Coarsening Rate of M₂₃C₆and MC Particles in a High Chromium Creep Resistant Steel

Abstract

The experimental coarsening rate of M 23 C 6 particles at 8008C was about three times greater than that calculated from the slightly modified Lifshitz-Slyozov-Wagner equation. Applying a simplified relation, the coarsening rate was deduced for M 23 C 6 particles in range 800-5508C. The calculated coarsening rates are compared to data in references.

Change of M23C6 Particles Size and Spacing by Tempering a High‐Chromium Creep Resistant Steel at 800–550 °C

“…The coarsening constant of M 23 C 6 particles ( k cT ) in the examined temperature range was calculated from Equation derived from the Lifshitz–Sljuzov–Wagner Equation for calculation of particles coarsening rate: with d t – particles size at tempering time t , d 0 – initial particles size, T – tempering temperature (K), S – atom content of chromium in solid solution in ferrite, γ – carbide particle matrix interfacial energy, Ω – volume of diffusing Cr atom, D – chromium diffusion rate D = D 0 exp(− Q / RT ), with D 0 = 3.7 × 10 −3 m 2 s −1 and Q = 267 kJ mol −1 ), k B – Boltzmann constant, and k c,1073 = 2.94 × 10 −27 m 3 s −1 – experimental coarsening rate at 800 °C …”

“…The particles stability depends of carbide solubility and matrix composition. In high‐chromium creep resistant steels with relatively low content of vanadium and niobium, VC is dissolved at ≈700 °C and NbC at ≈750 °C, while, chromium carbide M 23 C 6 with composition about Cr 18 Fe 3 Mo 2 C 6 is stable up to 800 °C due to the high‐atoms ratio Cr/C ≈60 in the steel. The surplus of chromium ensures virtually constant matrix and M 23 C 6 compositions from 550 to 800 °C.…”

“…In this work, the changes of the total number of M 23 C 6 particles, the number of particles intercepting mobile dislocations at 800 °C, the particles dissolution velocity at 750–550 °C, as well as the changes of average particles size and spacing were calculated for the steel 0.18C11.5Cr1.08Mo0.29V . The steel was annealed for 30 min at 1050 °C to obtain the complete dissolution of carbide particles in austenite, oil cooled to martensite, tempered at 800 °C and afterwards tempered at 750–550 °C.…”

“…The steel was annealed for 30 min at 1050 °C to obtain the complete dissolution of carbide particles in austenite, oil cooled to martensite, tempered at 800 °C and afterwards tempered at 750–550 °C. The base of the analysis are the average size and cumulative frequency of particles after 100, 334, 1334, and 3335 h of tempering at 800 °C, 100 h of additional tempering at lower temperature and the coarsening equation of M 23 C 6 particles in refs …”

High‐Temperature Phase Stability of 9Cr–W–Ta–V–C Based Reduced Activation Ferritic–Martensitic (RAFM) Steels: Effect of W and Ta Additions

“…At this juncture, it is also instructive to note that in addition to stable M 23 C 6 and MX type carbide phases, some high Cr‐compositions are also prone to precipitating the M 2 X phase, and this could lead to additional strengthening, if tempering treatment is carried out at relatively lower temperatures like 550 °C/823 K . Finally, the stable M 23 C 6 and MX carbide phases can also undergo coarsening during actual service at high temperatures …”