What kind of bolts and nuts are used for stainless steel flanges?
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Hello, everyone in this forum. Regarding stainless steel flanges, if carbon steel bolts are used, it will cause electrochemical corrosion, right? And if stainless steel bolts are used, will their strength be insufficient? What are everyone’s thoughts on this?
Carbon steel bolts should work; moreover, a loose-flange can have a stainless steel ring welded to it on top of the carbon steel flange.
The strength of bolts made of 35CrMOA can ensure the required performance. For corrosion protection, galvanizing or blackening can be used; galvanizing is more expensive, while blackening is generally sufficient in most cases!
Meets strength requirements, meets user demands!
Generally, 35CrMo is chosen; blueing treatment is sufficient. If there is a risk of leakage in acidic environments, it is recommended to use stainless steel bolts such as A193 B7 and B8
It is selected based on pressure; it is actually available on HG20592 as well. High-strength bolts are generally used, and to prevent electrochemical corrosion, the surfaces of the bolts and nuts can be coated, such as by spraying PTFE.
The strength of bolts and nuts made of stainless steel can meet the requirements
When the flange is exposed to high operating temperatures, care must be taken when selecting materials with large differences in expansion coefficients, as these differences can lead to issues related to expansion forces. For example: 1. In a combination of stainless steel flanges and carbon steel (or low-CrMo steel) bolts without insulation, the stress on the bolts increases during heating, exceeding the intended preload level; this results in increased deformation of the flange and stresses on the bolts reaching their limits. 2. In a combination of stainless steel flanges and carbon steel (or low-CrMo steel) bolts with insulation, during shutdowns after prolonged operation, the flange cools down faster than the bolts, causing the flange to contract more than the bolts, which can lead to leaks. Some preventive measures need to be taken, such as reducing the pressure first and removing the external insulation. Different types of bolts should be used for stainless steel flanges; as long as the appropriate measures are taken to ensure the required pre-tensioning force, it is feasible.
B8/8 and B8M/8M can be used, but temperature still matters
If the pressure is relatively low, PN1.6MPa allows for the use of A2-70 (which is an austenitic stainless steel)