Aluminum-alloyed lightweight stainless steels strengthened by B2-(Ni,Fe)Al precipitates

M Harwarth,G Chen,R Rahimi,H Biermann,A Zargaran,M Duffy,M Zupan,J Mola

doi:10.1016/j.matdes.2021.109813

Abstract

The age hardenability of Al-alloyed lightweight stainless steels with the base chemical composition Fe–10.5Cr–3Al (wt.%) and Ni concentrations in the range 3–15 wt.% was studied. Alloys containing 3% and 6%Ni exhibited almost fully ferritic matrix microstructures and a weak age hardening response. Alloys containing 9%, 12%, and 15%Ni, on the other hand, developed primarily martensitic microstructures. Differential scanning calorimetry measurements indicated the occurrence of an exothermic reaction in the approximate temperature range 375–625 °C. Dilatometry measurements indicated that the exothermic reaction was accompanied by a net contraction. Hardness measurements after aging for 5 min indicated significant hardening of alloys already at 350 °C due to the formation of B2-(Ni,Fe)Al intermetallic precipitates. The age hardening response was significantly superior to that of conventional precipitation-hardenable martensitic stainless steels. Tensile elongation in the aged condition was negatively influenced by the presence of soft ferrite regions. Processing conditions associated with a fully martensitic microstructure prior to aging are required to render a uniform age hardening response. Guidelines for the development of a new family of lightweight precipitation-hardenable steels with lower raw material costs and a higher corrosion resistance compared to the standard 18Ni maraging steels are provided.

Highlights

Due to its high strength and low cost, steel is the material of choice for a variety of engineering applications [1]
The vertical distance between the dilatometry curves for 3Ni and 6Ni alloys decreased at temperatures below approximately 500 °C
The following conclusions were drawn: 1. The solidification mode of the alloys was found by differential scanning calorimetry measurements to change from ferritic to austenitic as the Ni concentration increased

Summary

Introduction

Due to its high strength and low cost, steel is the material of choice for a variety of engineering applications [1]. The inherently low corrosion resistance can be enhanced by the addition of Cr, leading to the group of stainless steels for applications involving aggressive media or to increase the service life under atmospheric conditions [5]. The strength and ductility of stainless steels can be adjusted by the design of the microstructural constituents [5]. The latter design is aided by thermodynamic calculations or empirical approaches which take the effect of alloying elements on the microstructure development into consideration [6]. The empirical approaches are best exemplified by the common use of the Schaeffler diagrams to take into account the influence of alloying elements on the phases present at room temperature (RT) [6,7,8,9]

Methods

Results

Conclusion