Strange lack of tempering-colors on polished surface

M

MoTRoN

Joined
Jun 18, 2016
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I wanted to purpously discolour my blade while tempering. so i polished it after quenching. when hardening i used steele foil to prevent oxidation and decarburisation. The blade is 110WCrV5 Steele (aka 1.2519) and was oilquenched at 830°c and tempered at 280°c for 1h. I am very confident these temperatures are accurate, since i verified them with a additional k-type thermocouple. i also cleaned the bevels of the blade with breakclean to make the discoloration as uniform as possible and polished them to 600 grid with sandpaper by hand.
Strangly the spine of the blade which had not been polished or cleaned discolored perfectly to a deep purple, but the bevels only took on the slightest hint of blue.
it was difficult to photograph this, but i tried my best here:
https://ibb.co/iG3UWx
https://ibb.co/des1dc
i know the spine looks like it was marked with a sharpie but that is not the case.
has anybody got a idea why this happened? i have basically no idea and my best guesses are complete shots in the dark. maby the breakclean has some anty-oxydising agent in it?
 
Some possibilities are:
By putting the Brakeclean/Brakleen on the blade, the surface may have been protected from the oxygen. It is the oxygen that combines with the steel to make it blue.

A highly polished surface will have less total surface area for exposure than a rougher surface.

Surface oxides from HT in one hour may vary greatly depending on the available oxygen. A second temper would be wise.

You tempered for one hour only at 280°C/536°F. (That is a bit higher than most people use for a blade.)
I do agree that it seems like it would have made some coloration.

I would sand it very clean and do another temper with the blade as clean as possible. Wash well with dish soap, rinse in hot water, then wash off with denatured alcohol.
 
thanks for the response. the breakclean seemed to evaporate quite fast, so if it was actually responsible, it must have had some special anty-oxydising agents, as i stated before (kinda makes sense in breakclean). i will not temper again, since as you already noticed, 280° is quite high, and i dont want to soften the blade any more. according to a semi sketchy data sheet on the steel it should be around 60 HRC, but i don't want to take it any lower.
some slightly rougher areas did become more blue, so your argument that smoother areas have smaller surface makes quite a bit of sense. the temper was archived by electric heating, so the atmosphere did contain the necessary oxygen. the temper definitely caused discoloration on the spine, as can be seen on the photos above, sadly not on the bevels, where i wanted it.
 
Tempering it more/longer will only have a very small impact on the final hardness. After an hour you have reached the "leveling off point" where it takes much more time to see even a drop of 1 Rc.
 
I would also recommend two hour temper cycles for this steel. The higher alloy and type of carbide formers need longer tempers.

1.2519 steel is pretty much like Hitachi Blue #2 steel or V-Toku2 (Tokufu version).

It is normally tempered at 150°C/300°F to get a blade around Rc 62-63.
Some folks temper at 120-125°C ( 250°-260°F) to get Rc 65-64.
Your HT would have given you around Rc 60, as you had suspected from the online charts.

One thing I didn't mention in my first response is -
To get a blue color in tempering, you have to heat steel to 300°-325°C ( 570-620°F).
I have found the best beep blue color to be done on a knife is by using molten Nitre/Niter bluing salts. They are melted in a black iron or stainless tank and heated to between 315-340°C ( 600-650°F) and the very clean and dry blade is placed in them for 5-10 minutes. The blade is removed and immediately cooled and cleaned in running tap water (cold). The blade would drop to Rc 58-59 at the most. A 10 minute soak in the molten salts is not as bad as 2 hours at the same temperature. I do blades for 10 minutes in the low temperature salt pot and can detect no appreciable drop in hardness.
You can make an excellent nitre bluing tank from 4"stainless pipe and an old small pottery kiln. Weld a round bottom on the tank. Cut a hole in the kiln lid to put the pipe in. The pipe can extend as much as 6" above the lid. Wrap 2" of kaowool around any exposed pipe above the top. Change the kiln control tom PID and it can be set for any desired temperature. Put the TC ( monel/stainless covered) in the tank. Start heating with the PID set at 200°C and then raise to 300°C after reaching 200°C. After it levels off, set to the exact desired temp.
 
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