Which NDE technique is not suitable for detecting cracking in a 300 series stainless steel weld?

The technique that is not suitable for detecting cracking in a 300 series stainless steel weld is magnetic particle testing. This is primarily due to the fact that the 300 series stainless steels are non-ferromagnetic materials. Magnetic particle testing relies on the magnetic properties of the material being inspected; it can only be applied effectively to ferromagnetic materials. Since 300 series stainless steels do not exhibit ferromagnetism, magnetic particle testing cannot be employed to reveal surface cracks or discontinuities in these types of welds.

In contrast, liquid penetrant testing can effectively detect surface cracks regardless of the material's magnetic properties, making it suitable for stainless steel. Ultrasonic testing is also applicable, as it can detect internal and surface flaws in various materials, including non-ferromagnetic ones like 300 series stainless steel. Radiographic testing can provide information about internal weld integrity and can identify cracking as well, thus making it pertinent for the inspection of stainless steel welds.