Super High Strength Bolts

[kkmj]

I have a bolted joint with M24 12.9 bolts, we've experienced some joint seperation in a few situations and are considering a higher strength bolt so we can apply more pre-load. Inconel 718 or MP35 seem to rise to the top of the list. This joint sees about 10000 cycles in saltwater in two years and is then is replaced. Any words to the wise about issues with these higher strength bolts?

 

[SJones]

EEMUA 194, Clause 9.2. For N07718, same requirements, essentially, as N06625. If CP is applied (and it will need to be as the pitting resistance equivalent is too low for seawater), attention will have to be paid to hardness restrictions: usually 35 HRC (but could go to 40 HRC with care). The material will gall and disassembly could be difficult. Have you investigated correct bolt torquing procedures to improve tightness? MP35 has a high enough PREN for Northern latitude seawater - what temperatures are envisaged - that would preclude the need for CP and remove the risk of hydrogen embrittlement? I do not have any info as to the fatigue resistance of these materials.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[Riadsala]

Take care using 718 bolts. It is currently becoming very apparent that 718 is extremely sensitive to hydrogen embrittlement under CP. It is vital that there is no delta ferrite in the microstructure (follow API 6A718, however it may be necessary to tweak the heat treatment temperature and times to obtain hardnesses of less than 40 HRc). Preferably limit hardness to 35HRc for 718 bolting. It is then even more critical to ensure that the bolts are not over-torqued - that will lead to HE failure V. quickly (hours).

Riadsala

 

[arto]

Inco 686 is good in seawater, for crevice corrosion, etc