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Sep 17, 2020

How Do Filler Metals Provide Impact Toughness?


Filler metals gain their mechanical and chemical properties as well as operating characteristics from a blend of elements. To formulate filler metals that provide good impact toughness, it’s as important to add certain elements and minimize others to get the best results. And there is a specific testing process to ensure filler metal manufacturers have achieved the right balance.

So how is it all done?Image of Charpy V-Notch testing equipment in a welding lab

Adding and removing elements
Nickel is a popular choice of several elements that provide filler metals with enhanced impact toughness. It offers resistance to weld cracking by altering the weld’s microstructure in a beneficial way. The weld pool may be sluggish when welding with wires that contain more nickel and they may produce more spatter; however, the increased toughness is vital for critical applications. The advantages offset these issues. 

To provide the correct properties, filler metal manufacturers also reduce the amount of tramp elements including phosphorous and sulfur in the filler metal, since these elements negatively impact weld toughness. These tramp elements may be particularly detrimental in single pass applications because single pass welds typically exhibit higher dilution. This means there is greater addition of elements from the base metal mixed into the weld metal. Look for base and filler metals with as low of a phosphorous and sulfur content as possible — ideally no more than 0.03% combined — to prevent issues with loss of toughness.

Impact toughness testing
The Charpy V-notch (CVN) test is widely recognized as the industry standard for measuring a filler metal’s impact toughness. The test is performed on a piece of equipment consisting of a hammer at the end of a pendulum (shown above). The test involves several steps.

1. Typically, three to five specially sized weld specimens are machined from the weldment and feature a precision-machined notch.

2. A sample is placed in a chilled bath to bring it to the test temperature, which can range from cryogenic temperatures to room temperatures. Once a sample has reached the appropriate temperature, it is removed from the bath and placed into a fixture.

3. An operator releases a hammer-shaped pendulum of a set weight from a set height, allowing it to apply a consistent amount of energy to a weld specimen.

4. The pendulum breaks the weld specimen in the notched area as it moves along its fixed path.

5. A computer calculates the CVN impact value after measuring the highest point of the pendulum after contacting the weld specimen. The computer provides the impact values in units of energy absorbed, in foot-pound or joules. For example, 20 ft-lbs.\ @ -20°F.

6.The test is repeated with the remaining samples so that an average value can be determined.

The absorbed energy measurements of Charpy V-notch testing can be found on the filler metal data sheet and/or certificate of conformance often available from the manufacturer’s website. For critical applications, especially those requiring welds completed to code, it is important to always consult these data sheets and certificates of conformance to verify the product’s typical impact toughness properties meet or exceed the requirements of the application.

This article is the second in a three-part series about impact toughness. Read article one, What Is Impact Toughness? and article three, How to Maintain Impact Toughness During Welding.