Quickly verify whether austenitic stainless steel pipe fittings are solid-fusion treated

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 test cannot be. The article briefly introduces the solidification principle and elaborates on the site using the richter hardness tester on austenitic stainless steel pipe fittings for rapid hardness detection method to determine whether the pipe fittings for qualified solidification heat treatment criteria. The engineering application of the method shows that the effect is good.

0. Introduction

According to the relevant national standards, stainless steel pipe 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 pipe fittings. Still, some manufacturers, in pursuit of profits, reduce costs in stainless steel pipe fittings before leaving the factory without 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 cracking in the low-temperature state, 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 substantial economic losses.

As the average production of pipe fittings, solid fusion treatment after the need for pickling passivation, the solid fusion treatment, and unreliable fusion treatment of pipe fittings in the appearance is the same. The traditional way of stainless steel pipe fitting inspection is metallographic inspection. Still, in the construction site, the high cost of metallographic review, long time, could be more conducive to the full implementation of the site. For this reason, the author of the stainless steel pipe fittings of the metallographic organization, the manufacturing process, studied and finally chose to use the hardness test to identify how to ensure the quality and effectively reduce the detection cost.

1. Parameter determination

According to the provisions of SH/T3408-2012 and GB/T12459-2005, the hardness of austenitic stainless steel factory inspection requirements: Brinell hardness HB ≤ 190 for qualified, but due to the large size of the Brinell hardness tester, complex operation, field application difficulties, in general, on-site hardness testing using the Richter hardness tester (corresponding to the Richter hardness HL). Our application in the field is an HLN-200 type Richter hardness tester. Check GB/T17394-1998 Appendix B, Brinell hardness 190 is equivalent to the Richter hardness HLC523 when using C impact; when using the vertical down test, HL = HLC, so the test hardness value HL ≤ 523 for qualified.

1.1 Material testing

In the actual application process, due to the more types of austenitic stainless steel, hardness values vary; the real test value of the fittings can quickly meet the requirements. 0Cr18Ni9, as an example, the solid fusion heat treatment failed pipe fittings were sampled; the results are shown in Table 1.

Table.1 Unqualified pipe fittings Richter hardness testing

Pipe fitting’s No	Test value 1	Test value 2	Test value 3	Test value 4	Test value 5	Average value
1	420	419	392	404	424	411.8
2	403	403	406	412	415	407.8
3	392	400	372	389	406	391.8
4	402	405	372	384	424	397.4
5	443	446	412	442	443	437.2
6	430	426	410	438	429	426.6
7	396	41 7	416	438	404	414.2
8	418	406	419	387	409	407.8

Note: According to the standard measurement data dispersion requirements, the difference between the measured value and the average value can not be more significant than 15 because the pipe fitting is not solid-fusion treatment, and the internal metallurgical organization of the material is not uniform, so the dispersion value is substantial. The underlined data in the table must be qualified according to the standard requirements.

If the standard requirements of the hardness test value acceptance, the hardness of the above pipe fittings is qualified. Still, in fact, in the field application process, the batch of pipe fittings in the low-temperature state is generally cracked, and by the chemical composition analysis, the chemical composition of qualified. Therefore, only according to the standard requirements of the hardness of the inspection acceptance can it only partially meet users’ needs. Subsequently, several qualified pipe fittings were selected for hardness testing; the results are shown in Table 2.

Table.2 Qualified pipe fittings richter hardness testing

Pipe fitting’s No	Test value 1	Test value 2	Test value 3	Test value 4	Test value 5	Average value
1	363	361	360	362	368	362.8
2	352	361	363	348	364	357.6
3	351	363	356	347	360	355.4
4	362	362	360	364	352	360
5	360	358	348	359	364	357.8
6	364	357	362	372	363	363.6
7	365	371	349	363	353	360.2
8	373	364	362	362	348	361.8

Table 2 and Table 1 for comparison can be seen qualified pipe measurement dispersion is much smaller than the unqualified pipe, indicating that its uniformity improved. At the same time, to better compare, we use qualified, different manufacturers of some of the stainless steel pipe sampling measurements; the specific value is shown in Table 3.

Table.3 Stainless steel pipe fitting richter hardness test

Caliber/mm	Test value 1	Test value 2	Test value 3	Test value 4	Test value 5	Average value
273	376	374	368	360	371	370
323	372	358	379	358	344	362
219	277	289	287	301	265	284
114	324	334	322	336	333	330

From Table 3, it can be seen that the individual data dispersion is large. Analysis of the reasons, because the length of the pipe is larger, each pipe in the sampling according to the organization and uniform distribution of alloying elements, so the individual data dispersion is large. However, the hardness value is still closer to the solid fusion treatment of the pipe fitting, much smaller than the pipe fitting without solid fusion.

1.2 Modeling of inspection data

After the material inspection, 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 pipe fitting data

• (1) The average value of Richter’s hardness test is 411.9.

• (2) The variance of the unqualified pipe fitting test data is calculated to be 18.65.

1.2.2 Qualified pipe fitting data analysis

• (1) The average value of Richter’s hardness test is 359.9.

• (2) By calculation, the qualified pipe fitting testing data variance is 6.57.

1.3 Test analysis and conclusion

According to the principle of normal distribution, in the case of the same sample size, the larger the variance, the greater the data fluctuation, and the more unstable. The above calculation and the plot analysis show that the conflict of the data for qualified pipe fittings is small, while the opposite is true for unqualified pipe fittings.

In the 95% confidence level, the range of Richter hardness of unqualified pipe fittings is 370-450, and the content of Richter hardness of qualified pipe fittings is 340-380. there is a slight overlap between the two sides. Still, because the standard distribution curve can only approximate the actual situation on site, the extreme situation will not occur. Considering the need to ensure quality, it is finally determined that the critical value of Richter’s hardness of qualified and unqualified is 370. The average value of Richter hardness exceeds 370 for complete pipe fittings if objections exist 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 stainless steel pipe fittings. After the project is put into use, there are no more nasty accidents due to low-temperature cracking of stainless steel pipe fittings resulting in production stoppage and maintenance.

3. Conclusion

With the development of the market economy, counterfeit and shoddy products is gradually increasing, and metal material testing and inspection means must keep pace with the times in production practice and constantly explore innovative, new, and innovative. On the one hand, strengthen the inspection and acceptance links, and make every effort to eliminate the inflow of unqualified metal materials into the construction site; on the other hand, establish a self-restraint mechanism, establish a blocklist of manufacturers of complete materials, and strictly prohibit them from resupplying metal materials, so that there is no sales market for unable materials, to ensure the quality of the project fundamentally.

Author: Li Ningge

Source: China Stainless Steel Pipe Fittings Manufacturer – Yaang Pipe Industry (www.epowermetals.com)