Role of inductance in short circuit cycle in GMAW welding

Inductance controls the rise in amperage (current) between the time the electrode contacts the base metal and then pinches off. The higher the inductance is set, the longer the arcing period. Both short-circuit cycles shown in figure shows the same amount of time. As the inductance increases, the rise in amperage is hindered, thus increasing the amount of time the electrode wire is in contact with the base metal while decreasing the amount of time the open arc occurs, which increases the puddle fluidity. Provided the voltage and amperage are set correctly and if the inductance is set correctly, the puddle will be fluid with little spatter. Voltage and amperage (wire feed speed) are the primary parameters to set. Inductance can be thought of as the final stage to fine-tune the arc. If the inductance is set too high, the rise of amperage is hindered, and the electrode will have poor arc starting stability. The electrode wire will stumble, and the operator will feel the welding gun push back. If the inductance is set too low, the rise of amperage is not hindered, and the short-circuit cycle is fast and violent, producing a great deal of spatter. In worse cases, the short circuit is so violent that the electrode wire snaps back and fuses to the contact tip.

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Today’s modern power sources have a usable range of inductance and have eliminated the problem areas at the extreme high and low ends. For carbon steels, 30% inductance is sufficient to reduce spatter and provide good wetting at the weld edges. Inductance settings for stainless steels are set significantly higher in order to reduce spatter, and a 50% setting is desired. The higher inductance tends to ball the end of the electrode, which must be cut before restarting the arc.

Keep reading, happy welding

Thank you,

KP Bhatt

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Metallurgical and Mechanical factors responsible for hot cracks in Welds

The accepted theory of solidification cracking in welds as well as castings is that a coherent, interlocking solid network separated by essentially continuous thin liquid films is ruptured by tensile stresses arising from thermal contraction. If sufficient liquid metal is present near the cracks when they form, they can backfill or heal. If not, the cracks appear as open tears. For this reason, solidification cracks are also called hot tears, or, most commonly, hot cracks.

Several factors favor solidification cracking, some of which are metallurgical in origin and some of which are mechanical in origin. Metallurgical factors include:

(1) Freezing temperature range, the wider the freezing range for an alloy, the larger the area that is weak and susceptible to weld solidification cracking. The freezing range of an alloy can be increased either by intentional additions (e.g., Cu, Mg, or Zn in Al) or by the presence of undesirable impurities (e.g., S and P in steels or Ni-based alloys).

(2) Presence of low-melting eutectics, the effect of impurities, such as S or P in steels, is to widen the freezing range considerably, but without producing large volumes of liquid to permit backfilling. Such residuals tend to segregate to grain boundaries, where diffusion rates are almost always higher due to the more open nature of lattice structure in these areas, and form low-melting compounds that lead to cracking. In plain carbon steels, for example, FeS films would form and lead to severe cracking if not for intentional additions of Mn leading to preferential formation of harmless globules of MnS. For this reason, Mn/S ratio is carefully controlled in steels.

(3) Grain or subgrain structure in the fusion zone, coarse columnar grains are more susceptible to solidification cracking than fine equiaxed grains for several reasons. First, fine, equiaxed grains can better deform to accommodate contraction stresses, relieving stress across interdendritic boundaries. Second, liquid can be more effectively fed to incipient cracks to promote healing in fine-grained materials, where grain boundaries run in all directions. And, third, when the grain boundary area is greater, the concentration of potentially harmful low-melting point segregates is reduced

(4) Surface tension of the grain boundary liquid, If the surface tension between the solid grain and the grain boundary liquid is very low, a thin liquid film forms by wetting the grain boundaries. For this situation, susceptibility to cracking is very great. If, on the other hand, the surface tension of the liquid is high, it will tend to bail up to form globules and will not coat the grain boundaries with a continuous film. This greatly reduces the susceptibility to cracking. The effects of different surface tensions on liquid wetting and distribution is shown in figure below,

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Mechanical factors

(1) Contraction stresses, two internal sources of tensile stresses are generated are shrinkage as liquid transforms to solid and contraction due to the thermal coefficient of expansion.

(2) Degree of restraint, for the same joint design and material, the greater the degree of restraint of the workpiece by fixturing or other structure in the weld assembly, the more likely that solidification cracking will occur.

Reference: Principles of Welding, Robert W. Messler, Jr.

Keep reading, happy welding

Thank you,

KP Bhatt.