WEB Lightweight stainless steels strengthened by B2-(Ni,Fe)Al intermetallic precipitates
By adding Al to stainless steels, it is possible to obtain lightweight stainless steels. The simultaneous addition of Ni and Al to stainless steels with a body-centered cubic crystal structure favors the formation of ordered B2-(Fe,Ni)Al phase. In the present study, the formation temperature range and strengthening effect of B2 phase in martensitic Fe-10.5Cr-3Al-9Ni (wt.%) and Fe-10.5Cr-3Al-12Ni stainless steels is investigated. Differential scanning calorimetry (DSC) measurements of as-quenched martensitic steels during heating at a rate of 50 K/min indicated an exothermic reaction starting at approximately 350°C. A corresponding contraction was detected during continuous heating in dilatometry measurements. Scanning and transmission electron microscopy (SEM and TEM) examinations of tempered specimens confirmed that the exothermic reaction and contraction were due to the formation of B2 phase in martensite laths. After tempering at 500°C for 5 min, the average size of B2 precipitates in the 9Ni alloys was of the order of 50 nm. The formation of B2 precipitates was found to lead to a noticeable strengthening. For the 9Ni alloy, the hardness increased from the base level in the untempered condition of 405 HV 5 to 520 HV 5 after isothermal holding at 350°C for 5 min. The peak hardness of 600 HV 5 was achieved after tempering at 500°C. The strengthening of martensite by B2 precipitates was comparable for both alloys. This can be justified by the fact that the Ni content of both alloys was in excess of the stoichiometric Ni content required for the formation of NiAl. The strengthening concept applied to the 9Ni and 12Ni alloys could not be applied to another variant of the same alloy system in which the Ni content was reduced to 6%. This was due to the high ferrite potential of the latter alloy and the negligible fraction of austenite at high temperatures. As a result, only a small fraction of martensite could be obtained at room temperature and the hardening response of the predominant ferrite phase was very weak. Increasing the Ni content to 15%, on the other hand, resulted in the formation of retained austenite accompanied by a decrease in hardness. The results aid with the design of the chemical composition to obtain Al-alloyed lightweight stainless steels capable of precipitation hardening.