HV 0.05 hardness conversion

[blckwtr]

Is there a direct link between the value for HV 0.05 microhardness to the yield limit?

What can you say about the capacity in surface stress normal to the surface? No sliding...

 

[CosmoY]

blckwtr;

Sometimes the specimens will be too small to conduct the tensile test, so we often do hardness to estimate their mechanical property, fasteners especially.

Vickers hardness and tensile test all test the capacity of resisting to outer force.

So these is link with yield strength and hardness, I think.

 

[Goahead]

Microindentation hardness at that low load typically permits to test hardness of individual grains. In multiphased microstructures the result is not much informative for the whole area

http://www.welding-advisers.com/

 

[salmon2]

Yes, there is a correlation between hardness and tensile strength, both at macro scale.

Yes, you probably can not or should not correlate microhardness to macro strength, because the reason Goahead said and the indentation size effect (ISE) which means the smaller indentation, the bigger hardness read.

 

[TVP]

Hardness is not related to the "yield limit", but is related to the ultimate tensile strength. Search the Eng-Tips archives for more information.

 

[blckwtr]

The reason for asking, is that we have a part that is QPQ hardened, the material is AISI 4140 (ASTM A29). The part has a highly stressed surface (normal stress), but how much can tha surface withstand generally? Is it fair to say that the part can only withstand a stress level up to yield limit generally (on a macro level)? The part is 100 x 50 x 80 mm, and is loaded on the surface side 100 x 80 mm.

I don't know if I explained it very well, but I hope you know what I mean.

Summary; Is the part tougher to normal forces than a regular part in AISI 4140? How much can you say the surface can be loaded?

 

[dbooker630]

QPQ imparts wear resistance and corrosion protection. I would not expect improved impact or tensile strength as a result of the process but you should get improved fatigue life.

 

[CoryPad]

In general, localized normal stress must be three times the yield stress to initiate yielding. Surface hardening treatments like QPQ increase the local yield stress above the bulk/core yield stress, so you have an even higher resistance.

Regards,

Cory

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[blckwtr]

Hmm, that is interesting. I have heard that this value is closer to 2, but where can I find litterature on this specific subject?

 

[TVP]

CoryPad can provide references for the 3x value. Regarding the effect of QPQ treatment, it will depend somewhat on the depth of the diffusion zone. 4140 tends to form a shallow, but hard diffusion zone (maybe ~ 100-200 micrometers) which means that deformation may occur in the transition from diffusion zone to bulk material. This is a difficult phenomenon to quantify analytically, and typically requires physical/mechanical testing to validate.

 

[salmon2]

The qpq won't change the bulk properties, but only the thin layer of the surface which matters for wear/corrosion resistance. For your case wanting to do the stress analysis, I feel you should use the bulk properties instead of the <1 thousand layers' properties. If you agree to that, do a macro hardness, brinell or rockwell. There is correlation between them, ASTM E140. Or you have enough material for a sub-scale tensile test and you will get yield and tensile strength, but not elongation.

Roughly there are two formula to correlate hardness and ultimate tensile strength,
hardness = 3* strength //watch out the unit
or strength in psi = 500* brinell number

 

[salmon2]

The number for steel is 3. The number will be diffferent for different catogories of metal, such as steel or plastic, etc.

And I think it is coming from theoretical upper bound method or slip line method if I am not mistaken.

 

[CoryPad]

The factor of 3 I referred to (and the one salmon2 referred to) comes from slip-line field analysis of plane strain indentation coupled with the von Mises yield criterion. This is covered well in the book Metal Forming by Hosford and Caddell.

Regards,

Cory

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