Effect of N Content on Mechanical Properties and Microstructure of Alloy 690

The microstructures and mechanical properties of nitrogen bearing Alloy 690 have been systematically investigated. Alloy ingots with different N addition, range from 38 to 330wt.ppm, were melted using vacuum induction melting (VIM) plus electro-slag re-melting (ESR) double processing techniques. The forged and hot rolled different N content bars were solid solution treated between 1010°C and 1080°C, thermally treated at 715°C for different state mechanical property testing and microstructure study. Microstructure analysis indicated that nitrogen addition to Alloy 690 can effectively refine the solution treated austenite grains. This may be associated with titanium nitrides pinning the grain boundaries and hindering the grain growth during solid solution treatment. More nitrides, which are identified as TiN, were found on the grain boundaries and in the inside of austenite grains with increasing N contents of the alloy. The carbide precipitation at 715°C showed significant difference identified by SEM. At the level of 38, 100 and 220wt.ppm N, the chromium carbide Cr₂₃C₆ distribution on the grain boundaries appeared to be semi-continuous; when the N content reached 330wt.ppm, only few discrete type of carbides were observed. The tension testing results at room temperature of different N content alloys proved that both the ultimate tensile strength (UTS) and the yield strength (YS) enhanced about 50MPa when N content was raised from 38 to 330wt.ppm in this alloy; while the corresponding elongation (EL) and reduction in area (RA) adversely dropped about 5%. Room temperature hardness rose with increasing N content, well matched tensile strength. High temperature tension testing results at the range of 900∼1250°C showed that a severely hot ductility dip, representing by the values of the reduction in area (RA), existed in 300wt.ppm and 100wt.ppm nitrogen containing alloys at the lower end temperature range of 950∼1100°C. However, such ductility dip could be improved when the N content was at 220wt.ppm, and completely eliminated at 38wt.ppm N content. At the higher end temperature rang of 1150∼1250°C, the ductility of all 4 nitrogen bearing alloys did not show significant difference, even though the hot ductility of minimum 38wt.ppm N samples was preferable. Nitrogen content did not affect high temperature strength; the UTS values nearly had no change at the same testing temperature with different nitrogen bearing alloys. The carbide precipitation difference of the thermally treated alloy, induced by N addition, may affect Alloy 690 corrosion properties, which needs to be studied in future. The mechanical properties variation both at room temperature and high temperatures of different nitrogen bearing alloys in this study will be certainly beneficial to determine the practical processing routes of Alloy 690.