Weld tensile strength and weld testing

​Some people are surprised to learn that weldments on stainless steel components can actually be stronger than the base metal. How is welding strength calculated? 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 tensile strength is calculated. A non-destructive stainless steel pipe welding test or multiple tests can also be used on weldments. 

We went offsite to determine the tensile strength of stainless steel, as we generally don’t perform destructive testing in-house. The purpose of this test was to discover how a lack of weld penetration can ultimately affect tensile strength. The test was to prove to ourselves and our customers the value of a fully penetrated weld versus a non-penetrated weld when it comes to tensile strength. 

Tensile strength testing

Inferior weld joints on stainless steel tubing assemblies for gas carrying systems have greater potential for failure over time. The welding technicians at Axenics take the extra step to fully penetrate all of our welds. Tensile strength testing is an excellent way to demonstrate 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, we cut the tubing in half and then flattened it. To measure the strength of the weld, we used a calculation of the total area of the weld divided by the strength it took to break that weld.

Elements of testing for tensile strength 

• 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, resulting in “area.” 
• Once the machine stretches the sample, the amount of force applied is measured in pounds. 
• When the sample ultimately breaks, the machine records the measurement in pounds. 
• At that point, divide the “area” by the pounds of force. 
• This calculates the number of PSI required to break the weld.
• The results prove 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.

This exercise demonstrated that the welding process for the various sizes and types of tubing assemblies used in gas carrying systems, as well as all other applications, needs to be performed at the highest quality level. This will ensure that none of the equipment is compromised by catastrophic tube failure. Non-destructive testing generally ensures the top performance of weldments, but we’re willing to perform destructive testing to ease your mind. 

Methods of non-destructive testing (NDT) 

Axenics’ NDT services on stainless steel welds include but are not limited to:

• Visual testing
• Helium leak testing
• Ultrasonic testing
• Liquid penetrant testing

Visual weld testing

Visual weld inspection is a vital method of inspecting for potential leaks, as well as an opportunity to audit production consistency. Visual inspections are often combined with different forms of NDT, such as helium leak testing. Whether it’s for a small run of components or many repeated pieces, we’re testing to ensure quality and consistency.

We look for a range of characteristics during a visual inspection, and those characteristics are dependent on the type of material being used. While the standards can vary for each project, we’re often looking for potential problem areas, such as:

• Consistency in the materials
• Corrosion of materials
• Fissures in the welds
• Flaws on the surface of materials
• Banding distortion

Helium leak testing

Helium is the ideal element to detect cracks in welds because it is an inert gas made up of atoms, and has low viscosity, so the atoms can travel fast. Pressure testing and vacuum testing are two forms of helium leak testing used to ensure your components are free of weld defects. 

Our expert team knows exactly where flaws on a weld would be found. All of the instruments we use for NDT are regularly inspected and calibrated by our in-house technicians to make certain the machines are performing perfectly, ensuring 100% accurate test results for your process control systems.

Ultrasonic testing

The skill and training for ultrasonic NDT is more extensive than other NDT methods, and is overseen by our quality control department. Ultrasonic beams are safe for testers since they do not contain electromagnetic radiation. Ultrasonic testing locates the exact position of a weld discontinuity more accurately than even radiographic testing.

We establish a weld’s internal integrity with ultrasonic testing with the following process:

• An ultrasonic gauge transmits a beam of high-frequency ultrasonic energy into the component or material.
• The ultrasonic beam penetrates the entirety of the component or material.
• Then, two audible signals result from the gauge, alerting the tester.
• If the ultrasonic beam intercepts a discontinuity in the component, it reflects the location back to its point of origin and a third audible signal sounds
• This indicates there is a deformity beyond acceptable parameters. From that signal, we know the exact location and size of the deformity.

Clean technology advances are a boon for alternative energy manufacturers. Hydrogen fuel cells provide safer, cleaner solutions for companies that produce consumables. Water is the byproduct of hydrogen fuel cells, so forklifts and other vehicles with that technology installed can perform alongside food products without contaminating them.

Axenics provides NDT on materials and weldments for three-pass heat exchanger hydrogen fuel cells for a variety of vehicles, such as lift trucks or buses. An ultrasonic gauge sends high-frequency mechanical vibrations into a component or material. Once the wave has penetrated the entirety of the component, two audible signals result from the gauge. If there is a third audible signal, a deformity exists that goes beyond acceptable parameters. For example, a scratch on tubing cannot be greater than 10% of the wall thickness of the material, or it doesn’t meet quality standards.

Dye penetration testing

Weldments on hydrogen fuel cells are often the width of a single strand of human hair, so depth gauges may not detect a fault in a weld. A dye penetration test searches for minuscule cracks that visual and other testing methods cannot locate. Steps for this testing method include:

• Cleaning the material to remove any dirt, dust or potential contaminants and then drying the material
• A thin dye coats the material, and then the dye is wiped clean from the material
• Next, the material is sprayed with a developing powder
• The powder is wiped from the material
• If flaws exist, any remaining powder exposes them
• Depending on the type of dye used, a flaw is visible by the human eye under white light or black light

Cleanroom welding

Also noteworthy on this subject, NDT is pointless if the other steps in the manufacturing process occur in a “dirty” environment. Controlling contamination during every stage is a top priority while working with components used to produce high-purity products. Cleanrooms – such as the Class 100 cleanrooms in-house at Axenics – are the ideal purification solution for every aspect of production, from inspecting materials through final testing and packaging.

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