Micro-hardness and tensile testing were undertaken for the Q&T steels. The hardness and tensile properties after tempering for any combination of t and T exhibited a linear behavior when plotted against the Holloman Jaffe (HJ) parameter. A reduction in softening during tempering for HJ < 17.4 was observed, which was associated with the grain boundary pinning action from fine and uniformly distributed (Ti, Mo)C and M3C particles. The continuous decrease in hardness and tensile properties with increase tempering T or t was attributed to the combined effects of M3C coarsening, dislocation annihilation, and lath structure coarsening.
    To produce P110 steel that meets the minimum YS requirements from the API 5CT standard, tempering should be conducted at conditions where HJ < 18.4. Unless tempering t is sufficiently low, the tempering T should not exceed 650°C to acquire sufficient strength. In terms of HE resistance, the optimum carbide morphology can be obtained by tempering at 650°C for 45 min, where uniformly distributed fine particles (60-65 nm) with a high number density (51/µm2) and volume fraction (0.11) are produced.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.