When To Replace Bolts?

In case the bolts are under stress (ie torqued to recommended value). You should replace it every time you unscrew them (since at the recommended prestress you are usually between 60 to 75% of yield and thereby too near the plastic stage)

But as mentioned it is case (or rather pre-load) dependant.

Bolts are designed with such factor of safety with respect to allowable stress, It need not to be replaced till its failure. However in case of high temperature service, even if proper grade of bolts are used and temperature correction for allowable stress is not known then it is better to replace bolts say 50% or in worst case 33% on flange breakage.

You can reuse the same dismantled bolt if its length remains the same.

It means it has been stressed in the elastic zone only.

Of course you sould look at its surface status and replace it if it suffered corrosion.

Breaking a flange covers almost as much territory as breaking a joint. There are flanges that utilize studs versus bolts, so you pretty much cover any size or bolting configuration when you pose this question.

In the case of studs I have seen instances of the female threads pulling out of the base metal. The last time I saw this was on a set of control valve flanges that operated at 3600 PSI. These flanges had obviously been subjected to higher than design torque values. If the bolting had been replaced at the last opening the results would have been the same. The situation is further complicated by the fact that in the case of these valves an NDE is performed at every major outage. My limited knowledge of NDE leads me to believe that this particular indication would have had to be of particular interest to be noticed.

Getting back to the question of a routine replacement schedule, you have to realize that a general question involving a bolt that might cost 30 to 50 USD is a lot easier to answer than about bolts and studs that can cost in the range of 7,000 to 10,000 each.

My personal experience has been that bolting that is either expensive or critical are covered by the OEM, who will either recommend a blanket replacement schedule or adequate NDE recommendations (usually UT and mag part.) I don't think you are going to get much for guidance on the small stuff.

Any good craftsman will examine all the hardware that is part of his task and make recommendations where necessary to replace obviously flawed material. Before I would make a blanket policy of replacing all bolting hardware at every outage I would ask myself, am I willing to delay an outage to accomplish this? If I am not willing to take that action am I wasting my companies money?

In regards to the question asked by dsamples, there are many different strengths and torque requirements for fasteners across the board. If these are automotive fasteners, each orginazation has their own internal specifications for the replacement of fasteners. If there are interference threads on the bolts, then these bolts should be discarded and replaced after every use due to the loss of prevailing torque upon removal. Of course, if there is evidence of stress cracks or thread laps, then the strength has been compromised, and they should certainly be replaced. There are many unseen defects such as microstructure anomilies that cannot be detected without specialized equipment and processes, however, the old adage 'when in doubt, throw it out' is a safe guideline to be used.

dsamples,

A replacement schedule depends on a number of factors.

If the fasteners have been used in high-temperature applications, and unless you have the capability to test a representative few from the batch, it's a good idea to replace old with new since there may be a risk of hydrogen embrittlement. High temperature creep should also be considered but isn't as serious as long as the fastener's reference length is updated in the database.

If your company/facility does not employ QA or other means of control when bolting critical flanges, yes, the fasteners should be replaced after every disassembly. This should be done because you would have no idea whether the fasteners had been tightened properly. If they were too loose, fatigue failure may set in. If they were tight, they may have exceeded their yield point. In either case, there will be an increased risk of future failure if the same fasteners are used. Of course, if the cost of failure isn't high, you may wish to reuse the fasteners and deal with the joint separation when it occurs.

* please note that use of calibrated torque wrenches and following defined tightening patterns does not constitute QA or control!! On the contrary, it often leads to unexpected failure because it provides a dangerous false sense of security.