Effect of Ferrite Lattice Vacancies on Creep Rate of the Steel X20CrMoV121 in the Range 763–913 K

Abstract

The difference of the experimental and the calculated creep rates of the steel grade X20CrMoV121 with M 23 C 6 particles in temperature range 763-913 K was examined in terms of temperature, iron self-diffusion rate, glide stress, and number of ferrite lattice vacancies. The Ashby-Hornbogen creep equation is modified by adding a parameter that represents the effect of number of lattice vacancies; consequently, the difference of the experimental and the calculated creep rates is diminish to the level of average particles size assessment accuracy.

Glide Stress by Stationary Creep of Tempered Martensite with Polyhedral Particles

“…The effect of change of number of vacancies is negligible below 741 K . At higher temperature, the difference of experimental and calculated creep rate increased greatly and very similar was the growth of number of vacancies, while the effect of decrease of lattice glide resistance was by ≈70 times lower …”

“…By analysis of creep rate of equal specimens in temperature range 823–913 K, it was established that the difference of experimental creep rate and that calculated from Equation increased strongly with temperature. With substitution in Equation of λ with ( λ–d ) and addition of the parameter k v depicting the effect of increase of the number of lattice vacancies with temperature, the difference of calculated and experimental creep rate was greatly diminished …”

“…The difference of experimental and calculated creep rate in temperature range 823–913 K was diminished with completion of Equation with the parameter log k v = log k n × Δ T with k n , number of vacancies at 741 K and Δ T = T c – 741, T c , creep temperature and substitution of the parameter λ with ( λ–d ). By reliable values of other parameters, the difference of experimental and calculated creep rate was limited by the accuracy of assessment of average particles size and spacing …”

“…By absence of grain boundary voids in the creep gauge, the specimen stationary creep rate adjusts to the creep rate in pdm's with the lowest (critical) glide stress σ g and the dislocation gliding velocity g v . For a steel X20 with uniform distribution of carbide particles with the average size of d p = 147 nm and the average particles spacing of λ p = 537 nm, by creep stress σ c = 170 MPa, the creep rate at T c = 741 K was έ = 9.97 10 −11 s −1 . In pdm's with glide stress σ g = 4.03 MPa governing the creep rate, the average angle gliding direction‐creep stress direction in the poly‐grains creep specimen is ≈45°, the climb and creep step length the spacing (011) planes in creep direction cl s = cr sl = 0.2 / sin45 = 0.28 nm.…”

“…The glide and climb components of acting stress are deduced considering the particle as stress givers to the ferrite matrix with the mobile dislocations glide and climb directions dependent of the grains lattice orientation. The effect of change of number of lattice vacancies will be examined in the next article …”