Abstract
The stress corrosion cracking in expanded parts of stainless steel heat exchangers and its prevention were studied. Model test blocks of the exchangers were constructed with tubes expanded by about 3-15% and were exposed in 42% MgCl2 solution at 154°C.
The results obtained are as follows: -
(1) When both of tube and tube plate materials were stainless steels, no appreciable relation was found between the expanding ratio and the crack initiation.
TIG seal welding of the tube plate accelerated the crack initiation.
(2) Irrespective of the expanding ratio, the stress corrosion cracking was always initiated in a short time. The reason for this seems to be that the rate of increase of residual stress was not so large in the range of plasticity as in that of elasticity.
(3) When stainless steel tube was coupled with mild steel tube plate, the stress corrosion cracking did not occur at expanding ratio identical with the former.
That is because stainless steel was protected electrochemically by the sacrificial action of mild steel, the protecting current density of which was found to be 0.26mA/cm2 by calculation. This current density was approved by the electrochemical examination.
(4) As the stress corrosion cracking of stainless steel part influenced by expanding had almost no relation to the expanding ratio, it is desirable to apply cathodic protection to prevent the crack initiation.
(5) The results may be applied to practice. The condition in the test being more severe than in actual plants, the service life in practice will be much prolonged in comparison with the test results.
