Application and Characteristics of Low-Carbon Martensitic Stainless Steels on Turbine Blades

Hwa-Teng Lee1, Feng-Ming Liu2 and Wun-Hsin Hou3,+

1Department of Mechanical Engineering, National Cheng Kung University, No. 1, University Road, Tainan 701, Taiwan, R. O. China 2Department of Business Administration, Hsing-Kuo University, Taiwan, No. 600, Sec. 3, Taijiang Blvd., Annan District, Tainan, Taiwan, R. O. China 3R&D Center, Gloria Material Technology Corp., No. 35, Hsin Chung RD, Hsin Ting District, Tainan City, Taiwan 730, R. O. China

410M1 (0.17%C-11.6%Cr-0.18%Nb) and 410M2 (0.17%C-10.2%Cr-0.38%Nb-0.84%Mo-0.2%V-0.05%N) martensitic stainless steels are modiﬁed from the basic martensitic stainless steel 410 (0.12%C-12%Cr). They contain Nb and are utilized in the blades of turbines for generating power. This study investigates the heat treatment characteristics, microstructure and secondary hardenability of 410M1 and 410M2. The precipitation hardening of 410 occurs at 400°C but that of 410M1 or 410M2 occurs earlier at 300°C. The peak hardening of 410 occurs at 450°C but that of 410M1 or 410M2 occurs at 500°C. Clearly, addition of Nb improves the mechanical properties of steel at high temperature. Under quenching conditions, 410M1 and 410M2 are lath martensites. 410M2 contains not only Nb but also Mo, V, and N, which improve its secondary hardenability over that of 410M1. From the characteristic chart of quenching and tempering, the tempering softening and the increase in impact toughness of 410M2 are delayed as a high tempering temperature range of 650°C to 670°C is reached. This phenomenon is observed by FE SEM and proves that NbC-carbide with 20³40 nm are precipitated in the matrix. This investigation studies the effect of alloy design on its toughness, secondary hardenability, microstructure and applications. [doi:10.2320/matertrans.M2014307]

(Received August 25, 2014; Accepted January 14, 2015; Published February 20, 2015) Keywords: martensitic stainless steels, turbine blade, precipitation hardening

1. Introduction The addition of 0.5 mass% Nb to ferritic stainless steels that contain 1718 mass% Cr was studied. The steels thus Stainless steels combine superior resistance against high- formed were found to have a higher wear resistance than temperature corrosion with favorable mechanical properties; other steels that contain 1 to 3 mass% Nb, because form the they are often used at high temperature, along with super- smallest NbC particles and have both higher hardness and alloys. Stainless steels for elevated-temperature applications wear resistance.3) In another study of the effect of ageing on have various grades, including ferritic grade, martensitic the creep properties of 20Cr-25Ni-Nb steel, constant load grade, austenitic grade, precipitation-hardening grade, valve creep tests at 225 MPa performed on a material with two steels, and cast heat-resistant alloys. microstructures. Comparison of the creep behavior of Based on their strength and heat resistance, martensitic material that was aged for 500 h with that aged for 10,000 h stainless steels can be grouped as follows.1) revealed an increase in creep rate and ductility and a Group 1: Type 410, 403 and 416 (least strength and heat reduction in rupture life. The properties change with aging resistance) because the coarsening of a fine Nb(CN) matrix precipitates Group 2: Greek Ascoloy and type 431 increases the interparticle spacing and reduces the resistance Group 3: Moly Ascoloy and M152 to recovery, increasing the creep rate.4) Only fine Nb(CN) can Group 4: H-46 and type 422 (greatest strength and heat effectively improve creep properties. resistance) However, few studies of the applications of martensitic The addition of Nb to form H-46 improves its high- stainless steels that contain Nb have been performed. These temperature creep. Nb is a strong carbide-forming element, steels are widely used in the manufacture of power generation and small amounts of Nb are typically added to low alloy turbine blades. steels to improve their mechanical properties. In ferritic, Low-carbon martensitic stainless steels that contain Nb are austenitic and martensitic steels, the addition of Nb also used in turbine blades because Nb improves strength and improves for various purposes. For example, in 347, Nb is creep characteristics at elevated temperatures. International added to form Nb carbide and to prevent the precipitation of standards describe steel grades but the makers of turbine chromium carbide at grain boundaries, thereby improving blades modify their chemical compositions to stabilize their corrosion resistance.2) 18Cr-10Ni austenite has high nitrogen performance and improve service life. In this work, 410M1 content, and some studies have found that Nb interacts with and 410M2, which are modified martensitic stainless steels carbon and nitrogen more easily than does Ti, Zr and V, and that contain Nb, are used in power generation turbine blades thus forms Nb nitrides. Nb nitrides are finer than the nitrides and their microstructures and mechanical properties are that are formed with Ti, Zr and V, and they fix many studied. dislocations, effectively increasing yield and strength. Low-carbon martensitic stainless steels are used in various industries because of their low cost, high strength and +Corresponding author, E-mail: [email protected] toughness. Type 410 is basic, general-purpose stainless steel 564 H.-T. Lee, F.-M. Liu and W.-H. Hou that is used for bolts, pump shafts, and steam valves, at Table 1 Chemical composition of tested specimens (unit: mass%). room temperature. At high temperatures, this steel exhibits Steel 5,6) CMnSiP S NiCrMoNbV N favorable corrosion resistance and strength and so is Grade utilized to make turbine blades that will not be operated at Max. Max. Max. Max. Max. 11.5³ 410 ® ®®®® excessive temperatures. 0.15 1.0 1.0 0.04 0.03 13.5 With respect to the mechanical requirements of turbine 410M1 0.17 0.52 0.48 0.016 0.002 ® 11.6 ® 0.18 ®® blades, when the blades will be exposed to temperatures 410M2 0.17 0.39 0.39 0.020 0.008 0.46 10.2 0.84 0.38 0.20 0.05 above about 480°C for only a short period, their mechanical design need only take into account their short-term tensile properties. At the temperatures that exceed about 0.4 times the melting point of the material, the full range of effects tempering temperature is increased to 650°C. The amount become evident. Blades on spinning rotors in turbine engines of M7C3 carbide decreases as the tempering temperature slowly grow during operation and must be replaced before increases. At higher tempering temperatures, the finer M7C3 they touch the housing. When the operating temperature is to carbide aggregates to become coarse Cr23C6 carbide. exceed 480°C, the design process take into account such Tempering softening then reduces the hardness.12) properties as creep rate, creep-rupture strength, creep-rupture M2C carbide is usually absent from a Cr-C system but ductility, and creep-fatigue interaction. material that contains 12³17 mass% Cr forms M2C carbide Among metallic heat-resistant materials, 410 do not during tempering. This carbide is the intermediate phase 1) perform so well: its tensile strength and stress rupture of (Fe, Cr)3C before it is transformed to (Fe, Cr)7C3. The strength at elevated temperature are inadequate for its use in addition of Mo, N and V stabilizes the carbide and increases turbine blades at high temperatures. secondary hardening.9) Tempering in the appropriate temper- The basic steel 410 should be used at temperatures of less ature range causes secondary hardening by the precipitate than 400°C, but when some alloying elements such as V, Nb strengthening of M7C3 carbide. The elements Mo, W, Ti and and N are added to it, it can be used at temperatures up to Nb delay the coarsening of M7C3 carbide with increasing approximately 650°C. In martensitic stainless steels that temperature and bring forward the formation of M2C carbide, contain 13% chromium, adding just about 0.05 mass% Nb increasing secondary hardening.13) only slightly increases tempered hardness at a tempering 410M1 and 410M2 are formed by modifying 410 by temperature of below 600°C, but the increase is much greater adding the alloying elements of Nb, Mo, V and N to improve at 650°C, because fine NbC carbides precipitate in the matrix, their high-temperature mechanical properties. Most turbine and these carbides retard the recovery of dislocations.7) Nb- makers use 410M1 in the lower-temperature stage and containing alloys such as H-46 typically have a favorable 410M2 in the higher-temperature stage. Table 1 presents their stress-rupture characteristic (creep resistance) over short chemical compositions: 410M1 is similar to 410Cb but its Nb testing times (100 to 1000 h) but they lose this strength content exceeds that of 410Cb, so its strength at elevated advantage over periods of about 10,000 h or more. The temperature is greater. favorable effects of Nb additions on short-term stress-rupture 410M2 has a higher Nb content than 410M1 and other properties are attributed to the precipitation of finely alloying elements V, Mo and N, which improve its high- dispersed NbC. The favorable effects tend to decline as the temperature strength and creep strength. 410M2 is similar to tempering temperature is increased, and a more coarsely H46 but contains more carbon and alloying elements and is dispersed precipitate is therefore formed.1) typically used in turbine blades that are used at very high Since Mo can reduce the rate of formation of NbC, temperatures. The high alloying element content facilitates high-strength low-alloy steels are generally formed by the the macro-segregation of carbides, and the production of simultaneous addition of Nb and Mo to reduce the growth coarse grain and duplex grains, which defects are not rate of carbide and to increase its strength.8) acceptable in turbines. Preventing the macro-segregation of V and Nb very easily react with carbon to form fine alloy carbide is critical to the manufacture of 410M2. carbides. At elevated temperatures, these carbides can inhibit 410M1 is tensile-tested at 800°F (427°C) to determine the migration of grain boundaries and thereby improve the whether it meets the mechanical requirements for use in creep properties. The simultaneous addition of Nb and V is turbine blades. 410M2 requires a higher tensile strength and more effective than adding them separately.9,10) must pass a 1200°F (649°C) stress rupture test.14,15) In martensitic stainless steel, nitrogen can increase the high- The increase need for nuclear power plants has led to temperature strength by solid-solution strengthening and increases in the amounts of 410M1 and 410M2 used in turbine secondary hardening. Nitrogen atoms, nitrides or carboni- blades in such plants. To increase the turbine generation trides may be responsible for retarding the motion of austenite efficiency, 410M1 is gradually being replaced by 410M2. grain boundaries at elevated temperature and reducing the size This thesis study compares the thermal characteristics, of grains. In steel that contains 13% Cr, the amount of mechanical properties and micro-structures of 410M1 and chromium in the carbides and their lattice parameters decrease 410M2. as the nitrogen content of the steel or alloy increases, making the finer and more uniformly distributed.11) 2. Experimental At different tempering temperatures, 410 precipitates different carbide structures, such as M3C, M7C3 and M23C6; In this investigation, rolled round bars of 410M1 and the latter forms at 480°C and M3C disappears when the 410M2 with a diameter of 65 mm are used. Table 1 presents Application and Characteristics of Low-Carbon Martensitic Stainless Steels on Turbine Blades 565 the chemical composition. The bars are melted in an electrical air melting furnace and then remelted in the Electro-Slag Remelting Process (ESR). Reducing the oxide inclusion and sulfide contents not only improves the high-temperature brittleness but also reduces the transversal segregation and shrinkage, while refining the micro-structure, to extent the lifetime of the steel at severely elevated temperatures. The experimental procedure involves taken from the rolled bars, the pre-machine specimen size are 55 mm © 11 mm © 11 mm. The quenching lasts for 40 min and is performed at various austenitic temperatures, before forced air is used to cool the specimens to room temperature. Holding time in tempering is 80 min. Following heat treatment, to ensure that the test data are accurate, specimens are ground using a face grinding machine to remove the decarburized layer to a thickness of approximately 0.5 mm from each face, and then a 45° notch is machined to form a Charpy impact specimen. Before the impact test is performed, hardness is tested at both Fig. 1 Hardness of 410M1 after quenching and tempering. ends of each impact specimen. Micro-structural specimens are an impact specimen and ground using #180, #600 and #1000 sandpapers. After grinding, the specimens are polished using 3 µm and 1 µm diamond paste and etched using Vilella to observe their microstructures.

3. Results and Discussion

3.1 Quenching and tempering hardness The tempering curve of 410 low-carbon martensitic stainless steels reveals that secondary hardening occurs at 400 to 450°C, appearing to reach a maximum around 450°C.9) Figures 1 and 2 plot the tempering curves of 410M1 and 410M2. Secondary hardening occurred at approximately 300°C and the maximum hardness was not reached until around 500°C. When the tempering temperature of 410M1 exceeds the range for secondary hardening, the hardness becomes inversely proportional to the tempering temperature. How- Fig. 2 Hardness of 410M2 after quenching and tempering. ever, the tempering curve of 410M2 from 650°C to 670°C reveals a delay in the drop in hardness owing to the fine precipitation of the NbC carbide. The nano-sized alloy when heat-treated under the same conditions, the hardness carbide inhibits the slippage and climbing of dislocations and of 410M2 exceeded that of 410M1 by about 5 HRC, as exhibits a favorable anti-tempering softening characteristic. presented in Fig. 3. The greatest difference of about 10 HRC As the tempering temperature is increased above 700°C, the appeared following tempering at 650°C³750°C. Since obvious drop in hardness is caused by the growth of NbC 410M2 contained added alloying elements Mo, N, V and 1) carbide and the formation of coarser M23C6 carbide. more Nb than 410M1, 410M2 is better for use at elevated The amount of alloying elements that are dissolved in the temperatures because the alloying elements reduce the size of matrix increases with the quenching temperature. Since, the the carbide, increasing strengthening by precipitation. matrix dissolves more alloying elements, the amount of precipitation is increased, and so the hardness is higher. The 3.2 Micro-structural observation increase in the amount of tempering carbide precipitates The tempering hardness curves in Fig. 3 clearly demon- increases the Ms point and the retained austenite more easily strate the difference between 410M1 and 410M2, whose decomposes. Therefore, the tempering hardness increases cause is revealed by observation of the microstructures. with the quenching temperature. Figures 4(a), (b) and (c) present the microstructure of The specifications of 410M2 require that it undergoes a 410M1 following heat treatment under various conditions, stress rupture test at 650°C. At that tempering temperature, it observed under an optical microscope. Quenching at temper- is strengthened by NbC precipitation and exhibits better high- atures from 1100°C to 1180°C, it yielded lath martensite temperature creep. of medium size. However, quenching 410M2 at 980°C 410M1 did not exhibit a delay in tempering hardness produced extra-fine lath martensite, as presented in Fig. 4(d). softening. The two alloys have the same carbon content but The fine martensite represented a fine austenite grain 566 H.-T. Lee, F.-M. Liu and W.-H. Hou

Fig. 5 Microstructure of 410M1 that was quenched at 1180°C.

microstructure that is presented in Fig. 4(f ) and Fig. 5 reveals no grain growth and the quenching temperature at 1180°C is therefore acceptable. A comparison of the SEM microstructures of 410M1 and Fig. 3 410M2 has higher hardness than 410M1 and exhibits delayed tempering softening at 650°C. 410M2 in Fig. 6 indicates that 410M2 has rounder and coarser residual primary carbides. A comparison with quenching at 1150°C and tempering at various temperatures reveals that tempering at 550°C precipitates slim and rod-like carbides. When the tempering temperature is increased to 650°C, the carbides become coarse and round, and remain highly concentrated and distributed along the martensitic lath. When the tempering temperature is increased to 750°C, the carbides grow into coarser M23C6 carbides. Their concen- tration falls and the carbides become more scattered. Figures 7(d), (e) and (f ) present the microstructures of 410M2. The shapes of the tempering precipitated carbides are similar to those in 410M1 but the carbides are smaller than in 410M1, particularly following tempering at 750°C. Presum- ably, the addition of Mo reduces the rate of diffusion and the rate of carbide formation. 410M2 contains nitrogen, which can reduce the size of the carbide structures and refine the precipitated carbide. FE SEM was used to observe the microstructure of 410M2 that was tempered at 550°C. Figure 7(d) shows no fine NbC carbide precipitate in the matrix. When tempered at 650°C, the matrix contains fine NbC carbide precipitates sizes of 20 to 40 nm, as described in the literature.1) Figures 7(e) and (g) present the microstructures. The nano-carbides are easily Fig. 4 Quenched micro-structures of 410M1 and 410M2. Micro-structure observed following tempering at 650°C and became fewer of 410M2 following quenching at 980°C is extra fine, (a) 410M1 and more scattered as the tempering temperature was following forced air cooling at 1100°C; (b) 410M1 following forced air increased to 750°C. These microstructures reflect the delay cooling at 1150°C; (c) 410M1 following forced air cooling at 1180°C; in tempering softening to temperatures of 650°C to 670°C. (d) 410M2 following forced air cooling at 980°C; (e) 410M2 following forced air cooling at 1150°C; (f ) 410M2 following forced air cooling at 410M2 is strengthened by nano-carbide precipitation. 1180°C. The morphology of a martensitic structure is dominated by its carbon content. When the carbon content is below 0.5 mass%, lath martensite is formed. 410M1 and 410M2 are structure. At elevated temperatures, fine grains improved low-carbon martensitic stainless steels, with the same carbon toughness at room temperature but worsened creeping content and similar amounts of the main alloying element. strength. 410M2 should not be quenched at 980°C, and has Therefore, these steels have the same lath martensite better creep properties than 410M1. The sizes of lath quenched microstructures. The martensitic lath did not martensite following quenching from 1150°C to 1180°C are become too coarse due to over-quenching or too fine due to all similar. under-quenching and 410M1 can be quenched at a temper- The carbide of Nb has a high melting temperature and does ature from 1100°C to 1180°C. However, the martensitic laths not completely dissolve at 1100°C or 1180°C. Therefore, in the microstructure of 410M2 following quenching at some undissolved carbide was found on the grain boundaries 980°C are too fine, indicating the quenching temperature is and in the matrix following quenching at 1180°C. The too low. Comparing the tempered microstructure reveals that Application and Characteristics of Low-Carbon Martensitic Stainless Steels on Turbine Blades 567

Fig. 6 410M2 contains many residual primary carbides after quenching but the tempered carbide is finer than that in 410M1. the precipitated carbide in 410M2 is finer than that of 410M1 The Nb content of 410M2 exceeds that of 410M1 and 410M2 because 410M2 contains more of the added alloying elements also contains Mo, V and N. These elements easily combine Mo, V and N. At a high tempering temperature, these with carbon to form fine carbides, so 410M2 exhibits greater alloying elements exhibit delayed diffusion and they delay precipitation but have lower impact toughness at room the growth of the precipitating carbide. Hence, 410M2 temperature. 410M2 is used at elevated temperatures, so its outperforms 410M1 at elevated temperatures. The micro- creep property is more important than its impact property. structure that is observed by FE SEM demonstrates that the The tempering of 410M1 above 700°C rapidly increases its large amount of NbC carbide precipitated is responsible for impact toughness. Toughness is greatly increased because the delay in tempering softening. M23C6 carbide growth and tempering hardness are clearly reduced as the tempering temperature is increased. 3.3 Impact Test The toughness of 410M2 increases considerably only Figures 8 and 9 presents the results of the impact tests on when it is tempered at the lowest quenching temperature 410M1 and 410M2. When the steel is tempered at 300°C to of 980°C. This fact is explained by the microstructure in 500°C, impact toughness decreases as the tempering temper- Fig. 4(d). The grains are extra fine because the quenching ature increases. The impact toughness was lowest following temperature is minimal and a large amount of undissolved tempering at 500°C to 550°C, and is affected by secondary residual carbide at the grain boundary retards grain growth hardening that is caused by fine alloy carbide precipitation. during quenching. When 410M2 is quenched at 980°C and When the tempering temperature exceeds 600°C, the impact tempered at 700°C, the toughness is improved not only by toughness of 410M1 exceeds that of 410M2. the undissolved residual carbide, which easily becomes The amount of alloy carbide that is dissolved into matrix nuclei of M23C6, but also by the reduction of the dissolution increases with the quenching temperature. As the matrix of alloying elements in the matrix and of precipitation contains more alloying elements when the tempering temper- strengthening. ature is higher, the precipitation of alloy carbide increases the Fine grains improve the toughness of 410M2 which high-temperature strength but negatively affects toughness. can therefore be used under severe conditions. The creep 568 H.-T. Lee, F.-M. Liu and W.-H. Hou

Fig. 7 Increasing the tempering temperature transforms the M7C3 carbide rods into larger M23C6. No NbC carbide is precipitated in 410M1 and 410M2 upon tempering at 550°C. During tempering at 650°C, NbC carbides precipitate in the matrix of 410M2. These carbides form coarser carbide upon tempering at 750°C.

Fig. 8 Impact chart of quenched and tempered 410M1. Fig. 9 Impact chart of quenched and tempered 410M2. Application and Characteristics of Low-Carbon Martensitic Stainless Steels on Turbine Blades 569 resistance is more important than toughness and explains NbC carbide precipitation; the sizes of the carbides why 410M2 cannot be quenched at 980°C. Increasing the are 20 nm to 40 nm. The nano-carbides increase the quenching temperature grows the grains and dissolves more resistance to stress rupture and 410M2 is more suitable alloying elements into the matrix, increasing precipitation for use in turbines at high temperatures than is 410M1. strengthening, and thereby reducing toughness at room (4) NbC carbide precipitation clearly affects the tempering temperature. This effect is particularly evident in the curve from 650°C to 670°C. The temperature of 650°C tempering temperature range of 650°C to 670°C and is at which the stress rupture test is performed falls within caused by nano-carbide NbC precipitation. This phenomenon this temperature range. disappears when the tempering temperature rises above (5) 410M2 acquires an extra fine structure when quenched 700°C, consistent with the tempering hardness curves that are at 980°C. This steel has excellent toughness, but when presented in Fig. 9 and Fig. 2. used at elevated temperatures; its stress rupture property As the tempering temperature raises above 700°C, the is its most important property. Therefore, 410M2 should 410M2 toughness are increases, but remains below that of be quenched at temperatures above 980°C. 410M1, demonstrating that 410M2 is stronger at a high temperature. The toughness of 410M2 is less than that of REFERENCES 410M1 at room temperature, but owing to its better stress rupture properties, 410M2 is more suitable than 410M1 for 1) J. R. Davis: ASM Special Handbook-Heat-Resistant Materials, (ASM use in turbines at high temperature. International, 1997) p. 116. 2) H. Uno, M. Kobayashi, A. Kimura and T. Misawa: Stainless Steel’91, International Conference on Stainless Steel, (ISIJ, 1991) pp. 288294. 4. Conclusions 3) V. Kuzucu, M. Aksoy, M. H. Korkut and M. M. Yildirim: Mater. Sci. Eng. A 230 (1997) 7580. (1) The secondary hardening of 410 steel occurs upon 4) D. J. Powell, R. Pilkington and D. A. Miller: Stainless Steels’84, tempering at 400°C. The maximum hardness is reached Göteborg, (1984) pp. 382390. ’ following tempering at 450°C. 410M1 and 410M2 5) Heat Treater s Guide, Standard Practices and Procedures for Steel, (ASM International, 1988). exhibit not only secondary hardening at 300°C but also 6) G. Krauss: Steels: Heat Treatment and Processing Principles, (ASM a delayed maximum hardness at 500°C, because the International, 1993). addition of Nb increases precipitation hardening and 7) B. Baroux, P. Maitrepierre and B. Thomas: Stainless Steels’84, improves high-temperature tensile strength. Göteborg, (1984) pp. 115123. 8) W. B. Lee, S. G. Hong, C. G. Park, K. H. Kim and S. H. Park: Scr. (2) High-temperature tempering causes 410M2 to precip- fi Mater. 43 (2000) 319 324. itate re ne carbide, improving precipitation strengthen- 9) J. H. Davidson and J. B. Lindquist: Stainless Steel: Translated from ing, causing the hardness of 410M2 to exceed that Original French Version, (les editions de physique, 1993) p. 492. of 410M1 by approximately 5 to 10 HRC. The finer 10) T. Fujita and N. Takahashi: Trans. ISIJ 18 (1978) 269278. carbides cause 410M2 to exhibit greater precipitation 11) H. Leda: Process. Technol. 53 (1995) 263272. strengthening and resistance to tempering softening. 12) J. R. Davis: ASM Special Handbook: Stainless Steels, (ASM Interna- tional, 1997) p. 447. (3) When 410M2 is tempered at temperatures of 650°C to 13) T. Fujita, T. Sato and N. Takahashi: Trans. ISIJ 18 (1978) 115124. 670°C, tempering hardness softening is reduced and the 14) G.E. Material Specification, B50A790 rev C. 2001. rate at which the toughness increases is reduced by fine 15) G.E. Material Specification, B50A365 rev J. 2007.