Hardening bright mild steel using Eternite hardening powder.

Hardening bright mild steel using Eternite hardening powder.


Hardening bright mild steel using Eternite hardening powder. A part I am making in bright mild steel required its edge to be hardened so I needed to learn a new skill.  Why I hear you ask? Well, I made my ex copper, and internet guru brother Stephen, a chess set using aluminium and brass for the pieces when he retired. I have another younger brother, David, who asked if I would build him a Brian May ‘Red Special’ guitar? He is a Queen nut and also a talented guitar player. My own musical prowess ends when I turn on the radio!  My singing frightens the grandchildren, and sends people running for cover, so I was obviously the ideal man to ask…….

Since the skill sets required to make a replica Red Special guitar involves woodworking, a little electronics and some metal work David felt I was the ideal brother to make one! I now feel however that he conned me!  Rather than take the money he offered me I thought it was an ideal opportunity to make this a retirement gift and balanced up the brother issue. The fact that he has several years still to go matches nicely my current productivity rate!

The build will be covered on this website soon.

The guitar build will be the subject of a series of posts in the future.  With the Postman Pat van nearly completed and requiring good dry weather days to finish I decided to treat the guitar as a wet days project. One of the first task is to make the tremolo (does something twangy I believe 🙂 ).  This consists 3 metal elements which requires the part the strings connect to to be ‘floating’ on some motorbike strings. The contact point is a knife edged plate and the edge needs hardening. As Brian May himself points out it has lasted more than 50 years in his guitar and still going strong.

The hardening process.

To harden the bright mild steel you need to add more carbon. Some time ago I bought some Eternite hardening powder labeled “Shirley Aldred” from Chronos in the UK but I can’t find it on their website at the moment.  However if you are in the UK and want a supplier of a case hardening powder I did find this link. I have no personal experience of either the company or the product it sells but it may help you with at least one contact.

The process itself requires bringing the part to a temperature of between 900 to 1000 degree centigrade.  To judge when the correct temperature is achieved you tell by its colour but I had hoped to use the thermocouple and temperature facility on my digital multimeter which I use for my furnace.

Its all about colour…

However, as you will see on the video, the time taken to register the temperature, then move the part, made it impossible with just one pair of hands so I  had to fall back on the colour test (source Peter Wright’s Model Engineering, a Foundation Course p 48) a table of colour and temperature is listed below.  To differentiate between so many subjective  colours was less of a problem since my temperature requirement was the last one. The very bright red to bright orange red.

After it achieved the bright orange-red colour I quickly placed the part fully into a can holding the Eternite, burying it completely.  Agitation was required to ensure all the edging was in contacting with the powder. After letting it ‘soak’ for thirty minutes I pulled it out and quenched it, being surprised that it still held enough heat to sizzle.

To judge when the correct temperature is achieved

Colour                       Temperature (centigrade)

Barely red                           520

Dull red                               700

Blood red                            750

Cherry red                          800

Bright cherry                      825

Red                                       850

Bright red                            900

Very bright red                   950

Bright orange-red             1000

Challenges of using the colour test.

The reason for trying to use the multimeter is the ‘fluffiness’ of when something is either Blood red or Bright red A difference of 150 degrees.  When is it bright orange red?  In the end though, after a lot of heat, I do think I got to the bright orange red. What do you think?

If I had to reach a critical temperature within a narrow range I would have fired up my furnace and stabilised the temperature within it, then put the part in and let it soak.  This would have delivered a more scientific or more accurate reading I am sure.  In this case setting up the furnace seemed overkill.

I used Peter’s suggestion about using Brine water as a quench. The proportion of salt to water was found by using a tin can on scales. Resulting in an answer of 41.66g of salt to 500ml of water. The theory behind the brine solution is that the immersion of the metal causes bubbling at the edges whereas with the salt it stops this from happening.

After a rehearsal, to iron out any wrinkles (the pliers and my grip highlighted one!) heat was added. I wasn’t sure I could give enough flame with the first nozzle, and so it proved. What I wanted was a full flame, to immerse all the knife block in one go. So the nozzle was changed to the largest of my nozzles and the heating begun.

Hardening bright mild steel using Eternite hardening powder
Brian May’s Red Special guitar project and the Tremolo knife block

Have a look at the video


  1. Heat the metal to between 900 and 1000 deg celsius.
  2. Place the metal into a container with the carbon powder that allows total coverage of the part.
  3. Agitate the container to ensure all of the part is in contact with the powder.
  4. Leave in place for 30 minutes.
  5. Remove, then quench with oil or water as appropriate.
  6. Repeat the process if you require a deeper penetration.

As a matter of safety cover the powder  in the tin to exclude any air in case of fire. I checked after a further 30 minutes and there were still some red particles! I placed my covered container outside overnight just in case.

Having now completed the process I wonder why I never tried it before. I suppose I was worried about possible distortion to the part and they say it can be a problem. It has prompted me though to harden one of the spare camshaft I turned for the two Seal engines in build (pre Grandchildren…)

Please feel free to email me for any further information.


If you are looking for a source of the case hardening powder have a look at my eBay affiliate listings :-

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2 thoughts on “Hardening bright mild steel using Eternite hardening powder.

  1. Mike

    I hate to admit it but I always learn something new by watching your videos. Thank you for the detailed explanation



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