BRAZING Q&A BY ALEXANDER E. SHAPIRO Q: What etchants do you use to etch brazed joints to visualize the joint metal for metallography using an optical microscope? For instance, stainless steel brazed with BAg-24, or titanium Grade 5 brazed with TiBraze200? I cannot find etchant formulations in the literature, and I am having a hard time seeing the brazed joint properly. Many times we don’t have time to send samples for electron scanning microscope study. I may be overetching my samples, but I want to make sure that I am seeing the correct things. A: You are right. We rarely see optical metallography of brazed joints, etching procedure, and etchant formulations in the literature of the last two decades. Electron scanning microscopy is mostly used today, which does not require etching. However, for practical purposes, many companies still use optical metallography of polished and etched brazed joints. The following offers some practical information about etchants and other recommendations related to the polishing and etching procedure of cross sections of brazed joints. Etchant compositions for visualizing the microstructure of the most popular groups of brazing filler metals are presented in Table 1. Some etchant compositions and etching times are given in wide ranges. This is because successful etching depends on many factors, and the process parameters should be adjusted experimentally. This is not difficult to do, usually two or three iterations are sufficient. The most important factors that affect the etching process and reveal microstructure of both the joint metal and the base metal interface are the quality of polishing and the composition of the etched alloys. The base metal and the joint metal often have different hardnesses; therefore, polishing should be done without a relief (or “step”) from the base metal surface to the joint metal surface. In order to avoid this step, we recommend using only fine abrasive papers, changing the direction of polishing by 90 deg every two to three minutes, and frequently rinsing the polished surface under a tap-water stream to remove abrasive grains that can get stuck in soft joint metal. Final polishing with a diamond paste is used in cases where there is a big difference in hardness of the base and joint metals. An example of the microstructure of a titanium joint brazed with AWS BTi-5 (Ti- 20Zr-20Cu-20Ni wt-%) is shown in Fig. 1. The etching was done using Reagent #5. The etchant revealed the grains of the Table 1 — Etching Procedure and Reagents for Revealing Microstructure of Brazed or Soldered Joints of Most-Used Base Metals and Filler Metals No. Base Metal Brazing Etchant Formulation Etching Procedure 26 FEBRUARY 2014 Filler Metal 1 (NH4)2S2O8 1–20 g, H2O up to 100 mL. Immerse for 5–10 s. Add 3–5 drops of a saturated ammonia Rinse in cold water. solution immediately before application. 2 Stainless or Ag-based FeCI3 2–5 g, H2O, 100 mL (for joint metal), Deposit etchant for 10–60 s. carbon steels brazing filler ethanol 100 mL (for steel). Add 20–30 mL Rinse in cold water. Cu alloys metals H2O2 immediately before etching joints of copper alloys. 3 Acetic acid 5–10 mL, ethanol up to 100 mL Rub with a wad or Q-tip® for 20–120 s. Rinse in cold water. 4 Stainless or Cu-based Step 1: 4% solution of HNO3 in ethanol Immerse for 5–10 s. Rinse in carbon steels brazing filler Step 2: 50–60% solution of ammonia in water cold water. Cu alloys metals; Sn-Pb solders 5 HF 2–5 mL, H2O 100 mL (If a dark shadow Immerse or rub with a wad or appears, immerse in 5% HNO3 solution for Q-tip® for 10–30 s. Rinse in 2–3 s.) warm water. 6 Ti alloys Ti-based HF 1–5 mL, HNO3 1–5 mL, H2O 89–90 mL. Immerse or rub with a wad or brazing filler metals Use hot (60°–70°C) if cold etchant is not effective. Q-tip® for 2–3 min. Rinse in warm water. 7 Kroll’s reagent: HF 4 mL, HNO3 22 mL, H2O Immerse or rub with a wad or 100 mL Q-tip® for 2–10 s. Rinse in warm water. 8 Al alloys Al-Si brazing HF 1 vol part, HNO3 2 vol parts Immerse or rub with a wad or filler metals Q-tip® for 5–10 s. Rinse in warm water. Dry with ethanol. 9 Cu or Steel Sn-Pb, Sn-Ag, and HNO3 2–6 mL, ethanol 94–98 mL Immerse or rub with a wad or Sn-Zn solders Q-tip® for 20–60 s. Rinse in cold water.
Welding Journal | February 2014
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