A: The root of the problem lies in the fact that, unlike in SMAW, GMAW, FCAW, and SAW, in GTAW there is no relationship between filler metal deposition rate and welding current. In making a butt joint with a square preparation, the welder adds filler metal only to keep from having a concave bead shape, and when the joint closes, less filler metal is required so that dilution increases. As a result, dilution (percent of base metal incorporated into the fused metal) can range from near 0 to 100% in GTAW. A root opening of 1⁄16 in. maximum will result in high dilution for complete joint penetration, and dilution will increase as the root opening closes. To understand this effect on GTAW of a dissimilar metal joint, it is helpful to consider Table 1, which shows a typical carbon steel composition, a typical 304 stainless steel composition, a typical ER309/309L composition, and calculated fusion zone compositions at several dilution levels. Table 1 assumes that there is equal contribution to the dilution from each of the two base metals. One might think, since the ER309/309L composition is quite a bit lower in Cr than midrange (the range is 23.0 to 25.0% Cr in AWS A5.9), I have deliberately chosen an odd composition, but that is not the case. Suppliers of 309(L) rod for drawing into welding wire very commonly target the low end of the chromium range to keep the ferrite content of the wire low for ease of reducing the ingot or billet to rod. Likewise, suppliers of 309(L) rod commonly keep the nickel content to the high end of the nickel range (the range is 12.0 to 14.0% Ni in AWS A5.9), again to keep the ferrite content of the rod low. The ER309/ 309L composition shown in Table 1 is very typical of good quality commercial products. The calculations of Table 1 are shown on the WRC-1992 diagram (Ref. 1) with extended axes and the 1% martensite boundary in Fig. 1. A first tie-line is drawn from the carbon steel composition to the 304 composition. The midpoint of this tie-line represents the equal mix of the two base metals, which can be considered as a synthetic base metal. Then a second tie-line is drawn from this synthetic base metal to the ER309/309L filler metal composition. All possible dilutions of the synthetic base metal with the ER309/309L filler metal must lie along this second tie-line. The point labeled “Weld Metal” corresponds to 80% dilution. It lies along the second tie-line at 80% of the distance from the ER309/309L filler metal to the point labeled “Mix of Base Metals.” This weld metal composition lies below and to the left of the “1% Mn Martensite Boundary” on the diagram, predicting that the weld metal microstructure will be martensite and the weld metal therefore will be brittle. Such a composition would be expected to develop transverse cracks during a longitudinal face bend or root bend test, as were often observed by the inquirer. It can be appreciated that the composition indicated as “60% Dilution” in Table 1 will lie along the second tieline of Fig. 1 at 60% of the distance from the ER309/309L filler metal to the point labeled “Mix of Base Metals.” That point will lie inside the grey shaded “1% Mn Martensite Bound- STAINLESS Q&A 18 WELDING JOURNAL / JULY 2016 BY DAMIAN J. KOTECKI Q: We have been trying to qualify welders for complete joint penetration gas tungsten arc welding (GTAW) butt joints of 304 stainless to carbon steel according to the AWS D1.6/D1.6M:2007, Structural Welding Code — Stainless Steel. The steels are 1⁄8 in. thick. Although D1.6 does not provide for prequalified welding procedures of stainless to carbon steel, we thought that the AWS B2.11/ 8010: 2015, Standard Welding Procedure Specification (SWPS) for Gas Tungsten Arc Welding of Carbon Steel (M1/ P1) to Austenitic Stainless Steel (M8/ P8) through 10 Gauge, in the AsWelded Condition, with or without Backing, would be a logical starting point. We are using ER309/309L filler metal as in the SWPS. The joint design has been a square butt with a maximum root opening of 1⁄16 in. (the root often closes partially between tacks), as shown as joint type 1 in Fig. 1 of the SWPS. D1.6/D1.6M: 2007, Table 4.3, requires a face bend and a root bend for welder qualification. We have failed these tests with virtually every welder. The welds typically develop a number of transverse cracks during bending, and there is an occasional longitudinal crack. What is wrong? Table 1 — Dilution Calculations in Joining Carbon Steel to 304 with ER309/309L Typical Chemical Composition, wt% Component C Mn Si Cr Ni Mo Nb Cu N Carbon Steel 0.15 0.90 0.10 0 0 0 0 0 0.005 304 0.04 1.20 0.40 18.5 9.8 0.1 0 0.2 0.04 Equal Mix of 0.095 1.05 0.25 9.25 4.90 0.05 0 0.1 0.023 Base Metals ER309/309L 0.02 1.5 0.40 23.5 13.5 0.1 0 0.2 0.04 20% Dilution 0.035 1.41 0.37 20.65 11.78 0.09 0 0.18 0.037 40% Dilution 0.05 1.32 0.34 17.8 10.06 0.08 0 0.16 0.033 60% Dilution 0.065 1.23 0.31 14.95 8.34 0.07 0 0.14 0.030 80% Dilution 0.08 1.14 0.28 12.10 6.62 0.06 0 0.12 0.026
Welding Journal | July 2016
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