turbo38t,
To answer your question about repairing heavy equipment, it depends on where you work. Some places may use SMAW(stick) and some may use GMAW(mig) or FCAW(flux core) or a combination of all of them. 7018 rod is a very common rod for carbon steel welding. Typical amperages for 7018 are 90 amps for 3/32, 130 amps for 1/8, 170 amps for 5/32 diameter rod. Almost all your stick rod runs on reverse polarity(electrode positive). You may also encounter some types of wear resistant filler metals in the stick, and wire types for building up of a worn bucket for example. Your 60XX series rods are what is called a fast freeze rod which means the puddle tends to cool and solidify rather quickly. They are good for filling gaps in poorly fitted joints. You would most likely use them t fill a gap to make way for a rod such as 7018 although if you know how to weld with 7018 well you shouldn't need a 60XX rod.
A little info on carbon steel stick rods to know is that the first 2 numbers on the rod designate the tensile strength of the rod. 7018 means 70,000 tensile strength. The 1 means that it can be welded in all positions. A 2 would mean it can only be welded in the flat and horizontal positions such as a 7024 rod. The last number tells what the composition of the the flux coating is. The 8 in 7018 means that it is a low hydrogen, iron powder coating. Lincoln Electric website is a good place to go to find out more about consumables.
Don't worry too much about knowing all about filler metals. There are way too many to know them all. They will already have the equipment set up with the proper filler metal they will expect you to use. I'm sorry but depending on the workplace they may only use 2 or 3 different filler metals or they may use as many as a dozen or more so to prepare you for a situation like that would be impractical. There is no substitute for actual experience!
You will most likely be doing stringer beads or multiple pass stringers. You will not do well if you are asked to make a large fillet weld and try to pile it on. What you need to do is make the appropriate size stringer bead for the size of filler metal you are welding with and build up your weld with multiple stringers. Remember to stagger your stops and starts. Do not stop at the same spot when making multple pass welds. Overlap your welds by half the width of your previous weld. A good practice is to follow the edge, or toe of the previous weld when making the next weld. You will get the proper overlap when doing this.
As far as your question about spray arc.
That is a form of metal transfer using the mig process. It requires higher voltages in combination with mixtures of shielding gases. It is suited mostly for flat and horizontal positions because of the high deposition achieved.
If you are going to be weld testing, don't just jump on your test piece cold. Ask them for a piece of scrap metal so that you can set your machine. If at all possible do as much of the welding in the flat position. Turn the work piece to make welding as easy as possible unless they state that they want to see a vertical up weld, or an overhead weld.
If you're going to be taking your test with flux core you want to drag your gun as if you were stick welding instead of pushing it like when mig welding.
With all processes you want to end your welds by coming backward into the weld at the end or pausing to fill the crater. Do not just pull away at the end of a weld. Pause for a second or two to complete the weld.
One of the pointers I stress most about making good welds is the consistancy of your travel speed when welding. To make nice looking as well as sound welds your travel speed is one of the most important factors when welding. You need to concentrate on keeping the same speed of travel when making a weld. If you start off welding at one speed and while making your weld speed up or slow down while welding your weld will not look the way it should. That's another reason to practice on a piece of scrap. You need a few practice runs to get a feel for the machine you'll be using and adjust accordingly.
When you go in there don't try to act like you have been welding for thirty years and have seen and welded it all. They will will see right through you. Just stay calm try not to sound nervous and be humble.
Ask questions, try to be friendly and polite. Even if you're not the best welder they have tested you may beat out the other guy because of your personality. Supervisors want a worker that's easy to work with as well as do quality work.
I hope I have helped you. If you have any more questions I can also be emailed at roberthines33@knology.net
Just a little background on me:
I have been welding for 16 years. I am a certified pipe welder, certified welding instructor with the NCCER, and also work as a QA/QC inspector for a contractor at BP Chemicals.