Intercritical heat affected zone (ICHAZ) exposed to an intercritical peak temperature between AC1 and AC3 has been reported as one of the most susceptible regions to premature creep failures, for example, Type IV cracking, in creep resistant steel weldments. However, cracking mechanisms within the ICHAZ itself are not fully clear and needs further clarifications. Partial austenization and precipitate dissolution in intercritical heat treatment result with highly localized microstructure heterogeneity, which leads to large variations in creep deformation behaviors. Microstructure evolutions promoted by post weld heat treatment could make the situation even worse by accelerating creep strength degradation. Building a close correlation between these complex microstructures generated by the intercritical thermal cycles and mechanical properties, especially creep resistance at elevated temperature is challenging but essential.
* Experimental Procedures
In this study, Grade 91 steel (plate) was used as the base metal (BM). Three heat-treated specimens, designated as BM, IT, and ITT, were analyzed in comparison. The "BM" was normalized at 1100 °C for 30 min and tempered at 760 °C for 60 min. The "IT" specimen was exposed to an intercritical treatment at 860 °C for 5 min. Following the intercritical treatment, an additional tempering at 760 °C for 120 min was applied to the "ITT" specimen. Mechanical properties, including hardness, room-temperature tensile strength, and high-temperature creep strength, were evaluated for the three heat-treated specimens. Microstructure of the BM and heat-treated specimens before and after creep testing was characterized with a Zeiss AXIO optical microscope, a Hitachi S4800 field-emission scanning electron microscopy (FESEM), and a JEOL 6500F FESEM equipped with an EDAX electron backscatter diffraction (EBSD) system.
* Results and discussion
The BM shows a typical structure of tempered martensite. Fine precipitates (mainly M23C6 carbides) distribute along the prior austenite grain boundaries (PAGBs) and martensitic packets/blocks after tempering. The complex structures in the intercritical-treated specimen (IT) consist of 68.9% newly-transformed martensite (NTM) grains from partial austenization and 31.1% over-tempered martensite (OTM) grains. These OTMs are nearly free of internal sub-boundaries and coarse precipitates inside or along grain boundaries. Dissolution of precipitates led to a much lower precipitate density, comparing with that in the BM. Undissolved but coarsened precipitates are also observed inside those NTMs. Fine equiaxed grains are observed as the major structure in the ITT specimen after an additional tempering.
The BM has a moderate hardness of about 247 HV0.5. The IT specimen exhibits the highest hardness of 332 HV0.5, which is contributed from those NTM grains. The additional tempering led to the ITT specimen's lowest hardness of 178 HV0.5. The yield strength and ultimate tensile strength (UTS) of the BM are 611 MPa and 739 MPa, respectively. The elongation of the BM is 20 %. The IT specimen exhibits the largest strength with a yield strength of 690 MPa, an UTS of 1054 MPa, and the lowest elongation of 17 %. These high strength and low ductility are believed to be contributed from those new-transformed martensite grains. After additional tempering, the yield strength and UTS of the ITT specimen decreased significantly to 419 MPa and 596 MPa, respectively. Accordingly, the elongation of the ITT specimen increased to 30 %. Creep strengths of the three specimens tested at 650 °C and 100 MPa are compared. The BM shows the highest creep resistance with a minimum creep strain rate of 0.0013%/h. Creep curves of the IT and ITT specimens are not typical three-stage creep curve. The specimens quickly went into the tertiary creep stage with a very short secondary creep. The IT and ITT specimens fractured with a total life of 31.8 h and 19.2 h, respectively. The minimum creep strain rate of the ITT specimen is 0.49 %/h, which is twice of the 0.24 %/h of the IT specimen.
In this study, microstructure and mechanical properties of intercritical-treated structure in Grade 91 steel were characterized and tested. The partial austenization during an intercritical treatment led to transformation of new martensite (69.9%) in the treated specimen. 31.1% tempered martensite were over-tempered close to ferrite. Undissolved precipitates coarsened and remained in those newly-transformed martensite grains. The significantly increased hardness (332 HV0.5) and tensile strength (1054 MPa) were contributed from those transformed martensite grains. After an additional tempering at 760 °C, an obvious grain growth of those over-tempered martensite occurred. The fine newly-transformed martensite laths became fine equiaxed grains. Continuous grain growth pushed the coarsened precipitates together. These structural evolutions led to a lowered hardness of 178 HV0.5 and a reduced tensile strength of 596 MPa in The ITT specimen. The IT specimen shows a higher creep strength at 650 °C than that of the ITT specimen. But both the IT and ITT specimens exhibit extremely low creep resistance, comparing with the BM. Severe creep deformation was observed on the creep fractured specimens. Grain growth is still noticeable in the crept specimens even with this short-term creep test. Elongation of coarsened grains from the over-tempered martensite contributed to plastic fracture of the specimens. Inclusions have assisted nucleation and growth of cavities and micro-cracks.