The stainless steel pipes for condensers and heat exchangers are basically welded pipes. The investigation found that there are many cases of pitting corrosion in stainless steel pipes in the weld and its heat affected zone. At least 5 samples were investigated:
- 1) 125MW of a power plant in Zhejiang Unit TP304 stainless steel tube condenser.
- 2) TP304 stainless steel tube condenser in a power plant in Henan.
- 3) TP304 stainless steel tube heat exchanger in a unit refrigeration unit in Changsha.
- 4) 436L ferritic stainless steel tube heat exchanger in a chemical enterprise in Shandong Device.
- 5) Shanxi TY first thermal power plant TP304 stainless steel tube heat network heater.
Failure analysis of corrosion of TP304 stainless steel pipe of TY first thermal power plant heat network heater:
The first stainless steel tube heater of TY First Thermal Power Plant started to run. Only one heating period was found to have micro-leakage. At the beginning of the next heating period, the first heater was stopped and a second new stainless steel tube heater was opened. Heating ended. The heater was shut down and no leaks were found before the shutdown. After more than one month of heating, the hydraulic test was conducted to check for leaks. It was found that the first stainless steel tube heater had nearly two leaking tubes, and the second heater also had hundreds of stainless steel tubes leaking. A stainless steel tube corroded by the heater was sampled and found to be etched or perforated in the vicinity of the weld.
The leaked TP304 stainless steel pipe has only a few pits in the weld and nearby, some have been perforated, and there are basically no pitting pits in other places, indicating that the corrosion resistance of the weld and heat affected zone is significantly lower than that of the base metal, and the manufacturing quality is problematic. .
(3) Detection of chemical composition and mechanical properties
According to the analysis of the test results of a large number of stainless steel materials, the possibility of unqualified chemical composition and mechanical properties of stainless steel is extremely small. Due to the improvement of modern smelting technology, the stainless steel composition can be precisely controlled, the cost of removing impurities is low, and the cost of beneficial alloying elements is high. Therefore, the content of impurities in stainless steel is usually much lower than the specified value, and the contents of the beneficial alloying elements such as nickel, chromium and molybdenum are basically at the lower limit.
(4) Metallographic analysis
The transverse section of the TP304 stainless steel tube was taken, and the metallographic structure of the weld was observed under an optical microscope after being inlaid, ground and polished, and chemically etched. The microstructure of the weld of TP304 stainless steel pipe is austenite with a small amount of ferrite, and it is as-cast columnar and dendritic crystal. Since the harmful elements are easily segregated between the dendrites and the columnar crystals, the corrosion resistance at the weld is poor, which is a weak link of the stainless steel welded pipe.
(5) Weld corrosion ratio test
In order to quantitatively characterize the difference between the corrosion resistance of the austenitic stainless steel pipe weld and the base metal, the American Society for Testing and Materials Standard ASTM A249112 specifies the test method and the qualification criteria. The austenitic stainless steel welded pipe is cut into sections, half of which are welded, and half of which is a weld-free base metal. The thickness of the weld and the base metal is measured, and then placed in boiling 20% hydrochloric acid. When the base metal is thinned to the original When the thickness is 40-60%, the sample is taken out, and the thickness of the weld and the base metal is measured after cleaning. The corrosion reduction of the weld and the base metal is calculated, and the ratio of the corrosion reduction of the weld to the base metal is called Weld corrosion ratio R. If R is equal to 1, it indicates that the corrosion resistance of the weld and the base metal are equal; if R is greater than 1, the corrosion resistance of the weld is worse than that of the base metal. ASTM A249 stipulates that R<_1.25 is acceptable, and the R value can also be agreed upon by both parties. Take the unused spare tube and test the weld corrosion ratio according to S7 of ASTM A249. The test results are shown in Table 2 5. The weld corrosion ratio R of the TP304 stainless steel pipe is about 1.52, which is much larger than 1.25, and the weld quality is unqualified. Since this quality clause is a supplemental technical provision, ASTM A249 stipulates that when the standard is implemented, it is only implemented by the acquirer when specified in the contract or order. However, general power plant technicians are not familiar with the standard, and the terms are rarely written when ordering, which is prone to quality disputes.
(6) Corrosion mechanism and prevention and control method
Stainless steel welded pipes for condensers and heat exchangers are prone to pitting corrosion in welds and their heat-affected zones. The main reason is that untreated welds are as-cast columnar and dendritic crystals, and harmful elements are easily in dendritic and columnar Segregation between crystals, if the weld is in a disadvantageous position at the bottom, or in a special environment such as pickling, the weld and its heat affected zone are more susceptible to preferential corrosion. If the uniformity of the welded pipe material is to be improved, the microstructure and performance difference between the weld zone and the base metal are reduced or eliminated, the weld or the entire welded pipe needs to be cold worked and heat treated, and the weld can be made by rolling and recrystallization. The columnar and dendritic crystals are transformed into equiaxed crystals. 1721. The flattening after heat treatment in the weld can improve the microstructure and properties of the weld, but the internal weld should have sufficient height to ensure a certain amount of rolling; heat treatment Have enough time to stay.
Source: China Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)