Analysis of Internal Defects in Large Ring Forgings of 20# Steel

The types and causes of internal defects in large ring forgings of 20# steel were analyzed using macroscopic examination, metallographic examination, chemical composition analysis, and fracture analysis. The results indicate that the defect is a white spot caused by the internal stress generated during the cooling process of the steel after hot pressure processing.

20# steel diameter Φ500mm continuous casting round billet is cut, heated, and forged into a large circular forging. During ultrasonic testing, many dispersed defects were found in some areas of this batch of large circular forgings. Ultrasonic testing can detect defects such as coarse slag inclusions, cracks, white spots, and delamination in forgings, but it is generally impossible to determine the specific type. This article conducted an anatomical analysis of large circular forgings to determine the specific types of internal defects in this batch of large circular forgings with internal defects through chemical composition analysis, macroscopic inspection, and metallographic analysis.

1. Physical and chemical inspection

1.1 Macro morphology

The size of large circular ring forgings is Φ 870mm (outer circle) × Φ 192mm (inner hole circle) × 175mm thick; ultrasonic testing found a large number of dispersed defects in some areas about 80mm away from the end face, as shown in the fan-shaped area in Figure 1.
Cut the sector area in Figure 1 using gas cutting to obtain the sample. Then, the saw was cut horizontally from about 80mm from the end face. No coarse inclusions, slag inclusions, pores, or other defects were found on the sawing surface. However, multiple protruding particles with outward bulges were found on the sawing surface (i.e., the transverse section of the forging), some of which were smaller and generally sized at 1mm × Approximately 1mm, with a larger size of approximately 4mm × 4mm, all particles are metallic in color (consistent with other areas on the sawn surface). There are saw marks on the surface, protruding outward in a circular shape, irregularly distributed on the sawn surface of the forging, as shown in Figure 2.
Take the axial section of the circular ring forging for hot acid etching, conduct low magnification inspection after acid cleaning, and evaluate according to the GB/T1979-20# 01 standard: generally, the porosity is 1.0 level; No segregation defects were found, but multiple small cracks were found, with a length of about 0.5-1.5mm. The crack length direction is perpendicular to the axial direction of the circular forging, similar to the characteristics of white spot defects, as shown in Figure 3-4.

Figure.1 Non-destructive testing location of internal defects found in forgings

Figure.2 Protruding Particle

Figure.3 Macroscopic Morphology of Axial Profile of Ring Forgings

Figure.4 White Spot Crack Morphology on the Profile
Table.1 Chemical Composition of Large Ring Forgings (Mass Fraction, %)

Figure.5 Particle Profile Morphology

Figure.6 Morphology of Small Cracks on the Fracture Surface

Figure.7 Morphology of silver-white spots on the fracture surface

1.2 Chemical composition analysis

Samples were taken from large circular ring forgings and analyzed for chemical composition using a direct reading spectrometer. The results are shown in Table 1. It can be seen that the chemical composition of the large circular ring forging meets the requirements of the chemical composition specification for 20# steel in the GB/T699-20# 15 “High quality carbon structural steel” standard.

Figure.8 Morphology of Silver White Spotted Defects on the Cross Section

Figure.9 The microscopic fracture characteristics at the normal fracture surface are cleavage + dimples

1.3 Metallographic analysis of protruding particles

Dissecting the protruding particles in Figure 2 along the axial direction of the circular forging, an expanding crack was found at the particles, which had already expanded towards the sawn surface, causing the edges to protrude and forming a circular protruding particle; The metallographic structure around the particle is the same, both consisting of ferrite and pearlite, with a small amount of Weinstein structure in morphology, as shown in Figure 5. X-ray energy spectrometer analysis found no large inclusions or slag inclusions, such as furnace slag and protective slag, in these cracks.

1.4 Fracture inspection

Take 2 test pieces from the defect area of the circular ring forging, and perform fracture testing after grooving and quenching.
The fracture morphology of the axial surface of the circular ring forging is shown in Figure 6. Several small cracks were found in the fracture, and the crack length direction is perpendicular to the axial direction of the circular ring forging.
The fracture morphology of the transverse surface of the circular ring forging is shown in Figure 7. Silver white spots can be observed on its cross-section, which is elliptical and has a concave feature.
Through scanning electron microscopy analysis, the silver white spot-like defects on the cross-section are relatively flat small elliptical planes. The surface morphology of the silver white spot is characterized by unevenness, no edges or corners, and the convex parts are mostly curved surfaces, presenting a floating cloud like feature with visible ripple-like folds; There are strip-shaped depressions and small pores; No fracture features such as intergranular, cleavage, and dimples were found (see Figure 8). The microfracture characteristics at the normal fracture surface are cleavage + dimples, as shown in Figure 9.

2. Analysis and Discussion

• (1) The chemical composition of large circular ring forgings meets the requirements of the chemical composition specification for 20# steel in the GB/T699-20#15 “High quality carbon structural steel” standard; The hydrogen content in steel is relatively low. The matrix structure of the forging is blocked ferrite + a small amount of pearlite, which is the normal structure after the forging of 20# steel.
• (2) The ultrasonic inspection defects of circular ring forgings appear as protruding particles that bulge outward on the sawing surface. These protruding particles are circular and protrude outward, with sawing marks on the surface, and the color is metallic, consistent with other areas on the sawing surface. They are irregularly distributed on the sawing surface of the forging, indicating that the particles only protrude after sawing. After metallographic dissection, the cross-sectional morphology of the particle is a small crack that expands, causing its edge to protrude, forming a circular protruding particle. The above characteristics indicate that these small cracks exist in the forging, and gas with a certain pressure is stored in the cracks. When these cracks are located near the sawn end face, the metal layer on the near end face side of the crack is thinner and has lower strength. At this time, the gas pressure inside the crack causes the crack to expand, forming protruding particles similar to bubbles. Through X-ray energy spectrometer analysis, no large inclusions or slag inclusions such as slag and protective slag were found in these cracks, indicating that the generation of protruding particles resembling bubbles is not related to inclusions, slag, protective slag, and other slag inclusions, but rather to the gas in the steel.
• (3) The defects of circular ring forgings appear as small cracks in the low magnification inspection profile and the axial plane of fracture inspection, all of which have the characteristics of white spot cracks; Moreover, the length direction of the cracks is perpendicular to the axial direction of the forging (consistent with the transverse direction of the ring), indicating that the cracks have directionality. The length direction of the cracks is perpendicular to the direction of the forging under pressure, indicating that the generation of white spot cracks is related to forging.
• (4) In the transverse fracture of circular ring forgings, the defects are silver white spots, elliptical in shape, with concave features, and relatively flat elliptical planes. The micromorphology of silver white spots did not observe fracture characteristics such as intergranular, cleavage, and dimples; Instead, it presents floating cloud like and ripple like folds, one of the microscopic characteristics of white spot defects.
• Due to the presence of silver white spots on the fracture surface of the defects in the circular ring forging, the microstructure exhibits the characteristic of white spot defects; Small cracks appear in the low magnification and metallographic inspection sections and axial fracture inspection sections; These features all conform to the characteristics of white spot defects, and it can be considered that the defects found in the non-destructive testing of 20# steel large circular ring forgings are white spot defects.
• (5) White spots are caused by the combined action of excessive hydrogen content and internal stress in steel, which is mainly related to two factors: the hydrogen content in the steel and the internal stress generated during the cooling process after hot pressure processing; Moreover, the high hydrogen content is only a necessary condition for the formation of white spots, while the internal stress generated during the cooling process of steel after hot pressure processing is a sufficient condition for the formation of white spots. 20# steel belongs to ferrite steel and is generally difficult to form white spots. Moreover, the hydrogen content of the circular forging is already very low, indicating that the reason for the formation of white spots is not necessarily related to the hydrogen content of the raw material. The forging processing technology (such as fast cooling rate after forging) plays a main role in forming white spots.

3. Conclusion

The defect found in the non-destructive testing of the 20# steel large circular ring forging is a white spot defect. The cause of the white spot defect is not necessarily related to the hydrogen content of the raw material, and the forging process plays a major role in the generation of white spots.
Authors: Du Jiamei