Tempered martensite or bainite- toughness?

[pig]

Generally speaking my question is:
Which is toughness difference of a low bainite and a properly softened martensite as a blade spine in general?

I know, that the answer easily is just: It depends.

I specify a more precise example because of that:

Let's suppose a carbon steel 1 %C.
The max hardness supposed to be HRC 65.
The HRC 50 spine has got quenching and by differential tempering (somewhere around 700F).

On the other hand, we can get the same HRC about 50 with bainite quenching the spine to some temperature (probably to something around 650-700 F). Again the edge is supposed to be martensite.


The more specific question now is: Which is the difference in toughness between the two blades? The HRC 50 both.

Can a bainite be tempered and how?
Use of tempering a bainite?

In the both cases we temper the edge properly to somewhere 350-400F.

Here we supposed that we are able to do both bainite quenching and differential tempering right and succeed to do it in practice.



pig

 

[rlinger]

Pig, I followed as much as I could. All I could possibly add is 'I believe' tempering should not be below 400 F.. I base that on Mete's advise to me about that and I believe it.

RL

 

[mete]

If I understand your question, you want to compare an austempered blade with a hardened and differentially tempered blade. First bainite cannot be tempered or changed in any way after it is formed. Martensite on the other hand must be tempered and the tempering temperature picked to achieve desired properties.....In any one steel, the quenched and tempered (at 400F)will be harder by a few points (HRc) than the bainite but the bainite will be tougher. To get better blade toughness with martensite we can either differentially harden or diffentially temper, getting a hard edge and softer tougher spine...... As far as hardness you might get, with a certain steel, 55 with bainite , 58 with quench and temper ,and with differentially hardened or tempered you might get the same 58 on the edge with a spine of 50...... Anyway its your choice depending on the use of the blade and just how you want to heat treat.

 

[pig]

Thanks mete.

anyway its your choice depending on the use of the blade and just how you want to heat treat.

My problem was exactly thoughness as to BENDING. Which cracks or bends and stays bent first, both to my mind destroying the blade for future use badly.

Even the cracking looks more ugly we can' hope much for a balde bent and stayed bent a lot and then by force straightened.

I try to guess between lines. Is it so that we can't say that bainite has some extra advantage compared to much drawn back martensite?

In my example I presented a case with same hardness with bainite and 15 HRC drawn back martensite.



pig

 

[mete]

Bending and toughness are two different things. Toughness is related to impact resistance such as needed for swords or machetes, kukris. Bending is related to tensile strength. If you were going to use a knife for prying the easiest way would be to use a thicker blade . But what are the other conditions ? Impact + bending ? then I would pick bainite.

 

[pig]

Thanks again, mete.

I am a little bit lost with the terminology.

I know the term "tensile strenght", still I try to take into account the blade smith community special language too!
Too much balancing for me.

Ed Fowler's book page 28:

"Toughness: a knife is tough when it can be bent and still maintain its ability to be used as a knife.
Strenght:the ability of a knife steel to resist bending"


So, I tried to ask using "black smith English" to be understood.
I recalled this quotation because I noticed, reading it for the first time, the difference between metallurgy language.

To many conversions: Finnish->English->
Metallurgy English-> blade smith English-> ........

Sorry about the confusion, I did mean "tensile strenght".


Are there any published tables of tensile strenghts/steel/tempering amount and other similar steel charateristics tables based on tests?
(I did not mean only tabes. Curves or conversion approximation formulas from HRc as good. I am asking this because the curve HRc as the function of tempering temperature is always available but nothing else in my steel specs.)



pig

 

[mete]

That's easy since the hardness tests were developed to give an approximate measure of tensil strength. Just do a computer search for "hardness tables " you'll find many sources for the standard hardness conversion tables.Remember that they are all "approximate"....Toughness is a measure of impact strength measured various ways such as the ' Charpy V notch impact test'.