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-   -   Heat treating and tempering- temperatures and hardness values included (http://www.akfiles.com/forums/showthread.php?t=14475)

machinisttx 08-06-2006 11:30 AM

Heat treating and tempering- temperatures and hardness values included
 
I've noticed quite a few questions on the subject of heat treating and tempering, so I figured I'd post this for reference.

Here are some general temperatures based on carbon content
for .65 to .80 percent carbon content 1450F to 1550F
for .80 to .95 percent carbon content 1410F to 1460F
for .95 to 1.10 percent carbon content 1390F to 1430F
for 1.10 and over carbon content 1380F to 1420F

Some alloys commonly used for AK flats are 1050, 1080, and 4130. The last two digits of the alloy designation indicate the carbon content in one-hundredths of a percent, so 1050 has 5 hundredths of a percent or .5 carbon content. The particular alloy should be specified by either the maker or the supplier of the flat. Note-there is a color to approximate temperature chart, but I cannot find it within my reference material at this time.

For 1050, a temperature of 1475F-1550F is appropriate.
For 1080, a temperature of 1450F-1500F is appropriate.
For 4130, a temperature of 1600F-1650F is appropriate.

Quenching is required for proper hardening immediately after the steel reaches temperature. Do not waste time getting the part into the quenching medium! Oil(5 weight motor oil will suffice) is the preferred quench on 1050, 1080, and 4130. Water will work, but may cause cracks. When using water as a quenching medium, the addition of common dish soap will provide a better quench. The soap basically keeps the water from boiling away from the hot steel. Completely submerge the part, then agitate the part while quenching(provide some form of movement) to keep liquid flowing over the surface.

For tempering, these are approximate color to temperature indicators for plain carbon steel. Be certain to polish away all of the dirty black scale left from the heat treating process or it will impede your ability to correctly identify the color. It also helps somewhat to be in a medium lighting environment-not too bright and not too dark. Note-the color will appear almost like oxidation it should not glow the specified color

430F= very pale yellow
440F= light yellow
450F= pale straw-yellow
460F= straw-yellow
470F= deep straw-yellow
480F= Dark yellow
490F= yellow-brown
500F= brown-yellow
510F= spotted red-brown
520F= brown-purple
530F= light purple
540F= full purple
550F= dark purple
560F= full blue
570F= dark blue
640F= light blue

Here are the various temperatures for the individual alloys and the hardness after tempering.
Alloy 1050
600F-650F will give a hardness of around 40 Rockwell C scale, which is ideal for the purpose

Alloy 1080
400F will give a hardness of around 41 Rockwell C scale

Alloy 4130
800F will give a hardness of around 41 Rockwell C scale

link to original thread---- http://www.gunsnet.net/forums/showthread.php?t=276457

twistedneck 08-13-2006 02:13 AM

1 Attachment(s)
Good post machinisttx, just want to add that 800F is perfect and is a grey tempring oxide color. Comes after the blues.. Also, here is a chart of tempering temps vs. alloy vs. hardness..

allesennogwat 08-13-2006 05:41 AM

http://www.muggyweld.com/color.html

allesennogwat 08-13-2006 05:48 AM

Steel Color vs Temperature

2000F Bright yellow 1093C
1900F Dark yellow 1038C
1800F Orange yellow 982C
1700F Orange 927C
1600F Orange red 871C
1500F Bright red 816C
1400F Red 760C
1300F Medium red 704C
1200F Dull red 649C
1100F Slight red 593C
1000F Very slight red, mostly grey 538C
0800F Dark grey 427C
0575F Blue 302C
0540F Dark Purple 282C
0520F Purple 271C
0500F Brown/Purple 260C
0480F Brown 249C
0465F Dark Straw 241C
0445F Light Straw 229C
0390F Faint Straw 199C

allesennogwat 08-13-2006 05:51 AM

Steel exhibits different colors depending on temperature. Temperatures above 800F (427C) produce incandescent colors; the atoms in the steel are so energized by heat that they give off photons. Temperatures below 800F (427C) produce oxidation colors. As the steel is heated, an oxide layer forms on the surface; its thickness (and thus the interference color as light is reflected) is a function of temperature. These colors may be used in tempering steel.
If colors are a problem:


It's not always practical to use color to determine temperature. Five to ten percent of the male population are color-blind; further, colors of hot steel are much harder to judge in the sun if you do your heat treating outdoors, which you should unless you have a ventilation hood and chimney in your shop. Tempering can be done in an oven with an accurate thermometer. For hardening temperature, there are several solutions:

machinisttx 08-13-2006 06:38 PM

The vast majority of the heat treating I've done has been in a heat treating oven. I have occasionally had to flame harden one part or another, and always indoors(not really practical to wheel the torch 300+ yards from the toolroom to outside).

And yeah, the color isn't going to be of any significance to a person that is color blind, but they ought to know someone who isn't right?

Oh, most kitchen ovens aren't going to get hot enough for the tempering needed on something like an ak receiver.

Tatmike 08-13-2006 06:51 PM

How about a sticky?

allesennogwat 08-13-2006 06:51 PM

I bought a small heat treat oven/kiln made for a dentist's office.It's big enough for an AK receiver.

azhonkey 08-13-2006 08:08 PM

heat treat
 
i started to do a full heat treat on all my tapco flats about 7 months ago.bend the flat tig in the lower rails,then install it in a jig to keep it from warping.set the heat treat oven to 1600 degrees with a 5 mins. soak time.when it comes out of the oven the color is about the same as a october pumpkin.i then start to push it into a 5 gal bucket of 80 degree plus water at about 1 inch per second.reset the oven to 490 degrees ,back in the oven for another 30 mins at 490 degress.that receiver is as tough as the original.you can post all the numbers you want on heat treating,but these numbers work.just my two cents.

twistedneck 08-14-2006 06:21 AM

azhonkey, those numbers sound just fine.

When i soaked mine that long (5min) they scaled up pretty bad and grain size grew much larger. 4130 sheet only needs a minute at temp (1600) if that. too hot, too long you get huge grains and massive scale build up.

And when you quench, didn't warping start with the slow dunking action? I shot my recever in as fast as possible to get a big 'boom' of even quenching. i can make a banana out of a receiver by going slowly tip down into the quench.

490F tempering will work just fine, its making your receiver pretty hard. did you see the chart above? 490F for 2hrs = 45Rc. Not bad, but harder than needed (At 45Rc, its more likely crack in fatigue, however its better against wear, better strength, more elastic deformation)

If that formula is working fine, stick w/ it. You'll have one of the strongest AK's out there.

azhonkey 08-14-2006 08:29 AM

Quote:

Originally Posted by twistedneck
azhonkey, those numbers sound just fine.

When i soaked mine that long (5min) they scaled up pretty bad and grain size grew much larger. 4130 sheet only needs a minute at temp (1600) if that. too hot, too long you get huge grains and massive scale build up.

And when you quench, didn't warping start with the slow dunking action? I shot my recever in as fast as possible to get a big 'boom' of even quenching. i can make a banana out of a receiver by going slowly tip down into the quench.

490F tempering will work just fine, its making your receiver pretty hard. did you see the chart above? 490F for 2hrs = 45Rc. Not bad, but harder than needed (At 45Rc, its more likely crack in fatigue, however its better against wear, better strength, more elastic deformation)

If that formula is working fine, stick w/ it. You'll have one of the strongest AK's out there.

twistedneck
your right about the scale buildup,in future heat treats i probally will eliminate the five miniute soak time.i tryed the fast dunk and it warped the reciever,slowing down the quench,no more warping.the reciever is tiged to the jig about every inch along the top rails.pretty much learned all this by trial and error.

machinisttx 08-19-2006 11:30 PM

If the steel doesn't get above the critical temperature(meaning that it doesn't get hot enough for the molecules to re-align) and maintain it long enough, it won't harden. The soak time might or might not be necessary depending on whether you put the part in and then turn the oven on or if you let the oven get to temp then stick the part in.


I honestly would rather put the part in and then turn the oven on, let it get to temp, then soak for a couple minutes, then pull it out. Putting a cold piece of steel in a 1600 degree oven is bad business and can cause cracks. Some steels require a preheat step or two with soak times(think of it as intermediate temperatures) before you take the oven up to the final high heat temperature. 4150/4140/4130 aren't in that group, but it is something to consider.

If what you've got going works, don't change it. But, it's not necessarily going to work for someone else.


allesennogwat-

I've always been fortunate enough to use a large oven, the last one was a dual (independent)chamber job with around 18"x14"x24" chambers and all digital readout/setup.

twistedneck 08-20-2006 09:13 AM

Quote:

Originally Posted by machinisttx
If the steel doesn't get above the critical temperature(meaning that it doesn't get hot enough for the molecules to re-align) and maintain it long enough, it won't harden. The soak time might or might not be necessary depending on whether you put the part in and then turn the oven on or if you let the oven get to temp then stick the part in.

Good poitns MTX, Along that line, i put my receiver in the kiln at room temp, crank it to 1600F and wait. I turn on the argon at 800F and purge the kiln of oxygen. Some say 1550F is a good austenetizing temp for 4130.

The second my kiln reads 1620F I open the door and remove the receiver and brick (its inside a holding brick).. move over to salt pot, grab with tongs, slide vertically out of brick into quench.

That means i 'soak' for quite a long time going from say.. 1500F to 1600F (10min or so as the kiln heats up).

I tried opening the door @1600F and putting a receiver in. It warped like crazy, due to uneven contact with the hot holder brick. A lot of heat (150F) was lost from the kiln in the process.. so i still had to wait the 10min or so to get back up to 1600F. by that time it was scaled bad cause i didn't have a good argon purge.

azhonkey 08-20-2006 09:39 AM

Quote:

Originally Posted by machinisttx
If the steel doesn't get above the critical temperature(meaning that it doesn't get hot enough for the molecules to re-align) and maintain it long enough, it won't harden. The soak time might or might not be necessary depending on whether you put the part in and then turn the oven on or if you let the oven get to temp then stick the part in.


I honestly would rather put the part in and then turn the oven on, let it get to temp, then soak for a couple minutes, then pull it out. Putting a cold piece of steel in a 1600 degree oven is bad business and can cause cracks. Some steels require a preheat step or two with soak times(think of it as intermediate temperatures) before you take the oven up to the final high heat temperature. 4150/4140/4130 aren't in that group, but it is something to consider.

If what you've got going works, don't change it. But, it's not necessarily going to work for someone else.


allesennogwat-

I've always been fortunate enough to use a large oven, the last one was a dual (independent)chamber job with around 18"x14"x24" chambers and all digital readout/setup.

i don't put the reciever in the oven at 1600 degrees,i put it in a cold oven,set temp.,then turn on oven.the oven chamber is 10x12x14 and the oven is digital.

allesennogwat 08-20-2006 02:55 PM

There is a paste to prevent acaling of the steel.The paste is rated up to 1500 degrees F.As long as the receiver ois above 1250 F when it hits the quench it should harden.The thin sheetmetal does have a hard time staying hot from kiln to quench.I've tried 1550 F and can melt/burn the paste.Placing the receiver in a box of steel shavings is suppose to prevent scaling too as the shavings burn off all the oxygen around the receiver.

Years ago it was recommended to stress relieve welds in 4130 steel tubing.With the well controlled TIG welders and after much research this has been found not to be required.I used to take welded 4130 parts to the university and have them stress relieved in their kiln.Soak time is suppose to be important but on really large parts,like roll cages,I would use 900 F crayon mark the tubes one inch from the weld and heat the weld with a torch until the crayon melted.I was looking foe 1100 F at the weld to stress relive it.This is the kiln temp I used at the university.These days everybody says this really isn't required and doesn't do much good unless it's well controlled and timed.

machinisttx 08-20-2006 04:15 PM

Quote:

Originally Posted by twistedneck
Good poitns MTX, Along that line, i put my receiver in the kiln at room temp, crank it to 1600F and wait. I turn on the argon at 800F and purge the kiln of oxygen. Some say 1550F is a good austenetizing temp for 4130.

The second my kiln reads 1620F I open the door and remove the receiver and brick (its inside a holding brick).. move over to salt pot, grab with tongs, slide vertically out of brick into quench.

That means i 'soak' for quite a long time going from say.. 1500F to 1600F (10min or so as the kiln heats up).

I tried opening the door @1600F and putting a receiver in. It warped like crazy, due to uneven contact with the hot holder brick. A lot of heat (150F) was lost from the kiln in the process.. so i still had to wait the 10min or so to get back up to 1600F. by that time it was scaled bad cause i didn't have a good argon purge.

I've never had the pleasure of using a controlled atmosphere oven. I'll bet that it beats the heck out of getting cut up with that stainless tool wrap every time you have to heat treat a part.

azhonkey, it sounds like you've got a nice oven to work with.

allesennogwat--Crucible Tool Steel company still recommends that the welds be stress relieved. Even with tig, you're still going to impart stresses into the steel. They may not be enough to cause any problems, but they're still there.

I'm also thinking that there is another one of those pastes that is rated a little bit higher on the temperature, but I can't remember where I've seen it.

allesennogwat 08-20-2006 04:53 PM

I've seen some ceramic coatings on the internet that are rated to 2000 degrees F.I've never used them.I'm not sure they for heat treating and I don't know if they can be removed.Some info here,

http://www.timken.com/products/speci...etin%20102.pdf

twistedneck 08-20-2006 10:02 PM

Paste works, but it screws with quench rate. With a steel like 4130 (very quick pearlite nose, 2.2 seconds) you need to get down to that temp extremely fast, its easy with bare sheet metal, not sure about the anti scale paste.

And, despite a scaled appearance, 4130 isn't that high in carbon anyway so the driving force isn't as high as 1050 as some other receivers are made.

Descale (sandblast) doesn't get rid of decarb layer.. so its important to reduce it and the paste might be better than argon. MOst of my scale happens during transfer to quench, thats why i have an argon wand over the transfer brick and a blanket. keep it shield to the last minute.

Also, lesson learned: if you use a kiln with electrical elements these cause radiant heat direclty on to the part. that means the amibent air can be 1600F but the part temperature can be way higher due to radiation from the bright elements... shade you part or suffer a much higher rate of decarb from the higher temperture. that is another plus for torch heat treat.. that and less time above critical temp where scale forms.

camoguy 10-05-2006 01:08 PM

This is all quite interesting.. it is amazing the knowledge on these boards. I am learning more and more on electric and gas ovens every day. I use a coal/ charcoal forge and some technique I use wouldn't help you guys much. To heat treat a whole reciever I can either place the peice in a neutral part of the fire or a carborizing part..usually the later to keep scale down and it adds a bit of carbon to the skin of the 4130. Soaking it isn't a problem .. as soon as the whole of the receiver is up to a dull cherry (judging heat by sight takes alot of practice and it is person and equiptment specific. When I went from one of my older forges to newer one i melted everything until I got better use to the colour differences) take out and quench in brine front to back. I have found oil just doesn't get it to my liking , straight water may be to fast for a full receiver and I am not going to risk it. I have noticed 4130 does like a higher anneal then more simplier steels..weird. I can put it back on the forge and cover with charcoal with about a 1/4" sticking out and wait till I see blue moving to that edge. Take it out and quench in water.
I expect some scaling but if heavy is a problem I flux all sides..I prefer to not do this because I hate removing the flux later on these things. As to the ceramic spray that is for the furnace not your receiver....it helps to direct the heat away and it is hard to remove. Dont spray it on your receiver unless your trying to make a heat shield out of it. Great stuff if your making a new furnace ;)

artgtr 10-25-2006 07:46 PM

heat treating?
 
I got to say I am learning a lot from you guys and I'm getting ready to do a my 1st romy build and I'm using a tapco flat, after it's bent and rails in - heat treat, I don't have access to anything fancy, I do have a wood stove in my living room and I can get it pretty hot, so what do you guys think? I know the only stupid question is the one not asked. Thanks

camoguy 10-25-2006 08:04 PM

The wood stove could work, but I see many problems with that. Your stove is what I would call uncontrolled . Without a forced air source you have to soak your reciever to get it to a red stage(what that means is it take a longer time sitting in the fire) that will promote grain growth in the structure and that will run counter to what you want. Also without a forced air source the stove is working off of ambient oxygen .. an oxygen rich fire will promote fire scale on your receiver. I would really worry about warpage as well ...so in a nutshell I probably wouldn't do it.

artgtr 10-26-2006 04:25 AM

woodstove
 
Thanks, so what do you suggest as a simple and easy way to heat treat, Plumbers torch? Spot treat rivet areas? or don't bother to heat treat at all or send out to someone else who does this? I really appreciate the knowledge I'm gaining from listening to what you guys have to say. thanks.

machinisttx 10-26-2006 11:30 AM

Use a MAPP gas torch and heat it up until it loses magnetism, immediately quench in light oil such as transmission fluid or 5w motor oil. Polish all of the oxidation off so that you're left with bare steel. Heat it again with the torch until a dark grey oxidation forms on the steel surface. When that happens, remove the torch and let the receiver cool to ambient temperature.

artgtr 10-26-2006 07:14 PM

thanks alot and that sounds like a plan, I appreciate the help, thanks

twistedneck 11-23-2006 12:10 PM

FYI Lesson learned from heat treating a bunch of receviers now. If you are doing a full receiver heat treat you can save major warping headaches by stress releiving the receiver first for about two hours.

EDIT!

Stress relieving is not enough!!! You must fully anneal the receiver prior to full heat treat. Do this with the stainless foil MachinistTX recommends below. If you wrap it really tight and make double or tripple folds there wont be any way for scale to form.

To anneal, bring up temp to 1600F, then just turn off the oven and leave the receive in until the kiln is cool (at least a few hours). Then go ahead with Austenetize/Quench/Temper.

I kept having to get the stress relieve temp higher and higher to get rid of warping in the rail area, caused by less than perfect spot welding.. Finally i realized the stress relieve is simply not enough to reset those residual stresses.

machinisttx 11-25-2006 06:29 PM

Quote:

Originally Posted by twistedneck
Ideally, we would normalize or heat the entire reciver to 1600F then air cool and totally get rid of the residual stresses , but that high temp would create so much scale and decarb that it wouldn't be worth it for this thin sheet part.

You wouldn't have much problem with scaling assuming you were wrapping the parts with stainless steel "tool wrap". It's like aluminium foil, but about 500x better. :)

catman 12-06-2006 12:24 AM

Heat Treating and Oxidation Scale build-up
 
Garden Lime experiment

I'm not very experienced with heat treating but I've read a lot of old technical books and have found a lot of usable info.

Most old sources talk about using lime for preventing oxidation on high temperature steel. I bought a 50# bag of powdered (not granualar) garden lime at the farm store for $6.50. It is as fine as powdered sugar so don't breath it!

I filled a 6" diameter X 14" pipe section with lime. I put in a 2" diameter hardened hydraulic shaft covered the end in lime, the whole works was in a kiln, surrounded with fire bricks to prevent direct heat onto the 6" pipe, capped it with fire brick and put the lid on and plugged it in. More fire brick were added to the lid for insulation. I made sure no steel was in contact with the air or the pipe.

The steel reached about 2000 degrees ( vvveryyy hot!!!) based on the internal color and a digital pyrometer. 30 hours after I turned the kiln off the steel inside the lime was still over 200 degrees but no scaling, only a discoloration of the surface. The outer surface of the pipe was flaking the scale and looked nasty. The surface of the shaft had lime sticking in a thin coating to it that could be removed with a wire wheel or blasting with sand or glass beads. The lime itself was unchanged, no burning and fully reusable.

A person could also use a welded steel box to hold the lime, just don't make it air tight or we might hear about you on the evening news.

I didn't try this on any thin parts but it shouldn't be any different.

I think a person could make a slurry, like white wash, with lime and water to paint onto parts and into narrow openings of parts to be heated, too.

I've thought of using drill bits, broken broach, reamer, etc to see how a ground surface looks after heating in lime.

I haven't experimented with this anymore so it is up to you guys to try it and share your info with everyone. If you try it be sure to use something that can be scrapped so an early failure does not waste a good part. Have fun!

Uraijit 12-06-2006 08:48 AM

hmmm good info. Now, if only I had a kiln...

twistedneck 01-20-2007 04:45 AM

Quote:

Originally Posted by machinisttx
You wouldn't have much problem with scaling assuming you were wrapping the parts with stainless steel "tool wrap". It's like aluminium foil, but about 500x better. :)

machinisttx, you were right all along. That foil is awesome and works like a charm. Full anneal now before heat treat and its workin' great - zero scale after annealing.

machinisttx 01-22-2007 01:53 AM

Quote:

Originally Posted by twistedneck
machinisttx, you were right all along. That foil is awesome and works like a charm. Full anneal now before heat treat and its workin' great - zero scale after annealing.

Yep, it's expensive but it works! :)

igor 01-24-2007 06:41 AM

Quote:

Originally Posted by allesennogwat
I've seen some ceramic coatings on the internet that are rated to 2000 degrees F.I've never used them.I'm not sure they for heat treating and I don't know if they can be removed.Some info here,

http://www.timken.com/products/speci...etin%20102.pdf

I have never heard of those heat treatings ever being removed deliberatly, and afaik they are only really used to protect parts from excessive heats, IE Kiln walls, combustion chambers in engine heads, exhaust valves for combustion motors, and the inside and outside of exhaust manifolds on vehicles.

They are more for heat reflection, rather than absorption.

machinisttx 01-24-2007 10:34 PM

Quote:

Originally Posted by igor
I have never heard of those heat treatings ever being removed deliberatly, and afaik they are only really used to protect parts from excessive heats, IE Kiln walls, combustion chambers in engine heads, exhaust valves for combustion motors, and the inside and outside of exhaust manifolds on vehicles.

They are more for heat reflection, rather than absorption.

That is my understanding as well.

twistedneck 02-04-2007 03:19 AM

Another update to all who heat treat. Although stress relieving and annealing prior to heat treat (quench and temper) is helpful, its not always needed.

If you dont mind straightening a bit of warping after your final tempring is done, its no big deal to skip the pre austenetizing stress relieve or anneal steps.

bittwiddler 02-06-2007 08:29 PM

I've got a question about a receiver I think I messed up. I was working on my first AK flat and attempted to heat treat the trigger group holes with a propane torch. I was unable to get above a bright purple color on the holes but I continued with the quenching, reheating to a dull purple, and then air cooling just to learn what to do. I figured I would just use that receiver as a template and never build a gun from it. The next one I worked on I got a MAPP torch and that easily got the proper temperatures. Some people I've talked to said I should be able to go back with the MAPP torch and properly heat treat the first receiver and it would be fine. Is this correct or should I just continue to use it as a template? It's not like flats cost much but I figured if I could get a good use out of it then I would give it a try

twistedneck 02-06-2007 09:30 PM

Its fine if you re-do it. The only bad things that can happen when re-heat treating metal is grain growth due to to much time above critical temperature (1450F) and decarburization, also above critical temperature.

Since your propane torch never got the part above critical temp, then you don't have any issues. One of the great things about MAPP or Oxy torch heat treating is you get it hot quick avoiding all the negative effects above. Just make sure you don't get it too hot (beyond dull orange) not only can you get big grain growth and surface decarburization, but you will usually crack it upon quenching.

Remember when tempring, take it just past blue - blue = tempered martensite embrittlment range with 4130.


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