10901050

aAlloy 625 can be heat treated between these temperature ranges depending on specification needs. Generally the range is 950-1050°C.

aAlloy 625 can be heat treated between these temperature ranges depending on specification needs. Generally the range is 950-1050°C.

these lie between 2250°F (1232°C) and 1600°F (871°C). Due to their alloying elements these alloys have higher high-temperature strength, lower thermal conductivity, higher strain-hardening coefficient, higher strain rate sensitivity, and undergo rapid increase in strength with falling temperatures in the hot-working temperature range. Due to these characteristics, the hot-forming temperature range tends to be narrow. The upper temperature range is used for either reducing the cross-section thickness or forming or a combination thereof, and lower temperature ranges are used to develop specific properties after solution annealing. Table 7.22 gives the hot-forming and solution-annealing temperatures for the various alloys. For low degrees of hot deformation (less than 20%), the forming temperatures should be as close as possible to the lower limit, and the finishing temperature should be sufficiently lower than the final solution heat treatment temperature so as to develop proper microstructure and mechanical properties after annealing. For greater degrees of deformation, higher temperatures are recommended with appropriate reheats. Care must be taken during the last reheat to ensure that there would be enough reduction after the last reheat and that the hot finish temperature would be sufficiently below the solution annealing temperature.

After all hot working, it is recommended to give a full solution anneal and water quench for the Ni-Mo, Ni-Cr-Mo, and superaustenitic stainless steels to provide optimum corrosion resistance. Figure 7.8 shows the time-temperature -sensitization diagram for most of the Ni-Cr-Mo alloys, as determined by ASTM Method G-28A. As is evident, alloy 59 is the most thermally stable alloy in comparison to other Ni-Cr-Mo alloys like C-276, C-4, and alloy 22.

For Ni-Mo alloys, Fig. 7.9 shows the effect of higher iron content on retarding the kinetics of formation of embrittling 6 phase, as measured by isoimpact curves. The new alloy B-4 (UNS N10629) has solved both the cracking problem during

FIGURE 7.8 Time-temperature-sensitization diagram for Ni-Cr-Mo alloys as measured by ASTM G-28A test.
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