Determination and elimination of white spots in forging steel

The “white spot” in forging is a common product of metallurgy and forging. This defect is found in the forging produced in the smelting of materials. The white spot is distributed in the center of forging in group form, which is very harmful to forging. How to accurately determine “white point” in forging and take appropriate post forging heat treatment process to eliminate this defect is the forging enterprise’s production in forging Important quality assurance measures in production.

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White spot is a common dense defect in forgings. How to correctly determine the “white spot” of forgings by ultrasonic flaw detection is the starting point of our technical exploration. Through the analysis of the wave pattern characteristics of defects found in ultrasonic testing, combining with metallographic observation and breaking method of forging defects, the paper uses the wave type characteristics of ultrasonic defects The accurate judgment of “white point” of forging, and at the same time, certain technological measures shall be taken to eliminate the “white point” for forgings which have produced “white spot”.
Our company is a professional forging manufacturer, mainly engaged in the free forging of modules and structural parts. During the process of producing 5CrNiMo material, ultrasonic testing is carried out according to GB/T11880-89 technical conditions for die forging hammer and large mechanical forging press module. It is mainly reflected that large area dense defects appear in the longitudinal center of the die blank. In order to find out the defects, the qualified rate is very low. It is mainly reflected in the large area dense defects in the longitudinal center of the die blank The nature of the depression and the solution are explored comprehensively by our technical personnel. We first designed the solution as follows:

  • ① The defects of forgings are preliminarily located by ultrasonic detector;
  • ② Slice sampling of the blank in the defect position perpendicular to the longitudinal direction of the mold;
  • ③ Imaging and locating of defects found in ultrasonic examination on sampling sections by magnetic inspection;
  • ④ The observation of the metallographic specimen and the break hole by wire cutting for the defects of magnetic flaw detection imaging;
  • ⑤ Qualitative analysis of defects by metallographic observation;
  • ⑥ Based on qualitative analysis, the process solution measures are put forward;
  • ⑦ The process test of the unqualified blank by ultrasonic testing;
  • ⑧ The blank treated with new technology is re inspected by ultrasonic testing.

The following is a brief introduction to the practical operation process of our company’s process exploration

The flaw of forging is preliminarily located by ultrasonic detector

  • Forging size: 2000 × 400 × 185
  • Forging material: 5CrNiMo
  • Ingot type: 2.5t square ingot blanking (450 × 450 × 750)
  • Forging process: heating temperature 1180 ℃, initial forging temperature 1150 ℃, natural slow cooling by two fire, three upsetting and three drawing.
  • Annealing after forging: 880 ℃ for 6-8 hours, the furnace is cooled to less than 500 ℃.

The ultrasonic inspection of forgings is as follows:
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The red area in Figure 1 is the area of dense defects found in ultrasonic inspection of forgings, and the depth distribution accounts for about 30% of the thickness of forging billet.

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Figure 2 shows the dense defect wave pattern under 24DB attenuation scanning.
If the sensitivity of the flat bottom hole is determined by the adjustment of large flat bottom, the decibel value of the large flat bottom primary echo at 185mm of the module is 60dB (the amplitude height is set to 80%); the decibel difference between the flat bottom hole and the large flat bottom in the same sound range is Δ = 20lg (pb/pf) =20lg (2 λ x/ π D2f) (x ≥ 3n).
X-workpiece thickness DF – Minimum flat bottom hole dimension required to be detected λ = C / F
λ – wavelength MMC – wave velocity M / SF – frequency MZ
The frequency of 2.5p20-d probe is 2.5mz, λ = 5.9/2.5 = 2.36
After calculation Δ = 36.8db, the scanning attenuation should be 60-36.8 = 23.2db
Calculation of maximum equivalent value of dense defects:
The DB value of the maximum defect wave is 36.5db measured at 80% of the amplitude height.
Df= (2 λ x2f/10 Δ bf/20 π XB) the maximum defect equivalent is about Φ 1.75.

Sampling of slices on forging billet

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Fig. 3 shows the sampling of the defective forging billet. The thickness of the two sections is 16mm and 32mm respectively. After the slicing is flat grinding, no defects are found by naked eye (the grinding condition of the slice is shown in Figure 4).
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Imaging and locating of defects found in ultrasonic examination on sampling sections by magnetic inspection

Since the slice can not find any defects directly with naked eye after grinding, the measurement of fluorescence imaging location of magnetic flaw detection is adopted to observe and determine the defect location.
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Figure 5 is the magnetic particle inspection of the slice, and the inner defects of forgings in the red ring of Fig. 6 are shown under fluorescence.
Photo 7 and figure 8 are the distribution of dense defects in the forging marked with marker pen after the magnetic inspection of 16mm and 32mm sections respectively. Meanwhile, for the defects identified by magnetic flaw detection imaging, the metallographic observation samples and fracture observation samples are cut by wire cutting. We have divided the above two sections vertically again. Take No. 1 and No. 2 test blocks for imaging and positioning of defects for magnetic detection before making the positioning Metallographic sample cutting and observation of the cut-off.
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Fracture test on the defect part

After making sensitive fracture notch for the above two samples, the fracture surface is tested with the fracture surface as shown in the following figure:
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From the cross section of the above two fracture specimens, it can be seen clearly that the dense defects in the forging found by ultrasonic and magnetic prospecting are “white spots”.

Qualitative analysis of defects by metallographic observation

One 15 × 15 metallographic sample was taken at the defect found in magnetic inspection. After being abraded, the metallographic condition was observed under 100 times, 500 times and 1000 times optical microscope after being corroded by 4% nitric acid alcohol.
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From the above metallographic images, it can be seen that the dense defects in the forging found by ultrasonic inspection are actually an intergranular crack
According to the relevant data, the formation of “white spot” in forgings is mainly due to the excessive hydrogen content in raw materials (generally, the H content in steel is less than 2.1ppm), hydrogen atoms gather in the form of hydrogen molecules at the grain boundary at room temperature, and “break” the grain boundaries to form intergranular cracks due to the increase of its volume. This test by our company also fully verifies the above statements. At the same time, the metallographic view is used to analyze the above It is found that because the intergranular crack is formed by hydrogen molecules, there is no oxidation phenomenon at the crack. This crack should have the technological property of “welding” again under the external force. According to the characteristics of “white spot”, we re forge the forged parts which have produced “white spot”, and formulate special post forging heat treatment process to remove hydrogen content in raw materials The original “white spot” in the forging has been completely eliminated.

Conclusion

The density defects of forgings in ultrasonic testing are various. White spot is one of the basic forms of the internal dense defects of forgings. From our exploration, it is found that the equivalent value of single “white point” calculated is not large, but the actual area of “white point” is far larger than the calculated equivalent value from fracture surface. This indicates that “white spot” exists in forgings The harm of the white spot is very big, but from the other aspect of this exploration, the “white spot” can be eliminated by re forging after the intergranular crack “welding” and special hydrogen removal process is adopted.
Author: fuzongzhu

Source: Network Arrangement – China Pipe Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@pipelinedubai.com

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