Why specify a maximum strength? 4

[bruv]

This has been the subject of some debate at work, and nobody seems to have an answer that makes sense.

In various ASTM specs, eg A216, A217, and A352, some steel alloys are specified with a maximum tensile strength (as well as the usual minimum). If all chemical, mechanical and other requirements have been met, what is the need for specifying a maximum strength?

Thanks

Chris

 

[CoryPad]

Reasons to specify maximum strength:

1) to allow for controlled deformation or fracture (ability to calculate force, area, or stress if two of the three are known)

2) to control the allowable flaw size, which is proportional to (K/YS)^2. For constant K, any increase in yield strength decreases the allowable flaw size significantly. Also, for a given material + heat treatment, K usually decreases with increasing YS, so the allowable flaw size decreasing even more.

 

[etch]

I once got asked this and my senior metallurgist said that it had no practical use. Now, apart from the above reasons, there is also other reason that i went out and sourced.


Maximum strength, i.e until fracture, has uses in safety applications, where it is favourable for one rather cheap part to fail rather than the more expensive piece. This could be a link in a chain, commonly seen on your bycicle, its designe to break before your chain snaps, thus saving you buying a new chain.

 

[lgearhart]

Chris, note that the specifications you cited are all castings, and that one is for fusion welded applications while the other two are for pressure containing applications. This adds a lot of credibility to Corypad's explanation regarding ductility concerns. I would also see it as a useful 'red flag', noting when something is not quite right, and should be looked into.

Good luck!

Lee