Impact values - Charpy V Notch

[STEVEBR]

I recently Charpy V notch tested some 50Mn2 steel (CMn steel %C 0.47 - 0.55, % Mn 1.4 - 1.8 & % Si 0.17 - 0.37), in both the quenched and tempered and forged conditions. I have a Rp0.2 of 650 - 700N/mm² & Rm value of 900N/mm² & a % elongation value of between 16 - 20%. Values were taken at 20°C, -25°C & -40°C. Expecting to see charpy values in the region of 50J figures have been returned with values in the region of 10 - 20J.
Any ideas as to why the charpy values are so low?

 

[JoeTank]

I am not familiar with the materials designation by I've seen same thing happen several times with SA516-Grade 70. Problem was poor specimen preparation. The purchasing dept tried a new lab that had better prices than the regular guys. Problem had something to do with tooling not sharp enough. Tool marks were evident in the v-notch region. Upon retest at normal lab we obtained the more typical and expected result.

Joe Tank

 

[metengr]

STEVEBR;
I am not surprised at all because of the carbon content you reported at 0.47 to 0.55%. Carbon content is a major driver that affects CVN impact energy values. The higher the carbon the lower the CVN impact energy. This is one reason why pressure vessel steels are limited in carbon content to at or below 0.35%.

 

[redpicker]

As metengr says, at least in the as-forged condition, I am not surprised at the results. In the Q&T condition, however, you should be able to get better, at least at the highest test temp.

What section size are you quenching and what is your quench media? With only a C and Mn as alloying elements, you don't have much hardenability and with that high of a carbon, you probably can't water quench without running into a cracking problem, but with an oil quench up to about 1 inch (25mm) should get decent transformation so, after tempering, you should get in the 40-50 J range on your CVN. I wouldn't expect much at the lower temperatures; you'd need a different steel to get those to improve.

rp

 

[stanweld]

Like metengr and redpicker, I am not surprised at your results. In addition to the carbon content's adverse effects and heat treated condition, grain size and inclusions (number, size and shape) affect Charpy values. The austenitizing temperature range used may aid in grain refinement or coarsen the grain and you have not provided any details thereof.

Even with the best steel making and heat treating processes, I do not believe that the 50J impact value can be had at -40C.

 

[STEVEBR]

Thanks for all your replys - much appreciated. Will conduct a inclusion count and check grain size. Q&T details are hard to come by as this material was treated in China - They have been requested but so far are not forth coming.

 

[EdStainless]

Steve, what is the RT CVN? That might tell you if you have some serious inclusion or sample prep issues.

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[metengr]

Q&T details are hard to come by as this material was treated in China - They have been requested but so far are not forth coming.

When you evaluate the microstructure and hardness of a sample of the Q&T steel this will validate the type of heat treatment. I would be very cautious of any steel products supplied by China. The products (mostly boiler pressure parts) I have seen coming out of certain mills and fabrication shops has been marginal in quality, and I would not trust any material test reports. I believe in about 3-5 years the mills and fabrication shops will raise their level of quality, but for now buyer beware.

 

[MOB1]

The other factor which may affect impact properties is the reduction from the ingot during forging. The higher the better. Depending on the size of the finished forging a minimum reduction of four to one from the ingot would be necessary.

 

[stanislasdz]

1- Chek the proportions of inclusions in your steel
2- Chek the amount of Sulphur and Phosphorus in your steel
3- The Charpy valkue depend on carbon content see figure include (source : ASM HANDBOOK Heat Tretament )

http://img440.imageshack.us/img440/5585/charpy4ln.jpg

4- You can temper your steel to increase the value of charpy test