Formation Mechanism of Type IV Failure in High Cr Ferritic Heat-Resistant Steel-Welded Joint

The Influence of Thermal Cycles on the Microstructure of Grade 92 Steel

“…[7][8][9] However, it is apparent that there is no clear evidence for these steels that PAG size has a significant influence on properties, and indeed, where data are available, it appears that creep strength and ductility grain size are not a function of PAG size in tempered martensitic steels. One such example is the lack of evidence for a widespread susceptibility to reheat cracking as is documented for bainitic CrMoV steels.…”

“…It was reported that HAZ failures in 9 to 12 wt pct Cr steel welds can be present either in the fine-grain or intercritical region, [7] accompanied by a significantly shortened lifetime of weldments compared to bulk material samples. [10] A variety of microstructural factors, such as an inhomogeneous distribution of secondary precipitate particles, [8] the presence of an exceptionally fine-grain structure, [11] and excessively grown precipitate particles [12] were considered as the possible causes for Type IV failure. However, there is not yet consensus on whether a specific microstructural factor or a set of factors are dominant in causing HAZ failures.…”

“…Successful applications of these advanced characterization techniques on the microstructural characterization of the weld HAZ and the bulk materials of 9 wt pct Cr steels were reported in recent years. [8,17,18] However, there is still a lack of systematic investigations in which the microstructural gradient across the HAZ of 9 wt pct Cr steel welds was thoroughly evaluated and directly linked to the local thermal gradient during welding.…”

“…[7,19] These materials are typically produced by exposing the raw parent metal materials to controlled heat cycles that simulate weld thermal cycles using a Gleeble simulator or a dilatometer. [8,20] In these works, comparisons of creep behavior were made between the HAZ simulation materials that were exposed to heat cycles with various peak temperatures under a range of creep testing conditions. A shorter creep rupture life and higher creep strain were identified in material that was exposed to the thermal cycles with a peak temperature close to the Ac 3 temperature.…”

“…Evidence showing an inhomogeneous distribution of precipitate particles due to microsegregation was indicated after weld thermal cycle simulations with a peak temperature between the Ac 1 and Ac 3 temperatures. [8] However, there still remains a lack of systematic studies in which the observed microstructure in the HAZ of actual welded samples can be linked with corresponded simulated HAZ in order to understand the exact relationship between the microstructural gradient in the HAZ and local thermal histories experienced.…”