The rejection rate is 100%, and the crack of ring forging is caused by forging folding
During the processing of 18crmntih ring blank in a flange factory, it is found that there are cracks and scrap products with large crack defects and high scrap rate. The preliminary inspection of the crack defects shows that the crack is caused by the original crack of raw material bar. In view of this situation, the cracked parts were inspected, and the scrap situation was inspected at the processing site of the forging plant, so as to observe the macro defects and determine the causes of cracks.
Macro morphology of defective parts
Parts processing process: sawing cutting → induction heating → forging → rough turning (single side turning to remove 2mm), crack scrap found on rough turning smooth surface. This batch of bars finished forging more than 800 pieces, and about 200 pieces of scrap were found in the turning process after forging. After the actual confirmation at the processing site, it was found that about 200 pieces were cracked, and the rejection rate was 100%. The macroscopic appearance of the scrap is shown in Fig. 1 and Fig. 2, and the local crack is shown in Fig. 3. Several pieces are randomly selected from the scrap, as shown in Fig.4.
Figure.1 back of part
Figure.2 front of parts
Fig.3 local cracks
Figure.4 multiple scrap parts
Through macroscopic observation, it can be found that the cracks are located near the middle hole on the back of the parts, most of the cracks are about 40mm long, and their shapes are slightly arc-shaped. The shapes of the cracks on several waste products are similar, and the folding marks on individual parts are obvious. If the crack position corresponds to the section before forging, it should be a certain cross section inside the section. After ultrasonic testing, the possibility of crack at this position can be almost ruled out.
Physical and chemical examination
(1) Chemical composition
As shown in the attached table, the chemical composition of waste parts meets the requirements of the standard, which is not the cause of cracks.
Chemical composition of parts (mass fraction) (%)
(2) Dye penetration
Because the scrap rate after rough turning is 100%, it is speculated that there should be cracks on the forging before rough turning. Therefore, two parts were randomly selected for dye penetrant inspection among the parts that were not polished after forging, as shown in Fig.5 and Fig.6. After dye penetrant, there are many penetrant accumulations near the inner hole on the back of the parts, which indicates that there are cracks on the blank after rough turning.
Fig.5 parts after coloring penetration
Fig.6 local cracks after dye penetration
(3) Metallographic examination
As shown in Figure 3, samples are taken at the crack defect position of the part, and samples are prepared by grinding and polishing in the direction perpendicular to the crack. Under the optical microscope, the crack morphology is gradually observed along the crack extension trace from the outside. The crack has obvious opening at the beginning and gradually expands inward at a small angle, and it is generally lifted, and there is no opening at the end, and it extends in capillary shape. After amplification, obvious oxidation phenomenon can be seen in the crack, as shown in Fig. 7 ~ Fig. 12; After corrosion (4% nitric acid alcohol solution), the microstructure is normal ferrite pearlite with slight decarburization; The trace of metal flow line caused by forging deformation is clearly visible, and the crack propagation direction is parallel to the flow line as a whole.
Fig.7 crack tip (50) ×）
Figure.8 crack initiation (50) ×）
Figure.9 crack tip (200 mm) ×）
Fig.10 crack initiation (200) ×）
Figure.11 crack tip (after corrosion, 100 ×）
Figure.12 crack initiation (after corrosion, 100 ×）
On the macro level, the rejection rate is 100%, and the shape of defects on multiple scrap parts is consistent, and the distribution position is regular; The results show that there are cracks on the blank before and after rough turning; Microscopically, the crack begins with an obvious opening, which is nearly parallel to the surface and gradually extends inward, while the end of the crack gradually has no opening and extends in capillary shape; There are obvious oxidation and slight decarburization in the crack, and the propagation direction of the crack is parallel to the streamline trace of the forging metal. These characteristics show that the crack is a kind of forging folding, which can be reduced by controlling the reasonable blank volume, modifying the process and die shape, and regularly checking and replacing the die.
Authors: Liu Guijiang, Liu Xiaobin, Xu Qiang
Source: Network Arrangement – China Pipe Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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