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Fall 2024 Contents:                                                                                                Issue No. 64

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Effect of Arc Welding Process on Weld Strength

In this nugget, we review the effect of arc welding process including filler wire, shielding gas, and base metal, on weld strength.

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(Below is text of the article without figures; if you would like to download pdf copy with figures and tables, please click on the link above)

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One of the basic requirements of a weld is that the weld be strong enough to have sufficient mechanical strength over expected lifetime of the weld.  Although a very strong weld is not always a good choice, as strong welds may not have sufficient ductility and fracture toughness, so often it is a compromise.  Welds can also have other performance requirements such corrosion resistance, electrical conductivity, and hermeticity.  In autogenous welds where no filler is added, and the weld is made in an inert environment as with TIG welding, laser welding, and electron beam welding, the weld will be of similar strength as the base metal except for changes in strength due to annealing or hardening during cooling – will depend on weld chemistry.   However, there are many situations where a filler wire is added, and weld strength is affected by the welding process.  In this newsletter we will review the effect of multiple parameters on weld strength during arc welding with filler wire.

 

For this discussion we well review ER70S-6 filler using GMAW (MIG) and GTAW (TIG) processes; this wire is perhaps the most commonly used wire for welding conventional carbon steels.  Designation “E” indicates it can be used as an electrode for GMAW welding, and “R” indicates it can also be used as filler rod for GTAW welding.   “S” indicates that it is a solid wire and “6” is a code for chemistry of the wire as defined in by AWS 5.18 specification.  The number “70” indicates the minimum tensile strength of the weld deposit has to be 70 ksi.

 

Such a wire can be welded using multiple options for GMAW including globular transfer with 100% CO₂, short-circuiting transfer with a 75/25 Argon + CO₂ mix, and spray transfer with a 90/10 mix of Argon + CO₂.  Globular transfer with CO₂ introduces a lot of oxygen into the weld which combines with deoxidizers in the filler wire such as Mn and Si and are removed from the weld resulting in the formation of glass islands on the exposed surface of the weld; see Figure 1.  Reduction in Mn and Si from the weld metal reduces strength of the weld and increases ductility.  Short circuit transfer with 75/25 mix tends not to remove as much of the oxidizers and they remain in the fused metal, thus increasing strength of the deposited metal.   Table 1 lists the composition of the weld metal for selected welding process, along with published data for weld strength. 

 

 

Figure 1:  Photo shows glass islands (marked with arrows) that form on the surface of the weld bead as the oxygen combines with silicon and manganese.  The glass islands have to be removed prior to painting as the paint does not adhere well to the glass.

 

In addition to the two conditions that are commonly listed on data sheets, there are other conditions which can affect weld strength.  If the base metal being welded is not very clean, it may contain oxide inclusions which will get incorporated into the weld.  Oxygen from dissociated oxides will also bond with the deoxidizers resulting greater production of silica islands and a weld zone that has lower strength and greater ductility.  Another source of oxygen is surface oxides from steels that are not very clean, and rust on the surface gets incorporated into the weld, and dissociates to provide additional oxygen.

 

On the other hand, if the steel is welded with a 90/10 mixture of Ar/CO₂ in spray transfer mode, there will be even less scavenging of the deoxidizers and the weld will be even stronger than the values listed for the 75/25 mix.  Welds made with GTAW, which uses 

 

 Table 1: Table shows weld metal chemistry of key elements and strength values (highlighted rows) from a manufacturer’s published data; values could be different from other manufacturers.   Values in other rows are estimated strengths based on welding conditions.

 

100% Ar, will further increase retention of the filler constituents, resulting in an even stronger weld.  While GTAW may not have been used for the original weld, GTAW is often used for repairs on GMAW welds.

 

While the steel wire has relatively low carbon, of the order of 0.1%, it is not uncommon for the base metal steel to have higher levels of carbon such as A36 steel with 0.27% nominal carbon.  In a deep weld made in spray transfer mode a greater amount of carbon will be included into the fusion zone further increasing the weld strength.  If the steel being welded has other micro-alloying elements, the weld strength could further increase with addition of the alloying elements into the weld.  

 

Depending on the process, shielding gas mixture, and base metal conditions, strength of a weld made with a 70 ksi wire can range from 65 to 90 ksi.  The weld could be ductile and tough at one extreme, and strong and brittle on the other.  Weld strength can be measured by making fillet weld coupons under realistic conditions and testing them to failure; micro-hardness scans on a weld section can provide an estimate of variation in weld tensile strength within the weld zone.   A design engineer would be wise to conduct tests on coupons to get a realistic estimate of weld mechanical properties, rather than assume a number based on the nomenclature and published data.