Aint seen ne1 post one so I figured I'd write one real quick.

SMAW (Stick)

Carbon Steels:

* E6010, Run DCEP. Flat beads, deep penetration, common & cheap, good on poor preparation. Good for root passes, especially on pipe.
* E7018, Run DCEP or AC. Low-hydrogen, great for fill / cap passes, low skill involved, high quality welds.

Stainless Steels:

* 301-305 to 301-305 Stainless steels use E308-XX
* 301-305 Stainless to non-stainless steels use E309-XX
* 316 Stainless steel use E316-XX
* 321 & 347 Stainless steels use E347-XX

XX denotes -15, or -18 (EX: E308-15)
-15 runs DCEP & is easier out of position
-18 runs DCEP and AC. Has less arc blow & good for thicker metals.
Cast Iron:

* #55 (EniFeCl), stronger, less ductil & cheaper.
* #99 (ENiCl) high nickel, easier to weld

Pre-heat to 350*F, maintain 350-600*F between weld passes. Post heat, and insulating (sand / blanket / dirt) is recommended. Once welded, do not disturb for 48 hours.
I do not recommend welding turbo manifolds. Because the nickle will expand much more than the cast iron under excessive temperatures, it will eventually crack. (The best way I know to weld an exhaust manifold successfully is oxy-fuel / tig with a real cast iron filler.)
Aluminum:
E4043
I do not recommend. Requires too much experience, not enough common selection of rods.

GMAW (mig), and GTAW (tig) wire / filler

Carbon Steels:

* ER70S-2, Easy to weld all positions, most common, least cost. use C25 gas.
* ER70S-6, Most resilient to mill scale & rust, use C02 & higher currents.
* ER70S-A1, Even more expensive. Addition of Moly increases weld strength & corrosion resistance at high temperature.

Stainless Steels:

* For 301-305 to 301-305 stainless steels use ER308, or ER308H. The higher carbon content of ER308H rods increass strength at high temperature.
* For 301-305 stainless steels to carbon steel forgings use ER307.
* For 301-305 stainless steels to carbon steel castings, and general mild carbon steel use ER307, or ER309. 307 is stronger at high temperature, 309 is more resistant to corrosion.
* For 316 stainless steels use ER316, or ER316H. The higher carbon content of ER316H rods increass strength at high temperature.
* For 321 stainless steels use ER321 or ER347
* For 347 stainless steels use ER347

Aluminum
ER4043, ER4145, ER5356 are common.
Please consult a chart for the correct choice.


FCAW (flux-core, wire feed. sex welds)

Carbon Steels:

* ER71T-1M, DCEP. All position (no downhill), cheap, dual-shielded requiring C25 shielding gas, multi-pass is OK when flux is cleaned.
* ER71T-7, DCEN. All position (no downhill), cheap, no shielding gas, multi-pass is OK when flux is cleaned.
* ER71T-9, DCEN. ALl position (no downhill), cheap, dual-shielded requiring CO2 shielding gas. multi-pass is OK when flux is cleaned.
* ER71T-13, DCEN. All position (no downhill), cheap, no shielding gas, single pass only. If you have a chinese welder, or are welding under atleast 180amps. This is the flux-core wire you want to buy.

Note. All non-EXXX-13 FCAW welding is done in spray transfer. HF/chinese buzz-boxes can not achieve the current & voltage needed for spray transfer. They should be avoided at the gross amount of inclusions in the weld if weld quality is a concern. If you are using one... To get a quality weld you must use ERXXT-13 wire, and it can-not be multi-passed.


GMAW (mig) gas selections!
Short Circuit (low currents & voltage ranges). Primarily small boxes.
Carbon Steels use C25. High speeds, minimum spatter & distortion. Good apperance. Good control out of position. And virtually eliminates undercut if you're remotely in the ballpark of correct welding angles.
Stainless Steels use a tri-mix. 90% Helium, 7.5% Argon, 2.5% C02. No effect on corrosion resistance, minimal distortion. Good apperance, virtually no undercutting, small heat aaffected zone.
Aluminum use pure argon on sheet metal and, 75-25 mix of argon & helium on 1/8" and greater.
Spray transfer (high currents & voltage ranges)
Carbon Steels use argon + 3-5% oxygen. Improves arc stability, easily controlled weld puddle, nice bead, minimizes undercutting. Higher speed welding than pure argon.
Stainless steels use 99-98% argon, 1-2% oxygen. Use 9822 for thinner steels. Use 99-1 for thick steel.
Aluminum use pure argon for the least spatter. Use 65-35 argon-helium, and 75-25 helium-argon when even higher heat input is required.
Higher weld currents need more flow rates. For short circuit typically 10-15 CFH is good. For spray transfer 15-40CFH is typical, helium mixtures may require 40-50 CFH as it's lighter than air.


GTAW (tig) gas & electrodes!
Don't be a douche. Use 100% argon; or a helium mixture for DC & aluminum welding. Don't expect to get much penatration with DCEP, and there is no surface cleaning with DCEN. 15-20CFH with argon. 30-50CFH for helium mixes. If using a large cup (#8+), or a gas-lens the CFH requirement may be half of normal; all things depending. 10-15CFH.

* 2% thoriated (red) for DCEN steels, grinding dust is radioactive, but longest useable life
* 2% ceriated (orange) for DCEN steels and AC welding on inverter units, not radioative.
* 1-1.5-2% lanthanated (black, gold, blue) for DCEN steels. not radioative.
* Pure tungstens (green) for AC welding on transformer units.
* Zirconiated (brown) for AC xray ---- you'll never do.