High Cr Ferritic Steels Research Articles

Cr concentration dependence of overestimation of long term creep life in strength enhanced high Cr ferritic steels

Creep rupture data and microstructural degradation during aging of high Cr ferritic boiler steels with enhanced creep strength have been studied with special attention to prediction of long term creep rupture life. Tempered lath martensite structure in the high Cr ferritic steels remains unchanged during short term aging, whereas static recovery of the lath martensite structure proceeds when diffusion distance during aging becomes sufficiently long as is the case in long term creep. The static recovery brings about premature failure in long term creep and decreases in apparent activation energy for creep life. The decrease in activation energy is responsible for overestimation of rupture life reported in strength enhanced high Cr ferritic steels. The boundary from a short term region with high activation energy Q H to a long term region with low activation energy Q L moves towards longer time with decreasing Cr concentration. The difference in activation energy ( Q H − Q L) primarily determines the extent of overestimation of rupture life predicted from short term data. In general, the extent of overestimation is less serious at 9%Cr as compared to 12%Cr.

Premature Creep Rupture and Overestimation of Rupture Life in Modified 9Cr–1Mo Steel

Creep rupture life in the time range of 105 h and longer has been evaluated on the basis of creep rupture data available on modified 9Cr–1Mo (Gr.91) steel. The steel is strengthened by a tempered martensite lath structure, which degrades by means of strain induced recovery in short-term creep (Region H). The degradation leads to creep rupture. In long-term creep (Region L), static recovery of the martensite lath structure takes place in addition to the strain induced recovery, resulting in premature creep rupture and the consequent decrease in stress exponent n for rupture life. The premature creep rupture always occurs in Gr.91 steel, but the activation energy QL for rupture life in region L is usually close to the value QH in Region H in the steel. In such a case, long-term rupture life can be predicted correctly by simple extrapolation of short-term data. In some heats of Gr.91 steel, however, another region with low values of n and Q, namely Region L2, appears in the time range longer than 104 h. Simple extrapolation of short-term data results in overestimation of rupture life in Region L2. The static recovery and the consequent premature creep rupture are controlled by migration of sub-boundaries stabilized by precipitates. M23C6 as well as MX precipitates in Gr.91 steel have higher thermal stability than those in other high Cr ferritic steels, resulting in the later appearance or absence of Regions L and L2 in the steel.

Creep properties measurements of heat-affected zones of high Cr ferritic steel by SP creep test

The small punch (SP) creep test using a TEM disk-type specimen (ϕ3 × t0.25 mm) was applied to the fusion zone (FZ), the coarsened grain heat-affected zone (CGHAZ), the fine grain HAZ (FGHAZ), the tempered HAZ (THAZ) and the base metal (BM), respectively, which were removed from the joint of 8Cr–2W–VTa steel welded by an electron beam welding, in order to investigate the creep properties of such local regimes. Experimental results revealed that the SP creep rupture of the FGHAZ was accompanied by creep void formation even if the rupture time was relatively short and its creep rupture strength was inferior to those of the other regimes. This reduction in rupture strength seemed to be attributable to the microstructural changes during the welding thermal cycles, which were represented by the annihilation of clear lath martensitic structure and the change in precipitate distribution.

Change in Hydrogen Desorption Characteristics of Mod.9Cr–1Mo Steel due to Creep

Hydrogen is usually trapped by vacancy, dislocation, grain boundary, precipitate and void in steels and it is easily released from these trap sites by heating the steels. However, the desorption temperature ranges vary according to the kinds of trap sites and they can be separated in principle, because the binding energy between hydrogen and each trap site is different. It seems that this interesting nature of hydrogen is available for determining microstructural changes and defect formation processes during creep. In this study, the change in hydrogen desorption characteristic due to creep was investigated to discuss the applicability of this technique as a nondestructive procedure for evaluating creep damage accumulation of high Cr ferritic steels. The gauge and grip portions of creep ruptured specimens with different rupture times were charged with hydrogen by means of cathodic electrolysis. Subsequently, the thermal desorption spectroscopic (TDS) analysis was applied to those hydrogen-charged samples for measuring hydrogen desorption curve. Experimental results revealed that a clear peak appeared on the hydrogen desorption curve at around 65°C for the as-tempered. The peak temperature and the peak height of the gauge portion increased significantly resulting in the increase in amount of desorbed hydrogen as the creep rupture time increased, although those of the grip portion showed almost no difference. These changes in hydrogen desorption characteristic seemed to be attributable to the formation of geometric damages such as a creep cavity and microcrack rather than the microstructural changes, because the re-normalizing had almost no influence on the hydrogen desorption curve of the gauge portion.

Irradiation-Induced Hardening and Embrittlement of High-Cr ODS Ferritic Steels

Abstract The dependence of irradiation temperature, neutron dose, and Cr content on irradiation hardening and embrittlement has been investigated for oxide dispersion strengthened (ODS) ferritic steels irradiated from 290 to 600°C up to the maximum neutron dose of 0.75 dpa. Irradiation hardening and embrittlement of the materials strongly depend on the irradiation temperature. While neutron dose increases from 0.01 to 0.21 dpa, the ODS steels with Al show a saturation of irradiation hardening but the no-Al ODS steel has an increase in the hardening. The higher the Cr content, the more significant the irradiation hardening and embrittlement of ODS steels irradiated below 420°C. Interestingly, the ODS ferritic steels exposed to neutrons show little change of tensile elongation, compared to 9Cr JLF-1. Furthermore the ductile-brittle transition temperature (DBTT) shift of ODS steels increases with the increasing irradiation hardening, indicating that the increase of yield strength due to radiation induced hardening increases the DBTT and thus a reduction of hardening is effective to reduce the DBTT shift.

Temperature and stress dependence of creep life of welded joints in strength enhanced high Cr ferritic steels

AbstractOverestimation of creep life is a serious problem that happens in long term creep of welded joints made of 9–12%Cr ferritic steels. Creep rupture data of welded joints made of grades 91, 92 and 122 steels were subjected to multiregion analyses paying attention to change in activation energy Q for creep rupture life. A region with a low Q value was found in long term creep of the welded joints. Therefore, long term creep rupture life of the welded joints can be overestimated when it is evaluated by temperature acceleration tests based on a high Q value of short term data. The decrease in activation energy was closely related to the change in fracture location from base metal to heat affected zone. Activation energy and stress exponent for creep rupture life take similar values both in base metal and in the welded joint, though the transition to the low Q region starts at shorter time in cross-welded joints. This fact suggests that creep rupture life can be formulated in a unified manner irrespectiv...

Change in contribution of block boundary to macroscopic strength of 10Cr–1Mo–1W–VNbN steel due to creep

Outstanding strength performance of high Cr ferritic steels is attributable to the combined strengthening mechanisms of matrix and various grain boundaries. However, it is by no means easy to separate the contributions of such strengthening factors and quantitatively understand them because of extremely fine and complicated microstructures. In this study, the instrumented indentation test was carried out to clarify the change in contribution of “block” during creep. The materials used in this study were turbine rotor steel (Fe–10Cr–1Mo–1W–VNbN). The indentation test was applied to the as-tempered and creep damaged specimens under the maximum loads ranging from 1 to 1000 mN. The test results revealed that the decrease in contribution of block was the predominant factor controlling the material deterioration, namely, softening at the early stage of the creep life. This decrease in block's contribution was caused by the decrease in resistance of the block boundary to deformation.

Dependence of Precipitation Behavior and Creep Strength on Cr Content in High Cr Ferritic Heat Resistant Steels

It is known that high temperature tensile strength increases with increasing Cr content in Cr containing heat resistant steels. Recently, however, it was found that long-term creep strength decreased with increasing Cr content in the heat resistant steels containing 8.5-12%Cr. In this study, precipitation behavior of M23C6 carbide and the Z phase after creep tests was investigated using two kinds of high Cr ferritic steels (9Cr and 10.5Cr). As a result, 10.5Cr steel exhibited larger average particle size of M23C6 than 9Cr steel irrespective of creep stress levels, but the amount of M23C6 carbide was almost the same in both steels. On the other hand, the amount of the Z phase became large in 10.5Cr steel compared with 9Cr steel. These experimental results indicate that high level of Cr content accelerates precipitation and coalescence rate of both M23C6 carbide and the Z phase, resulting in degradation of long term creep strength in 10.5 Cr steel compared to 9Cr steel.

高Ｃｒフェライト系耐熱鋼における長時間クリープ強度に対するＲｅの効果

There has been a great demand for superior heat resistant steels in order to raise the thermal efficiency of fossil-fuel power plants and to reduce CO2 emission to the global environment. To this end, by using the d-electrons concept, 9–12% Cr ferritic steels were designed for use of a steam turbine rotor operated in the USC power plants at the steam temperature of 620 to 650°C. The crucial issue for the design is to suppress the deterioration of the long-term creep strength by alloying. First, the Re addition was found to give a beneficial effect on the creep strength of a 10%Cr–4%W steel. Then, the creep tests were performed with the six Re-free and 3.5% W ferritic steels to get an optimum Cr content in the range of 8.5 to 11.5%. As the result, it was found that an excess amount of Cr yielded a detrimental effect on the creep properties, and the 9% Cr steel was the best in view of the very long-term creep strength tested in the condition of 650°C, 98 MPa. Subsequently, a series of creep tests was conducted with the steels by fixing at 9% Cr but by varying the W content from 2 to 4% and the Re content from 0 to 0.5%. From the prolonged creep tests for more than 40,000 h, it was shown that the 9Cr–4 W–0.5Re steel had the longest creep rupture life among all the high Cr ferritic steels so far developed in the world.

Microstructures and Type-IV Creep Damage of High Cr Steel Welds

Creep strength of welded joints in high Cr steels decreases due to the formation of Type-IV creep damage in the heat-affected zone (HAZ) during long-term use at high temperatures. This paper aims to elucidate the processes and mechanisms of Type-IV failure. Creep tests for the welded joints with different groove configurations of Mod.9Cr-1Mo steel were conducted. Distributions of Type-IV creep damages in HAZ of these welds were measured quantitatively, and were compared with FEM computations using damage mechanics analysis. For the welded joints with double U groove, creep voids were observed mostly at 20% below the surface of the plate, and scarcely near surfaces and center of thickness. For the welded joints with single U groove, creep voids were observed inside the plate thickness more than 3mm below the surfaces. From the comparison of experimental damage distributions with FEM analysis, it is considered to be important to take the stress triaxiality into account for the prediction of damage location and fracture life of high Cr ferritic steel welds.

High-temperature strength analysis of welded joint of RAFs by small punch test

Type IV creep damage has recently been a worldwide issue for high Cr ferritic steels. The small punch (SP) creep test has been successfully applied to evaluate this damage of low alloy ferritic steel by the author’s group. However, the heat affected zone (HAZ) of fusion reactor material welded by electron-beam (EB) welding is so thin that it is not easy to evaluate its mechanical properties by the conventional SP test. In this study, the SP test using a further miniaturized specimen was applied to the EB welded joint of reduced activation ferritic steel (RAFs), for evaluating high-temperature tensile properties of the HAZs. As the result, the σ y and σ B of the tempered HAZ at 873 K were estimated to be as low as 275–300 MPa and 325–340 MPa, respectively.

Review of allowable stress and new guideline of long-term creep strength assessment for high Cr ferritic creep resistant steels

A large drop in creep rupture strength in the long-term is a noticeable phenomenon for high Cr ferritic creep resistant steels, since it may result in overestimation of 100,000 h creep rupture strength, and allowable stress. A program for assessment of long-term creep strength and review of current allowable stress of high Cr ferritic creep resistant steels has been conducted in Japan. Provisional rules for revised allowable stress and new life prediction method for those materials have been set up at the beginning of 2005. In this paper, the contents of those rules will be presented.

Microstructural Changes due to Secondary Precipitation Hardening of Martensitic Creep Resistant Steel X20CrMoWV 12 1 (AISI 422)

After development of the well-known T/P91 steel grade in the early 80’s and its long industrial experience since early 90’s, it has been necessary to develop new martensitic creep resistant steels to answer the demand of the power generation industry. New USC (ultra-super critical) boilers require materials with advanced creep properties to reach severe steam parameters. Addition of W to the steel has been found by many researches to be effective to increase creep rupture strength at high temperatures and already used in some developed steel grades such as T/P92, T/P122 and AISI 422 for the USC boilers. Recently, long-term creep strength of the advanced high Cr ferritic steels has been argued regarding the instability of their microstructures at high temperatures over 600 °C. This microstructural instability seems to be enhanced with increasing Cr content or with substitution of Mo by W in the steels. The aim of this paper is concentrated on the investigation of the microstructural development of the studied steel using the Jominy end-face quench test. Different hardness profiles from this test were introduced.

Prevention of the overestimation of long-term creep rupture life by multiregion analysis in strength enhanced high Cr ferritic steels

Long-term creep rupture life is often evaluated from short-term data by a time–temperature parameter (TTP) method. However the conventional TTP methods sometimes fail in understanding creep rupture behavior of strength enhanced high Cr ferritic steels and overestimate creep rupture life in long-term creep. In the present paper, creep rupture data of seven kinds of heat resistant steels with different W and Cr concentrations have been analyzed. The conventional TTP method like Orr–Sherby–Dorn analysis evaluates long-term creep rupture life assuming a unique value of activation energy for all the creep rupture data. This analysis is called single region analysis in this paper. The single region analysis can represent well the creep rupture data of steels containing less than 8% Cr. The creep rupture analyses of steels containing more than 8% Cr exhibit that apparent activation energy changes from a high value in short-term creep region to a low value in long-term creep region. In each case a creep data was divided into several data sets, and then the conventional single region analysis was applied to each divided data set. This analysis is referred to as multiregion analysis. The multiregion analysis describes very well all the data points, whereas regression curves of the single region analysis deviate from the data points, resulting in overestimation of long-term rupture life. The difference between the two activation energies of short-term and long-term creep increases with increasing Cr concentration. Therefore, the overestimation due to singles region analysis is expected to be more serious at higher Cr concentration.

Review of allowable stress and new guideline of long-term creep strength assessment for high Cr ferritic creep resistant steels

Review of allowable stress and new guideline of long-term creep strength assessment for high Cr ferritic creep resistant steels

OS1303 高Cr実用フェライト系耐熱鋼のクリープキャビティ生成挙動(高クロム鋼の高温強度と寿命評価,オーガナイズドセッション)

火力発電用ボイラ設備に実用化されている高Crフェライト系耐熱鋼母材部のクリープ損傷過程では,クリープキャビティの発生を伴う場合もあるが,その発生に着目した例は少ない。そこで,火SUS410J3材を用いて,クリープ中断材におけるクリープキャビティの生成状況について調査した。その結果より,本耐熱鋼はクリープ寿命の消費とともにクリープキャビティの増加および粗大化・連結を伴い,低い破断延性を示すことがわかった。また,クリープキャビティはBとNからなる介在物の廻りにできやすい傾向が認められた。

OS1302 高Crフェライト系耐熱鋼の析出挙動とクリープ強度に及ぼすCr濃度依存性(高クロム鋼の高温強度と寿命評価,オーガナイズドセッション)

一般にCrを含む鋼では、Cr量の増加とともに高温強度は向上するが、高Cr(8.5-12%Cr)系フェライト系耐熱鋼の長時間破断強度はCrが多いほど低下することが知られている。本研究ではCr量のみが異なる2種類の鋼について、応力部、応力無し部それぞれにおいてCr量の違いによるZ相およびM_23C_6炭化物相の析出挙動について調べた。その結果、応力の有無には関係なくCr量の多い鋼ではZ相の析出量が多くなるとともに、M_<23>C_6炭化物の平均粒径が大きくなることがわかった。一方、M_<23>C_6炭化物の析出量はCr量によってほとんど違いは見られなかった。以上から、Cr量が多いことで各相の析出、粒成長が促進され、このことが長時間側での強度低下の一因となっていると考えられる。

OS1309 TEMディスクSP試験片による高Cr耐熱鋼のクリープ特性評価(高クロム鋼の高温強度と寿命評価,オーガナイズドセッション)

HAZ局所領域のクリープ特性評価技術の開発を目的として、TEMディスクサイズ試験片を用いたSPクリープ試験の検討を行っている。本研究では、P91鋼の母材部を対象にして、有限要素法によるSP試験片の応力解析を実施し、得られた結果と実際の実験結果を比較した。SPクリープ曲線、最小クリープ速度および試験片断面形状を比較したところ、摩擦係数が0.2のとき両結果が比較的よく一致した。また、ネッキング部での定常平均相当応力σ_SとSPクリープ試験の負荷荷重(P)の間にP=0.45σ_Sの関係があり、SPクリープ破断試験結果をσ_Sで整理すると単軸クリープ破断試験結果とよく一致した。

(Fe, Cr)3O4 Spinel Layer as the Key to Solving the Accelerated Oxidation of High Cr Iron Alloy in High-temperature Steam

The amount of hydrogen dissolved into oxide layers on Fe–10Cr alloy after exposure to steam at 750°C was measured with thermal desorption spectroscopy (TDS). It was found that the dissolved hydrogen existed mainly in (Fe, Cr)3O4 spinel layer formed on the alloy surface. This result is related closely to the fact that oxidation of high Cr ferritic steels is accelerated remarkably by the existence of steam at elevated temperatures. Using a diffusion system of (Fe, Cr)3O4/CoO, cation diffusivity was investigated at 750°C in argon atmosphere or in steam. As a result, the diffusivity of Fe ion in (Fe, Cr)3O4 became larger in steam than that in argon atmosphere, indicating that hydrogen (proton) promotes Fe ion outer diffusion from Fe–10Cr alloy. This is direct evidence that high temperature steam accelerates oxidation of Cr containing steels.

Assessment of Strain Energy by Measuring Dislocation Density in Copper and Aluminium Prepared by ECAP and ARB

In order to estimate the strain energies of pure Cu and pure Al prepared by ECAP and ARB processes, dislocation densities remaining in these metals after each process were analyzed by the Warren-Abervach method based on the Williamson-Hall plot from a series of data taken by conventional X-ray diffractometry. The measured dislocation density was higher by several times in pure Cu than in pure Al regardless of ECAP and ARB, resulting in the higher strain energy stored in pure Cu than in pure Al. Also, the dislocation density was found to be lower in the f.c.c. phase in pure Cu than that in the lath martensite phase in high Cr ferritic steels. Furthermore, it was observed that the amount of stored strain energy in pure Al depended considerably on its purity.