Hardening of carbon steel with time

[Paulettaa]

Dear All,

I am a mechanical engineer and have problem understanding something related to metallurgy so please help me.
Is there a mechanism based on which a carbon steel becomes harder and harder as time goes by?
I mean suppose that I have a steel with yield stress 250 MPa and I put it aside in a warehouse which is a place with normal temperature and after two years I have the same steel which has a higher strength like 260 MPa? Is it possible and what could be the cause of it?

Warm Regards

 

[Ron]

Not likely. More likely a variation in measurement. How are you determining yield strength? I assume by your description that you are using hardness as a measure of yield strength. At best, this is an approximation. Many factors can affect variations in hardness at any point where it is measured.

 

[ornerynorsk]

Subtle changes can be observed in drawing quality steels over months-to-years. Nippon/Sumitomo, for one, makes reference to this in one of their catalogs. We've also experienced this with AKDQ tube in severe bending applications where we cannot store material more then several months without encountering problems. Someone with greater metallurgical knowledge than I would have to explain the mechanism behind it. Have never experienced this in any other so-called carbon steel.


It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[metengr]

There is a mechanism called strain age embrittlement for carbon and low alloy steels. It is a function of prior processing and the material can age at room temperature, resulting in changes to tensile properties. You can look up this mechanism in carbon and low alloy steels.

https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=kts&NM=392

 

[MagBen]

Carbon precipitates. That was why some magnetic grades specify carbon content < .015% to prevent so-called "magnetic aging" that leads to increase in coercivity. What we do is to age the material at 100C for 100 hours to make sure the physical property (covercivity) increases less than 5%.

 

[EdStainless]

And with yield strength many things happen.
Even minor changes in residual stresses can alter the measured Yield Strength. And stresses will change with time.
Other things such as direction that the samples were cut, sample size, temperature of testing, and strain rate can all alter the apparent yield strength by small amounts.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

A change from 250 MPa to 260 MPa is within the normal variation within a sample, and it is well within the measurement uncertainty (either directly by tensile test or indirectly by hardness test).
Besides, field hardness testing is done wrong > 80% of the time, for a long list of reasons.
I am aware of only one physical phenomenon that can harden steel at room temperature, the one metengr cited and ornerynorsk alluded to, strain age embrittlement. Carbide or nitride reactions will not occur in any measurable way.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Dhurjati Sen]

Dear Paulettaa,

So, what did you understand?

Regards.

DHURJATI SEN

 

[Paulettaa]

Thank you all for your replies.

I asked this question since we had a problem for approving a plate of carbon steel (SA-516 Gr. 60). The plate had their certificates all indicating that the specimens tested had the acceptable values in tension tests. All the tensile stresses were in the range 415-550 MPa. But when we sent the specimens to laboratory the tensile properties were higher than those indicated by the manufacturer test reports. For the yield stress it was OK since there is no maximum yield stress in the specification. However, when it comes to the tensile stress it is a problem since the laboratory shows a tensile strength of 563 MPa. Please note that we sent the specimens to another laboratory and the results were the same. I studied what you suggested as the possible root of this phenomenon (strain aging). I think this mechanism looks a little strange for this case since these plates are not cold formed and they are all normalized.
1- Is strain aging a possible phenomenon in normalized plates? (no cold working after normalization)
2- Plastic deformation is key to strain aging. Are the plates of hot rolled subject to strain aging? or should the plastic strain be due to cold work?
3- I have heard nitrogen atoms can diffuse in a steel alloy an it can increase the surface hardness of steels. Is it possible that nitrogen diffusion in the air increase the hardness and even yield strength of steel or does it diffuse in conditions different from room temperature?

Regards

 

The plot of stress-strain from the tensile test should be revealing. Please post it if you have it.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[metengr]

1- Is strain aging a possible phenomenon in normalized plates? (no cold working after normalization)

Not for SA 516 plates for certain structural steels yes.

2- Plastic deformation is key to strain aging. Are the plates of hot rolled subject to strain aging? or should the plastic strain be due to cold work?

They can be depending on the finishing temperature.

3- I have heard nitrogen atoms can diffuse in a steel alloy an it can increase the surface hardness of steels. Is it possible that nitrogen diffusion in the air increase the hardness and even yield strength of steel or does it diffuse in conditions different from room temperature?

No

 

[MagBen]

Not necessary "strain" aging. Room temperature aging is well known in magnetic industry, where the materials are fully annealed.
Being practically insoluble in Fe at room temperature, C atoms tend to remain dissolved as intersitials after finial anneal. Because of its high mobility, C can eventually precipitate as fine carbide particles, even at room temperature.

 

[Dhurjati Sen]

Dear Paulettaa,

Actually all your questions have already been answered. You have been enlightened regarding different mechanisms that lead to higher hardness / strength in carbon steel with the passage of time. (refer your original post first part)

Regarding the second part of your question, again you have been specifically informed that such a small change in strength / hardness is a matter of no concern.

Regards.



DHURJATI SEN