Quick Verification for Solution Treatment of Stainless Steel Pipe Fittings
Posted: 03/22/2021 10:55:44 Hits: 16
For austenitic stainless steel pipe fittings that have not been subjected to solution heat treatment or it is not in accordance with the solution process before leaving the factory, the inspector can't know whether they are well treated based on the appearance on site. The principle of solution treatment and austenitic steel pipe fittings being quickly tested by the Leeb hardness tester are explained in this article, and the criterion for judging whether the stainless steel pipe fittings have undergone qualified solution heat treatment is given. The engineering application of this method shows that the effect is good.According to the relevant national standards and regulations, stainless steel pipe fittings must be subjected to solution heat treatment to refine the grain structure and increase the corrosion resistance and low temperature resistance of the pipe fittings before leaving the factory. However, stainless steel pipe fittings are not solution heat treated or the process is not in strict accordance with the requirements before they leave the factory, because some manufacturers pursue profit and low costs. After unqualified stainless steel pipe fittings are used in petrochemical projects, they usually crack at low temperatures, causing pernicious production accidents like oil spills as well as suspended production and maintenance of oil and gas processing equipment, processing plants or gathering stations, serious safety hidden dangers and great economic losses.
Usually, the pipe fittings need to be subjected to the solution treatment and then pickling and passivating, but the stainless steel pipe fittings with solution treatment and those without solution treatment basically have the same appearance. The traditional inspection method of stainless steel pipe fittings is metallographic inspection. However, the high cost and long time of metallographic inspection at the construction site are not conducive to full implementation on site. After studying the metallographic organization and manufacturing process of stainless steel pipe fittings, the method of hardness testing is adopted for identification at the end, which not only ensures the quality, but also effectively reduces the testing cost.
1. Determining the parameters
According to the provisions of SH/T 3408-2012 and GB/T 12459-2005, the hardness requirements for austenitic stainless steel pipe fittings for delivery inspection are as follows: Brinell hardness HB should be lower than or equal to 190. However, the Brinell hardness tester is large and can't be operated easily. It's difficult to use it on site. Generally, the Leeb hardness tester is used for hardness testing on site (corresponding to the Leeb hardness HL). What we use on site is the HLN-200 Leeb hardness tester. According to appendix B in GB/T17394-1998, Brinell hardness 190 is equivalent to Leeb hardness H LC523 when the C-type impact is used. When vertical downward inspection is used, the HL is equal to the HLC. Therefore, the inspection hardness value HL should be less than or equal to 523.
1.1 Material inspection
In the actual application process, because there are many types of austenitic stainless steels and their hardness values are different, the actual test values of stainless steel pipe fittings can easily meet the requirements.
The hardness of the above-mentioned stainless steel pipe fittings are qualified, if they are inspected according the hardness test value required by the standard. In fact, in the field application process, these stainless steel pipe fittings generally cracked at low temperature, but the chemical composition analysis showed that their chemical composition was qualified. Therefore, it could be seen that the hardness required by the standard couldn't fully meet the requirements of use. Later, a batch of qualified stainless steel pipe fittings were selected for hardness testing.
After making a comparison, it could be seen that the measurement dispersion of qualified pipe fittings was much smaller than that of unqualified pipe fittings, indicating that its uniformity was better. At the same time, we conducted spot checks on some qualified stainless steel pipes from different manufacturers for better comparison. The dispersion of individual data was too great. The pipeline had long length; each pipeline was considered according to the uniform distribution of the organization and alloy elements during the random inspection, resulting in the individual data having a great dispersion. However, the hardness value was still relatively close to that of pipe fittings with solution treatment, which was much lower than pipe fittings without solution treatment.
1.2 Test data modeling
After the material inspection was completed, the statistical data was sorted; the concentration or dispersion of the data distribution was judged, and the model was modeled according to the normal distribution.
1.2.1 Data analyses of unqualified pipe fittings
(1) After calculation, the average value by the Leeb hardness test was 411.9.
(2) After calculation, the variance of inspection data of the unqualified pipe fitting was 18.65.
1.2.2 Data analyses of qualified pipe fittings
(1) After calculation, the average value of the Leeb hardness test was 359.9.
(2) After calculation, the variance of inspection data of the qualified pipe fitting was 6.57.
1.3 Inspection analyses and conclusionAccording to the principle of normal distribution, when the sample size was the same, the greater the variance was, the greater the fluctuation of the data became, and it was more unstable. Through the above calculation combined with the drawing analysis, it was concluded that the data variance of qualified pipe fittings was small, while the situation of unqualified pipe fittings was the opposite.
With 95% confidence, the Leeb hardness range of unqualified pipe fittings was from 370 to 450, and the Leeb hardness range of qualified pipe fittings was from 340 to 380. There was a slight overlap between the two, but considering that the normal distribution curve could only approximate the actual situation on site and extreme cases would not occur. The critical value of Leeb hardness for qualified and unqualified was finally determined to be 370 while the quality was ensured, that is, if the average value of Leeb hardness exceeded 370, the pipefitting was unqualified. If there was any objection, further testing could be carried out.
2. Application effects
After determining the qualified range of Leeb hardness, the stainless steel pipe fittings used in the project were checked and accepted according to the Leeb hardness, and the Leeb hardness test method was used for verifying solution treatment. After the project was put into operation, there were no more vicious accidents that caused the shutdown and maintenance due to the cracking of stainless steel pipe fittings at low temperature.
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