Heating and Forge Welding?

[SoCal Dave]

I just read an article by Dave Smucker on Heat Treatment of Tool Steel. As I was reading it, I was wonder if there is a minimum temperature that tool steel needs to reach to forge weld for a blacksmith with a 3 lb hammer and anvil. I would image that at room temperature it would be impossible to weld two pieces of tool steel together by hand. However, as the steel is heated, is there a grain structure that is reached that allows for forge welding to take place with less stress and deformation? Or, do we want to reach a certain grain structure before forge welding? Do we want to forge weld at a specific grain structure. Dave indicated that after forging we don't want to just heat the steel up to much otherwise we start increasing the large grain structures. Clearly, I am trying to understand what is going on while heating and quenching. If we heat treat, anneal, harden, and temper at certain temps, then it seems to me that we would forge weld at certain temps and grain structures and what would that be? Any help in answering these question would be appreciated.

[ThomasPowers]

Forge welding depends on three things: Cleanliness, Temperature and Pressure.

Max any of these out and you can get "solid phase welding": Vacuum welding, Forge welding and Galling are examples where one of the three is maximized. So get all three a good as you can and you can get a good weld without pushing one to the max.

Note that different alloys weld best at different temps so a plain high carbon steel welds at a lower temp than a very mild steel.

And I don't think "grain structure" has anything to do with it---unless you really mean the different phases iron can be at different temperatures.

If this topic interests you may I commend "Solid Phase Welding of Metals" by Tylecote; you should be able to ILL it at your local public library. Unfortunately I'm not taking it on my vacation as I hope instead to pick up a copy of John Percy's "Metallurgy: The Art Of Extracting Metals From Their Ores, And Adapting Them To Various Purposes Of Manufacture" whilst in the UK.

[dablacksmith]

there is a reason you normalize steel ... and the temperatures you reach forge welding are one of them ... normalizing is done to refine the grain structure .... forge welding is done at tempratures much higher than normally reccomended for high carbon steel forging.. that is why you dont skip the normalizing steps...

[Maillemaker]

The grain structure that forms above critical is called austentite. However, grain structure is not what we're concerned about when forge welding.

Arc/electric welding is a little different. If pieces of carbon steel are not pre-heated, the sudden temperature change can alter the grains around the weld, making the steel brittle.

[SoCal Dave]

Ideally speaking, would you want to forge weld when the entire piece has reached an austentite state? Is this why you let the piece soak in the forge to ensure the entire piece has reached and austentite state? What happens when the entire piece is not at an austentite state? Again, it seems there is a lot of technical aspects to annealing, hardening, and tempering of different types of steel, but not on when is it best to forge weld and what condition/phase the steel should be in for forge welding. Also, I've heard for years that in arc/electric welding the weld can be stronger than the steel.

[Rich Hale]

Dave It seems you are bogged down in details that in the real world do not really enter the big picture,,as far as home shop forgeing is concerned. As you mentioned there is a lot going on inside a stgeel ;piece being forge welded. In the actual shop piece being welded, the outcome of the finished product depends on some basics. The steel in fact must reach welding heat, If not it simply does not weld. After the weld as far as I can see it simply does not matter wot state the grain structure is in. It then becomes my job to put it in a proper state for further work. Normalizing does that. The heat treatment steps takes it on into the other steps to make a usable end product. My suggestion is for you to do some shop trests. Forge weld a fairly long bar of high carbon steel. forge it into a workable size..like quarter inch lx one inch or so. Cut off a few inches, Heat to non critical, quench in proper medium for you steel and then break it open and examine the grain..take notes and markd the piece. Then take another few inches and normalize, harden and break it also..Check grain size make notes and mark piece. Then following guidlelines for heat treat of the steels you have welded do the best you can at heat treating the piece correctly. Break and take notes as above. Review your notes and comparre grain sizes on each piece. Some answers come from books some from shops, a combination of the two helps me a lot. It may help you with this also.

[Rich Hale]

A simple answer to yoru last questioon is that if you do not heat the entire bar to forge welding heat it will not weld properly.

[Frosty]

You can get diffusion welding at almost any temp above absolute zero. If the pieces are matched, polished and clean well enough, contact is all that's needed, well, okay time too. Father's gage blocks used to check instruments and occasionaly set them up in special circumstances were stored wrapped in oil paper to keep them from making contact with each other or they'd weld.

Diffusion welding happens when two pieces of metal are brought close enough together to allow electrons to transfer. A metal is a metal because it has electrons in the valence shell that are not strongly held, it's also why metals are conductors. As electrons begin changing sides the rest of the atoms follow and the bond is as strong as the weakest metal. Given time for enough atoms to exchange that is.

Forge welding uses heat because heat is brownian movement accelerated, the molecules and atoms are bouncing faster and farther. This increases space between atoms making the material softer, more forgable, and with the increased energy electrons will begin exchanging at a less matched condition, Matching is also easier because the metals are soft and will crush together.

The hotter the attempted weld, the less sensitive to clean and matching it takes. Liquid of course making it pretty automatic.

Frosty the Lucky.

[pkrankow]

You can get diffusion welding at almost any temp above absolute zero. If the pieces are matched, polished and clean well enough, contact is all that's needed, well, okay time too. Father's gage blocks used to check instruments and occasionaly set them up in special circumstances were stored wrapped in oil paper to keep them from making contact with each other or they'd weld.
...
Frosty the Lucky.

One of the profs at school had a set of blocks that he claimed this was what had occurred. They would not slide on each other, and could not be pulled apart.

He had other blocks in the set that were so highly polished that the oil film created enough tension that I could not PULL the blocks apart, but they would easily slide over each other and part. These were rather small blocks, with faces about 1 inch square and very near mirror polished, although the oil may have been the only reason they were not mirror.

We used the blocks for teaching/learning micrometers because they were laser engraved with the exact dimensions.

Phil

[saintjohnbarleycorn]

thanks frosty and others I never thought of it on that level.

[rockstar.esq]

Dave, years ago I was in a welding class where they talked about the weld being stronger or as strong as the parent material. Many, many years after that I was in a structural materials course. They explained that the weld may indeed be "stronger" as measured in terms of shear, bending, torsion, or elasticity. The problem is that the weld never acts the same as the parent metal under all of those conditions. It's very common for a welded joint to fail in the parent material NEAR the weld. Like a lump in the gravy it may well be made of the same stuff, but there are times where it's presence signifies failure.

In the context of a homogeneous solid, welding is going to mix things up. The normalizing is an effort to re-sort the deck.

[itsme]

I'm in over my head in this discussion, but Frosty's explination is the most cogent I have encountered. The only thing I have ever forge welded is the axel of a trailer when I failed to grease it. I am trying to understand what is required, so I will ask this question:

If I heat rod up enough, I can hammer it from round to square, or draw it out. If I want to fold it back on itself, is anything else required besides heat and pressure?

Again, if I am asking in the wrong place, just ignore the post.  Thanks for your discussion.

[Maillemaker]

Clean fire, clean steel.  A flux of some kind, like borax or silica sand helps by forming a glassy layer over the steel that oxygen cannot penetrate.  Your fire should be deep and free of clinker and slag.
 

[SoCal Dave]

I have forge welded many times in a coal forge.  I get the metal to a white hot stage.  In gas forge, I find it difficult to get the metal to a white hot state and therefore unable to gas forge weld.  It seems to me that if it is necessary to get the metal to a white hot state to forge weld in a coal forge, it goes that the metal needs to be at the same white hot stage when coming out of a gas forge, or any kind of forge. If all other conditions are the equal.  The metal doesn't care who is heating it up.  

[Steve Sells]

I rarely need to get above Orange and I hand weld except for wrought iron I need yellow/white.

[Rich Hale]

I have never seen white hot color in any of the coal or gas forges I have used,,,,may have to do with ambient light. Wot I call dull redd may look bright in shade.

 

If you are having trouble with forge weldin in a gasser is it the gasser,,,operator or both?

 

I use twenty mule team for flux..when it looks like the metal is hot all the way through and the same color as inside of gasser and the flux looks like it is molten it is ready to weld.

[SoCal Dave]

Rich:  From what you have indicated, I must be taking my metal pieces way past the forge welding stage.  I see the flux flow all the time.   I usually get the metal at least to a yellow before I try hammering. If I am taking the temp to high, is it effecting the metal to the point that it won't forge weld from contamination or changing of the composition of the metal?????   

[Steve Sells]

over heating can ruin the steel, also more time at high heat = more risk for oxidation = scale formation

[doc]

The two big inhibitors to forge welding in a gasser are one....... not enough heat and two......too oxidizing an atmosphere. The lack of heat is often caused by the oxidizing atmosphere.  

[ThomasPowers]

It is interesting to note that many if not most of the Pro's doing fancy pattern welding use propane forges.  Just ones designed/tweaked for forge welding.

[basher]

I would like to add to all this that time is also an important factor in forge welding.
Temp , exclusion of oxygen, material prep and pressure are all very important .but a high temp soak post welding and also thermocycling to allow grain growth across the weld boundries are also important as is soaking the piece to insure the entirety of the billet(inside and out) is at welding temp.
All the best owen

[thingmaker3]

Back a hundred or so years ago there were folk in Scotland experimenting with welding polished steel by keeping it at 900C in an oxygen-free environment for two hours. They routinely got good welds from a variety of steels.

 

I would love to try this myself, but hydrogen atmospheres in porcelain tubes kind of scare me.

[Frank Turley]

Getting back to the original question, a cute little book came out of England first printed in 1916, "The Value of Science in the Smithy and Forge." At that time, they were beginning to look at metals through microscopes, and this book had a photomicrograph of the line of a forge weld. It showed some "slag inclusions" (dirt) along the line of the weld. It also showed grain growth along the line, the grains being larger than that of the parent material. I don't have a lab, so I can't say that normalizing is going to perfectly even out the grain structure. I suspect that it would help. At all events, a forge weld performed on high carbon steel in a coal forge situation is most often weaker than that of the parent material because of the inclusions and grain growth.

 

When oxy or arc welding is performed, one obtains puddles which freeze, but then there is the heat affected zone to deal with. When the puddles freeze, you get a different crystalline structure than the parent stock, I believe termed "dendritic." This structure is not always forgable, as it is liable to crack or crumble.

 

When thinking about coal forge welding weakness, we refer to the cowboy adage, "I don't care how weak it is, as long as it's strong enough."