Does different grain formation make different hardness number?

[Nanda Dhiyan]

I did transverse vickers hardness test on a carbon steel pipe SA210 A1 from a boiler, the result shows that the hardness is increased by almost 100VHN on the outside of the pipe. From the micro structure photo, I can see that the grain formation is different between the outer and the inner side. The inner side grain formation is kind of in row formation, but in the outer side the grain formation is kind of randomly placed. Can anyone explain to me does grain formation affect the hardness number?

the outer side micro structure:

the inner side microstructure:

 

[0707]

Yes! Fine-grain steels do not harden quite as deeply and have less tendency to crack than coarse-grain steels of similar analysis.

luis

 

[EdStainless]

Though in most cases the difference is hardness is not a direct result of the difference in grain size but a result of the different thermo-mechanical exposure that lead to the change in grain size.
The other possible factor is contamination picked up from the environment.

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P.E. Metallurgy, consulting work welcomed

 

[Dhurjati Sen]

You don't seem to be a Materials guy. What do you mean by "row formation" and "randomly spaced" ? These are not metallurgical terms.

If you want better interpretation of the hardness values, provide a little more about the failure history of the tube.

This seems to be a water wall tube? Or is it a bank tube?

Something awful has taken place with this boiler, else why would you take metallographs / microstructure of the internal as well as the external of the tube and also measure hardness ?!?!



DHURJATI SEN
Kolkata, India

 

Different grain size or 'formation' in a low strength CS boiler tube could have a minor effect on hardness, but nowhere near 100 Vickers points. The absolute hardness of some of these tubes can be barely over 100 HV when new.

Something else is going on here and it is not grain structure. More and better information is needed.
Oh and make sure the hardness testing was performed competently. This includes sample preparation.

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

 

[0707]

Nanda Dhiyan water tube hardness on the outside of the pipe seems to me that you are in presence of a hot spot. Which is usual in the water tubes of a boiler. IMO the outer side has a thicker layer of carbides precipitation than in inner side, in consequence the high hardness in the outer side, points to localized overheating, maybe the case.

luis