Feasibility of heavy section pieces made of X 2 Cr Ni Mo N 25-7-4 superduplex steel

Abstract

Sensitivity to quenching of X 2 Cr Ni Mo N 25-7-4 superduplex stainless steel (UNS 31260) is such that between 54000 and 360 °C h −1, the impact strength drops by more than 260 J and the transition temperature at 50% crystallinity (FATT) rises by more than 200 °C. The tensile characteristics remain practically constant except for reduction of area which drops by 20%. Resistance to localized corrosion in sea-water environment remains unaltered. Embrittlement is caused by the formation of sigma intermetallic phase by decomposition of the ferrite. This may happen both during the forging process or during the post heat treatment of heavy section forged pieces made of X 2 Cr Ni Mo N 25-7-4 steel. Forging of such pieces is a delicate operation and their impact characteristics may be heterogeneous and very poor: Charpy V-Notch (CVN) at +20 °C = 45 J and FATT = +70 °C. Embrittlement being reversible, for pieces up to 150 mm thick, homogeneous and high characteristics can be obtained by quenching process on pieces featuring a condition the closest to their final dimensions. Such a piece, in the shape of a “U” featuring an external diameter of 600 mm, 350 mm high and 150 mm thick, incorporating a 90 mm diameter bottom hole, has been manufactured at industrial scale by forging of a billet 650 mm diameter and 400 mm high, followed by machining and quenching process to a thickness in the range of 150 mm. The following was obtained in any area of the piece: CVN at +20 °C ≥ 180 J and FATT < −30 °C. For thicknesses in excess of 150 mm, the impact strength shows an abrupt drop. The transition temperature and homogeneity of the impact strength characteristics are no longer controllable; they may vary in a significant manner, for a similar type of piece, from any single piece to the next. A specific electrochemical test in 4M HCl environment has been developed, to establish the link between the quenching rate and severity of the corrosion in order to know, a posteriori, the local quenching rate in a piece, and assess the impact strength in that location.