Concrete tie wire is the same as mechanic wire, low-alloy mild carbon steel. You must be careful about what you buy, if it's mfg'ed anywhere except US, EU or Canada, it is normally made from whatever $#@* they had laying around. The first thing any good welder looks for is consistency in their consumables, avoid the hassles and possible dangers of working with unknowns and spend a few dollars on a known alloy. A good low-alloy mild carbon steel oxy-fuel / TIG rod can be had from most filler metal mfg's such as Lincoln R-45 and other X-45 designations (45 ksi deposit). I'm not discounting the fact that mechanic's wire or tie wire can actually work well, the problem is that unless it comes with an alloy certification, you don't know how much of what's in it and it can contain any number of contaminants that are detrimental to the weld and/or highly toxic when heated to welding temperatures.
Contaminants are an issue with weld quality because while the weld may look good, it can in fact have very low structural strength. Think of metal a block of salt; the block is made up of individual grains of salt bonded to each other in order to form a larger block. The weakest point of the salt block, just as with any metal, is at the joint where the grains meet. Many welders will go to stainless steel filler rods to make welds because the higher Ni & Cr content makes the filler flow easily and will make bonding to dissimilar or higher alloy metals much easier but in most cases it is not a good choice. The major problem with using high Ni Cr (stainless) fillers is that unless the weldment is properly post-weld heat treated (normalized), there is a "brittle" zone crated within the HAZ (heat affected zone) that is subject to stress & impact failure. The second major problem is when high Ni Cr fillers are used on non-stainless alloys is caused by the different thermal expansion rates that cause increased stress around the weld zone. The use of high Ni Cr filler alloys came about as a manner of making welds on high-alloy and unknown steels as well as cast irons and dissimilar metals and if done properly, their use can be benficial in some cases but also result in catastrophic failures in others where the application or method is incorrect.
Years back, there wasn't a large choice of filler rods commonly available and anyone visiting a welding store will find the most common filler metals and usually staff that has no clue about properly matching alloys to applications and specific variations of fillers within the same specification. Normally in stick electrodes you'll find 6010, 6011, 6013, 7014, 7018, 308, 309 & 310. Most welding shop staff couldn't begin to explain the difference between McKay E-6010 and Lincoln E-6010 or the Lincoln E-6010 5P+; all the spec yet the 5P+ is as different in it's running characteristics from a standard E-6010 as is night from day. The same applies to E-6011 & E-6013 that come in a variety flavors from standard to those tailored to run on the little low-voltage AC buzz-boxes and again, the difference is as night to day. The filler metal industry has exploded since the late 1980's responding to industry request for tailored and specialty filler metals. The 6IMP (non-nickel cast iron rods) changed the whole world of cast iron welding and kicked off several other choices of non-nickel stick electrodes for cast iron use as well as electrodes tailored to specific varieties of cast iron.
I know that's off the topic of this post but I'm trying to make the point that there is no reason to not match the base alloy as closely as possible with the filler metal. As I stated before, sometimes it's a $#@* shoot trying to figure out what the base alloy is and when you're dealing with unknowns, it becomes a guessing game at which point the best course of action to take is using a middle-of-the-road generic filler and following the common generic pre-heat & post-heat methods.
JCKelly said, "Whenever you ask someone to repair a gun part, well, if he doesn't regularly repair antique/replica guns, he will do as he is accustomed to do."
That's a very true statement but the fellow who is accustomed to reaching for the 3xx series filler rather than trying to correctly match the alloys is not the best choice to be doing the particular work. Go to an autobody shop and you'll find them trying to make weld repairs on anything made of metal using ER 70S-6; I've also seen many of these alleged welders running ER 70S-6 without any shielding gas and trying to weld steel to aluminum! At a bare minimum, a good welder will start with a spark test to give a rough indication of the steel alloy; professional shops will have an alloy testing unit that gives a fairly reliable indication of the primary elements.
I remember many years back, one of my uncles was a "welder" trained at FWC in the early 1950's. Forty years later he was still reaching for coat hangers, putting up with the popping, spitting and making the same weld a dozen times because it kept cracking; he flat-out refused to spend $8 for a two-pound box of filler rods but had no problem wasting $40 worth of gas and spending a whole day trying to make one little weld hold. Most metal coat hangers are made from higher alloy material, generally the only industry still using them is the uniform suppliers and they spec a tempered alloy to reduce damage loss rates. It's a good alloy for the application but it is work hardened through a cold drawing process and thus normally contains a good amount of Mn in the alloy that will be left in the weld deposit. Yes, it will increase strength of the weld but it will also work harden again as you do whatever needs doing to blend the weld; a high Mn weld put onto otherwise soft steel will present the same types of failure issues as are common to the use of high Ni Cr fillers.
It goes beyond the choice filler metal too because you can change the properties of the weld with the type of flame used and the method in which the weld is made. When oxy-fuel welding, using a carburizing (soft) flame ads carbon to the weldment; using an oxidizing flame burns carbon out of the weldment; back welding reduces strength while push welding increases strength; cover-filling produces a much weaker joint than flow-filling. When using SMAW, GTAW or GMAW processes, the filler must also match the process being used in that a stick electrode designed for flat & horizontal welding only will not produce a quality weld in vertical-up or overhead just the same as a rod designed for V-down will not produce a quality weld if it's run V-up; yes, you can "make the weld" but it will not be a quality weld no matter how good it "looks". If you're going to run "hot", you must also run "fast" or the weldment will fail the v-notch & bend testing; if you're going to run low & slow, the weld will most often fail the those tests as well because the weld lacks penetration.
Avoid any welder who is in a "habit" because each job must be taken for what it is. I did field work and a "typical day" would was at 5am SMAW welding a cast steel steering component on 350 ton haul truck, at 9am GMAW welding a broken 304 stainless agitator assembly inside of a 6,000 gallon tank, 1pm brought a cast-iron gear box housing and 7pm I make it back to the shop to find an aluminum body dump truck with the tailgate ripped off and the owner-operator begging me to get it fixed so he could go to work the next day.
