Cr Ferritic Heat-resistant Steel Research Articles

Effect of groove configuration on Type-IV creep damages in Gr.122 steel welds

Creep strength of high Cr ferritic heat-resistant steel welds decreases more rapidly than base metals due to Type-IV creep damage formed in the heat-affected zone (HAZ) at high temperatures. This paper aims to elucidate the effect of groove configuration of welds on the formation and evolution of Type-IV creep damage. Creep tests using base metal, simulated HAZ and two-types of weld joint; gas tungsten arc and electron beam welds, of the ASME Gr.122 steel were conducted. Decreasing the groove angle and width of HAZ was effective in prolonging the creep life; however, it could not prevent Type-IV failure. Type-IV creep voids were mostly observed in the area about 1.0–3.0 mm below the specimen surfaces, whereas their detailed distributions were dependent on the groove configurations. Both the distribution of the multiaxial stress state and creep strain in HAZ can be considered to influence the formation of Type-IV creep damage. Damage mechanics analysis taking the multiaxial stress state into account can predict fairly well the distribution of Type-IV creep voids and failure times of the weld joints without regard to the groove configurations.

Effect of Cooling Rate on Microstructure of Simulated CGHAZ for Modified High-Cr Ferritic Heat-Resistant Steel

The microstructure and carbide precipitate of simulated coarse grain heat affected zone(CGHAZ) in modified high Cr ferritic heat-resistant steel at different cooling rates have been investigated by means of thermal simulator, optical microscope, SEM and TEM . It was found that the microstructure of CGHAZ of testing steel was mainly lath martensite and δ-ferrite under the different welding thermal cycles. However, the prior austenite grain size reduced with increasing the cooling rate. Furthermore, with increasing the cooling rate, the amount of carbide precipitate inside laths of martensite increased, and the size and morphology of precipitates have changed from elongated and coarse to needlelike and fine.

The isochronal δ → γ transformation of high Cr ferritic heat-resistant steel during cooling

Thermal simulation technology was employed to investigate phase transformation in heat-affected zones (HAZ) of high Cr ferritic heat-resistant steel. The simulated continuous cooling transformation diagram was established based on the experimental results obtained from different cooling rates in the range of 0.02–60 °C/s. A theoretical model considering the site saturation nucleation at grain boundaries has been applied to calculate the austenite fraction as a function of cooling rate. It is found that both the austenite fraction and grain size decrease with the increase of cooling rates. The calculated results are mostly consistent with the experimental data.

Evaluation of long-term creep damage in high Cr ferritic steel welds

The creep strength of high Cr ferritic heat-resistant steel welds decreases due to the formation of Type IV creep damage in the heat affected zone (HAZ) during long-term service at high temperatures. In order to elucidate the processes of Type IV creep damage, creep rupture and creep interruption tests using ASME Gr.91 and Gr.122 steel welds were conducted. It was found that creep voids formed at an early stage (0.2 of life) and coalesced to form a macro crack at 0.8 of life for the Gr.91 steel weld. On the other hand, for the Gr.122 steel weld, a small number of Type IV creep voids formed at 0.5 of life, increased slightly until 0.9 of life and rapid crack growth occurred after that. Differences of creep damage behaviour between the Gr.91 and Gr.122 steel welds are discussed. The Type IV creep damage distributions obtained were compared with analytical results using the finite element method and damage mechanics.

Kinetics of isochronal austenization in modified high Cr ferritic heat-resistant steel

Employment of high Cr ferritic steels as a main structural material is considered as a way to achieve economical competitiveness of main steam pipe and nuclear reactors in power plants. Differential dilatometry and microstructure observation were employed to investigate the isochronal austenitic transformation of the modified high Cr ferritic steel. The kinetics of the isochronal austenitic transformation were described by a phase-transformation model involving site saturation (pre-existing nuclei), diffusion-controlled growth, and incorporating an impingement correction. The experimental results and kinetic analysis indicate that an increase of the heating rate promotes the diffusion-controlled austenitic transformation. The dissolving degree of precipitates during the austenization process affects the activation energy for diffusion and the undissolved precipitates lead to an increase of the onset temperature of the subsequent martensite transformation upon cooling.

Effect of Minor Compressive Deformation in Austenite on Martensitic Transformation in Modified 9-12%Cr Ferritic Steel

A modified high Cr ferritic heat-resistant steel was developed and subjected to the thermomechanical treatment. The effect of applied stress and temperature on martensitic transformation and microstructure was studied by using high-resolution differential dilatometer and optical microscope. Applied stress raises the amount of defection as dislocation and grain boundary defects, thus elevateMs. Applied stress also lead to the increase of transformation time due to the mechanical stabilization of austenite. Increase of applied stress or temperature would enhance these above effects.

Effects of Two-Step Tempering Treatment on the Microstructural Formation of T91 Ferritic Steels

As a representative type of high Cr ferritic heat-resistant steels, T91 steels (ASME SA-213 T91/P91) has been recognized as the preferable materials and widely used in high-temperature structural components such as header and main steam pipe in advanced power plants. For the service condition is tempered martensites, its corresponding microstructure and mechanical performance are mainly adjusted by the tempering treatment. After exploring the size and number of MX and M23C6 precipitating particles and the width of martensitic lath as a function of tempering temperature, it is recognized that the high tempering temperature leads to an increase of secondary hardening effect, while the low tempering temperature brings a high dislocation density and a small martensitic lath. Hence, a two-step tempering treatment was developed after the traditional normalizing process, in which the T91 steels sample was firstly tempered at a low temperature in order to form some precipitates and then tempered at a high temperature. Those firstly-formed precipitates would pin the dislocations and martensitic laths on the subsequent tempering process, which finally leads to more precipitates, higher dislocation density and smaller martensitic lath width than that obtained from the traditional tempering process.

Evaluation of creep crack growth properties of Gr. 92 steel weldment

High Cr ferritic heat resistant steels are used for boiler components in ultra-super critical thermal power plants. In weld components of these steels, Type-IV creep damages formed in the fine-grained heat-affected zone (HAZ), causing their creep strength to decrease at high temperatures. To assist the standardization of the testing method for creep crack growth in weld components, we conducted round robin tests using ASME Grade 92 steel 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 weld joint of Gr. 92 steel. The effects of specimen configuration, temperature, load and stress triaxiality conditions on the crack initiation and growth properties were investigated.

Evaluation of long-term creep damage in high Cr ferritic steel welds

The creep strength of high Cr ferritic heat-resistant steel welds decreases due to the formation of Type IV creep damage in the heat affected zone (HAZ) during long-term service at high temperatures. In order to elucidate the processes of Type IV creep damage, creep rupture and creep interruption tests using ASME Gr.91 and Gr.122 steel welds were conducted. It was found that creep voids formed at an early stage (0.2 of life) and coalesced to form a macro crack at 0.8 of life for the Gr.91 steel weld. On the other hand, for the Gr.122 steel weld, a small number of Type IV creep voids formed at 0.5 of life, increased slightly until 0.9 of life and rapid crack growth occurred after that. Differences of creep damage behaviour between the Gr.91 and Gr.122 steel welds are discussed. The Type IV creep damage distributions obtained were compared with analytical results using the finite element method and damage mechanics.

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.

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.

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)

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.

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.

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.

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。

184 非平衡科学的手法に基づくW添加9Cr鋼におけるクリープ・疲労重畳条件下でのき裂成長寿命則(学生賞I)

184 非平衡科学的手法に基づくW添加9Cr鋼におけるクリープ・疲労重畳条件下でのき裂成長寿命則(学生賞I)