Decarb, Unreliable Hardening of Fire Strikers

The Problem

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Here's a QUESTION that I got on alt.crafts.blacksmithing on 12/21/2011. I had asked for input from the folks on that group (I had told them that I had some ideas, but wanted to hear their ideas, first):

"--- What I want to hear about is what you -think- you learned about (Decarb) and how you figure you know more than squat about it. LOL :) But I can wait on that tho.
>
> What I don't want to wait on is an answer to this question...
>
> 1) Pete, did you do any careful spark testing?
>
> Alvin in AZ---"
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SHORT ANSWER BACKGROUND:

Hello, Alvin.

ans. 1) I don't have any sense for "spark testing" at all. I don't know whether the problem is my eyes or my brain. When this practice has been demonstrated to me, I don't even see things the way the demonstrator says he sees them. otoh, I do make sparks all the time with my strikers and I notice real quick if they don't work right.

I have a long answer and a short answer to your basic question. The long answer is for later.

Background:
Hot oxygen and hot carbon like each other a lot. They like to make carbon dioxide. If I don't take appropriate steps to control this phenomenon, the outer surface of the part will loose some of its carbon. Significant loses can be as little as a few thousandths of an inch to as much as 1/8", from input I have received.
-I always assumed that scale forms faster in the fire than decarb occurs, so I never paid it much heed.
-Apparently, the higher the carbon content in steel, the more rapid the decarb.
-Decarb appears to slowed, but not stopped by other alloying elements.

Finally, the Short Answer:

The Long Answer-

This has to do with my problems getting strikers to harden and make sparks while demonstrating at Laura Days in Sep. 2011. I have had this problem from time to time. This time I decided to dig into the problem seriously.

Some Background:
Carl Nordquist bought some MSC 3/16" X 1/4" W1 (the stuff I have been using) and had some problems making strikers that work properly, too, a year or so ago. He gave me a piece of his stock to try at the Ulmelund, Wi. threshing show in 2010, and it worked fine for me.

I thought my (Sep. 2011) problem was my most current order from MSC so I contacted them but they had no answers so they sampled me 2 bars of their current stock to try. I made a couple of strikers from that stuff in my shop, under well controlled conditions, and they worked properly.
Summary Conclusion from that MSC-samples test:
1. I am creating more decarb sometimes by heating the steel slowly or heating it to red several times as I make the striker, as I demonstrate the non magnetic properties of the steel, and as I otherwise waste time with the part hot while running my mouth, etc..
2. I haven't been taking enough material (scale) off after hardening to get through the decarb. ***See note toward bottom of article.

Random piece of information on the subject:
I think I actually talked to Howard Clark about decarb while at the Spring Valley, Mn. Tunnel Mill event in Oct, 2011. I told him about my striker issues.
I think he said that decarb could go in as far as 25 thou. Or was it Mike Blue? No, I think Howard.

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Reply to Kieth Johnson after I asked him for his input:
Yes, I was thinking that MSC had mixed up steels, too, as had Carl.
I am not very good at spark testing, but I tried to test various bars of this stuff and could not see any difference. It just occurred to me that I might be causing my own problem by heating the steel a couple of times when I demo. Decarb is what I am thinking. When you read the spec for W-1, it says that you have to be concerned about surface decarb, but I am not sure how deep and how much occurs in my application. Maybe I should have simply filed or ground into some of those "bad" strikers to see if they were hard 20 or 30 thousandths below the surface. I need to do some testing. I did google decarb, but most of what I found related to decarbing cylinder heads on cars and trucks. One entry was about someone asking Howard Clark about it. The guy wondered if decarb was keeping the tip of his knife from being hard enough. Howard suggested that the decarb only went in a few thousandths of an inch. Also, I think I have heard other blacksmiths say that their strikers need to used some to get the best sparks. Maybe that's a decarb issue, too. In any event, I need to do some testing before I cry wolf to MSC any more. Pete Stanaitis ----------------

From: Keith Johnson [mailto:keith@greatriverforge.com] Sent: Monday, September 26, 2011 11:16 AM To: Pete & Sheri Subject: Re: NMM Sept. 2011 Newsletter Hi Pete, Nope, I have no idea. Sounds like it could be a mix-up in steels on their part. What does a spark test tell you compared to something that you know works? Thanks for the tip, Keith Yes, I am aware of brine for a quench. I think it's a bit faster for the first 200 or 300 degrees (less steam formation), then slower than water after that. The only thing I use salt water for is quenching S1, believe it or not. It was George Dixon's method for getting a couple more points of hardness. And, for demonstrating, I am simply using my slack tub, as you might imagine. In an earlier email, you suggested grinding down some bad strikers to look for hardness. I immediately threw the bad ones out so I wouldn't get them mixed with good ones. I have already been kicking myself for doing that. I do have most of the stick left the gave me the trouble, so, hopefully I can recreate the problem, then do as you suggest. ***Note; I recently found those 2 "bad" strikers. I still need to test them by annealling, grinding (to get rid of any accumulated decarb) and re-hardening in one step (no fooling around). After rehardening, I need to: 1. test surface hardness after only shining up the face 2. Grind away 25 thou and test again. Is there a difference in hardness between 1 and 2? -------------------------------------------------------------------------------------------------------------------

Actually, Tom Latane' quenches small sections of W1 in oil and feels he get full hardness that way. I haven't tried it on the strikers, though. And I probably won't for demonstrating to the public anyway. Pete Stanaitis ----------------

From: Keith Johnson [mailto:keith@greatriverforge.com] Sent: Monday, September 26, 2011 6:16 PM To: Pete & Sheri Subject: Re: NMM Sept. 2011 Newsletter BTW Pete, you mentioned quenching in water. Fine for demonstrations but did you know a 5% rock salt solution is much safer, more even quench? Keith. -----------------------------------------------------

December 18, 2011, notes: "750 failure" comment from Al Lindholm: He says that steels like W1 fail in normal use from decarb that occurs in service if they are subjected to working temps as low as 750 degrees F.

One of the rec.crafts.metalworking guys said he has seen "bark" of decarb as thick as about 1/8".
Several rcm'ers commented about wrapping steels in stainless steel foil for heat treatment to minimize/eliminate oxygen from getting to the surface while at heat. A couple of guys commented about adding some carbon inside the foil for the same purpose.
One guy mentioned keeping blower air from hitting the steel when heating in a forge.
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Here's a list of things I have probably been doing wrong when I have had failures.
-Heating the steel too slowly. I would heat it up a little, while talking about what I am going to do.
-Heating the steel to non-magnetic, then handing a magnet to someone in the crowd, so they can feel that the steel has, indeed, become non-magnetic. This process might take a couple of heats.
-My demonstrating forge is a "rivet forge" style, having no firepot. So, I need to really keep on top of the fire to make sure there is plenty of fuel between the air blast and the work. Sometimes, when there's a big crowd and/or I am in a hurry for whatever reason, I don't pay close enough attention to this. I guess I figure that the part (the striker) is small enough that I don't have to be too fussy about fire quality. This probably isn't much of an issue for S-hooks, etc., but could create problems for the W1 strikers.
-The issue isn't only made worse during the hardening process, it applies to actually forming the striker, too. It usually takes 4 or 5 heats to make the striker, since I am usually explaining the process, as I go. Since the air blast is a crank blower, it is easy to just stop cranking while I look for a tool, too. This adds extra time-in-the-fire to the piece. This tool-hunting is brought on by my demonstration technique, because I am constantly changing from one project to another.
-Sometimes I will purposely NOT quench a striker after heating it, for several demonstration reasons. After I finish the striker, I often reheat it to non-magnetic to normalize it, then simply set it on the edge of the forge to air cool. Once the striker has cooled well below glowing, I cool it and then ask the audience if they think my newly made striker will work. Of course, they almost always answer in the positive. So, I quickly just barely rub the oxide off the working edge, dry it off and attempt to make sparks. Of course there aren't any. Now is when I start talking about heat treating and the movement of the carbon in the iron. So that's another opportunity for decarb.
-As just mentioned, above, I don't even take a couple thousandths off the working edge when i clean off the scale. I do have a hand cranked grinder on my work table. Sometimes I use it to scratch off the scale, again, taking very little material away. So, here's another opportunity to take care of decarb in the future.

-When MSC sent me a couple of sample bars or the 3/16 X 1/4 W1 to try, I made 2 strikers in my shop and they both worked normally.
Why? Because I didn't do ANY fooling around during the shaping and hardening process. For hardening, I carefully took them just up the non-magnetic and quenched. Also, I took the strikers to my Icker belt grinder to remove the scale. I'll bet I took off at least 25 thou in a single pass over the 36 grit belt.

-Note: I do not temper my strikers at all.

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January 3, 2012:
I made 10 strikers today. I made each one complete with hardening, one at a time, just as I would when demonstrating before the public, except that I did not stop to "show" anything to anybody. Also, I worked as efficiently as I could, so as to need a minimum of heats to get the job done. I used a magnet to check for "non magnetic" and quenched quickly. I ground the hardened working edge on the Ickler belt grinder to clean off the scale (and probably took off 10 to 15 thou of metal, too. I tested all the strikers for their ability to generate a good shower of sparks with the same hard rock.
Eight of the ten strikers worked just fine. As I looked back on the process, I am pretty sure that in both "failure" cases I hesitated when going into the quench. Lesson to be learned: go up a tiny bit hotter above nonmagnetic before quenching and make sure the tongs are holding part at the right angle so the working surface is parallel with the water in the slack tub.
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