A Method for Estimating Residual Creep Lives of Cast HK-40 Reformer Tubings

Abstract

Results of short term creep rupture tests of creep damaged cast HK-40 reformer tubings, which were removed from ICI steam reformers after about 40000 to 70000 hours of service, were analyzed correlating to the various metallographic factors. It was found that area fractions of voids and fissures, which were characteristic of the third stage of creep degradation of HK-40 alloys, bore a good correlation with the creep rupture property expressed in the from of Larson-Miller parameters. Through analysis of the results, empirical equations for estimating the residual creep lives were proposed as follows:
logσ=-0.7374+0.2994P₀-0.01130P₀²
P₀=P+0.60+0.60V
P=T×10^-3 (logt+15)
where,
σ: scheduled operating stress (kg/mmmm₂)
T: scheduled operating temperature (°K)
V: area fraction of voids (%)
t: residual life (h)
The only one inspection to be involved in the present estimation is the metallographic quantitative analysis of area fractions of microfractures as voids and microfissures. A criteria for the renewal of the existing reformer tubings was established to be an area fraction of voids and fissures of 1%, when the residual life would fall short of one year.