Tensile & Yield Mins Calculated from Tested Brinell hardness - not physical testing - Acceptable 2

[65CCJ]

Has anyone ever used heat-treated steel certification having Tensile and Yield strength minimums calculated from the tested Brinell hardness value rather than a physical strength test? Is this accurate enough to say steel meets the certification minimums if the conversion meets the min?
The material specs required on the drawing are:
AISI 4340 with tensile, yield, and hardness callout.
TENSILE: 142200 PSI MIN.
YIELD: 127980 PIS MIN.
HARDNESS: 290-340 HB

Is it acceptable practice to calculate the values from the hardness?

 

[weldstan]

Yield strength cannot be calculated/estimated fromm BHN test values.

 

[weldstan]

Also the hardness only tests the surface not the through thickness strength.

 

[mrfailure]

Note too that tensile conversion from hardness per ASTM A370 is considered an estimate, not an absolute. For that reason, I never certified in such circumstances unless testing both tensile and hardness samples.

 

[EdStainless]

Nope, the only specs that I know of that allow something similar are for special products like some wire.
If the spec has tensile requirements then tensile testing is required.

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P.E. Metallurgy, consulting work welcomed

 

[MagBen]

I ever met with one or two spec that allows to convert hardness to tensile for some fine bar or wire product and very thin gaged strip product where tensile testing is challenging. But of course, it is based on mutual agreement, not on a general industrial standard. "better than nothing" rule applied i guess.

Even when converting to tensile, the accuracy depends heavily on materials and the material conditions (e.g. annealed vs heavily cold worked).

 

[mrfailure]

As a practical matter, I often have to convert Vickers hardness to tensile strength for steel springs because tensile testing cannot be performed. This is used for characterization but of course not certification.

 

[65CCJ]

This website had good info regarding using hardness to determine mechanical properties. [URL unfurl="true"]https://materion.com/about/new-at-materion/hardness-and-strength[/url]

Hardness is shown to correlate well with tensile strength but only very loosely to yield strength and ductility. For steel, hardness and tensile strength are roughly proportional (see ASTM A 370-68 Steel Tables). The drawing that generated my question was for a small finished part where checking hardness was the only way to determine if the heat treatment was performed properly. I learned that we have always used hardness to verify the tensile for this particular part. As mrfailure wrote, converting hardness to tensile is a practical matter when tensile testing cannot be performed.

 

[weldstan]

For finished parts, hardness testing is often used to assure mechanical tensile properties have been met. For what you are planning, it is an excellent test to assure part integrity.

 

[mfgenggear]

on AISI 4340 Steel using AMS specification the material has to be certified by the specification. which includes hardness and tensile test.
if additional tensile test are required during heat treat, sometime tensile are manufactured from any certified AISI 4340. or if it is required from the same material it must be specified
on the engineering drawing or heat treat specification. tensile must be manufactured from the same material as the parts and are manufactured from the raw stock.
if that can not be done permission must be give to use any certified material.

 

[mfgenggear]

also the hardness is always specified on aerospace parts, and hardness is inspected 100% at heat treat if the geometry allows, if geometry is not conducive again test samples must be used.

 

[mrfailure]

I do wonder why the committees developing standards do not also specify hardness if the finished product would be impractical to tensile test. For example, wire specifications usually only call out tensile properties but springs made from the wire usually have to be microhardness tested unless they are really large. That said, I do find the hardness conversion for springs anyway does show excellent correlation.

 

[MagBen]

@mrfailure: no surprise to see excellent correlation (empirical equation: HV = 3 UTS) for springs. but for fully annealed materials, especially for coarse-grained materials, the relationships are quite different. The main reason is the work-hardening effect from the hardness indentation. This work-hardened effect is trivial compared with already hardened springs. In this case HV and yield strength still have a good relationship because yield is actually very close UTS. For fully annealed material, the yield occurs before work hardening, the additional work hardening caused by the indentation will likely lead to HV>3yield strength, while the less working hardening from indentation than that for tensile will lead to HV < 3 UTS.