Correctly understand the magnetic stainless steel / customer complaints that a batch of purchased stainless steel valve accessories have corrosion cracking phenomena after using for a period of time, so the quality objection is raised. The material of the valve accessories purchased by the customer is CD4MCu (00Cr25Ni6Mo2Cu+N). It is a duplex stainless steel with high strength, wear resistance, corrosion resistance, and good resistance to stress corrosion. It is widely used in the manufacture of water pump accessories. It can be said that it is absolutely no problem to use in the working environment described by the customer. However, after on-site inspection, the corrosion phenomenon was confirmed to be stress corrosion cracking, which was really puzzling, so it was decided to sample the test material first. After testing, the results are surprising. The material of these cracked accessories is not CD4MCu at all, but 304 stainless steel. The parts purchased by the customer are castings sold by the factory. The material is absolutely no problem, so the customer is notified and asked whether the parts have been replaced. After verification, the customer confirmed that the assembly department of his company had indeed changed the parts and changed the original parts of CD4MCu to 304. The reason is that CD4MCu is magnetic. The person in charge of assembly thinks that the magnetic stainless steel is stainless steel iron Corrosion is not good, so the accessories of 304 material were changed without authorization. CD4MCu contains about 60% ferrite and is magnetic, but it is precisely due to the dual-phase structure of ferrite plus austenite that greatly increases its stress corrosion resistance. The assembler knew nothing about stainless steel and changed the 304, which has poor resistance to stress corrosion, resulting in this accident. "Magnetism is stainless steel iron, corrosion resistance is not as good as stainless steel", this concept is generally found in people's perception, especially those who do not have professional knowledge of stainless steel. But in fact, whether or not it is magnetic is not a criterion for judging the strength of corrosion resistance. Each stainless steel has its special purpose. Whether it is ferritic stainless steel, martensitic stainless steel or duplex stainless steel, these magnetic stainless steels have their own special places, considering the cost-effectiveness, in some places, these stainless steels are more suitable than austenitic stainless steels.