Thermal Bluing Issue

[LMS]

I am practising bluing screws. I am using a soldering station with good temp control (I have adjusted the thermostat to increase the temp beyond the 400°C limit). I have a stainless steel bowl with a bed of brass shavings and a thermocouple to measure the actual temp in the bed. My issues are:

1. The temp I found to get to blue was well above the slated 310°C. 

2. I was at something like 370°C to get a decent blue.

3. I am only able to blue the head of the screw (the surface that I had polished).

4. Even with cheating the thermostat, I was only able to get the bed up to 458°C. That still was not sufficient to blue the remainder of the screw, it only got this to a straw colour.

Can anyone advise on what I'm doing wrong, or need to do differently to blue the whole screw. Also, why is the temperature so far above what's quoted for bluing steel?

All advice/insights will be gratefully received.

 

Edited 10 hours ago by LMS

[tomh207]

I would change the container to a thick walled brass one to start with. Looking at you screw pictures they don’t look shiny enough, really it’s best to do a black polish on them before trying. Also make sure once polished they are scrupulously clean, I dunk my experiments in acetone before bluing.

 

i think the thin walled stainless container to leach away the heat from the shavings, most successful bluing videos for example use thick walled brass to hold a good deep amount of shavings which acts as a better holder of heat.

 

Tom

[LMS]

Thanks for the reply. I polished the head of the screw using 1500 to 10000 lapping paper and a screw polisher. This is why I think the head blued and the rest of the screw didn't. Silly question, how do you polish the whole screw inc threads, if that's what it needs to blue? 100% clean, Elna red and heated ultrasonic bath + IPA rinse. Finally, there is definitely heat leak as there's about a 30c difference between the brass bed and the soldering station temp. Having said that, the bed temp was stable. I will try to search out a better brass pot, but I haven't found one yet, hence the stainless steel bowl. 

[mbwatch]

Are you certain the screw was not plated with nickel in the first place? Polishing the head may remove the plating and expose the steel to oxidizing while the rest of the screw is protected from oxidation.

I have made this mistake...

An unplated unpolished screw will turn blue but not look very nice.

I agree about the container. It is much too big and acting as a heat sink. To get decent results I kept reducing my pan until I settled on a 3cm steel cup.

At the end of this thread is what I ended up using over a flame. Not exactly useful on a hot plate though. https://www.watchrepairtalk.com/topic/30935-bluing-hands-with-a-spirit-lamp/#findComment-264421