Can tempering go wrong?

Hi!
That's my question, regarding knife-making. I mean, if I do the
hardening of the blade with rather less-than-professional equipment
and I put the knives in the oven for an hour or two at the given
temperatur, can it still be brittle? Even if the hardening prosess was
far from perfect and the edge might not be that hard, it should at
least not break first time it gets some stress - or would it?
Thnx for your help!
Falko
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Reply to
Falko Fritz
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It shouldn't be.
There are various failure modes for chemical problems (such as hydrogen, oxygen or nitrogen embrittlement) where leaving hot metal in an active atmosphere for too long. These are usually only a problem for esoteric metal alloys, particularly titanium.
There may be hard spots in the blade, caused by uneven temperatures during tempering. Watch out for how you support the blade within the oven, or for any inward leaks of cold air directly onto the blade. It can somethimes be worth putting a thin metal shield plate around the workpiece - just a disposable piece of scrap sheet.
You're more likely to have warping problems, then brittleness problems. Watch for an even temperature distribution.
Reply to
Andy Dingley
Assuming you are using a common high carbon steel- I doubt if it really matters so much what you use for equipment so long as you reach critical temps for the steel. If you were at the proper temp when you quenched it should get hard. You should've done at least a file test to see if it was hard before you tempered. You are the only person here right now who knows what you mean by "far from perfect hardening". Just going on what youve said so far I think you need to concentrate on getting the hardening part down before going any further. But to answer your question, sure you can temper for brittle, and in those spots that reached temp at hardening, they would temper to brittle during tempering. And yes it would break the first time stressed. If you do not want the blade to be brittle, then you need to reevaluate your tempering temps for the steel you are using, and how those temps are being determined (judging by color, trusting an oven thermostat?) Good luck
Forger
Reply to
Forger
I use a toaster-oven and a mercury-type oven thermometer, it goes to 650F (340C) ...that's higher than my junky toaster oven. ;)
Like Forger says getting to the right temperature and holding there for an hour is all that's needed. the steel don't know if it's a second-hand-store toaster oven or a computer controlled furnace. :)
It depends. ;) You hate that answer as much as I do? :/ But that's the answer here... sorry.
Can you come back with more detailed information? :)
Like Forger says, you need to do a "file test" on the heat treated part before going any further with the heat treating process.
Later you can get away without the file-test, but early-on it's a must-do. Right after it comes from the quench check with the corner of a file to see if it's "file hard". If not you didn't get the steel hot enough before the quench or the quench wasn't quick enough.
What heating equipment are you using?
For sure-> what steel is it you are working with? US or DIN standards will work, composition won't hurt. ;)
Forget what the oven thermomstat is pointed-at ;) go by what a good oven thermometer is telling you.
Also please know that brittle is "a type of failure" not a discription of how much strength.
Ice and glass are both brittle but glass doesn't ice-pick so easy, it's harder and stronger. ;)
My knife blades are brittle too, but harder than the tip of an ice pick and at least as "strong", both. :)
What does the steel do when pushed-past its elastic-limit? Bend and stayed bent? Break? Some of both of those?
How much "pressure" did it take to push the steel past its elastic limit? Was that pressure suddenly applied or slowly applied?
That information is available in certain good books. :) You game for a little studying? ;)
I'm a weird knifemaker :/ all I care about is edge taking and edge holding.
Alvin in AZ
Reply to
alvinj
You've already got the basic answers from other posts but just to reiterate a point or two. If it's through hardend then it's *hard*- and brittle. If it's not brittle out of the quench then you didn't get it hardend. I'll hazard a guess that even if differentially hardened, the hard parts are still brittle. Secondly and I know this from recent painful experience - if you don't temper it hot or long enough then it can still be brittle. SNAP. Really sucks... Then there's topics like grain size and how that plays a part in it all but that's best left to the experts. I'm not qualified.
GA
Reply to
Greyangel
(this foolish google-account oviously doesn't support the reply-function in some way, so I'm not posting in rigth thread. sorry4that.)
Hi!
..some detailed information then: It's about tempering some laminated blades of mild steel with swedish steel (No. 1770, C = 0,7%, Si = 0,4%, Mn = 0,7%, P < 0,035%, S < 0,05%). I heated them in the forge to what I judged to be about 840°C (by colour) and quenched them in oil. The heat was more or less a good guess and somewhat uneven, so I'd be surprised if it worked out well for the whole edge..
Thnx for your help so far! :o) Falko
..still a lot to learn..
Reply to
Falko Fritz
When I have to harden parts that have to be heated evenly, I place them into a section of water pipe that I lay into the forge fire. I get a good fire going, add some coke, then the pipe with the parts in it, then add more coke over that and blow gently. Using a thermocouple, I can keep the temp. controlled within 25 degrees F pretty easily. And using a gently blast, the parts seem to almost "dissapear" into pipe since everything is the same color.
Pete Stanaitis -----------------------
Falko Fritz wrote:
Reply to
Pete & sheri
This shouldnt be a problem. You didnt say if its a coal or gas forge youre using, but it really doesnt matter. Get yourself a magnet, fasten it to a wire. Keeping it near the forge, slowly bring the blade up to temp, keeping it moving. Try to keep the entire blade at the same color, and as it gets red move the magnet against it, pull the magnet away and heat the blade to a slightly brighter red, move the magnet against it again. Keep doing this until the magnet doesnt stick. Thats the color you want, now just make the entire edge at least a third of the way up that color (or the entire blade if thats what you desire to harden) and quench. I usually try to keep the area near the tip alittle cooler until just before I quench, to keep from overheating it. After hardening test the edge with a file, it shouldnt bite into it. Do your temper twice, keeping in mind the advice already giving about that. Try this on some scrap peices, see how it works for you.
Forger
Reply to
Forger
Ok, AISI/SAE calls that "1070". :)
"1069" is almost the same except 1069 has a lower Mn content range.
Does "1770" mean? 1-7-70 1=what? and .7%Mn? and .70%C?
That'd be perfect... if it weren't laminated in mild steel. :/ No kidding, Falco, Forger's got it right except the mild steel gets in the way of the non-magnetic properties of the "1070".
(GA, fig 6-2 page 161) the mild steel won't get non-magnetic until it reaches ~1650F (depending on its true carbon content) and the 1070 needs only to get to 1400F. 1350F is about when the 1070 will get non-magnetic. Will a guy need a temperature controlled setup?
Going by the arrest point is going to be tough... not very much surface to compare and 1070's arrest point is hard to see both? :/
Alvin in AZ ps- While going over that graph 6-2 notice some of the lines are for temperatures going up and some of the lines are for temperatures going down. :) (same with the magnified veiw fig 6-8 page 171;) (you knowed that already right?)
Oooo... seen fig 6-7 page 170 again... it's one of my favorites I remember it being a major light bulb turn-er-on-er for me. :)
Quenching the "1035" at 1550F results in fine grained martensite and ~1575F insures getting the steel's highest, as quenched, hardness.
See how the numbers match exactly with fig 6-8 page 171? :)
Imagine the different look of fig 6-7 and the different temperatures needed if it was 1084 instead of the 1035 pictured, simply eliminate the white spots... -and change 1550F to ~1375F going up -and change the 1475F to 1350F going down. -the 1290F (Ar1 line) is the same for both 1035 and 1084. :) -the 1350F (Ac1 line) is the same for both 1035 and 1084. :)
I think of c=cooling and r=rising but since it's french and not english it's friggin backwards. ;)
So really, c= rising and r= cooling. ;) ---------------------------
Reply to
alvinj
Ok Alvin I can agree with that, somewhat, only my take on the book method is alittle different. First assuming his forge is coal, clear off the grate leaving a clean area roughly the size of that book, and mound up the coal in a circle of slightly larger diameter. Open the book to the middle, +/- 4%, with any ignition source available light several pages on each side. Place burning side down directly over grate. When the outer edges turn black and the fire begins to burn its self out, slowly add some blast. The book should at this point become fully engulfed, if not stop the blast, remove charred pages and start again. Once the book is engulfed begin crowding the coal in. WHen the coal begins to burn you can crank up the heat, cremating the book and get down to business. Once the book is totally gone, heat the laminated blade to nonmagnetic and quench :)
You know I'm joking with you Alvin:) I'm sure it cant be done that way, I probably just got lucky a few hundred times with my laminated blades :)
Forger (read em if ya got em)
Reply to
Forger
So, non-magnetic for mild steel is too hot for the 1070 and the grain will coarsen in it... bad. (no- Cr, V, W or Mo to stop it BTW;) Unless a guy had something to compare it to, how would a guy know?
1095 will let you know by oozing carbon into the grain boundries and the edge will be crumbly unless drawn to above 425F.
What steels were laminated with what steels there, Forger? What temperature/s were they drawn to?
Alvin in AZ
Reply to
alvinj
Alvin maybe he has some way of applying the knowledge of your post but I gotta tell ya, without an oven I think those numbers were just numbers. If you know of how he can apply that using just his eyesight and his forge I would love to learn it myself.
Reply to
Forger
Yeah! Tai Goo mentioned that method and it sounds more simple and effective than anything else I've seen/heard of. Been thinking about this - A good camp fire gets pretty hot all on it's own. Why not just have small bon fire and then rake out the coals evenly and lay the pipe into and over it. I know the fire is hot enough for heat treating without adding air if it's burning well. I've seriously thought about doing that in my back yard but I worry about smoking up the neiborhood. May be a good supply of charcoal would do the trick... naw.
GA
Reply to
Greyangel
Wish I had one. Two grand for the one I want but I'm serious about trying to make one from scavanged parts. See what happens. I paid attention to the part about higher carbon/lower temps. The most drastic example for me was that I started playing with that Pandrol stuff right off the bat and non magnetic is pretty much orange heat with that stuff. Then one day I had some of that straight carbon creeper or whatchamacallit material and discovered it was already non magnetic and was still a dull red (in shaded daylight).
This is bad... I knew the chart you were talking about as soon as I read your comment. ;)
GA
Reply to
Greyangel
Perhaps a guy without a $2000 temp sensor would put a hunk of plain 1070 scrap or stock in with the sandwich, and he could check that for non-magnetic-status, and decide that it was time to quench the sandwich when that happened (if they were arranged to be heated the same, & looked to be the same color). Just a thought.
Reply to
Ecnerwal
If you're building from scratch, spend the money on the controller and the matching thermocouple. That's the heart of it, the rest is just brick and a hot wire. IIRC, I got a 2500 F controller for about $425 and the thermocouple was part of the kit. Not the "build a helicopter for $25" but still reachable by the hobbyist. You get what you pay for, heat treatment makes or breaks a blade. High tech where it counts.
Charly
Reply to
Charly the Bastard
Actually what I had in mind was just a tempering rig made from modified toaster oven parts... I like your idea though. should be pretty simple to build the box and wiring and add a commercial controller...
GA
Reply to
Greyangel
Another trick I've seen him use is to first coat the blade in some oil (he was quenching in corn oil), and then keep stuffing little chunks of wood into the pipe. The idea being to keep all the oxygen away from the blade by giving it something else to attack. The blade came out of the hardening process without any scale at all.
- ken
Reply to
Ken Rose
Thats an excellent thought, if I get a chance today I'm gonna try it!
Reply to
Forger
You don't need anything fancy for tempering unless you are working with D2 or HSS and want to take advantage of their secondary hardness hump. (~900F)
The lack of a temperature controlled oven is what's holding me back from using A2. Yes I could "stick it in the fire" but to get A2's full potential a guy needs to "go by the numbers". :/
Very un-blacksmith like I know, but A2 won't be any better than 52100 without it, so why bother?
One of the guys on r.c.m (metalworking) claimed to control his by hand using a thermocouple and meter, did that for years before getting a good-used controller cheap enough. The price came down on them. :)
That's my plan for A2 and some VascoWear I already have.
I'm not going to use the electric-rig that many times that I can't do it by hand. :) I already have a meter with the resitor on it and face that reads too 1400C.
On the face in tiny letters it sez- Thermocouple Pt/Pt13%Rh, Ext.Res.10ohms, SPI Instuments Co.
Got it used for $2 at an industrial-type second-hand-store. ;)
Agreed. :)
Alvin in AZ
Reply to
alvinj

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