The steel has to be bright red hot in order to forge weld. Work the bar(s) flat and fold over to about a 60deg angle, then flux. Reheat until the flux bubbles hot then work from the fold out to the endl slowly tapping and rapping. If you hit too hard the parts will bounce apart, if it's too cold they wont weld. It really doesn't take much hammer pressure to weld two pieces together, it's more a combination of proper prepartation and heat with just enough pressure to bring the two faces together.
More fire. Bright yellow to start, back in the fire at orange. Repeat. Steel cools off really fast on the anvil, think five to ten seconds max per pass. Use the heaviest hammer you can comfortably swing, then let it do the work, you just steer. Get some refractory bricks and build walls in the forge to get a deeper fire. The most common problem with old Cavalry-type forges is too shallow a coal bed with too much surface area to concentrate the heat. I've watched lots of hopefuls struggle with this one and wonder why they keep getting cold shuts instead of fusion. You can actually chimney the top a bit with the last course of bricks and scavenge a little from the exhaust. This doesn't need to be a permanent install, dry stack works just fine.
K-mart steel is probably A36. Most common stuff is these days. A36 is slightly harder to weld than 1020 or 1018 as it has a narrower heat range before it disentigrates.
You don't really need anything special for flux. Plain old dirt will do fine. Especially if there is a fair bit of clay in it.
I can tell from the comment above that you haven't ever really gotten it hot enough. Else you'd have overheated and it would actually melt, or just short of melting it would disentigrate when you hit it. That would be a very bright yellow. Just a bit too hot.
When sparks first start jumping off the metal...around mid yellow, its hot enough. That would be the carbon starting to leave town.
These sparks are distictly different than other colder sparks, in that they appear more jagged and sparkly.
You can also tell when you are close by the flux (dirt) melting and flowing a bit.
Another tip: Long as you have gotten flux between, its good to bring the sides very close together just before getting a welding heat.
Another poster has mentioned the forge fuel composition and configuration. An easier but less roomy way is to wet and bank the fine coal around the center. Reducing the fire to just enough size for your steel.
Pack the sides up and keep it wet just back from the fire. Let it burn long enough to form a good layer of coke. It needs to be deep enough to form a neutral or carborizing oven all around the steel. That is, not too much oxygen present or it will oxidize too much for welding.
Figure on ruining a few pieces. But keep experimenting. It takes the right combination of knowing when its hot enough and not pounding it to death to get it right. Takes some practice but then gets fairly easy.
Thanks- I will try this stuff- I think my heat was high enuf, but too much chrome in the steel- well I will try the RR spike, interesting on the dirt, I know the old Japanese used to use rice straw- thanks-
About the heat.... have you actually melted a bit of your bar? If not, you haven't been too hot yet.
Try that out, understanding it will ruin an inch or so of your bar. Then, heat again after removing the burned up part and somewhere short of melting when you see sparkly sparks start to jump off your piece its ready to weld.