How is welding strength calculated?

Some people are surprised to learn that weldments on stainless steel components can actually be stronger than the base metal. How is weld strength calculated exactly? One way is via tensile strength.

Tensile strength is a destructive testing method that demonstrates the maximum stress a material withstands before failing or breaking. Pulling or stretching the material to its breaking point is how to calculate the tensile strength.

There are also several methods of non-destructive testing (NDT) utilized regularly by technicians at Axenics to verify the top performance of stainless steel weldments (such as visual testing, helium leak testing and ultrasonic testing). However, destructive testing is sometimes requested by industries like life sciences, chemical manufacturing, medical device and alternative energy that transport highly-volatile chemicals. For those industries, we perform orbital welding in a cleanroom setting on a variety of stainless steel tubing.

We generally don’t perform destructive testing in-house, but in order to better serve our customers, we went offsite to determine what is the tensile strength of steel. The purpose of this test was to discover how a lack of weld penetration can ultimately affect tensile strength. The goal was to determine the value of a fully penetrated weld versus a non-penetrated weld.

Tensile Strength Testing

Tensile strength testing is an excellent way to show the importance of good welding practices. We tested 316L Stainless Steel, 1.00” dia. x .065” wall tubing, conducted on an equal sampling of both fully-penetrated and non-penetrated weld joints.

To prepare the sample, cut the tubing in half, and then flatten it. To measure the strength of the weld, there is a calculation using the total area of the weld by the strength it took to break that weld.

Consistency in the size of the weld or the tube does not matter. The formula takes the width of a weld sample and measures by the height of the weld. This gives you “area.” Once the machine stretches the sample, the amount of force applied measures in pounds. When the sample ultimately breaks, the machine gives you the measurement in pounds. At that point, take the “area” and divide by the pounds of force. This calculates the number of PSI required to break the weld.

The results show the fully penetrated samples break at a much higher ultimate strength PSI than the partially penetrated samples. On all samples tested, the breaking point on the non-penetrated welds was at the weld joint. On the fully-penetrated samples, the point of the break was generally located outside of the weld joint itself, on the heat affected area of the tubing.

The test demonstrates the necessity of weldments performed at the highest quality level to ensure a tubing system is not compromised by a failure in one piece of tubing. Again, NDT generally ensures the top performance of weldments, but know we’re willing to go all the way to destructive testing to ease your mind.

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