Austemper Medium

[duurza]

I've been reading up on different hardening treatment methods and the one that really intrigues me is Austempering; mainly because it was recently discovered. From most of the articles/books that I've read, the typical quench medium is a salt bath. That seems to be widely used but unless you can afford the equipment for it, it doesn't seem very feasible for home DIYers.

What I am really interested in knowing is, could liquid metals be used instead of a salt bath? For example, tin-lead (~63% tin %weight, ~37% lead %weight - eutectic composition) (basically solder material for electronics although they use 60/40) melts at 183 °C (around 361 °F). That is below the martensite start temperature (assume 400 °F) which would allow one to easily see if the temperature is too low (solid bits start to appear). There are also many low temperature liquid metals; it's just tin-lead seems to be the cheapest and easiest to work with.

From what I've learned from my Heat Transfers courses, metals have thermal conductivities that are typically higher than their convection coefficients when in liquid form (convection depends on buoyant forces and metals aren't very good at that). Due to this property, the bulk thermal transfer during a quench should be conduction which simplifies the process as preheating to achieve a certain viscosity is not needed. Also, since most metals don't typically boil below 1000 °C, one would not need to worry about any vapor jackets.

What do you guys think? Has anyone tried this before? I know that molten lead is used in some cases to soak steel at the austenizing temperature so it definitely wouldn't be damaging to steel. I can't find any articles on whether or not tin would damage iron but there doesn't seem to be a reason why it would.


Side note:
Currently, I'm finishing up profiling a sword from O1 tool steel by hand. I'm going to give Austempering (aiming for 100% bainite) a try for the following reasons:

1) Quenching and tempering have been done for 3000 years or more. It just doesn't seem that exciting now that we have a very good understanding of what goes on during the hardening process.

2) Running a molten solder bath so far (at least on paper) seems safer than running a salt bath. Correct me if I'm mistaken but the salts used are sodium and potassium nitrate? Those are powerful oxidizers and without the proper equipment, I don't feel safe using them. I'm also not going to purchase any equipment as I don't foresee myself making more than a blades a year.

3) As stated above, I'm almost done my sword which is made from O1. Since O1 is not as tough as something like 5160. I figured that it would be best to maximize toughness and sacrifice a bit of hardness. This tradeoff seems worth it to me.


Thoughts or comments on this would be nice!

 

[timgunn1962]

Liquid metals can be used and certainly have been used in the past.

Lead is quite good in that it does not tend to form intermetallic compounds with the iron, but its melting point is higher than we tend to want. Many of the common low-melting-point metals do (including Tin). Intermetallic compounds tend to form at the grain boundaries and drastically reduce the toughness of the workpiece.

About the best liquid metal for the job seems to be Lead-Bismuth Eutectic. Bismuth, like lead, does not form the intermetallic compounds. It also expands on solidification, unlike other metals. LBE neither expands nor contracts on solidification/melting, which should make it safer/easier to use. Verhoeven refers to use of LBE for Heat Treatment of steels.

Bismuth is expensive though and is also very dense, making it very expensive to fill a useful-sized quench vessel. This factor alone tends to make salts the preferred medium among bladesmiths: you made the point about affordability in your first paragraph.

 

[Stacy E. Apelt - Bladesmith]

IIRC, Murray carter uses a lead bath. I also think Sam Salvatti used one a while back.

 

[mete]

Recently discovered ? HA, HA HA !! where have you been ?
Of course quenching in oil that's near the flash point is very dangerous. That leaves salt or metal. The techniques and safety procedures must be learned. You must seriously study the methods getting advice from those who use it .
For me it's not for the average maker and the gain in blade performance is not great compared to quench and temper.
Originally the technique was developed to minimize warping in complex shaped parts.

 

[duurza]

Liquid metals can be used and certainly have been used in the past.

Lead is quite good in that it does not tend to form intermetallic compounds with the iron, but its melting point is higher than we tend to want. Many of the common low-melting-point metals do (including Tin). Intermetallic compounds tend to form at the grain boundaries and drastically reduce the toughness of the workpiece.

About the best liquid metal for the job seems to be Lead-Bismuth Eutectic. Bismuth, like lead, does not form the intermetallic compounds. It also expands on solidification, unlike other metals. LBE neither expands nor contracts on solidification/melting, which should make it safer/easier to use. Verhoeven refers to use of LBE for Heat Treatment of steels.

Bismuth is expensive though and is also very dense, making it very expensive to fill a useful-sized quench vessel. This factor alone tends to make salts the preferred medium among bladesmiths: you made the point about affordability in your first paragraph.

Thanks for your suggestion. I read Verhoeven's book a month or so ago but only remembered that it briefly touched on LBE. It is definitely something that I will consider. I probably won't need the eutectic composition as the melting point is way too low. I could probably add more lead and increase the liquidus point for the whole system to approximately 350 °F which would suffice for both martempering and austempering applications. That would reduce the cost. I also did a quick google search and I'm not sure how accurate one of the sources is but it shows that the price of bismuth has been falling (http://www.metalprices.com/p/BismuthFreeChart). It looks like it is less than 7 USD / pound which is the price of Tin (the last time I checked).

Regarding salt bath vs molten metal bath, I will definitely not be trying the salts. The material may not be more expensive but the equipment certainly would. Also, there is also the danger involved since those will readily decompose. At least for molten metals, I would just need to avoid breathing in any fumes which should not be difficult as I would be working in an open space and I will be wearing a respirator.

Also, once I'm done with austempering, I could cast the metal into something.

 

[duurza]

Recently discovered ? HA, HA HA !! where have you been ?
Of course quenching in oil that's near the flash point is very dangerous. That leaves salt or metal. The techniques and safety procedures must be learned. You must seriously study the methods getting advice from those who use it .
For me it's not for the average maker and the gain in blade performance is not great compared to quench and temper.
Originally the technique was developed to minimize warping in complex shaped parts.

According to wiki, Austempering was studied and understood around the 1930's so its relatively recent compared to quench & temper (https://en.wikipedia.org/wiki/Tempering_(metallurgy)#History) =).

Also, I know austempering isn't for the average maker; however, each year I try to do a science project. For 2016, it just so happens to be making a sword and since I don't project the material cost to exceed $750 in total, it definitely is worth a try. I know O1 isn't as tough as something like 5160 so it seems more appropriate to maximize toughness while sacrificing minimal hardness. I'm referencing this TTT (http://www.cashenblades.com/steel/o1.html) and it seems like austempering at 500 °F would give a hardness of 56 which is plenty hard for a sword.

Initially I wanted to make my sword from 5160 but I can't seem to find a supplier near Toronto that has it. Steels with at least 0.5% C that are available that I'm aware of are tool steels. Ordering from the USA is an option but our dollar is so weak right now that it's just discouraging. I figured that if I might as well use a steel that is more readily available in case I make a mistake. I am basically having the same problem as in this thread (http://www.bladesmithsforum.com/index.php?showtopic=2202).