Duplex stainless steels for structural weight saving – relationship between mechanical properties and microstructure

Duplex stainless steels (DSS) have always been an exciting area of interest for researcher, stainless steels producers, fabricators and end-users. The use of DSS as structural and corrosion resistant materials has been increasing markedly in recent years. The fine austenite / ferrite microstructure of these materials promotes an excellent combination of toughness and mechanical resistance, desirable for many applications in chemical and petrochemical industries. For that reason DSS are good candidates for making lighter structures in a large range of applications such as building and construction, oil and gas and the energy segment.

Scientifically, the dual phase of the alloy makes the relationships between microstructure and mechanical properties more difficult to understand compared to single-phase materials. Particularly, tensile strength in combination with elongation and toughness properties need to be improved in order to better substitute austenitic stainless steels. New lean DSS with low content of nickel and molybdenum give opportunities for innovative substitution to achieve equivalent performance at a lower cost via weight reductions.  

The aim of this work is to better understand tensile properties and impact toughness in relation with the architecture of the two phases within the microstructure: amount and chemical composition of phases, geometrical distribution. Experimental work will be carried out on industrial or laboratory casts having various compositions going from pure ferrite to pure austenite.  

Concerning the impact properties, ferrite is the brittle phase, controlling the fracture behaviour. We will first consider the effect of Ni, Cr and Mo alloying content on the DBTT of ferritic stainless steels. Thermodynamic calculations could be done to estimate the alloying content required to provide fully ferritic structure with 0, 2 and 4% of Ni.  Subsequently, the introduction of the ductile phase i.e. austenite phase, with different amounts, morphologies and chemical compositions will be addressed. DSS laboratory grades will be cast and thermo-mechanical treatments will be done to achieve different microstructure. The resistance of ferrite/austenite interface is certainly crucial; it will be studied by changing the direction of solicitation on anisotropic microstructures. In the same way, mechanical tests will be performed on the laboratory castings, varying the austenite content and stability.

The results of these tests will be analyzed by mechanical analysis, especially the strain partitioning which is important to analyse the tensile behaviour. This aspect will be measured locally using nano-indentation technique on cold drawing wires. Cold deformation can be obtained by cold drawing stainless steel wire from 5.5mm to around 2 mm in order to have 20 to 90% of cross section reduction. This can be done on the drawing machine of the research centre in Ugine. Different duplex grades can be studied in order to investigate the influence of chemical composition (N, Ni, Mo) on the strain-hardening coefficient of the austenite and ferrite. An austenitic grade will be added as a reference.

Nano-indentation measurements should give accurate data in both phases, because of the elongated and very thin structure (a few microns or less) obtained after cold drawing. On the same samples, the amount of a' (bcc) martensite formed during drawing will also be measured using magnetic balance or X ray diffraction. For phases identification and dislocation densities measurements, a specific tool ACOM (Automatic Crystal Orientation Mapping) operating in the TEM can be used in the Simap Lab. In a second step, mechanical modelling will be carried out in order to describe the plastic deformation of a two-phase material.

- A.F. Padilha and R.L. Plaut in Duplex Stainless Steels Edited by Iris Alvarez-Armas et Suzanne Degallaix-Moreuil  Wiley 2009  Chapter 3.

- K. Cho and J. Gurland, Metallurgical Transactions A, Vol 19A, August 1988, pp 2027-2040