6.3.1 Processes
Bladder accumulator shells shall be made from one of the following processes:
⎯ seamless tube manufactured by extrusion or other processes and subsequently hot or cold drawn,
⎯ billets, pierced hot or cold (back extruded) followed by hot and/or cold drawing, or
⎯ plates, cupped hot or cold followed by hot and/or cold drawing.
The bodies shall be made only by processes, including heat treatment, that have been shown to produce accumulator bodies that are free from cracks or other flaws.
Where closure of a tube is necessary in order to form a neck or a base, this shall be carried out by forging, cupping or spinning.
Ends shall not be welded on and metal shall not be added at any time during or after the closing operation. Ends may be machined to accept the required end fittings.
The chemical analysis of the material shall be recorded on material data sheets.
In the case of welded shells or end caps, refer to the requirements of EN 13445-4.
6.3.2 Heat treatment
Following completion of all forming operations except the machining of the end bores, each accumulator shell shall be uniformly heat-treated and the operations involving heating shall be carried out in temperature controlled furnaces.
Quenchants shall either be oils commonly used for this purpose or liquids with a similar quenching severity as oils. The choice of quenchant shall take into account the type of steel, size and thickness of accumulator shell and changes in profile and section.
The temperature of the bath, concentration of quenchant (where quenchants other than oil are used), time of immersion and the accumulator shell temperature upon withdrawal shall be controlled to ensure that all necessary metallurgical changes have been completed and that the possibility of quench cracking is minimized.
Where normalizing or tempering is involved, the accumulator shell shall be uniformly cooled in still air.
The heat treatment temperature and times shall be recorded.
6.3.3 Verification of mechanical properties 6.3.3.1 Tensile test
One test specimen shall be taken from every batch, except where the number in the batch exceeds 200 shells, in which case one sample shall be taken in every 201 or part thereof.
The test sample shall comprise either:
⎯ material from a finished accumulator shell, or
⎯ in the case of accumulator shells of large capacity (>300 mm diameter) where justification has been made in the Technical Documentation File that destructive testing is unnecessary, a sample shall be cut from one end of an accumulator shell prior to closure. The sample shall be in the form of a ring of sufficient size to provide the requisite test pieces, and subjected to a heat treatment such that its mechanical properties are representative of the accumulator shell in the batch.
The tensile specimen shall either be:
⎯ made from a strip cut longitudinally from a finished accumulator shell or test ring with its form or dimensions in accordance with EN 10002-1, or
⎯ if the dimensions of the accumulator body are such as to preclude the manufacture of a standard test specimen then a non-standard specimen may be used with the agreement of the notified body.
The cross-section shall either be formed by a portion of the wall of the accumulator body, or when the wall thickness permits, be circular with a diameter of not less than 7 mm for the central portion.
The test results shall comply with the requirements of the Technical Documentation File and be recorded.
NOTE A batch is defined as a quantity of up to 200 shells plus shells for destructive testing, of the same nominal diameter, thickness, length and design made from the same steel cast and subjected to the same heat treatment for the same duration of time.
6.3.3.2 Impact test
One test sample shall be taken from every batch, except where the number in the batch exceeds 200 shells, in which case one sample shall be taken in every 201 or part thereof.
The test sample shall comprise either:
⎯ material from a finished accumulator shell, or
⎯ in the case of accumulator shells of large capacity (>300 mm diameter) where it has been agreed that no destructive test shall be carried out, a sample shall be cut from one end of an accumulator shell prior to closure. The sample shall be in the form of a ring of sufficient size to provide the requisite test pieces, and subjected to a heat treatment such that its mechanical properties are representative of the accumulator shell in the batch.
Testing shall be carried out in accordance with EN 10045-1.
NOTE A batch is defined as a quantity of up to 200 shells plus shells for destructive testing, of the same nominal diameter, thickness, length and design made from the same steel cast and subjected to the same heat treatment for the same duration of time.
6.3.3.3 Post heat treatment hardness test
Each accumulator shell shall be subjected to a hardness test in accordance with EN ISO 6506-1, EN ISO 6506-2 and EN ISO 6506-3 in order to check the homogeneity of the heat treatment batch.
The test shall be carried out on the cylindrical portion of the accumulator body and the surface shall be prepared by light polishing. The wall thickness shall not be reduced to below the minimum calculated value for e (see Table 3.2-1).
6.3.4 Visual and ultrasonic examination 6.3.4.1 Visual examination
Each accumulator shell shall be visually examined for internal and external defects in accordance with the requirements of EN 13018. General indications on visual criteria can be found in EN 1968:2002, Annex C and used as a guideline. No defect shall be accepted.
In particular, each accumulator shell shall be examined for neck folds in the hemispherical region. If neck folds are visible, the shell shall be deemed not to comply with this European Standard.
6.3.4.2 Wall thickness ultrasonic examination
Each accumulator shell shall be ultrasonically examined (manually, semi-automatically, or automatically) to confirm the minimum wall thickness specified on the drawing, in accordance with the requirements of EN 1968:2002, Annex G, test sensitivity G.3.2 a). The thickness shall be recorded in the test report.
This ultrasonic examination is not necessary provided that the accumulator shells are made from seamless tube where the wall thickness has already been verified, and that the tube closure process is proven not to reduce the wall thickness.
NOTE Ultrasonic examination cannot replace the hydrostatic pressure test described in 7.5.
6.3.4.3 Surface defects ultrasonic examination
In the case of accumulator shells of large capacity (>300 mm diameter), each shell shall be ultrasonically examined (manually, semi-automatically, or automatically) for internal and external surface defects in accordance with the requirements of EN 1968:2002, Annex G, test sensitivity G.3.2 a).
Accumulator shells of smaller capacity (<300mm diameter) can also be ultrasonically examined for surface defects.
NOTE Ultrasonic examination cannot replace the hydrostatic pressure test described in section 7.5.
6.3.4.4 Repair
Surface imperfections of accumulator shells can be repaired by grinding or machining. It is important that the repaired areas are smoothly aligned with the shape of the shell.
Any surface imperfection in an accumulator shell, having a depth greater than 5 % of the minimum wall thickness, or a length greater than 25 mm, shall be repaired. General indications on repair criteria can be found in Annex C of EN 1968:2002 and used as a guideline.
After repair, wall thickness examination shall be repeated.
6.3.4.5 Minimum wall thickness
Surface imperfections of accumulator shells leading to a wall thickness below the minimum wall thickness specified on the drawing shall be considered as defects, and accumulator shells shall be deemed not to comply with this European Standard.