HMS Thorn by Kevin Kenny - 1:48 scale - Swan-class - David Antscherl practium

[AON]

A very good video that shows the process quite clearly.

Thank you for that... but I have few questions about the process you followed.

 

In your first stage annealing (softening) process you do not quench the part.

Why?

Do you not want to freeze the microstructure in its new homogeneous crystal phase?

 

After re-hardening you Temper the scraper in your (toaster) oven.

At the 14:01 minute mark you mention putting the part in an oven at 250° to 300°F for 1 hour

At the 14:48 minute mark you say 350°-400°F for 2 hours

Which did you do?

With a hacksaw blade likely being a rather thin piece of 1095 grade steel wouldn't 350°F for 10 minutes be adequate? 

 

You quenched in oil.

What oil did you use?  Motor oil, cooking oil? 

Is there any concern of flash ignition?

Some cooking oils flash ignite at 600°F and the part is rapidly cooling from about 1500°F or less.

Some motor oils flash ignite at 400°F to 600°F.

I understand oil is a slower quench and less stressful on the steel.

Why not hot water?

I believe cold water might be too much shock and crack the metal.

 

Would it be better quenching the blade holding it vertically in the quenchant and swishing it about gently to get an even (front and back) and efficient (heat transfer) quench?

 

Thank you Kevin.

 

[Kevin Kenny]

Lots of very good questions,

let me say that the process was based on my research. I do not pretend to be an expert in this field,

 

quenching to soften. It was particularly mentioned that steel that is brought up to a slow heat should not be quenched as it will become brittle. They specifically suggest that it should be left to cool at room temperature. Apparently this is different for gold and silver and specific only for steel.

 

tempering. There are different opinions about the temperature and the time it should stay at that temperature. The objective here is again to slowly bring the piece up to a low red heat. This temperature is supposed to be around 250 to 300. when this happens and to check that the correct temperature has been reached and you have a magnet check the steel to see if it has lost all its magnetic properties. This is the critical temperature.  At this point it is dunked  in the oil. The type of oil does not seem to be important except for environmental issues. At 300 degrees there is little chance of fire. Some use cooking oil some use old discarded motor oil.  Water is not recommended as it will again make the metal brittle. The time spent  in the oven  is just to get to temperature slowly back to the 250 to 300 degrees. Once achieved You simply turn off the over and let it cool naturally providing your wife does not come around and want to know what you are doing in her oven. This was my big problem as we were renovating her kitchen at the time hence the use of the toaster oven. The swishing of the steel in the oil is an old holdback from my drinking days as i was a rum salesman.

[AON]

Are your degrees  F or C?

 

In your research did you find the T-T-T diagram for the material to understand what happens in what time frame?

[Kevin Kenny]

Degree F not sure what TTT diagram is. 

 

[Kevin Kenny]

Just looked TTT up and this is getting far to technical for a simple mind like mine. Having read it i have no way of knowing what type of steel  the hacksaw blade is made up of  so no way of knowing how long it should be heated to and the exact temperature. But i am always will to learn. Thats the purpose of my videos. 

[druxey]

I've found heating the hacksaw steel to bright cherry red and air cool will soften it nicely. For the quantity of molding that I need to run the blade does not really need to be re-hardened, so don't bother with that process.

[AON]

Thanks D. So you do not quench at all.

 

Kevin

Hack saw blades are usually 1095 carbon steel.

 

TTT is the Time-Temperature-Transformation diagram. It shows at what temperature and over what time the microstructure changes Austenitic to Ferritic + Cementitic)

 

I believe low red heat is not 200 to 300 F it is more like 1000 F and well above the flash temperature of some oils.

Edited June 18, 2023 by AON

[Kevin Kenny]

Good to know, but in any case thats why the fire extinguisher is always close at hand. In case. 

[Kevin Kenny]

I guess based on this it was around 700-800 degrees F

[AON]

Kevin

The chart is SAE not Imperial so that would be about 1000°F.

 

The fire extinguisher needs to be for oil.

 

What type oil did you use?

 

I believe Druxey's method was correct and I will explain later tonight after our company has left ... or tomorrow.

 

Alan

 

[AON]

Good evening Kevin. I will do my best to explain what it all means...

 

I was taught to read these diagrams 50 years ago so if there is a professional metallurgist out there that can do this better than me please step in.

 

I will explain the Isothermal Transformation Diagram (T-T-T) as simply as I can in three postings - this being the first.

Hacksaw blades are made from high carbon steel (C1095) and this is the Time-Temperature-Transformation diagram for it.

On the left is a scale for temperature. On the bottom a scale for time in seconds. On the right a hardness scale in Rockwell C (Rc) where Rc15 is soft and Rc58 is hard... so soft is towards the top of the graph.

 

There are three swerving curves in the middle of the diagram. The one on the left is the start of transformation from one structure to the next. The one on the right is completed transformation. The one is the middle is 50% transformation. You can interpolate 25% and 75% from that.

 

You must heat the blade above the "As" line, a horizontal line at about 1375°F to completely transform into Austinite. So you heat the high carbon steel blade to a Red colour which would be above 1400°F... in the "A" or Austenitic zone and to soften the material you want it to cool at a rate that will cross all the transformation lines in the upper "F+C" zone. Once it crosses that far right line it is fully transformed and cannot become anything else.

F+C is Ferrite (soft) and Cementite (hard). The higher you are the more Ferrite is formed and the softer the material will be. This is the Pearlite phase.  The lower you are the more Cementite is formed and the  harder the material will be. This is the Banite phase. At the very bottom of the graph is the M zone... Martensite. This is very hard and brittle.

 

The hacksaw blade is very thin and so heats up and cools down very quickly.

 

What you did was oil quench from 1400+°F to warm in possibly 10 seconds (?) or more. If you draw a line from 1500°F to 10 seconds (red line on the diagram) you see the material passes through the knee of the transformation curve but does not completely transform. Instead it passes into the "M" zone and becomes mostly Martensite, hard and brittle at possibly HRC66.

Edited June 21, 2023 by AON

[AON]

In this figure we are showing air cooling from 1500°F to warm in 600 seconds = 10 minutes (?). We pass completely through the transformation curve. Where this happens is in the Pearlite phase and is about a hardness of HRC40.  Once it completely cross that third line it cannot become anything else. This is considerably softer and easier to machine than Martensite.

I believe this is what Druxey described he does. As his scrapers are not for production use he does not bother to re-Harden, Temper and Age his scrapers.

 

If you were to do this it would be a case of heating the scraper to 1500°F, allowing it to air cool to become warm to touch, only because it was worked on and any localized heating may have changed the microstructure in the area. Then Temper it by re-heating it to 275°F in an oven and holding at that temperature for more than one hour to phase to achieve a hardness of about HRC63 to 65. Finally you would shut off your oven and let everything cool naturally in the oven to room temperature to Age it, make it more ductile, less brittle.

 

If you did do all this you would have a harder/tougher/longer lasting scraper... but is it necessary?

Edited June 21, 2023 by AON

[AON]

and lastly...

Most industrial heat treat companies soften their steel by cooling it very slowly, over hours of computer controlled heating temperature drops in a heat treat furnace. This forces it to cross the transformation lines higher on the curve so the material is even softer (HRC25?). This would be ideal but we hobbyists do not have the means or time to do this.

From what I've learned today about home quenching in oil I would personally avoid it. Hot water should work as well but the blade is so thin that allowing to air cool shouldn't take an unreasonable amount of time.

Edited June 21, 2023 by AON

[Kevin Kenny]

Thank you Alan. Ill have to read it about 5 times to understand in layman's terms. What was interesting in the videos on YouTube is that the test to achieve the correct heat was  arrived at by testing the magnetic properties of the red hot steel. They claim its only when the steel has lost its magnetic properties that it has achieved the correct temperature to be made soft, and they just let the piece cool at normal room temperature on the work table.  Going back to my colour chart above thats between 1300-1400 degrees F. 

[AON]

Yes. Above the "As" line.

But when going by colour one person's red can be another's orange!

This is the case for my wife and I. It has to do with the number of cones in your eyes.

So it is better to go a little higher then to find you were to low... but we aren't building rockets so maybe it isn't so important.

[Kevin Kenny]

I have included some of these changes in the video and replaced it with a new version. 

 

[Kevin Kenny]

The magic of the iPad and a monitor 

[AON]

That's freaking ingenious!

[Kevin Kenny]

 

[allanyed]

Great video Kevin

What is extra nice is that you show the mistakes and explain what causes these and how to avoid them.

Thanks for sharing. 

Allan

[Kevin Kenny]

 

[Kevin Kenny]

[TBlack]

Kevin,

the Turbo Carver looks great, but is quite expensive and requires an air compressor besides. Do you think the Foredom could do the same job as well?

Tom

[Kevin Kenny]

The Foredom is a totally different tool. It runs at around 20,000 rpm and is full of power. The Turbo carves runs at 400,000 rpm and has no power what so ever. You paint with the turbo while you cut with the Foredom. You will have to use one to understand what i mean. I have long had a compressor so that cost is not of concern for me. The hand piece is around $200. 

[AON]

The Foredom is like a Dremel in my mind and my Dremel is occassionally possessed by evil spirits when carving with proper carving bits.

 

I cannot imagine the difference when using an exceptionally high speed Turbo Carver that sounds like it is designed for the job.

[Kevin Kenny]

Just arrived .009 thick 

[Kevin Kenny]

Also found a good tube of Prussian blue and some new paint brushes that turned out to be useless.

[druxey]

For painting frieze details you need to invest in top quality sable brushes. Try Winsor and Newton Series 9 or Rosemary & Co. Expensive? Yes, but properly cared for they will 'point' beautifully and last a lifetime.

[Kevin Kenny]

Starting to figure put how to paint the scrolls. Using acrylic paint. 000 paint brush. Pencil in the scrolls on the blue acrylic and painting over the pencil lines.  . Any suggestions

[stuglo]

On 7/19/2023 at 8:08 PM, Kevin Kenny said:

The Foredom is a totally different tool. It runs at around 20,000 rpm and is full of power. The Turbo carves runs at 400,000 rpm and has no power what so ever. You paint with the turbo while you cut with the Foredom. You will have to use one to understand what i mean. I have long had a compressor so that cost is not of concern for me. The hand piece is around $200. 

You've  reawakened my interest in this tool. What would you suggest as the basic burrs and accessories based on your experience ?