Austenitic stainless steel pipe fittings are not factory or not in strict accordance with the solid fusion process for solid fusion heat treatment, site acceptance from the appearance of the inspection can not be carried out. The article briefly introduces the solidification principle, and describes the site using the Richter hardness tester on the austenitic non-inductive steel pipe parts for rapid hardness detection method, giving a judgment whether the stainless steel pipe parts for qualified solidification heat treatment criteria. The engineering application of the method shows that the effect is good.
According to the national standard Huai specification, stainless steel fittings must be solid-fusion heat treatment before leaving the factory, in order to refine the grain organization, increase the corrosion resistance and low-temperature resistance of the fittings, but some manufacturers to pursue profits, reduce costs, in stainless steel fittings before leaving the factory is not solid-fusion or not strictly according to the process requirements for solid-fusion treatment directly from the factory. Unqualified stainless steel pipe fittings in petrochemical projects after application, usually in the low temperature state cracking, oil leakage, oil running and other malignant production accidents, resulting in oil and gas processing units, processing plants or gathering stations shutdown maintenance, resulting in serious safety hazards and huge economic losses.
As the normal production of pipe fittings, after solid solution treatment also need to be pickled and passivated, so the solid fusion treatment and not solid fusion treatment of stainless steel pipe fittings in appearance is basically the same. Traditional stainless steel pipe fittings inspection method for metallographic inspection, but in the construction site, the high cost of metallographic inspection, long time, is not conducive to the full implementation of the site, the author of the stainless steel pipe fittings of the metallographic organization, manufacturing process was studied, and finally chose to use the hardness test for identification, not only to ensure the quality, but also effectively reduce the cost of detection.1、Parameter determination
According to the provisions of SH/T 3408-2012 and GB/T 12459-2005, the hardness of austenitic non-inductive steel pipe parts factory inspection requirements for: Brinell hardness HB ≤ 190 for qualified, but due to the large size of the Brinell hardness tester, complex operation, site application difficulties, in general, the site hardness detection using Richter hardness tester (corresponding to the Richter hardness HL). We applied in the field is HLN-200 type Richter hardness tester. Check GB/T17394-1998 in Appendix B, Brinell hardness 190 is equivalent to the use of C impact when the Richter hardness H LC523, when using vertical downward inspection, HL = HLC, so the test hardness value HL ≤ 523 for qualified.
1.1 Material inspection
In the actual application process, due to the variety of austenitic stainless steel, hardness values vary, the actual test value of stainless steel pipe fittings can easily meet the requirements.
If the hardness test value acceptance according to the standard requirements, the hardness of the above stainless steel pipe fittings is qualified, and in fact in the field application process, this batch of stainless steel pipe fittings in the low-temperature state is generally cracked, and by the chemical composition analysis, its chemical composition qualified. Therefore, only according to the standard requirements of the hardness of the test, acceptance can not fully meet the requirements of use. Subsequently, a batch of qualified stainless steel pipe fittings were selected for hardness testing
Table 2 and Table 1 for comparison can be seen that the measurement of qualified fittings dispersion is much smaller than the unqualified fittings, indicating that its uniformity to better. At the same time, in order to better compare, we have used qualified, different manufacturers of some stainless steel pipe for sampling and measurement.
From Table 3, it can be seen that the dispersion of individual data is large. Analysis of the reasons, because the length of the pipe is large, each pipe in the sampling according to the organization and uniform distribution of alloying elements to consider, so the individual data dispersion is large, but the hardness value is still closer to the solid-fusion treatment of the pipe fittings, much smaller than the unconsolidated pipe fittings.1.2 Modeling of inspection data
After the material inspection is completed, the statistical data are organized, and the concentration or dispersion of the data distribution is judged and modeled according to the normal distribution.
1.2.1 Analysis of unqualified fittings data
(1) The average value of Richter’s hardness test is 411.9.
(2) The variance of the unqualified fittings testing data is calculated to be 18.65.
1.2.2 Qualified pipe data analysis
(1) Calculated, the average value of the Richter hardness test is 359.9.
(2) Calculated, the variance of qualified pipe fittings risk test data is 6.57.1.3 Test analysis and conclusion
According to the principle of normal distribution, in the case of the same sample capacity, the larger the variance, the greater the fluctuation of the data and the more unstable. Through the above calculation and combined with the plot analysis, it is concluded that the variance of the data for qualified fittings is small, while the opposite is true for unqualified fittings.
At 95% confidence level, the range of Richter hardness of unqualified fittings is 370-450, and the range of Richter hardness of qualified fittings is 340-380. There is a slight overlap between the two sides, but considering the fact that the normal distribution curve can only approximate the actual situation on site, the extreme situation will not occur, and considering the need to ensure quality, the critical value of Richter hardness between qualified and unqualified is finally determined as 370 The average value of Richter's hardness exceeds 370 for unqualified pipe fittings, if there are objections to further testing.
2、Application effect
After determining the Richter hardness qualified range, the stainless steel pipe fittings used in the project according to the Richter hardness for acceptance, using the Richter hardness test method for solid fusion verification. After the project is put into use, no more malicious accidents due to stainless steel pipe fittings cracking in the low-temperature state and leading to production stoppage and maintenance.
3、Conclusion
With the development of the market economy, counterfeit and shoddy products are gradually increasing, material testing and inspection means must keep pace with the times, in production practice, and constantly explore innovation, pushing the new. On the one hand, strengthen the inspection and acceptance links, and make every effort to eliminate the inflow of unqualified materials into the construction site; on the other hand, establish a self-restraint mechanism, establish a blacklist of unqualified material manufacturers, and strictly prohibit them from supplying materials again, so that there is no sales market for unqualified materials, to fundamentally ensure the quality of the project.