Microstructural characteristics and impact toughness in YS690MPa steel weld metal for offshore structures

Abstract

The fine-grained mixed microstructure of acicular ferrite (AF) and bainite in YS690MPa steel weld metal contributes to attain high-impact toughness. The morphology and evolutionary mechanism of fine-grained mixed microstructure in this weld metal were investigated. Single or multiple AF grains were nucleated on complex inclusions by forming Mn-depleted zones, where Mn spontaneously diffused into Ti oxide inclusions due to the cation vacancies. It is in good agreement with the theoretical calculation by first principle. The bainite nucleated on austenite grain boundary and then assisted the pre-formed AF to partition the austenite grain into small and separate regions. Furthermore, the later formed ferrite nucleated on the broad surface of pre-formed ferrite plates and grew in those small regions with limited grain size. All of them resulted in the formation of fine-grained mixed microstructure, which provided excellent impact toughness in this weld metal with dimples and quasi-cleavage fracture surface combination.

Influence of rotation frequency and rotation diameter on mechanical properties and microstructure of weld metal produced by MCAW-RE

“…The metallographic examination revealed a microstructure where the presence of primary ferrite (PF), acicular ferrite (AF), and ferrite with second phase (FS) are observed. These results are in accordance with the available literature for weld metals of the class 700 MPa [16,38,[42][43][44][45][46][47][48]. In addition, the comparison of the columnar and reheated regions (Fig.…”

Significant Improvement in Strength and Toughness of Nanoscale Precipitate–Strengthened Steel by Direct Quenching and Tempering Process

“…Unlike the coarse precipitates formed in the high‐temperature austenite region, nanoscale precipitates form at intermediate temperature in the ferrite region. [ 7–9 ] The precipitation behavior is controlled by the solubility product of microalloying elements at a given temperature. [ 10 ] Therefore, it is valuable to optimize the precipitation behavior of carbonitrides by tailoring the tempering temperature.…”

Investigation into the Impact Toughness of API 5L X80 Steel Weldments and its Relationship with Safe Welding Procedures

“…In addition, PWHT did not promote any significant changes (<4%) to the tensile strength (Table 3). The weld metal presented a refined microstructure composed of acicular ferrite (AF), primary ferrite (PF) and ferrite with a second phase, as expected for 700 MPa class steels [53][54][55][56][57][58][59][60][61][62][63][64] . This microstructure provided Charpy-V results much greater than the minimum required 45 , both in the as welded and heat treated conditions, as shown in Figure 7.…”