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Cracking analysis of stainless steel flange

The chemical composition of cracked flange was analyzed...

Stainless steel flange, as the name implies, is forged from stainless steel flange, and because of its material and process differences, stainless steel flange often cracking phenomenon, the following cracking problem gives the following analysis recommendations.

1 Physical and chemical analysis

The chemical composition of cracked flange was analyzed.

The analysis results show that the chemical composition of the flange and welding material conform to the relevant standard requirements. Brinell hardness test was carried out on the outer surface and sealing surface of the flange neck, and the Brinell hardness test value met the requirements of JB4728-2000 standard.

On the premise of qualified chemical composition, hardness testing, pipeline installation, technical requirements and operating specifications of pressure test, pipeline flange cracks in the process of pressure test, indicating that the defects are related to the internal structure of the flange or other unknown factors.

2 crack property analysis

2.1 surface crack morphology

The leaking flange was cut off from the pipeline and inspected. It was found that there were cracks on the inner surface of the flange, the length was 3 mm and the width was 0.5 mm. Rust stains were found on the cracks and their vicinity. More cracks were also found on the inner surface of the bolt hole of the flange. Many non-metallic inclusions were found on the sealing surface of the flange and other locations, and some were non-gold. The inclusion formed a small pit on the surface of the flange seal surface because of the dropping of inclusions. The pit is 2.0 mm in size and 0.8 mm in size. There are small cracks visible to the naked eye on the edge of the pit.

Macroscopic inspection of other austenitic stainless steel flanges of the same manufacturer and batch shows that many flanges have defects of varying degrees, and some flanges have obvious cracks on the surface visible to the naked eye. Macroscopic examination results further show that flange leakage is related to flange itself.

2.2 section crack morphology

In order to further explore the real cause of flange leakage, a flat welded flange with small pits on the sealing surface was randomly selected from the same manufacturer and batch, which was formed by non-metallic inclusions falling off. The flange was dissected and divided into three parts of uneven size. Careful observation of the flange section showed that four of the six sections had naked eyes. Visible cracks and cracks almost run through the entire section.

One of the sections was selected, and the cracks were observed after roughing, fine grinding and corrosion, which almost ran through the whole section. The crack has a certain width. Most of the crack voids contain black non-metallic inclusions. One of the inclusions is about 3.0 mm away from the flange sealing surface, and its diameter is about 2.5 mm. There are obvious repairing welding marks from the upper part of the inclusion to the sealing surface. The repairing welding area is about 380 mm2, and the repairing welding depth is up to 3.0 mm.

In this new process pipeline, the components used in the pipeline are not old-fashioned products, but newly purchased products. A number of cracks containing non-metallic inclusions were found in the profile of France, which can be preliminarily determined that the cracks are related to a large number of non-metallic inclusions. The weld marks on the profile are a very puzzling and abnormal phenomenon.

2.3 metallographic structure analysis

In order to further explore the causes of cracks, to find out whether the newly purchased flanges have actually undergone repairing welding before assembly welding to the pipeline, the flange section crack location was metallographically inspected.

The flange section was finely ground and polished, and the cracks were observed under a 100-fold microscope. The longer cracks contained large strip-like non-metallic inclusions (see Fig. 2) and dot-like non-metallic inclusions dispersed in the section. The microstructure contains dendritic austenite, which is a typical austenitic stainless steel welded joint microstructure.

3 Determination of crack properties

The analysis results of crack morphology, fracture surface, micro-morphology and metallographic structure show that the crack is a forged crack along the top of inclusion in the production process of flange. In order to eliminate the obvious cracks on the flange surface, the repair welding was done before flange.

4 crack cause analysis

4.1 the existence of non-metallic inclusions is the root cause of cracks.

Because there are a large number of non-metallic inclusions in the forging blank of flange manufacturing, volume inclusions are forged repeatedly to become area inclusions, resulting in forging cracks along the top of inclusions. The larger inclusions become larger cracks after forging, and the smaller inclusions become smaller cracks after forging. Most of these cracks are buried cracks.

Some of the buried cracks in the forging blank become surface cracks after machining, which is the reason why there are more micro-cracks in the flange internal and external surfaces and bolt holes. For the flange with large surface size cracks, in order to let the products out of the factory, manufacturers adopt the methods of grinding, eliminating surface cracks, repairing welding, re-machining, etc. to make the flange look no obvious defects. This is the reason why the flange repair area is 380mm2 and the repair depth is 3mm.

4.2 the forging crack is further expanded under test pressure, resulting in flange leakage.

A large number of non-metallic inclusions will reduce the plasticity and toughness of materials. Slag inclusion in the crack causes stress concentration at the top of the crack. The flange can not bear external loading. As a result, the flange cracks and leaks when the test pressure reaches 0.6 MPa during the pressure test. 5 measures taken

(1) replace all the flanges.

(2) Continue to search for the same manufacturer and batch of other pipe fittings used in this device, and adopt corresponding inspection means to confirm whether they are qualified.

(3) in addition to rechecking its chemical composition and mechanical properties, the new flange should also be examined macroscopically.



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