Let's start with cold & hot rolled steel ... more often than not, you will not find cold rolled in a hardware store because it is dimensionally correct and quite a bit more expensive than the el cheapo hot rolled which never quite right dimensionally. Common cold rolled presents a major problem of being stressed from the rolling process because unless it's spec'ed as being post-roll heat treated (even more expensive), it's not. When you try to weld it, the heat from the weld will play heck with the stress already in the steel as well as compound the problem by adding stress created by the weld shrink.
The common generic GMAW (MIG) filler wires for mild steel are actually alloyed quite a bit to not only produce a better weld via the GMAW process but also many are made to comply primarily with the SAE standards for automotive repair which means they deposit a harder than normal weld as compared with the common generic E60xx/E70xx series SMAW (stick) electrodes.
There is also another problem with GMAW and that's the "cold start" meaning both work and filler are cold when you hit the trigger and it takes a certain amount of time for everything to come up to working temperature. GMAW is thought to be quite handy and dainty but in reality the starting point of the weld is like the top of a zipper joint if it's not done correctly and you go into it cold. When you're talking the small cross sectional area of a tang, you must not only pre-heat but also come into the weld with hot wire. Pre-heat of the work should be at least 450°F - fixture the pieces on a piece of scrap steel that extends past both sides of the joint and pre-heat the entire assembly. Get the weld started on the scrap then come into the joint with the wire running properly and continue right on out the other side of the joint and break the arc on the scrap to ensure complete fill and eliminate post-fill cratering. If you stuck the work to the scrap, just use a cut-off wheel to cut the weld staying away from the work about 0.010" and break the remaining weld by bending. Overfill the joint too, too much is better than not enough, it comes off easy with the 4.5" angle grinder. Flip the work over, groove the backside until you're into the weld from the front side - this is especially important when running coated/cored filler so as to ensure you are not trapping slag in the joint and repeat the welding process. With the exception of certain specific FC & DS GMAW wires, make sure you are running it EN (Electrode Negative) so as to put the heat on the work and not on the wire as this will eliminate loosing the shielding. If you have too much drag angle on the electrode, you'll get a high bead but less penetration, keep that wire dang near perpendicular and don't rush it, let it work.
No matter if you used cold rolled steel or not, you should still post-heat treat the weld anyway. Bring the whole piece of work up to around 500°F and beat the $#@* out of it with a hammer. You don't need to forge it into a different shape but you want to peen-relieve the stress as much as possible. Normally I peen with a pneumatic hammer as soon as the arc is broken but a tang only requires the use of a small 3-6oz hammer and you can peen as soon as you break the arc but watch that flying slag as it will most certainly find your eyes and nasal passages. You want to lightly peen the entire weld plus a little past it on both sides. You need not hit it so as to deform it but be real close to using that much of a striking force. It never hurts to run a full post-weld heat treat either, I'd say a two hour soak then keep the cooling rate around 75-80°F per hour max.
12L14 can be welded quite easily because the Pb content is only a mere 0.15-0.35% and it boils off and comes out as Pb vapor or oxide trapped in the slag if using a flux type electrode. The only time you'll have an issue welding 12L14 is when using a gas-shielded process and not running hot enough or using the right technique and you'll trap the oxides in the weld instead of floating them out on the deposit puddle.
Birddog6,
It's not your liner. The liner only houses the wire and when it's bad, you'll find the wire get jumpy/sticky resulting in a jerky sputtering weld ... trust me, you'll know when the liner is screwed up. The shielding gas flow through between the outer shield of the whip and the liner, not through the liner itself. If you were loosing gas or have poor/disrupted flow at the nozzle, you'll see little (or sometime huge) bubbles in the weld deposit where you lost shielding - turn the gas off and the weld will look like spray-in foam and be about as strong as the foam too. That's a little excessive but it'll give you a good idea what to look for. If you're not seeing it with your eye, run a bead of weld, grind then sand half of it off and look at it with a 30x loupe. If don't see any porosity, go to the next step, warm the work up to 220-240°F and dunk it in used c-c oil letting it cool completely in the oil bath. Wipe it clean with a dry rag then slowly re-heat the piece back up the 220'ish range while closely watching the weld. If you have porosity, you'll see little spots of oil cooking out of the weld. If you have interbead cracking, you'll see that too in the form of thin lines of oil. Porosity is a shielding/contamination issue, interbead cracking is an alloy, heat and/or stress issue.
Based on what you said thus far, I'd hazard a guess that you may have gotten hard wire (most of this stuff is made overseas and depending on the brand name, it could be anything wrapped on that spool). Reason I am quick to blame the wire here is because you said you didn't have any problems until the wire was changed and that's usually a good indication it's either the wire, a setting issue or the machine just happened to $#@* at the same time. When you changed the wire, did you clean all the electrical contact connections between the machine, wire feed assembly and whip? If yes, did you accidentally swap polarity? GS solid wire needs to run EP (electrode positive).
When blowing out the whip, always blow in the direction the wire runs first, then reverse the airflow but only after you have removed the whip from the machine so as not to blow all the crud back into the feed assembly. Check the o-ring/gasket seals on the gas side connection at the whip on a regular basis (don't get suckered into buying the $6+ each "special" o-rings in the welding supply section of the hardware/autoparts store, it's "inert" gas, common generic buna/viton o-rings work just fine and a thin coating of non-drying silicone ball valve grease will keep the whip from binding in the socket (very thin coating so as not to plug the gas ports).
For everyone, do yourself and avoid buying rod/wire at the big box stores because unless it's McKay, Lincoln or Harris, it's likely generic junk - not to lump all the generic's together because some of it is good certified wire that simply got wrapped onto smaller spools. The first-filled at the wire plant are the 500 pound drums/coils then 60-120 pound and so on down to the smaller packages. If the plant is not a continuous feed multi-wrap where smaller sizes can be wrapped without stopping the line, then they wind up with a lot of remnants which are often sold to re-wrapping companies resulting in being able to obtain good wire under at a generic price. I know Hobart brand is owned by Miller but avoid their filler materials - I've had nothing but problems with Hobart.
