Blue Brittleness?

L

lcf

Joined
Nov 28, 2006
Messages
57
If this has been answered 100 times I apologize in advance.
On page 100 of the ASM HTG. It has a small paragraph on blue brittleness. It deals with brittleness caused from steel being heated in the 445-700F range, (or the blue zone). It says that this may INcrease tensile and yield, BUT DEcrease ductility and impact strength, and parts should not be subjected to impact loads. (Information above accurate without copying direct).
Does this mean that "blue backing" a blade is doing something that could be harmful if the blade is used for impact?? IE chopping, as in competition cutting?
 
mete or Kevin - want to field this one?
From what I understand about Blue Brittleness, it is not a concern we need to deal with.
Stacy
 
We had serious discussions about that in the past .Two different phenomina can occur.But with modern steel making and better alloys it's not a big problem .It's such a little problem that for details I'd have to research it again !Every steel is different and each has it's own optimum HT, that is a balance between hardness and toughness.Of course if you want a chopper pick the alloy that has high toughness to start with !
 
Gabe, thanks for sharing the key to steel artical.
Have been, and still am, reading to get to the answer I'm looking for.
Mete, how far in the past was the discussion, so that I could look in the archives for the thread?
Thanks
 
Run the Google search: temper embrittlement site:bladeforums.com and it will show up.
 
lcf said:
If this has been answered 100 times I apologize in advance.
On page 100 of the ASM HTG. It has a small paragraph on blue brittleness. It deals with brittleness caused from steel being heated in the 445-700F range, (or the blue zone). It says that this may INcrease tensile and yield, BUT DEcrease ductility and impact strength, and parts should not be subjected to impact loads. (Information above accurate without copying direct).
Does this mean that "blue backing" a blade is doing something that could be harmful if the blade is used for impact?? IE chopping, as in competition cutting?
Click to expand...

In order to get the blue back on a blade in the seconds it takes with a torch you are actully often going well beyond the range for TME (tempered martensite embrittlement 450F-550F) so I don't see it as an issue. Also the steel selection would have a great deal to do with ammountof concern you should have. For instance L6 has a noticeable drop in impact toughess within that range while still losing hardness, whereas O1 only levels out for a bit before continung to gain toughness. Since this is mostly a result of certain alloying elements reacting to the processes occuring in that range most of the simpler steels smiths work with are near the source of concern that other may be.

The uncontrolled and impercise nature of termpering with a torch tends to save you from this issue since you are not nailing an exact range throughout the whole piece long enough for the phenomenon to prevail. Now using an oven or salts for the entire blade within the TME range is another story and while for the msot part in knife use applications you would never notice it, if the blade were destroyed under lab conditions it could be measured.

In short don't lose too much sleep over it;)
 
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