How to Deal with Cold Cracks of UT Flaw Detector?

Ultrasonic detector has high echo height, wide wave amplitude, and can produce multiple peaks. When the probe is translated, the reflected wave appears continuously with fluctuating wave amplitudes. When the probe rotates, the wave peaks have an up and down movement.

Cracks are a major and dangerous defect that not only reduces the strength of the welded joint but also becomes the origin of structural fracture due to the sharp notch at the tip of the crack. Cracks are divided into three types: thermal crack, cold crack, and reheat crack.

Reasons for thermal cracks generated by UT flaw detector

The cooling rate of the molten pool during welding is very fast, leading to segregation;
The uneven heating of the weld seam generates tensile stress.

Preventive measures:

Limit the content of elements prone to segregation and harmful impurities in base metal and welding materials, mainly limiting sulfur content and increasing manganese content;
Increase the alkalinity of the electrode or flux to reduce impurity content and improve segregation;
Improve the welding structure, adopt a reasonable welding sequence, and increase the freedom of welding seam shrinkage. Ultrasonicdetectors are used.

The causes of cold cracks in ultrasonic flaw detectors

When the weld material has high hardenability, it is prone to cracking when subjected to welding tensile stress during the cooling process.
During welding, if the cooling speed is very fast, hydrogen may not have enough time to escape and remains in the weld seam. Hydrogen atoms combine to form hydrogen molecules, which enter the microscopic pores of the metal in a gaseous state and create significant pressure. This pressure causes the local metal to experience high stress and form cold cracks.
Cold cracks are likely to form when welding tensile stress occurs simultaneously with the accumulation and quenching embrittlement of hydrogen.

Preventive Measures:

Preheating before welding and slow cooling after welding to allow sufficient decomposition of austenite in the heat-affected zone within a suitable temperature range, avoiding the formation of hardened structures and reducing welding stresses.
Timely post-weld annealing and hydrogen removal treatment to eliminate stresses generated during welding and ensure the timely diffusion of hydrogen to the external environment.
Selecting low-hydrogen electrodes, alkaline fluxes, or austenitic stainless steel welding rods and wires. Dry the welding materials as specified and clean the groove thoroughly.
Strengthening protection during welding and ensuring proper cleaning of the welded surface to prevent hydrogen ingress.
Using appropriate welding specifications and adopting a rational welding sequence to improve the stress state of the welded components.

BKN adheres to a philosophy of professionalism, precision, and strength, focusing on online and offline automatic ultrasonic flaw detection of stainless steel pipes. It has achieved remarkable results in this field and has become a leader in the industry. Feel free to contact us for further information.