Analysis of Flaw Detection Defects for Large Thick Wall Ring Forgings Used in Water and Electricity
After rough machining of two pieces A668 steel thick walled ring forgings for 63t hydropower, UT inspection was conducted, and it was found that the defects exceeded the standard. The location and distribution of the defects were the same, forming a circular shape. To analyze the cause, a forging was dissected and sampled for macroscopic, compositional, metallographic, and inclusion inspection and analysis. It was found that the main reason for the failure of UT inspection was porosity and incomplete forging during the forging process. Based on the inspection results, develop a return forging plan for another forging, and the flaw detection results after forging meet the customer’s requirements.
Two large thick walled ring forgings forged from 63t A668 steel ingots were subjected to smelting forging heat treatment, rough machining, and UT testing. End face inspection, 300mm from the outer circle, 90-750mm deep, 400mm wide circular area, dense defects, equivalent Φ3-Φ6. If it exceeds the standard, it will be judged as unqualified. Both forgings failed UT inspection, and the defect location and distribution were the same, forming a circular shape. So, one forging was selected from two forgings for anatomical sampling and defect analysis.
2. Experimental analysis
Both forgings failed UT inspection, and the defect location and distribution are shown in Figure 1.
2.1 Anatomical sampling
According to the flaw detection report, mark and saw. After sawing the section, the flaw detection personnel conducted another flaw detection to confirm the location of the defect and take the test block for the flaw detection. Take horizontal and vertical low-magnification test blocks from the defect test block.
Figure.1 Schematic Diagram of Flaw Detection Defects
2.2 Test results
Macroscopic examination results: The overall organization is not dense, with large and tightly arranged loose points forming a honeycomb shape, and the rating is generally loose at level 4.0.
Take samples from the horizontal low magnification test block to analyze composition, gas, metallography, and inclusions. The composition and gas content are shown in Table 1.
Table.1 Chemical Composition Inspection Data (Mass Fraction) %
|Finished Product Composition||0.28||0.68||1.11||0.008||0.001||0.15||0.02||0.19||0.03||0.015||0.06|
Inspection results: The components in the defect-dense area are inspected, and the C content exceeds the upper limit; Hydrogen content in finished products (0.91-1.32) ×10-6, slightly different hydrogen elements at different positions; Oxygen and nitrogen are relatively stable and have good repeatability.
Whether H can cause defects is mainly considered from the following aspects: whether the composition of the steel grade is sensitive to H, which is the understanding and understanding of the material. A668 is C-Mn steel, and it can be judged from its composition that it is not sensitive to H; For the prevention of white spots, it is generally solved through hydrogen expansion heat treatment, and once white spots occur, there is no way to remedy them.
The hydrogen pressure theory was proposed by Bennek et al. and further improved by Zapffe. The hydrogen pressure theory suggests that high-strength hydrogen pressure in the internal micropores is an important reason for hydrogen embrittlement damage in steel. The solubility of hydrogen decreases with temperature, and during the cooling process, hydrogen atoms are precipitated at weak grain boundaries. After reaching a certain amount, they combine to form hydrogen molecules, forming huge internal stress due to volume expansion. The continuously increasing internal stress leads to the initiation of cracks called white spots.
High magnification inspection
The metallographic examination results are shown in Figure 2. Inspection results: The metallographic structure is pearlite + ferrite, with 3 defects and an abnormal structure. An obvious needle-like ferrite was found, suspected to be a Widmann structure.
Figure.2 Metallographic Structure 200 ×
There are two types of Weinsteinite structure: coarse austenite grains caused by overheating, which grow from the coarse austenite grain boundaries to the feathery ferrite inside the grains. This type of structure is very harmful; The other type is needle-like ferrite that grows freely from the austenite grain boundaries and within the austenite grains. The austenite grains in this structure are not coarse and have little impact on the material’s properties. The influence of Weinsteinite on material properties is mainly due to the coarse austenite grains, which severely cut the matrix and significantly reduce the strength and impact toughness of the steel.
The results of the inclusion inspection are shown in Figure 3. There are D-type and DS-type inclusions, multiple D-type and grade 1, and DS-type and grade 1.5 with more severe inclusions.
Figure.3 Inclusion Inspection Results
The composition of the refining slag system for smelting is shown in Table 2, and the slag system belongs to the high alkalinity slag system.
Table.2 Refining slag system (mass fraction) %
The high alkalinity slag system is beneficial for removing Class B inclusions, but the removal effect for Class D and DS could be better. The high alkalinity slag system has high CaO activity, and a small amount of C reduction reaction of CaO in the slag will occur during the VD process:
(CaO) + [C] = [Ca] + [CO];
The reduced Ca combines Al and O in the molten steel to form a spherical oxide mCaO·Al2O3. If Ca combines with steel grade S, it will generate CaS with poor deformation ability. Therefore, to solve the problem of spherical inclusions, reducing the Ca content of the steel grade and reducing the activity of CaO in the slag is necessary. The most effective method to reduce CaO activity is to increase the content of SiO2 in the slag. According to the CaO-SiO2 binary phase diagram, when C/S>3, there will be free CaO present, preferably controlled between 2-3. So, it is necessary to select slag systems with different alkalinity based on the technical requirements of the steel grade being smelted.
The core rod is formed by drawing, upsetting, and expanding, paying attention to the insulation time and the deformation amount per heat. Strictly control the reduction and feed rate during the core rod elongation process to ensure compaction and forging penetration. The deformation diagram is shown in Figure 4.
Figure.4 Elongation deformation diagram
During the upsetting process, to prevent deformation and folding of the inner hole, a local upsetting deformation method is selected, as shown in Figure 5. Pay attention to the deformation and changes in the inner hole, and promptly flip the surface if there is an inverted trapezoid; Adjust the insulation temperature and time for the last heat according to the size of the deformation to avoid coarse grains. By using high-temperature diffusion and large reduction forging methods, defects are eliminated.
Figure.5 Schematic diagram of local upsetting cloth anvil method
Through anatomical sampling of forgings and experimental analysis, it was found that the main reason for UT flaw detection failure was a looseness and incomplete forging during the process; by analyzing the experimental results, such as segregation of large steel ingots, hydrogen-induced defects, understanding of Weinsteinite structure, and discussing the influence of slag system on inclusions. Another piece has been developed with a return forging plan, which involves returning to the furnace for re-forging. After forging, the flaw detection meets the customer’s requirements and has been successfully delivered.
Author: Wen Peijian