Ferritic Heat Resistant Steels Research Articles

Changes in Anodic Polarization Behavior of 12%Cr Ferritic Heat-Resistant Steels due to Creep

In order to investigate the possibility of electrochemical material characterization method as a procedure for remaining-life prediction of 12%Cr ferritic steels, this method was applied to three kinds of 12%Cr ferritic steels used in boiler and steam turbine and the changes in their anodic polarization behavior due to creep were measured. Experimental results revealed that the peak current densities “Ip1” and “Ip2” appeared at specific potentials during anodic polarization curve measurement in 1M-KOH solution. These peak current densities corresponded to the preferential dissolutions of several kinds of precipitates, respectively. The Ip2, which corresponded to the selective dissolution of M23C6 and Laves phase, increased more significantly with creep than thermal aging in all three steels. On the other hand, the effect of applied stress was not clearly reflected on the change in Ip1 corresponding to the selective dissolution of copper precipitate in the boiler steel, although it increased with thermal aging and creep as well as the Ip2. Therefore, it was expected that these peak current densities were useful indicators for estimating a temperature and applied stress working in high-temperature components of 12%Cr ferritic steels. Additionally, it was also revealed that the ΔIp2, which was increment of Ip2 from the as-tempered, increased with increasing creep life fraction and its variation showed almost no steel dependence.

Metallurgical investigation and modelling of deterioration of creep rupture strength in heat affected zone of heat resistant ferritic steel

Heat resistant ferritic steels containing 9–12% chromium have been widely used as materials such as the main steam pipes for fossil fired boilers in thermal power stations. In these steels, creep rupture strength of welded joints deteriorates compared with base metals. The fracture due to this phenomenon is called Type IV cracking. Although this phenomenon occurs in the fine-grained heat affected zone (HAZ) in welded joints and causes limitations of design stress in fossil-fired boilers, its mechanism is not yet perfectly understood.To clarify the mechanism of deterioration of creep rupture strength in HAZ of heat resistant ferritic steel, we have surveyed the affect of weld thermal cycle on the creep rupture strength and the microstructural change at high temperature.As a result, it is found that the creep rupture strength deteriorated in HAZ heated to a peak temperature near 1273 K, at which M23C6 type carbides are partially dissolved and the growth rate of fine particles is faster. The mechanism to explain this phenomenon is discussed from the viewpoint of the growth of precipitates, such as M23C6 and MX, during long-term heating.

Stress Dependence of Microstructural Evolution in Heat Resistant Steels

The state of the microstructure of ferritic heat resistant steels during creep was evaluate by the system free energy, which composes mainly chemical free energy, surface energy and elastic strain energy, and its stress dependence was expressed quantitatively by using a relaxation time. The steels used in this study were P91 (9Cr-1Mo-C-N-V-Nb) steel and P92 (9Cr-Mo-W-C-N-V-Nb-B) steel. The obtained results are as follows: (1) the relaxation time of elastic strain energy was expressed as a function of stress and temperature, (2) surface energy of P92 scarcely decreased during creep due to the formation of the Laves phase, and (3) the relaxation time of the chemical free energy in P92 was larger than that in P91.

Results of a Japanese round robin program for creep crack growth using Gr.92 steel welds

Abstract High-Cr ferritic heat-resistant steels are commonly used for boiler components in ultra-super critical thermal power plants operated at about 600 °C. In the welded joints of these steels, Type-IV cracks initiate in the fine-grained HAZ during long-term use at high temperatures, causing their creep strength to decrease. To assist the standardization of the testing and evaluation method for creep crack growth (CCG) in the welded components, we conducted round robin tests (RRT) using 9Cr–0.5Mo–1.8 W–V–Nb steel (ASME Grade 92 steel) welded joint as part of the VAMAS TWA31 collaboration. The CCG tests were carried out using the CT specimen and the circumferentially-notched round bar specimen for both the base metal and welded joint of Gr.92 steel. Testing was performed at four different laboratories. The effects of specimen configuration, temperature, load, and stress triaxiality conditions on the crack growth rate and fracture life were investigated.

Creep property of carbon and nitrogen free high strength new alloys

The carbon and nitrogen free new alloys which were composed of supersaturated martensitic microstructure with high dislocation density before the creep test have been investigated systematically. These alloys were produced from the new approach which raised creep strength by the utilization of the reverse transformed austenite phase as a matrix and intermetallic compounds such as Laves phase and mu-phase as precipitates during heating before the creep test. It is important that these alloys are independent of any carbides and nitrides as strengthening factors. The high temperature creep test over 700 °C exceeds 50,000 h, and the test is continuous. Creep behavior of the alloys is found to be different from that of the conventional high-Cr ferritic steels. The addition of boron to the alloy pulled the recrystallization temperature up in the high temperature, and it became a creep test in the un-recrystallization condition, and the creep property of high temperature over 700 °C was drastically improved. The minimum creep rates of Fe–Ni alloys at 700 °C are found to be much lower than those of the conventional high Cr ferritic heat resistant steels, which is due to fine dispersion strengthening useful even at 700 °C in these alloys. As a result it became clear that the value for 100,000 h was exceeded at 700 °C and 100 MPa calculated from the Larson-Miller parameter at C = 20.

Numerical simulation of long-term precipitate evolution in austenitic heat-resistant steels

Numerical simulation of the long-term precipitate evolution in five different austenitic heat-resistant steels–NF709, Super304H, Sanicro25, CF8C-PLUS and HTUPS–has been carried out. MX and M 23C 6 are predicted to remain as major precipitates during long-term aging in these steels. While the average size of MX is maintained below several ten nanometers during the aging, that of M 23C 6 exceeds 100 nm after 100,000 h of aging at 700 ∘C. The addition of 3 wt% Cu produces very fine Cu-rich precipitates during aging in Super304H and Sanicro25. It is found that the amount of Z phase starts to increase remarkably between 1000 and 10,000 h of aging at the expense of MX precipitates in the steels containing a high nitrogen content. However, the growth rate of Z phase is relatively slow and its average size reaches at most a few ten nanometers after 100,000 h of aging at 700 ∘C, compared with 9%–12% Cr ferritic heat-resistant steels. The simulated precipitation sequence and precipitate size during aging are in general agreement with experimental observations.

PS15 環状切欠試験片を用いたひずみ制御高温クリープ・疲労重量条件下におけるP92鋼のき裂成長寿命の繰返し速度特性(フェロー賞対象ポスターセッション)

W strengthened 9Cr ferritic heat resistant steel has been applied to boiler materials for the most efficient electric power plant technology and it will be used under the conditions of high temperature creep-fatigue multiplication. In this study, mechanical tests under the conditions of high temperature creep-fatigue multiplication were conducted for W strengthened 9%Cr ferritic heat resistant steels (ASME-Code P92) with various stress holding time conditions. Based on these results, the characteristics of load frequency of crack growth life were investigated by analyzing cycle-dependent and time-dependent mechanisms on the characteristic of load frequency of crack growth life.

102 W添加9Cr鋼のクリープ・疲労重畳条件下におけるき裂成長寿命則(学生賞I,一般講演)

102 W添加9Cr鋼のクリープ・疲労重畳条件下におけるき裂成長寿命則(学生賞I,一般講演)

A Method for Materials Selection in Reliability-Based Design Diagram of Structural Components under Creep-Fatigue Damage Mode

A new method for materials selection in reliability-based design was applied to the high temperature structural components under creep-fatigue damage mode. The creep-fatigue interaction law was formulated in terms of creep time fraction (creep damage) and fatigue cycle fraction (fatigue damage) using pure creep and pure fatigue material properties referring NIMS creep and fatigue datasheets for ferritic heat resistant steels such as 1Cr-1Mo-0.25V steel forging, 12Cr-1Mo-1W-0.3V steel bar and 2.25Cr-1Mo steel plate. The statistical distribution under the creep-fatigue damage mode was fitted well with the two-dimensional log-normal distribution function but the scatter bands of creep-fatigue data were considerably larger compared with the cases using pure creep and pure fatigue damage distributions. Once the ratio of creep damage to fatigue damage was fixed, the confidence limit could be restricted within more reasonable range for design application. The diagram was expressed by the relationship between creep stress vs. fatigue strain range for those three materials and the apparent superiority of 12Cr-1Mo-1W-0.3V steel bar to other materials was demonstrated. Thus this diagram was proved to be an effective tool for decision making of materials selection and structural reliability-based design even at the very initial stage of design process, that is, the front-loading design of components.

Metallurgical investigation and modelling of deterioration of creep rupture strength in heat affected zone of heat resistant ferritic steel

Metallurgical investigation and modelling of deterioration of creep rupture strength in heat affected zone of heat resistant ferritic steel

クリープ疲労したフェライト系耐熱鋼(12Cr-2W)のサブグレイン組織解析

クリープ疲労したフェライト系耐熱鋼(12Cr-2W)のサブグレイン組織解析

Characterization of precipitate distributions for ferritic heat-resisting steel with superior to creep-fatigue property

A new observation method was developed for evaluating precipitate distributions on the electropolished surface of ferritic heat-resisting steel, using field emission scanning electron microscopy (FE-SEM). FE-secondary and backscattered electron images of the electropolished surfaces clearly reveal fine precipitates, since the electropolishing surfaces are very flat and not influenced by martensite matrix surface unevenness. The intergranular or transgranular precipitate distributions for each of two types of chromium carbide and Laves phase, which were identified from backscattered electron images, were quantitatively evaluated for the material with superior to a creep-fatigue property, before and after a creep-fatigue testing, respectively.

131 W添加9Cr鋼のクリープ・疲労重畳条件下におけるき裂成長寿命則(材料力学III)

131 W添加9Cr鋼のクリープ・疲労重畳条件下におけるき裂成長寿命則(材料力学III)

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.

溶接熱影響部におけるクリープき裂成長挙動に及ぼす局所応力多軸効果

Under the multi-axial stress field, creep crack growth is considered to occur in brittle manner. From the view point of practical use, it is important to conduct the characterizations of the creep crack growth rate and its life under the multi-axial stress field to predict the life of creep crack growth. In the present study, for the welded joint of the W-added 9Cr ferritic heat-resistant steel, ASME Code Case 2179/ASTM A335 P92, creep crack growth tests and 3-dimensional elastic-plastic creep finite element analyses were conducted to clarify the effect of stress multi-axiality on creep crack growth. As a result, the creep ductility of welded joint was found to decrease and shift to the creep brittle region as compared with that of base metal due to the large value of stress tri-axial factor TF.

Characterization of high temperature long-term fatigue properties for advanced ferritic heat-resisting steels

Characterization of high temperature long-term fatigue properties for advanced ferritic heat-resisting steels

Development of new 11%Cr heat resistant ferritic steels with enhanced creep resistance for steam power plants with operating steam temperatures up to 650 °C

The goal of developing new heat resistant 11%Cr ferritic–martensitic steels with sufficient creep and oxidation resistance up to 650 °C was pursued within a joint project following an alloying concept based on physical metallurgy principles. The highest creep strength combined with good oxidation resistance was achieved for a Ta-alloyed test melt (11 wt.% Cr, W, Co, Mo, V, 0.09 wt.% Ta, relatively high contents of C and B). The microstructural evolution during creep was investigated by transmission electron microscopy for the Ta-alloyed melt in comparison to a sister melt where Ta was exchanged for 0.04%Nb. It is proposed that fine particles of types MX and M 23C 6 (M: metallic element, X: interstitial elements) are the cause of the outstanding creep resistance of the Ta-alloyed melt.

Characterization of Structural Embrittlement of Creep Crack Growth for W-Added 12%Cr Ferritic Heat-Resistant Steel Related to the Multiaxial Stress

In components under static creep loading condition, the multiaxial stress fields appear due to the plastic constraint and they produce a more brittle type cracking behavior. From a practical standpoint, the characterizations of creep crack growth rates under the multiaxial stress field are important to improve the methods for creep life extension. In this paper, creep crack growth tests were conducted using round bar specimens with sharp circular notches for tungsten-added 12%Cr ferritic heat-resistant steel (W12%Cr steel), and the effect of multiaxiality on creep ductility and creep crack growth rate were investigated. Furthermore, three-dimensional elastic-plastic creep finite element analyses were conducted to clarify the effect of multiaxiality on creep crack growth.

Effect of carbon content on creep rupture strength and microstructure in heat affected zone of heat resistant ferritic steel: alleviation of decrease in creep rupture strength in heat affected zone of heat resistant ferritic steel

The effect of the carbon content on the creep rupture strength and the microstructural change in weld heat affected zone (HAZ) during high temperature services was investigated in order to alleviate the decrease in the creep rupture strength in HAZ of heat resistant ferritic steels. The test specimens were prepared from 9Cr–3Co–3–W–V, Nb ferritic steel plates with carbon content ranging from 0˙005 to 0˙1%. After the simulated HAZ thermal cycle treatments at the peak temperature of 1273 K was applied on these specimens, creep rupture tests, aging tests and microstructural examinations were conducted. As a result, it was clarified that the creep rupture time of simulated HAZ became longer and the decrease in the creep rupture strength in HAZ was alleviated by decreasing the carbon content. Then the mechanism to explain the effect of the carbon content was discussed from a viewpoint of the growth of precipitates, such as M23C6 and MX, during long term heating.

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。