Analysis of Brazing Imperfections by Metallographic Investigation / Optical Microscopy – Part 1
In everyday practice we can encounter problems connected with brazing imperfections – often manifested as leaking. In order to identify the failure root causes, usually a detailed examination needs to be perform. Optical metallography is one of the most important tools for such an analysis.
The purpose of this article is to present the basic principles for performing analysis of brazing joints, and to show what information can be obtained from the investigation.
Before starting to work with an optical microscope, the part to be examined must be prepared appropriately. There are several steps in this procedure:
- Cutting out smaller pieces to about 30mm to 40mm with a band saw
- Embedding the cut sections in epoxy resin
- Step grinding with sand paper (220 grit, 500 grit, 1000 grit)
- Polishing with diamond suspension (6 µm, and 3 µm) and with oxide polishing suspensions (OP-S)
- Microscopic observation in non-etched conditions – mainly to determine if the quality of polishing is sufficient
- Microscopic observation after etching – most common etchant: 0.5% hydrofluoric acid, (10 – 15 seconds immersion)
There are usually two steps in a metallographic examination.
The first one is to evaluate the brazing quality. It is usually performed when introducing a new product. In such case the manufacturer should define the way the sample is cut for microscope observations. In most cases the tube to header and the fin to tube joints are examined. Such exemplary cutting is shown in Fig. 1: