PRODUCTION • PROCESSING • HANDLING
Table 2. Comparison of commonly used materials for storage tanks with equivalent TMCP steels
elements for TMCP compared to normalised steels. Tis is based on the Hall-Petch relationship, which provides an improvement of strength and also toughness if the grain size of a microstructure gets reduced. Te combination of a fine-grained microstructure and a very lean chemical analysis, which is typical for TMCP steels, leads to several additional advantages when it comes to further processing of the plates. Subsequently the advantages for different production steps are illustrated.
ADVANTAGES OF TMCP PLATES DURING FABRICATION
Te first step during fabrication of storage tanks is the cutting and weld edge preparation of the delivered plates. Due to their lean chemical analysis, TMCP steels show only a very low susceptibility to hardness increase. Tis means there is no need for any preheating before the thermal cutting process.
Te second step during fabrication of storage tanks is the bending of the shell plates. Like all other production steps (especially welding), bending shifts the impact transition temperature of the material to higher temperatures, due to the deformation rate after cold forming. Tis is equivalent to a reduction of the material’s toughness. TMCP plates provide a significantly higher toughness level and a lower impact transition temperature than normalised steels (Fig. 4). Te advantage for the fabricator is a safer production process arising from the higher toughness reserves and lower susceptibility to brittle fracture.
THE WELDING ADVANTAGE Te most challenging process during the fabrication of storage tanks is the welding of the bended shell
Te advantage of TMCP steels becomes clear when the formula is plotted into a diagram for different steels (Fig. 5). Te diagram shows that a thermo-mechanically rolled steel with a yield strength of 355 MPa, such as SA841-A-1 or P355ML2 (which are equivalent to normalised SA516-70 or SA537-1) can be easily welded with no preheating up to 40mm plate thickness, while the normalised grades would need a preheating in the range of 60-100°C. Furthermore, a TMCP steel with a yield strength of 420 MPa, such as SA841-B-2 or P420ML2, which are equivalent to quenched and tempered SA537- 2, might also be welded with no preheating up to 40mm plate thickness, if the hydrogen and heat input are carefully controlled. Te benefit for the fabricator is that no preheating before welding becomes necessary, which results in a faster fabrication, less energy consumption and lower costs. Tis advantage of TMCP in comparison to normalised steels arises from their lean chemistry, which is represented by the CET value in the given equation.
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segments. Generally this production step consists of three separate processes, which are preheating, welding and PWHT. For normalised material it is usually mandatory to
carry out a preheating to avoid excessive hardening and cold cracking after welding. Regarding the European codes EN1011-2 gives the following formula to calculate the necessary preheating temperature:
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