T91 alloy tube, T91 alloy tube is a new martensitic heat-resistant steel developed by the National Like Tree Ridge Laboratory and the Metallurgical Materials Laboratory of the American Combustion Engineering Company. It is based on 9Cr1MoV steel to reduce the carbon content, strictly limit the content of sulfur and phosphorus, add a small amount of vanadium, niobium elements for alloying. According to ASTM213/A213M-85C, the chemical composition of the T91 alloy tube is shown in Table 1.
The German steel number corresponding to the T91 alloy pipe is X10CrMoVNnb91, the Japanese steel number is HCM95, and the French steel number is TUZ10CDVNb0901.
ALLOY PIPE A335 . How to define alloy pipes?
Alloy pipes are tubular with higher percentages, than standard carbon steel pipes, of alloying elements as Molybdenum (Mo), Chromium (Cr), Nickel, etc. Actually, the ASTM A335 covers “low-alloy” steel pipes, i.e. pipes that have a total amount of alloying elements below 5%. The addition of higher percentages of alloying elements (example Nickel and Chromium) transforms the steel into higher alloys, like stainless steel, duplex, up to super-alloyed materials like Inconel, Hastelloy, Monel, etc.
Alloy steel pipes are used in the energy industry for high temperature and very low-temperature service (cryogenic), or for applications with very high pressures.
ASTM A335 alloy steel pipes fit ASTM A234 WPx series buttweld fittings (WP5, WP9, WP91) and A182 Fx forged fittings and flanges (A182 F5, F9, F11, F22, F91). All these materials have similar chemical and mechanical properties and can be joined or welded.
Each alloying element in the T91 alloy tube plays a role in solid solution strengthening
diffusion strengthening and improving the oxidation resistance and corrosion resistance of steel, and the specific analysis is as follows.
(1) Carbon is the most obvious element of solid solution strengthening in steel, with the increase of carbon content, the short-term strength of the steel rises, plasticity, toughness declines, for T91 martensitic steel, the rise of carbon content will accelerate the carbide spheroidization and aggregation speed, accelerate the redistribution of alloying elements, reduce the weldability, corrosion resistance and oxidation resistance of steel, so heat-resistant steel generally hopes to reduce the carbon content, but the carbon content is too low, the strength of the steel will be reduced. Compared with 12Cr1MoV steel, T91 alloy pipe has a carbon content reduction of 20%, which is determined by considering the influence of the above factors.
(2) T91 alloy tube contains trace amounts of nitrogen, and the role of nitrogen is reflected in two aspects. On the one hand, the solid solution strengthening effect, the solubility of nitrogen in steel at room temperature is very small, the thermal impact zone of T91 alloy tube after welding in the process of welding heating and post-weld heat treatment, there will be a solid solution and precipitation process of VN successively: the austenite tissue that has been formed in the heat affected region during welding heating due to the dissolution of VN, the nitrogen content increases, and then the degree of supersaturation in the normal temperature tissue is increased, and there is a fine VN precipitation in the subsequent post-weld heat treatment, which increases the stability of the tissue and improves the lasting strength value of the heat affected zone. On the other hand, the T91 alloy tube also contains a small amount of A1, nitrogen energy and its formation of A1N, A1N in the 1 100 ° C above a large number of dissolved into the matrix, at a lower temperature and re-precipitation, can play a better diffusion strengthening effect.
(3) The addition of chromium is mainly to improve the oxidation resistance and corrosion resistance of heat-resistant steel, when the chromium content is less than 5%, 600 °C begins to oxidize violently, and when the chromium content reaches 5%, it has good oxidation resistance. 12Cr1MoV steel has good oxidation resistance below 580 °C, the corrosion depth is 0.05 mm/a, and the performance begins to deteriorate at 600 °C, and the corrosion depth is 0.13 mm/a. T91 chromium content increased to about 9%, the use of temperature can reach 650 °C, the main measure is to make the matrix dissolved more chromium.
(4) Vanadium and niobium are both strong carbide forming elements, which can form fine and stable alloy carbides with carbon after addition, and have a strong diffusion 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 normalized + high temperature tempering, the normalizing temperature is 1040 °C, the holding time is not less than 10 min, the tempering temperature is 730 ~ 780 °C, the holding time is not less than 1h, and the final heat treatment after the tissue is tempered martensite.
2.3 Mechanical properties
T91 alloy tube has a tensile strength of ≥585 MPa, a yield strength of ≥ 415 MPa, a hardness ≤ 250 HB, an elongation (standard circular specimen with a standard pitch of 50 mm) of ≥20%, and a permissible stress value [σ] 650 °C = 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
It can be seen that the weldability of T91 is poor. Problems with T91 welding
3.1 Generation of hardened tissue in the heat affected zone
As can be seen from Figure 1, the critical cooling rate of T91 is low, the austenite stability is very large, and normal pearlite transformation is not easy to occur during cooling, so that the martensitic transition occurs when cooling to a lower temperature. Because of this, T91 has a great tendency to harden and cold crack.
Since the various tissues in the heat affected zone have different densities, coefficients of expansion and different lattice forms, they will inevitably be accompanied by different volume expansion and contraction during heating and cooling; On the other hand, due to the uneven and high temperature characteristics of welding heating, the internal stress of the T91 welded joint is very large.
For T91, austenite is very stable and needs to be cooled to a lower temperature (about 400 °C) before it becomes martensitic. The coarse martensitic structure is brittle and hard, and the joint is in a state of complex stress. At the same time, during the weld cooling process, hydrogen diffuses from the weld to the near seam area, and the presence of hydrogen promotes martensitic embrittlement, and the result of its comprehensive action is that it is easy to produce cold cracks in the hardening area.
3.2 The grains in the heat affected zone grow
The welding thermal cycle has a significant impact on the grain growth of the heat affected zone of the weld head, especially in the immediate vicinity of the fusion zone where the heating temperature reaches the highest. When the cooling rate is small, a coarse bulk ferrite and carbide structure will appear in the welding heat affected zone, which will significantly reduce the plasticity of the steel; When the cooling rate is large, due to the coarse martensitic structure, the plasticity of the welded joint is also reduced.
3.3 Generation of softening layers
T91 alloy tubes are welded in the quenched and tempered state, and the softening layer in the heat affected zone is inevitable, and it is more severe than the softening of pearlite heat-resistant steel. When using specifications where both heating and cooling speeds are slower, the degree of softening is greater. In addition, the width of the softening layer and its distance from the fusion line are not only related to the heating conditions and characteristics of the weld, but also related to preheating and post-weld heat treatment. Harbin Boiler Factory has done experiments to obtain the hardness curve of the T91 welding heat affected zone, see Figure 2.
3.4 Stress corrosion cracks
T91 alloy pipe before the post-weld heat treatment, the cooling temperature is generally not less than 100 ° C, if cooled at room temperature, and the environment is relatively humid, it is easy to appear stress corrosion cracks. German regulations: must be cooled to less than 150 ° C before post-weld heat treatment. In the case of thick workpiece, the presence of fillet welds and poor geometry, the cooling temperature is not less than 100 °C. If it is cooled at room temperature, it is strictly forbidden to get wet, otherwise it is easy to produce stress corrosion cracks. Welding process of T91 alloy tube
4.1 Selection of preheating temperature
The Ms point of T91 alloy tube is about 400 °C, and the preheating temperature is generally selected at 200~250 °C. The preheating temperature should not be too high, otherwise the cooling rate of the joint is reduced, which may cause carbide precipitation at the grain boundary and the formation of ferrite structure in the welded joint, thereby greatly reducing the impact toughness of the steel welded joint at room temperature. The lower limit of the preheating temperature can be well explained from the insertion test done by the Harbin Boiler Plant.
The latch test rod adopts T91 alloy tube, diameter 8 mm, depth 0.5 mm, the bottom plate is made of 13CrMo steel, thickness 20 mm, the test is carried out without preheating, preheating 150 °C, preheating 200 °C, preheating 250 °C. The electrode is J707. The welding current is 165~170 A, the arc voltage is 21~267 V, and the test results are shown in Table 2. conclusion
(1) T91 alloy pipe relies on the principle of alloying, especially with the addition of a small amount of niobium, vanadium and other trace elements, high temperature strength, oxidation resistance compared with 12 Cr1MoV steel has a greater improvement, but its welding performance is poor.
(2) The latch test shows that the T91 alloy tube has a large tendency to cold crack, and the preheating is 200~250 °C and the interlayer temperature is 200~300 °C, which can effectively prevent the generation of cold cracks.
(3) Before the heat treatment after T91 welding, it must be cooled to 100~ 150 °C, and the heat preservation is 1 h; Tempering temperature 730 ~ 780 °C, holding time is not less than 1 h.
(4) The above welding process has been applied to the 200 MW, 300MW boiler manufacturing and production practice, achieving satisfactory results and obtaining greater economic benefits.
P91 alloy tube production and manufacturing method
P91 alloy pipe is rolled with a solid pipe blank after perforation.
According to different production methods, it can be divided into hot-rolled pipe, cold-rolled pipe, cold-drawn pipe, extruded pipe and so on. Alloy tubes
P91 alloy tube
1.1, hot-rolled P91 alloy pipe is generally produced on the automatic pipe rolling unit.
The solid tube blank is inspected and removed from the surface defects, cut into the required length, centered on the end face of the perforation end of the tube blank, and then sent to a heating furnace for heating and perforation on the perforation machine. At the same time as the perforation is constantly rotating and advancing, under the action of the roll and the head, a cavity gradually forms inside the tube blank, called a capillary. It is then fed to an automatic pipe rolling mill to continue rolling. Finally, the whole wall thickness of the uniform machine is determined by the sizing machine to meet the specifications. The production of hot-rolled P91 alloy pipes by continuous pipe rolling units is a more advanced method.
1.2. If you want to obtain a seamless tube with a smaller size and better quality, you must use the method of cold rolling, cold drawing or combination of the two. Cold rolling is usually carried out on two-roller rolling mills, where steel pipes are rolled in a circular hole shape consisting of a variable section round hole groove and an immovable tapered head. Cold drawing is usually carried out on single-chain or double-chain cold-drawn machines of 0.5 to 100T.
1.3. The extrusion method places the heated tube blank in a closed extrusion cylinder, and the perforated rod moves together with the extrusion rod to extrude the extruded parts from the smaller mold hole. This method produces steel pipes with small diameters.
P91 steel is equivalent to the national standard 10Cr9Mo1VNbN.
Features of P91 alloy tube:
It not only has high oxidation resistance and high temperature steam corrosion resistance, but also has good impact toughness and high and stable long-lasting plasticity and thermal strength. When the use temperature is lower than 620 °C, its permissible stress is higher than that of austenitic stainless steel. Above 550°C, the recommended design permissible stress is approximately twice that of T9 and 2.25Cr-1Mo steels.
Application of P91 alloy tube:
It can be used as a subcritical and supercritical boiler wall temperature ≤ 625 °C high temperature superheater, steel pipe for reheater, and wall temperature ≤ 600 °C high temperature container and steam pipeline, and can also be used as a nuclear power heat exchanger and oil cracking device furnace pipe.
Standard for P91 alloy tubes: ASTM A213 ASTM A335
Tensile strength: ≥585 (MPa)
Yield strength: ≥415 (MPa)
The chemical composition of P91 steel is confirmed, see Table 1
Table 1 Chemical composition of P91 steel
C 0.08~0.12 ; Mn 0.30~0.60 ; Si 0.20~0.50 ; P ≤0.02 ; S ≤0.01
Cr 8.0~9.5 ; Mo 0.85~1.05 ; V 0.18~0.25 ; Nb 0.06~0.1 ; N 0.03~0.07