Cause analysis of black belt defect of 316L forged flange
In the article, the problem of “black band” with strong magnetism and easy rusting on the surface of 316L forged flange was found by using macroscopic inspection, chemical analysis, metallographic analysis and scanning electron microscope analysis, etc. to find the cause of strong local magnetism and easy rusting of 316L forged flange. The black belt part and the parent material boundary is clear, and the metallographic organization of the black belt part is ferrite, Fe element is more than 90%, judged to be the involvement of foreign metal. Through the analysis of the actual production conditions, 316L forged flange “strip-like black belt” is in the steel casting process involved in the formation of foreign metal. Reasonable control of the influence of operational factors in the casting process, can completely avoid the production of similar defects.
316L standard grade 022Cr17Ni12Mo2, with good plasticity, toughness, weldability and corrosion resistance, corrosion resistance in oxidizing and reducing media are better, can be used to make acid-resistant equipment, such as corrosion-resistant containers and equipment lining, transport pipelines, nitric acid-resistant equipment parts, etc. [1,2,3]. A customer in the production of 316L forged flange individual forged flange local area of high magnetic and rust “black band” problem, this paper through the detection and analysis, to find the cause of the defect.
- (1) The 316L forged flange macroscopic inspection, mainly using the naked eye on the surface of the flange produced for inspection.
- (2) The 316L forged flange sample analysis, in the SPECTROLABM10 photoelectric direct reading spectrometer to detect its chemical composition, compared with the chemical composition obtained in the standard range of the steel grade.
- (3) Make standard metallographic samples and observe the microstructure under AxinImagerAim type metallographic microscope. Specific experimental methods refer to GB/T12938-1991 “metal microstructure test method”.
- (4) Scanning electron microscope analysis was performed on the 316L flange sample, and the difference in composition between the normal part and the magnetic part was compared according to the energy spectrum image obtained.
The customer’s production plant visit, its flange products as shown in Figure 1, 316L forged flange appeared on the strip of black lines. The dark part of the circle is a strong magnetic part, the maximum width of the black line is about 3.5mm, length 35-40mm.
Chemical composition analysis
According to the customer’s 316L forged flange sampling test, its chemical composition as shown in Table 1, fully in line with the standard requirements.
Figure 2 is the normal part of the 316L forged flange and the defective part of the metallurgical morphology comparison. Can be seen: strong magnetic defective parts and normal parts have obvious boundaries; strong magnetic defective parts darker color, and more sulfide inclusions, metallographic organization for the ferrite organization; normal parts lighter color, no inclusions gathered phenomenon, metallographic organization for the austenite organization.
Figure.1 Sample macroscopic morphology
Table.1 316L forged flange chemical composition (mass fraction, %)
Figure.2 Defects in the metallographic morphology
Figure 3 is the normal part of the 316L forged flange austenite organization, grain size of 5.0 grade, no mixed crystal phenomenon. Figure 4 shows the ferrite organization of the defective part with strong magnetism, and the grain size of ferrite is 8.0. The grain size of the defective area is obviously smaller than that of the normal area.
Scanning electron microscope analysis
The magnetic and non-magnetic areas on the 316L flange were scanned and analyzed by SEM to compare the differences in chemical composition, as shown in Fig. 5 and Fig. 6.
The results of the scanning electron microscopy can be seen by comparing the energy spectrum of the defective area with the normal area: the defective area is mainly a heterogeneous metal with Fe mass fraction over 90%, and the content of base elements such as Cr and Ni is very low. The mass fraction of Fe in the normal part was about 69%, and the rest of the elements did not differ much from the tested chemical composition.
Figure.3 Tissue morphology of normal parts
Figure.5 Magnetically strong defective parts of the scanning electron microscope image
Table.2 316L forged flange magnetic strong defects at the energy spectrum composition (mass fraction, %)
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Table.3 316L forged flange normal parts of the energy spectrum composition (mass fraction, %)
Discussion and analysis
According to the analysis of 316L forged flange sampling, the flange local magnetic strong and easy to rust defects is the part of the Fe mass fraction of more than 90% of the foreign metal, metallographic organization for the ferrite, and the part and the parent material boundary is clear, the preliminary analysis is in the steel solidification process involved in the formation of foreign metal.
After communication with the customer, a large number of 316L forged flange site inspection, showing local magnetic defects in the overall proportion of 0.25% of the flange, combined with the plant’s production process and past production experience, to determine the plant’s production process itself does not lead to the possibility of defects. The preliminary judgment is that the cause of this quality accident is that the site operator did not clean the ingot mold carefully before pouring, resulting in the Fe foreign metal involved in the steel.
For customers in the production of 316L forged flange individual forged flange local area with high magnetic and rust “strip of black belt” problem, the use of macroscopic inspection, chemical analysis, metallographic analysis and scanning electron microscopy analysis and other detection means of 316L forged flange defect samples for testing, found that the black belt parts that are magnetic parts The maximum width of the black band is about 3.5mm and the length is 35~40mm, and the composition analysis of the flange sample shows that the composition is controlled within the range required by the national standard. According to the metallurgical analysis, the defective organization with strong local magnetism is ferrite organization with grain size 8.0, while the normal part of the flange sample is austenite organization with grain size 5.0. Comparing the results of the energy spectrum of the defective and normal parts of the sample, the defective parts are mainly heterometallic with Fe mass fraction over 90%, while the Fe mass fraction of the normal parts of the sample is about 69%. The analysis shows that the cause of this black band defect is the involvement of foreign metal in the casting process. It was judged that the quality accident occurred because the site operator did not carefully clean the ingot mold before casting, which resulted in the involvement of Fe containing foreign metal in the steel. Therefore, the influence of operational factors in the casting process can be reasonably controlled to avoid similar defects.
Figure.6 Scanning electron microscope image of normal part
Author: Wang Sentinel
Source: China Forged 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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