Structural integrity issue of welded 9Cr steels components used in the steam boilers, for example premature Type IV cracking, has been one of the biggest challenges of using these steels at higher service temperature above 625 °C. Failure of these welded components has been frequently reported to be caused by a low creep resistance of a heterogeneous region, called intercritical heat affected zone (ICHAZ). Although the typical thickness of the ICHAZ generated by arc welding processes is narrow (no more than 0.5 mm), large structural variations exist in this small region. It is worthy of further clarifying creep rupture location within the ICHAZ itself. In this work, two ICHAZs with different microstructures under two peak temperatures below the critical AC3 temperature were simulated by the Gleeble thermo-mechanical simulator. Their creep strengths at an elevated temperature of 650 °C were tested. Detailed structural analyses were also conducted to reveal their creep fracture mechanisms.
* Experimental Procedures
In this study, Grade 91 steel (plate) was used as the base metal (BM). The base metal was normalized at 1100 °C for 30 min and then tempered at 760 °C for 1 hour. Heat-affected zone simulation was conducted with the Gleeble 3500 thermal-mechanical simulator. To simulate intercritical microstructure, the specimens were heated up to 860 °C and 900 °C with a heating rate of 100 °C/s, held for 1 second and cooled in air. The two simulated specimens were named as LT-ICHAZ with a peak temperature at 860 °C and HT-ICHAZ with a peak temperature at 900 °C. After that, the simulated ICHAZ specimens were post-weld heat treated (PWHT-ed) at 760 °C for 2 hours. Creep testing of the PWHT-ed ICHAZ specimens (gauge length: 70 mm, cross-section: 6 mm×6 mm) was conducted at 650 °C with a stress level of 100 MPa. Vickers hardness measurement with a load of 0.5 kgf was conducted on the base metal and the simulated ICHAZ specimens before and after creep testing. The microstructure of the base metal and HAZ was characterized by a Zeiss AXIO optical microscope and a Hitachi S4800 field-emission scanning electron microscope (FESEM). The polished HAZ specimens for FESEM analysis were etched with 10 % Nital for 15 min. Electron backscatter diffraction (EBSD) analysis on the base metal and the simulated ICHAZ under the as-polished condition was conducted on a JEOL 6500F FESEM equipped with an EDAX EBSD system under 20 kV accelerate voltage and a step size of 0.15 µm for the ICHAZ specimens.
* Results and discussion
A dilation curve analysis clearly shows there is an obvious martensitic transformation during cooling in the HT-ICHAZ specimen with a peak temperature of 900 °C. There is only a slight fraction of martensitic transformation occurred in the LT-ICHAZ specimen with a peak temperature of 860 °C close to the AC1. The Ms temperatures analyzed with the derivation of dilation curves are 423 °C and 441 °C for the LT-ICHAZ and HT-ICHAZ, respectively. The HT-ICHAZ exhibits a highly transformed structure with only a few islands of untransformed tempered martensite from the BM. New PAGBs free of precipitates are also observed. The LT-ICHAZ shows a typical mixture of untransformed tempered martensite and newly-transformed martensite. Based on the location of the previous PAGBs in the LT-ICHAZ, austenitization preferentially nucleated from the previous PAGBs and grew towards the martensite matrix. The grain size in the HT-ICHAZ is 1.38 µm, which is smaller than 2.06 µm in the LT-ICHAZ. The normalized KAM value (1.09°) is higher than 0.88° in the LT-ICHAZ. After PWTH, The LT-ICHAZ maintains a structure of tempered martensite with a small fraction of recrystallized grains. However, the HT-ICHAZ exhibits a structure of highly-recrystallized grains instead of tempered martensite. The fraction and size of coarse precipitates in both specimens are comparable, but their distributions are dramatically different. The majority of M23C6 carbides distributed along matrix grain boundaries in the LT-ICHAZ, but in the HT-ICHAZ many precipitates stay inside the grains. The LT-ICHAZ fractured after testing for 575.5 hours, which is comparable to creep strength of Grade 91 base metal. However, the HT-ICHAZ exhibits an extremely low creep resistance with a creep rupture time of only 15.8 hours. The LT-ICHAZ shows a typical three-stage creep, but the creep curve of the HT-ICHAZ contains only primary and tertiary stages with almost no secondary creep. This indicates continuous deformation (or creep degradation) occurred in the HT-ICHAZ during testing. Both specimens exhibit ductile failures with obvious necking in the middle of the specimens. The elongation of the HT-ICHAZ is 45.8 %, which is larger than 26.4 % of the LT-ICHAZ.
Two different creep behaviors in the simulated ICHAZs of Grade 91 steel were observed and studied to further clarify creep deformation/rupture mechanisms within the ICHAZ. There is only a small amount of partial austenitization occurred in the LT-ICHAZ exposed to a peak temperature close to AC1. Highly austenitized structure transformed into new martensite in the HT-ICHAZ exposed to a peak temperature close to AC3. Different tempering behaviors of the simulated ICHAZs under a typical PWHT at 760 °C led to significant structure variations among the two ICHAZs. The excellent creep resistance of the LT-ICHAZ was benefited from the maintained tempered martensite structure from the base metal. Extensive recovery/recrystallization of the martensite matrix in the HT-ICHAZ after the PWHT caused its poor creep strength. This faster structural degradation was favored by the less pinning effect and reduced C solute concentration from the undissolved and coarsened M23C6 carbides. The results in this work show that the Type IV cracking likely occurs in the high-temperature end of the ICHAZ.