Study of Failure Mechanism and Improved Procedure for Dissimilar Welding between T91 and T22 Cr-Mo Steels(II)

Cr-Mo steels have been widely used in high temperature and pressure environments due to their excellent weldability, high temperature mechanical properties and corrosion resistance. In order to reduce the capital investment, dissimilar weldments made by Cr-Mo steels with different compositions are often used, and the facility is always examined by nondestructive methods to ensure the absence of welding defects. However, prolonged exposure at elevated temperatures may result in various failures due to the changes in microstructure as well as mechanical properties of Cr-Mo steels. This study was to evaluate the failure mechanisms of dissimilar welds between 9Cr-1Mo and 2.25Cr-1Mo steels (T91-T22) after prolonged service in the high temperature/pressure environment. Cracks were observed in the dissimilar welds and mostly were adjacent to the fusion line adjacent to the T22 base metal. It was determined that the occurrence of these cracks was closely related to the migration of carbon atoms from Cr-lean T22 steel to Cr-rich Ni-based filler and T91 steel. It is this carbon migration phenomenon that results in the formation of a decarburized band, i.e., a soft zone, in the T22 heat-affected zone adjacent to the fusion line. Meanwhile, there may exist a carbon-rich band in the high alloy region (such as T91 steel or Ni-based filler metal. Carbon in this C-rich region will form chromium carbide and increase the hardness but lower the ductility. In order to this type of cracking after prolonged high temperature usage, Ni-base filler is used to form a buttering layer on T22 to prevent the carbon migration. However, based on real industrial experience, this Ni-base buttering layer is unable eliminate the carbon migration completely. Consequently, several severe breakdowns occurred in the recent years in Taiwan and resulted in significant economic losses and safety concern. The major approach of this study is to develop buttering filler materials with varying carbon contents and alloying elements, i.e., V, Nb, and Ti. Butter layers with higher carbon contents will change the diffusion direction of carbon atoms and prevent the formation of the soft zone in the T22 heat-affected zone. Meanwhile, the addition of V, Nb, and Ti may form carbides and prevent the diffusion of carbon atoms from the high carbon buttering layer to the Ni-based filler metal, and the formation of the hard zone. In-service heat treatment (ISHT) as well as microstructural and compositional characterization will be conducted to evaluate the effect of the modified welding procedures on the properties of the weldments. Results of the feasibility study confirmed that the high carbon/alloying buttering layer could eliminate the formation of the soft zone and the hard zone. Therefore, it is believed that this project can be very successful in preventing the formation of cracking problems in this type of dissimilar welds. The major objectives and procedures of this 2-year study include: to evaluate the failure mechanism of the dissimilar welds between T22 and T91, to develop an optimum welding procedure and filler metal selection in order to retard the carbon migration and the related formation of soft zone and hard zone. Various filler metals will be developed and used to butter the Cr-lean T22 steel prior to the welding, and subsequent In-Service Heat Treatments (ISHT) will be applied to simulate the prolonged exposure in real production situation. Optical microscopy, microhardness testing, bend testing, EDS analysis and SEM fractography wll be used to study the microstructures, mechanical properties, and chemical characteristics of these T22-T91 dissimilar welds. Finite element analysis (FEA) will be used to evaluate the thermal stress in the weldments of varying geometry and thickness during heating and cooling cycles as well as in the normal operation conditions, in order to confirm the effectiveness of the modified welding procedures.

Project ID:PB9907-10767
External Project ID:NSC99-2221-E182-019