Prakt. Met. Sonderband 38 (2006) 351 
nless steel At the interface of larger copper gussets and the stainless steel an additional phase is 
daries. observed. In case of short time V2A-pickle etching it is best visible (Fig. 7a). EBMA 
ocipitations measurements shows that compared with the stainless steel plate Cr is enriched and Fe 
e (Fig. 6d), and Ni are reduced in this phase (Fig. 7b). The formation of this phase can be explained 
rner of the by the dissolution of the stainless steel during the brazing process. Fe and Ni show higher 
Cu up to solubility in copper than Cr which results in a phase with higher Cr content. Microhardness 
ses rapidly measurements were performed and the stainless steel matrix shows 125 HV; 1, Whereas 
m this data the Cr-rich phase exhibits about 130 HVoo1. Due to the small grain size of the additional 
also Ni are phase the microhardness measurements are not exact, but the values are relatively low 
can form a and therefore carbide, oxide or o-phase can be excluded. From the EBMA- and 
ns that the microhardness results this phase can be described as a chromium-rich ferrite [6]. 
atching: V2A-pickle I 
(a) gs Phase atthe (b) 
' braze-steel 
interface 
20 un li 
Fig. 7: Investigations at the interface steel-copper. (a) LOM; (b) EBMA of the phase 
tching) located at the interface Cu - stainless steel and the steel matrix 
Further investigations were carried out in a region where several stainless steel plates 
were brazed together (Fig. 8a, b). In comparison to the gusset regions the thickness of the 
braze-layers are much thinner (up to 10 um). The Cu-diffusion into the stainless steel can 
also be observed but there is no Cr-rich phase (Fig. 8c). 
From Fig. 8a, b it is clear visible that the examined heat exchanger is a double-wall type, 
which means that two, about 180 um stainless steel plates are used for one wall. For 
connecting the plates also Cu braze is used. Following the general concept of the double- 
wall heat exchanger there should be some space between the plates and in case of a 
leakage the liquid comes out at the side of the heat exchanger and will not contaminate 
the second medium. 
| . 
Prem 74 
. 500 pm 200 pr. 50 um 
indaries. . . 
Fig. 8: Cu-brazed stainless steel plates at the outer region of the heat exchanger 
(a) cross section, polished: (b) cross section. etched: V2A-pickle; (c) detail of b 
9 
1D) 
'd) ha) La)