In the process of manufacturing the DN6200 composite plate elliptical head (composite plate head for short), a company experienced multiple cracks. The base material (outer layer) of the head is 16MnR carbon steel with a thickness of 20mm, the inner wall material of the head is 316L stainless steel, the thickness is 4mm, and the diameter of the head is 6200mm.

　　Because the size of the head is too large, and the existing steel plate width specification is 2.2mm, and in order to make full use of the stainless steel scrap, the stainless steel plate is first welded by hand arc welding, and then three stainless steel plates and three carbon steels are exploded. Weld it into a composite board, and then use manual arc welding to splice it into a single composite board blank.

　　The processing method of the composite plate elliptical head is cold spinning. In order to find the reason for the cracking of the composite plate head, so as to take effective countermeasures and avoid similar accidents in the future, the engineer analyzed the cause of the composite plate head cracking.

　　1 Analysis of the inner wall of the composite plate head

　　The damage to the inner wall of the head only occurs at the splicing welds of the stainless steel plates 1 and 2, but there is no problem at the splicing welds of the composite plate of the head. In addition, the cracks on the splicing welds of stainless steel plates all appear in the center depressions of the welds, and all the cracks are mainly caused by overload tearing, which is caused by excessive plastic deformation.

　　Analysis of the outer wall of the composite plate head On the carbon steel side of the outer wall of the composite plate head where cracking occurs, there are visible excessive deformation, necking and cracking, and their positions correspond to the splicing welds of the stainless steel plates 1 and 2 of the inner wall. Observation shows that the main crack surface of the composite plate corresponds exactly to the splicing weld position of the inner wall stainless steel plates 1 and 2, and the crack has steps in the process of propagation, which is caused by the intersection of cracks developed at different levels. Therefore, the cracks that occur during the cold spinning of the composite plate head are multi-crack source overload tearing. However, in the process of crack propagation, after encountering cracks of different layers and passing through the steps, they change the direction and return to the original development direction of the main crack surface, which corresponds to the splicing weld of the stainless steel plate on the inner wall. It shows that the splicing weld area of the inner stainless steel plate is the weakest part of the head composite plate. The main fracture has an obvious stress rest line, which is a fibrous plastic fracture produced by repeated excessive loading and deformation. It starts from the inner wall of the stainless steel plate and extends to the carbon steel substrate. Carefully observe the stainless steel plate, you will find that there are many crack sources, so it can basically be determined that the main fracture is a multi-crack source.

　　Microanalysis

　　2 Metallographic analysis of carbon steel substrate

　　The morphology of the voids is similar to that of the carbon steel as the base material of the composite plate before spinning, but in comparison, the number of cavities in the composite plate after cold spinning is higher than that of the composite plate before spinning, and the diameter is enlarged. The main reason for this phenomenon is that during the cold spinning process of the composite plate head, the carbon steel base material undergoes cold work hardening due to repeated force spinning, resulting in dislocation accumulation and changes in the metallographic structure. The change in the number and size of the voids is the original cause of material tear failure.

　　2.2 Metallographic analysis of splicing welds of stainless steel wrench arc welding

　　The manual arc welding splicing seam of stainless steel plate is double-sided welding and double-sided forming without repair welding. The weld on this side combined with the carbon steel base plate is the second weld, and the weld on the inner wall surface of the head is the first weld. There is a geometric defect of less than 2mm in the middle of the two welds. This is due to the lack of penetration during the second weld, leaving a linear welding defect continuously distributed along the center of the weld.

　　The above-mentioned welding defects have become the crack inducing source during the spinning forming process of the composite plate head. After the above-mentioned defects are further enlarged, the four corners of the defects can be seen to have preferentially induced cracks. It can be seen that the linear welding defect of the stainless steel plate is the main source of the internal tearing of the composite plate head.

　　2.3 Scanning electron microscopy analysis The fracture sample was observed and analyzed by scanning electron microscopy. It can be seen that the characteristic morphology of the fracture of the stainless steel composite plate is a plastic equiaxed dimple, which belongs to overload plastic fracture. The fracture characteristic morphology of the carbon steel substrate is also a plastic dimple, but the dimple size changes greatly. Moreover, there are many large dimples of abnormal size, and their appearance is related to the unevenness of the material itself. Those cavities found in the metallographic structure are a major source of large dimples.

　　to sum up

　　1) Before the composite plate is formed, the continuous distribution of linear defects in the double-sided manual arc welding center of the stainless steel plate is an important source of cracking during the processing of the DN6200 composite plate head. The cracks under the repeated force during the spinning process of the hydraulic press eventually caused the head to break and fail due to overload.

　　2) A large number of cavities in the carbon steel substrate are the source of abnormally large dimples during overload fracture, and are one of the important reasons for the uneven material properties. At the same time, it is also the place where the carbon steel substrate is most prone to cracking. According to analysis: the main reason for the increase in the number of cavities and the increase in diameter is that during the cold spinning process of the composite plate head, the repeated external force (pressure) causes the carbon steel substrate to undergo cold work hardening and cause dislocation accumulation. And changes in metallographic organization.

　　3) In view of the above problems, it is recommended that, on the one hand, the number and diameter of cavities should be controlled as far as possible for the carbon steel substrate, so as to reduce the potential hidden dangers caused by the increase of cavities caused by cold working. On the other hand, it is necessary to ensure the welding quality, especially when the stainless steel plate is spliced by hand arc welding, the splicing welds must be strictly inspected to ensure the quality and eliminate hidden dangers of internal defects.