Heat resistant austenitic stainless steels such as 0.4C-25Cr-35Ni-1Nb, HP-Nb alloys are commonly used in high temperature applications (>850°C) for applications such as ethylene pyrolysis tubes and furnace rolls. HP-Nb alloys exhibit high temperature corrosion resistance and good creep properties due to the high volume fractions of MC niobium carbides, as well as, M7C3 and M23C6 chromium carbides. However, these carbides can form a continuous network after casting or during service which leads to poor ductility and weldability. Previous studies have reported that HP-Nb alloys are susceptible to hot cracking in the Heat Affected Zone during fabrication following the pathway of the continuous carbide network along the grain boundaries and interdendritic regions. As the microstructure and concomitant carbide continuity is a result of the casting conditions and alloy composition, a thorough study is needed to explore the solidification of these alloys. Although the solidification of Ni-Nb-C and Ni-Cr-C ternary systems are well documented, there is limited information concerning the Ni-Cr-Nb-C quaternary system where both chromium and niobium carbides form. Therefore, the objective of this study is to improve the understanding of the solidification in the Ni-Cr-Nb-C quaternary system in order to understand the relation between composition, microstructure and mechanical properties of HP-Nb alloys.
The solidification of HP alloys has been studied through the development of an alloy matrix with systematic variations of carbon, silicon, niobium, titanium and tungsten. Experimental alloys were prepared at the University of Alabama and poured into 45lb wedge shaped blocks for a total of 18 unique chemistries. Industrial centrifugally cast HP-Nb material was also included in the study in order to compare to the experimental alloy matrix for a total of 30 alloys. Samples were extracted from the center wedge or cast tubes and metallographically prepared to a final polish of 50nm colloidal silica. Differential thermal analysis (DTA) was conducted using a Netzsch STA 409 with 500mg ± 50mg samples and a heating and cooling rate of 10°C/min in order to determine the liquidus and eutectic transformation temperatures. Scanning electron microscopy (SEM) was conducted using a FEI Scios Dual Beam microscope with an accelerating voltage of 15kV and a beam current of 13nA using an electron backscatter detector. Backscatter electron images were processed via a quantitative image analysis program written in MATLAB to extract phase fractions of each constituent. Electron microprobe analysis (EPMA) was conducted using a Joel JXA-8900 with an accelerating voltage of 15kV and beam current of 30nA for phase identification and to determine the partition coefficients for representative alloys. Thermo-Calc Scheil solidification modeling was conducted using the TCNi9 database. Room temperature tensile testing was conducted in duplicate for each alloy according to ASTM E8. Weldability was assessed through longitudinal varestraint testing with a sample geometry of 6” x 1” x 0.25” and an imposed strain of five percent.
Results and discussion
Fractography of the room temperature tensile samples revealed that fracture consistently occurred through continuous carbide networks along dendritic boundaries. The mechanism of cracking in HP-Nb alloys was found to initiate with microcracking within the M7C3 carbides after which, the continuous carbide network provides a pathway for crack propagation. However, the cracking preferentially propagated through M7C3 carbides instead of MC carbides. Previous studies have conducted nanoindentation on each carbide where, a hardness of 14.5 ± 2.5 GPa was observed for the M7C3 carbides and 25.7 ± 3.6 GPa for the MC carbides. Because hardness and tensile strength are directly correlated, a lower hardness indicates a lower tensile strength and is therefore likely the reason why cracking initiated in the M7C3 carbides. Longitudinal varestraint tests revealed that cracking primarily occurred in the base metal and low temperature heat affected zones, associated with insufficient ductility to support the residual stresses from welding.
Volume fraction data measured from the quantitative image analysis procedure was used in combination with EPMA compositional data to calculate the experimental eutectic points for the niobium and chromium eutectic carbides. These points were then plotted to represent the nickel rich corner of the Ni-Cr-Nb-C liquidus projection. Solidification modeling was then conducted assuming nonequilibrium conditions for chromium and niobium and equilibrium conditions for carbon and were plotted along with the experimental liquidus projection. From this, increasing the nominal concentration of niobium was found to increase the volume fraction of MC carbide and a decrease in the volume fraction of the M7C3 carbides and increasing the nominal concentration of carbon resulted in an increase in M7C3 carbides as well as total eutectic constituent.
Longitudinal varestraint testing and room temperature tensile tests were observed to have the same fracture mechanism which initiated in the chromium rich carbides and propagated through the continuous carbide network. Fracture was found to preferentially initiate within the chromium carbides instead of the MC carbides, associated to the lower hardness and therefore lower tensile strength.
An experimental Ni-Cr-Nb-C liquidus projection was developed and revealed that the addition of niobium results in an increase in the volume fraction of MC carbide and a decrease in the volume fraction of the chromium carbide. The addition of carbon resulted in an increase of the eutectic constituent through a minor increase in the volume MC carbides and a significant increase in the volume fraction of chromium carbides. Statistical regression techniques reveal a correlation between increasing volume fraction of M7C3 carbides and decreasing ductility, where no correlation is observed with increasing volume fraction MC carbide. The results of this study show that high volume fractions of chromium carbide are detrimental to the room temperature ductility and therefore weldability of HP-Nb alloys.