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Jul 2, 2014

Know the Basics to Get the Best Results from Metal-Cored Wire


When used with the right applications, metal-cored wire can help minimize costs, improve quality and increase productivity in the welding operation. Like any filler metal, metal-cored wire has unique characteristics, benefits, limitations and applications where the wire is best suited. Knowing when and how to use this wire can help companies achieve the best success with the product.

This article discusses the most appropriate applications for metal-cored wire, some of the characteristics to consider when choosing this filler metal and tips for welding successfully with metal-cored wire.

Metal-cored wire can help minimize costs,
improve quality and increase productivity
in the welding operation when used in the
right applications. Knowing when and how
to use this wire can help companies achieve
the best success with the product.

Understanding the basics 

Metal-cored wire is a tubular wire filled with metallic powders, alloys and arc stabilizers, each of which offer distinct benefits such as lowering oxidation, providing higher impact strengths and reducing silicon deposits in the final weld.                         

The materials inside metal-cored wire can vary, depending on the desired properties and characteristics of the filler metal. Metal-cored wire is similar to flux-cored wire in that both have tubular construction and offer a higher deposition rate than solid wire. However, unlike flux-cored wire, metal-cored wire does not contain any slag-producing elements. This feature makes it more efficient, because more of the wire ends up deposited in the joint as weld metal. The lack of slag also contributes to the time- and cost-saving benefits of metal-cored wire, since it can be used to minimize pre- and post-weld activities such as grinding, chipping the slag or removing spatter.

While metal-cored wire offers a weld deposit similar to solid wire, its tubular structure causes the wire to operate differently than solid wire, which (as its name implies) is solid throughout the entire cross section. These different structures give metal-cored wire different arc and weld profile characteristics that can lead to significant benefits in the right application. 



Where metal-cored wire excels
Metal-cored wire, because of the way that it is manufactured, is easily alloyed and available in many different chemistries, making it suitable for welding a wide variety of base metals. The wire can be used in many of the same applications that employ solid wire, but applications that benefit most are single-pass welds three inches or longer in flat or horizontal position using the spray transfer mode.                         

Using the spray transfer mode maximizes the benefits of metal-cored wire because this mode allows the welding operator to move faster. The spray transfer mode also generates little to no spatter, further enhancing the cleanliness of the metal-cored wire and minimizing the amount of post-weld cleanup.                        

Other applications that are well suited to metal-cored wire include those prone to burn-through; components presenting poor fit-up; and jobs where aesthetics are important.

The benefits of metal-cored wire
Because of the physical structure and the commonly used spray transfer mode, metal-cored wire produces a broad, cone-shaped arc, resulting in a wide penetration profile (compared to the more finger-like penetration of solid wire). This arc shape in turn creates a consistent bead profile that bridges gaps easily and accurately without burn-through. Another feature of metal-cored wire is the smaller droplets of liquid metal transferred across the arc, which result in less turbulence in the weld puddle.             

Metal-cored wire offers faster travel speeds and higher deposition rates when compared to solid wire, resulting in increased productivity for welding operators. It’s also known to help minimize weld defects such as porosity, lack of fusion and undercut, which means using metal-cored wire may help reduce reject rates.                     

In addition, metal-cored wire has an increased ability to weld through mill scale and rust and still produce very little spatter, so it often helps eliminate the time and cost of activities before and after welding such as grinding, sand blasting or applying anti-spatter compound.    

Consider the cost factors

While metal-cored wire offers advantages in many applications, it is not always the best
product to use. There are some factors to consider when deciding if it’s the most cost-effective choice.        

Metal-cored wire is more expensive than solid wire, so in applications where the advantages are not being utilized, that extra cost may not pay off in greater efficiency or productivity. Applications where the additional cost may not be justified include welding in the short circuit mode of transfer, welding out-of-position, and applications with a low operator factor (percentage of time in an operation actually spent welding).                

The spray transfer mode used with metal-cored wire
generates little to no spatter, leaving an clean,
consistent weld that requires little to no cleanup.

 

For the higher cost of metal-cored wire, welding operators have the ability to get a higher deposition rate, but if the application can’t utilize that benefit the company isn’t really seeing any advantages for the extra cost.       

Another consideration when weighing the pros and cons of metal-cored wire is shielding gas. High argon content gas (usually 90 percent argon/10 percent CO2, but mixtures range from 75 to 95 percent argon with the remainder CO2) must be used to achieve the beneficial spray transfer. However, argon is a more expensive gas, so this is another cost consideration when choosing metal-cored wire.                     

Companies should keep in mind that welding is only one step in the production process, and changes to that portion may require changes in other areas to avoid other issues such as product flow and inventory management. When productivity increases in the welding portion the rest of the process has to be able to handle that increase to realize a cost savings.

Tips and techniques for welding with metal-cored wire

Even though the physical characteristics of solid wire and metal-cored wire are different, the welding technique is basically the same. Here are a few differences:

•    Use V-knurled drive rolls. Since tubular wire can crush easily when there is too much tension, use V-knurled drive rolls, which have small teeth to grip and guide the wire. That way less tension is needed to feed the wire down the liner. 


•    Longer stickouts won’t cause erratic transfer. With metal-cored wire, the contact tip to work distance — the gap between the welding gun and the base material — can be slightly longer than it is with solid wire. The recommended gap for the best performance and arc stability with metal-cored wire is between 1/2 inch and 1 inch, depending on wire diameter. Also, the general rule of thumb is the distance should increase as the diameter of the wire increases. 


•    Using a larger wire diameter is OK. When switching to metal-cored wire from solid wire, welding operators typically can use one wire diameter larger. Since metal-cored wire has a broader metal transfer, the heat is not as concentrated and there is less chance of burn-through. The wire also is better at bridging gaps and providing consistent sidewall fusion.  


•    Less need to manipulate the welding torch and puddle. The wider metal transfer/arc cone with metal-cored wire allows welding operators to make larger beads without the need to weave or manipulate the puddle. 


•    Be careful with storage. Just as with any filler metal, proper storage is important. Metal-cored wire can pick up moisture in the chemical powders used to fill the wire and at the seams, which are left when the tubular wire is formed. Take care to store it in a dry place at room temperature. If filler metal is stored in cold temperatures, such as outside in a truck bed during winter, bring it inside and allow it to reach room temperature before welding with it to avoid condensation forming on the wire. 


Other factors to note

The high argon shielding gases used with metal-cored wire impact the duty cycle of the gun. A welding gun is rated for a specific type of gas, so typically a 100 percent duty cycle rating at a specific amperage refers to using the gun with 100 percent CO2. Because CO2 does a better job of cooling the gun than argon does, the duty cycle can be reduced by 30 to 50 percent with high argon gas. Be sure to have a gun with a high enough amperage rating to account for a reduction in duty cycle when using metal-cored wire.               

Also, the high argon gas spray transfer method used with metal-cored wire tends to result in lower visible smoke generation. These lower smoke levels can lead to noticeably more radiant light, since there is nothing in the area to minimize the light generated by the arc. The higher amperages often used with metal-cored wire also contribute to the increase in radiant light. Welding operators should take extra care to cover exposed skin, possibly increase welding lens shade and, when necessary, use screens in the area around the welding operation.

Making the right choice for the application 

The selection of the right filler metal for the job is an important consideration. Metal-cored wire allows for greater travel speeds and higher deposition rates, but it also costs more than solid wire. Knowing when it’s most advantageous to use metal-cored wire can help increase productivity and save money by allowing welding operators to weld more efficiently, deposit more weld metal, reduce quality issues and spend less time cleaning welds.