Microstructural investigations on simulated intercritical heat-affected zone of boron modified P91-steel

Abstract

Conventional P91 and boron modified P91-steels were considered in the present investigation. Using Gleeble, weld intercritical heat-affected zone (ICHAZ) was processed. Conventional microscopy was done along with electron back-scattered diffraction for both specimens. Lath boundary of martensite was more, and preferably oriented for P91B-ICHAZ than P91-ICHAZ. Lattice strain was high and lath mobility was low for P91B-ICHAZ in comparison to P91-ICHAZ. Large fraction of ferritic grain structure (39.1%) and small fraction of fresh martensitic grain structure (9.5%) in P91-ICHAZ, whereas small fraction of ferritic grain structure (10.7%) and large fraction of fresh martensitic grain structure (32.7%) in P91B-ICHAZ was observed. However, tempered martensite exhibited meager variation. These differences between ICHAZ simulations were attributed to the presence of 100ppm-boron in P91B-steel.

A novel approach to envisage effects of boron in P91 steels through Gleeble weld-HAZ simulation and impression-creep

“…It is well described in literature that the inherent creep strength of PM/B-PM is derived from solid solution, precipitate or dispersion, dislocation, and grain/subgrain boundary strengthening. 4,5,15,16,23 In general, precipitates in these steels are M 23 C 6 , M 23 (C,B) 6 , and other particles 39 decorated along PAGBs and at lath boundaries as observed in Figure 5(a) and (b) as white colored dots labeled as black dotted circles. Other than these, very fine and nano-sized MX particles (not discernable in EBSD micrographs) also contribute a minute fraction that is both intra-and inter-granularly located.…”

“…Consequently, other than strengthening induced by solid solution due to abundant martensite, the presence of fine precipitates along with PAGBs in B-PM was the mechanism responsible for enhanced strengthening induced by precipitates grain/sub-grain boundaries, and dislocations as reported in previous works. 15,16,23 Phase transformations in terms of overall microstructural evolution that occurred during simulation of singular HAZ specimens and subsequent PWHT had a strong linkage to T P and hold time at T P . 11,12,17,18,40 Therefore, improved creep functionality of B-PM and its subzones of HAZ over standard P91 counterparts could be explained by microstructural manifestations such as evolution of lattice strain.…”

“…14 In this regard, the addition of a low concentration of boron (90–100 ppm) in P91 steel as an alloying element has shown notable improvement in creep strength of parent metal and its weldments. 15 It was claimed that free boron segregates to prior-austenite grain boundaries (PAGBs) and combines with M 23 C 6 carbides. Thereby hindering their growth during creep, which eventually delays the inception of the tertiary creep stage.…”

“…It further consists of mainly three sub-HAZ realms, that is, CGHAZ, FGHAZ, and ICHAZ. 15 Proximity among sub-HAZ regions limits the implementation of macrostructural examination by conventional creep testing to evaluate creep behavior of singular HAZ. Generally, specimens cut from actual cross weldments are put for conventional long-term uniaxial creep tests that reveal either average creep property or endurance of weakest subzone in P91 welded joints.…”

Fatigue fracture behavior of high-strength low-alloy steel for flexible marine riser in the high cycle fatigue regime

“…The blue lines represent the high-angle grain boundaries (15°) and the red lines represent the low-angle grain boundaries (2°-15°). Martensitic lath boundaries, sub-grain boundaries, CSL boundaries all contribute to the misorientation angle [28,29]. Martensitic lath boundaries and sub-grain boundaries belong to the high-angle grain boundaries, and CSL boundaries belongs to the low-angle grain boundaries.…”