Application of High Tensile (E-550 Grade) Structural Plates for Built-up Columns and Beams in Boiler Structure

Abstract

In the past Boiler Structure has been designed using mainly carbon steel plates of Grade E250 or E350. This paper presents and analyses the application of High Tensile steel (E550 Grade as per IS: 2062) in Heavy structure like Boiler Supporting Steel Structure. Boiler supporting Structure consists of major weight contribution in Columns and Beams which are stress governed, the objective is to optimize the structural quantity through application of High Tensile Steel with higher yield stress. This paper describes the complete analytical approach adopted in analysis & design, conclusion and recommendation. Latest structural plate rolling technology provides excellent combination of high strength, Toughness and weldability for High Tensile Steel, these properties of steel plate are most critical mechanical properties for steel structure design. The key microstructural control is obtained in the plate rolling mills through on-line heat treatment immediately after accelerated cooling during Thermo-Mechanical Controlled Processing (TMCP). Thermo-Mechanical Controlled Processing technology is recent development applied in latest steel structural plate rolling mills for rolling with thickness up to 120mm. Niobium (Nb) as a micro alloy in the steel enables to achieve a substantial grain refinement, very fine-grained microstructure improves toughness and increases the yield strength. The TMCP process enables the required tensile properties in structural plates with leaner chemical composition. Leaner chemical composition, especially lower carbon content and lower carbon equivalents improves weldability. Improved weldability benefits the fabricator for fabrication of plate fabricated built-up profiles which is the main requirement in Heavy Structure like Boiler Supporting Steel Structure. Detailed analysis and design of typical 660 MW coal fired Boiler structure has resulted in 25% structural weight saving in case of High Tensile steel application (E550 Grade) as compared with commonly used carbon steel (E250/E350 Grade). Also, this analysis concludes that use of high tensile steel should be limited to Columns and Beams only which are stress governed and where yield strength is the main design criteria.