Grain size affects yield strength

[BluntCut MetalWorks]

How grain size affects yield strength could be a useful fact to know/aware of. It won't give you definitive yield strength value for particular specimen but will provide a reasonable value within empirical range. In simplistic view...

ref: http://en.wikipedia.org/wiki/Grain_boundary_strengthening

Key formula (sigma_y = sigma_zero + Ky / sqrt(d)), where sigma_y = yield strength (YS)

Example 1: Mild Steel
Using Hall-Petch constants table (on wiki page) (sigma_zero=70MPa, k = 0.74 MPa sqrt(meter)) and compute YS for grain diameter of 100um vs 1um

YS_100 = 70 + 0.74 / ((100)^-6)^1/2) = 70 + 0.74 * 316 = 304MPa
YS_1 = 70 + 0.74 / ((1)^-6)^1/2) = 70 + 0.74 * 1000 = 810MPa (~117K psi) <= 166% stronger than sample with grain diameter = 100um
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Example 2: 52100 at 60-67rc range, where known YS_15 (grain diameter ~15um) is 2000MPa
I couldn't find concrete sigma_zero & k values but worked backward with very conservative estimated values: sigma_zero =~1500, k = ~2

YS_1 = 1500 + 2 / ((1)^-6)^1/2) = 1500 + 2 * 1000 = 3500MPa <= 75% stronger 52100 with grain diameter=15um​

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Otoh, it could be a meaningless number in a larger scheme of knife-making.

 

[jdm61]

A complex way of describing what many of us have been told.

 

[BluntCut MetalWorks]

Please do share HP coefficient values for variety of steels & alloys. Hard to dig up these numbers so we can quantitatively figure out yield strength for certain grain size.

Maybe you or someone can shed insight (formula & coefficients) for plasticity affects by grain size? I am searching but only found vague references. :thumbup:Thanks.

A complex way of describing what many of us have been told.

 

[jdm61]

No, that is the COMPLICATED part. What we have been told is that small aus grain makes for tougher blades.

Please do share HP coefficient values for variety of steels & alloys. Hard to dig up these numbers so we can quantitatively figure out yield strength for certain grain size.

Maybe you or someone can shed insight (formula & coefficients) for plasticity affects by grain size? I am searching but only found vague references. :thumbup:Thanks.

 

[tiguy7]

That must be why I like powdered steels.

 

[BluntCut MetalWorks]

Touche'

Well, not a surprise, I don't listen very well. To see for myself. My tests indicated strength gained but loss quite a bit of impact toughness - still very chippy at 60rc. Either that or my samples are bad I will do some more reverse-chop (stick of seasoned oak whacks the edge) at 58rc zero grinded paring knife to see what happen...

No, that is the COMPLICATED part. What we have been told is that small aus grain makes for tougher blades.

You betcha - hard to lose when outcome assure winning :thumbup: HSS grain around ~12um, Cold PM low carbon grain can be as low as 7um. Nifty awesome for heavy tonnage of carbide volume.

That must be why I like powdered steels.

edit: at 58rc, test blade is still very stiff & brittle. I can cleanly plier small chunks off the edge and clean break the tip with a vise. Yep, nice crunchy sound, didn't set from bend at any point at all. Tomorrow, I will temper the remain of this 52100 blade at 600F and then 650F after that. Go back to my 3rd post, exact reason why I asked for plasticity affects by grain size

 

[james terrio]

*never mind*