Evo nesto o vijcanim spojevima i na sto treba paziti.... jako nije uvijek najbolje:
C/P iz jednog pdf-a
In conclusion:• Hydrogen embrittlement, is to some degree, unpredictable and it is sensible to, whenever possible, specify fasteners that are, to a large degree, inherently less prone to thistype of failure. Experience indicates that if property class 12.9 fasteners are electroplated, measures can be taken to reduce, but not fully eliminate, the risk from hydrogen embrittlement. In some applications, the consequences of joint failure as a result of brittle fracture are such that a campaign change to replace all potentially defective fasteners from a product is deemed essential even when the failure rate is very low. Usually the extent of the problem is unknown when a decision needs to be taken whether to replace the fasteners. It may be possible to identify a batch of fasteners affected and reduce the extent of the campaign but this can often be problematic. Typically, the cost of the fasteners is minimal compared to the other costs involved in such a campaign. Considering that there are other valid options available, high strength fasteners (10.9 and 12.9) should not be electroplated. Where a fastener user speci es such a product it is crucial they recognise that it is not, and cannot be, risk free.• For most applications, property class 10.9 fasteners gives the best compromise between strength and brittle fracture risk. If you are a fastener manufacturer, consider limiting the core hardness of property class 10.9 fasteners to HRC 36 (the permitted range in the standard is HRC 33 to HRC 39) to further reduce the risk of brittle fracture.• On 10.9 fasteners use zinc ake type coatings (Geomet, Delta Protekt, etc) or mechanical zinc plating instead of electroplating.In my experience 12.9 socket head cap screws are frequently used in the UK particularly, not because such strength is needed in the application, but because 8.8 or 10.9 socket head cap screws are not readily available. The 12.9 fasteners arethen tightened to a torque value applicable for an 8.8 fastener. There is no technical advantage in using a property class 12.9 fastener when it is tightened to a level that a lower strength fastener could sustain. (For example the fatigue endurance strength of a typical 12.9 fastener is the same as that for a 10.9 or an 8.8 fastener.) Hence, using a 12.9 fastener and tightening it to a torque value suitable for an 8.8 fastener does not reduce the risk of failure when compared to using an 8.8 fastener. A property class 12.9 is susceptible to HE and SCC problems given the right conditions. This risk is not widely appreciated by users of fasteners.Stronger fasteners are certainly not necessarily better in some circumstances.
