There could be a number of issues involved.  First, 5160 is a very poor choice for forge welding.  I personally don't use it in a forge welding situation.... it's simply does not play well with others, or with itself. I suspect that in itself is a major contributor to your failure in this instance.

Since there are subtle differences in how each type of steel forge welds to another, I'd recommend getting some 1080 or 1084, and some 15N20.  There's a big reason that many of us use it as our standard mix for Damascus.... the steels are very compatible with each other, which also means they are some of the easier to forge weld and manipulate.

  When you said that you "tack weld the ends"  does that mean both ends?  If so, that could also be an issue.  Since steel expands and contracts as it heats and cools.... if you weld both ends together, the individual pieces will usually bow when heated....creating more difficulty for you to overcome when attempting to forge weld.

  Time AND Temp are everything when it comes to forge welding.  When those two factors are correct, steel will almost forge weld itself....not literally, but close.  There have been times when I've had two billets in the forge at the same time, failed to pay attention that they were touching....and they literally welded together where they were touching..... so much so that I couldn't twist them apart, and had to resort to the angle grinder to separate them.   That also means that you don't have to hit it hard, nor fast.  When the time and temp are correct, it only requires light blows, and as you "feel" the steel getting stiffer, increase the force of the hammer blows.  Many times if you strike too hard/fast, the surfaces will literally "slip" over each other with welding.

  I think that if you were to get yourself some 1/8" X 1" 1080 and  the same size 15N20, but thinner..... say .050-.080" thick, then at first make yourself a billet of 5-6 layers X 4-6" long to learn on, you'll have much better success, and less of a learning curve.    When you stick weld the "stack".... weld only the handle end (I use old pices of rebar for billet handles), and then two stick weld beads on each edge side... one approx in the middle of the length.....and another near the end opposite the handle...but NOT on/over the actual end.  I will look around my pics and see if I can find an image, and post it latter.

The surface of the billet was just beginning to melt, so I pulled it out and rapidly hit it until it was too cool. I repeated this several times. After that I started to forge on the edges. That was when the welds started to pull apart and leave cracks all along the edge of the billet. I have no idea what is causing this to happen. Is the billet not clean enough, or is the forge too hot?  

I can't be sure with seeing, but it sounds like you were too hot.  You generally cannot actually see the surfaces of the billet melting, which leads me to believe.... too hot, for too long.  And finally, those "cracks all along the edge" sounds to me like the steel was "burnt", again, indicating too hot for too long.  One key indicator that I often use when teaching forge welding, is that the billet is ready when you pull it out of the forge, and it is SLIGHTLY "smoldering" or "fuming".... I've found that is the best indicator I can tell people to look for....but beware... ONLY SLIGHTLY.  Too far and you're burning the steel.    

  I think you are in the general area, and unless you have someone around who you can learn from, it's just going to be trial and error.  Start with different steel types as I mentioned. 

 You might also consider a pyrometer for your forge.  I've found that it's often a great learning aid for forge welding.... allowing you to learn what colors YOUR forge looks like at what temps.. as just another tool to help you learn.

Thanks so much, Ed.

I had no idea about 5160 not being a good steel to use for forge welding. I had been tack welding both ends. I will have to change that.

I currently don't have any access to steels, and ordering some will take a month or two to get here. We do have some 1095. Is this any good for forge welding, or should I just wait and order some?

I will have to practice getting the heat correct on the forge.

1095 will work.... but I'd suggest not practicing/learning by welding it to itself.... although it will work in terms of forge welding, I've always found that welding a given steel to itself gives sort of a false sense of how things work in forge welding, and besides, there will be no contrast in a finished/layered piece when etched.   

If you can get it at all,15N20 is my recommendation for the second material in the mix.  It's just much easier on you from a learning standpoint.

  I know it seems like a huge learning hill to climb right now, but just keep at it, and learn from mistakes..... and of course, keep asking questions!  😉     

Thank you very much for all of your help and suggestions. I really appreciate your support and encouragement. I will try to source some 15N20.

Thanks again,

Andrew

 Absolutely!  Please feel free to keep me informed on your progress!  PM, forum post, or email me if I can be of help.  

Posted by: @andrew-beckendorf

I currently don't have any access to steels, and ordering some will take a month or two to get here.

While it is certainly not as good a choice to combine as the 15N20 that Ed recommends, if you are in a bind you might consider trying to source some of metal band packing straps and/or older large non-bimetallic band saw blades.  Both of these are often thrown out after use and can forge weld into a billet with acceptable contrast.

Needless to say grind all surfaces to be welded right before assembly.

Sorry for the late reply. Thank you for all the responses. I will have to see if I can find some band saw blades. How will I know if they are bimetallic?

I just ordered some steel. Hopefully it will arrive sometime in June.

Thank you all again.

Andrew