Download Hardness Model for Creep-Life Assessment of High-Strength Martensitic Steels and more Lecture notes Materials Physics in PDF only on Docsity!
Materials Science and Engineering A 510–511 (2009) 154–
Contents lists available at ScienceDirect
Materials Science and Engineering A
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m s e a
Hardness model for creep-life assessment of high-strength martensitic steels
Fujimitsu Masuyama
Kyushu Institute of Technology, 1-1, Sensui-cho, Tobata, Kitakyushu 804-8550, Japan
a r t i c l e i n f o
Article history: Received 9 January 2008 Accepted 30 April 2008
Keywords: Heat resistant steels Creep Hardness Precipitations Structures Life assessment
a b s t r a c t
The development of creep-life assessment technology for creep-strength enhanced ferritic steels such as Grades 91 and 92 is strongly demanded by power-plant operators. However the degradation and failure mechanisms of these high-strength steels with martensitic structure have not yet been well clar- ified due to the complicated interaction among dislocation structures, precipitates, solute atoms and stress/strain conditions. The simple hardness measurement technique has been extensively applied to assess the material conditions and to detect creep deterioration. In this study based on the hardness changes measured on the specimens creep-tested and thermally aged at various conditions, a concept of strain-induced softening and stress-induced softening in the martensitic steels is proposed and dis- cussed. The creep-softening mechanisms in the martensitic structure which is composed of lath matrix, lath boundaries, block boundaries, packet boundaries, prior-austenite grain boundaries, precipitates and dislocations under the uni-axial or localized multi-axial stress/strain conditions are to be considered to establish a hardness model for creep-life assessment. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
Elevated steam temperature and pressure conditions are essen- tial for increased efficiency in thermal power plants, while frequent shutdowns and startups and/or load swing operations are unavoid- able in response to changes in demand for electric power. Materials having high-creep strength, high-thermal conductivity, and a low expansion coefficient must be selected for high-temperature com- ponents to withstand such strenuous conditions. In consideration of these requirements, high-strength 9–12% Cr steels featuring a number of martensite structures have been developed from the 1980s through to the present, and these have been introduced for practical application. The first Grade 91 steels (9Cr–1Mo–V–Nb), first used in the 1980s, have already experienced high-temperature, high-pressure service for nearly 20 years, and life assessment is an urgent topic. It is also extremely important to predict the remain- ing life and long-term creep strength reliability of high-strength steels containing tungsten, such as the more recently developed Grade 92 steels. While considerable research has been conducted on these subjects, there still remains much to be elucidated. In actual installations, hardness is used to determine and assess degradation for materials currently in service, and while this method is practi- cal, creep degradation and damage phenomena are rather complex, and it is not necessarily the case that hardness alone can be used as a reliable index for degradation. However, from the results of detailed structural observation thus far, it is considered that creep
E-mail address: [email protected].
degradation and damage are directly related to lath structure and precipitate behavior. Also, given that creep degradation and damage can be detected in terms of changes in hardness, the present author proposes a hardness model for creep-life assessment, incorporating precipitate and lath structural changes.
2. Changes in precipitates due to heating and creep
High-strength 9–12%Cr steels use primarily Mo and/or W, V, Nb and N as the alloy elements, and, since they are tempered martensite, as-received hardness is around 220 HV. This hardness declines by only about 5% when subjected to thermal aging for 30,000 h at 650 ◦C, but declines by 20–30% due to creep after more than 10,000–30,000 h M 23 C 6 and Laves can be observed as precipitates at the tempered martensite grain boundaries using optical microscopy, and transmission electron microscopy (TEM) allows observation of metal-metalloid phases (MX) as well. MX is extremely fine, and does not show any major change in particle diameter resulting from thermal aging or creep. In contrast, M 23 C 6 and Laves show coarsening due to thermal aging and creep [1]. Fur- thermore, while the martensite lath is quite fine in the normalized and tempered conditions, coarsening occurs as a result of thermal aging and creep, finally transforming into equi-axis subgrain. In case of the precipitates and precipitation sites observed on creep rup- tured specimen of 9Cr–3W–3Co–V–Nb steel there are four kind of grain boundaries in the structure, i.e., prior-austenite, packet, block, and lath, while prior to the creep test M 23 C 6 is seen at all the grain boundaries and MX is observed at the lath grain boundaries and within the lath. During the creep test, a new Laves precipitate is
0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.04.
seen at the grain boundaries excepting the lath boundary of grip portion (corresponding to thermal aging without any stress load). Since the grain boundary is unclear on the loaded parallel portion after creep, Laves and M 23 C 6 are observed both at the grain bound- aries and within the grains (thought to be the locations of the former grain boundaries). It is known that the sizes of these precipitates change due to heating and creep. According to particle diameter dis- tribution measurement results [2] for MX, M 23 C 6 , and Laves on the grip and parallel portions of 10Cr–1Mo–1W–V–Nb steel, average values on the grip portion were approx. 30 nm for MX, 80 nm for M 23 C 6 , and 350 nm for Laves, while on the parallel portion these were approx. 50 nm, 130 nm, and 400 nm, respectively. The dis- tributions were generally normal, with M 23 C 6 demonstrating the greatest decline in peak height, and with the peak grain diameters for MX and Laves moving in the direction of increase. Looking at the precipitate particle diameter distribution measurement results [3] in the case of thermal aging at 600 ◦C, Laves precipitation is not seen prior to heating, with the distribution occurring due to the heating. With respect to the Laves diameter distribution, the peak location moves rapidly toward the maximum in conjunction with heating time. Specifically, the average particle diameter that is 70 nm at around 1000 h changes to 400 nm at around 33,000 h. In contrast, the average particle diameters of MX and M 23 C 6 , approx. 50 nm and 80 nm respectively, do not exhibit any major changes due to heating. From the foregoing, Laves undergoes considerable coarsening even without the stress, while it can be seen that growth without stress for MX and M 23 C 6 is quite limited, and that even the effect of stress on MX is quite small. In this context, M 23 C 6 appears particularly sensitive to the stress. For tungsten-containing steels, M 23 C 6 and MX precipitates at the grain boundaries, and MX also precipitates within the grains, with Laves precipitating at the grain boundaries only. Precipitation and growth of M 23 C 6 is promoted by the stress, but the contribution of stress to Laves precipitation and growth is slight. The containing of tungsten serves to inhibit the growth and coarsening of M 23 C 6 and MX [4]. Thus, the effects of temperature and time are substantial on Laves precipitation and growth, such that changes are expected to be about the same as for thermally aged material [4]. While M 23 C 6 experiences coarsening at high-temperatures, the effect of heating time is comparatively slight, and this coarsening is considered to accompany the advance- ment of creep life. The fact that precipitation at the grain boundaries is in a near-saturated state from before the creep test is thought to be the cause of the strong dependency on changes in the lath struc- ture that occur due to the stress or the progress of creep. Also, since the growth of M 23 C 6 and MX is inhibited in cases where tungsten is contained as an alloying element, this may actually serve to stabilize the lath structure.
3. Changes in lath structure due to precipitation
Precipitation behavior and lath structure are closely related, and these are deemed to have a major influence on the creep strength and life of high-strength 9–12% Cr steels. Consideration is thus warranted as to how the lath structure changes during creep, and how precipitates are involved. According to comparative study [5] on the structure of 9Cr–1Mo–V–Nb steel with 25% tensile strain applied at 820 ◦C with that of as-received material, as-received (nor- malized and tempered) material has extremely high-dislocation density, with an approx. 1 m lath structure, but deformation at high-temperatures results in dramatic lowering of the dislocation density, and substantial growth in lath size (subgrain size) with coarsening can be seen. Also, while it is thought that precipitates originally existed at the lath boundaries or other grain boundaries, these are dispersed within the grains, and the alignment of precip- itates suggests that they are on the original grain boundaries. This type of structure is also observed after creep [6], and it is consid-
ered to be caused by movement of the lath boundaries or subgrain boundaries during creep. It has already been reported [7] that the size of this lath or subgrains is related to creep resistance, the min- imum creep rate increases in proportion to the cube of the grain size. The lath size is also strain-dependent [8]. This increases pro- portionately to strain until the strain reaches approx. 0.1. It appears that necking occurs in cases where the strain is above approx. 0.1, and, although the lath spacing is shown as a constant value in order to separate the location of structural observation from the necking area, it is considered that there is actually a wide range in which the proportional relationship holds. It is clear that such lath boundary movement or grain growth is induced by the stress and the result- ing strain. Based on consideration [8] of this process, precipitates are strongly bound with the dislocations (serving to increase inter- nal stress) moving as a result of the action of stress and reaching the lath boundaries to form dislocation networks. When these disloca- tion networks are swept by the movement of the lath boundaries, it is thought that they are absorbed, thus increasing the interface energy.
4. Changes in hardness due to lath structure
Considering the growth of the lath structure, given some form of change to the precipitate or interaction between the precipitate and the dislocation, the dislocation could shift away and reach the lath boundary, thus being the direct cause of the boundary move- ment. The lath size would be inversely proportional to the applied stress (lath size being greater with lower stress), and as a result the lath structure or subgrains that had grown would have a major effect on hardness. Especially in the case of ruptured material, it is predicted that hardness will decline with lower load stress. With respect to the subgrain size distribution as well, while a normal distribution applies to cases of high-stress above approx. 100 MPa, the distribution in the case of low stress is characterized by two maxima, and mixing of coarse and fine grains is reported [9]. Hard- ness, on the other hand, is proportional to the square root of the dislocation density remaining after creep deformation, such that a relationship can be established between hardness and subgrain size. Accordingly, because grain growth behavior differs depending on stress, it is predicted that differences will also occur in terms of changes in hardness. As a result of investigation [6] into the influ- ence of subgrain size and dislocation density on the hardness of 9Cr–1Mo–V–Nb steel, it has been clarified that increased subgrain size and reduced dislocation density cause hardness to decline. As subgrain size and dislocation density are directly related to creep resistance, measurement of hardness can be understood to enable estimation of creep resistance.
5. Hardness model for creep-life assessment
Consideration thus far clearly indicates that hardness declines due to creep, and that the amount of decline is strongly related to the creep-life consumption. The high-strength 9–12%Cr steel in the present context also experiences reduced hardness due to ther- mal aging, but the amount of the reduction is small, and is actually extremely slight under the stress conditions. That is, the hardness drop in the creep test materials (interrupted and ruptured mate- rials) is based on structural changes dependent upon stress, and there is no doubt that the assessment of creep life from hardness measurement values has a firm basis in materials science. Fig. 1 presents hardness measurement results for 9Cr–1Mo–V–Nb ther- mally aged or crept specimens of base metal and welds (including interrupted specimens, and using minimum hardness in the heat affected zone for welds), taking the Larson–Miller parameter as a variable. Thermal aging was performed from 550 to 675 ◦C in 25 ◦C
softening under the elastic limit or multi-axial stress conditions take place with greater hardness drop.
6. Conclusion
Research concerning improved reliability of long-term creep strength and life assessment after long-term service of high- strength 9–12%Cr steel is extremely important now that this family of materials is being widely used for high-temperature equipment. Creep strength reliability and material life are highly influenced by structural stability, and factors involved in structural stability should be approached from various directions. Here, hardness is linked to precipitation behavior and lath structural changes, which govern structural stability, and creep-life assessment by means of hardness has been considered. As a result, it was clarified that M 23 C 6 is a precipitate closely connected with creep life, that its growth and coarsening are affected by lath structural recovery and growth due to stress, and that growth accompanies reduced dislo-
cation density, thus reducing hardness. The softening mechanisms due to creep are different depending on stress, with the occurrence of either (a) strain-induced softening at stress above the elastic limit, or (b) stress-induced softening at stress under the elastic limit.
References
[1] H. Cerjak, V. Foldyna, P. Hofer, B. Schaffernak, in: A. Strang, D.J. Gooch (Eds.), Microstructural Development and Stability in High Chromium Ferritic Power Plant Steels, IOM, London, 1997, p. 145. [2] P. Hofer, H. Cerjak, B. Schaffernak, P. Warbichler, Steel Res. 69 (1998) 343. [3] H. Cerjak, P. Hofer, B. Schaffernak, ISIJ Int. 39 (1999) 847. [4] F. Masuyama, N. Nishimura, A. Sasada, CAMP-ISIJ 11 (1998) 1245. [5] F. Masuyama, N. Nishimura, in: H. Oikawa, et al. (Eds.), Strength of Materials, JIM, Tokyo, 1994, p. 675. [6] K. Sawada, K. Maruyama, Y. Hasegawa, T. Muraki, Key Eng. Mater. 171–174 (2000)
[7] A. Orlowa, J. Bursik, K. Kucharova, V. Sklenicka, Mater. Sci. Eng. A245 (1998) 39. [8] T. Endo, F. Masuyama, K.S. Park, Tetsu-to-Hagane 88 (2002) 526. [9] K. Suzuki, S. Kumai, H. Kushima, K. Kimura, F. Abe, Tetsu-to-Hagane 86 (2000)
