Influence of aluminum as alloying element on the microstructure of stainless steels with predominantly austenitic matrices

• Applicant: Professor Dr. Javad Mola, Ph.D.
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: from 2015 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 274385700

In the first phase of the project, Al in quantities up to 7 mass-% was added to stainless steels to obtain steels with austenitic (six alloys), duplex (one alloy), and ferritic (one alloy) matrix microstructures. For steels containing austenite, the temperature dependence of tensile properties was determined. Al addition was associated with a clear increase in the stability of austenite evidenced by a decrease in the Mdγ→αʹ temperature. This effect correlates well with the SFE-raising effect of Al. Analysis of the temperature dependence of tensile properties and strain hardening behavior for austenitic and duplex alloys enabled to establish interrelationships among glide planarity, linearity of the strain hardening rate, and tensile elongation. The ferritic steel containing 7 mass-%Al showed embrittlement caused by the formation of B2-(Ni,Fe)Al intermetallics. B2 intermetallics could also be induced in the duplex alloy by an aging treatment. One of the objectives of the second phase of the project is to design Al-alloyed steels exhibiting maximum tensile elongation near room temperature. Insights from the first phase of the project will serve as guidelines for the design of the chemical composition. The design is based on the recognition that C, Cr, Ni, Mn, and Al decrease the Mdγ→αʹ temperature. Reportedly, all of these elements except Al have a similar effect on martensite start (Ms) temperature. Given the consistency of the effect of alloying elements on Mdγ→αʹ and Ms temperatures, Al addition to stainless steels is expected to decrease the Ms temperature. Therefore, one of the objectives of the second phase of the project is to re-examine the reported influence of Al on Ms temperature. Another objective of the second phase is to study the formation of B2 intermetallics in Al-alloyed ferritic stainless steels containing Ni. The straightforward dilatometry-based method introduced in the first phase will be used to study B2 precipitation in a series of alloys containing both Ni and Al. This knowledge is an aid in the design of duplex stainless steels with B2-strengthened ferritic constituent. Quenching and Partitioning (Q&P) processing is an effective method of obtaining 3rd generation advanced high-strength steel properties in steels with martensitic-austenitic microstructures. Successful application of Q&P requires that the precipitation of cementite in martensite, a process that competes with the C enrichment of austenite, is inhibited. Al is regarded as an alloying element opposing the precipitation of cementite during austempering of TRIP-assisted steels. The last objective of the proposal is to study the influence of Al on the precipitation of paraequilibrium cementite in the martensitic constituent of martensitic-austenitic stainless steels and on the C enrichment of austenite.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr.-Ing. Horst Biermann