P420ML2——Weldable fine-grain structural steels for pressure vessels

Usage ：These fine-grain structural steel grades are characterised by a minimum yield strength of 275-460 N/mm2, by good weldability and high resistance to brittle cracking. These grades are used above all for manufacturing vessels for pressurised gas (LPG, butane and propane tanks), pressure vessels, steam boiler parts, pressure piping, compressors ect.The user of these steel grades must make sure that his calculation, design and processing methods are appropriate for the material. The welding technique used must be suitable for the intended application and comply with the state-of-theart. The fine-grain strucutral steel grades rolled under normalised conditions offer good cold-forming and hot-forming properties without negatively affecting the material properties. These steel grades are therefore also suitable for cold-bordering and cold-flanging as well as for coldbending and cold-folding, considering the bending radii according to DIN 6935 for a steel grade of corresponding strength.With cold-forming above 5% or hot-forming outside of the temperature range of 850–1.000 °C as well as after excess times, the steels rolled under normalised conditions should be subsequently normalised at 900–950 °C. The holding time after temperature equalisation should be approx. 1 minute per millimetre of plate thickness.

Welding ：The fine-grain structural steel grades of this series can be perfectly welded both manually and using automatic equipment by means of all known welding processes. However, the quality of the weld joint depends on the welding process, the welding conditions and the selection of the correct filler metals. The welding wires and electrodes approved in this strength category must be used as filler metals. Basic coated elctrodes are recommended for manual welding. In general, pre-heating prior to welding or torch-cutting is not necessary. At outside temperature below +5 °C, preheating to approx. 150 °C should be carried out. Subsequent normalised or stress relief annealing with the grades rolled under normalised conditions is required only if called for in the regulations or if operating and manufacturing conditions make a reduction of the residual welding stresses appear useful. The thermo-mechanically rolled steels are not suitable for normalizing; stress relief annealing is possible. With stress relief annealing at 530–580 °C, the holding time should be 2 minutes per milimetre of plate thickness, not fall short of 30 minutes and not significantly exceed 60 minutes. Excessively high annealing temperatres or excessively long annealing times bear the risk of reducing the strength values to below the condition of delivery. In addition to this, the guidelines for processing these steel grades according to "Stahl-Eisen-Werkstoffblatt" (Iron and steel material sheet) 088 must be observed.