Selection of fasteners for atmospheric pressure systems
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For atmospheric pressure systems such as air ducts and flue ducts, how should fasteners be selected? I see that Q235-B is sometimes required; are there any other materials available as alternatives? (Should the bolts and nuts be made of the same material?) What grade is generally used, and what are the common technical requirements? Is it better to use single-headed or double-headed bolts, or hex head bolts?
Look for the pipeline material grade table; it contains detailed instructions on the use of fasteners for pipelines at various pressure levels
For atmospheric pressure systems, such as air ducts and flue ducts, the standard pipe grade tables are not included:o
According to HG20592, bolts are only suitable for pressures up to 1.6 MPa; above 1.6 MPa, double-headed bolts must be used. Moreover, when selecting the material, the grade of the nut should be one lower than that of the bolt or stud. Materials such as Q235A/B and 8.8/8.0 are commonly used for medium and low-pressure pipelines; sometimes 30# and 40# steel is also employed. Inside equipment, stainless steel grades such as 304, 316L, or 2522 are used as well. It is estimated that processes like those using acetic acid may require Ti materials. For ordinary medium and low-pressure pipelines, Q235 and 8.8/8.0 are sufficient. I have only a superficial understanding; I hope the teachers can offer some guidance.
For normal-pressure systems, Q235 material is sufficient. The bolt and nut are made of the same material, either a single-head bolt or a hex head bolt. Using double-headed bolts on-site is sometimes done for ease of installation, allowing both sides to be tightened simultaneously.
Question: ① It is said that bolts and nuts should have different hardness levels, with the stud having a higher hardness than the nut. Is it mandatory, necessary, or is there room for choice? Q ② Regarding the Q235 material, it is said that the standard for Q235A has been revoked – can it still be used? ； Is everyone still using it: Q