The impact of welding on the creep properties of advanced 9–12% Cr steels

Abstract

The term "long-term creep properties" for heat resistant steels is mainly reflected by the 100.000 hour creep rupture strength at elevated testing temperature. Often, results of high stress, short-term creep tests are extrapolated to this 100.000 hour target value. Results of longterm creep tests are rather rare because of high testing costs and the time consuming testing procedure. Especially, long-running crossweld creep tests have not been performed in a sufficient extent so far, although, the heat-affected zone of crosswelds of ferritic chromium steels is known as a possible weak point. The long-term creep properties of crosswelds is linked to microstructure of the heat-affected zone of 9-12% chromium steels. The formation of heat-affected zone microstructures is studied by dilatometry, in-situ X-ray diffraction using synchrotron radiation, optical microscopy as well as most advanced electron microscopic methods. Results of crossweld creep tests up to duration of 40,000 hours are directly linked to the heat-affected zone microstructure at the location of fracture. The most predominant failure mechanism at lower stress levels and long term duration is Type IV cracking in the fine-grained heat-affected zone region of crosswelds. The failure mechanism is discussed in detail in this paper.

The effect of ferrite phases on the micromechanical response and crack initiation in the intercritical heat‐affected zone of a welded 9Cr martensitic steel

“…These are further classified, according to their characteristic microstructure into coarse‐grained, fine‐grained, and intercritical (IC‐HAZ). Under complex operational loading conditions, failures can often initiate within the IC‐HAZ, a phenomenon known as type‐IV cracking …”

Microstructure and Mechanical Properties of Modern 11%Cr Heat-Resistant Steel Weld Joints

“…In the CGHAZ near the fusion line, the temperature is significantly above transition temperature A C3 [ 29 , 30 ]. High peak temperature of the welding cycle in the zone contributes to dissolution of precipitates in the matrix, mainly M 23 C 6 particles [ 29 , 31 , 32 ]. This dissolution results in disappearance of the boundary pinning effect and leads to the formation of coarse austenite grain ( Figure 6 ).…”

Technical Issues in Fusion Welding of Reduced Activation Ferritic/Martensitic Steels for Nuclear Fusion Reactors

“…Reduced dislocation density may be also an indicative of recovery occurred in the FGHAZ during PWHT. The FGHAZ is characterized with low hardness and moderate toughness because of weak solid solution strengthening compared to CGHAZ 27) . Tsukamoto et al 28) addressed this observation.…”