high-Cr Martensitic Steels Research Articles

Root Cause of Degradation in the Creep Strength of Martensitic Steel

Creep curves of Grade 91 and 92 steels were analyzed by applying an exponential law to the temperature, stress, and time parameters to investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel. The activation energy (Q), activation volume (V), and Larson–Miller constant (C) were obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening occurs because of dislocations that are swept out of the sub-grains, and this is followed by strengthening owing to the rearrangement of M23C6 and the precipitation of the Laves phase. Heterogeneous recovery and subsequent heterogeneous deformation start at an early stage of transient creep near several of the weakest boundaries because of the coarsening of the precipitates; this results in the simultaneous decreases in Q, V, and C even in transient creep. Further, this activity triggers an unexpected degradation in strength because of the accelerated formation of the Z-phase even in transient creep. The stabilization of M23C6 and the Laves phase is important to mitigate the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for Cr-Mo systems because Fe2Mo is easily coarsened at approximately 600 °C compared to Fe2W in Grade 92 steel.

Analysis of the Degradation in the Creep Strength of High-Cr Martensitic Steels

To investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel, the creep curves of Grades T91, T92, and P92 were analyzed along with the experimental steels of 9Cr-1W and 9Cr-4W by applying an exponential law to the temperature, stress, and time parameters. The activation energy (Q ), activation volume (V ), and Larson-Miller constant (C ) were obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening occurs due to dislocations that are swept out of the sub-grains, which is followed by strengthening due to the rearrangement of M23C6 and the precipitation of the Laves phase. After Q  reaches a peak, heterogeneous recovery and subsequent heterogeneous deformation begin at an early stage of transient creep in the vicinity of several of the weakest boundaries due to coarsening of the precipitates. This activity triggers an unexpected degradation in strength due to the accelerated formation of the Z-phase. Stabilization of M23C6 and the Laves phase is important for mitigating the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for the Cr-Mo systems because Fe2Mo is easily coarsened at ~600 °C as compared to Fe2W. Lowering the hardness and Si content also prevents excess hardening due to the Laves phase, which also mitigates the degradation. The online monitoring of creep curves and the QVC  analysis render it possible to detect signs of long-term degradation under targeted conditions within a relatively short period.

Analysis on Degradation in Creep Strength of 9Cr-W Martensitic Steel

In order to clarify the creep mechanism of high Cr martensitic steel, creep curves of 9Cr-1W and 9Cr-4W steels were analyzed applying an exponential law to the temperature, stress, and time parameters. The activation energy, Q, the activation volume, V, and the Larson-Miller constant, C, are obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening by swept dislocations out of sub-grains occurs followed by strengthening due to the rearrangement of M23C6 and the precipitation of Laves phase. After Q reaches a peak, heterogeneous recovery and subsequent heterogeneous deformation begin at an early stage of transient creep in the vicinity of some weakest boundaries due to coarsening of the precipitates, which triggers the unexpected degradation in strength due to the accelerating coarsening of precipitates. Stabilizing not only M23C6 but also Laves phase is important to mitigate the degradation of rupture strength of martensitic steel. The above creep mechanism for martensitic steel can be applicable to the explanation for the degradation in long term rupture strength of high Cr martensitic steel, Grades 91 and 92.

Enhancing the performance of a long-life modified CANDLE fast reactor by using an enriched 208Pb as coolant

The investigation of the utilization of enriched 208Pb as a coolant to enhance the performance of a long-life fast reactor with a Modified CANDLE (Constant Axial shape of Neutron flux, nuclide densities, and power shape During Life of Energy production) burnup scheme has performed. The analyzes were performed on a reactor with thermal power of 800 MegaWatt Thermal (MWTh) with a refueling process every 15 years. Uranium Nitride (enriched 15N), 208Pb, and High-Cr martensitic steel HT-9 were employed as fuel, coolant, and cladding materials, respectively. One of the Pb-nat isotopes, 208Pb, has the smallest neutron capture cross-section (0.23 mb) among other liquid metal coolants. Furthermore, the neutron-producing cross-section (n, 2n) of 208Pb is larger than sodium (Na). On the other hand, the inelastic scattering energy threshold of 208Pb is the highest among Na, natPb, and Bi. The small inelastic scattering cross-section of 208Pb can harden the neutron energy spectrum. Therefore, 208Pb is a better neutron multiplier than any other liquid metal coolant. The excess neutrons cause more production than consumption of 239Pu. Hence, it can reduce the initial fuel loading of the reactor. The selective photoreaction process was developing to obtain enriched 208Pb. The neutronic was calculated using SRAC and JENDL 4.0 as a nuclear data library. We obtained that the modified CANDLE reactor with enriched 208Pb as coolant and reflector has the highest k-eff among all reactors. Meanwhile, the natPb cooled reactor has the lowest k-eff. Thus, the utilization of the enriched 208Pb as the coolant can reduce reactor initial fuel loading. Moreover, the enriched 208Pb-cooled reactor has the smallest power peaking factor among all reactors. Therefore, the enriched 208Pb can enhance the performance of a long-life Modified CANDLE fast reactor.

On effect of rhenium on mechanical properties of a high-Cr creep-resistant steel

9–12%Cr martensitic steels are perspective materials for critical components of new high-efficiency power plants working at ultra-supercritical parameters of steam. Addition of 0.2% rhenium in the experimental steel improved the short-term creep strength at 650 °C. Comparison of kinetics of tungsten depletion from the matrix in different high-Cr martensitic steels showed that rhenium in the experimental 10Cr-3Co-3W-0.2Re steel did not lead to retaining an increased amount of solute W in the ferritic matrix during both aging and creep at 650 °C. At the same time, the precipitation of the high fraction of the fine Laves phase particles provided the effective particle strengthening.

Relation between Sub-grain Size and Dislocation Density During Steady-State Dislocation Creep of Polycrystalline Cubic Metals

The sub-grain size, d, during steady-state dislocation creep of polycrystalline metals is theoretically formulated to be inversely proportional to the dislocation density, ρ, which is defined as the number of dislocations swept out of a sub-grain divided by the cross-sectional area of the sub-grain. This dislocation density differs from the typically observed dislocation density inside a sub-grain after unloading, ρ_ob. In the current work, the ρ_ob values inside sub-grains in steadily crept specimens of Al, Cu, Fe, Fe–Mo alloy, austenitic stainless steel, and high-Cr martensitic steel reported in the literature were used to evaluate the relation ρ_ob=ηρ. It was confirmed that η≈1 for pure metals (regardless of the type of metal) crept at high temperatures and low stresses or for long durations and η>1 for Mo-containing alloys and martensitic steel crept at low temperatures and/or high stresses. Moreover, it is suggested that the condition η>1 corresponds to a state of excess immobile dislocations inside the sub-grain. The theoretical relation d_ob (≈d)∝η∙〖ρ_ob〗^(-1), where d_ob is the observed sub-grain size, essentially differs from the well-known empirical relation d_ob∝〖ρ_ob〗^(-0.5).

Aberrant P91 and its inspection, monitoring and life assessment

There have been many cases of power utilities around the world finding aberrant’ or ‘abnormal’ P91 base and/or weld metal components in their power plants. Although it is now well known that correct heat treatment is critical in achieving full strength of high Cr martensitic steels, it is clear that, for various reasons, many materials suppliers, manufacturers or welding companies have failed to realise this criticality. As a result, there is a worldwide problem with P91 base metals and/or whole welded joints that have not been heat treated to the Standards specifications and/or best practices, resulting in hardness levels below the acceptable limit and/or the microstructure not in the fully martensitic condition resulting in premature Type IV failures. To ensure the safe operation of such components, it is therefore important to generate long-term creep strength data on the typical aberrant microstructures often found in high temperature plant. The first part of the paper discusses this issue. To make matters worse, traditional NDE techniques are failing to find damage in high Cr martensitic steel components, including the more vulnerable components containing aberrant microstructures, at an early stage in life and as a result plant operators are finding it difficult to assure the safe performance of such components. In view of this, an international industrial community, led by ETD, started a three-year duration project on the development and demonstration on ‘normal’ P91 and P92 welded pipes, tested in controlled laboratory conditions, of new and more sensitive NDE techniques to detect such damage early in life and relate it to the component remaining life. Some of these findings will be discussed in this paper. A new project, open to new partners, on the testing of pipes containing ‘aberrant’ microstructure and further validation of some of the NDE techniques found to be more promising has now been launched.

Detection of creep degradation during pressure vessel testing using electromagnetic sensor technology

9%Cr steels have been in use in power generation for over 20 years. Over this time, there have been a number of incidents of failure by Type IV cracking at welds initiating sub-surface, making surface inspection procedures unsuitable for crack detection. Replication to identify creep cavitation damage can potentially give an early warning of imminent cracking, but has proved unsuccessful in high Cr martensitic steels. Electromagnetic (EM) inspection can provide fast inspection of welded components with minimal surface preparation and some increase in inspection depth. This paper shows the results of accelerated creep tests carried out on a P91 pressure vessel, with EM inspections during regular test interruptions. The results show that EM inspection gave indications of damage in the latter stages of creep cavitation. EM is considered to have the potential for quick assessment of component through thickness damage, flagging up locations of potential problems for more detailed investigation.

Microstructure and Mechanical Properties of a Tempered High Cr Martensitic Steel

Microstructural and mechanical studies have been performed in a high Cr martensitic steel Firth-Vickers (FV535) to analyze the tempering of martensite. Nanoindentation technique was used to determine the hardness and elastic modulus through systematic measurements on martensite and tempered martensite. On the other hand, microscopic studies were carried out to analyze the material in the same condition as received and subsequently observe the microstructural modifications after heat treatment. The precipitation presented in the last stage of tempering was observed by transmission electron microscopy. The results showed the effect of the martensite decomposition on the mechanical and nanomechanical properties of FV535.

Effects of Dislocation Substructure on Creep Deformation Behavior in 0.2%C-9%Cr Steel

In this paper, microstructural evolution under creep deformation was investigated by using a simple 0.2%C-9%Cr steel to reveal effects of dislocation substructure on creep strength in addition to creep deformation behavior in high Cr martensitic steels. Whereas strain versus creep rate curves are composed of simple primary and tertiary creep stages on the steel tested at higher applied stress condition, complicated variation in creep rate are found on the curves examined at lower stress levels under a temperature of 873 K. Formation of cellular dislocation network substructure is observed in the early periods of the primary creep stage on the steel tested at a lower stress. Such substructure is stable until onset of acceleration creep. The degradation of the substructure is due to coarsening and condensation of the carbides existing along the lath boundaries. An increase in yield stress at elevated temperature and internal back stress in the whole course of the transient creep region are provided by the cellular dislocation network substructure having highly dense dislocation walls where intralath dislocations move and integrate in the early periods after loading. The complicated variation in creep rate at the lower stress conditions is caused by accumulation and partial disappearance of dislocations at cell and lath boundaries. It can be concluded from above results that dislocations act a key role for strengthening against creep deformation.

M5C2 carbide precipitates in a high-Cr martensitic steel

The precipitate phases in an advanced 11% Cr martensitic steel, expected to be used at 650 °C, have been investigated to understand the effect of precipitates on the creep-rupture strength of the steel. M23C6 and MX precipitates were dominant phases in this steel. Needle-like precipitates with a typical length of 180 nm and width of 20 nm; and metallic-element compositions of 53–74Fe, 16–26Cr, 3–18Ta, 2–8W, and 2–4Co (at%); were observed mainly within the martensite laths of the normalized-and-tempered steel. The needle-like precipitates have been identified as monoclinic carbide M5C2, which is not known to have been reported previously in high chromium steels, or in heat-resistant steels those have been normalized-and-tempered. This indicates that the formation of M5C2 carbides can occur in heat-resistant steels produced under appropriate tempering conditions, and that this does not require long-term isothermal aging or creep testing, in all cases.

IRRADIATION EFFECTS OF HT-9 MARTENSITIC STEEL

High-Cr martensitic steel HT-9 is one of the candidate materials for advanced nuclear energy systems. Thanks to its excellent thermal conductivity and irradiation resistance, ferritic/martensitic steels such as HT-9 are considered for in-core applications of advanced nuclear reactors. The harsh neutron irradiation environments at the reactor core region pose a unique challenge for structural and cladding materials. Microstructural and microchemical changes resulting from displacement damage are anticipated for structural materials after prolonged neutron exposure. Consequently, various irradiation effects on the service performance of in-core materials need to be understood. In this work, the fundamentals of radiation damage and irradiation effects of the HT-9 martensitic steel are reviewed. The objective of this paper is to provide a background introduction of displacement damage, microstructural evolution, and subsequent effects on mechanical properties of the HT-9 martensitic steel under neutron irradiations. Mechanical test results of the irradiated HT-9 steel obtained from previous fast reactor and fusion programs are summarized along with the information of irradiated microstructure. This review can serve as a starting point for additional investigations on the in-core applications of ferritic/martensitic steels in advanced nuclear reactors.

A new approach to improve creep resistance of high Cr martensitic steel

A modified 9Cr–1Mo steel was cooled to 200°C from the normalizing temperature and then directly heated to the tempering temperature. It was found that the time to rupture at 650–700°C for the steel heat-treated at 200°C increased three times over than that of the modified 9Cr–1Mo steel conventionally normalized and tempered. The microstructure of the improved steel was tempered martensite and the size of martensite blocks was larger than for the conventional treatment. The hardness of the improved steel was adequately recovered after tempering. Aging tests showed that the particle sizes of Cr23C6 and VN type carbonitride in the improved steel were finer in the conventional steel. The above-mentioned heat treatment was applied to the reduced activation martensitic steel F-82H and the improvement was confirmed.

Remaining life assessment issues in high Cr martensitic steels and development of new innovative tools for damage monitoring and integrity assessment

The relatively newer high Cr martensitic steels such as P91 have now been in use in power industry for over twenty years. Over this time, there have been a number of incidents of cracking and failure in components made from P91 steel, both in thick and thin section equipments mainly due to creep damage. The thick section components have been usually failing due to Type IV cracking associated with the weldments while thin section components have been failing due to higher than expected levels of steam oxidation resulting in enhanced metal loss, increase in metal temperature above design, creep cavitation and cracking. However, it has not been possible to detect early stage creep damage/cavitation in high Cr martensitic steels using conventional replication type methods. This is because unlike the low alloy CrMoV steels, spherodisation of microstructure does not occur in high Cr martensitic steels and cavitation clearly visible by traditional methods only appears later in life when the material is about to fail. Thus there has been a need to develop new tools and more sensitive methods for integrity and damage assessment in these steels. European Technology Development (ETD) together with its industrial collaborators from Europe, Japan and North America have been looking at the development of tools and methodologies for early stage damage detection and life prediction as a part of its international multi-client project ‘P91 Integrity’. The tools which have shown successful results are portable Scanning Force Microscopy, laser guided hardness tester and more innovative use of ultrasonic probes for detecting early stage creep damage. This paper discusses the issues involved and describes some of the developments in this project.