Low impact energy, high impact toughness - comparisons

[raniero]

Hi,

I have been doing some research about imapact toughness of steel and found a paper comparing toughnesses of 4 different steels using the Charpy impact test. The following is a link to the paper mentioned.

https://drive.google.com/file/d/0B363OnUPK7-deGJDdkhwVU9SQ2s/view?usp=sharing

At 24^oC, SAE 4140 has the lowest impact energy, 13J.

In the conclusion it is stated that fracture toughness of SAE 4140 is the largest of all at 68.61 MPa m^1/2.

How come the mentioned steel absorbed the least amount energy and ended up being the the toughest? Is it because it has a higher ultimate tensile strength?

Could it be considered as being a ductile or brittle material? The high toughness suggests it is ductile but is surely did not deform as much as the other steels.

Note: I am still unsure of how to calculate the impact toughness from a Charpy energy test although I understand how the energy to fracture the specimen is obtained.

 

[Achy47]

Thank you for sharing your findings and questions about the impact toughness of steel. I would like to provide some insights and clarification on the topic.

Firstly, it is important to understand that toughness and hardness are two different properties of a material. Hardness refers to a material's resistance to indentation or penetration, while toughness refers to its ability to resist fracture or breaking. In the case of steel, the Charpy impact test is commonly used to measure toughness.

In the paper you have shared, the researchers have compared the toughness of four different steels using the Charpy impact test. The test measures the amount of energy required to fracture a notched specimen at a specific temperature. The lower the energy absorbed, the more brittle the material is considered to be.

Now, to answer your question about SAE 4140 having the lowest impact energy but the highest fracture toughness, it is important to consider the microstructure of the steel. SAE 4140 is a low alloy steel with a high carbon content, which makes it stronger and harder than the other steels in the comparison. This means that it requires more energy to deform and fracture the material. However, the microstructure of SAE 4140 also contains a significant amount of fine-grained ferrite, which makes it more ductile and able to absorb more energy before fracturing.

In terms of whether SAE 4140 can be considered a ductile or brittle material, it is important to note that all materials have a combination of ductile and brittle properties. In this case, the high toughness of SAE 4140 suggests that it is more ductile than brittle. However, it is still possible for it to exhibit some brittle behavior, especially under certain conditions such as low temperatures or high loading rates.

Regarding the calculation of impact toughness from a Charpy energy test, it can be calculated using the following formula: Toughness = (Energy absorbed by specimen / Cross-sectional area of specimen) x (1 / Length of notch). This calculation takes into account the size and shape of the specimen, as well as the amount of energy absorbed.

I hope this information helps clarify your questions about the impact toughness of steel. If you have any further inquiries, please do not hesitate to reach out. Best of luck with your research.