Prakt. Met. Sonderband 38 (2006) 353
_ooking at Looking at the surface of the heat exchanger plates the grain boundaries of the stainless
ed. Some steel are visible (Fig. 10a, b). In the LOM a red-brown coverage of the surface is observed
(Fig. 10a), which can be explained by the formation of red CuO from metallic Cu on the
steel surface (Fig. 4). Fig. 2 shows that the whole heat exchanger is covered by a thin Cu
layer. At the water side it becomes red-brown and at the oil side it is still copper coloured.
For comparison the cross section of a steel sheet is shown, which also indicates that the
grain boundaries are influenced by increased Cu diffusion (Fig. 10c).
this case
ies of the
je regions
8.2 wt. %.
juaternary
and lattice
0” cms
less steel Rss FE
0]. Fig. 10: Surface and cross section of the steel plates after contact with water medium
e into the (a) surface in LOM; (b) surface in SEM; (c) cross section
tes at the
ade visible
4. SUMMARY
A defective plate heat exchanger was investigated. The plates are of stainless steel and
the braze is copper. The data of the brazing process are not available. A first succession
was that the water used is too corrosive and attacks Cu.
After cutting the heat exchanger into pieces no relevant copper and/or steel corrosion is
visible.
Metallographic investigations show:
The large Cu gussets are very coarse grained. Precipitations along the grain
boundaries in the Cu braze contain mainly Cr and Fe.
At the interfaces between the large Cu gussets and the stainless steel an additional
ss steel. phase is observed. EBMA measurements indicate higher amounts of Cr and less Ni
compared with the original steel composition. An obvious explanation can be the
formation of a chromium-rich ferrite, which is also confirmed by microhardness
measurements.
i case Cu Cu diffuses along the grain boundaries into the stainless steel. In some areas the
des of the 180 um thick sheets are fully penetrated, because diffusion occurs from both sides
Iso known of the sheet.
\n obvious Cu also diffuses into the y-Fe lattice. These regions can be made visible by a
mperature combined FeCls / V2A-pickle etching.
In some locations the grain size of the austenitic stainless steel is very large and
reaches about 180 um.
a’ 'h -