Weld metal deposited by flux cored arc welding that exhibited a combination of high strength and toughness was studied. Microstructural characterization revealed it contained primarily bainitic ferrite with a fine packet size in the as-deposited metal and mainly nonaligned ferrite in the reheated zones, which were concentrated near the root of the weld. A new type of spherical inclusion is reported with an average size of 311 nm in diameter that exhibits a shelled structure mainly rich in Al, Mg, and O in the core, and Mg, O in the outer shell. It is suggested the good properties stem from a combination of fine inclusion size, low content of interstitials, and small ferrite packet size. Instrumented impact testing indicates that grain refinement in reheated zones near the root of the weld improve the Charpy impact energy; however, fracture initiation energy is similar to the top of the weld. Introduction ABSTRACT This research focuses on the use of a novel flux-cored arc welding wire formulation, which appears to depart from the typical mechanisms of microstructural development, resulting in outstanding weld metal strength and toughness. The traditional strategy for achieving a combination of high strength along with good lowtemperature toughness in high-strength weld metals is to promote an acicular ferrite microstructure (Refs. 1–4). This microstructure consists of fine interlocking ferrite needles, with high grain boundary misorientations to promote grain boundary strengthening together with crack deviation during cleavage fracture at low temperatures (Refs. 5, 6). The nucleation of acicular ferrite occurs intragranularly in austenite on inclusions, and commercial weld consumables rely on Ti and Al additions to form inclusions such as TiOx, TiN, and MnO. Al2O3 (Refs. 7–9). The nucleation of acicular ferrite depends on achieving a large volume fraction of inclusions with a diameter between 0.2 and 2 μm, where the ideal size is close to 0.4 μm (Refs. 10–13). It has been shown that achieving this structure typically occurs when the weld metal oxygen content is close to 200 ppm, where lower oxygen concentrations fail to produce the acicular ferrite, while higher values form excessive amounts of large oxide inclusions that are ＞ 1 μm in diameter and nucleate cracks (Refs. 14, 15) and deteriorate toughness (Ref. 16). Considering the influence of chemistry and cooling rate on the thermodynamics and kinetics of inclusion formation (Ref. 17), successful application of welding consumables using Ti additions requires careful control of welding parameters that influence the chemistry in the weld pool, particularly Ti, O, and N content. This can limit the operating window for some consumables to achieve the desired acicular ferrite microstructure depending on these chemistry additions in the electrode. However, recent developments have shown that excellent toughness and strength may also be achieved with a complex combination of ferrite with martensite/austenite islands, martensite, degenerated pearlite, and upper bainite (Ref. 18). High-toughness weld metals based on large fractions of ferrite with nonaligned second phase and little acicular ferrite microconstituents were produced; however, this was limited to a tensile strength of 480 to 651 MPa (Ref.19). Alternative microstructures are of interest since they may offer reduced levels of interstitial oxygen and nitrogen, which will help to improve low-temperature toughness; however, these elements are normally required in forming inclusions that nucleate acicular ferrite. The general consensus is that toughness during impact testing is limited in the upper shelf region by the volume fraction of nonmetallic inclusions, and by the type and morphology of microconstituents during brittle fracture in the lower shelf (Refs. 2, 20, 21). Since a ferrite structure with aligned second phase dominates at lowoxygen contents, the toughness is limited by the larger unit crack length path during brittle fracture (Refs. 22–24). Weld metal deposits that achieve Charpy impact energy values of 300 J at –50°C (Ref. 25) are possible through optimizing oxygen and Ti content to control the formation of TiO2, which nucleates acicular ferrite. However, there are a few techniques discussed that do not rely on acicular ferrite structures and do not use Ti additions. This investigation examines the use of a flux cored arc welding consumable with a nominal tensile strength of more than 825 MPa, which does not utilize Ti additions or promote acicular ferrite formation. The weld metal can be deposited with 100% CO2 shielding gas, while containing low interstitial content with good low-tempera- WELDING JOURNAL 15-s WELDING RESEARCH Characterization of High-Strength Weld Metal Containing Mg-Bearing Inclusions Microstructural analysis of flux cored welds using a 4% Ni steel consumable exhibits both high strength and toughness BY A. P. GERLICH, H. IZADI, J. BUNDY, and P. F. MENDEZ KEYWORDS Flux Cored Microstructure Phase Formation Oxide Inclusions Instrumented Charpy Magnesium A. P. GERLICH is with University of Waterloo, Mechanical and Mechatronics Engineering, Waterloo, Ontario, Canada, H. IZADI and P. F. MENDEZ are with University of Alberta, Chemical and Materials Engineering, Edmonton, Alberta, Canada, J. BUNDY is with Hobart Brothers, Troy, Ohio.
Welding Journal | January 2014
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