Creep fracture in tempered martensitic steels

Abstract

As long term laboratory creep data became available the original estimates of the allowable design stresses for creep strength enhanced ferritic steels (CSEF) were reduced. Thus, even in properly processed steel, the long term performance and creep rupture strength reported is below that originally predicted from a simple extrapolation of short term data. In addition, the typical alloy compositions for these steels specified by Codes provide reasonable strength but can steels can exhibit brittle creep behavior. This inherent brittle behavior results in notch weakening and the need to invoke weld strength reduction factors. Moreover, creep brittle behavior, and the associated micro void development, promotes burst rather than leak type fracture in components. The existence of significant densities of voids further complicates in-service assessment of condition and weld repair of these steels. It is now clear that the levels of ductility required in engineering applications necessitate proper control of composition (including trace elements), steel making and processing and all heat treatments. This paper examines background on the development of creep voids in martensitic steels and discusses metallurgical and stress state factors which promote brittle behavior.

Analysis of the Degradation in the Creep Strength of High-Cr Martensitic Steels

“…Moreover, RA tends to recover for a longer time duration. The decrease in ductility is mainly caused by the pre-existing BN particles (Gu et al, 2014;Parker & Siefert, 2018;Abe et al, 2018). Breakdown in rupture strength of Gr.IIIa definitely occurs in the degradation region of RA, however the primary cause for the breakdown in strength should be investigated separately from BN, because the center of Gr.IIIa is positioned at a higher stress in Figure 12 than the center of the RA = 0 zone.…”

“…Though the figures are omitted, a spool line for RA = 70% of T92/MJV is ~10 000 h and is not so largely dependent on temperature and that for MJU lies between those for MJP and MJT. Though the pre-existing BN particles are considered to be a major factor for the low ductility in Grade 92 (Gu et al, 2014;Parker & Siefert, 2018;Abe et al, 2018), the above mentioned facts indicate that factors other than BN affect the low ductility of Grade 92. Normalizing temperature, Hv, C and Ni contents for MJU are 1050 °C, 226, 0.10%, and 0.27%, respectively, and 1050 °C, 234, 0.092%, and 0.15% for MJV, respectively.…”

Creep deformation measurement of ex‐service 12% Cr steel over nonuniform stress fields using digital image correlation

“…The material selected for this work is virgin and service-exposed X20 steel supplied in the form of 34 mm thickness piping offcuts from a local power utility. The reason for the selection of X20 as a test material is because it is widely adopted in aging coal-fired power plants in South Africa [11] which allows access to aged material with an extensive history of up to 20 years of service exposure. Although X20 was superseded by X10CrMoVNb9-1 (P91) in the 1970s, [23] similarly aged materials are not easily acquirable for the latter steel.…”

“…This is in agreement with similar studies on 11% Cr steels by Gupta et al [37] and suggests significant cavitation and cavity growth in the high damage, low stress test compared to the higher stress tests. [11] However, it should be noted that freshly nucleated cavities have diameters in the order of <1 μm [32] and so have been excluded from the current analysis. This is because both pre-existing and large creep cavities (>1 μm) in the ex-service materials have been shown to be principle contributing factors to failure at the relatively high stresses employed in this study.…”

“…[9] In some instances, replication is unable to characterise the damage of aging alloys due to limited void development such as in the case of the 12% Cr steel X20CrMoV12-1 (X20 hereafter). [10,11] In the authors' previous work, [9,12] micro-grain and M 23 C 6 precipitate growth were found to be effective microstructural damage sensors of ex-service (removed from service) X20 from main steam piping. Quantitative evaluation of these sensors, like hardness and void density measurements, are subject to high degrees of scatter given the microstructural inhomogeneities.…”