Strain strengthening equipment: must the thickness of the cylinder heads be identical?
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For strain-hardening equipment, is it necessary for the thickness of the cylinder heads to be consistent? Does ASME VIII-1 Appendix 44, item 44-2(e), which states that “the shell and heads shall have a uniform normal thickness,” mean that “for strain-hardening equipment, the thickness of the cylinder heads should be the same”? However, this requirement is not specified in the draft version of GB/T18442.7; some domestic manufacturers follow it while others do not. What is the reason for this?
The theoretical basis for strain strengthening is to increase the tensile strength of a material through strain; it is best for all parts of the container to have consistent thicknesses and to be continuously connected. If the thickness is discontinuous, it will result in partial strengthening and partial lack of strengthening. The instructor mentioned this issue during the strain strengthening technology training.
However, the draft standard GB/T18442.7 does not specify that the nominal thickness of the head should be the same as that of the cylinder; moreover, the head itself exerts a constraint on the cylinder, preventing the cylinder section connected to the head from being fully strengthened. In practice, when measuring strain, manufacturers do not take readings at the section of the cylinder connected to the head that is closest to it; as a result, the head and the cylinder section adjacent to it are not fully strengthened. This is true unless it is a spherical tank.
That makes sense; strain strengthening technology is still in the experimental research stage in our country, with not many manufacturers carrying out actual production, and experience is being accumulated gradually.
It is not necessary; only consistency in the cylinder body is required, while there is no requirement for the cylinder body and the head to have the same thickness.
So how should we understand the provisions of the ASME standards?
In fact, many manufacturing plants are using this technology in equipment production; there are sling pads on the cylinder body, and that area is not reinforced either, so it often fails when stress assessment is conducted. Yet many manufacturing plants use 273 MPa*3, which is quite aggressive
You can understand it as 1: the thicknesses between different tube sections are the same, and 2: the thicknesses of the different plates making up the end caps are the same.