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Explosion bounded clad plates for vesselheads
According the European design codes vesselheads must be heat treated after cold forming due to the properties of the carbon steel. The ASME design code does not have this requirement (in most cases).
Heat treatment parameters (for example, temperature, soaking time) must be suitable for the carbon steel as well for the stainless steel clad layer and weld seam incase of large diameter heads. However in practice this will not always be considered or recognised as a problem. Mechanical and corrosion properties will change due to heat treatment.The questions is: are the corrosion properties of the explosion bounded plates taken into account after heat treatment by the pressure vessel part producer?
Explosion bounded clad plates are often a combination of carbon steel base material and stainless steel clad material which are both supplied with different heat treatment conditions and mechanical properties.
The carbon steel is chosen because of the mechanical properties and is commonly supplied in the normalised or normalised/tempered condition. The clad material is chosen because of the corrosion resistance properties and is commonly supplied in the solution annealed condition. After cold forming of vesselheads, a normalising heat treatment is necessary for the carbon steel when the extreme fibre elongation is more than 5percent from the rolled condition according most European design codes.
Carbides and inter metallic phases can originate in the stainless steel clad material during normalising heat treatment. Therefore the corrosion resistance of the stainless steel clad material will be less after the normalising heat treatment than the corrosion resistance in the delivered solution annealed condition. Do engineering companies take this into account?
Example
Material:
Base material:
WStE355 according DIN17102
Clad material: 6Mo
Thickness:
20mm base material + 3.5mm clad material
Delivery condition clad: Explosive bounded clad
Because the deformation by cold forming is more than 5percent a normalising heat treatment is required for the carbon steel. The carbon steel and clad steel materials are usually heat treated according different temperatures and parameters as described below:
WStE355 Normalised on 920–940°C and air cooled.
6 Mo
Quench annealed on 1150–1200°C and water cooled.
We have carried out two different heat treatments on the clad plate to investigate the influence on both materials.
Heat treatment 1:
Temperature: 1080°C
Soaking time: 20–25 minutes
Cooling medium: Air
Results:
–
Grain boundary precipitation's observed in stainless steel.
–
Corrosion test G48-92 Method A of stainless steel clad material.
Heat treatment 2: Quench annealed followed by tempering
Quench annealed:
Temperature: 1180°C
Soaking time: 10 minutes
Cooling medium: Water Cooling
Tempering:
Temperature: 580°C
Soaking time: 35 minutes
Medium: Air
Results:
– No grain boundary precipitation’s observed in stainless steel.
– Carbon steel: Tempered martensite structure.
Conclusion
In order to guarantee good quality vesselheads the supplied base and clad material should both be suitable for one heat treatment.
Enter 55 or at www.engineerlive.com/iog
Richard Hamersma is responsible for the Engineering Department and Project Management with Antonius Vesselheads BV, Maasbracht The Netherlands. www.antonius.nl
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