Microstructure and properties of nippon fire-resistant steels
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JMEPEG (1999) 8:606-612
Microstructure and Properties of Nippon Fire-Resistant Steels W. Sha, F.S. Kelly, and Z.X. Guo (Submitted 1 March 1999; in revised form 19 May 1999) The microstructure and mechanical properties of two fire-resistant steels manufactured by Nippon Steel were investigated. Microstructural observation was carried out using optical microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). It was found that the good hightemperature strength and creep properties of these steels are due to the high lattice-friction stresses, which are a result of the very fine distribution of metal carbide (MC) precipitates and molybdenum in solid solution. In addition, a strong secondary wave of precipitation at approximately 650 °C was observed. This lattice friction stress maintained strength up to 600 °C.
Keywords
creep, fire resistant, microstructure, steel, strength
1. Introduction In recent years there has been a large increase in the number of multistory buildings constructed with steel frames due to a decrease in raw material prices and better productivity in the steel industry. Correspondingly, there has been an increase in research into fire engineering, with major findings being included in the publications of comprehensive Eurocodes EC3 and EC4 (Ref 1, 2). The main requirement of fire safety regulations is to ensure the safety of occupants and fire fighters, and in practice, to offer some protection to the building itself (Ref 1). It is well documented that conventional structural steels retain approximately 50 to 60% strength at around 550 °C (Ref 35). Fire protection is needed because the steels cannot withstand the temperatures experienced in real fires. Tests have been carried out on large-scale constructions at Cardington Laboratory in England as part of a European collaboration coordinated by the UK Building Research Establishment (BRE) and British Steel. Most of the steelwork involved was unprotected. Maximum steel temperatures between 690 and 1060 °C were recorded (Ref 6). If these temperatures are reached, then the limiting stress of the steel falls below the working stress and failure ensues. Thus, it is necessary to protect steelwork and ensure that such temperatures are not reached within the specified fire resistance period. The main forms of fire protection are rigid boarding, spray, intumescent paint, concrete encasement, and flexible blanket; the first three dominate the market (Ref 4). Application of fire protection is a labor intensive activity. Most of the cost is derived from manual application rather than the material itself, and it can cost around 50% of the steel price (Ref 7). This means that reducing the amount, for example, thickness, of apW. Sha, School of Civil Engineering, Queen’s University of Belfast, Belfast BT7 1NN, UK; F.S. Kelly, Peter Brett Associates, Bridge Studios, 107A Hammersmith Bridge Road, London W6 9DA, UK; and Z.X. Guo, Department of Materials, Queen Mary and Westfield College, Mile End Road, London E1 4NS, UK. Contact e-m
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