T91 alloy steel pipe

T91 alloy steel pipe is a type of steel pipe. T91 steel is a new type of martensitic heat-resistant steel developed in collaboration between the National Elephant Ridge Laboratory and the Metallurgical Materials Laboratory of Combustion Engineering Corporation in the United States. It is based on reducing the carbon content of 121MoV steel, strictly limiting the content of sulfur and phosphorus, and adding a small amount of vanadium and niobium elements for alloying. According to ASTM213/A213M-85C, the chemical composition of T91 steel is shown in Table 1. The German steel grade corresponding to T91 steel is X10CrMoVNNb91, the Japanese steel grade is HCM95, and the French steel grade is TUZ10CDVNb0901. Table 1 Chemical Composition% of T91 Steel

T91 alloy steel pipe element content

C 0.08-0.12

Mn 0.30-0.60

P ≤ 0.02

S ≤ 0.01

Si 0.20-0.50

Cr 8.00-9.50

Mo 0.85-1.05

V 0.18-0.25

Nb 0.06-0.10

N 0.03-0.07

Ni ≤ 0.40

The various alloying elements in T91 steel play a role in solid solution strengthening, dispersion strengthening, and improving the steel's oxidation resistance and corrosion resistance. The specific analysis is as follows.

① Carbon is the element with the most obvious solid solution strengthening effect in steel. With the increase of carbon content, the short-term strength of steel increases, while plasticity and toughness decrease. For martensitic steels such as T91, the increase of carbon content will accelerate the spheroidization and aggregation rate of carbides, accelerate the redistribution of alloying elements, and reduce the weldability, corrosion resistance, and oxidation resistance of steel. Therefore, heat-resistant steels generally hope to reduce the carbon content, but if the carbon content is too low, the strength of the steel will decrease. Compared with 12Cr1MoV steel, T91 steel has a 20% reduction in carbon content, which is determined by considering the influence of the above factors comprehensively.

② T91 steel contains trace amounts of nitrogen, and the role of nitrogen is reflected in two aspects. On the one hand, it plays a role in solid solution strengthening. At room temperature, the solubility of nitrogen in steel is very low. In the heat affected zone of T91 steel after welding, the solid solution and precipitation process of VN will occur successively during welding heating and post weld heat treatment. The austenite structure formed in the heat affected zone during welding heating increases the nitrogen content due to the dissolution of VN, and then the degree of supersaturation in the normal temperature structure increases. In the subsequent post weld heat treatment, small VN precipitates, which increases the stability of the structure and improves the durability strength value of the heat affected zone. On the other hand, T91 steel also contains a small amount of A1, which can form AlN with nitrogen. A1N only dissolves into the matrix in large quantities above 1100 ℃ and re precipitates at lower temperatures, which can have a good dispersion strengthening effect.

③ Adding chromium is mainly to improve the oxidation resistance and corrosion resistance of heat-resistant steel. When the chromium content is less than 5%, severe oxidation begins at 600 ℃, while when the chromium content reaches 5%, it has good oxidation resistance. 12Cr1MoV steel has good oxidation resistance below 580 ℃, with a corrosion depth of 0.05 mm/a. The performance begins to deteriorate at 600 ℃, with a corrosion depth of 0.13 mm/a. The chromium content of T91 is increased to about 9%, and the operating temperature can reach 650 ℃. The main measure is to dissolve more chromium in the matrix.

④ Vanadium and niobium are both strong carbide forming elements, which can form small and stable alloy carbides with carbon after addition, and have a strong dispersion strengthening effect.

⑤ The addition of molybdenum is mainly to improve the thermal strength of steel and play a role in solid solution strengthening.

2.2 Heat treatment process

The final heat treatment of T91 is normalizing+high-temperature tempering, with a normalizing temperature of 1040 ℃ and a holding time of not less than 10 minutes. The tempering temperature is 730-780 ℃ and the holding time is not less than 1 hour. The final microstructure after heat treatment is tempered martensite.

2.3 Mechanical Properties

The tensile strength of T91 steel at room temperature is ≥ 585 MPa, the yield strength at room temperature is ≥ 415 MPa, the hardness is ≤ 250 HB, the elongation (standard circular specimen with a gauge length of 50 mm) is ≥ 20%, and the allowable stress value [σ] is 650 ℃=30 MPa.

2.4 Welding performance

According to the carbon equivalent formula recommended by the International Welding Society, the carbon equivalent of T91 is calculated as

It can be seen that T91 has poor weldability.

Problems during T91 welding

3.1 Formation of hardened microstructure in heat affected zone

From Figure 1, it can be seen that T91 has a low critical cooling rate, high austenite stability, and is not prone to normal pearlite transformation during cooling, resulting in martensitic transformation when cooled to lower temperatures. Due to this, T91 has a high tendency towards quenching and cold cracking.

Due to the different densities, expansion coefficients, and lattice forms of various tissues in the heat affected zone, there will inevitably be different volume expansion and contraction during heating and cooling processes; On the other hand, due to the uneven and high temperature characteristics of welding heating, the internal stress of T91 welding joints is very high.

For T91, austenite is very stable and needs to be cooled to a lower temperature (about 400 ℃) to transform into martensite. The coarse martensitic structure is brittle and hard, and the joint is in a complex stress state. At the same time, during the cooling process of the weld, hydrogen diffuses from the weld to the near weld area, and the presence of hydrogen promotes martensitic embrittlement. As a result of its combined effect, cold cracks are easily generated in the quenched zone.

3.2 Grain growth in heat affected zone

The welding thermal cycle has a significant impact on the grain growth in the heat affected zone of the welded joint, especially in the fusion zone adjacent to the highest heating temperature. When the cooling rate is low, coarse blocky ferrite and carbide structures will appear in the welding heat affected zone, resulting in a significant decrease in the plasticity of the steel; When the cooling rate is high, the formation of coarse martensitic structure can also cause a decrease in the plasticity of the welded joint.

3.3 Generation of Softening Layer

When T91 steel is welded in the quenched and tempered state, the formation of a softening layer in the heat affected zone is inevitable, and the softening is more severe than that of pearlite heat-resistant steel. When using standards with slow heating and cooling rates, the degree of softening is greater. In addition, the width of the softened layer and its distance from the fusion line are not only related to the heating conditions and characteristics of the welding, but also to preheating, post weld heat treatment, etc. Harbin Boiler Factory has conducted experiments and obtained the hardness curve of T91 welding heat affected zone, as shown in Figure 2.

3.4 Stress Corrosion Cracking

Before post weld heat treatment, the cooling temperature of T91 steel is generally not lower than 100 ℃. If it is cooled at room temperature and the environment is relatively humid, stress corrosion cracking is prone to occur. German regulations require cooling to below 150 ℃ before post weld heat treatment. In the case of thick workpieces, presence of fillet welds, and poor geometric dimensions, the cooling temperature should not be lower than 100 ℃. If cooled at room temperature, moisture is strictly prohibited, otherwise stress corrosion cracking may occur.

Welding process of T91 steel

4.1 Selection of preheating temperature

The Ms point of T91 steel is about 400 ℃, and the preheating temperature is generally selected between 200-250 ℃. The preheating temperature should not be too high, otherwise the cooling rate of the joint will decrease, which may cause carbide precipitation at grain boundaries and the formation of ferrite structure in the welded joint, greatly reducing the impact toughness of the steel welded joint at room temperature. The lower limit of preheating temperature can be well explained from the pin test conducted by Harbin Boiler Factory.

The pin test rod is made of T91 steel, with a diameter of 8 mm and a depth of 0.5 mm. The base plate is made of 13CrMo steel, with a thickness of 20 mm. The test is conducted under the conditions of no preheating, preheating at 150 ℃, preheating at 200 ℃, and preheating at 250 ℃. The welding rod used is J707. The welding current is 165-170 A, and the arc voltage is 21-267 V. The test results are shown in Table 2.

Table 2 T91 Pin Test Results

Experiment

Conditional sample

Stress level

/MPa fracture time

/Min

Not preheating 1 303.8 9 9

2 186 8 237

3 176.4 8.3 1440 Not Broken

Preheat to 150 ℃ 4 421.4 8.1 1260

354.8 120 Not Broken

Preheating at 200 ℃ 6 465.2 8.6 1440 without interruption

7 482.7 8.1 438

8 539 7.9 313

Preheating at 250 ℃ 9 539 8.2 1440 without interruption

10 600 8.0 1440 Not Broken

From the above experimental results, it is known that the critical stress of T91 steel welded joints is 176.4 MPa without preheating conditions; When preheated to 150 ℃, the critical stress is 354.8 MPa, which is 85.4% of the yield limit of T91 steel at room temperature of 415 MPa; When preheated above 200 ℃, the critical stress is greater than 460 MPa, exceeding the yield limit of T91 steel at room temperature. Therefore, in order to avoid cold cracks during welding of T91 steel, the preheating temperature must not be lower than 200 ℃. Germany stipulates a preheating temperature of 180-250 ℃, and the US CE company specifies a preheating temperature of 120-205 ℃.

4.2 Selection of interlayer temperature

The interlayer temperature should not be lower than the lower limit of the preheating temperature, but just like the selection of preheating temperature, the interlayer temperature should not be too high. The interlayer temperature during T91 welding is generally controlled at 200-300 ℃. France stipulates that the interlayer temperature shall not exceed 300 ℃. According to US regulations, the interlayer temperature can be between 170 and 230 ℃.

4.3 Selection of starting temperature for post weld heat treatment

T91 requires post weld cooling below the Ms point and holding for a certain period of time before tempering treatment. The post weld cooling rate is 80-100 ℃/h. If not insulated, the austenite structure of the joint may not be completely transformed, and tempering heating will promote the precipitation of carbides along the austenite grain boundaries, which is very brittle. However, it is not allowed to cool T91 to room temperature before tempering after welding, as there is a risk of cold cracking when the welded joint cools to room temperature. For T91, the optimal starting temperature is 100-150 ℃, and holding for 1 hour can basically ensure the completion of tissue transformation.

4.4 Selection of tempering temperature, constant temperature time, and tempering cooling rate

T91 steel has a high tendency for cold cracking, and under certain conditions, delayed cracking is prone to occur. Therefore, welded joints must be tempered within 24 hours after welding. The structure of T91 after welding is lath martensite, which can be changed into tempered martensite after tempering, and its performance is superior to lath martensite. When the tempering temperature is low, the tempering effect is not obvious, and the weld metal is prone to aging and embrittlement; If the tempering temperature is too high (exceeding AC1 line), the joint may undergo austenitization again and re harden during the subsequent cooling process. Meanwhile, as mentioned earlier in this article, the determination of tempering temperature also takes into account the influence of the joint softening layer. Generally speaking, the tempering temperature for T91 is 730-780 ℃.

The tempering constant temperature time after T91 welding should not be less than 1 hour to ensure that its structure is completely transformed into tempered martensite.

In order to reduce the residual stress of T91 steel welded joints, the cooling rate must be controlled to be less than 5 ℃/min. The welding process of T91 steel can be represented in Figure 3.

① Preheat to 200-250 ℃; ② Welding, interlayer temperature of 200-300 ℃; ③ Cooling after welding at a rate of 80-100 ℃/h; ④ Holding at 100-150 ℃ for 1 hour; ⑤ Tempering at 730-780 ℃ for 1 hour; ⑥ Cooling at a rate not exceeding 5 ℃/min

Application Example of 5 T91 Steel in Guangdong Province's Internal Thermal Power Plant

The First Welding Training Center of Guangdong Electric Power Bureau has conducted welding process evaluation for the butt joint of T91 small diameter pipes with a diameter of 42 mm × 5mm. The preheating temperature adopted is 200 ℃, cooled to 150 ℃ after welding, and tempered after 1 hour of insulation. The tempering temperature is 750-780 ℃, and the insulation time is 1 hour. The heating and cooling speed is less than 5 ℃/min. After welding, the sample was subjected to visual inspection, fracture inspection, non-destructive testing, tensile and bending tests, and the results were all qualified, which also indicates that the above welding process is effective.

The above welding process has been successfully applied to the outer ring of the high-temperature reheater in Shajiao A Plant and Meixian Power Plant. After the application of T91 steel in these power plants, the frequency of accidents caused by overheating and other factors has been greatly reduced.

6 Conclusion

① T91 steel relies on the principle of alloying, especially with the addition of trace elements such as niobium and vanadium. Its high-temperature strength and oxidation resistance are significantly improved compared to 12Cr1MoV steel, but its welding performance is poor.

② The pin test shows that T91 steel has a significant tendency towards cold cracking. Selecting preheating at 200-250 ℃ and interlayer temperature at 200-300 ℃ can effectively prevent the occurrence of cold cracks.

③ Before T91 post weld heat treatment, it must be cooled to 100-150 ℃ and held for 1 hour; the tempering temperature is 730-780 ℃, and the holding time is not less than 1 hour.

④ The above welding process has been applied in the manufacturing and production practice of 200 MW and 300 MW boilers, achieving satisfactory results and significant economic benefits. Steel pipe is a long strip of steel with a hollow cross-section and no seams around it. Steel pipes have a hollow cross-section and are widely used as pipelines for transporting fluids, such as oil, natural gas, coal gas, water, and certain solid materials. Compared with solid steel materials such as round steel, steel pipes are lighter in weight when their bending and torsional strength is the same. They are an economical cross-sectional steel material widely used in the manufacture of structural and mechanical parts, such as oil drill rods, automotive transmission shafts, bicycle frames, and steel scaffolding used in construction. Using steel pipes to manufacture circular parts can improve material utilization, simplify manufacturing processes, save materials and processing time, such as rolling bearing rings, jack sleeves, etc. Currently, steel pipes are widely used for manufacturing. Steel pipes are still an indispensable material for various conventional weapons, such as gun barrels and cannon barrels, which require steel pipes for manufacturing. Steel pipes can be divided into circular pipes and special-shaped pipes according to their cross-sectional area and shape. Due to the fact that the circular area is the largest under the condition of equal circumference, a circular tube can transport more fluid. In addition, when subjected to internal or external radial pressure, the circular cross-section is subjected to more uniform force, therefore, the vast majority of steel pipes are circular. Formula for calculating the weight of alloy pipes: [(outer diameter wall thickness) * wall thickness] * 0.02466=kg/meter (weight per meter)