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?

 

[EdStainless]

The hardness is often used later in process as a quick check, having the reference to the original is helpful.
Hardness testing is the least expensive test that there is.
The strength is limited by the elongation requirements in tensile testing. If you go too strong it will not pass the ductility .

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[TugboatEng]

On a second note, hardness only measures the strength on the surface. Especially at larger diameters hardness can vary quite a bit from the surface to the core due to issues with quenching.

 

[IRstuff]

There are treatments specifically for increasing surface hardness while leaving bulk properties unchanged.

https://en.wikipedia.org/wiki/Case-hardening

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[weldstan]

Brinnel hardness is proportiona to UTS, not yield strength. And will you tensile test each forging; for example, which the spec does not require or perform a hardness test. which is simple, far cheaper and easy to do.

 

For many materials the hardness limit in ASTM standards is useless or superfluous.

Fun fact: hardness is not a material property

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

 

[SteveHooker]

Tensile testing is performed on a test coupon representative of the product, but not necessarily on the product itself. Hardness test limits normally apply to the product. In a perfect world the tensile test does represent the product, but that is not always the case. This is evident on castings where the cooling rates of the test coupon and the product do not always match.

 

[BenMacier]

It is shocking if not scary to read some of the answers to this initial post.
I imagine people thinking they can't hurt anyone by posting something here, until the person follows a dangerous advice and gets hurt or injured someone else.

If you are welding this material, you will want to check its hardness because most welding processes will increase it locally.

* Finding a solution is great * Knowing how to implement it is fantastic * Believing it is the only one and best is naive ?

 

[kingnero]

Nothing shocking or scary here.
The OP is asking about material specs, you bring welding into the equation.

 

On a second note, hardness only measures the strength on the surface.

Hardness [testing] only measures the hardness on the surface.


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

 

[EdStainless]

In many metals that we work with the hardness and UTS are tightly correlated.
And in many alloys that is not true.
As they say YMMV.

Hardness is just fast and easy. On a large job we may do 6 tensile tests and 200 hardness checks.
You use the tool that fits.

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[mrfailure]

To ensure proper heat treatment and tempering, many material specification specify minimum yield and tensile strength and maximum hardness. This is actually very common (for example, in ASTM specifications like A540).

 

[romychaj]

"Fun fact: hardness is not a material property":

why not? what is "material property" in this sence?

 

[david339933]

It is shocking if not scary to read some of the answers to this initial post.

Give me a break......

 

In many metals that we work with the hardness and UTS are tightly correlated.

It is not that 'tight' at all, and it certainly not a law of nature. It is a best-fit relationship that permits approximation. I will accept hardness as a substitute for a tensile test only if the result meets the spec PLUS the estimated error in the H-UTS relationship.

It gets worse with ASTM E140, which labs apply willy-nilly without declaring it, and which the amateur metallurgists I am surrounded by think is something that is absolutely precise (these are the same engineers who will measure things to 0.00001" precision using everyday tools).

I am constantly butting heads with adult engineers who do not understand the inherent and practical messiness of metallurgical processes, especially welding.

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

 

Uncalled for, david339933

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

 

romychaj,

I refer you to George Vander Voort.

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

 

[romychaj]

thank you. which of his 300 publications would you suggest?

 

[EdStainless]

Yes, many people don't understand the accuracy and precision of hardness testing.
Look at your reference blocks.
See that they are often marked +/-2.5
I don't care if your machine reads 35.6, you can't know the hardness with that level of accuracy.

If you have a material that is single phase, fine grained, and has uniform properties through the section; and you measure hardness with a large load/large indenter method then you will find hardness correlating very well with UTS. Every deviation from these criteria will introduce more error into the correlation.

Hardness isn't a property in the sense that it can't be defined by absolute measures (like UTS or elongation at fracture). The same can be said for yield strength.
These are engineering constructions that correlate very well with information that we can use.

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I don't care if your machine reads 35.6

That's what I call the Digital Delusion ... trained people who should know better will believe (and report) anything and everything they see on a digital display. PMI instruments are a case in point.

Like having a digital watch having a display showing hundredths of a second will help you get to that meeting on time, or a digital weigh scale will help you with your diet. It goes on and on, at work and at home.


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