Yoshiatsu Sawaragi

The Development of a New 18-8 Austenitic Stainless Steel (0.lC-18Cr-9Ni-3Cu-Nb, N) with High Elevated Temperatures Strength for Fossil Power Boilers

ABSTRACT The economical austenitic materials with high elevated temperature strength are required for superheater tubes in Japanese fossil power boilers. In this work, the effect of alloying elements such as Cu, Ti, Nb, N and Ni on the elevated temperature strength properties and structural stability for 0.1C-0.15Si-18Cr-10Ni steel were systematically investigated. The main results are as follows. (1) The addition of Cu more than 2.5% contributes to improve the creep rupture strength remarkably, because of fine and spherical Cu-rich phase which precipitates coherently in an austenite matrix during creep. And these Cu-rich phase particles produce the deterioration of creep rupture ductility. (2) The addition of slight amount of Nb or Ti is effective for further improvement of creep rupture strength for 3%Cu containing steel. Especially, Nb also contributes to improve the creep rupture ductility of Cu containing steel because of grain refining effect. (3) The addition of about 0.1%N in Cu containing steel is effective to increase elevated temperature tensile strength without remarkable deterioration of ductility and toughness. (4) The developed steel, of which composition is 0.lC-18Cr-9Ni-3Cu-0. 4Nb-0.08N, has more than 30% higher creep rupture strength at 600~700°C than ASME SA-213 TP347H. This excellent strength is based on the finely precipitated particles such as Cu-rich phase, NbCrN, Nb (C,N) and M23C6. The microstructural stability is also sufficient. (5) Based on these considerations, it may be concluded that this steel can be promising materials for boiler tubes from the economical and practical points of view.

The oxidation and carbon transfer behaviour of Ni-27Cr-5Mo-5W based Alloys in an impure Helium atmosphere at high temperature

The corrosion behaviour of a series of Ni-27Cr-5Mo-5W alloys with various element additions in an impure helium atmosphere at 1273 K has been studied. Although the extent of oxidation was small, significant amounts of carbon transfer occurred. The carbon transfer behaviour was influenced by alloy composition with the effects of chromium and aluminium being particularly marked. The effect of Cr was interpreted by the carbon activity-oxygen potential equilibrium diagram for Cr and a Ni-Cr alloy and that of Al was interpreted by consideration of possible deterioration of surface scales due to the volume increase expected in the internal oxide zone where Al2O3 stringers precipitate.

A new austenitic alloy containing 3.5% vanadium resistant to coal gasifier corrosion environment

A new alloy containing 26.5% Cr, 39% Ni, 3.5% V, 0.3% Nb and 0.02% N was developed for heat exchangers of coal gasification power plants. This alloy is expected to be resistant to both high temperature sulfidation and aqueous corrosion during down time. Laboratory-melted alloys of various compositions were prepared to optimize the alloy composition with respect to mechanical properties. The Greeble test, tensile test, creep rupture test, Charpy impact test for aged materials, and the Trans-Varestraint test were performed. Changes in the microstructure of the alloys upon ageing were investigated by optical and electron microscopy, along with chemical analyses of the precipitates.