15CrMoR is a chromium-molybdenum alloy steel widely used in Chinese pressure vessel standards (GB/T 713) and also recognized internationally. The designation indicates a nominal composition of 0.15% carbon, 1.0% chromium, and 0.5% molybdenum. This Cr-Mo alloy system provides exceptional high-temperature creep rupture strength and resistance to hydrogen attack, making it suitable for reactors, hydrocrackers, and other heavy-wall pressure vessels operating at temperatures up to 550°C.
Creep Rupture Mechanisms and Data
Creep rupture is the time-dependent failure of a material under sustained stress at elevated temperature. For 15CrMoR, the creep rupture strength is governed by three primary mechanisms: dislocation glide, grain boundary sliding, and carbide coarsening. At operating temperatures of 450–550°C, the steel’s bainitic or ferritic-bainitic microstructure (achieved through quenching and tempering) contains fine carbides of the type M₇C₃, M₂₃C₆, and Mo₂C that pin dislocations and retard grain boundary movement. Standard creep rupture test data for 15CrMoR, based on long-term tests (up to 50,000 hours or extrapolated to 100,000 hours), indicate that at 500°C the average creep rupture strength for 100,000 hours is approximately 120 MPa. At 540°C, this value drops to about 80 MPa, and at 570°C it falls below 50 MPa. The allowable design stress values derived from these creep rupture strengths, with appropriate safety factors (typically 1.5 against creep rupture for Grade 1 materials), enable engineers to compute minimum wall thicknesses for high-temperature pressure vessels.
Factors Affecting Creep Rupture Strength
Several variables influence the actual creep rupture strength of 15CrMoR in service. Chemical composition control within the specification (Cr 0.80–1.25%, Mo 0.45–0.65%) is critical because even small deviations in chromium and molybdenum content significantly affect carbide precipitation and coarsening rates. Heat treatment plays a decisive role: quenching from 900–960°C followed by tempering at 660–740°C produces a tempered bainite structure with optimal creep resistance. Over-tempering reduces strength, while under-tempering reduces toughness. Service exposure over decades may lead to temper embrittlement due to phosphorus segregation, especially in thick sections. Furthermore, welds in 15CrMoR require careful control; the creep rupture strength of weldments is often lower than that of base metal due to the formation of coarse-grained heat-affected zones (CGHAZ) and local carbide precipitation. For this reason, PWHT at 660–690°C is mandatory for all 15CrMoR vessels to restore ductility and creep properties in the weld region. Advanced design codes such as ASME Section VIII Division 2 and GB/T 150 incorporate creep-fatigue interaction rules for 15CrMoR when cyclic operation is expected.
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