Tempered Martensite Ferritic Steel Research Articles

Modified Z-phase formation in a 12% Cr tempered martensite ferritic steel during long-term creep

The formation of modified Z-phase in a 12Cr1MoV (German grade: X20) tempered martensite ferritic (TMF) steel subjected to interrupted long-term creep testing at 550°C and 120 MPa was investigated. Quantitative volumetric measurements collected from thin-foil and extraction replica samples showed that modified Z-phase precipitated in both the uniformly-elongated gauge (fv: 0.23 ± 0.02 %) and thread regions (fv: 0.06 ± 0.01 %) of the sample that ruptured after 139 kh. The formation of modified Z-phase was accompanied by a progressive dissolution of MX precipitates, which decreased from (fv: 0.16 ± 0.02%) for the initial state to (fv: 0.03 ± 0.01%) in the uniformly-elongated gauge section of the sample tested to failure. The interparticle spacing of the creep strengthening MX particles increased from (λ3D: 0.55 ± 0.05 μm) in the initial state to (λ3D: 1.01 ± 0.10 μm) for the uniformly-elongated gauge section of the ruptured sample, while the thread region had an interparticle spacing of (λ3D: 0.60 ± 0.05 μm). The locally deformed fracture region had an increased phase fraction of modified Z-phase (fv: 0.40 ± 0.20%), which implies that localised creep-strain strongly promotes the formation of modified Z-phase.

Boron segregation and creep in ultra-fine grained tempered martensite ferritic steels

Abstract The creep strength of ultra-fine grained tempered martensite ferritic steels with 9 to 12 wt.% chromium is significantly increased when small amounts of boron (in the 100 wt-ppm regime) are added. In the present note, experimental evidence from the literature is reviewed and previous explanations for the role of boron are summarized. The explanation for the effect of boron on creep strength must leave space for the simultaneous effect of boron on ductility. A scenario is suggested where boron segregates to micrograin boundaries which often have subgrain-boundary character. Boron can decrease the subgrain-boundary energy, decrease the velocity of subgrain-boundary migration, hinder knitting reactions between free dislocations and subgrain-boundaries and thus contribute to creep strength. This decrease of intensity of interface plasticity contributes to a decrease in ductility. Boron may moreover decrease the strength of the interface and thus promote brittleness. Further work is required to fully account for the role of boron on the mechanical properties of tempered martensite ferritic steels.

On the nature of internal interfaces in tempered martensite ferritic steels

Abstract This paper presents results on the evolution of microstructure in 12% Cr tempered martensite ferritic steels during heat treatment and creep. Prior austenite grain size and micro grain size measurements were performed using optical microscopy, and scanning and transmission electron microscopy. Moreover, orientations of micro grains are determined using TEM and SEM. Prior austenite grain sizes, micro grain dimensions and orientation relationships between micro grains are interpreted in terms of processes associated with the formation of martensite and ferrite during heat treatment and creep. The elongated micro grains (in the interior of prior austenite grains) can have high- or low-angle boundaries. The hierarchical evolution of internal interfaces is described. Micro grain boundaries most often represent (i) variant boundaries separating individual former martensite variants and (ii) subgrain boundaries (associated with recovery processes during tempering and creep). The present results also show that there is a need to study (i) the evolution of dislocation density during heat treatment and creep and (ii) the precipitation of carbides during tempering and creep in relation to the different types of internal interfaces.

TEM replica analysis of particle phases in a tempered martensite ferritic Cr steel after long term creep

Tempered martensite ferritic steels (TMFSs) have been and are being used for critical components in high temperature plant operating in the 600 °C range. They are exposed to creep conditions for long time periods, exceeding 100,000 h. In the present study we investigate a 12% Cr TMFS, after creep-testing at 550 °C at 120 MPa for 139,000 h. We had previously investigated this material in the TEM using thin foils. We now use an extraction replica technique to analyze four particle families: M23C6, MX, Laves-phase and Z-phase, considering statistically relevant numbers of particles (between 120 and 720). We show how EELS mapping can help in identifying Z-phase particles and use Cr-V-maps to differentiate between the four particle families. The chemical evolution of particles is investigated. The experimental results are discussed in the light of previous thin foil data and with respect to predictions from computational thermodynamics. The strength and weakness of thin foil and replica procedures are compared. Improvements for thermodynamic databases are suggested.

Nucleation of W-Rich Laves Phase Nanoparticles in Tempered Martensite Ferritic Steel During Long-Term Aging at Elevated Temperature.

The nucleation of W-rich Laves phase nanoparticles was studied during long-term aging at elevated temperature in tempered martensite ferritic steels (TMFS). The TMFS was fabricated by vacuum induction melting (VIM) process. The long-term aging tests are interrupted at various stages to simulate the different level of nucleation of intermetallic phases. In the present work, we employ scanning electron microscopy (SEM), Auger electron microscopy (AES), and transmission electron microscopy (TEM) to study the formation nucleation of Laves phase particles. We investigated the preference of Laves phase particles to nucleate next to M23C6 micrograin boundary carbides due to the segregation of W from the matrix to the micrograin boundaries.

Strain dependent constitutive model and microstructure evolution of a novel 9Cr martensitic steel during high-temperature deformation

The flow deformation behavior and microstructure evolution of a novel tempered martensite ferritic steel G115 were systematically investigated under temperatures ranging from 625 to 675 °C and stain rates of 5.2 × 10−5–5.2 × 10−3 s−1. The strain dependent constitutive model was established to reflect the flow deformation behavior of G115 steel. The corresponding material constants and the activation energies for this steel were determined under the different strains (0.005–0.07). The flow stresses predicted by the exponential equation show a good agreement with the experimental data, which suggests the stress-strain relationship of G115 steel can be accurately described using the exponential equation. Fine M23C6 carbides and Cu-rich phase particles have been characterized for G115 steel under different deformation conditions. The size of M23C6 carbide increases by a factor of 1.6 while that of Cu-rich phase particles increases in 2.4 times when the temperature increases from 625 °C to 675 °C. In addition, the number density of Cu-rich phase particles decreased drastically at 675 °C. Regular martensite lath structure still existed at 625 and 650 °C, but breakup of the martensite laths occurred at 675 °C. This was mainly strongly related to the evolution of M23C6 carbides and Cu-rich phase particles. The fine Cu-rich phase particles with a high number density contributed to slowing down the degradation of microstructure in G115 steel.

M <sub>23</sub>C <sub>6</sub>-Assisted Laves Phase Growth Mechanism in a Novel 9Cr-3W-3Co-1CuVNbB Steel Under Creep Deformation

Laves phase particles are an important precipitate in tempered martensite ferritic steels that can strongly affect the microstructure and mechanical properties. We systematically investigated the evolution of Laves phase in G115 steel. Experimental results showed that the amount of Laves phases increases, whereas that of M23C6 carbides decreases as creep deformation proceeds. Further characterizations were conducted to thoroughly explore the evolutionary mechanism of Laves phase. Interestingly, a hybrid Laves-M23C6 particle was observed, and Cr segregation occurred near the remaining M23C6 carbides. Further, we found that C and Cr were also segregated near a Laves phase that was formed. The Cr concentration of the Laves phase was much lower than that of M23C6 carbide, but slightly higher than that of the ferrite matrix. The result confirms that Laves phase is transformed from M23C6 carbide, and it can also explain reasonably well the decrease in M23C6 carbide during creep deformation. We also proposed a M23C623C6 carbides, which is considered to be an interface-controlled M23C623C6-assisted Laves phase growth model was established assuming that the Gibbs energy is balanced at the M23C6/Laves phase interface. Furthermore, a guideline for development of novel tempered martensite ferritic steels was proposed. Decreasing the Si content of the material can effectively postpone Laves phase nucleation. In addition, the fraction of the Laves phase decreases, which helps improve the creep strength.

Deformation-mechanism-based creep model and damage mechanism of G115 steel over a wide stress range

The effects of applied stress (150–250 MPa) on the creep deformation behavior and creep damage (stable and unstable deformation) were studied systematically for a novel tempered martensite ferritic steel G115 at 650 °C. A creep model involving three deformation mechanisms (grain boundary sliding, dislocation glide, and dislocation climb) was applied to fully understand the creep deformation behavior of G115 steel over a wide range of applications. Subsequently, the deformation-mechanism-based creep model was validated for G115 steel over a wide range of stresses (120–250 MPa) and temperatures (625–675 °C). Further, in the stable deformation regions, the martensite laths have no significant change under high-stress conditions, whereas the martensite laths obviously coarsen and micro-cavities formed under low-stress conditions. Four types of precipitates can be characterized after creep. M23C6 carbide and MX carbontride are pre-existing in the initial microstructure. Cu-rich phase was precipitated at 650 °C after very short-term creep (6.87 h), whereas Fe2W Laves phase formed after long-term creep (4404.78 h). The Cu-rich phase particles are cut by a dislocation shearing mechanism and become too small to be stable, leading to the dissolution of the Cu-rich phases after long-term creep. The fine Cu-rich phase particles can significantly retard the recovery of martensite laths in the early stage of creep or during short-term creep. In the unstable deformation (necking) regions, martensite cracking and martensite fracture are the main micro-damage features during creep at high stresses, whereas the growth of micro-cavities and micro-cracks are the main features at low stresses. Ductile fracture is the dominant fracture mode of G115 steel at 650 °C above 150 MPa.

Transient creep behavior of a novel tempered martensite ferritic steel G115

The transient creep deformation behavior and dislocation evolution in G115 steels were investigated in the temperature range 625–675 °C for applied stresses of 120–220 MPa. The transient creep curves showed that creep rate decreased with increases in time and creep strain. In this study, a novel method was proposed to determine the transient creep strain and the transient creep time. The strain decreased with increase in applied stress between 625 °C and 675 °C. A phenomenological constitutive equation was used to characterize the transient creep curves of G115 steels. The results showed that the constitutive equation for the G115 steel had a high precision. An internal stress was introduced to study the interactions of dislocations and precipitates, and a mechanism-based equation for transient creep in G115 steels was derived. The dependences of normalized strain and transient creep time on applied stress and temperature were analyzed in detail to better understand the transient creep deformation mechanism. It was concluded that dislocation annihilation at the grain boundaries was the dominant rate-controlling mechanism in the transient creep deformation of G115 steels. It was observed that the dislocation structures became more complex and no obvious textural features occurred after the transient creep deformation. The calculated dislocation density decreased at 650 °C relative to the initial value, which was mainly attributed to the annihilation processes occurring at the grain boundaries and the subsequent formation of subgrain boundaries.

Microstructure evolution and fracture mechanism of a novel 9Cr tempered martensite ferritic steel during short-term creep

In this work, the microstructure evolution and fracture mechanism of a novel 9% chromium tempered martensite ferritic steel G115 were investigated over the temperature range of 625–675°C using uniaxial creep tests. The creep curves consist of a primary transient stage followed by an apparent secondary stage, and an accelerated tertiary creep regime. The relationship between the minimum creep rate and the applied stress followed Norton's power law. Based on the EBSD analysis, there were no obvious textural features formed after creep deformation, and with the increase in creep time, the number of subgrains slightly increased, and then sharply increased, indicating dynamic recrystallization (DRX) occurs after creep deformation. In addition, three types of precipitates can be observed after creep deformation: W-rich Laves phase, Nb-rich MX, and Cu-rich precipitates. The Nb-rich MX with a square shape and Cu-rich precipitates with an ellipsoidal shape remain very stable. However, the W-rich Laves phases distributed mainly on the grain boundaries have rod-like, chain-like, and bulky shape, which are coarsened significantly. Representative fractographs of the G115 steel after creep deformation exhibit significant necking with an elliptical shape. A dense array of deep and equiaxed dimples appear in the central region under the tested creep conditions. Ductile fracturing is the dominant fracture mechanism during short-term creep deformation.

A multi-scale crystal plasticity model for cyclic plasticity and low-cycle fatigue in a precipitate-strengthened steel at elevated temperature

In this paper, a multi-scale crystal plasticity model is presented for cyclic plasticity and low-cycle fatigue in a tempered martensite ferritic steel at elevated temperature. The model explicitly represents the geometry of grains, sub-grains and precipitates in the material, with strain gradient effects and kinematic hardening included in the crystal plasticity formulation. With the multiscale model, the cyclic behaviour at the sub-grain level is predicted with the effect of lath and precipitate sizes examined. A crystallographic, accumulated slip (strain) parameter, modulated by triaxiality, is implemented at the micro-scale, to predict crack initiation in precipitate-strengthened laths. The predicted numbers of cycles to crack initiation agree well with experimental data. A strong dependence on the precipitate size is demonstrated, indicating a detrimental effect of coarsening of precipitates on fatigue at elevated temperature.

Micromechanical finite element modelling of thermo-mechanical fatigue for P91 steels

In this paper, the cyclic plasticity and fatigue crack initiation behaviour of a tempered martensite ferritic steel under thermo-mechanical fatigue conditions is examined by means of micromechanical finite element modelling. The crystal plasticity-based model explicitly reflects the microstructure of the material, measured by electronic backscatter diffraction. The predicted cyclic thermo-mechanical response agrees well with experiments under both in-phase and out-of-phase conditions. A thermo-mechanical fatigue indicator parameter, with stress triaxiality and temperature taken into account, is developed to predict fatigue crack initiation. In the fatigue crack initiation simulation, the out-of-phase thermo-mechanical response is identified to be more dangerous than in-phase response, which is consistent with experimental failure data. It is shown that the behaviour of thermo-mechanical fatigue can be effectively predicted at the microstructural level and this can lead to a more accurate assessment procedure for power plant components.

Microscale deformation of a tempered martensite ferritic steel: Modelling and experimental study of grain and sub-grain interactions

In this paper, a finite-element modelling framework is presented with explicit representation of polycrystalline microstructure for a tempered martensite ferritic steel. A miniature notched specimen was manufactured from P91 steel with a 20,000h service history and tested at room temperature under three point bending. Deformation at the microscale is quantified by electron back scattered diffraction (EBSD) before and after mechanical loading. A representative volume element was developed, based on the initial EBSD scan, and a crystal plasticity model used to account for slip-based inelastic deformation in the material. The model showed excellent correlation with the experimental data when the relevant comparisons were made.

The nucleation of Mo-rich Laves phase particles adjacent to M23C6 micrograin boundary carbides in 12% Cr tempered martensite ferritic steels

We study the nucleation of Mo-rich Laves phase particles during aging and creep of 12wt.% Cr tempered martensite ferritic steels (TMFS). Recently, in Isik et al. (2014) we reported that Laves phase particles tend to form at micrograin boundaries of TMFSs after Mo and Si had segregated from the ferritic matrix to these internal interfaces. In the present work, we employ transmission electron microscopy (TEM) and atom probe tomography (APT) to study the formation of Laves phase particles. We investigate the preference of Laves phase particles to nucleate next to M23C6 micrograin boundary carbides. Our results suggest that this joint precipitation effect is due to the combined segregation of Mo and Si from the matrix to the micrograin boundaries and Si and P enrichment around the growing carbides.

Classification of creep crack and cavitation sites in tempered martensite ferritic steel microstructures using MTEX toolbox for EBSD

Tempered martensite ferritic steels exhibit a complex, hierarchical microstructure. Previous work has demonstrated the utility of electron backscatter diffraction (EBSD) analysis methods to classify crack and cavitation sites in terms of the substructural units and their boundaries. However, manual, interactive application of such methods is time consuming. The open-source MTEX toolbox for MATLAB allows scripting and automation of EBSD data analysis procedures. Here we present a validation of MTEX-based methods for identification of prior austenite grains, blocks and packets against the results of manual analysis for two example datasets, one exhibiting cracking and the other, cavitation.

On the nucleation of Laves phase particles during high-temperature exposure and creep of tempered martensite ferritic steels

This paper reports on the formation of an Mo-rich Laves phase during high-temperature exposure and creep of a tempered martensite ferritic steel with 12wt.% Cr and 1wt.% Mo. The material was exposed to 550°C for time intervals between 864 and 81,984h. For comparison, a few creep tests were carried out at 550°C and 120MPa (duration between 864 and 12,456h). All tests were interrupted after specific time periods and microstructures were investigated using transmission electron microscopy and atom probe tomography. Laves phase formation occurs during both heat treatment and creep. Creep stress and strain have no significant effect on the early stages of Laves phase formation. In the present work we show that prior to Laves phase nucleation Si and Mo segregate to micrograin boundaries, where subsequently Laves phase particles appear next to M23C6 carbides.

Characterization of the creep properties of heat resistant 9–12% chromium steels by miniature specimen testing

A miniature specimen geometry is presented that allows to extract small samples for tensile creep tests directly from critical components of steam generators in power plants like e.g. superheater tubes. In this way, material can be tested which has been subjected to the full processing chain of the component. The specimens then exhibit all microstructural modifications that are present in the component after the shaping process and heat treatment, and representative mechanical properties are determined. Similarly, specimens can be extracted from pre-corroded test pieces or used components after service to determine the impact of complex aging/loading/oxidation conditions during service on the mechanical properties of the material. The applicability of the miniature specimen test method is demonstrated in a comparative creep study involving standard and miniature specimens of P91 tempered martensite ferritic steel. Test results indicate satisfactory accuracy/repeatability of the method. Comparison with large scale specimen data reveals an influence of specimen size on the obtained creep behavior. These size effects need to be considered for a correct interpretation of results from miniature specimen creep tests.

Advanced scanning transmission stereo electron microscopy of structural and functional engineering materials

Stereo transmission electron microscopy (TEM) provides a 3D impression of the microstructure in a thin TEM foil. It allows to perform depth and TEM foil thickness measurements and to decide whether a microstructural feature lies inside of a thin foil or on its surface. It allows appreciating the true three-dimensional nature of dislocation configurations. In the present study we first review some basic elements of classical stereo TEM. We then show how the method can be extended by working in the scanning transmission electron microscope (STEM) mode of a modern analytical 200 kV TEM equipped with a field emission gun (FEG TEM) and a high angle annular dark field (HAADF) detector. We combine two micrographs of a stereo pair into one anaglyph. When viewed with special colored glasses the anaglyph provides a direct and realistic 3D impression of the microstructure. Three examples are provided which demonstrate the potential of this extended stereo TEM technique: a single crystal Ni-base superalloy, a 9% Chromium tempered martensite ferritic steel and a NiTi shape memory alloy. We consider the effect of camera length, show how foil thicknesses can be measured, and discuss the depth of focus and surface effects.

On the nature of internal interfaces in a tempered martensite ferritic steel and their evolution during long-term creep

Orientation relationships between ferrite micrograins in a 12% Cr tempered martensite ferritic steel are characterized before and at intermediate times during long-term creep (120 MPa at 550 °C) up to ∼140,000 h. Orientations inherited from the martensitic microstructure during tempering deviate significantly from the well-known/idealized orientation relationships. The observed relationships between micrograins persist throughout creep. The spatial densities of former sub-block, block, and packet boundaries all decrease during creep, suggesting that coarsening takes place on all microstructural scales.

Study on the applicability of the measurements of magnetoelastic properties for a nondestructive evaluation of thermally induced microstructure changes in the P91 grade steel

The paper presents the results of the investigation of the influence of duration and temperature of tempering process on the magnetoacoustic properties of P91 tempered martensite ferritic steel. During tempering of martensitic steels the dislocation density, which initially is very high, systematically decreases what is confirmed both by analysis of the width of X-ray diffraction peaks and hardness measurements. Magnetic hysteresis loops undergo a systematic change resulting in the monotonous decrease of magnetic “hardness” of the investigated samples. The influence of the tempering time on the Barkhausen noise (BN) and magnetoacoustic emission (MAE) signals intensity was investigated in order to verify the possibility of application of those methods for a nondestructive monitoring of heat treatment quality. Barkhausen noise signal envelopes change their shape very strongly during the initial stages of tempering, yet the overall signal intensity is weakly affected. As for the magnetoacoustic emission a very strong increase of its intensity is observed for almost all the tempering times and temperatures. It was concluded that the MAE signal intensity is a suitable parameter for the evaluation of tempering time, as for the BN signal the envelope shape analysis would be necessary for such a purpose.