JOURNAL OF MATERIALS SCIENCE 35 (2 0 0 0 ) 375 – 380 Solidification mode and residual ferrite in low-Ni austenitic stainless steels A. DI SCHINO Materials Engineering Centre, University of Perugia, Terni, Italy M. G. MECOZZI, M. BARTERI Centro Sviluppo Materiali, Italy J. M. KENNY Materials Engineering Centre, University of Perugia, Terni, Italy E-mail: kenny@unipg.it The solidification modes of two new classes of austenitic stainless steels with a low content of Ni are shown. Their chemical composition is similar to that of the standard AISI 304 and AISI 316, except for the content of nickel, manganese and nitrogen. It is found that standard formulas for predicting the residual ferrite can be fairly well used in the prediction of the solidification mode while they do not work in predicting the residual ferrite content. In particular, it is found that ferrite is the first phase to solidify for values of the equivalent ratio (calculated according to the formulas developed by Hammar and Svensson) greater than 1.50, otherwise austenite is the first phase to solidify. A new set of equations for predicting the residual δ -ferrite in these new classes of materials is determined via multivariable linear regression. The influence of the steel solidification mode on the material structural transformations during heat treatment is also shown. C  2000 Kluwer Academic Publishers 1. Introduction Nickel, Ni-alloys and, in particular, nickel containing austenitic stainless steels have been indispensable for the progress of technology during the past 80 years. Our modern technology and high standard of life would not have been achieved without nickel containing alloys. Due to the high cost of nickel and to the increasing attention to nickel allergic reactions, more and more laboratories and industries are trying to develop a new class of austenitic stainless steels with a low content of nickel and with mechanical and corrosion properties comparable to those of standard nickel based materials [1]. In the development of a new austenitic stainless steel, modifying the chemical composition of a standard one, a detailed knowledge of the effect of this change on the solidification mode and ferrite content is necessary. Metallurgists have been very active in developing ba- sic scientific research on solidification of various metal- lic materials. In particular, in the field of stainless steels, as steel makers move to the continuous casting of in- creasingly highly alloyed steels, it becomes more and more important to determine the solidification modes, since these determine the castability, the hot workabil- ity and the room temperature structure. Four mechanisms or modes explain the stainless steels solidification, namely: mode A: L → L + δ → δ → δ + γ mode B: L → L + δ → L + δ + γ → δ + γ → γ mode C: L → L + γ → L + γ + δ → γ + δ → γ mode D: L → L + γ → γ where L , δ and γ represent liquid, ferrite and austenite respectively. These modes are illustrated schematically on a ver- tical section through the Fe-Ni-Cr phase diagram in Fig. 1. The solidification sequence and the subsequent transformation characteristics determine both the level of segregation and the distribution of the residual fer- rite. Segregation is more deleterious in steels solidify- ing as primary austenite (mode D), since segregation at grain boundaries will not be redistributed by solid state transformations as it occurs with modes A, B and C [2]. Moreover the ferrite present in these last three modes may be dendritic or interdendritic depending on the solidification mode. The dendritic ferrite formed as a primary phase is not enriched in solute elements, un- like the interdendritic δ-ferrite, which forms as a result of segregation. In this paper the solidification modes and the resid- ual ferrite content of two new families of low-nickel austenitic stainless steels are examined. These new al- loys are mainly characterised by a strong Ni reduc- tion, which is compensated by manganese and nitrogen addition. 2. Literature equations for predicting solidification mode and residual ferrite Complex austenitic stainless steel compositions can be reduced to simple Fe-Ni-Cr ternary alloys by the use of Cr and Ni equivalent compositions. Various sets of equations are available for predicting both the so- lidification modes and the residual ferrite content in 0022–2461 C  2000 Kluwer Academic Publishers 375