Nitre Blued Damascus Blades Too Soft!

B

berkut

Joined
May 25, 2004
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Does nitre bluing alter the Rc of the blade and makeke for a blade that is not sufficiently hard to make a good working tool. Seems to me that the bulk of the steel that goes into mosaic damascus blades is 1085/95 and 15N20 also known as L6. To achieve 58/60 Rc the tempering temperatures of 1085/95 are in the 570 -600 Deg F range. to achieve 58/60 Rc the tempering temp of 15N20/L6 is in the 280 - 320 Deg F range.

From brownells catalogue nitre Salts act in the 570 - 650 Deg F range. It appears that the bright blue colors only appear in the upper end of this range.

!085/95 tempered at 650 deg results in a hardness of only 55 Rc and 15N20 at this temp will result in a mere 45 Rc. Both of these figures are way below the industry standard of 58/60Rc.

Am I completely off the mark here? Can anyone throw some light on this for me. I am very attracted to these highly colored blades but wonder whether they make reliably knives?
 
Berkut - Welcome to the forums... And I sure hope you get a decent answer soon 'cause now you've got me doubting some of the blades I've seen and lusted after... :eek:
You pose a very logical question... let's see what you get for an answer!
Good luck!
 
There was an article in Blade magazine a couple of issues ago about this very subject. The short answer is yes, blade temper is effected if nitre blue is used. That being said, let me expound on what I've discovered. Most will agree that TIME AND TEMP are of the utmost importance where steel is concerned, and with this in mind I did some experiments with nitre blue.........
My findings were:
-On blades of average thickness (3/16" to 1/4") with distal tapers, you can expect to loose one or two points of Rc hardness by the two minute mark with nitre blue.

-At the three minute mark Rc hardness drops dramatically.....to the point that I consider the knife a poor cutting tool.

Now, in defense of those (including myself) who use nitre blue on collector pieces, it is common practice among Knifemakers to ensure a customer(s) is/are fully aware of the drawbacks to "blue" blades. More often than not I have customers who request bluing, and do not care that the blades are not "high performace" cutters. They are more interested in the "Wow" factor that the bluing adds to the piece.
So, is there a loss of performance? Yes. How much? That depends on the Bladesmith who produced it, and how much they know about the process. Unless your working at the upper temp range of nitre blue, in two minutes about all you'll get is a splotchy blue, mixed with dark straw coloring on a blade. At the upper end of the nitre blue's working range you can achieve a nice blue color, but you have to be paying very close attention to what your doing. I would encourage anyone who is contemplating purchasing a "blue" knife, to ask questions, and make their decisions based on the answers they receive.
Please don't think I'm being snid or rude, but there are a couple of corrections that I would like to make about the steels mentioned. Most mosaic is 1080 or 1084 (for those who still have some), and 15N20, which is a very different animal from L-6. (15N20 is basically 1075 with 1.5-2% nickel) Generally the tempering temp for a mosaic of these materials is 400F-475F depending on the desired end product. I'm not poking fun at you, or trying to be rude, but I always smile when some mentions "industry standards". A competent Bladesmith, can manipulate the blade geometry (and other factors) that will enable blades in the 57/59Rc range to out perform blades of poorer design that are in the 59/60Rc range. It really boils down to the Bladesmith producing the knife. But, back to the question at hand........there IS a performance drop that occurs when a blade is blued using Nitre Blue, is it a bad thing? I suppose that depends on each individual. I would not expect someone to purchase a "blue" blade for a using knife, if for no other reason than the bluing is not a very durable finish. Even with a lot of handling there will be some wearing of a blued finish. On the other hand, a knife that has been well executed and finished off with Nitre blue is a beauty to behold!
 
Very well said Ed. I will back you up till your nose bleeds. I personally have done no testing with nitre blued blades because Its not hard to know the results of a too hot temper even if its just a few minutes in the bath. The blade is so thin in most cases that it will soften slightly along the cutting edge right where we need it to be hard. Those dazzling blued blades are very collectable and too pretty to use anyway but I think its best to make a blade for use first and looks second. I use Oxynate #7 from Brownells because it saturates at 292 F and cant lower the hardness. For the best blue colors though nothing can beat Nitre Blue except maybe heat coloring and we all know what that does.
 
Whao... ask and ye shall recieve...

Most illuminating answers...

I knew normal handling degraded a blued finish, but I didn't know the application process could so radically alter the properties of the blade...
So far, it's only been used as decorative process on guards and such on the knives in my collection... Looks good, fer sure...

Thanks for the info guys!
 
Well said Ed, I totally agree, some interesting points you have brought up.
I have started making a few fixed blades this year and I had one with an 11" blade of 1084 that would not fit in my oven I use to temper my folder and smaller blades in. I just thought I would use my forge and watch the colors.
Well I turned it a nice dark straw color with a few purple spots along the spine, I thought this would be fine, Boy was I wrong. I had Hank Knickmeyer rockwell it and it tested 48 to 50, not hard enough in my book. So I sharpened it up and proceeded to cut up oak fire wood and cut alot of cardboard, it preformed very well :confused: But I would not feel right selling a blade that was not heat treated properly. I am now using nitre blue salts to temper my blades at the proper temp and time with very good results.
I do like the blue blades but not on a using knife.

Don Hanson
 
Holy smokes Don - you're cranking out 11" blades these days? Let's see some pics!!

Thanks Ed, for the very informative post. I remember the trhead you started over on CKD on this point. Very educational.

Roger
 
Hi Roger, I haven't finished one yet but will show pics when I do.
I'll try to get some pics of my Blade show pieces posted soon.

Don
 
Not to disagree Ed but I too have been playing with nitre bluing and you are right to a point. You said the secret is time and temp. After the test that I participated in at Ed Schemp's last November with nitre bluing I ran sime tests here at my place.

At Ed's we were playing with What colors different alloys would turn at the same temps and time. We welded op some bars and cut slices off of them. These were then polished and etched and then put in nitre salts at 425 degrees and let soak for twenty to forty minutes. With a thirty plus minute soak all the alloys turned a shade of blue with only subtle differences. since we were only looking for color differences no one checked the hardness of the samples. So I took this one step farther. I made a blade out of some L-6 and hardend and tempered as I normally would. Then sharpened it and cut some rope. I recorded the number of cuts made at an average of twoo hundred cuts. I then put this same blade in my nitre salt tank which I had stabilized at four hundred degrees F. I let it soak for a full one hour tempering cycle, removed it and let the blade cool to room temp in air. I the repeted the rope cutting. I found very little if any difference in the cutting on rope. I do not have a hardness tester so I cannot give any specifics on rockwell hardness but I also don't see how a soak in salt at the same temp as the blade was originally tempered at woul do any harm to the blade. MY theory here is that the nitre salt, being an aggressive oxidizer simply accentuates the oxide layers that would normally form in the air at much higher temps.

So the long and the short of it is. I do not disagree that a blade that is blued at six hundred degrees +- a few will be softer than what is normally used for a good cutting blade but I do think that a nice blue is achieveable without sacrificing hardness.
 
Hi Bill!

Your right on the money. The soak at a temp which is at or below the tempering temp of the blade isn't going to effect anything. When I made the statement about things depending on the Bladesmith, and how much he/she knows about the process, the issues you bring up are what I was referring to. As you and I both know, there are ways around most problems........;)
The problem is that most won't take the time and effort necessary to discover the "work around". As a general rule, I suspect that most who haven't worked extensivley with Nitre Blue will take it as gospel that it must be used at it's recommended working temps (570-650F), and will therefore produce blades that I would consider too soft. Now, stop giving away everything! How are we gona be able to keep our images as Miracle workers if you you keep tellin' them all the secrets! :D
 
Now, stop giving away everything! How are we gona be able to keep our images as Miracle workers if you you keep tellin' them all the secrets!

Well I could tell you but then I would have to kill you.:eek:

I just had to reply because of the picture of my blue blade in the gallery. I wouldn't want anyone thinking that I was selling soft blades. :)

Bill
 
I would like to thank all of you who have posted on my earlier thread for the interesting info. I apologise for posting what is essentially the same post a second time. Being new to the forum when I posted the first time I thought that I had glitched and that nothing had been posted.

The treating of damascus and particularly mosaic damascus blades in Nitre salts to achieve an array of brilliant colors many of which are very striking seems to have become very popular.
Are these blades Just for show?
The data set out below indicates that the steels commonly used in mosaic damascus require tempering temperatures in the 300 - 600Deg F range to aquire a working hardness of 58 - 60 RC. These temperatures are below those required to achieve the blue range of colors by Nitre Bluing. This means that by heating them to the temperatures required to generate these colors the blades would be further tempered and would most likely have an average hardness in the low 50'S.
Is this good enough? Is the common standard of 58 - 60 RC now a thing of the past? Is 'pretty' now surpassing practical?

FACTS:
L6/15N20

Tempering temps: 300 F Hardness 57 RC
500 F Hardness 53 RC
600 F Hardness 50 RC

1085/1095

Tempering temps: 450 F Hardness 62 RC
600 F Hardness 57 RC
650 F Hardness 55 RC

Data from Heat Treaters Guide 95 Edition (ASM International)

NITRE BLUING

Colors: Straw to Gold lower temps in the 570 F range
Blue's higher temps in the 650 F range

Data From Brownells (Suppliers of Nitre salts)

Let us here what you think about this!

Berkut
 
Maybe this is what Steven Dick was referring to when he recently wrote about ABS members catering to a collectors' market where performance is relegated to a secondary consideration. Obviously I can't speak for him, nor would I even presume to suggest that he was indeed talking about fancy blued blades, but I have to admit that I myself have had some concerns about this.

It is very clear from the words written by the bladesmiths above that performance remains a very real - and primary - concern of theirs. They are very forthcoming and honest about their methods, and have performed some enlightening performance tests on their blades. I would not hesitate to place my utmost confidence in their work. That said, I don't know if all makers share the same concerns.

Further, with regards to rated smiths in the ABS, I can't help but wonder about the ethics of putting an "MS" stamp on a blade that a maker has deliberately compromised.

A bladesmith has to work very hard to earn his Mastersmith rating, and it's something to be extremely proud of. It means that he has passed rigorous performance tests, and has met the very high standards of fit and finish that the ABS expects of its masters.

If a customer buys a knife that bears the M.S. stamp, I feel that the customer should expect a certain level of quality consistent with the ABS standards. If, however, a maker were to place that stamp upon a blade that he had knowingly compromised, a blade that would not stand up to the highest levels of performance that his known abilities will allow, then I think it would be a disservice to the customer and a slight to the meaning and status of mastersmith.

I mean no disrespect to anyone - I simply mean to articulate that I, as an apprentice in the American Bladesmith Society, and a collector of forged knives, have certain expectations of the performance characteristics of a stamped blade.

Let me also add that I firmly believe that a bladesmith should be encouraged to pursue not only performance, but also beauty in his work. His artistic vision should be expressed however he sees fit. If, however, his artistic exploits materially compromise the performance of a blade to a level below that with which he would otherwise be satisfied, then he should not create the impression, either explicitly or implied, that the knife is not everything it is expected to be. Giving a substandard blade the M.S. stamp would, I believe, create that impression.
 
For some reason, I get the feeling that there is some intention in this thread to drive home the idea that a Nitre-Blued blade is sub-par. Two well known knife makers have said that this isn't the case if done correctly. There are a lot of steps in forging a blade that if done incorrectly will yield a sub-standard blade. They don't need to be of any color for this to happen. I think that trying to draw the conclusion that "Blue is Bad" is a poor rule of thumb when choosing a knife. But, I do think that it could be an area of discussion with the Knife Maker. To sum it up in a simple manner without charts and figures, The proof is in the pudding.
Greg
 
'For some reason, I get the feeling that there is some intention in this thread to drive home the idea that a Nitre-Blued blade is sub-par'

mckgreg, I am very grateful for the open and informative manner in which people have posted in this thread. My particular thanks to Ed Caffrey. I must state clearly that I do not have the agenda that you suggest.

Firstly let me say that some of these colored blades are so beautiful it almost brings tears to my eyes. However, being an individual of enquiring nature and having heard some rumours of the high temperatures involved in nitre bluing I went digging for further information. At first glance the information suggests what it suggests. (A little knowledge can perhaps be a dangerous thing.) I am glad to hear that there is more to it than meets the eye.

I am, however still not entirely convinced. This may well be due to my own ignorance in that I am not able to grasp all that Ed has written. For my part the jury is still out. I would like to hear the opinion of more makers experienced in this field.
 
A knife maker that can make damascus, heat treats his own blades, and uses Nitre-Blue to color his blades, WILL ALSO know that if he gets the bluing salts to a temperature that is higher than what he tempered at, will sacrafice his optimim blade hardness. If you are unsure of the makers ability to make a knife, whether blued or not, do as you would when not buying a blued blade, check him out. I have read this thread a few times and if you are in doubt, then don't buy it, but don't think that blued blades are soft.
Another thing, when you make damascus using the steels that require a high tempering temperature and one that requires a low tempering temperature, they will both change and be different due to the carbon migration and the amount of each material used in the mix. If it were true that at 475 degrees I would get a 57 Rc from the 52100 and a 52 Rc from the 15N20 , then we would still have the hard and soft serrated edge effect. The people making damascus with L6 and 15N20 would have either chipped edges in spots or rolled edges in spots, depending on the tempering temperature. I hope I have helped in alleviating your worries about the Nitre- Blued blades.
 
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