Study on Microstructure and properties of TP304H austenitic heat resistant steel boiler tube

In order to study the microstructure and properties of TP304H austenitic heat-resistant steel which has been used for a long time under high temperature and high pressure and overheated tube burst, the metallographic structure analysis, electronic bell test, SEM energy spectrum analysis and mechanical property test at room temperature were carried out respectively. The results show that Cr23C6 is precipitated in the microstructure of TP304H austenitic heat-resistant steel under high temperature and high pressure, and σ phase is precipitated along the grain boundary, which results in the decrease of impact toughness, plasticity and oxidation resistance of TP304H austenitic heat-resistant steel, resulting in grain boundary poor chromium in solid solution and increasing the susceptibility of intergranular corrosion. The more σ phase precipitates, the lower the strength of TP304H steel.

Austenitic steel has high thermal strength and excellent oxidation resistance, high temperature range, good cold forming and welding performance, so it is widely used as boiler tube. The common grade of austenitic heat-resistant steel is ASME A213 TP304H, which has good corrosion resistance after solution treatment. Figure 1 and Figure 2 show the common microstructure of new austenitic heat-resistant steel ASME A213 TP304H (the etchant is aqua regia solution), which is austenite twin structure.

Figure. 1 TP304H new tube structure 600X

Figure 2 TP304H new tube tissue 400X

Microstructure of TP304H steel after long term operation

The No.8 boiler of a 300MW unit was put into operation in 1997 and has been operated for 850000 hours. The TP304H steel pipe of reheater was sampled and analyzed.

Metallographic structure

The metallographic structure of the pipe bend was analyzed after being eroded by aqua regia solution. The metallographic structure of the fire facing surface is austenite and carbide. More carbides are precipitated in austenite grain boundary and grain boundary. Besides carbide, some σ phase is precipitated on the grain boundary, as shown in Fig. 3. The metallographic structure of the back fire surface is austenite. Some carbides are precipitated in austenite grains and grain boundaries, and twin substructure can be seen (Fig. 4).

Fig. 3 Fire facing structure of pipe bend 400X

Fig. 4 Structure of fire surface of pipe bend 400X

Electron probe test

After the sample was corroded by copper sulfate and hydrochloric acid solution (Fig. 5), the electron probe test was carried out with JXA-8800R electron probe.

Fig. 5 600X after corrosion of outer arc to fire surface

Figure 6 shows the line scan photo of the sample, and Figure 7 shows the energy spectrum line scanning line. According to the analysis, the chromium content of precipitate phase is 26.010% – 39.387%, which is significantly higher than that of matrix (16.848% – 20.448%), and silicon content is slightly higher than that of matrix; iron content of precipitation phase is 54.722% – 66.712%, lower than that of matrix (67.832% “- 70.893%); nickel content of precipitation phase is 3.006% – 5.015%, which is significantly lower than that of matrix (8.038%” – 9.817%). The results show that the precipitates are rich in chromium and poor in nickel.

Fig. 6 Scanning of energy spectrum lines (bil) on fire surface

Fig. 7 Scanning spectral line of sample energy spectrum

As shown in Fig. 8, the precipitates are rich in chromium and poor in nickel and iron.

Fig. 8 Energy spectrum scanning photos

Energy spectrum analysis of scanning electron microscope

The samples were analyzed by amray L830 scanning electron microscope and Northern energy spectrometer. Figure 9 shows the energy spectrum analysis spectrum of the sample matrix, the analysis results are: Fe: 69.7%; Cr: 20.95%; Ni: 7.92%. It can be seen that there are Fe, Cr and Ni alloy elements in the matrix of TP304H steel pipe, and the iron element is the main element, followed by the chromium element. This result is consistent with the chemical composition of TP304H steel.

Fig. 9 Energy spectrum of sample matrix

Figure 10 shows the spectrum of precipitates in the sample. The results show that Fe: 36.91%; Cr: 59.11%; Ni: 3.73%. The precipitates mainly contain iron and chromium alloy elements, and the chromium content is very high, but there is no nickel or a small amount of nickel, which indicates that the precipitate is not carbide, but iron chromium compound, and the content of the precipitate accords with the composition range of the precipitate phase. σ phase is a kind of intermetallic compound with approximately equal proportion of Fe and Cr atoms. The composition of σ phase in Fe Cr alloy can vary in a wide range and can be expressed approximately by the molecular formula FeCr.

Fig. 10 Energy spectrum of precipitates in the sample

Mechanical properties at room temperature

The impact test and tensile test were carried out on the samples at room temperature. The results show that the impact toughness of the sample is 96.4-117.5j/cm2 when the content of precipitates is less than 2.5%. With the increase of the content of precipitates, the impact toughness decreases sharply. When the content of precipitates is 23.2%, the impact toughness is only 25.1j/cm2, which decreases by about three quarters. The results of mechanical property test at room temperature show that the yield strength is 545mpa, the tensile strength is 640-695mpa, the reduction of area is 29.0% – 41.0%, and the tensile strength value is higher than the standard value of TP304H steel.

Microstructure of TP304H steel tube burst failure tube

The appearance of the tube burst of the pipe bend under the reheater tube is shown in FIG. 11, the operation time is 80000 h, the material is TP304H, and the specification is D63 mm x 4 mm.

Fig.11 Appearance of tube burst of reheater
The metallographic structure of the blast hole is austenite (the etchant is aqua regia solution). There are many cracks on the grain boundary. More carbides are precipitated in the austenite grain boundary and on the grain boundary. Besides carbide, more σ phase (Fe Cr metal compound) is precipitated on the grain boundary (Fig. 12 and Fig. 13).

Fig.12 Structure of reheater tube burst 400X

Fig.13 Structure of reheater tube burst 400X
Fig. 14 shows the electron microscope photos of the tube burst of reheater. The results of EDS analysis of precipitates are: Fe: 27.11%; Cr: 67.23%; Ni: 2.61%.

Fig.14 Microstructure of reheater tube burst 600X

Analysis and discussion

(1) After normal heat treatment, TP304H austenitic heat-resistant steel maintains austenite structure at room temperature, and twin substructure can be seen in the grain, which belongs to non-magnetic steel. After long-term operation at high temperature and high pressure, the microstructure and properties of TP304H austenitic heat-resistant steel decrease. The microstructure consists of austenite and carbide, and carbides precipitate along grain boundary. The back fire microstructure of burst tube and cut tube is the same.
Cr23C6 precipitates mainly from supersaturated austenite solid solution when austenitic heat-resistant steel is heated at 400-850 ℃. Of carbides. When the temperature is higher than 730 ℃, the Cr23C6 on the grain boundary is isolated and granular. When the temperature is lower than 730 ℃, the distribution of Cr23C6 along the grain boundary is continuous.
(2) When TP304H austenitic heat-resistant steel has been working at high temperature and high pressure for a long time and superheated tube burst occurs, carbide precipitates from supersaturated austenite solid solution as well as σ phase along grain boundary in the microstructure of the fire facing surface and the crack mouth.
The precipitation of σ phase in TP304H austenitic heat-resistant steel, on the one hand, reduces the impact toughness and plasticity of the steel; on the other hand, due to the formation of chromium rich σ phase, the poor chromium zone in solid solution occurs, resulting in uneven oxidation and intergranular corrosion, resulting in the decrease of oxidation resistance and the increase of susceptibility to intergranular corrosion.
(3) For Cr Ni austenitic heat-resistant steel, Cr and Ni dissolve in γ – Fe to form single-phase austenite at room temperature. In practical application, due to the influence of alloying and working environment and other complex factors, the transformation temperature of γ (austenite) – δ (ferrite) decreases, or γ – α (ferrite) transformation occurs. Because δ (α) is body centered cubic structure, the amount of carbon dissolved is very low, and some carbon precipitates. Carbon is easy to combine with Cr and Fe, so alloy carbides are formed, and Cr carbides are preferentially formed. However, chromium nickel austenitic heat-resistant steel generally has low carbon content, and Cr can be completely dissolved in δ (α). Therefore, excessive Cr (i.e. the part of Cr that does not form carbide with carbon) will form intermetallic compound σ phase with Fe.
(4) TP304H austenitic heat-resistant steel with solid solution treatment is used in practical application. Solid solution is an unbalanced supersaturated state, and both C and Cr are supersaturated in austenite. At a sufficiently high temperature (to provide the power needed for phase transition), phase transition will occur. With the change of environmental conditions (mainly temperature), carbides and σ phases will precipitate. According to the analysis, it can be inferred that there are at least three possible ways to precipitate carbide and σ phase in austenitic heat-resistant steel

• γ – γ + δ (ferrite) + carbide – γ + α + carbide (1)
• γ – γ + α (ferrite) + carbides – γ + α + carbides (2)
• γ – γ + α + carbides (3)

(5) The factors influencing the formation of σ phase are as follows:

• 1) High chromium and low nickel austenitic heat-resistant steel has low stability of austenite and the tendency to form σ phase is the largest. Other alloying elements such as Al, Si, Ti, W, V, Nb, Mn can promote the formation of σ phase; the increase of carbon and nitrogen content will inhibit the formation of σ phase.
• 2) It is not easy to appear σ phase in austenitic heat-resistant steel with single austenite structure. However, when M23C6 (mainly Cr23C6) carbide precipitates in austenite heat-resistant steel besides austenite, the probability of σ phase will be greatly increased. Moreover, when ferrite exists in austenitic heat-resistant steel, the ferrite contains a large amount of chromium required for the formation of σ phase, and the diffusion and aggregation of chromium in ferrite is easier than that in austenite, so the nucleation of σ phase is easy to form and grow in ferrite and promote the formation of σ phase.
• 3) Phase A is usually formed during long-term aging at 500-800 ℃. This is because higher temperature aging is conducive to the diffusion of chromium, and then high temperature heating D phase will begin to dissolve, when heated to above 920 ℃, the dissolution is complete. The operating temperature of TP304H austenitic heat-resistant steel furnace tube used in thermal power plant is 600-800 ℃, and the designed working life is lo 10000 h, which is conducive to the formation of σ phase.
• 4) Cold working deformation increases the tendency of forming σ phase in steel and reduces the temperature of forming σ phase; for steel with chromium content less than 20%, such as TP304H austenitic heat-resistant steel furnace tube applied in thermal power plant, it is beneficial to form σ phase at pipe bend and other parts with large deformation.

Conclusion

• (1) After normal heat treatment, TP304H austenitic heat-resistant steel maintains austenite structure at room temperature, and twin substructure can be seen in the grain, which belongs to non-magnetic steel.
• (2) After long-term operation at high temperature and high pressure, the microstructure of TP304H austenitic heat-resistant steel tube is aged and its properties are decreased. The microstructure is austenite and carbide, and carbide precipitates along the grain boundary.
• (3) When TP304H austenitic heat-resistant steel has been used for a long time under high temperature and high pressure and overheated tube burst, M23C6 (mainly Cr23C6) carbide will be precipitated from supersaturated austenite solid solution, and σ phase will be precipitated along grain boundary. The precipitation of σ phase will reduce impact toughness, plasticity and oxidation resistance of steel, and make grain boundary poor chromium in solid solution Intergranular corrosion sensitivity.
• (4) The content of σ phase in the burst of TP304 austenitic heat-resistant steel tube is much higher than that in the long-term operation but no tube burst occurred. It shows that the serious precipitation of carbide and d-phase is caused by overheating and high stress, and the microstructure and properties are seriously aged and reduced. Due to the hardness and brittleness of σ phase itself, the grain boundary is usually weakened when the σ phase precipitates and accumulates along the grain boundary. Therefore, when the amount of σ phase precipitation is large, the cracks are easy to generate and expand along the bite phase under stress, which makes the thermal strength of the steel decrease significantly, showing brittle failure. This feature is consistent with the actual explosion of TP304H austenitic heat-resistant steel furnace tube.

Author: Zhao Yongning, Yue Zengwu

Source: China Boiler Tube Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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