Research On Weldability Of Dissimilar Steel Of Power Station Boiler Super-heater Tubes

In an electric power station of China, the dissimilar welded steel joints between 12CrlMoV pearlite heat-resistant steel and OCr17Ni12Mo2 austenite 'stainless steel of the third level super heater tubes in boiler cracked along the pearlite steel HAZ after operating at high temperature for 5.7× 104 hours . This paper researched the weldability of this kind of dissimilar metal joints and the mechanical properties and microstructure changes after service based on this accident, and at the same time analyzed the reasons for early failure.By the means of drawing test, it shows that pearlite steel's tensile strength is lower than austenite steel's, and after operating at high temperature, the strength of all kinds increases and flexibility decreases, which is apt to make joints crack fragilely. Through microstructure observation, it shows that there exists a marstenite transition after welding immediately; and during the course of operating at high temperature, the cementite in pearlite steel is ballized as leads to the decrease of metal's creep-resistance; at the same time carbon atoms diffuse from pearlite steel with low alloys to austenite welded metal with high alloys, namely carbon migration, which result in carbon-enriched zone in austenite steel side and carbon-depleted zone in pearlite steel side. Part of the carbon atoms migrated from pearlite steel solve in the marstenite base metal, and the surplus carbon atoms form M23C6 carbon oxide along the weld interface. The "penisular-like zone" which is rich in pearlite steel ingredient can be observed, and it deteriorates the properties of weld metal and increased the interface stress of joints. All in all, under the factors above, the depleted zone in pearlite steel would crack fragilely for formation of creep-flaw in low stress during operating at high temperature before designed expiration date.