The austenitic stainless steel pipe fittings have
not been subjected to solid solution heat treatment or strictly according to the solid solution process before leaving the factory, and can not be visually inspected during field acceptance. The article briefly introduces the principle of solid solution, elaborates the rapid hardness test method of austenitic stainless steel pipe fittings using a Leeb hardness tester, and gives the criteria for determining whether stainless steel pipe fittings have undergone qualified solid solution heat treatment. The engineering application of this method shows that the effect is good.
According to the relevant national standards, stainless steel pipe fittings must be subjected to solid solution heat treatment before leaving the factory to refine the grain structure and increase the corrosion resistance and low temperature resistance of the pipe fittings. However, some manufacturers have sought to reduce profits and reduce costs before stainless steel pipe fittings leave the factory. Leave the factory without solid solution treatment or strictly follow the process requirements. After the unqualified stainless steel pipe is applied in a petrochemical project, it usually cracks at low temperature, and vicious production accidents such as oil leakage and oil running will cause oil and gas processing equipment, processing plants or gathering stations to stop production and maintenance, causing serious safety Hidden dangers and huge economic losses.
Due to the normal production of pipe fittings, pickling and passivation are required after solution treatment, so the stainless steel pipes that have undergone solid solution treatment and non-solid solution treatment are basically the same in appearance. The traditional stainless steel pipe inspection method is metallographic inspection, but at the construction site, metallographic inspection is costly and time consuming, which is not conducive to full implementation on site. The author has studied the metallographic structure and manufacturing process of stainless steel pipe fittings, and finally chose In order to identify the hardness, it not only guarantees the quality, but also effectively reduces the testing cost.
According to the provisions of SH / T 3408-2012 and GB / T 12459-2005, the hardness requirements for austenitic non-inductive steel pipe factory inspection are: Brinell hardness HB≤190 is acceptable, but because the Brinell hardness tester is relatively large, The operation is complicated and the field application is difficult. In general, the hardness test on the site uses a Leeb hardness tester (corresponding to the Leeb hardness HL). We applied the HLN-200 Leeb hardness tester in the field. Refer to Appendix B in GB / T17394-1998. The Brinell hardness 190 is equivalent to the Leeb hardness H LC523 when using C-type impact. When vertical downward inspection is used, HL = HLC, so the inspection hardness value HL≤523 is acceptable. .
1.3 Inspection analysis and conclusion
According to the principle of normal distribution, when the sample size is the same, the larger the variance, the larger the fluctuation of the data and the more unstable it is. Through the above calculations combined with drawing analysis, it is concluded that the variance of the data of qualified pipes is small, while the situation of unqualified pipes is the opposite.
With 95% confidence, the Leeb hardness range of unqualified fittings is 370-450, and the Leeb hardness range of qualified fittings is 340-380. The two sides overlap slightly, but considering that the normal distribution curve can only approximate the actual situation on site, extreme conditions will not occur. At the same time, considering the need to ensure quality, the critical value of the Richter hardness of qualified and unqualified is finally determined. 370, that is, those whose average hardness on the Richter scale exceeds 370 are unqualified fittings. If there is any objection, further testing can be carried out.
After determining the Leeb hardness qualification range, the stainless steel pipe used in the project is checked and accepted according to Leeb hardness, and the Leeb hardness test method is used for solid solution verification. After the project was put into use, there were no more vicious accidents that caused production stoppage and maintenance due to cracking of stainless steel pipe fittings at low temperature.
With the development of the market economy, counterfeit and inferior products have gradually increased, and the means of material inspection and inspection must keep pace with the times, and constantly explore innovation and innovation in production practices. On the one hand, we strengthened the inspection and acceptance process and tried our best to prevent unqualified materials from flowing into the construction site; on the other hand, we established a self-restraint mechanism and established a blacklist of unqualified material manufacturers. It is strictly forbidden to re-supply, so that there is no market for unqualified materials. Guarantee project quality.
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