Charpy Energy vs. Hardness

[demsha]

Hi All,

The ANSI Z359.18 subcommittee is struggling with the practicality of doing charpy tests on small components that may be forged or stamped that cannot be machined into a standard Charpy sample (10mmx10mmx100mm).

I am wondering if there is a correlation between the charpy energy and the Rockwell Harness number, and whether I could specify a series of RHN vs. temperature as an alternative to Charpy testing.

Any reference materials or suggestions would be greatly apprecaited.

 

[metengr]

There is no general correlation between CVN impact energy and hardness for metals. I have seen hardness, tensile strength and CVN impact data plotted on a single graph for specific steels, but no general correlation for metals.

 

[OGMetEngr]

Impact energy correlates with more than just hardness. It has an inverse correlation with strength (and hardness), carbon content, and grain size. Also, some things affect the DBTT while others affect the upper shelf energy (some affect both).

 

[OGMetEngr]

I suppose, as metengr said, in theory you could make a correlation for each alloy at a set grain size and manufacturing/thermal history (manuf./thermal history to reduce possibility of embrittlement). As a practical exercise, this would be a difficult thing to accurately do and manage.

 

[demsha]

To OGMetEngr;

When you say inverse correlation, is there any coefficent associated with the formula? Can you send me that formula?

Thanks for all.

 

[rms1956]

There is no real correlation that I am aware of. As OGmetengr stated the relationship between hardness and impact strength for a given material is the obvious one, typically the harder the material is the more brittle it is.

The only sub-size CVN specs that I have seen are where someone has changed to an energy per unit area requirement as opposed to just the typical energy requirement. Is testing a full sized specimen totally out of the question, machine it from the raw material in the same final condition?

 

[redpicker]

Perhaps OGMetEngr should have said "inverse relationship" instead of "inverse colleration", since he later says making such a correlation would be difficult to develop and manage.

rp

 

[stanweld]

Subsize specimens are permitted by ASTM Specs, ASME Codes, etc. Why is there a need for full size only?

 

[redpicker]

As stated above, there are sub-sized CVN specimens that are recognized, While you can't really just go with the absorbed energy per unit area of the specimen, as the geometry effect is much more complicated than that, they can be helpful.

In general, if the section is too small to obtain a sub-sized CVN, a Charpy test is not needed. Sure, thin materials can be brittle, but the CVN test is used to measure toughness under conditions of high constraint, and if the material is thin, you usually won't get a lot of constraint. In these cases, the ductility results from the tensile test (% El) will usually let you know if the material is too brittle for the application.

rp