021s.pdf

Welding Journal | January 2014

A B would minimize lattice strains, making it difficult to detect martensite via XRD. Regardless, the presence of a significant fraction of austenite may be beneficial during fracture, and the nickel content (3.51%) of the weld metal likely provides an austenite stabilization effect (Refs. 41, 43). The addition of Ni to weld metal has long been known to improve low-temperature toughness of weld metals, particularly below –30°C (Refs. 44–46). For example, more than a 100°C decrease in the ductile to brittle transition temperature can be achieved when only 3.5% nickel is added to steel (Ref. 47). The Charpy toughness values and impact transition temperature achieved in Fig. 8 are comparable to those observed in a 9%Ni steel, despite using a much lower nickel content (Ref. 41). Reducing the fraction of interstitials, in addition to the presence of nickel in solution is also known to increase the cleavage fracture strength and lower the brittle transition temperature dramatically. The premise is that nickel improves the cohesive strength of the ferrite lattice itself, which contributes to the enhanced fracture properties (Ref. 48). The present work suggests that an additional enhancement may occur due to a change in the distribution of microconstituents as well, since nickel is an austenite stabilizer. For example, when MA phase does not contain martensite but rather is dominated by austenite, this may also enhance toughness properties (Ref. 49). The high fracture toughness values obtained at low temperatures are also promoted by the fine-grained ferrite microstructures produced in the welds in combination with small-diameter oxide inclusions. The fine ferrite sizes with fewer aligned microstructures in reheated zones contributed to the higher fracture energy values. Aligned ferrite grains are typically separated by boundaries with low-angle misorientation (Ref. 50), and do not promote crack deviation during cleavage fracture. When the width of the ferrite laths or size of the packets are reduced (as shown in Figs. 1 and 2), and few aligned carbides are present, the cleavage fracture stress increases dramatically (Ref. 51). Since the FS(A) microconstituents that dominate the upper portion of the weld exhibit a fine packet size, this contributed to the fracture toughness in the top regions of the weld (containing mostly the as-deposited material), reducing the unit crack path during fracture (Refs. 23, 52). Both J1d and the total impact energy are slightly higher for the bottom region of the weld, and this trend is explained by increased fraction of reheated material with a microstructure that contains a lower fraction of aligned ferrite/carbide phases. The difference in toughness between top and bottom is more pronounced at lower temperatures, where cleavage fracture dominates and the finer microstructures with fewer aligned ferrite microconstituents result in higher fracture energies. WELDING JOURNAL 21-s WELDING RESEARCH Fig. 10 — Particles observed on fracture surface of Charpy sample tested at –18°C. Fig. 11 — Facets observed on the quasi-cleavage fracture surface of Charpy sample tested at –62°C. Fig. 12 — Instrumented Charpy impact data showing force and displacement during impact from the A —Top region of weld; B — bottom region.


Welding Journal | January 2014
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