Bolts Grade

[nisha1980]

Hi

I have specified Hilti HSC-I safety anchors to fix structural door frame to the concrete wall. the grade of the bolts are specified as min 8.8/S bolts.these will be zinc plated countersunk bolts and still exposed. .

they advised they will use 12.9 grade bolts . My question is 12.9 the high tensile bolts is ok in terms of strength (UTS is 1200Mpa ). But the type of application usually use for mechanical assembly .

these property class 12.9 are susceptible to hydrogen pick up, possibly leading to delayed brittle fracture . hydrogen pick up can occur from pickling process used in galvanising or from rust formation in service. Specialist advise should be sought before specifying these bolts . and these are precision bolts and suitable for mechanical assembly. Use as fitted bolts ( from steel designers manual)

We do not use galvanising option , we can go with Zinc plated one.

Please let me know if you know about this and can we accept to use 12.9 grade bolts without considering the application?


Thanks in Advance

 

[hokie66]

What do you mean by "structural door frame"? I doubt there is a reason not to use the 12.9 bolts, but if this is exterior, you might want to reconsider the plated bolts.

 

[nisha1980]

Thanks for your reply . this is steel door frame and is in exterior . we use Zinc plated countersunk socket head screws of grade 12.9. We do not galvanised ones. .


I am having a doubt zinc plating will promote the risk of hydrogen embrittlement in steel . And also will it lead to delayed brittle fracture with rust formation

Thanks

 

[nisha1980]

HI ALL

can anyone give me a reply whether this substitution (use of 12.9 instead of 8.8) is acceptable or not ?

Thanks in Advance

 

[nisha1980]

12.9 bolts have a high ultimate tensile strength but a low ductility (they're more brittle) . IS THIS TRUE?

 

[hokie66]

Yes, that last statement is true. Whether it is important in your situation, I'm not sure.