ASME SA213 T91: How Much Do You Know?
Background & Introduction
ASME SA213 T91, the steel number in the ASME SA213/SA213M standard, belongs to the improved 9Cr-1Mo steel, which was developed from the 1970s to the 1980s by the U.S. Rubber Ridge National Laboratory and the Metallurgical Materials Laboratory of the U.S. Combustion Engineering Corporation in cooperation. Developed based on the earlier 9Cr-1Mo steel, used in nuclear power (can also be used in other areas) high-temperature pressurized parts materials, is the third generation of hot-strength steel products; its main feature is to reduce the carbon content, in the limitation of the upper and lower limits of the carbon content, and more stringent control of the content of residual elements, such as P and S, at the same time, adding a trace of 0.030-0.070% of the N, and traces of the solid carbide-forming elements 0.18-0.25% of V and 0.06-0.10% of Nb, to refine the grain requirements, thereby improving the plastic toughness and weldability of steel, improve the stability of steel at high temperatures, after this multi-composite reinforcement, the formation of a new type of martensitic high-chromium heat-resistant alloy steel.
ASME SA213 T91, usually producing products for small-diameter tubes, is mainly used in boilers, superheaters, and heat exchangers.
International Corresponding Grades of T91 Steel
|
Country |
USA | Germany | Japan | France | China |
| Equivalent Steel Grade | SA-213 T91 | X10CrMoVNNb91 | HCM95 | TUZ10CDVNb0901 | 10Cr9Mo1VNbN |
We will recognize this steel from several aspects here.
I. Chemical Composition of ASME SA213 T91
| Element | C | Mn | P | S | Si | Cr | Mo | Ni | V | Nb | N | Al |
| Content | 0.07-0.14 | 0.30-0.60 | ≤0.020 | ≤0.010 | 0.20-0.50 | 8.00-9.50 | 0.85-1.05 | ≤0.40 | 0.18-0.25 | 0.06-0.10 | 0.030-0.070 | ≤0.020 |
II. Performance Analysis
2.1 The role of alloying elements on the material properties: T91 steel alloying elements play a solid solution strengthening and diffusion strengthening role and improve the steel’s oxidation and corrosion resistance, analyzed explicitly as follows.
2.1.1 Carbon is the most apparent solid solution strengthening effect of steel elements; with the increase in carbon content, the short-term strength of steel, plasticity, and toughness decline, the T91 such steel, the rise in carbon content will accelerate the speed of carbide spheroidization and aggregation speed, accelerate the redistribution of alloying elements, reducing the weldability, corrosion resistance and oxidation resistance of steel, so heat-resistant steel generally want to reduce the amount of carbon content. Still, the strength of steel will be decreased if the carbon content is too low. T91 steel, compared with 12Cr1MoV steel, has a reduced carbon content of 20%, which is a careful consideration of the impact of the above factors.
2.1.2 T91 steel contains traces of nitrogen; the role of nitrogen is reflected in two aspects. On the one hand, the role of solid solution strengthening, nitrogen at room temperature in the steel solubility is minimal, T91 steel welded heat-affected zone in the process of welding heating and post-weld heat treatment, there will be a succession of solid solution and precipitation process of V.N.: Welding heating heat-affected zone has been formed within the austenitic organization due to the solubility of the V.N., nitrogen content increases, and after that, the degree of supersaturation in the organization of the room temperature increases in the subsequent heat treatment of the weld there is slight V.N. precipitation, which increases the stability of the organization and improves the value of the lasting strength of the heat affected zone. On the other hand, T91 steel also contains a small amount of A1; nitrogen can be formed with its A1N, A1N in more than 1 100 ℃ only a large number of dissolved into the matrix, and then re-precipitated at lower temperatures, which can play a better diffusion strengthening effect.
2.1.3 add chromium mainly to improve the oxidation resistance of heat-resistant steel, corrosion resistance, chromium content of less than 5%, 600 ℃ began to oxidize violently, while the amount of chromium content up to 5% has an excellent oxidation resistance. 12Cr1MoV steel in the following 580 ℃ has a good oxidation resistance, the depth of corrosion of 0.05 mm/a, 600 ℃ when the performance began to deteriorate, the depth of corrosion of 0.13 mm / a. T91 containing chromium content of 1 100 ℃ before a large number of dissolved into the matrix, and at lower temperatures and re-precipitation can play a sound diffusion strengthening effect. /T91 chromium content increased to about 9%, the use of temperature can reach 650 ℃, the primary measure is to make the matrix dissolved in more chromium.
2.1.4 vanadium and niobium are vital carbide-forming elements. When added to form a fine and stable alloy carbide with Carbon, there is a solid diffusion-strengthening effect.
2.1.5 Adding molybdenum mainly improves the thermal strength of the steel and strengthens solid solutions.
2.2 Mechanical Properties
T91 billet, after the final heat treatment for normalizing + high-temperature tempering, has a room temperature tensile strength ≥ 585 MPa, room temperature yield strength ≥ 415 MPa, hardness ≤ 250 HB, elongation (50 mm spacing of the standard circular specimen) ≥ 20%, the permissible stress value [σ] 650 ℃ = 30 MPa.
Heat treatment process: normalizing temperature of 1040 ℃, holding time of not less than 10 min, tempering temperature of 730 ~ 780 ℃, holding time of not less than one h.
2.3 Welding performance
In accordance with the International Welding Institute’s recommended Carbon equivalent formula, T91 steel carbon equivalent is calculated at 2.43%, and visible T91 weldability is poor.
The steel does not tend to reheat Cracking.
2.3.1 Problems with T91 welding
2.3.1.1 Cracking of hardened organization in the heat-affected zone
T91 cooling critical speed is low, austenite is very stable, and cooling does not quickly occur during standard pearlite transformation. It must be cooled to a lower temperature (about 400 ℃) to be transformed into martensite and coarse organization.
Welding produced by the heat-affected zone of the various organizations has different densities, coefficients of expansion, and different lattice forms in the heating and cooling process will inevitably be accompanied by different volume expansion and contraction; on the other hand, due to the welding heating has uneven and high-temperature characteristics, so the T91 welded joints are enormous internal stresses. Hardened coarse martensite organization joints that are in a complex stress state, at the same time, the weld cooling process hydrogen diffusion from the weld to the near-seam area, the presence of hydrogen has contributed to the martensite embrittlement, this combination of effects, it is easy to produce cold cracks in the quenched area.
2.3.1.2 Heat-affected zone grain growth
Welding thermal cycling significantly affects grain growth in the heat-affected zone of welded joints, especially in the fusion zone immediately adjacent to the maximum heating temperature. When the cooling rate is minor, the welded heat-affected zone will appear coarse massive ferrite and carbide organization so that the plasticity of the steel decreases significantly; the cooling rate is significant due to the production of coarse martensite organization, but also the plasticity of welded joints will be reduced.
2.3.1.3 Generation of softened layer
T91 steel welded in the tempered state, the heat-affected zone produces an inevitable softening layer, which is more severe than the softening of pearlite heat-resistant steel. Softening is more remarkable when using specifications with slower heating and cooling rates. In addition, the width of the softened layer and its distance from the fusion line are related to the heating conditions and characteristics of welding, preheating, and post-weld heat treatment.
2.3.1.4 Stress corrosion cracking
T91 steel in the post-weld heat treatment before the cooling temperature is generally not less than 100 ℃. If the cooling is at room temperature and the environment is relatively humid, it is easy to stress corrosion cracking. German regulations: Before the post-weld heat treatment, it must be cooled to below 150 ℃. In the case of thicker workpieces, fillet welds, and poor geometry, the cooling temperature is not less than 100 ℃. If cooling at room temperature and humidity is strictly prohibited, otherwise it is easy to produce stress corrosion cracks.
2.3.2 Welding process
2.3.2.1 Welding method: Manual welding, tungsten-pole gas-shielded, or melting-pole automatic welding can be used.
2.3.2.2 Welding material: can choose WE690 welding wire or welding rod.
Welding material selection:
(1) Welding of the same kind of steel – if manual welding can be used to make CM-9Cb manual welding rod, tungsten gas shielded welding can be used to make TGS-9Cb, melting pole automatic welding can be used to make MGS-9Cb wire;
(2) dissimilar steel welding – such as welding with austenitic stainless steel available ERNiCr-3 welding consumables.
2.3.2.3 Welding process points:
(1) the choice of preheating temperature before welding
T91 steel Ms point is about 400 ℃; preheating temperature is generally selected at 200 ~ 250 ℃. The preheating temperature can not be too high. Otherwise, the joint cooling rate is reduced, which may be caused in the welded joints at the grain boundaries of carbide precipitation and the formation of ferrite organization, thus significantly reducing the impact toughness of the steel welded joints at room temperature. Germany provides a preheating temperature of 180 ~ 250 ℃; the U.S. C.E. provides a preheating temperature of 120 ~ 205 ℃.
(2) the choice of welding channel / interlayer temperature
Interlayer temperature shall not be lower than the lower limit of the preheating temperature. Still, as with the selection of preheating temperature, the interlayer temperature can not be too high.T91 welding interlayer temperature is generally controlled at 200 ~ 300 ℃. French regulations: the interlayer temperature does not exceed 300 ℃. U.S. regulations: interlayer temperature can be located between 170 ~ 230 ℃.
(3) the choice of post-weld heat treatment starting temperature
T91 requires post-weld cooling to below the Ms point and hold for a certain period before tempering treatment, with a post-weld cooling rate of 80 ~ 100 ℃ / h. If not insulated, the joint austenitic organization may not be fully transformed; tempering heating will promote carbide precipitation along the austenitic grain boundaries, making the organization very brittle. However, T91 cannot be cooled to room temperature before tempering after welding because cold Cracking is dangerous when its welded joints are cooled to room temperature. For T91, the best post-weld heat treatment starting temperature of 100 ~ 150 ℃ and holding for one hour can ensure complete organization transformation.
(4) post-weld heat treatment tempering temperature, holding time, tempering cooling rate selection
Tempering temperature: T91 steel cold cracking tendency is more significant, and under certain conditions, it is prone to delayed Cracking, so the welded joints must be tempered within 24 hours after welding. T91 post-weld state of the organization of the lath martensite, after tempering, can be changed to tempered martensite; its performance is superior to the lath martensite. The tempering temperature is low; the tempering effect is not apparent; the weld metal is easy to age and embrittlement; the tempering temperature is too high (more than the AC1 line), the joint may be austenitized again, and in the subsequent cooling process to re-quench. At the same time, as described earlier in this paper, determining the tempering temperature should also consider the influence of the joint softening layer. In general, T91 tempering temperature of 730 ~ 780 ℃.
Holding time: T91 requires a post-weld tempering holding time of at least one hour to ensure its organization is wholly transformed into tempered martensite.
Tempering cooling rate: To reduce the residual stress of T91 steel welded joints, the cooling rate must be less than five ℃ / min.
Overall, the T91 steel welding process in the temperature control process can be briefly expressed in the figure below:
Temperature control process in the welding process of T91 steel tube
III. Understanding of ASME SA213 T91
3.1 T91 steel, by the principle of alloying, especially adding a small amount of niobium, vanadium, and other trace elements, significantly improves high-temperature strength and oxidation resistance compared to 12 Cr1MoV steel, but its welding performance is poor.
3.2 T91 steel has a greater tendency to cold Cracking during welding and needs to be pre-welding preheated to 200 ~ 250 ℃, maintaining the interlayer temperature at 200 ~ 300 ℃, which can effectively prevent cold cracks.
3.3 T91 steel post-welding heat treatment must be cooled to 100 ~ 150 ℃, insulation one hour, warming and tempering temperature to 730 ~ 780 ℃, insulation time of not less than one h, and finally, not more than 5 ℃ / min speed cooling to room temperature.
IV. Manufacturing Process of ASME SA213 T91
The manufacturing process of SA213 T91 requires several methods, including smelting, piercing, and rolling. The smelting process must control the chemical composition to ensure the steel pipe has excellent corrosion resistance. The piercing and rolling processes require precise temperature and pressure control to obtain the required mechanical properties and dimensional accuracy. In addition, steel pipes need to be heat-treated to remove internal stresses and improve corrosion resistance.
V. Applications of ASME SA213 T91
ASME SA213 T91 is a high-chromium heat-resistant steel, mainly used in the manufacture of high-temperature superheaters and reheaters and other pressurized parts of subcritical and supercritical power station boilers with metal wall temperatures not exceeding 625°C, and can also be used as high-temperature pressurized parts of pressure vessels and nuclear power. SA213 T91 has excellent creep resistance and can maintain stable size and shape at high temperatures and under long-term loads. Its main applications include boilers, superheaters, heat exchangers, and other equipment in the power, chemical, and petroleum industries. It is widely used in the petrochemical industry’s water-cooled walls of high-pressure boilers, economizer tubes, superheaters, reheaters, and tubes.
