352 Prakt. Met. Sonderband 38 (2006) 
After brazing only a small amount of Cu remains between the plates (Fig. 8). Looking at I 
the structure of the stainless steel very large recrystallised grains are observed. Some ‘ 
crystals are limited by the sheet thickness (Fig. 8b, c). ! 
3.3 COPPER DIFFUSION INTO THE STAINLESS STEEL 
During the preparation of metallographic samples different etchants were applied. 
After an FeCl; etching, which only attacks the Cu phase, V2A-pickle is used. In this case 
interesting etching structures become visible (Fig. 9). Along the grain boundaries of the 
stainless steel bright areas can be seen. At the edges they are thicker and in the regions 
between the sheets thinner. 
The Fe-Cu phase diagram [7] shows that at 1096°C the Cu solubility in iron is 8.2 wt.%. 
During cooling the solubility decreases. Exact data for the Cu solubility in the quaternary 
system Fe-Cr-Ni-Cu are not available. 
In the case of Cu-diffusion into steels it is distinguished between grain boundary and lattice 
diffusion. For Cu in y-Fe at 1100°C the lattice diffusion is calculated with 9.4 x 10? cm?/s 
and 1.68 x 10° cm?/s for the grain boundary diffusion [8]. 
From surface alloying experiments it is known that Cu can precipitate in the stainless steel 
during cooling [9] and similar experiences are known from welding procedures [10]. 
In the demonstrated case Cu diffuses along the grain boundaries from the braze into the 
stainless steel. During cooling the copper solubility decreases and Cu precipitates at the 
stainless steel grain boundaries. Some Cu remains in the steel, which can be made visible 
by the etching procedure 
100 Mu 
Fig. 9: FeCl / V2A-pickle makes visible the Cu influenced regions in the stainless steel. 
(a) and (b) Cu-enriched zones; (c) detail of b 
In principle some Cu diffusion during brazing is acceptable but in the presented case Cu 
diffusion occurs locally through the whole steel plates (about 90 um from both sides of the 
sheet). This phenomena of excessive braze diffusion into the basic material is also known 
as a type of high-temperature corrosion and called “liquid metal embrittlement”. An obvious 
explanation for the advanced Cu diffusion into the steel is that the brazing temperature 
was too high. 
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