Looking for ways of hardening small parts without decarburizing the outside

[Bented]

Could you achieve the same result by flooding the electric furnace chamber with argon or some other inert gas to displace all the oxygen?
Is that impractical in an oven not designed as such?

Many gasses are inert at room temperatures but will react with steel at elevated temperatures. Some inert gasses are used for quenching purposes.
As far as I know a high vacuum reduces the effects of atmospheric born elements on the work, nature abhors a vacuum so one is even more difficult to achieve in a furnace.

[J Tiers]

Some inert gases are/were used in the process of vacuum degassing of steel at the mill, so there are some that would work. Argon probably being one, as it is used as an inert shield gas for welding.

No vacuum furnace here (must remedy that, of course). I saw methods for bleeding argon into the furnace recently, I think it was "This old Tony" when I browsed through to see if he had done anything interesting.

Vacuum is of course perfect.... if no gases no oxygen, so no oxide scale. Displacing it with an inert gas would do just as well as long as it really is inert. Or, ammonia would work, it would do a bit of nitriding along with the displacing of oxygen and creation of a reducing atmosphere. The burn-off of leakage might be a issue.

You asked (-:

I was just playing along! Don't spoil it. 😁

[dian]

did you temper them?

[Forestgnome]

I always just use straight boric acid. Warm the part and dip it in the boric. Reheat until the boric melts and repeat until it's coated well. When you're done heat treating boil it in water to remove the coating.

[J Tiers]

did you temper them?

No. They see no particular stress, so I left them with no tempering. They are not extremely hard, since I can get a slight cutting with a sharp file, but in general the file skids. Probably in a condition similar to some tempering of a very hard part. IIRC, the spec on the material said max harness was about RC56. Not stupidly hard.

[JRouche]

"Looking for ways of hardening sma"ll parts without decarburizing the outside"

Good question. . All parts go through a de-carb when heating. I have SS wrap, it still happens. JR

[dian]

No. They see no particular stress, so I left them with no tempering. They are not extremely hard, since I can get a slight cutting with a sharp file, but in general the file skids. Probably in a condition similar to some tempering of a very hard part. IIRC, the spec on the material said max harness was about RC56. Not stupidly hard.

i hope they are not going to crack on you. they are small, but still.

im not criticizing what you did, it will serve the purpose. however its a good example of what happens "at home". recommended holding time at austenizing temp. is 10-30 minutes. you probably didnt do that. so you wind up with around 55 hrc. by dropping them in cold water you "supercooled" the tiny parts creating a lot of (unneccessary) stresses. after a 170°c temper w1 should be around 65 hrc. therefore "max. hardness of 56 hrc" is strange.

btw, im still strugling with the american steel designations. some tables list w1 as 1.15%c, some as 0.7-1.5%c. some say 0.15% cr/ni/mo/v/w some say 0-0.15%. it obviously makes a lot of difference. so the question is what it was in the first place. the din system is more difficult to remember but is more precise.

edit: the pieces might get harder if you put them in a freezer for a while. (martensite finish temp. is way below zero @1.15c.)

[J Tiers]

Not an issue regardless. The as-quenched hardness is directly related to carbon percentage. These definitely did not end up glass-hard, so the RC56 appears quite believable. In any case that was the spec of the material.

I would not expect a 10mm OD x 5.05 mm ID tube to crack regardless, at a hardness that is really a "straw temper". Not a shape to do that, even with the knurled stress risers.

[dian]

if you have spares, try it, just for fun. plase it on an anvil and hit it lightly with a small hammer.

hardened parts dont crack because they are hard. it has nothing to do with it. they crack: 1) because they lack fracture toughtess/impact toughness/ductility. depending on what exactly happened to the steel it can very well be "soft" and brittle. 2) because of internal stresses. not only from martensitic transformation but also simply from quenching.

btw, meaning of glas hard: cracks when you drop it. (brittle, not neccessarily hard, just like glass.)

[J Tiers]

..........

hardened parts dont crack because they are hard. it has nothing to do with it............

Of course they crack "because they are hard"..... for hardened parts. And what you said is really the same thing.

"Hardness" in steel is essentially the raising of the yield point to be closer to the ultimate tensile strength (that is not a "definition", but it is true). The part, if quite hard, does no longer deform significantly, it simply breaks. The tensile strength may be more than it is when "soft", but still, the two get closer together. The elongation and area reduction are also less, as these are all linked. It would be difficult to get just to deformation point and have no portion of the piece go over the ultimate tensile strength for parts of any size that are quite hard.

The locked-in stresses due to cooling may be just close to the ultimate strength, so an added external stress can add with them and go over the ultimate strength. Or, in some shapes and sizes of work, the ultimate strength may be exceeded by the internal stress, and it cracks in hardening, or when it cools.

Incidentally, the RC56 hardness corresponds generally with a carbon percentage of around 0.40 %. That is also near the point at which you probably should not water harden the steel, as higher carbon may crack in hardening due to the violence of the quench. Oil is usual for percentages over 0.5%, especially for larger parts or more complex shapes.

If you want to see this effect dramatically, harden some music wire, which will generally harden "glass hard". Then, if you drop it on the floor without tempering it, the wire will likely shatter into two or more pieces. If tempered back toward blue temper, it will be "springy" and will not shatter if dropped. It may, if very hard, shatter under conditions that glass itself may not shatter from.

[dian]

i completely agree, except for the first paragraph . i whish i were that simple.

tensile and yield stress have not much to do with brittleness. its another property of steel. its futile to muse over how properties of steel are correlated, because it depends first on the individual situation and second on how you measure/define them. one example is the good, old banana diagram (hardness vs. "ductility"). if it were universally true, most modern steel products would not be possible. another example is an allloy as common as 4340: depending on the particular structure achieved it can have a high fracture toughness and a low impact toughness (actually energy) or vice versa while ys/us stays the same. (there are 9 ways to measure "brittleness"/"toughness" i can think of right now.*) ci can be soft and brittle and so can steel when in an unfavorable condition.

btw, max. hardness with w1 is 67-68 hrc (as quenched).

* just thought of another one: drop it on the floor.

[JRouche]

Hmm, dunno? What is this post? The Jerry and Dian web talk? Love it.. JR

[dian]

Yes, I know about stainless steel wrap and piece of paper. Don't have any SS wrap here, need to get these parts done for a guy I know, he brought them to me because I can do them rapidly. But I did not know about some of them needing to be hardened until I had them ready (nice surprise!).

Parts are small cylinders, about 10 x 15mm, and they are knurled. I need the knurling to be hardened with the rest of the part, and obviously they cannot be knurled after hardening. Parts are made of W1.

They do not need to be glass hard, but do need a resistance to deforming, and they need wear resistance. The material is what it is, it is supplied material, or I would think immediately of using A2.

I do not have the oven up and running yet, so these will be torch hardened.

So, I have "heard of" various "home remedies" for the hardening vs decarb problem. Some suggest coating them with soap (?). Some suggest heating somewhat and then dipping in sugar before heating the rest of the way. Some suggest dipping in thick oil first. Some say to use the "reducing portion" of the flame, others, and I agree, say that part is not as hot, although it might be "hot enough".

I could heat them inside a capped piece of pipe, but I'd not know if they were hot, and I doubt getting the pipe open before the parts are too cold to harden.

Any word on what actually works?

this is from a manufacturers site:

[BCRider]

Hmm, dunno? What is this post? The Jerry and Dian web talk? Love it.. JR

I know, eh? But at least it has a lot of good proper HSM shop talk information going back and forth on heat treating. Far more in tune with what we should be reading instead of magic money mining off the interwebz and following one ship captain's lack of proper skills and the (mis) use of video advertising that arrives in the mail......

[J Tiers]

All I can say is if you wanna put in a comment, have at it... dian is not the only person on the forum.....

The "customer" has not picked up the stuff yet, but on the other hand none of the hardened ones have cracked yet, despite all the stress risers on the OD (knurling). Given that they are only at the equivalent of a purple or blue temper, I would not expect them to crack.