PHASE CHEMISTRY AND PRECIPITATION REACTIONS IN MARAGING STEELS .4. DISCUSSION AND CONCLUSIONS

This article summarizes our studies of phase chemistry and precipitation reactions in a variety of maraging steels. The roles of different phases and alloying elements are investigated by comparing the behavior of different steels. The phases considered are Ni3Ti, Fe7Mo6 mu phase, Fe2Mo Laves phase, omega phase, Ti6Si7Ni16 G phase. ''Z phase,'' austenite, and alpha matrix. The alloying elements discussed are Ti, Al, Mo, Si, Mn, Ni, Cr, and Co. By comparing the aging behavior of both commercial steels and model alloys, a major role of Co is confirmed to be the lowering of the matrix solubility of Mo. Of the two main hardening elements in maraging steels (namely, Ti and Mo), Ti is much more active than Mo in the very early stage of precipitation. The main Mo-rich precipitate found in this work was Fe7Mo6 mu phase instead of Laves phase. The precipitation of Mo is modified by the presence of Ti. Omega phase appears only in Ti-free alloys, especially when aged at a low temperature. The quantity of Ni-containing precipitates and the presence of Cr in the steels change the austenite reversion behavior. Other phases, such as G phase and ''Z phase,'' contribute to age hardening in different types of maraging alloys.