Extrapolation of Charpy values for lower temperatures 2

[scides]

Here is some background on the material before I get to the question:

From GB/T700-06 Q235
Gr A 21J @ +20C, Carbon .22%
Gr B 21J @ 0C, Carbon .20%
Gr C 21J @ -20C, Carbon .17%

The graph in the link below shows that there can be a fairly wide temperature range between maximum impact energy and 0 impact energy.

http://www.steeluniversity.org/content/html/eng/default.asp?catid=156&pageid=2081271546

Is the DBTT the entire range from when the impact energy starts to decrease until 0? Or is it just the temperature where the decrease starts?

In the case of Q235 above is it possible to approximate the impact resistance of Gr B at -20C?

 

[metengr]

scides;
The DBTT is the inflection point of the curve where the CVN energy value is plotted as a function of test temperature. You can even further define a DBTT based on the appearance of the fracture surface of the impact test specimens - for example one can select 50% flat fracture (brittle fracture) based on the plot of flat fracture appearance versus test temperature.

 

[metengr]

To answer your second question

In the case of Q235 above is it possible to approximate the impact resistance of Gr B at -20C?

No, but you can anticipate the CVN will be less than 21 J but probably above 10J at -20 deg C. Since the carbon content has a direct influence on CVN impact energy curve, the higher carbon steel would most likely be below 21J at -20 deg C based on data provided and heat treatments are the same for all three Grades of steel. I can't tell you the approximate value because of insufficient data.

 

[scides]

Thanks metengr.

Q235 is similar to A36 with regards to processing and properties. The chemistry is where things vary a little. Grades A and B allow higher P and S content than grade C.

I've been reading more since you replied and have a few more questions:

1) Which value should I be using for reliable -30C design: DBTT, NDT, or FATT(as you mentioned above)?

The DBTT seems to be the most conservative. It sounds like I should definitely stay above the NDT. And the FATT should be somewhere in the middle.

2) For a moderately stressed low cycle structural part is fatigue as much of an issue as brittle failure at -30C? It seems like fatigue & toughness are related but I'm having trouble putting together exactly how.

 

[metengr]

scides;

1) Which value should I be using for reliable -30C design: DBTT, NDT, or FATT(as you mentioned above)?

I would not necessarily define any of the above as being more conservative over the other. Any of the three criteria mentioned above suitably define notch toughness and can be used to set a threshold for minimum notch toughness at a minimum design temperature. For the nil ductility temperature (NDT), this value is based on drop weight tear testing and has been used for many years in the pressure vessel and nuclear industry to quantify minimum notch toughness. I would go with DBTT using CVN impact specimens. My rationale is that CVN impact testing is cost effective and provides adequate information regarding notch toughness behavior.

For a specific value regarding minimum CVN impact energy, I would use 20 ft-lbs (27 J) at the minimum temperature of interest.

2) For a moderately stressed low cycle structural part is fatigue as much of an issue as brittle failure at -30C? It seems like fatigue & toughness are related but I'm having trouble putting together exactly how.

Yes, fatigue would be as much of an issue. However, don't mix fracture toughness and fatigue resistance. These are different material behaviors, and each needs to be accounted for separately. You cannot make any assumptions that increased toughness means increased low cycle fatigue strength. Both have to be dealt with separately in design.

 

[scides]

"For a specific value regarding minimum CVN impact energy, I would use 20 ft-lbs (27 J) at the minimum temperature of interest."

How did you arrive at 27J? Is there a reference or standard that reccomends CVT impact energy values for different applications? I've found values ranging from 18J to 27J. 27J seems to be the most common value. This is for construction equipment that is not subjected to shock loading.

 

[metengr]

How did you arrive at 27J? Is there a reference or standard that reccomends CVT impact energy values for different applications?

Just experience.

 

[scides]

I was worried you were going to say that.

Thank you very much for your help!

 

[SJones]

27J is the result of a Master Curve correlation with fracture toughness assessments. See if the attached helps you any further in your application.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04