Hydrogen Embrittlement of Tough Pitch Copper by Brazing
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Hydrogen Embrittlement of Tough Pitch Copper by Brazing Testing the effects of six variables statistically shows factors that can cause, enhance, or have no effect on embrittlement BY E. BELKIN AND P. K. NAGATA ABSTRACT. Hydrogen embrittlement cuprous oxide particles. some were not. of ETP (Electrolytic Tough Pitch) Above 374 C the critical temper Two time periods at temperature copper due to brazing has been ature of water, steam is the only form were used to evaluate the effect of studied. The authors have found that in which water exists, and the pres time. Ten cylinders comprised a run. the major causative factor of hy sures generated can reach as high Five replications were made of each drogen embrittlement when using as 80,000 psi (5.52 X 102 MPa). The run and they were made consecu fluxed brazing alloys was the water of phenomenon usually manifests itself tively. hydration in one of the flux constitu as large fissures and cracks (see The experimental cylinders are ents. Time at brazing temperature Fig. 1), although only oxide deple shown in Fig. 3. Each of the four holes was found to strongly influence the tion may be observed (Fig. 2). in the cylinder was stopped by a con depth of embrittlement, a longer time Hydrogen embrittlement does not ical shaped piece of ETP copper leading to a greater depth of em seriously affect the tensile strength or (Fig. 4). These cones were lightly brittlement. Different brazing filler elongation of ETP copper, but does held in place so that the gas pressure metals give varying depth of em affect its fatigue strength (Refs. 5,6). in the holes would need to be a little brittlement. Bright dipping the copper Therefore, it must be avoided in any greater than atmospheric pressure in and manually handling the brazing structure that may be subject to order to escape. This was done in an alloys do not significantly increase or cyclical stress or vibration. effort to confine the gas products to decrease the amount of em In general, hydrogen embrittle some degree but not confine them so brittlement. ment of brazed copper was thought to as to cause a steam explosion. occur only during furnace brazing Three brazing filler metals were Introduction where the furnace gas was too reduc used. Their compositional limits are ing for that temperature (Refs. 7,8). in Table 1. The hydrogen embrittlement of However, this study has shown that The flux is a general purpose braz ETP (Electrolytic Tough Pitch) cop embrittlement can occur in circum ing flux. Its composition was deter per is a well known phenomenon stances other than by heating in mined by the Westinghouse R&D (Refs. 1,2). Hydrogen diffuses into reducing furnace gases. laboratory and the results are shown the copper as atomic hydrogen and in Table 2. Prior to the test the flux reacts with the cuprous oxide par Experimental Procedure was stirred so that a homogeneous ticles to produce copper and water. mixture was achieved. This can occur at temperatures as low The experiments were run with ETP The cylinders were bright dipped as 150 C but is usually encountered copper cylinders each having four for three minutes to remove any oils at higher temperatures (above 400 C) holes to allow evaluation of four vari or surface oxides. The bright dip anal (Ref. 3). ables under common conditions. The ysis is shown in Table 3. Below 374 C the phenomenon three brazing filler metals tested con A test run consisted of 10 cylinders. generally manifests itself as an oxide sisted of two alloys that require flux Since the bright dipping operation depletion or as holes at the sites of under normal brazing conditions in air leaves some hydrogen adsorbed in and a so-called fluxless alloy. The two the surface of the copper, some of the brazing filler metals that require flux specimens were subjected to a E. BELKIN and P. K. NAGATA are with the were tested with and without flux. vacuum of 10_e torr and held for 24 Materials and Process Engineering group, The tests with flux were evaluated with hours at 200-300 F (93-144 C). This Large Rotating Apparatus Division, wet and dried flux. procedure will rid the copper of much Westinghouse Electric Corporation, East Pittsburgh, Pa. 15112. All the cylinders were bright of the adsorbed hydrogen (Ref. 10). It Paper was selected as alternate tor the dipped. Some were vacuum treated preceded the experiment by no more 55th AWS Annual Meeting held at Hous prior to testing to remove hydrogen than 36 hours. ton, Texas, during May 6-10, 1974. adsorbed during bright dipping and The variables tested are listed 54-s | FEBRUARY 1975 Fig. 1 — Hydrogen embrittlement. Brazing filler metal B wet flux, 4 min at brazing temperature, x200, reduced 28% Fig. 2 — Oxide depletion. Brazing filler metal C, 1 min at brazing temperature, x200, reduced 28% below: .19 Diameter x 1.25 Deep 1. One minute vs four minutes at 4 Ho Ies Equal Iy Spaced brazing temperature. As Shown On .5 Diameter 2. Flux with physical water and water 125— — of hydration vs flux with water of Bo 11 Circle. hydration only. Chord = .354- 3. Bright dipped vs bright dipped and vacuum treated. Vacuum treat ment removes the effect of bright dipping. 4. Body oils vs no body oils. Fig. 4 — Stoppers for cylinder holes 5 Wet flux vs no flux. 6. Brazing filler metal A vs brazing filler metal B. above are used to analyze data, to The five test runs were made in the find the standard deviation, and to following manner: test hypothesis regarding the vari 1. The flux was stirred to achieve ables in an experiment. homogeneity. The effects that were studied were 2. Eight parcels of wet flux, 0.20 time at brazing temperature, wet vs grams each, were weighed out and dry flux, vacuum treatment vs no placed in the appropriate holes. vacuum treatment after bright dip 3. Four 1 X 1/16 in. (2.54 cm X 0.16 ping, presence of body oils, presence cm) diam pieces of each brazing of flux, and brazing filler metal A vs filler metal were placed in their brazing filler metal B. The six vari proper places. The pieces of braz ables are examined in Tables A4, A5, ing filler metal were degreased A6 and A7 in Appendix I. Their iden and not manually handled except tification numbers with "—" and "+" where noted in Table 4. level assignments are given in Table 4. The copper cones were placed in 5. The results are shown in Table 6. the holes. Significance occurs when the F- 5. The samples were induction braz value of a variable (last column on ed at 10 kHz using the procedure right in Tables A4-A7 in Appendix I) below: exceeds the 95 percent point of F1i64 a. Heating time from room (Fi,32 in Table A7) (bottom line in temperature to 1400 F(760 C): tables). This means that the prob 1 minutes ability that the variable has no effect b. Time at temperature: 1 or 4 on the depth of embrittlement is less minutes than five percent. c. Air cool This probability could be de 6. The samples were then cut in half Fig. 3 — Experimental cylinders creased by using a one percent along their axial length and metal significance level, but this would in lographically sectioned. Prohas- crease the probability of missing real ka's etchant was used to etch the effects. samples. Its constituents are H20 eral, oxide depletion did not exceed Figure 5 (see Appendix 1) is a sug (distilled). 70 ml; HCI (reagent 0.5 mils or 0.0005 in. (0.0127 mm) so gested "four dimensional plot" of the strength), 25 ml; Fe (N03)3, 5 gm. that its contribution to the total 16 average embrittlement values The experimental results are pre measurement is negligible. The data found in Table A4 of Appendix 1. sented in Table 4. The data are mea are analyzed in Appendix I using Since four dimensions cannot be surements of the depth of embrittle Yates' Algorithm, ANOVA (Analysis of presented, a cube having two data ment and/or oxide depletion. In gen- Variance) tables, and the F-test. The points at each corner is used. WELDING RESEARCH SUPPLEMENT! 55-s Table 1 — Brazing Filler Metals and Composition (wt %) Brazing filler metal Ag Cu Zn P Cd Impurities Equivalent specifications QQ-B-655, Class FS BAg-6 A 49-51 33-35 14-18 — 0.15 Max AWS 5.8-69 BAg-6 QQ-B-655. Class FS BAg-la MIL B 15395A Gr. 4 B 49-51 14.5-16.5 14.5-18.5 — 17-19 0.15 Max. AWS 5.8-69 BAg-la C 14.5-15.5 Remainder — 4.8-5.25 0.15 Max MIL-S-15395, Gr. Ill QQ-S-561, Class III A AWS 5.8-69 BCuP-5 QQ-B-655, Class FS BCuP-5 spread. However, this practice causes (a) Table 3 — Composition of Bright Dip Table 2 — Composition of the Flux hydrogen evolution at the surface of the copper and may cause some ad sorption of hydrogen into the copper Item Dilution Weight % Constituents Weight % matrix. Upon heating with or without brazing alloy and flux, this adsorbed H S0 96% H SO„ 61 hydrogen causes some embrittle KBF, 25 2 4 2 + 4% H20 ment in the former case and in K2B40, 35 HN03 70% HNO3 30.5 creased embrittlement in the latter. K2B407(5H20) 9 + 30% H20 H20 31 The effect of adsorbed hydrogen is HCI 37.5% HCI 1.0 shown in Tables A1 and A5 in Appen + 62.5% H20 H 0 7.5 dix I.