Why measure Hardness in addition to Strength? 3

[A_Alrumaidh]

Why do material specifications (ASTM/ASME) limit the maximum Hardness value, and not Yield Strength?

I know that steels with high strength -above a certain limit- indicate a microstructure that is more susceptible to cracking. So why not represent microstructure by Yield Strength? Both hardness and yield strength are proportional and related to plastic deformation.

In addition to the tensile test, asking for the hardness test is an additional cost. What additional information do we get by measuring hardness?

 

[romychaj]

you got me confused. are you calling uts and elongation that depend on temperature, speed, length, shape, (what else?) not to mention problems with attaching to the apparatus "absolute" in contrast to hardness? it seems wrong as hardness measurements are carefully adapted to the material in question. also uts (and total elongation) is not a very interesting property anyway. you must be talking about some particular definition of "material property". please advise on what it is.

is the statistical significance (e.g. variance) in hardness measurements lower than for uts, toughness, endurance limit or abrasion?

edit: "... mechanical properties, such as strength, hardness and toughness" (george van der vort).

 

[bobjustbob]

I am going to check the hardness on my batch regardless of if it is in the material spec or not. In some cases I check as quenched hardness prior to temper as in in process quality check. I can do that out on the floor as soon as the parts are cool. The tensile bars need to be processed by the lab, sent for machining and then tested. In our case that didn't happen until the next day unless I am expediting.

Hardness is also a way to check more material on the heat treat batch. I cannot pull a tensile bar off of each piece or even 10% of pieces when I am heat treating 40 tons of castings a day. I can take 10% of the load and do a brinell check.

 

[EdStainless]

When I pull a tensile, I can control the sample size and shape, the temperature, the speed, and other such variables.
If I take a lot of tube or wire and pull 20 tensiles I am basically measuring the repeatability of my equipment.
After all I am measuring a load. The UTS will repeat.
Not so with yield, it may be quite different. And it depends on how I choose to calculate it.
With hardness I can move from grain to grain on the sample and get different values. Are they all 'right' or none of them?
If I have a fine-grained material and a hardness test that covers a large enough are at least I get values that converge on the average.
The real underlying issues are you measuring something that has a calibration standard, and what is the scope (scale) compared to the structure of the material. Even microhardness tests cover very small areas, whereas tensile testing is averaging the entire cross section of the sample.
But an average what? Aren't all hardness tests based on some arbitrary selection of indenter and load?
There are even some properties with real physical definitions that are incredibly hard to measure, such as thermal conductivity.
Every technique yields a different result. This one is maddening.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

Distribution of alloy elements and impurities in the ingot is not homogenous.
Mechanical working of the ingot into shapes is not uniform.
Heat treatment is not uniform.
Processes like surface decarburization and precipitation of minor phases occur.
Sampling of test coupons is not representative of the entire material (e.g., keel blocks on castings).
Testing heavily involves human factors (e.g., determination of YS mentioned by EdS).
When it comes to evaluating materials with multiple structures (e.g. welds), expert knowledge and judgment are necessary to find the best version of the truth.

I'm sure I've left out some more obvious ones.

There's many a slip 'twixt the cup and the lip.



"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[weldstan]

When I worked in the Tin Mill over 50 years ago, I had occasion to perform a failure analysis on our product that failed to meet the cold forming of the production parts by our customer. After thoroughly reviewing the involved materials only the temper hardness could be defined as the cause. All of the hardness tests of the good material (no failures) and the failed materials were within the temper grade specified for the product. Material with hardness greater than 0.5 of the max superficial hardness for material where no failures occurred had a 25% failure rate. Material with hardness of 1 point above had a failure rate of over 80%. A 100% failure rate was seen when hardness was 1.5 points higher or greater. This was an important lesson learned - that minute changes in properties can have great impacts on materials' end uses.

 

[replica]

I think hardness value is a real issue when it is criteria for rejection. I think in this situation, Tensile strength should be a criteria for rejection....not the hardness.

 

[mfgenggear]

Gentlemen
It really depends on the material, heat treat requirements wall thickness.
The depth that a material will harden.
Low alloy steels like 4340 will have deep penetration of hardness.
In my type of projects carburizing and low alloys are very accurate with hardness test
When required tensile test can be of the same lot # of material and heat treat.
And parts are verified for 100% hardness and extra tensile test requirement. When it is required. On case harden steels a micro hardness of the case depth is very accurate. As well as the hardness on the surface. Si it really depends on the project and requirements.