Characterization of as-welded microstructure of heat-affected zone in modified 9Cr–1Mo–V–Nb steel weldment

Journal Article · Mon Aug 15 04:00:00 EDT 2016 · Materials Characterization

DOI:https://doi.org/10.1016/J.MATCHAR.2016.05.024· OSTI ID:22689581

Wang, Yiyu; Kannan, Rangasayee; Li, Leijun

Non-equilibrium microstructure of the heat-affected zone (HAZ) in the as-welded modified 9Cr–1Mo–V–Nb pipe steel (P91) weldment deposited by gas tungsten arc welding (GTAW) and flux core arc welding (FCAW) has been characterized by field-emission scanning electron microscope (FESEM) and electron backscatter diffraction (EBSD). The heterogeneous structures in the sub-layers of the as-welded HAZ are attributable to phase transformations caused by the welding thermal cycles and the local structure variations in the as-received base metal. Coarse-grained heat-affected zone (CGHAZ) has a prior austenite grain (PAG) size of 20 μm. Fine uniformly-distributed precipitates and a higher fraction of MX carbonitrides are observed in the CGHAZ. Fine-grained heat-affected zone (FGHAZ) consists of the finest grains (1.22 μm measured by EBSD, 5 μm PAG size), coarse undissolved M{sub 23}C{sub 6} carbides within the PAG boundaries and fine nucleated M{sub 23}C{sub 6} particles within the martensite laths. Inter-critical heat-affected zone (ICHAZ) consists of partially austenitized grains and over-tempered martensite laths. EBSD kernel average misorientation (KAM) map in the FGHAZ close to the ICHAZ illustrates the greatest local strain variations with a moderate normalized KAM value of 0.92°. The majority (88.1%) of the matrix grains in the CGHAZ are classified as deformed grains by EBSD grain average misorientation (GAM) evaluation. The FGHAZ close to the ICHAZ has the most recrystallized grains with an area fraction of 14.4%. The highest density variation of precipitates within grains in the FGHAZ originates from the inhomogeneous chemistry in the base metal. - Highlights: •A comprehensive characterization of the as-welded HAZ of P91 weldment is conducted. •Structural features in the each layer of the HAZ are quantified by EBSD. •Structural heterogenities in HAZ are due to welding cycle and base metal structure. •FGHAZ contains the finest grain structure and largest precipitate density variation.

• OSTI ID: 22689581
• Journal Information: Materials Characterization, Journal Name: Materials Characterization Vol. 118; ISSN 1044-5803; ISSN MACHEX
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
AUSTENITE
BACKSCATTERING
CARBIDES
CARBONITRIDES
ELECTRON DIFFRACTION
FIELD EMISSION
GAS TUNGSTEN-ARC WELDING
HEAT
HEAT AFFECTED ZONE
LAYERS
MARTENSITE
MICROSTRUCTURE
PHASE TRANSFORMATIONS
PRECIPITATION
SCANNING ELECTRON MICROSCOPY
STEELS
WELDED JOINTS