No correlation between hardness and ultimate tensile strength !
No correlation between hardness and ultimate tensile strength !
(OP)
I have a metallurgical problem : when I take different parts from a steel bloc i don't found correlation between hardness (HB) and ultimate tensile strength (UTS)
How can I explain this ?
How can I explain this ?





RE: No correlation between hardness and ultimate tensile strength !
RE: No correlation between hardness and ultimate tensile strength !
Run the statistics, make sure to account for precision of each test, and you have good results.
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube
RE: No correlation between hardness and ultimate tensile strength !
The data as represented fits neatly in a band,do not expect to get a straight line relationship.
RE: No correlation between hardness and ultimate tensile strength !
RE: No correlation between hardness and ultimate tensile strength !
Explaining it is simple, stanislasdz. Hardness testing is a compressive test, tensile testing is a tensile test (surprisingly!). Compressive tests are pretty much (although not totally) unaffected by inherent flaws in the material. Tensile testing is affected.
Take a titanium aluminide as an extreme example. Compressive strength is huge, but the brittleness of the sample due to cracks formed on solidification causes the UTS to be rubbish.
I should point out that this is the TiAl one company I worked for made, the stuff can be very good, apparently.
RE: No correlation between hardness and ultimate tensile strength !
1) You mentioned 'different parts of the block' - metallic materials are rarely perfectly homogeneous in both composition and properties.
2) Tensle tests inherently have a certain amount of scatter.
3) Hardness tests have measurement error, especially if indentation size is visually measured. This increases at smaller indentation size; i.e. higher hardness (where this data is).
Hardness-UTS correlations (and hardness conversions) should always be taken with a large grain of NaCl. The many points collected to generate these curves have some scatter, and the curves are 'best-fit'.
Also, you are looking at data points within fairly small parts of the respective HB and UTS ranges. The scatter will be visually exagerrated.
RE: No correlation between hardness and ultimate tensile strength !
Thanks a lot Brimstoner !
RE: No correlation between hardness and ultimate tensile strength !
If this particular data set was plotted on log-log paper and extrapolated back to the origin, it would look like a smooth fit.
In the case of hardness vs UTS, the UTS test data is usually from a sample that is long enought to fit in a test rig, while the hardness test zone is only a surface phenomena . Considering how a material is cooled down following N+T , the outer surface zone cools much faster than the inner zones, one would expect a different microstructure at the surface vs internal zones. Review a typical TTT plot and correlate that to a temp vs time vs depth thru the cooled part ( nased on a conductive heat transfer model) and see for yourself.