b'over the whole test temperature range,resistance behavior was observed in theFigure 1. Fracture toughness of 12 Cr as presented in Figure 1. It is a notablethree 9Cr steels. Both of the two-stepsteel (HT-9 steel) heat-3 and heat-result that the HT-9 heat-3 withouttemperings after water quenching,4 after various thermomechanical treatments consisting of water-nitrogen addition after two-step500C/1h + 650C/15min andquenching (WQ) and two-step tempering treatments are able to main- 600C/1h + 750C/15min, yieldedtempering. Note that S indicates the tain their fracture toughness over 200high fracture toughness with the threesecond tempering was for 15 minutes MPam in the high-temperature range9Cr steel compositions. Third, additioninstead of typical 30 minutes.of >200C. When compared to those ofof nitrogen or other modification in Fast Flux Test Facility (FFTF) and Idahocomposition did not improve the frac-National Laboratory (INL) HT-9 steels,ture resistance of the high-Cr steels. That the fracture toughness of the HT-9 steelis, the addition of nitrogen (~0.044%)It is possible to improve heat-3 actually improved with thoseto the original HT-9 composition isthe high-temperature TMT routes while that of the HT-9 steelactually detrimental to the fracturemechanical properties heat-4 did not show any improvement.resistance of the alloy. Similarly, theof high-Cr steels by The single-step tempering at 600C9Cr steel-3 contains relatively smallersimply changing the also resulted in high fracture toughnessamount of molybdenum (0.5% insteadfinal thermomechanical for both HT-9 steels. Considering theof ~1.5% for the other two 9Cr alloys)schedule, without benefit of a higher strength that canand additional ~0.21% vanadium andincreasing any be obtained with a lower degree ofhighest amount of nitrogen (0.045%),manufacturing cost.tempering, it is recommended that thebut this compositional modification, HT-9 steels are tempered at 600C withfrom tradition 9Cr-1Mo steels, actually or without a short additional temperingreduced fracture toughness of the alloy, at 750C. Second, similar fracturein particular, at low temperatures.2019|AFC ACCOMPLISHMENTS 195'