Oils for Hardening

I don't need to know the ins and out of different oils Just a simple what oils do you find best for quenching when hardening steel.

I only want to know from those that have used oils to hardened steel

Edited By Dalboy on 19/11/2022 11:56:26

The best, and traditional oil for that is whale oil but I doubt you'd find any of that these days.

For simple at home use, any mineral oil will do but if you use old car oil (which is perfectly ok for home use) be prepared for it to flame over upon the initial quenching.

I always use oil rather than water for silver steel to eliminate the potential for cracking.

Tug

When I harden oil-hardening steels I use a thin oil (low viscosity), I have also used rapeseed oil.

Thor

I have read that used motor oil is good because of the carbon content but I have no facts to back that up.

I have used ISO 32 hydraulic oil and red diesel for hardening silver steel - couldn't tell any difference in results between the two.

Nigel B.

Whale oil is very good

Dirty motor oil has lots of carbon and can be good

First, catch your whale!

For what little I have done, any light oil that has been to hand. Hydraulic oil for agri machinery so likely ISO 32.

It’s only used to slow the rate of cooling, cf water or brine.

Thank you all

I am sure I read somewhere that Jojoba oil had a very similar composition to whale oil. I have never tried it maybe someone else has?

If you're going to the time and trouble of making a number of your own cutting tools (like me) then you might as well use a proper quenching oil that's formulated for the purpose. There are a few brands available in small quantiites at reasonable cost. The one I have used is-

Quench oil

At £22 for 5 litres; doubtless you could find a cheaper one. This one doesn't smoke or flame up too much, or smell.

if it's just a one-off I guess there are other options, as detailed in some of the other posts.

Regards,

Clive

Bad news, you probably do need to understand. What other people do only helps if you're doing exactly the as them!

What sort of hardening is needed?

Heating certain steels to a certain temperature and holding it for a certain time causes the internal structure of the metal to change. If the temperature is then reduced rapidly, the modified structure is caught, and the properties of the steel are changed. The goal is to create a desired combination of hardness and toughness.

Water is a good quench, but it's too fast for many steels, which it makes very hard and brittle with a high-risk of cracking and distortion. On the other hand, some steels need very rapid cooling, in which case brine is often used. Water quenching requires skill, for example if the metal isn't enthusiastically stirred during the quench, steam forms an insulating jacket around the metal which then cools too slowly and doesn't harden. Or the metal cools too quickly and shatters.

Oil quenches slow down cooling compared with water, making it easier to avoid over-hardening, cracking and distortion. They don't form an insulating steam jacket, making the need for stirring less critical.

There is no single best oil for quenching. For predictable reliable results it depends on the alloy and whatever combination of properties the quench is intended to achieve. Each type of oil absorbs a certain amount of heat at a certain speed, and one of them is optimal for the job in hand. This is important if the metal is being hardened to a specification, less so if 'man in shed' simply wants to turn a soft steel into something harder, and isn't worried about hitting a Rockwell number!

For amateur purposes affordability is usually more important than the exact result. Best avoid light oils like Petrol, Paraffin or Diesel because of the fire hazard. Plain motor oil is quite popular, and some prefer used oil. Used oil has been modified by a good hot thrashing in an engine, effectively making the oil lighter (slower cooling), and it's cheap! On the downside used oil is dirty and a bit of a health hazard, but the muck helps if the intent includes leaving a black protective finish on the steel. Otherwise, vegetable Oils, ISO32 and generally not too sticky oils without additives are a reasonable bet for amateur purposes.

The way to know for sure is to identify the type of steel, decide what the result needs to be, and then look up the heat treatment. It will recommend temperatures, holding times, the type and volume of quench needed, and any subsequent heat treatment needed. It's quite common for the quench stage to produce an over hardened, highly stressed, brittle product, and for that fault to be corrected by tempering. Industry have lots of choice. In addition to straight oils as already mentioned, they look to powders, emulsions, and a wide range of synthetics.

Steels vary tremendously. Silver steel is good stuff: the alloy is formulated to produce reasonable results from water quenching without over fussy time and temperature constraints. Only problem is the price! Cheaper metals are fussier. The worst I know of is HSS which has incredibly fussy heat treatment requirements; the process is essentially too difficult to follow without special equipment.

I always like to ask 'what could possibly go wrong'. I don't think the type of oil is critical for amateur purposes. More important to know what the steel is, to have a torch or oven able to heat it quickly to the required temperature, to judge that temperature with sufficient accuracy and hold the metal at temperature until the internal changes are complete, and then getting the object into a sufficiently large quantity of quench to remove the heat at the right rate. ( Large rather than small!)

Dave

I use whatever motor oil is around in the shed. Or hydraulic oil if I have it around. It all works.

More important than the type of oil is the technique you use. Use a large enough container of oil that the oil will not be overly heated by the hot job. And when plunging the job into the oil, don't just toss it in a and leave it stationary. Hold the job in pliers or on a piece of wire and vigorously move the job up and down while moving it around in a circle in the can of oil. This helps stop pockets of stationary oil vapour forming around the job and insulating it from the cold liquid oil.

Same applies if using oil to do oil blacking of parts rather than hardening.

And yes always have flat sheet of metal on standby to put over the tin of oil if it catches fire in the process. Using clean engine oil I have not had it catch fire, but it could. I also keep a fire extinguisher handy and do any kind of torch work or welding etc outside the main workshop just in case. I do not trust fire in a workshop.

Found this to be quite instructive on the subject of quenching.

**LINK**

regards Martin

Clive ,That's cheaper than c**p engine oil, what's not to like.