arc tracks

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I did this today as a quick demo/proof.  3/8 by 2 inch flat bar about 6 inches long.  I put a few arc tracks in the center area then buffed off the scale.  After being put under the coupon bending die, the fractures are evident.  This is a reminder to novices that arc tracks are to be avoided.
Reply:I knew they were frowned upon, but never was actually SHOWN why.  Thanksvery much
Reply:I've seen steam pipe welds that were 50 years old that had arc marks all over them, they looked like the welders used to intentionally strike outside the bevel, then drag over and start welding. Those welds held for 50 years.I'm sure arc marks are detrimental on thin wall pipe, but I don't know how much they matter on thicker stuff. Just think how much pressure you would have to have to make a pipe bulge like it was in that bending machine.
Reply:I can think of a couple situations where arc strikes are very serious.  The OP's original post is a good example.  The type of steel and the amount of stress that the arc strike location sees are very critical.  An arc strike on a ductile piece of mild steel that is just sitting without any load on it is not a big deal.But high strength steels will almost certainly form cracks in and around an arc strike.  If the material is thin these can be through cracks that would leak if the steel in question was used in a pipe transporting fluid or gas.The worst problem comes from arc strikes on steel that sees stress that cycles.  Take a structural beam in a bridge for example.  The pounding of passing cars and trucks applies hundreds or thousands of stress cycles per day.  A crack originating from an arc strike can grow a tiny bit with each passing vehicle, if the stress created by the load of the passing vehicle is high enough.  Just the sun shining on steel can cause the temperature to rise 100°F in some parts of the world.  The steel wants to expand as it warms and contract as it cools.  If it can do so freely then there's no problem.  But if the steel is fixed, say by welds to other pieces of steel, then this expansion and contraction caused by heat absorbed from sunlight, can cause stresses great enough to enable small cracks to grow.Another example might be a pipe in a power plant that warms and cools over and over again as some equipment or pump cycles on and off to pump cooling fluid.  Stress is created in the pipe every time the pipe heats up; it wants to expand and contract with the change in temperature.  Most welded pipes are restrained from moving with thermal stress.  So when they are heated, they try and expand and can't.  The stress created by this can cause cracks to grow and eventually lead to the pipe leaking or even exploding.Lastly, small cracks can become the starting point for corrosion.  Steels, even stainless steels, that don't normally rust, can corrode because a small crack or pitt will hold moisture.  This trapped moisture will enable rusting to start under conditions where a flat smooth piece of steel won't rust.  Pitting or crevice corrosion is like cancer.  It starts small, but if left untreated will ruin a piece of steel eventually.  So avoiding or repairing arc strikes can prevent rust from getting a start on a piece of steel.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Yes, and those pipes you saw happened to operate under conditions of low stress, or were made from a low strength steel that didn't crack when the arc strike occurred.  50 years ago, low strength steels were typical; and designers used thicker, heavier sections to compensate.  Today steel designers have adopted higher strength steels and lower factors of safety in designs.  Higher strength steels are lighter, easier to transport, and pipes and beams made from them take less raw material to manufacture.  Improvements in engineering and use of computer modeling makes it possible to design structures to be just a slight amount bigger and stronger than needed to do the job they were meant for.  There's less need to guess, and then oversize steel pipes and beams to get the job done and be safe.But all of this means that fabricators need to use more care as there's little or no margin for mistakes like arc strikes.  No one is likely able to gage the consequences of making an arc strike at the moment it happens.  It takes difficult, expensive, time-consuming testing to determine what an arc strike would do to any given piece of steel.  No one can know just how likely a crack is to form on any given piece of steel by simply looking at it,  or just what kind and size of stress that steel will see in use.  It's better for all welders to assume that it is unsafe to make arc strikes outside the weld joint.  If an arc strike does happen, then they need to assume it's going to cause a problem, and repair it. Originally Posted by TimmyTIGI've seen steam pipe welds that were 50 years old that had arc marks all over them, they looked like the welders used to intentionally strike outside the bevel, then drag over and start welding. Those welds held for 50 years.I'm sure arc marks are detrimental on thin wall pipe, but I don't know how much they matter on thicker stuff. Just think how much pressure you would have to have to make a pipe bulge like it was in that bending machine.