Causes and Prevention of Cracks in CE3MN Duplex Stainless Steel Castings
For the CE3MN cast super duplex stainless steel used in the valve body of 30″-300 Lb up-mounted ball valve,the causes of crack formation in the CE3MN duplex stainless steel castings during cutting the as-cast risers before solid solution treatment were investigated by using the FactSage8.2 thermodynamic calculation software and FSstel professional database combined with optical microscopy(OM),scanning electron microscopy and accompanying energy dispersive spectroscopy(SEM&EDS),X-Ray diffraction(XRD)and other microstructure characterization instruments in this paper.The results demonstrate that in the processes of slow heating or slow cooling within the sensitive temperature range of a series of brittle precipitated phases,a large number of the Cr and Mo element rich intermetallic compound sigma phases,which detrimental to the performance of CE3MN duplex stainless steel castings,may generate due to severe segregation of chemical components,and the intermetallic phases dispersedly distribute in the ferrite matrix in forms of short rod-like and fine needle-like structures,or distribute at the boundaries of α/γ phases in forms of long chain-like,banding-like,coral-like,(σ+γ2)cellular shape-like,long needle-like widmannstatten structure and so on.Production practice further showed that the solid solution treatment was carried out in advance to eliminate the possible as-cast brittle σ phases and inhibit the formation and expansion of cracks before cutting the risers of CE3MN duplex stainless steel castings,or firstly using sawing machine to cut the as-cast risers under cold condition and then carry out the solid solution treatment,the temperature factor causing cracks could be fundamentally avoided.
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