What happens molecularly when tempering

[Display Name]

Ok I know that there is somewhere I can find this info but I am either not using the right key words to search it and I've been searching here and google for about three hours now, so please do not just reply "use the search bar". You don't have to give me a detailed explanation if you know there is a link to the info or know which keywords to use for searching that will suffice (links to reliable sources being preferred)

Anyways the question is, when tempering hardened steel, why do different temperatures affect hardness. Like what is happening molecularly or whatever that makes it softer but tougher at say 400 degrees whereas 300 will be harder but more brittle (temperature numbers just given as examples). What voodoo is going on that when tempering at 300 the hrc value only drops a point or so but at just 100 degrees more it drops 5 points and so on.  Also does time play a huge factor. Example, let's say I take a knife of O1 at 65 hrc quenched and double temper it at 450 and I take another identical knife and temper it at 450 for 12 hours. Would there be a difference in hardness or toughness towards one or the other. I read somewhere that if you leave steel to temper at a temperature that would leave a straw color, if tempered at two hours, normally and left it in there longer it can turn a color that would resemble as if you were to temper it at a higher temp, say blue, but it didn't say if this actually reflected a change in the hardness as well. 

Also, embrittlement tempering range. If I understand this right it is the temperature at which a particular steel will get softer but also more brittle. What is this temperature for say O1 and 440 series stainless as those are the two steels I use mainly besides sometimes (read rarely) ats 34. 

I really appreciate any insight you can offer. I know it's a long read and I apologize if it has been asked before and could have searched it. I tried to search it but couldn't really find anything other than "tempering at 300 will give you a harder blade than tempering at 500" kind of answers. I know that but I want to know the why. Thanks again for any help. 

[ThomasPowers]

First have you read the Wiki article on tempering? https://en.wikipedia.org/wiki/Tempering_(metallurgy)

Next you must remember that these processes are not 'all or none'; but more a statistical basis---eg For a certain alloy heated to a certain temperature for a certain time and quenched in a certain quenchant: 98% of the austenite will form martensite.  If you heat to a lower temp for a set time, a high percentage of that Martensite will convert to tempered martensite, if you cycle several times more will convert but it's more of a  first time 90% converts, second time 90% of the 10% that was left converts, third time 90% or what was left after the second time converts and so on till you get down into ridiculous numbers.

Time at temp does have an effect but it's a much lower one than cycles  (ie if you hold it at tempering temp after it has fully come up to temp for decades it will have about the same effect as doing say 2 more cycles)  Now certain alloys have certain "needs"; some of the high alloy steels profit from a cryo quench to deal with retained austenite *after* a tempering cycle and then requiring another tempering cycle to take the martensite just created and temper it.

Not an easy or clear thing at all.

Temperature colours on steel are due to the thickness of the oxide layer and so holding steel at a temp for long times will generally cause the layer to thicken and change colour; but may not do much for tempering as long time is generally measured in hours and tempering effects may need years...

[beech]

Or the blueprint on here in articles might help if you haven't found it already.

 

[Display Name]

Well thank you for your reply. I did read the wiki article. But it still didn't really explain what is happening when different temps are used. But I did do a batch of knives yesterday starting at 65c as quenched and started around oh 10a.m. Took them out this morning at 10 and they come out at about 61ish so no noticeable difference that I noticed. I took one knife and broke it and the grain structure seemed very nice and even and fine. Very hard to even tell there were individual grains. I then took another one and pitched it like a baseball at the concrete wall and nothing to the knife but a nice chip in the wall so pleasantly pleased there. 

 

@beech I tried opening that link you posted and it just bring up a list of threads for blueprints including the one in the link. I tried even searching it and it just keeps bringing me to the same place each time. 

[Buzzkill]

You need to scroll down.  It's there.

[Display Name]

Haha so it is

[Display Name]

Well I finally finished reading the blueprint. It's a long read especially when the kids keep pulling you away. Very informative and answered everything I was curious about. Thanks a lot. 

[beech]