Bruce made the point that this is a subject that has been pounded into the ground. IT NEEDS TO BE! Then it needs to be dug up and pounded again. It's important! What else is important is that there is always more than one way to achieve a satisfactory end to a need. Another thing that needs to be brought out is that 4130 is nothing more or less than a lower grade of chrome-moly steel, and other than carbon content is no different from the other 41XX steels. It does not harden as readily as 4140 or 4150, which is why it is used in aircraft, the other two would be far too brittle. It is not a "magic" material, nor does it have any qualities that make it too far different from any other chrome-moly.

I agree that 4130 is NOT anything special among the family of chrome moly steels. It happens to be a good one for aircraft use. It is also interesting to see how the strength increases when it is "normalized" and instructive to see how the "normalization" temperature differs from the recommended temperatures for "tempering."

In my experience, all steel with any significant amount of carbon in it can easily embrittle next to a weld. I welded and rewelded many structures before I realized that even a little bit of carbon made a difference.

As with any higher carbon steel, heating past the transition temperature and then quenching will "harden" the steel. The percentage and quality of the grain level changes that cause the steel to harden depends a great deal on the speed of the quench. An air quench is slowest and generally gives less hardening than the more rapid quenchs. A water quench is probably the quickest because water pulls a LOT of heat out quickly when it changes state. Water quench will harden any moderate to high carbon steel quite effectively. Sometimes too effectively. The faster the quench and the thinner the section the more likely warpage will mess up your job. Oil quench is a nice intermediate quench. It is faster than air and slower than water. It is used for many steels where dimensional stability and lack of warpage is important, even though it won't usually "harden" quite as much. With the hard comes also, unavoidably "brittle." Hard steels will crack and chip before they bend or deform.

This "brittleness" can be alleviated by a "normalization" or a "tempering" process. Both of these processes are similiar, differing only in degree and not in kind. This ability to bend or elongate before breaking is called "toughness." Normalizing or tempering "draw down" the hardness, increasing the toughness. Like everything in aviation, it is a compromise. You give up hard for tough and vice versa. The normalized state for 4130 alloys is a nice compromise. The remaining hardness allows a significant increase in strength while easeing almost all of the brittle. It is several times as strong as the "annealed" or "soft" state, but still will give and elongate a lot before it actually breaks. Perfect for structural applications.

Welding thick sections would probably not cause a problem. However, a quick weld on thin wall tubing is a different matter. Most aircraft are constructed of tubing that is between .030 and .050 inches thick. This is really quite thin. Thin sections lose heat more quickly than thicker sections. When you weld metal you have to bring the metal in the joint to the point where it melts and runs together. Since the transition temperature required for hardening carbon steels is always less than the melting temperature, there is always a region near the weld where the "hardening" temperature was reached. A quick and precise weld, like TIG or MIG, will heat a narrow zone quickly to the molten point. Since the heated region is very small, it also cools quickly, by radiation, by conduction to the air, and by conduction into steel a short ways away from the weld that was NOT heated. This rapid cooling near the weld acts line a quench and causes a narrow hardened region to form alonside the actual weld. This region is quite brittle and will easily crack unless the weld itself is "normalized." This can easily be done with an O/A torch because the "normalizing" temperature range is fairly large and is easily identified by the "red" glow of the steel when it reachs the correct temperature range. This temperature is commonly referred to as "red hot."

The welding process is totally independent of the means of heating the metals at the joint. Anything that gets them hot enough to melt and flow together will create a weld. The appropriate penetration is controlled by controlling the depth of the molten puddle. In welding, ALL WELDING, the control of the molten puddle is the key to strong and consistent welding. When you learn to read and control the puddle you know how to weld. I repeat, it does NOT matter where you heat comes from.

However, with moderate to high carbon steels in thin sections, the way the heat is applied and removed and the rate of removal can be critical to the quality of the weld. It is also quite easy to over heat the puddle and literally burn the carbon out of the steel. Either burning carbon out of the puddle, or using a smoky flame and adding carbon to the puddle, can result in a change in the alloy when the puddle cools and a resulting major change in the strength characteristics of the metal within and near the weld.

O/A allows careful control of the puddle and the puddle temperature by simple manipulation of the torch and the distance of the puddle from the tip of the inner cone of the flame. The welding equipment is the least expensive and the materials cost is quite low. It allows for easy control of the rate of application of heat and, more importantly, the rate of removal of heat after the weld. Aircraft welding requires a little practice controlling the puddle, but is a skill that can be easily learned by almost anyone with a little proper direction and a few hours of practice.

TIG also allows careful control of the puddle and the puddle temperature, by manipulation of either a foot control or a finger control temperature adjustment. The welding equipment is relatively expensive but the materials cost is quite low. It does allow for very precise control of the heat and the puddle, but does not work well for broad area heating or heat removal rates. Generally TIG welds should be "normalized" after completion with an O/A torch or with an oven large enough to heat the entire structure to the "normalizing" temperature of "red hot." Aircraft quality welding with TIG is not too difficult for anyone who has learned puddle control with an O/A torch. You have to learn the temperature control by finger or foot, rather than by changing the distance from torch to joint.

MIG allows the least control of the heat and the puddle. Temperature control is usually varied by stopping the weld and changing the setting on the box. The filler rod/wire feed rate is constant and you vary you puddle size by varying your rate of movement along the weld. The puddle size varies the penetration. Moveing too slowly will blow holes through the tubing. Moving too rapidly will lay a nice bead on top of the metal without any strength that results from the depth of penetration. The constant rate of heat application does not allow normalization at the time of welding, and does require that the welds be heated to the "normalizing" temperature either with an O/A torch or in an oven large enough to hold the entire structure and bring it to a "red heat" for normalization.

Therefore, I see no reason for a homebuilder, who is going to weld up one fuselage, to invest in both O/A equipment for the required normalization process and some other more expensive technology for doing the actual welding itself. Since the O/A equipment is needed in any case, and it is also the easiest technique to learn and apply for the actual welding, why spend the extra money unless you do have a production line and plan on welding many fuselages. Then the time savings realized by the electrical welding processes can be beneficial. If you, like most homebuilders, have more time than money, don't fool around with the electrical stuff for welding up your airplane.

Alan Swanson wrote:

I am just learning to weld, with the intention of welding the landing gear and other metal parts on a Pietenpol. To practice, I am running beads on 16 ga steel, with Oxweld 32CMS rod, oxy-acetylene. When I add the rod, I get showers of fine sparks much like you get from a grinding wheel. The rod is new, and I cleaned it carefully with acetone before using. What am I doing wrong?

You got some really good questions. I will try to summarize the important ones.

• Set pressures for both Acetylene and Oxygen to around 5 psig or a little less. You can usually read such pressures easily on the acetylene regulator. You may NOT be able to read the oxygen regulator easily at those pressures. If not, set the acetylene pressure just under 5 psig. Then crank the oxygen pressure down until you get NO oxygen. Light the torch and open the red valve as far as you can without the flame blowing away from the tip and going out. Then open the green valve halfway. Start turning up the oxygen regulator until the orange in the flame disappears and you see a hard blue cone in the middle. If the red valve is not all the way open, you can now open it some more. If you get a "soft" feather on the cone, turn up the oxygen pressure a bit more. When the red valve is open as far as you can get it and the oxygen valve is half way open and you just barely get the "cone" in the flame, then the regulators are set for welding.

• Filler rod. The best rod for a beginner to use when welding 4130 is an E70S-6 rod. This rod is slightly more expensive, but is much easier to weld with because it helps keep the puddle quiet. The key to all welding is controlling the little puddle of molten metal.

• Tip to use. The flame, when it is adjusted should make a relatively quiet roaring sound. If the roaring gets hard or suddenly increases in volume, you have the gas valve too far open. Close the oxygen, then close the acetylene down a little and readjust the flame until the feather just goes away. If it takes more than fifteen or twenty seconds to get the puddle started with the flame just starting to get noisy, go to the next size larger tip. If it melts a puddle in much less than ten seconds, go to the next size smaller tip. That range gives you the best control over the puddle.

• Flame adjustment. The perfect flame for welding 4130 or mild steel is one where there is exactly enough oxygen to burn all of the acetylene and NONE left over. Excess oxygen will quickly corrode the hot molten iron in the puddle. The iron oxide will spark and jump out of the puddle. If it doesn't get out of the puddle it will weaken the weld. Excess acetylene will leave carbon in the molten puddle. This will harden the steel where it was melted and make it excessively brittle. This will cause the weld to fail in service. If you are welding stainless steel use the same welding rod, but after you adjust your flame, tweak the acetylene valve just enough to you barely start to see a little feather at the tip of the cone. This is a "reducing" flame and works better for stainless. Tweaking the gas a little to far DOWN so that the feather disappears and then a little more, gives you an oxidizing flame and will ruin your welds. You can see it because the cone get harder edged and starts to shrink a little. What you want is a relatively long soft blue inner cone right on the tip with the edges just on the verge of starting the feather. That is the best welding flame.

Testing your work. The old pipefitters test is a good one. Take a short length of the largest tubing you can find. Cut four pieces that are mitered to the center. You can do that by taking a piece of tubing about an eighth of an inch longer than its diameter and then cutting it diagonally to get six pieces of tubing with two forty five degree cuts. Now you have two saddles and two HALF saddles. Arrange these pieces to make a funky looking ball. Drill an eighth inch diameter hole in the center of one of the full saddles. You have to drill the hole BEFORE you start welding.

Now weld these pieces together into a ball. Enlarge the hole and screw a fitting into it that will accept a tire valve. Pressureize your ball to about 30 psig and dunk it in a bucket of water. If you see bubbles, you flunk your pipefitters test.

Now comes the HARD part of the test. Without messing up the air fitting, put the ball in the vise and flatten it as flat as you can. Then pressurize it again to 30 psig and put it back into that bucket of water. If you don't see any bubbles then, you passed the test!

Even when you flatten the joint in a vise, it should not break either in the weld or next to it.

When you can pass this test easily and automatically, then you are an aircraft welder!

I have posted my findings and opinions so many times that some might want to put ZZZ's on my thread. First off, you do not want or need to use anything but mild steel rod for welding 4130 whether it be with Oxy/Acet or TIG. The usual filler wire for acetylene welding is E70S-2. Dash 3 can be used but it isn't as easy as -2. I like E70S-6 for both acetylene and TIG work because it flows well and contains a very high level of di-oxidizers to help keep the molten puddle quiet by eating up the contaminants.

There are those who use -2D. I tried it and found nothing to recommend it over -6. Some like the "vacuum melt" formulation but at $50+ a pound it doesn't offer enough advantage for its high price. Several have said that the only thing to use is 4130 filler wire--- an inexperienced welder will usually run into problems with 4130 and there are no benifits to its use for assembling tube fuselages, landing gear and engine mounts. If something in your project needs to be assembled with 4130 filler your instructions will say so and that part will likely be heat treated after welding.

The last choice is stainless. I have tried all the 300 series and some of the 400 series stainless filler rod with TIG on 4130. Again I found little advantage over E70S-6 and stainless filler has some idiosyncrasies of its own when used on 4130 which in some cases can lead to short fatigue life and cracking in the joints.

Mild steel has the "give" needed for the tougher 4130 to move as the weld cools and reduces the opportunity for cracks to begin as the shrinking filler metal tugs at the toe of the junction between the tube and the fillet (that is not a French word).

The only useful application for a MIG box on aircraft structure is perhaps in tacking the tubing together prior to welding. Even then, the tacks should be kept small enough to weld over. I wonder if anyone ever told this to the people building those fine certified production tube-and rag aircraft at American Champion Aircraft - home of the Citabra, Decathlon, and Scout. I had a chance to visit there plant near Rochester, WI (an interesting experience if you're ever in the area. The entire fuselage is MIG welded using, if I remember correctly, the Lincoln SP125 wire feed mig welder (the street price is around $600). I've had sufficient experience with MIG that I'd have no qualms using MIG to put together a 4130 fuselage (but personally, I use TIG - a little more graceful and more of an art. The Lincoln SP125 is a nice unit and is high on my list of items to get. My other unit (besides the OA torch) is a Lincoln 175 Square Wave TIG ( for the money it can't be beat ).

The American Champion has had serious problems in the field with cracking next to the welds. Even though they were welded under very carefully controlled conditions by expert MIG welders with much better MIG equipment than the average homebuilder is attempting to use.

There is NO inexpensive MIG welder that has acceptable heat control for welding thin wall steel tubing safely. They tend invariably to overheat at both the start and end of the weld bead. Since most of the welds placed are only a fraction of an inch long, this burning of the base metal at the start and end of each weld is dangerous at best. Also the consistency of technique requires a lot of experience to gain before aircraft structures can be safely welded with MIG.

Even after you gain the experience, unless you go over the entire structure and normalize all of the welds, you will have an area near each weld where the strength characteristics of the steel have been seriously impaired, making it prone to crack at that point.

I didn't believe that either, until I found myself rewelding a lot of my welds. Normalized them properly and haven't had to reweld one since! Theory be damned, I KNOW what happened to ME.

The best way by far to learn how to weld to aircraft standards is to start welding with an O/A torch. When you learn to do an acceptable weld with that, it is a relatively easy step to doing acceptable welding with a GOOD TIG rig. When you can do TIG well, you probably know enough about the metal and how it welds and flows to learn to weld it acceptably with MIG.

It is ironic that it is easiest to learn how to lay a bead with MIG, even a cheap one, but HARDEST to learn how to do consistent quality aircraft standard welding with a MIG.

Incidently, unless you have a VERY large oven available, you will need the O/A rig handy with a rosebud to do the normalization after the welding is complete with either TIG or MIG.

Since a top quality brand new O/A rig can be had, complete with a supply of required gasses, anywhere in the country for something under $600, and the CHEAPEST acceptable MIG or TIG outfit will come to at least $700 and likely more, one wonders why everyone wants to MIG their airplane! Especially since the O/A rig is a LOT easier to use for aircraft standard welding.

I will never understand it.