charpy C-notch vs V-notch

[M Wadel]

how does these 2 correlate? i was having a look over at crucible and saw they measured one steel in C-notch and another company uses V-notch. ok lets say 2 steels both get 100J but one was C-notch and the other one V-notch, which is tougher? and how much tougher? can you calculate this?

bonusquestion: is charpy C-notch the same as U-notch?

 

[Larrin]

Charpy C-notch is used only by Crucible, it's numbers are pretty close to an unnotched charpy, but it ensures that it breaks in the same place every time, rather than at the most stressed point. The C-notch is the same depth as the V-notch but it is more of a round notch, and is much wider. They use C-notch because V-notch isn't very good for tool steels and stainless steels, because every steel would be like 1-5 ft. lbs. or even less.

Bonus question: C-notch and U-notch are different.

 

[M Wadel]

thanks for clearing that up

 

[Cliff Stamp]

how does these 2 correlate?

Not well, toughness ratios can change significantly when moving from one impact test to another because they have different fracture mechanics.

is charpy C-notch the same as U-notch?

They look like the letters, a U-notch is deeper than it is wide.

Charpy C-notch is used only by Crucible ...

These are standard impact tests not defined or solely used by Crucible, C,U,keyhole and prefracture, then there are also torsional and izod.

They use C-notch because V-notch isn't very good for tool steels and stainless steels, because every steel would be like 1-5 ft. lbs. or even less.

Tool steels have v-notch values up to 240 ft.lbs. Even the very brittle steels like T15 have v-notch values of 3-4 ft.lbs, see Byson's book on heat treatment of tool steels and "Tool Steels" by Krauss.

-Cliff

 

[Larrin]

These are standard impact tests not defined or solely used by Crucible, C,U,keyhole and prefracture, then there are also torsional and izod.

C-notch is not a standard impact test, it was designed by Crucible.

 

[Larrin]

Tool steels have v-notch values up to 240 ft.lbs. Even the very brittle steels like T15 have v-notch values of 3-4 ft.lbs, see Byson's book on heat treatment of tool steels and "Tool Steels" by Krauss.

That might be possible for annealed T15, but T15 gets 3-4 ft. lbs. in Charpy C-notch, it couldn't possibly get the same in CVN. Crucible die steels get 7-15 CVN and they're rarely hardened above 52 Rc. CVN is rarely performed on anything that's 60+ Rc, though maybe I haven't looked hard enough.

 

[Cliff Stamp]

C-notch is not a standard impact test, it was designed by Crucible.

To say something has a standard means it is precisely defined and accepted, that means someone else can do it and expect to get the same results because it is constrained by the defination. Crucible may have origionated that geometry, someone had to for each of them. However they are not the only ones using it. You will find charpy c-notch values for steels made by other manufacturers in other countries.

....maybe I haven't looked hard enough.

Bryson lists Charpy v-notch values for a wide range of tool steels though at times I think he is using torsional data based on other referenes such as ASM's Tool Steels. I wrote him awhile back asking for references for each result. The standard impact samples are however much larger than knives and thus they don't always give representative results. The shallow hardening steels for example have non-martensite cores which drastically increase their impact values but this won't happen in a knife edge obviously.

Generally the reason why charpy and izod tests, irregardless of notch geoemtry, and tensile tests, are critized for hard steels is that they are very sensitive to grain alignment which is why often torsional testing is advocated instead. Torsional testing also shows clearly embrittlement regions which charpy/izod often ignore (but not always - see Bain for example). There has been much discussion of this on rec.knives as well as on Swordforums. Roberts and Cary also discuss much of the issues in detail, Alvin has quoted it in bulk on rec.knives.

-Cliff

 

[M Wadel]

Charpy C-notch is used only by Crucible, it's numbers are pretty close to an unnotched charpy

ive only found unnotched charpy using 7x10mm rods (but i havent looked that much tho) instead of the standard 10x10mm, will those give similar results, 10x10 C-notch and 7x10 unnotched that is?

 

[Cliff Stamp]

In general you would not compare different impact tests in that manner as they don't tend to be consistently different.

-Cliff

 

[Satrang]

You cannot correlate impact testing using different notch configurations due to the change in fracture mechanics as Cliff mentioned above. The North American Die Casting Association did some informal testing trying to correlate V-notch and larger radius notches and found you could not make a straight line correlation. This work was done by Oak Ridge National Lab and was an internal study by NADCA. The reason V notch is not used on tool steels is the values are not within the certfied range of the Charpy testing machine. It is equivalent to certifying your car going 3 miles an hour using your speedometer. It gets quite bouncy and innacurate at the low numbers. Crucible started using C notch for that reason and it is true anyone can use their specimen geometry to duplicate their testing. The Japanese established the U-notch and the Europeans standardized on un-notched due to their use of the larger Charpy machines 300 ft/lb capability vs the US 200 ft/lb capability (average). As Cliff mentioned above also, the numbers are sensitive to things like grain direction, carbide banding, carbide size, etc. This is why charpys always should have the notch direction stated. Longitudinal numbers will be higher than transverse numbers, etc. The numbers for tool steels are typically done longitudinally which means the fracture is perpendicular to the grain flow.