All alloys which, due to their properties and characteristics, are specifically suitable for the construction of containers for the transport and storage of pressurized liquids and gases are identified as boiler quality steels.
These products require special materials, such as P355 steel, which tolerate a substantial difference between the internal and the external pressure of the container and which have considerable resistance to impact and corrosion.
Boiler quality steels must be selected so that they conform with the standards defined by strict directives such as PED (in Europe and a few other countries) and ASME (mainly applied in the United States and Canada). Manufacturers must in fact observe the parameters indicated starting immediately from the choice of the raw material and continuing up to the passing of the final test, which certifies its safety of use.
The different types of P355 steel and other boiler quality steels, such as A516 (A516.60 and A516.70) and SA 516 (SA 516.60 and SA 516.70) according to the ASME classification, are used for the construction of pressure tanks and vessels, heat exchangers and components required for storage under pressure, such as valves.
P355 steel can be further classified into p355nl1, p355nl2, p355nh, P355gh and p355nh. The additional letters that follow the nomenclature indicate the use of the alloy at high (nl) and low (nh) temperatures.
The P355ngh and P355nh steels are used in particular in the petrochemical industry for the construction of pressure tanks, heat exchangers and piping, which must tolerate high temperatures in service, for example for the transport of hot liquids.
The P355nl steels on the other hand are suitable for constructing pressure vessels to be used in environments characterised by low temperatures. The difference between P355nl1 and P355nl2 steels lies in the temperature level tolerated, lower in the latter.
From the point of view of mechanical properties, these steels are equal in terms of tensile strength and elongation, while the yield strength changes (slightly higher in P355NH steel). This difference has an effect on the load capacity of the individual alloys belonging to this class and, consequently, on the possible applications.
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