Study On Aging Precipitation Behavior Of Ferritic Stainless Steel With Untralow C And N Contents Alloyed With Ti, Nb And W

As materials used in the automotive exhaust system, the microstructure and property of ferritic stainless steels with ultralow C and N contents have direct relations with working temperature and service life of the exhaust system. To meet requests of high working temperature of the exhaust system, alloying of ferritic stainless steels is an important and effective way. In this thesis, ferritic stainless steels with ultralow C and N contents alloyed with Ti, Nb and W were fabricated using vacuum arc melting method and followed by solid solution, quentching and aging treatments at different temperatures. Mechanical properties of the materials were tested by means of micro-hardness measurement and strength measurement. In addition, precipitation behavior, chemical composition and microstructure of precipitates were studied by means of scanning electron microscopy, energy dispersive X-ray spectroscopy and transmission electron microscopy. The conclusions are as followings:1. The Vickers microhardness of the ferrite stainless steel with content of W, Ti and Nb of 1.45, 0.21 and 0.45?wt.%? reaches 198.3HV0.5 and the tensile strength of the material reaches 553.2MPa when aging at 600?. Meanwhile, the material has very good elongation?>18%?. The hardness, yield and tensile strength of the ferritic stainless steel decrease when the content of W increases to 1.6?wt.%? and the contents of Ti and Nb decrease to 0.16 and 0.35?wt.%? simultaneously.2. Ti, Nb and W are uniformly distributed in the grain interior and boundary regions and there is no obvious boundary segregation while the ageing temperature is between 550?and 600?. When the aging temperature reaches 700 ?, Ti remaines evenly distributed. However, Nb and W segregate obviously at the grain boundary regions.3. When aging temperature is between 550 ? and 600 ?, there are two kinds of precipitates in the ferrite stainless steel while the content of W element is 1.06 1.45?wt.%? and the contents of Nb and Ti are 0.24 0.21, 0.45 0.48?wt.%?respectively. One is?Ti,Nb? C with irregular granular morphology, the other is Laves phase Fe₂?Nb,W? with strip morphology. Both the two precipitates have consistent crystallographic orientation relationship with the ferrite matrix, indicating coherent or semicoherent interfaces between the two phases and the ferrite matrix, which can effectively strengthen the stainless steel material. The strengthening effect decreaseswhen aging temperature increases to 700 ?, since the quantity of?Ti,Nb?C decrease while the quantity of Fe₂?Nb,W? increases. Meanwhile the sizes of the two precipitates increase.4. The Laves phase Fe₂?Nb,W? still exists in the ferritic stainless steel when the content of W increases to 1.6?wt.%? and the contents of Ti and Nb decrease to 0.16 and 0.35?wt.%? simultaneously. Meanwhile, WC precipitates with complex morphology appear. The quantity of Fe₂?Nb,W? decreases and the quantity of WC increases when aging temperature is raised to 700 ?. Meanwhile, the size of WC increases to submicron or micron range, consequently the mechanical properties of the ferritic stainless steel degradate.