What is Welding? And, What Makes Welds Crack?

Let’s begin with a short protect your butt statement; Welding may be dangerous. it is always recommended that certified welders be utilized. Welding; alloys, tool steel and stainless may be tricky and caution is to be observed; especially if the metal has been hardened. Appropriate protective gear, and adequate ventilation is required. There are fumes, potentially harmful light rays, intense heat, and big and heavy sharp things that you should be concerned with. This is intended to be informational only, NOT INSTRUCTIONAL! It is furnished to introduce Non-welders to the topic so that they may have a frame of reference if the topic of welding arises. That statement is there to ensure that I don’t get hurt by welding.

When you weld steel, you are heating it to a temperature at which it will melt. You are adding a filler metal (weld rod or wire). The filler and base metal will melt together, and when the assembly begins to cool, the base metal and the filler should solidify and bind together.

The filler metals may be in the form of “rods” that come sealed in a container and are manually fed into the weld areas as they are consumed. Or, they may be in the form of a wire spool which allows the wire to feed automatically. Often, when you see notes that suggest rod be used instead of wire, they may simply be suggesting that to minimize heat input, since wire lends itself to heavier deposits due to the increased speed.

The following contribute to potential failure “cracking” of the weld. Dramatic and uncontrolled temperature changes are not good. Going from cold temperatures to extreme heat, then back to cold is not conducive to sound welds. Whenever possible, bring the temperature on the materials to be welded up. Ideally you will want to know the tempering temperature of the steel you are welding, and make sure you stay under that, but, it is not impossible to preheat if you don’t know the tempering temperature. If the steel is below 30RC hardness, you may generally assume a preheat of around 600F will not be detrimental to the base metal. If the steel is 40RC or higher, you may want to stay below 300F. These preheat statements are only to illustrate that some warming of the unit is better than none. Preheat also serves to evaporate atmospheric moisture present on the metal from the environment. Second to dramatic temperature change, moisture is the most troublesome condition. As the weld freezes (solidifies), hydrogen from the moisture gets trapped in the weld. Hydrogen Entrapment is contributory to cracking of the weld. Weld rods come to you in sealed containers, intended to keep them dry. Once you open the package any assurance of dryness is over. Prior to using any remaining rods from an opened container, you may want to preheat them in an oven to evaporate any residual moisture.

You do not need furnace gages to determine temperatures when you are field welding. Wax crayons or Tempil Sticks are available; they will begin to melt at, or near, indicated temperatures. You may minimize heat input by using small diameter weld rods. Further, you may “skip-and-back-step” as you are laying down the weld. Weld a little, skip forward with no weld, then weld a little more. If your parts don’t line up, it is not good to force alignment and then tac both ends of the weld seam so the pieces can’t move. As you “run” a weld bead, you ideally want the stresses to run ahead of you so they exit out the far end of the weld. If you forcibly tac them, you prohibit movement, which will eventually lead to serious warp or fracture.

Chiller bars may be used to dissipate heat. Placed adjacent to the weld, they act to absorb some of the heat (heat sink) and slow the rate of cooling. You may use several “stringer-beads” instead of laying down a weld bead that looks like a heavy hemp rope.

Moisture, heat, forced alignment that disallows movement, contaminants on the metal, such as grease, oil and rust, are all impediments to successful welds. Chemical elements contained in the steel may inhibit successful welding. Carbon levels, sulfur, phosphorus, and other elements may diminish your chances for a successful weld. In maintenance areas condition are seldom conducive to sound welding. Certified, or very experienced welders will know what cautions to observe. The “Standard Low-Hydrogen Method” is something you may want to familiarize yourself if welding discussions could be part of your occupation. 

In parting, during the nearly half of a century that I have been working in maintenance repair, qualified welders have always been in demand; great demand. Welding is very much an art-form, and a talented welder is a joy to behold. If you are of the age that you are still considering employment options, male or female, you may want to consider welding school. 

-Howard Thomas, December 14th 2018

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What is Fatigue Failure?

Let’s say you are leaving the house and you see your new credit card on the counter. You replace the expired card in your purse or wallet, but there are no scissors to cut up the old card. So, you start bending it back and fort until it breaks in half; then pitch it. You have just induced a fatigue failure.

So too with steel. A piece of steel that undergoes repeated motion (twisting, bowing, vibration, flex) will at some point fail. Conditions may encourage that failure to be earlier than expected. A nick or gouge at the surface, or an inclusion or defect (foreign element) within the steel may be the likely culprit.

In my experience, most industrial steel shaft failures are caused by fatigue. The failure may begin at the surface of the shaft (surface initiated), or, it may begin from inside (internal). Most common, will be surface initiated. Surface nicks are called “STRESS RISERS”. Think “A chip in the windshield of your car.” If not smoothed out, that nick will eventually become a crack that runs outward until the windshield fails. A Stress Riser is a break in the surface continuity of an item. Through repeated external forces, the surface-initiated nick becomes a fracture; progressing internally through the steel until a point of catastrophic failure. The shaft cracks in half, while the machine is still running. Not good!

If there are inclusions within the metal (microscopic tramp elements), they may contribute to an internally initiated fatigue failure.

What can we do to minimize fatigue type failures? First and foremost, make sure you are working with high quality materials. In the case of steel, make sure it has a high degree of cleanliness, free of internal defects. There are methods of steel production that insure your steel has excellent core integrity. Those are generally referred to as Clean-Steel-Production-Methods. Those methods, such as; Melting in an electric furnace, vacuum degassing, inclusion shape control, stirring, limiting tramp elements, etc. are available to people who require steel that has undergone refining processes. Certainly, you would want to employ those processes for materials that would be used in critical service.

Those processes address the internal portion of the steel. What about the surface? You can process the steel so as to minimize any roughness, nicks or gouges on the surface. If it is a shaft, you may want to insure you have a polished surface finish, even if the tolerance requirements of the application do not require a precision finish. Note that a highly polished surface not only resists surface-initiated fatigue failure, it provides a certain degree of corrosion resistance. Caution should be observed so that you do not get such a smooth surface that required lubricants will not adhere to the shaft. Most commercially available polished shafting will have a surface finish of about 15 micro. When you start getting into finishes much brighter than that, you may want to check into lubrication requirements.

If your finished part has contour changes (keyways, step-downs, grooves, etc), make sure the sharp corners have been radiused and if possible, even smooth out the contour.

FOOD FOR THOUGHT; “Most heavy industrial shaft failures are fatigue related. Toughness resists fatigue failures. Clean Steel Production Increases Toughness.”

-Howard Thomas, December 3rd 2018

 

 

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