Recently as a part in the ECSC-project "NEWSCC" a u-bend test program in neutral dilute Chlorides (Cl-) was performed. Earlier presented data of stress corrosion resistance on stainless steel in the same environment have mostly been based on constant load testing. The results from these two test programs differ even though they have been performed at the same environmental conditions and in the same equipment. The ranking of alloys inside the two material groups of austenitic and duplex stainless steel were not changed, but the ranking between both groups were changed. In the u-bend test, a constant strain test, the austenitic stainless steels received better results compared to the duplex stainless steel in the previously performed constant load test program. Another result was that AISI 304L steel experienced cracking at a temperature as high as 125°C in 1% Chloride (Cl-) solution, while experiences from service shown given a maximum service temperature between 40 and 60°C. Mechanical testing including relaxation tests at actual testing temperature was done in order to evaluate the differences. The austenitic stainless steel experienced up to 30% relaxation and the higher alloyed duplex steels only about 10%. The differences in relaxation behavior must be considered during evaluation of u-bend test programs. The high degree of relaxation of AISI 304L may also explain the improvement for u-bend specimens compared to constant load test data and service experience. The difference in relaxation behaviour is also important when a test method shall be chosen for simulating an actual application. For example constant strain samples such as u-bends are the most representative for conditions where a strain causes the load, for instance in a non stress relieved bent heat exchanger tube. However, the huge difference in relaxation for different stainless steels in a u-bend will not give representative results for a constant load application, for instance stress relieved heat exchanger tubing or pressure vessels.
Keywords: stress corrosion cracking, stainless steels, constant load, u-bends, relaxation