Determination of white spots in forging steel and measures to eliminate them

Forgings in the “white spot” is a common product of metallurgy and forging, the defect is in the smelting of materials bred in the forgings found, “white spot” is commonly distributed in the form of a group in the more central part of the forgings, the forgings are very harmful, how accurate in the forgings How to accurately determine the “white spots” and take appropriate post-forging heat treatment process to eliminate this defect, is an important quality assurance measures in the production of forgings for forging enterprises.

“White spot” is a common dense defect in forgings, how to use ultrasonic flaw detection as a non-destructive testing method to correctly determine the “white spot” of forgings is the starting point of our technical exploration, through the analysis of the waveform characteristics of the defects found by ultrasonic flaw detection, combined with the forging defect at the metallographic observation. Forging defects at the combination of metallographic observation and interruption method to confirm the nature of the defect, the use of ultrasonic waveform characteristics of defects on forgings “white spots” of accurate determination, while the forgings have produced “white spots” to take certain process measures to eliminate the “white spots”. “White spots”.
In the production of 5CrNiMo material die billet forgings, according to GB/T11880-89 “die forging hammer and large mechanical forging press with module technical conditions” for ultrasonic detection of its pass rate is very low, the phenomenon is mainly reflected in the longitudinal center of the die billet part of a large area of intensive defects, in order to clarify the nature of defects and find a solution, our technical staff made a more Comprehensive exploration, we first designed the problem solving ideas as follows:

• ① Use ultrasonic flaw detector on the forging defects for preliminary positioning.
• ② In the defective parts perpendicular to the longitudinal die billet slicing samples.
• ③ Magnetic flaw detection on the sampling section of the ultrasonic flaw detection of defects found in the image positioning.
• ④ The magnetic flaw image positioning of the defects with wire cutting to take metallographic observation. specimens and interrupted mouth observation.
• ⑤ Qualitative analysis of defects with metallographic observation.
• ⑥ Propose process solutions on the basis of qualitative analysis.
• ⑦ The implementation of process testing with ultrasonic flaw detection failed die blanks.
• ⑧ Reinspection of the mold billet with ultrasonic flaw after treatment with new technology.

The following is the actual operation process of this process exploration of our company:
1. Use ultrasonic flaw detector for preliminary positioning of forging defects.

• Forging size: 2000 × 400 × 185
• Forging material: 5CrNiMo
• Ingot type:2.5T quadrilateral ingot down (450×450×750)
• Forging process: heating temperature 1180℃, starting forging temperature 1150℃, natural slow cooling by two fires, three upsets and three draws.
• After forging annealing: 880 ℃ insulation 6-8 hours furnace cooling to 500 ℃ or less out of the furnace.

The ultrasonic flaw detection of the forgings is as follows:

Figure 1 red area for the forging ultrasonic flaw detection found dense defect areas, depth direction distribution of about 30% of the thickness of the forging billet.
Figure 2 shows the wave pattern of dense defects under 24dB attenuation scan.
If the large flat bottom to adjust the sensitivity of the Φ2 flat bottom hole, measured the module 185mm at the sound range of the large flat bottom echo decibel value of 60dB (wave amplitude high set to 80%); the same sound range Φ2 flat bottom hole and the large flat bottom decibel difference is Δ = 20lg (PB/Pf) = 20lg (2λX/πD2f) (X ≥ 3N)
X-workpiece thickness Df-minimum flat bottom hole size required to be probed λ=C/f
λ-wavelength mmC-wave speed m/sf-frequency MZ
2.5P20-D probe frequency is 2.5Mz then λ=5.9/2.5=2.36
Calculated Δ=36.8dB, the scanning attenuation should be 60-36.8=23.2dB
Calculation of the maximum equivalent value of dense defects:
The decibel value is 36.5dB measured at about 80% of the wave amplitude of the maximum defect wave.
Df = (2λX2f/10ΔBf/20πXB) to calculate the maximum defect equivalent Φ1.75 or so.
2. Slice sampling on the forging billet

Figure 3 shows the defective forging billet for slicing and sampling, the thickness of the two slices are 16mm and 32mm respectively, the slices are directly observed by the naked eye after flat grinding without any defects (the situation after slicing and grinding is shown in Figure 4).

3. Using magnetic flaw detection on the sampling section ultrasonic flaw found defects in the image positioning
As the slices by the flat grinding directly with the naked eye can not find any defects, so the subsequent magnetic flaw detection fluorescence positioning measures to observe and determine the location of defects

Figure 5 shows the magnetic particle inspection of the slices, and the red circle in Figure 6 shows the internal defects of the forgings under fluorescence.

Figure 7, Figure 8 is the magnetic flaw detection of 16mm and 32mm two slices, respectively, with a marker to mark the distribution of dense defects inside the forging, while the defects for the magnetic flaw detection image positioning with a line cut for metallographic observation specimens and interrupted the mouth to observe the specimens, we again on the two slices for vertical division, take the 1, 2 test blocks for magnetic flaw detection defect imaging positioning and then cut and interrupted the metallographic specimens. The mouth to observe.

4. The defective parts for fracture test
In the above two specimens made after the fracture sensitive notch, the interruption of its test, the fracture of the fracture profile is shown below:

From the above two fracture specimens can be clearly seen in the section of ultrasonic and magnetic detection of the forging internal dense defects are “white spots”.
5. Qualitative analysis of defects with metallographic observation
The defects found in the magnetic flaw detection to take a 15 × 15 metallographic specimens, specimens ground by 4% nitric acid alcohol corrosion in 100 times, 500 times, 1000 times under the optical microscope to observe the metallographic situation.

From the above metallographic pictures can be seen, ultrasonic flaw detection found in the forging internal dense defects is actually an intergranular crack.
From the relevant information, the formation of “white spots” in the forgings is mainly due to the excessive amount of hydrogen in the raw material (generally require the H content of steel ≤ 2.1ppm), hydrogen atoms at room temperature in the form of hydrogen molecules gathered at the grain boundaries, due to the growth of its volume and “broken “The intergranular crack is formed at the grain boundary due to the growth of its volume, and our test completely verifies the above statement. According to this feature of “white spot” formation, we reforged the forgings with “white spot” and developed a special post-forging heat treatment process to remove the hydrogen content in the raw material. The original “white spots” in the forgings were completely eliminated.

Conclusion

Forgings in the ultrasonic flaw detection presents a variety of dense defects, “white spots” is one of the basic forms of dense defects within the forging, from our exploration found that by calculating the equivalent value of a single “white spot” is not large, but from the fracture can be The actual area of “white spots” is much larger than the calculated equivalent value, which indicates that the existence of “white spots” in forgings is very harmful, but from the other side of this exploration, “white spots “However, from the other side of this exploration, “white spots” in forgings can be eliminated due to non-oxidizing grain boundary cracks, by reforging the resulting intergranular cracks after “welding” and a special exhaust hydrogen removal process.

Author: Fu Zongzhu

Source: China Forgings Manufacturer – Yaang Pipe Industry (www.epowermetals.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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