CN103866163B - A kind of nickel chromium cobalt molybdenum refractory alloy and tubing manufacturing process thereof - Google Patents

Abstract

The invention relates to a nickel-chromium-cobalt-molybdenum heat-resistant alloy and a pipe material manufacturing process thereof, belonging to the technical field of heat-resistant alloys. The chemical composition of the alloy The weight percent of the chemical composition of the alloy is: Cr: 21-23%; C: 0.05-0.07%; Mn: ≤ 0.3%; Co: 11-13%; Mo: 6.0-9.0%; Ti: 0.3 -0.5%; Al:0.8-1.3%; W:0.1-1.0%; B:0.002-0.005%; Zr:0.03-0.15%; Nb+V:0.2-0.6% and Nb0.01-0.05%; Cu: ≤0.15%; P:＜0.008%; S:＜0.002%; N:≤0.015%; Mg:0.005-0.02%; Ca:≤0.01%; As:≤0.01%; Pb:≤0.007%; Bi:≤ 0.001%; the balance is nickel and unavoidable impurity elements. The advantage is that the room-temperature mechanical properties, high-temperature mechanical properties, low-cycle fatigue properties and durability of the produced large-diameter thick-walled boiler tubes are all higher than the ASME standard requirements and the CCA617 alloy technical requirements; the HAZ liquefaction cracks and stress relaxation cracks are greatly reduced. Occurrence, HAZ durable fracture strength and toughness after long-term aging are also excellent.

Description

A nickel-chromium-cobalt-molybdenum heat-resistant alloy and its pipe manufacturing process

technical field

The invention belongs to the technical field of nickel-based heat-resistant alloys, and in particular provides a nickel-chromium-cobalt-molybdenum heat-resistant alloy (C-HRA-3) and its pipe manufacturing process, which can be used for advanced ultra-supercritical thermal power units with 700°C steam parameters The use of large-diameter thick-walled boiler tubes and related piping.

Background technique

According to statistics from the China Electricity Council, in 2013, the total installed capacity of electric power in the country reached 1,247.38 million kwh, of which thermal power installed capacity reached 862.38 million kwh, accounting for about 70% of the total installed capacity. On the one hand, China has abundant coal reserves. Compared with other energy sources, coal-fired power generation is cheap and economical; on the other hand, traditional coal-fired power generation technology brings more CO ₂ and other pollution than other energy sources gas emissions. In order to further reduce coal consumption, improve thermal efficiency and reduce emissions, Europe, the United States, Japan and India are developing ultra-supercritical thermal power units with 700 °C steam parameters. my country also launched a national plan for the development of ultra-supercritical thermal power units with 700 °C steam parameters in 2010. . Steam parameters include steam temperature and steam pressure. Studies have shown that the increase of steam temperature can significantly improve the thermal efficiency of the unit.

Heat-resistant materials are the main "bottleneck" problem that restricts the development of thermal power units to higher parameters, and large-diameter boiler tubes and headers are the "bottleneck of bottlenecks." 700°C Steam Parameters The temperature of the steam in the boiler of the ultra-supercritical thermal power unit is gradually raised to 700°C, and candidate heat-resistant materials that meet the requirements of use are required in each temperature range. Studies have shown that P92 martensitic heat-resistant steel can be used for the manufacture of some large-diameter boiler tubes below the steam temperature of 620°C; the new steel grade G115 researched and developed by Liu Zhengdong and others at the General Iron and Steel Research Institute can be used for large-diameter boiler tubes with a steam temperature of 620°C-650°C. Boiler tube manufacturing; and the candidate material for large-diameter boiler tubes in the steam temperature range of 650°C-700°C must use nickel-based heat-resistant alloys. Studies have shown that CCA617 nickel-based heat-resistant alloy is the preferred candidate material for large-diameter boiler tubes in the steam temperature range of 650°C-700°C.

Inconel617 (hereinafter referred to as 617 alloy) is a nickel-chromium-cobalt-molybdenum solid-solution strengthened alloy. This alloy has been included in the ASME boiler and pressure vessel specification ASMECodeCase2439. Due to the wide upper and lower limits of the element composition range, the mechanical property data is scattered. , and the long-term creep performance data also need to be improved urgently, Jutta of Germany ThyssenKrupp et al. optimized the narrow composition range of Cr, Co, Al, Ti, C, Fe, Mn, Si and B elements in the alloy, especially using the "B metallurgy" strengthening mechanism, and successfully developed a alloy with higher durability than Inconel617. CCA617 alloy (also known as 617B alloy) (see Jutta et al. published in recent years). In terms of industrial practice, the American Wiman-Gordon Company has trial-produced 617 alloy thick-walled boiler tubes with a size of Ф378mmOD×88mmAWT; the German Vallourek Mannesmann Steel Tube Company has produced CCA617 boiler tubes for the European 700°C AD plan.

Utilizing the 700°C test platform of the E.ONScholven power plant in Germany, the European AD700 plan tested the CCA617 alloy large-diameter thick-walled boiler pipe. After the test, circumferential cracks appeared in the welding heat-affected zone (hereinafter referred to as "HAZ"). Microscopic analysis shows that the cracks propagate along the grain. Due to the accumulation of precipitated phases during service, the local residual stress is too high and the grain boundaries are weakened. It belongs to stress relaxation cracks, also known as strain aging cracks. Research practice shows that CCA617 alloy has a strong tendency of stress relaxation cracks. At the same time, even with low input energy during welding, liquefaction cracks such as welding microscopic solidification cracks are very easy to occur for thick-walled parts. The emergence of two kinds of cracks had to delay the EU's plan to establish a 700°C ultra-supercritical coal-fired demonstration power station in 2014, and it is urgent to further optimize and evaluate the CCA617 alloy.

Patent document 1: CN102686757B discloses an austenitic heat-resistant alloy, the composition of which contains by weight percentage: carbon: <0.15%; silicon: <2.0%; manganese: <3.0%; nickel: 40-60%; cobalt: 0.03-25%; Chromium: 15-28%; Molybdenum: <12%; Neodymium: 0.001-0.1%; Boron: 0.0005-0.006%; Nitrogen: <0.03%; Oxygen: <0.03%; Phosphorus: <0.02%; Sulfur: <0.005%; Aluminum: <3%; Titanium: <3%; Niobium: <3%; the remainder consists of iron and impurities. Excellent resistance to liquefaction cracking of HAZ during welding under suitable conditions.

Patent document 2: CN103080346A discloses a nickel-chromium-cobalt-molybdenum alloy, which includes by weight percentage: chromium: 21-23%; iron: 0.05-1.5%; carbon: 0.05-0.08%; manganese: ≤0.5%; Silicon: ≤0.25%; Cobalt: 11-13%; Copper: ≤0.15%; Molybdenum: 8.0-10.0%; Titanium: 0.3-0.5%; Aluminum: 0.8-1.3%; Phosphorus: <0.012%; Sulfur: <0.008 %; Boron: >0.002 and <0.006%; Niobium: >0-1.8%; Nitrogen: ≤0.015%; Magnesium: ≤0.025%; Calcium: ≤0.01%; Vanadium: ≤0.6%; of impurities. Adjusting the range of elements and stress relief annealing at 980°C for several hours can eliminate the tendency of stress relaxation cracks.

Although patent 1 eliminates the liquefaction cracks at the HAZ during welding by adding the characteristic element neodymium (Nd) and other elements, the upper and lower limits of other components such as C, Mo, Al, Ti, Nb, etc. are too wide. For the future 700 Pipeline applications with strict mechanical properties in ℃ high steam parameter environment are still not safe enough, and the strain aging cracks of this alloy are not considered.

Although patent 2 mentions welding stress cracks, it does not consider solidification cracks or liquefaction cracks during welding of thick-walled parts, and the durable strength performance of thick-walled parts such as large-diameter pipes still needs to be improved.

It should be noted that various viewpoints have been put forward on the causes of welding liquefaction cracks and stress crack tendencies in different materials at the HAZ, but the countermeasures to deal with the two crack tendencies have not been comprehensively considered from the perspective of the same material.

In addition, the toughness of aging-type heat-resistant alloys with high strength will be greatly reduced after long-term use, so at present, large-diameter boiler tubes use solid-solution-strengthened heat-resistant alloys, but solid-solution-strengthened alloys still need to increase their durability to meet the future 700℃ Safe application of relevant pipelines in the environment of steam temperature parameters.

Based on the above-mentioned status quo, under the guidance of the theories of "multi-element composite strengthening" and "selective strengthening", combined with the principle of "nickel-based alloy welding solidification metallurgy", according to the service conditions of heat-resistant alloys for ultra-supercritical thermal power units at 700 °C Based on the research of CCA617, the alloy composition is further optimized, so that the alloy has higher durability, good aging toughness, and no tendency of liquefaction cracks and HAZ stress cracks during HAZ welding. According to laboratory research and industrial trial production practice, the inventor of this patent proposed the smelting, thermal processing and tube making procedures of using the heat-resistant alloy of the invention to manufacture large-diameter boiler tubes, especially the optimal chemical composition range and optimal thermal processing process And the best heat treatment process system. The enterprise brand of the General Iron and Steel Research Institute of the invented heat-resistant alloy is C-HRA-3.

Contents of the invention

The object of the present invention is to provide a nickel-chromium-cobalt-molybdenum heat-resistant alloy (C-HRA-3) and its pipe manufacturing process, involving the optimum chemical composition range, optimum thermal processing process and optimum heat treatment process of the alloy, suitable for Manufacture of large-diameter boiler pipes and related pipes for 700°C ultra-supercritical thermal power units. This alloy has good HAZ welding crack resistance, good long-term aging toughness and excellent high temperature creep strength and other comprehensive properties.

It should be noted that good HAZ resistance to weld cracking specifically means excellent resistance to both liquefaction cracking of HAZ and tendency to stress relaxation cracking of HAZ.

The present invention includes three parts, one is narrow composition range matching and precise control technology based on "multi-element composite strengthening" theory, "selective strengthening" and "nickel-based alloy welding solidification metallurgy" theory; the other is based on large The smelting and optimal thermal processing manufacturing process of the caliber thick-walled boiler tube industrial production practice; the third is the optimal heat treatment process of the large-diameter thick-walled boiler tube based on the industrial production site. The above three parts as a whole provide a heat-resistant alloy and production method with comprehensive performance for large-diameter thick-walled boiler tubes of ultra-supercritical thermal power units in the steam temperature range of 700 °C, which surpasses previous research results. Innovations have been achieved both theoretically and practically.

1. C-HRA-3 alloy of the present invention and its narrow range chemical composition and precise control

The heat-resistant alloy of the invention belongs to the solid-solution strengthening type, contains a small amount of primary carbide in the solid-solution state, and precipitates a small amount of strengthening phase γ' phase after long-term aging. Therefore, the strengthening mechanism of the alloy of the present invention includes: solid solution strengthening, grain boundary strengthening, and precipitation strengthening, wherein the precipitation strengthening includes carbide strengthening and γ' phase strengthening. A large amount of γ' phase precipitation will lead to a significant decrease in toughness after long-term aging and increase the sensitivity to strain aging cracks, so this method is not used in the present invention.

The present invention adopts strengthening mechanism as follows:

(1) Add solid solution strengthening element W to replace part of Mo: the melting point of W element is 800°C higher than that of Mo element, and its atomic radius is larger than that of Mo element , while the thermal diffusivity of W is lower than that of Mo.

(2) Prevent the decomposition of primary carbides and the aggregation and coarsening of secondary precipitated carbides.

The alloy of the present invention in the solid solution treatment state contains a certain amount of grain boundary carbides (called primary carbides), mainly M ₂₃ C ₆ type carbides, which will decompose after aging for 8000 hours and transform into secondary carbides. Destabilize the grain boundaries of the alloy. Therefore, adding primary carbide-forming elements Nb and V to generate more stable primary carbides (Nb, V)C can improve the long-term durability.

Another study shows that B not only segregates the grain boundary in the alloy to reduce the grain boundary energy to strengthen the grain boundary, but also fills the vacancies around the carbides at the grain boundary, hinders the diffusion of C atoms inside and outside the carbide, and reduces the coarseness of the carbides at the grain boundary. conversion rate. Zr element and B element are both trace grain boundary strengthening elements, so the inventors believe that Zr element also has a certain effect on slowing down the coarsening of grain boundary carbides. Therefore, the addition of Zr element can not only form B-Zr composite strengthening, but also slow down the coarsening rate of grain boundary carbides, which is the characteristic element of the alloy of the present invention.

The present invention improves the resistance to liquefaction cracks during HAZ welding and stress aging cracks during service as follows:

Based on the understanding of the existing technical literature, the liquefaction cracks of austenitic heat-resistant alloys during HAZ welding are mainly directly related to trace elements such as P, B, S, etc., and the Cr content has an indirect effect on them; The accumulation of HAZ precipitates causes excessive local stress, but the specific mechanism is not yet clear.

Under modern advanced smelting technology, through the study of HAZ liquefaction cracks in alloys where C, B, Zr, P and S elements coexist:

(1) The crack occurs at the HAZ grain boundary close to the melting boundary, and there are more carbides on the grain boundary.

(2) There are melting traces around the grain boundary carbides on the crack section at the HAZ, and trace elements B and Zr are enriched around the carbides.

(3) There is a small amount of ZrS phase on the grain boundary melting fracture surface.

(4) The degree of influence of B and Zr on the liquefaction cracking of HAZ is affected by the grain boundary carbides in the alloy. The larger the size, the greater the number and the more concentrated the distribution of carbides, the more significant the adverse effects of B and Zr.

At the same time, by observing the stress relaxation crack at the HAZ, it was found that:

(1) The hardness around the crack at the HAZ is higher than that of the base metal, and the precipitation strengthening is obvious under long-term service.

(2) The crack is an intergranular fracture type, and there are many carbides distributed on the fracture grain boundary.

(3) The enrichment of B and Zr was found on the grain boundary section and around the grain boundary carbide.

The present inventor also researched the tendency of HAZ cracks to different grain size series and found that: when the grain size series was less than 3 grades, the tendency of liquefaction cracks and stress relaxation cracks at the HAZ was more obvious; At level 5, the tendency to resist two kinds of cracks is excellent. The inventor believes that the smaller the grain size series (the larger the grain size), the fewer the grain boundaries, the more concentrated the trace elements and carbide segregation grain boundaries after welding, and the greater the tendency to liquefaction cracks. On the contrary, the smaller the grain size, the more grain boundaries, the more dispersed the element segregation and the grain boundary carbide distribution, and the smaller the tendency of the two kinds of cracks. However, the smaller the grain size series, the higher the long-term creep strength of the alloy and the lower the low-cycle fatigue performance. Considering the welding performance of the alloy, the grain size of the finished tube is also very important to ensure the performance of the alloy of the present invention.

Through the above analysis, the following results are obtained:

Trace elements B, Zr, S and other trace elements, grain boundary carbides and grain size series are the main factors affecting the formation of two types of cracks in the alloy. The influence of grain boundary carbides is obviously related to C element and the elements that form carbides; when adding trace element Zr, the S content should be strictly controlled to prevent the formation of low melting point ZrS phase. It can be further speculated that if the S element is strictly controlled, the harmful effect of the Zr element on welding cracks can be eliminated.

Therefore, the heat-resistant alloy of the present invention is effective by adopting the following means:

(1) Reduce the content of P and S elements; reasonably match the content of B-Zr;

(2) Control grain boundary carbides, especially delay the aggregation and coarsening of carbides precipitated over a long period of time;

(3) Use a reasonable heat treatment system to obtain a suitable grain size series.

Considering the occurrence of HAZ liquefaction cracks and HAZ stress relaxation cracks comprehensively based on the above factors, the alloy design should satisfy the following formula:

Y1＝10×C+5×B+0.5×Zr+P+S

Y2＝(4V+Nb)/C

The symbol of the element in the formula is set as the content of the weight % of the element. Set the Y1 parameter between 0.536-0.754, and the Y2 parameter between 11-40. In addition, the grain size level of the alloy before welding is preferably around 3-5, which can ensure the durable strength and Toughness, and can reduce the occurrence of liquefaction cracks and HAZ stress cracks in HAZ during welding.

It should be noted that the aggregation and coarsening of grain boundary carbides after long-term aging not only affect the durable strength of the alloy, but also affect the tendency of HAZ stress cracks. Therefore, the composite addition of carbide-forming elements V+Nb is another aspect of the alloy of the present invention. feature element.

The chemical composition weight % of nickel-chromium-cobalt-molybdenum heat-resistant alloy of the present invention is: Cr: 21-23%; C: 0.05-0.07%; Mn:≤0.3%; Co: 11-13%; Mo: 6.0-9.0%; Ti:0.3-0.5%; Al:0.8-1.3%; W:0.1-1.0%; B:0.002-0.005%; Zr:0.03-0.15%; Nb+V:0.2-0.6% and Nb0.01-0.05% ;Cu:≤0.15%;P:＜0.008%;S:＜0.002%;N:≤0.015%;Mg:0.005-0.02%;Ca:≤0.01%;As:≤0.01%;Pb:≤0.007%; Bi: ≤0.001%; the balance is nickel and unavoidable impurity elements, and the parameter Y1 shown in the following formula (1) is between 0.536-0.754, and the parameter Y2 shown in the formula (2) is 11- Between 40.

Y1＝10×C+5×B+0.5×Zr+P+S…..(1)

Y2＝(4V+Nb)/C.....(2)

The symbol of the element in the formula represents the content of the weight % of the element.

The reasons for limiting the composition of the heat-resistant alloy of the present invention are as follows:

Newly added or removed elements:

Zr: The characteristic element of the present invention, not only forms B-Zr composite strengthening to improve the durable strength, but also has a strong affinity with S, which can be used as a S scavenger to reduce the harm of S elements. In nickel-based heat-resistant alloys, adding B and Zr at the same time has a better effect on improving its performance. B and Zr mainly exist on the grain boundary, and their functions can be considered in three aspects: one is to improve the grain boundary structure, that is, B and Zr atoms are enriched on the grain boundary, which will fill the vacancies and lattice defects at the grain boundary , to slow down the diffusion process of grain boundary elements, reduce the speed of dislocation climbing, thereby improving the durable strength of the alloy; second, B and Zr can be distributed around M ₂₃ C ₆ to inhibit the early aggregation of carbides and delay the formation of grain boundary cracks The third is that the B and Zr distributed on the grain boundary can change the interface energy, which is conducive to changing the shape of the second phase on the grain boundary, making the second phase morphology easy to spheroidize, and improving the grain boundary strength, that is, improving the transgranular state of the alloy. Transition to intergranular fracture temperature. The present invention requires the compound addition of Zr and B, and the Zr content of the present invention is 0.03%-0.15%.

Nb+V: The composite addition of Nb-V is another characteristic element of the alloy of the present invention. Nb and V are not only carbide forming elements, but also can enter γ′ and replace part of Al and Ti, promote γ′ phase forming elements, and delay the growth process of γ′ phase aggregation. Studies have shown that NbC and VC precipitated at high temperatures have higher stability than M ₂₃ C ₆ and M ₆ C carbides, and are evenly dispersed, not easy to aggregate and coarsen, improve the creep strength of the alloy, and prevent welding cracks Tendency to have better resistance. V itself has a lower thermal diffusivity than Co and Cr in the nickel matrix, which is beneficial to improving the high temperature creep strength of diffusion creep alloys. On the other hand, Nb and V have oxidation resistance to damage the alloy, especially cyclic oxidation, so the composition range must be strictly controlled. The range of Nb+V elements in the alloy of the present invention is 0.2-0.6%.

W: The distribution ratio of W in the γ and γ′ phases is 1:0.88, respectively. When discussing the functions of W and Mo, people often pay attention to their commonality, but careful comparative studies show that the functions of these two elements are not equivalent. W has a lower thermal diffusivity than Mo element, and the solid solution strengthening effect is stronger. W tends to segregate in the dendrite stem region during solidification, while Mo tends to segregate in the interdendritic region. The ability of W segregated in the dendrite stem and C segregated between the dendrites to form M ₆ C during the solidification process is lower than that of Mo segregated between the dendrites, which can significantly reduce the residual stress caused by carbide aggregation during the welding solidification process and improve the welding performance. performance. Simultaneously, when W and Mo are added with an equal atomic percentage, the tendency of W to form a μ phase is significantly smaller than that of Mo, but too much W element will form Laves harmful phase, which affects the stability of the structure and the impact toughness after long-term aging. The present invention The alloy contains 0.1-1.0% W element.

Mg: Mg is an element that is segregated at the grain boundary. When added to the superalloy, it can mainly play the following roles: (1) form a high melting point compound MgS with harmful impurity elements such as S, etc., to purify the grain boundary and make the grain boundary The concentration of S, O, P and other impurity elements is significantly reduced, reducing their harmful effects, further purifying and strengthening the grain boundaries. (2) Improve and refine grain boundary M ₆ C carbides, make them distributed in granular form, effectively inhibit grain boundary sliding, reduce grain boundary stress concentration, and prevent intergranular crack propagation; (3) Enter γ′ phase and carbide , increase the long-range order of the γ′ phase and the antiphase domain boundary energy; (4) improve the creep fracture plasticity and life, Mg segregates to the grain boundaries of the alloy, and continuously segregates to the pores with the formation of creep pores The surface reduces the surface energy of the pores, thereby reducing the growth rate of the pores. Because the burning loss of Mg is relatively serious, the yield is relatively low, and there is an optimum content range of Mg in the alloy, so the content of Mg in the present invention is controlled at 0.005-0.02%.

Si: Harmful elements in superalloys, enriched in grain boundaries, reduce grain boundary strength, and Si will promote the precipitation of σ phase and Laves phase, especially in the process of welding solidification to promote the precipitation of Laves phase, expanding the temperature range of solidification liquid , easy to form welding solidification cracks. Therefore, no Si element is added in the present invention, but the smelting residual Si content is limited to Si≤0.03%.

Fe: The matrix of nickel-based heat-resistant alloy is a single austenite phase. Fe element is not an austenite-forming element. Adding Fe will seriously damage the high-temperature performance and deteriorate the stability of the structure. At the same time, Fe will form spinel FeCr ₂ O ₄ , which reduces the integrity of α-chromium oxide, thereby reducing high temperature corrosion resistance. Excessive Fe will also lead to the formation of harmful TCP phase or Laves phase, so Fe is not added in the present invention, and the Fe content in the charge during smelting is strictly controlled within 0.05%.

The role of original elements (referring to the original elements in Inconel617 alloy):

C: C in heat-resistant alloys mainly forms carbides, and through the granular discontinuous carbides precipitated at the grain boundaries during the aging process, it can prevent sliding along the grain boundaries and crack expansion, and improve the durable life. Excessive C content will form excessive carbides, which will increase the susceptibility to HAZ liquefaction cracks and stress relaxation cracks of the alloy. In the present invention, the C content is controlled at 0.05-0.07%.

Co: It is mainly dissolved in the γ matrix, and a small amount enters the γ′ phase, and the distribution ratio between the γ and γ′ phases is 1:0.37. The main function of the Co element is to strengthen the matrix by solid solution, which can reduce the stacking fault energy of the γ matrix, the stacking fault energy decreases, and the probability of stacking faults increases, making it more difficult for the cross-slip of dislocations, so that the deformation A larger external force is required, which is manifested as an increase in strength; and the stacking fault energy is reduced, the creep rate is reduced, and the creep resistance is increased. At the same time, Co element can also reduce the solubility of γ′ forming elements Ti and Al in the matrix, thereby increasing the number of γ′ precipitated phases in the alloy and increasing the dissolution temperature of the γ′ phase. These effects have an effect on improving the creep resistance of the alloy. significantly. In addition, in polycrystalline alloys, Co can also increase the solubility of Cr, Mo, W, and C in the γ matrix, reduce the precipitation of secondary carbides, and improve the morphology of grain boundary carbides. Generally, 10-20% of Co is added to nickel-based superalloys. In the present invention, the content of Co is controlled at 11-13%.

Cr: Cr is an indispensable alloying element in nickel-based heat-resistant alloys. Its main functions are as follows: 1) Anti-steam oxidation and hot corrosion elements: Cr forms Cr ₂ O ₃ dense oxidation during the service process of superalloys The film protects the surface of the alloy from oxidation and hot corrosion caused by O, S, and salt. At present, the Cr content of alloys with better thermal corrosion resistance is generally higher than 15%; the Cr content of 700°C steam oxidation resistance is generally higher than 20%. 2) Solid solution strengthening: Cr in the superalloy γ matrix causes lattice distortion, resulting in elastic stress field strengthening, which increases the strength of γ solid solution. (2) Precipitation strengthening: mainly M ₂₃ C ₆ carbides, the carbides are mainly distributed at the grain boundaries, and the granular discontinuous carbides evenly distributed at the grain boundaries can effectively organize the grain boundary slip and migration, thereby increasing the creep strength of the material. On the other hand, the harmful effect of high Cr promotes the formation of σ phase, which destroys the long-term stability of the alloy structure. Based on the above considerations, in order to ensure the anti-steam oxidation and hot corrosion performance and strength at 700 °C, the Cr content range is controlled at 21-23%.

Mo: Mainly enters the γ matrix for solid solution strengthening. The distribution ratios of Mo in the γ and γ′ phases are 1:0.33, respectively. Its atomic radius is quite different from that of Ni, and the addition of these elements can improve the interatomic bonding force, increase the recrystallization temperature and diffusion activation energy of the alloy, thereby effectively improving the durable strength of the alloy. Mo is also a carbide-forming element, mainly forming M ₆ C carbides, and the granular M ₆ C carbides distributed along the grain boundaries play an important role in improving the high-temperature durability of the alloy. However, the segregation coefficient K value of Mo is less than 1, and it is easy to segregate between dendrites during solidification, and combine with C segregated here to form M ₆ C carbides. Excessive carbide aggregation will cause excessive local residual stress. Weld solidification cracks appear. In addition, Mo is easy to promote the formation of harmful phases of TCP, mainly forming μ phase, and a higher Mo content has an adverse effect on the coal ash corrosion resistance of the alloy. Based on the above considerations, the content of Mo element in the alloy of the present invention is controlled to be 6.0-9.0%.

Al: The main element forming the γ′ phase, the distribution ratio between γ′ and γ′ is 1:0.24. Al and Al are important elements to improve the surface stability of alloys. It is generally believed that high Al is beneficial to improve the oxidation resistance of alloys. However, when the amount of Al exceeds the upper limit, harmful β-NiAl phase may appear. The Al content of the alloy of the present invention is limited to 0.8-1.3%;

Ti: The main element that forms the γ' phase, and the distribution ratio between the γ' and γ' phases is 1:0.1. In the γ′ phase, Ti can replace part of Al, reduce the solubility of Al, and promote the precipitation of γ′. Ti is also an important element to improve the stability of the alloy surface. It is generally believed that high Ti is beneficial to improve hot corrosion resistance. However, when the amount of Ti exceeds the upper limit, a harmful phase of η-Ni ₃ Ti may appear. Ti is also a carbide forming element and promotes the formation of MC carbides. The content of Ti in the alloy of the present invention is controlled at 0.3-0.5%.

Mn: A small amount of Mn added to the superalloy melt can be used as a refining agent to generate MnS through the chemical reaction of Mn and S, reducing the harmful effect of S. Mn can improve the hot workability, high temperature corrosion resistance and welding of nickel-based alloys, etc. It's all about it. Adding less than 0.93% Mn to HastelloyX alloy can improve the welding performance. But in general, Mn is a harmful element in the alloy, and Mn will also segregate to the grain boundary, weaken the bonding force of the grain boundary, and significantly reduce the durable strength. Therefore, in the present invention, the Mn content is controlled at Mn≤0.3%.

B: B is the most widely used grain boundary strengthening element in superalloys. B has the most obvious impact on the durability and creep properties of superalloys, and usually has an optimal content range. Its solubility in the γ phase is extremely low, and it does not enter the γ′ phase. The B segregated between the grain boundaries and dendrites fills the gaps in these regions as an interstitial element, slowing down the diffusion process, thereby reducing cracking between grain boundaries and dendrites In addition to the tendency, it also delays the coarsening rate of carbides, and the compound addition of Zr element has a better effect. In the present invention, the B content is controlled at 0.002-0.005%.

S: Although S can be infinitely soluble in liquid nickel, its solubility in solid state is very small, and it is easy to form a low-melting point grain boundary eutectic phase, which greatly deteriorates the hot workability and high-temperature durability of the alloy. Generally, the S content in the alloy is less than 0.008%, but when the Zr element is added to the alloy, the nickel-based alloy is easy to form ZrS low-melting point compound at the grain boundary and dendrite area at the end of welding solidification. When the S content is lower than 1ppm, it can completely avoid B, The influence of Zr element on welding performance, but the cost of smelting is increased. Therefore, under the conditions of existing smelting technology, the lower the S content, the better.

P: P is an element that harms liquefaction cracks at the HAZ, and the lower the content, the better. Nickel-based alloy tubes for power stations mostly adopt advanced vacuum induction and vacuum self-consumption smelting dual or triple process, and the P content is completely controllable to meet the requirements.

In addition, the lower the five harmful elements, the better, and the content of hydrogen and oxygen should be strictly controlled to keep them as low as possible. Low hydrogen and oxygen content plays an important role in formulating the production process and ensuring the final performance of large-diameter pipes.

2. The smelting and manufacturing process of the heat-resistant alloy C-HRA-3 pipe of the present invention:

(1) Smelting: VIM+VAR or VIM+ESR or VIM+ESR+VAR smelting process can be used, or other suitable process can be used for smelting.

(2) Hot working performance parameters of the alloy:

The homogenization annealing process for smelting alloy ingots (or electrode rods) is 1190-1230°C, and the annealing time is determined according to the size of the ingot. After annealing, steel ingots or electrode rods are made of steel pipes by hot extrusion or skew rolling and piercing.

Fig. 1 is the true stress-true strain curve of the alloy of the present invention, when the deformation temperature is lower than 1100°C, the deformation resistance increases sharply. Fig. 2 is a high-temperature plasticity diagram of the alloy of the present invention after homogenization annealing. Figure 3 is the recrystallization diagram of CN617 alloy after homogenization annealing. When the temperature is lower than 1050℃, the recrystallization rate is very low. Considering the above considerations and the heat of deformation in the extrusion process, the optimum temperature range for deformation heat processing of the alloy of the present invention is 1100°C-1200°C.

(3) Process method of large-diameter thick-walled tubular tube insulation sheathing process: Since the optimal thermal processing temperature range window of the alloy ingot billet of the present invention is narrow (100°C-150°C), the billet size is relatively large, and the contact time between the billet and the extrusion cylinder Longer, if the thermal insulation treatment is not carried out, the temperature will drop too fast during the thermal processing, lower than the optimum deformation temperature, and surface wrinkles will appear in the billet making process (another patented treatment), which will seriously affect the alloy yield and product Quality, so the alloy tube of the present invention needs to be heat-insulated during billet and extrusion, so as to isolate the heat transfer between the billet and the extrusion cylinder. When the alloy billet of the present invention is made, the thermal insulation sheathing process adopts thermal insulation cotton and thin steel plate composite sheathing treatment: the thermal insulation material is a commercially available aluminum silicate ceramic fiber blanket, with a thickness of 12mm; the thin steel plate is No. 45 carbon steel, with a thickness of 3mm. The heat preservation method of the alloy pipe of the present invention is only wrapped with aluminum silicate ceramic fiber blankets, and covered with two layers of aluminum silicate ceramic fiber blankets. The billet ingot that is about to be prepared is coated before heating, and then quickly coated after it is out of the furnace. layer. Fig. 4 is a three-dimensional metallographic photograph of the 1/2 wall thickness of the heat-resistant alloy large-diameter thick-walled boiler tube in the extrusion state of the present invention.

3. The optimal heat treatment process of the heat-resistant alloy C-HRA-3 pipe of the present invention:

Studies have shown that the grain size not only affects the durable life of the alloy, but also affects its weld crack susceptibility. The larger the grain size, the higher the durability, and the poorer the solderability. When formulating the solution treatment temperature, the grain size and the resolubility of precipitated phases are mainly considered, but the optimal heat treatment process of the alloy of the present invention also considers the requirements of welding performance. The changes in grain size after different solution temperatures and holding times are shown in Figures 5-7, and the solid solution of precipitated phases and the precipitation of carbides at grain boundaries are shown in Figures 8-10. When the solution temperature is about 1150℃, the grain size is small and there are more undissolved precipitates; when the solution temperature is 1175℃, the grain size is appropriate and contains a small amount of undissolved carbides at grain boundaries.

To sum up, 1175°C±10°C/water quenching (the holding time can be adjusted according to the size of the pipeline) is selected and determined as the best heat treatment system for the heat-resistant alloy of the present invention, and the number of grain size grades is 3-5.

The performance of the heat-resistant alloy boiler tube of the present invention:

The mechanical properties of the heat-resistant alloy boiler tube of the present invention produced on an industrial scale according to the above-mentioned optimal composition design, optimal thermal processing technology and optimal heat treatment process are:

Mechanical properties at room temperature: when the test temperature is 23°C, sampling along the longitudinal direction of the pipeline, R _m (σ _b )≥750MPa; R _p0.2 (σ _0.2 )≥310MPa; A (δ _0.5 )≥60%; Z (Ψ)≥ 63%. Sampling along the transverse direction of the pipeline, R _m (σ _b ) ≥ 740MPa; R _p0.2 (σ _0.2 ) ≥ 305MPa; A (δ _0.5 ) ≥ 58%; Z (Ψ) ≥ 60%.

Impact performance at room temperature: when the test temperature is 23°C, the impact energy A _KV of samples taken along the longitudinal direction of the pipeline is ≥260J; the impact energy A _KV of samples taken along the transverse direction of the pipeline is ≥250J; the impact toughness A _KV ≥65J after aging for 8000 hours at 700°C; It is a Charpy V-shaped incision.

High-temperature mechanical properties: when the test temperature is 700°C, samples are taken longitudinally along the pipeline, the tensile strength R _m (σ _b )≥540MPa; the yield strength R _p0.2 (σ _0.2 )≥190MPa; the elongation A (δ _0.5 )≥ 60%; reduction of area Z (Ψ) ≥ 55%. Sampling along the transverse direction of the pipeline, R _m (σ _b )≥535MPa; R _p0.2 (σ _0.2 )≥185MPa; A (δ _0.5 )≥60%; Z (Ψ)≥50%.

Low cycle fatigue performance: when the test temperature is 700°C, the strain waveform is a triangular wave, the cyclic strain ratio is -1, the strain rate is 1×10 ^-3 /s, and the total strain amplitude is 0.5%, the number of fracture cycles N _f is 10000- 15000 times.

Durable strength performance of the heat-resistant alloy C-HRA-3 large-diameter thick-walled pipe of the present invention: 100,000 hours of extrapolated 100,000-hour durable strength at 700°C according to the ASME code is ≥ 140 MPa.

Table 1 summarizes the properties of the alloy of the present invention compared with those in the standard.

Table 1 Alloy performance of the present invention and performance contrast in the standard

Beneficial effects of the present invention: the room temperature mechanical properties, long-term impact properties, high temperature mechanical properties and durable performance of the alloy of the present invention are all higher than the requirements of the 617 alloy in the ASME standard, and also higher than the durable strength of the newly invented CCA617 alloy pipe (document Reported value), more importantly, the HAZ of the alloy of the present invention is resistant to liquefaction cracks and HAZ stress cracks, as well as durable strength and toughness, and has good comprehensive performance. Therefore, the C-HRA-3 heat-resistant alloy of the present invention is satisfied at 700 ° C The actual needs of the construction of ultra-supercritical thermal power units with steam parameters are the preferred materials for related pipelines.

Description of drawings

Fig. 1 is the true stress-true strain curve of the heat-resistant alloy of the present invention.

Fig. 2 is a thermoplastic diagram of the heat-resistant alloy of the present invention.

Fig. 3 is a recrystallization diagram of the heat-resistant alloy of the present invention.

Fig. 4 is a three-dimensional metallographic photo of the heat-resistant alloy large-diameter thick-walled pipe of the present invention in the extruded state at 1/2 wall thickness.

Fig. 5 is a metallographic photo of the heat-resistant alloy of the present invention after solution treatment at 1150°C.

Fig. 6 is a metallographic photo of the heat-resistant alloy of the present invention after solution treatment at 1175°C.

Fig. 7 is a metallographic photo of the heat-resistant alloy of the present invention after solution treatment at 1200°C.

Fig. 8 is a scanning photo of undissolved carbides at grain boundaries of the heat-resistant alloy of the present invention after solution treatment at 1150°C.

Fig. 9 is a scanning photo of undissolved carbides at grain boundaries of the heat-resistant alloy of the present invention after solution treatment at 1175°C.

Fig. 10 is a scanning photo of the grain boundaries of the heat-resistant alloy of the present invention after solution treatment at 1200°C.

detailed description

The present invention is specifically described below through specific examples, but the present invention is not limited by the examples.

Table 2 has listed the typical chemical composition of 12 kinds of comparison alloys, wherein 1# is the heat-resistant alloy C-HRA-3 alloy composition of the present invention, and 2# is the chemical composition optimization control of the alloy of the present invention produced by the General Institute of Iron and Steel Research. Forgings, 3# is Germany Jutta et al. developed the latest CCA617 alloy composition, and 4# is the 617 alloy composition produced by the American Weiman-Gordon Company. Alloys 5# to 12# are alloys whose smelted chemical composition does not meet the conditions specified in the present invention. Except for forgings of 2# alloy, the other 11 alloys are smelted by industrial-scale VIM+VAR process. The homogenization annealing process is 1200 ℃, and the holding time is 30h. After annealing, the steel ingot (or electrode rod) is made of large diameter by vertical hot extrusion method. Tube, the tube size specification is outer diameter Ф460×wall thickness 80mm. The billet making process is treated with thermal insulation cotton and thin steel plate composite wrapping; the hot extrusion molding is wrapped with aluminum silicate ceramic fiber blanket.

Table 2 Comparative examples of heat-resistant alloy (C-HRA-3) of the present invention

The performance test results of the heat-resistant alloy (C-HRA-3) examples 1# to 4# of the present invention are shown in Table 3 to Table 6. It can be seen from the table that the room temperature mechanical properties, high temperature short-term mechanical properties, impact toughness after long-term aging, and long-term durable strength properties of the heat-resistant alloy of the present invention are all better than the prior art alloys.

Table 3 The room temperature mechanical properties of heat-resistant alloy (C-HRA-3 alloy) embodiment of the present invention

Table 4 The 700°C high-temperature short-term mechanical properties of the heat-resistant alloy (C-HRA-3 alloy) embodiment of the present invention

Table 5 Room temperature impact energy (J) of steel heat-resistant alloy (C-HRA-3 alloy) embodiment of the present invention after aging at 700°C

Table 6 Durable fracture time (h) of the heat-resistant alloy (C-HRA-3 alloy) of the present invention under different stresses at 700°C

11 furnaces of alloy pipes with other components except 2# forging in Table 2 were welded with the same specification welding wire SNi6617 (2.4627) and tungsten inert gas shielded (TIG) narrow groove welding with low input energy, and HAZ was performed on each welded joint Liquefaction crack observation, HAZ stress relaxation crack observation, HAZ creep rupture strength observation and toughness evaluation.

Among them, the observation of HAZ liquefaction cracks takes the circular welded longitudinal surface samples of pipes, grinds, polishes and corrodes them, and inspects them with an optical microscope to observe whether there are HAZ liquefaction cracks.

HAZ stress relaxation cracks and toughness are subjected to a 1000h aging test on a simulated 700°C steam parameter environment test bench. Before the welded joint is tested on the machine, a flaw detection inspection is performed, and the welded parts are tested again. HAZ stress relaxation cracks The longitudinal surface of the annular welded joint is taken, and after grinding, polishing, and corrosion, it is inspected by an optical microscope to observe whether there are HAZ stress relaxation cracks; a V-shaped notch test sample is taken from the welded joint and tested at room temperature. Impact test to evaluate toughness.

HAZ Endurance Fracture Strength Evaluation Obtain enduring strength samples at welds, and carry out enduring strength tests under the conditions of 700°C and 300MPa.

The above HAZ performance research test results are shown in Table 7. It should be noted that "no" in Table 7 means that no cracks are found; "yes" means that cracks are found; "high" means that the enduring strength is good; "low" means that the enduring strength is low; stipulated conditions.

Table 7

It can be seen from Table 7 that no liquefaction cracks and stress cracks in the HAZ were found in the finished pipe with an outer diameter of Ф460 mm and a wall thickness of 80 mm produced by the large-scale industrial production of the 1# alloy with optimized chemical composition, and the enduring strength and toughness after long-term aging It is also excellent, especially suitable for the installation, construction and service requirements of pipelines related to ultra-supercritical thermal power units with 700°C steam parameters.

Claims (3)

1. A nickel-chromium-cobalt-molybdenum heat-resistant alloy, characterized in that, the chemical composition weight % is: Cr:21-23%; C:0.05-0.07%; Mn:≤0.3%; Co:11-13%; Mo :6.0-9.0%; Ti:0.3-0.5%; Al:0.8-1.3%; W:0.1-1.0%; B:0.002-0.005%; Zr:0.03-0.15%; Nb0.01-0.05%; Cu: ≤0.15%; P: <0.008%; S: <0.002%; N: ≤0.015%; Mg: 0.005-0.02%; Ca: ≤0.01%; As: ≤0.01%; Pb: ≤0.007%; Bi: ≤0.001%; the balance is nickel and unavoidable impurity elements; And the parameter Y1 shown in the following formula (1) is between 0.536-0.754, in addition, the parameter Y2 shown in the formula (2) is between 11-40: Y1＝10×C+5×B+0.5×Zr+P+S...(1) Y2=(4V+Nb)/C...(2) The symbol of the element in a formula represents content by weight% of the said element. 2. nickel-chromium-cobalt-molybdenum heat-resistant alloy according to claim 1, is characterized in that, Mechanical properties at room temperature: when the test temperature is 23°C, samples are taken longitudinally along the pipeline, R _m (σ _b )≥750MPa; R _p0.2 (σ _0.2 )≥310MPa; A(δ _0.5 )≥60%; Z(Ψ)≥ 63%; horizontal sampling along the pipeline, R _m (σ _b )≥740MPa; R _p0.2 (σ _0.2 )≥305MPa; A(δ _0.5 )≥58%; Z(Ψ)≥60%; Impact performance at room temperature: when the test temperature is 23°C, the impact energy A _KV of samples taken along the longitudinal direction of the pipeline is ≥260J; the impact energy A _KV of samples taken along the transverse direction of the pipeline is ≥250J; after aging for 8000 hours at 700°C, the impact energy A _KV ≥65J; Charpy V-shaped incision; High-temperature mechanical properties: when the test temperature is 700°C, samples are taken along the longitudinal direction of the pipeline, the tensile strength R _m (σ _b )≥540MPa; the yield strength R _p0.2 (σ _0.2 )≥190MPa; the elongation A(δ _0.5 )≥ 60%; reduction of area Z(Ψ)≥55%; transverse sampling along the pipeline, R _m (σ _b )≥535MPa; R _p0.2 (σ _0.2 )≥185MPa; A(δ _0.5 )≥60%; Z( Ψ)≥50%; Low cycle fatigue performance: when the test temperature is 700°C, the strain waveform is a triangular wave, the cyclic strain ratio is -1, the strain rate is 1×10 ^-3 /s, and the total strain amplitude is 0.5%, the number of fracture cycles N _f is 10000～ 15000 times. 3. A process for manufacturing pipes using the heat-resistant alloy claimed in claim 1, characterized in that the process and its control technical parameters are: (1) Smelting: use VIM+VAR or VIM+ESR double or VIM+ESR+VAR triple process for smelting, or use other suitable processes to smelt steel ingots or electrode rods; (2) Thermal processing: the homogenization annealing process is 1190-1230 ° C, and the steel ingot or electrode rod is manufactured by vertical hot extrusion after annealing, and the thermal processing temperature range is 1100-1200 ° C; (3) Thermal insulation wrapping should be carried out during billet making and hot extrusion molding; thermal insulation cotton and thin steel plate composite wrapping should be used for billet making; only thermal insulation cotton wrap should be used for pipe extrusion; (4) Heat treatment system: solution treatment temperature is 1165-1185°C, water quenching and cooling, to ensure that the grain size level is 3-5; Durable strength performance of the large-diameter thick-walled pipe: 100,000 hours of extrapolated 100,000-hour durable strength value ≥ 140MPa at 700°C according to ASME specifications; It is used for boiler tubes of ultra-supercritical thermal power units with steam parameters of 700°C.