Why Do Stainless Steel Flanges Crack?

Stainless steel flanges will crack during use, so what are the reasons for cracks in stainless steel flanges? Let’s analyze them together.
 
Physical and chemical analysis
First, the chemical composition of the cracked flange was analyzed. The analysis results showed that the chemical composition of the flange and welding materials met the requirements of relevant standards. A Brinell hardness test was conducted on the outer surface and sealing surface of the flange neck. The Brinell hardness test value complies with the requirements of the JB4728-2000 standard. Under the premise that the chemical composition of the material and the hardness test are qualified, the pipeline installation meets the technical requirements, and the pressure test operation specifications are met, cracks appear in the pipeline flange in the pressure test process, indicating that the defect is related to the internal structure of the flange or other unknown factors.
 
Analysis of crack properties
The leaking flange was cut from the pipeline and a comprehensive macroscopic inspection was conducted. It was found that there were cracks on the inner surface of the flange, which had a length of 3mm and width 0.5mm. Rust spots were produced around the cracks. More cracks were found on the inner surface of the bolt holes of the flange. Many non-metallic inclusions were found on the sealing surface of the flange and other places. Some non-metallic inclusions appeared on the sealing surface of the flange, forming small pits. The maximum pit was 2.0mm. The smallest pit is 0.8 mm, and there are tiny cracks visible to the naked eye on the edge of the pit.
 
Determining the nature of cracks
According to the above analysis results of crack morphology, fracture characteristics, micromorphology and metallographic structure of the flange, it is shown that the crack occurs in the flange production process and is a forging crack along the tip of the inclusion. To eliminate the obvious cracks on flange surfaces, the flange was repaired and welded before leaving the flange factory.
 
Analysis of causes of cracks
The volumetric inclusions are repeatedly forged into regional inclusions due to the presence of a large number of non-metallic inclusions in the forging blank in the forging process, resulting in forging cracks along the tips of the inclusions in the material. The inclusions after forging turn into larger-sized cracks, and the smaller inclusions turn into smaller-sized microcracks after forging. Most of these cracks are buried cracks. After the forging blank is processed, some buried cracks turn into surface cracks. This is the reason why many micro-cracks appear on the inner and outer surfaces of flange holes and bolt holes during macro inspection. For large cracks on the surface of the flange, to make the product leave the factory, the manufacturer has taken measures such as polishing, repair welding and then processing to eliminate surface cracks, so that there are no obvious defects on the surface of the flange. This is why the repair welding area of the flange has an area of 380mm2 and the repair welding depth 3mm.
 
The taken measures
(1) Replace all problematic flanges; continue to search for other pipe fittings from the same manufacturer and batch used in this device, and use corresponding inspection methods to confirm whether they are qualified.
(2) For newly replaced flanges, in addition to retesting their chemical composition and mechanical properties, macroscopic inspection should also be conducted to evaluate their geometric dimensions and surface finish, and to check whether there are abnormal geometric discontinuities.
The above is the content sharing about stainless steel flanges, I hope it can be helpful to everyone.