Microstructural Evolution And Mechanical Properties of FCAW Joints In 9% Ni Steel Prepared With Two Types of Ni-Based Weld Metals

The microstructural and mechanical evaluation of 9% Ni steel with Flux-Cored Arc Welding was performed with two different Ni-based weld metals: Inconel 625 and Hastelloy 609. Weld metals showed the microstructural change depending on the temperature gradient and crystal growth rate for each region during the cooling after welding. At the bottom of the weld metal, which is rapidly cooled in contact with the cold base metal, a cellular/planar growth was exhibited due to a large temperature gradient and low crystal growth rate. While, columnar dendrites were exhibited in the central region cooled relatively slowly and precipitates were observed in the interdendritic region. In the low-temperature toughness test, the absorbed impact energies were 89 and 55 J for Inconel 625 and Hastelloy 609, respectively. When Inconel 625 is used as the weld metal compared to Hastelloy 609, the high content of the γ stabilizer and martensite start temperature decreasing elements leads to the formation of a thicker γ-phase layer and thinner martensite layer in the transition region. In addition, high content of these elements suppresses the martensite transformation and maintains the stability of the weld joint interface even at low temperatures, resulting in the higher absorbed impact energy.

EBSD characterisation of grain size distribution and grain sub-structures for ferritic steel weld metals

Microstructural characterisation of engineering materials is required for understanding the relationships between microstructure and mechanical properties. Conventionally grain size is measured from grain boundary maps obtained using optical or electron microscopy. This paper implements EBSD-based linear intercept measurement of spatial grain size variation for ferritic steel weld metals, making analysis flexible and robust. While grain size has been shown to correlate with the strength of the material according to the Hall–Petch relationship, similar grain sizes in weld metals with different phase volume fractions can have significantly different mechanical properties. Furthermore, the solidification of the weld pool induces the formation of grain sub-structures that can alter mechanical properties. The recently developed domain misorientation approach is used in this study to provide a more comprehensive characterisation of the grain sub-structures for ferritic steel weld metals. The studied weld metals consist of varying mixtures of primary ferrite, acicular ferrite, and bainite/martensite, with large differences observed in hardness, grain size, grain morphology, and dislocation cell size. For the studied weld metals, the average dislocation cell size varied between 0.68 and 1.41 µm, with bainitic/martensitic weld metals showing the smallest sub-structures and primary ferrite the largest. In contrast, the volume-weighted average grain size was largest for the bainitic/martensitic weld metal. Results indicate that a Hall–Petch-type relationship exists between hardness and average dislocation cell size and that it partially corrects the significantly different grain size—hardness relationship observed for ferritic and bainitic/martensitic weld metals. The methods and datasets are provided as open access.

Yield Load Solutions for SE(B) Fracture Toughness Specimen with I-Shaped Heterogeneous Weld

The objective of this work was to investigate the fracture behavior of a heterogeneous I-shaped welded joint in the context of yield load solutions. The weld was divided into two equal parts, using the metal with the higher yield strength and the metal with the lower yield strength compared to base metal. For both configurations of the I-shaped weld, one with a crack in strength in the over-matched part of the weld and one for a crack in the under-matched part of the weld, a systematic study of fracture toughness SE(B) specimen was carried out in which the crack length, the width of the weld and the strength mismatch factor for both weld metals were varied, and the yield loads were determined. As a result of the study, two mathematical models for determination of yield loads are proposed. Both models were experimentally tested with one strength mismatch configuration, and the results showed good agreement and sufficiently conservative results compared to the experimental results.

Microstructure features and formation mechanism in a newly developed electroslag welding

Electroslag welding (ESW) is known to show higher heat input than electrogas welding (EGW), resulting in poor low-temperature toughness. However, a newly developed ESW (dev. ESW) method using low-resistivity slag bath exhibited excellent low-temperature toughness as a result of lower effective heat input than conventional EGW, as demonstrated by the faster cooling rates measured in weld metals and estimated using finite element method analyses. This led to much shallower molten pool in the dev. ESW, resulting in much finer columnar grains and thinner centerline axial grains. High cooling speed in the dev. ESW method appeared to contribute to increased acicular ferrite proportion. The uniform microstructure with large acicular ferrite proportion and small number of inclusions in the weld metal permitted the dev. ESW weld metal to possess little variation in Charpy impact energy across the center of weld metal.

Comprehensive Analysis of Cold-Cracking Ratio for Flux-Cored Arc Steel Welds Using Y- and y-Grooves

This study investigates various factors that influence the cold-cracking ratio (CCR) of flux-cored arc welds through Y- and y-groove tests. Factors affecting the CCR include the alloy component, diffusible hydrogen content, microstructure, hardness, and groove shape. In weld metals (WMs; WM375-R and WM375-B) of a low-strength grade, the diffusible hydrogen content has a more significant effect on the CCR than the carbon equivalent (Ceq) and microstructure. However, the combined effects of the microstructure and diffusible hydrogen content on the CCR are important in high-strength-grade WM. The CCR of the WM increased upon increasing Ceq and the strength grade because hard martensite and bainite microstructures were formed. Moreover, y-groove testing of the 500 MPa grade WM revealed a more significant CCR than that of the 375 MPa grade WM. Therefore, in high-strength-grade WMs, it is necessary to select the groove shape based on the morphology in the real welds.