APRIL 2015 / WELDING JOURNAL 61 An Enduring Mystery The Puzzle and Its Importance Failure to reproduce experiments is unacceptable in science and so, when the same grade of steels are welded under the exact same welding conditions, it would be absurd to expect weld-to-weld variations in geometry. In reality, this totally unexpected behavior was the norm when the same grade of steel with minor variations in composition was welded (Refs. 6–10). Figure 6 shows cross sections of two welds fabricated using the exact same procedure from the same grade of steel that are strikingly different (Ref. 11). The main difference in the steels was the amount of sulfur, which differed by 130 parts per million (ppm) by weight. Finding a solution to this longstanding puzzle (Refs. 6–10) was important because the weld geometry affects its performance; however, the solution remained elusive for decades (Refs. 8–10). A Promising Hypothesis A team of scientists at the Rocky Flats plant, a former nuclear weapons production facility near Denver, Colo., (Refs. 8–10) first presented a promising solution to this mystery in the early 1980s. They proposed a hypothesis to explain why a small amount of selenium (Ref. 6) or sulfur (Ref. 7) in steel significantly increases the depth of penetration. They considered how sulfur affects the surface tension of liquid steel, weld metal spin, convective heat transfer, and the resulting weld pool geometry. Figure 7 shows the surface tension of pure iron decreases with temperature. The same trend is observed for steels (very low sulfur). However, when a small amount of sulfur is present, the surface tension decreases overall and increases with temperature as shown in the figure. At temperatures close to the boiling point, the surface tension decreases with increase in temperature. Sulfur and many other alloying elements such as oxygen, nitrogen, selenium, and tellurium have a tendency to migrate to the surface of the liquid steel. They all affect the surface tension in a manner similar to sulfur and are called surfaceactive elements (Ref. 12). Directly under the heat source, the liquid metal has the highest temperature and lowest surface tension when the steel contains practically no sulfur. Since liquids flow from low to high surface tension regions, hot liquid steel moves sideways from the middle to the edge of the weld pool and melts metal there. It then turns downward as shown in Fig. 8A. As a result, the weld pool becomes wide and shallow. Small additions of sulfur change the flow pattern completely. Hot liquid under the heat source now has a higher surface tension than that in the cooler regions — Fig. 7. So, on the surface of the weld pool, the weld metal rushes to the middle then moves downward to the bottom of the weld pool. The downward flow of the hot metal in the middle of the weld pool works like a thermal drill and a deep weld pool forms, as shown in Fig. 8B. The Rocky flats team also showed that selenium affected the shape of the weld pool just like sulfur. The hypothesis the Rocky Flats team proposed provided a plausible explanation. However, in order for their theory to gain traction, direct proof of changes in the flow of liquid weld metal was required. Since metals are opaque and the liquid weld metal is very hot, this was not an easy task. They added some tiny alumina particles that floated on the surface of the weld (Ref. 9) and used a high-speed camera to film their motion during welding. The evidence was now at hand. Sulfur does change the flow pattern of liquid metal (Ref. 9). Insightful and elegant, their work inspired many other researchers. Helpful but Incomplete The work at Rocky Flats explained why a small amount of sulfur or selenium changed the shape of welds for the conditions of their welding. But after more than a decade, when experiments were conducted to cover more extensive welding conditions, it was found that sulfur does not always change the shape and size of the weld pool, although it does so in many cases (Ref. 11). So, the mystery actually deepened. The powerful spark of a new idea incubated at the Rocky Flats plant still required more work for a deeper understanding of when sulfur changes the shape and when it does not, and why. The Rocky Flats team explained the role of selenium and sulfur assuming convection as the mechanism of heat transfer. This mechanism, valid only when velocities within the weld pool are large, was indeed valid for their experiments. However, the assumed convective heat transfer mechanism is not always valid, because the velocities are small for certain welding conditions. A rigorous understanding of the role of surface-active elements for a specific welding condition requires mechanistic insight of heat transfer achievable through a combination of experiments Fig. 8 — A — Pure iron flows sideways from the middle, making the weld pool wide and shallow; B — when a small amount of sulfur is added, the alloy goes downward in the middle of the weld pool resulting in a deep weld pool. Fig. 7 — Variation of surface tension with temperature (Ref. 12).
Welding Journal | April 2015
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