Recall the blistering or ruptures that can be experienced when you braze non-OFE Copper in Hydrogen (see Blog Post October 7, 2009). As a follow-up post, we want to briefly discuss “stringers.” Stringers are actual holes or leaks paths in the material that follow grain directions and are intrinsic to many grades of materials that are used in vacuum applications. Surprisingly, stringers can be up to inches in length. See the Scanning Electron Microscope (SEM) photo below (Fig. 1 and Fig. 2):
Most metals can have stringers, including low vapor pressure metals. Some of these include:
● Iron (Fe)
Intrinsic leak paths can also be found in cast metals and are generally the result of gas and oxide inclusions. Fortunately some materials are stringer free, such as OFE Copper.
When designing for high vacuum applications, you should avoid thin wall sections that are perpendicular to the grain direction in the material. It is a good practice to specify grain direction on your component drawing and also specify the raw material form. For example, bar stock and tube have grains running axially or length-wise and sheet stock has grains running horizontally.
When thin weld flanges or other thin components are needed and have features in both axes or multiple directions (see Fig. 3), you may want to consider forming or punch pressing the component from think sheet stock. The drawing process will allow the grains to stretch and follow the “bends” or features in parallel eliminating the possibility of a perpendicular stringer.
Also, when very thin sections of metal are being used, say less than 0.015”, there’s always potential for a leakage path through an inherent stringer and the risk can be greatly increased if this material is “over-brazed” resulting in “erosion” (see Fig. 4), where the parent material is “thinned” and/or a significant portion is brought into solution with the molten braze filler/braze alloy.