Silicon Bronze Nuts 1

[jay165]

Our current specifications call for Si-Br nuts made out of ASTM Alloy 651 (UNS 65100) per ASTM F467. Per ASTM, the nuts must be cold formed and have a minimum hardness of 75 Rockwell B. We were told by our supplier and the leading manufacturer that we are the only entity that requires these type of nuts. We would like to revise our specifications to allow Alloy 655 (UNS C65500) hot formed nuts per ASTM F467 (the same material as the stud bolts). These nuts are approximately 1/3 to 1/10th the cost of the 651 nuts. We are about to purchase the bolting for a large repair job that involves thousands of bolt and nut sets.

Does anyone have any experience with using either type of material and whether the added expense is justified?

 

[jay165]

I should add that these nuts are for Cu-Ni piping in seawater service.

Thanks

 

[unclesyd]

I’ve seen the same specifications that you are using on purchase orders. 651 nuts (F467) with 655 (468) studs or bolts. I haven’t heard of any problem with supply. As you stated they maybe a price differential that warrants a closer look. I’ll check on this Monday with my good buddy who went fishing this weekend without me.
One of the trade names for these materials are low silicon bronze (651) and high silicon bronze (655).

I would be quite interesting to know who developed this specification. May I ask why you are changing out the bolting?

I have several galvanic series tables and nowhere are the alloys compared to the Cu-Ni series of alloys. I’ll have to leave some messages to see if I can get some better data. I don’t know whether the purpose of the cold forming is to get the hardness of the nut closer too but less than the hardness of the stud or they may be a coupling problem. Still more digging.

http://www.copper.org

is a very good source of information on copper based alloys.

 

[jay165]

unclesyd:

Thanks for your reply and appreciate your efforts.

We are changing out the bolting because the original bolts are teflon coated carbon steel and are rusting away to nothing. Don't know why they went with carbon steel except for cost. Turns out the nuts are what drove the price up. We were told the manufacturing process for alloy 651 nuts is more expensive than 655.

The service is inside ballast tanks on a large semi submersble structure in the Gulf of Mexico. The bolts are submerged in seawater, at least part of the time and are in moist seawater laden air the rest of the time. They are isolated electrically from anode protection inside the ballast tank - so the corrosion rate is extremely high.

Can't get an answer as to why the higher grade material for the nuts was put in the specs, either. Thought it was maybe a problem with stripping threads, but was told that was not a problem, even when "unspecified" Si-Br nuts were used on earlier vessels.

Thanks again!

 

[unclesyd]

My connection at the nut house says that he now quotes all Si-Br as per ASTM F467 655 nuts with ASTM F468 655 studs or bolts. He says that he is 99% sure all these fasteners are destined for offshore use. At onetime he was quoting both ways 651/655 and 655/655. I don’t know how the industry classifies these materials. A lot of time in the fastener industry high grade equates to availability and what’s in stock.

Being outside the umbrella of cathodic protection would be one of the concerns as you mention and like I stated I don’t have data on the potential between Si-Br/Cu-Ni. It the same vane, if someone was worrying about there being a problem in this area they would have specified Cu-Ni fasteners, except for the cost

The only problem is unspecified has a lot of meanings. If you had a sample a quick analysis would tell a lot. You might check your purchase orders and see what was supplied as non-spec.

I don’t think it’s a mechanical consideration as having nut and bolt or stud of equal hardness gh it isn’t the most desirable. Though in screws there is marked difference in high and low Si-Br.

Things to remember about fasteners is quantity, and lead time are very important in fastener procurement especially in alloys. No one has the inventory of 10 yrs ago. Source is another can of worms.

Let me do a little more checking.

 

[unclesyd]

Sorry for the delay.

As you know the root of your problem is the F467 specs for the nut. When you specify the hardness of 75 Rb that immediately drives the price up as it takes cold forming to get the numbers. They don’t tell you but a lot depends on the starting material or how the manufacturer handles it. You could use a proof load test instead of the hardness values. I been told that there is no problem meeting the proof load test.
Most purchase orders now only call out either 651/655 or the 655/655 nut and bolt combinations without the F467/F468 specs. Or with only the F468 spec. The nuts are of commercial quality and either 651 or 655 is supplied. Even if your are required to use F467 nuts I see no problem with using 655 instead of 651. These call outs were for very similar applications.

Who has jurisdiction over the equipment and what determined the call out of your specification?
I have a very good idea what your system is for and I can’t find the jurisdiction.

Have you done an analysis of the piping system to determine what’s needed for the connection?

Looking at the prices of Cu-Ni makes the Cu-Si look cheap. Glad the strike is over.

If allowed by both the jurisdiction and analysis of the system, I would seriously think about calling out F468 655 studs with commercial nuts of either 651/655. Get a quote on a set this way and compare. There other suppliers of the fasteners get you supplier to get quotes from others. Give as much lead time as possible.

Two of my “underwater sharecropper” friends say the use 651/655 interchangeably. Neither remembers seeing any specs on the bag of bolts.

If we keep reasearching this problem I think we will meet.

 

[jay165]

Syd:

Thanks for your reply and your info. We will ask for quotes for both types of nuts and decide if it's worth the difference. The feedback I'm getting is that the price is more dependent on timing and available stock.

Thanks again for your help!

Jay