Is it ok to re-temper a blade for the colour?

[Josh B]

Hi guys,

First-time knifemaker here. In the midst of Covid-19 lockdown, I saw it as the perfect opportunity to develop a new hobby.

I filed a knife out of a piece of 1075, successfully heated and hardened it, and tempered in my home oven. I was aiming for 200C but my kitchen's old gas oven from the 1980s was a little unpredictable and the nearest I could stabilize without going below 200C was around 230C (give or take some fluctuations of 5C either way), at which point I did 2x 2 hour tempering cycles and got that nice bronzy-coloured oxide coating. Unless I'm mistaken, that colour indicates a successful temper - a bit hot maybe, but I'm led to understand that 1075 is fairly forgiving and as long as it doesn't go blue it should basically be fine (happy to have any misconceptions corrected here!).

I'm a big fan of this bronze colour, it looks great, and I was wondering if it's feasible or advisable to give it another temper cycle after the final grind and polish to get that colour back?

Will this additional temper cycle have any adverse effects on the integrity of the blade?

Will it look good? i.e. will the knife maintain its shiny polished finish but with the bronze colour superimposed on top?

How durable would the oxide coating be, and how long would such a coating be expected to endure on a knife applied to general kitchen use? On that note, would such a coating be food-safe?

Cheers,
Josh

 

[Mikel_24]

Excesive tempering cycles will soften the steel more than it is desirable and you will end up with poor edge retention.

I would suggest you ask the details in the Knifemakers area of this forum.

If you want to give it nice finish, you could sand to whichever grit you feel like and then etch with ferric chloride and tumble with some abrasive media. There is a nice video in Youtube from BladeHQ demostrating the process.

Mikel

 

[KingMC]

Moved from General Knife Discussion

 

[Stacy E. Apelt - Bladesmith]

Temper color is literally skin deep ... and the skin is only a micron or two. It will rub off easily in use.

Just sand it to the final grit and leave it shiny or etch in FC or something ( mustard or vinegar) to get a darker oxide. Even these etches are not very permanent.

 

[Drew Riley]

Excesive tempering cycles will soften the steel more than it is desirable and you will end up with poor edge retention.

Depends on what you mean by "excessive" and "soften", but this isn't exactly true. As long as he doesn't exceed the desired tempering temperature (~400F or so, I'm assuming), there shouldn't be any problems with multiple cycles. Granted, there may be some concern about overshooting the temps with a household oven, as "400" doesn't necessarily mean "400", but letting the oven pre-heat up to temp and monitoring with a secondary thermometer should alleviate those concerns.

As far as tempering to achieve the surface coloring, I wouldn't bother. After some light to moderate use, the finish will be rubbed off and just look "unfinished".

 

[Stacy E. Apelt - Bladesmith]

I agree with Drew. Additional tempering cycles for any normal length won't soften the blade any amount you can detect. Even a ten hour temper won't affect a blade noticeably. Temperature is much more a factor than time by a power of 10. What that means is that 10° increase does the same as 10 hours extra tempering.