According to the relevant national standards and regulations, stainless steel pipe fittings must be subjected to solid melt 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, in order to pursue profits and reduce costs, some manufacturers directly leave the factory without solid melting before the stainless steel pipe fittings leave the factory or without solid melting treatment strictly in accordance with the process requirements.After unqualified stainless steel pipe fittings are used in petrochemical projects, they usually crack at low temperatures, and vicious production accidents such as oil spills and oil runs occur, resulting in the shutdown of oil and gas processing plants, treatment plants or collection and transportation stations for maintenance, causing serious safety risks and huge economic losses.

 Since the normally produced pipe fittings still need to be pickled and passivated after the solution treatment, the stainless steel pipe fittings that have been solid-melt treated and the stainless steel pipe fittings that have not been solid-melt treated are basically the same in appearance.The traditional inspection method of stainless steel pipe fittings is metallographic inspection, but at the construction site, the high cost and long time of metallographic inspection are not conducive to the full implementation on the site. The author has studied the metallographic organization and manufacturing process of stainless steel pipe fittings, and finally chose to use hardness testing for identification, which not only ensures the quality, but also effectively reduces the cost of testing.

 1. Parameter determination

According to the provisions of SH/T 3408-2012 and GB/T 12459-2005, the hardness requirements for the factory inspection of austenitic stainless steel pipe fittings are: Brinell hardness HB≤190 is qualified, but due to the large size of the Brinell hardness tester, the operation is complex, and the on-site application is difficult. Under normal circumstances, the on-site hardness tester is used for on-site hardness testing (corresponding to the Richter hardness HL).What we use in the field is HLN-200 Richter hardness tester.Refer to Appendix B in GB/T17394-1998. The Brinell hardness of 190 is equivalent to the Richter hardness H LC523 when using C-type impact. When using vertical downward inspection, HL=HLC, so the test hardness value HL≤523 is qualified.

 1.1 Material inspection

 In the practical application process, due to the large types of austenitic stainless steel and different hardness values, the actual test values of stainless steel pipe fittings can easily meet the requirements.

 If it is accepted according to the hardness test value required by the standard, the hardness of the above-mentioned stainless steel pipe fittings is qualified, but in fact, during the on-site application process, this batch of stainless steel pipe fittings is generally cracked at low temperature, and after chemical composition analysis, its chemical composition is qualified.Therefore, the inspection is only carried out according to the hardness required by the standard, and the acceptance cannot fully meet the requirements of use.Subsequently, a batch of qualified stainless steel pipe fittings were selected for hardness testing. The results are shown in Table 2.,

 Comparing Table 2 with Table 1, it can be seen that the measurement dispersion of qualified pipe fittings is much smaller than that of unqualified pipe fittings, indicating that its uniformity is better.At the same time, in order to better compare, we conducted spot checks and measurements on some stainless steel pipes from different manufacturers that have been qualified for use. The specific values are shown in Table 3.

 As can be seen from Table 3, the dispersion of individual data is too large.Analyze the reason, because the length of the pipeline is large, each pipeline is considered according to the uniform distribution of tissue and alloy elements during spot inspections, so the individual data is too discrete, but its hardness value is still close to that of the solid-melt-treated pipe fittings, much smaller than that of the non-solid-melt pipe fittings.

 1.2 Inspection data modeling

After the material inspection is completed, the statistical data are collated, the centralized or discrete status of the data distribution is judged, and the normal distribution is modeled.

 1.2.1 Data analysis of unqualified pipe fittings

 (1) After calculation, the average value of the Richter hardness test is 411.9.

 (2) After calculation, the variance of the inspection data of unqualified pipe fittings is 18.65.

1.2.2 Data analysis of qualified pipe fittings

(1) After calculation, the average value of the Richter hardness test is 359.9.

(2) After calculation, the variance of the risk test data of qualified pipe fittings is 6.57.

1.3 Inspection analysis and conclusion

According to the principle of normal distribution, in the case of the same sample size, the greater the variance, the greater the fluctuation of the data and the more unstable it is.Through the above calculations and combined with drawing analysis, it is concluded that the data variance of qualified pipe fittings is small, while the situation of unqualified pipe fittings is the opposite.

 With 95% confidence, the Richter hardness range of unqualified pipe fittings is 370-450, and the Richter hardness range of qualified pipe fittings is 340-380.The two sides overlap slightly, but considering that in fact the normal distribution curve can only approximate the actual situation on the site, extreme situations will not occur, and taking into account the need to ensure quality, it is finally determined that the threshold value of qualified and unqualified Richter hardness is 370, that is, the average value of Richter hardness exceeds 370 for unqualified pipe fittings, if there is any objection, further testing can be carried out.

 2. Application effect

 After determining the qualified range of Richter hardness, the stainless steel pipe fittings used in the project are accepted according to the Richter hardness, and the Richter hardness testing method is used for solid melt verification.After the project was put into operation, there were no more vicious accidents caused by the cracking of stainless steel pipe fittings at low temperature, which led to the shutdown of production and maintenance.

 3. Conclusion

 With the development of the market economy, counterfeit and shoddy products have gradually increased. Material testing and inspection methods must keep pace with the times, and constantly explore and innovate and innovate in production practice.On the one hand, strengthen the inspection and acceptance link to make every effort to prevent the flow of unqualified materials into the construction site; on the other hand, establish a self-restraint mechanism, establish a blacklist of manufacturers of unqualified materials, and strictly prohibit their re-feeding, so that unqualified materials have no sales market, and fundamentally ensure the quality of the project.