Cr Martensitic Steel Research Articles

Evolution of strengthening factors during long-term aging at 650 °C in advanced 10% Cr heat-resistant steel

The effect of long-term aging for 1000…∼40,000 h at 650 °C on the tensile strength at ambient temperature was studied in a low-nitrogen and high-boron 10%Cr martensitic steel. In order to establish the reason for increasing the yield stress and ultimate tensile strength after 10,000 h of aging, the evolution of strengthening factors was analyzed. A decrease in the substructure and solid solution strengthening during long-term aging is compensated by an increase in the dispersion strengthening due to the precipitation of V-rich MX carbonitrides.

Evolution of the tempered lath structure of the 12%Cr steels with low N and high B contents during creep

9-12%Cr martensitic steels are promising materials for elements of boilers, tubes, pipes, heaters and steam blades for fossil power plants, which are able to operate at ultra-supercritical parameters of steam. Creep tests were carried out for two 12%Cr and 9%Cr steels at 650°C. After tempering at 750-770°C, a tempered lath structure was revealed in all the steels studied. The 12%Cr-Ta steel had the smallest lath width among the steels studied that provided the longest creep rupture time after creep test at 650°C/120 MPa. During creep, the applied stress causes the lath growth and the formation of subgrains instead of martensitic laths. The experimental sizes of subgrains and equilibrium subgrains, calculated on balance of driving and retarding forces and using the dependence of the size of subgrains on the applied stress during creep, were compared to estimate the contributions of secondary particles and applied stress to the growth of subgrains during tempering and creep in the different steels.

Investigation on solidification and phase transformation of heterogenous interface structure in dissimilar metal weld between high Cr heat-resistant steel and nickel-based alloy 617

The present study characterized heterogenous interface structure in dissimilar metal weld between a high Cr martensitic heat-resistant steel and nickel-based alloy weld metal (WM), and revealed its solidification and phase transformation processes during welding. The heterogenous interface structure consists of a martensitic peninsula structure and a partially mixed zone (PMZ) of austenitic stainless steel. The K–S relationship between peninsula structure and its surroundings indicates: 1. phase transformation of (δ-Fe) to (γ-Fe) of peninsula occurs only after previous austenitic PMZ has solidified; 2. the subsequent WM solidifies epitaxially on (γ-Fe) of peninsula. Due to that the liquidus temperature of PMZ is lower than that of base metal (BM), the peninsula structure can form in the early stage of solidification by quick freezing of liquid BM.

The mechanism of thermal oxide film formation on low Cr martensitic stainless steel and its behavior in fluoride-based pickling solution in conversion treatment

(Attention for corrosion science, the abstract must not exceed more than 100 words)In this research, first a multi-analytical approach was carried out to propose a mechanistic model for the formation of the thermal oxide film on low Cr stainless steel. It describes the effects of kinetically and thermodynamically driven phenomena on the growth of a Fe-Cr complex oxide film at 950℃. Second, the behavior of the thermal oxide film in the presence of fluoride ions, as a relevant pickling agent in conversion treatment, was investigated. It was shown that fluoride ions enter the oxide film and form a complex with Fe and preferentially dissolve hematite islands through a complexation mechanism.

Influence of thermomechanical fatigue loading conditions on the nanostructure of secondary hardening steels

Abstract Dual hardening steels reach their well-balanced mechanical properties in terms of strength and toughness through the combination of secondary hardening carbide and intermetallic particle precipitation. This characteristic profile makes them well suited for hot-work applications. In this study, out-of-phase thermomechanical fatigue tests, recreating operating conditions present during hot-work applications, were performed on a dual hardening steel and a 5% Cr martensitic hot-work tool steel. Via high resolution analysis utilizing atom probe tomography and transmission electron microscopy, the behaviour of the different precipitate populations under combined thermal and mechanical loading conditions were compared. Coarsening of the different precipitates and partial dissolution of the intermetallic compounds was observed. It could be shown that with rising maximum fatigue test temperature, the dual hardening steel reaches an increased lifetime caused by its higher tempering resistance.

Primary creep regeneration in 10%Cr martensitic steel: In-situ and ex-situ microstructure studies

Primary creep regeneration (PCR) is a phenomenon observed during stress-varying/cyclic creep loading conditions where a load reversal might clear the previous strain hardening memory and cause the regeneration of the primary creep regime (i.e. a period of high creep strain rate) upon reloading. In this study, in-situ and ex-situ microstructural examinations, including transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), neutron and synchrotron X-ray diffraction were conducted to better understand the responsible mechanisms of PCR for a 10%Cr martensitic steel at 600 °C. Our experimental evidence indicated that the PCR phenomenon is related to the change of dislocation density due to activation of dislocation generation and recovery mechanisms, formation and relaxation of dislocation pile-ups, as well as bowing/unbowing of dislocation-lines during stress-varying creep loading conditions. These mechanisms could explain the observed creep strain accumulation in the steel during the examined stress-varying creep loading conditions reported in the current and previous studies. The presented mechanistic description of the PCR phenomenon and the reported experimental observations for the microstructural and mechanical parameters can provide a basis for the formulation of physically-based models to describe the creep behaviour of the steel under high-temperature stress-varying creep loading conditions.

Cr Segregation and Impact Fracture in a Martensitic Stainless Steel

The fracture surfaces of a 10.5 wt.% Cr martensitic stainless steel broken in Charpy tests have been investigated through X-ray photoelectron spectroscopy (XPS). The specimens have been examined in two different conditions: as-quenched and heat treated for 10 h at 700 °C. The trends of Fe/Cr ratio vs. test temperature are similar to the sigmoidal curves of absorbed energy and, after both ductile and quasi-cleavage brittle fractures, such ratio is always significantly lower than the nominal value of the steel chemical composition. Cr segregation does not occur on a macroscopic scale but takes place in microscopic zones which represent weaker spots in the steel matrix and a preferred path for moving cracks. Small area (diameter 300 µm) XPS measurements evidenced a higher density of such microscopic zones in the inner part of probes; this is explained by the different diffusion length of Cr atoms in the external and inner parts during quenching from austenitic field which has been calculated through FEM simulations. No significant differences of Cr concentration were observed in fracture surfaces of probes with and without heat treatment. The results highlight how Cr segregation plays a role not only in the intergranular mode of fracture but also in the quasi-cleavage and ductile ones.

The investigation of primary creep regeneration for 10%Cr martensitic steel: Unified constitutive modelling

An elastic-viscoplastic constitutive material model is developed for the representation of the creep response of a 10%Cr steel under cyclic loading conditions. It has been shown that the model is able to describe primary creep regeneration (PCR), i.e. the incidence of a period of high creep strain rate following a stress reversal. The developed model is a variant of the well-known Chaboche viscoplastic constitutive model, and employs a bi-term power-law equation to represent the stress-regime dependence of the viscoplastic strain rate response. Back stress and drag stress are used to describe the kinematic and isotropic hardening/softening behaviour of the material, respectively. The evolutions of back stress and drag stress consider contributions from strain hardening, dynamic softening, static recovery and cyclic hardening/softening.The effectiveness of the developed model for describing the sensitivity of the PCR behaviour to different loading parameters (e.g. reverse-loading magnitude and duration) and to represent the effect of PCR activation on the overall strain accumulation behaviour of the material is discussed. Furthermore, the predictive capability of the model is demonstrated for describing the response of the material during two independent benchmark tests; a stress-varying creep and a low-cycle fatigue experiments.

Creep behaviour of a high chromium martensitic steel under stress varying creep loading conditions: Primary creep regeneration (PCR)

Primary creep regeneration (PCR) is an important phenomenon observed for some alloys loaded at high-temperature and under stress varying loading conditions. For example, for a specimen deforming in the secondary creep regime, a stress reversal might result in an enhanced creep rate upon reloading due to full/partial re-activation of the primary creep stage (i.e. primary creep regeneration). The extent of PCR activation is expected to be dependent on different loading parameters (e.g. magnitude and duration of reverse loading) and the characteristics of the loaded alloy. In this study, a comprehensive testing program was designed and conducted to investigate the PCR behaviour of an advanced 10%Cr martensitic steel at 600 °C. The experimental observations characterized the influence of different loading parameters on the extent of PCR activation. It was found that the inelastic strain accumulation during reverse loading periods is the most important parameter governing the extent of PCR activation after stress transients. Furthermore, the gathered experimental data was analysed to understand the effect of different loading parameters on anelastic recovery behaviour of the steel. It was shown that a larger amount of strain recovers after unloading for tests with higher forward stress magnitudes and/or larger amounts of pre-accumulated inelastic strain.

The Role of Deformation in Coarsening of M23C6 Carbide Particles in 9% Cr Steel

The kinetics of the growth of M23C6 carbide particles in 9% Cr martensitic steel with a high boron content is studied under conditions of long-term aging and creep. It is shown that the grain-boundary diffusion is a mechanism that controls the coarsening of these carbides during the aging. Deformation accelerates the coarsening of particles during creep, which can be explained by additional diffusion along dislocation lines.

Comparison of primary creep regeneration and anelastic recovery behaviour of 316H austenitic and 10%Cr martensitic steels

ABSTRACT Primary creep regeneration (PCR), i.e. re-occurrence of the primary creep stage for a material loaded under stress-varying creep conditions, is an important consideration for the design and assessment of high-temperature components operating under cyclic loading conditions. This study reviews observations from a comprehensive creep experiment program aiming to understand the PCR behaviour of two high-temperature steels with very different metallurgical-microstructural characteristics at their respective application temperatures; namely 316H austenitic stainless steel at 650°C and a 10%Cr martensitic steel at 600°C. The program included twelve (stress-varying) creep tests for each steel which were systematically designed to characterise the PCR and anelastic recovery response of the steels under various loading scenarios. The experimental observations showed that the PCR behaviour of both steels is similarly sensitive to the parameters of the loading profile and PCR is more pronounced for e.g. larger reverse-loading magnitudes or longer reverse-loading durations. Detailed analysis however showed that the early PCR response is different for the two steels and the 10%Cr steel showed significantly higher creep rates immediately after a stress-transient. Similarly, it was found that although the anelastic recovery response of both steels shares similar sensitivity to the parameters of the prior-loading profile, the extent of anelastic recovery is less significant for the 316H steel. The observed PCR and anelastic recovery behaviour of the two steels have been interpreted based on their dislocation-structure and microstructural features.

Effect of tungsten on creep properties of martensitic 9Cr steel

The effects of tungsten on the creep behavior and microstructure of tempered martensite 9Cr steel with different W concentrations ranging from 0–2.5 wt% were studied. The matrix consisted of tempered lath martensite structure and M23C6 located along the grain boundary or inside the grain. As the W concentration increased, the number of M23C6 increased while the particle size decreased. Also, with the increasing W concentration, the minimum creep rate and elongation decreased while the time to rupture increased. The rise in W concentration led to the refinement of M23C6, which enhanced the creep strength but reduced the creep plasticity.

Effect of tantalum on short-term creep of a 12%Cr-3%Co-0.07%Ta martensitic steel for steam blades

9-12%Cr martensitic steels are prospective materials for elements of boilers, tubes, pipes, heaters and steam blades for fossil power plants, which are able to work at ultra-supercritical parameters of steam. Decreasing N content and increasing B content together with increasing Cr content required the optimization of ferrite-stabilizing and austenite-stabilizing elements to avoid the formation of δ-ferrite at high temperatures. Addition of 3-4%Co, 0.01%B, 0.80%Cu and 0.07%Ta as austenite-stabilizing elements can compensate increasing Cr content up to 12% at decreasing N content to 0.003% and provides the δ-ferrite in the amount less than 10%. Two Co-containing 12%Cr steels were developed and investigated. The main difference between these steels is Ta addition; one of the 12%Cr steel studied contains 0.07%Ta, other 12%Cr steel is Ta-free one. Creep tests at 650°C and the applied stresses ranging 180-80 MPa with a step of 20 MPa for both Co-containing 12%Cr steels were carried out. Addition of Ta in the Co-containing 12%Cr steel provide the increment in the creep time to rupture at both high and low applied stresses. This increment in the creep properties due to Ta addition was explained by decrease in coarsening rate of Ta-rich MX carbonitrides during creep at 650°C.

Near-threshold fatigue crack growth behavior of 10% Cr martensitic steel welded joint with 9% Cr weld metal in high temperature air

The near-threshold fatigue crack growth (FCG) behavior under different stress ratios (R) of a 9%–12% Cr steel welded joint was studied in 538 °C air. The most significant oxide-induced crack closure (OICC) occurred in weld metal (WM) due to the lowest Cr content, resulting in the highest threshold value (ΔKth) of WM. The base metal (BM) with coarser grains and denser lath structures had more obvious roughness-induced crack closure (RICC) and more severe crack deflection, which led to a higher ΔKth of BM than heat affected zone (HAZ). The ΔKth of WM, HAZ and BM decreased as R increased.

XPS study of Cr segregation in a martensitic stainless steel

Cr martensitic steels are promising materials for structural applications in future nuclear fusion reactors. Because the embrittlement after tempering treatments can be a serious problem, the fracture mode of a steel with 10.5 wt% of Cr treated at 700°C for 18 h has been investigated through Charpy tests in the temperature range from −100°C to +150°C. X‐ray photoelectron spectroscopy (XPS) analyses carried out on the fracture surfaces evidenced the segregation of Cr in both ductile and brittle (quasicleavage) fields. The unexpected result indicates that Cr segregation weakens the atomic bonds; thus, the fracture path in both the cases corresponds to the zones with higher Cr content.

Effect of prior deformation on the subsequent creep and anelastic recovery behaviour of an advanced martensitic steel: Unified constitutive modelling

This paper presents an elastic-viscoplastic constitutive model formulation for the representation of different aspects of high-temperature deformation behaviour of an advanced martensitic steel under various loading conditions. The model is a variant of the well-known Chaboche constitutive model and uses a bi-term power-law equation for describing the stress dependency of the viscoplastic strain rate. Isotropic and kinematic hardening/softening considerations have been incorporated through the employment of drag-stress and back-stress concepts. The evolution equations for drag-stress and back-stress consider contributions from strain hardening, dynamic strain softening, and static recovery. The effectiveness of the model formulation has been examined for the representation of the deformation behaviour of a 10%Cr martensitic steel at 600 °C. It has been demonstrated that the developed model could provide a fairly accurate description for i) constant-load creep, ii) constant-strain stress-relaxation, iii) anelastic recovery, and iv) negative creep behaviour of the alloy. More importantly, evidence on the capability of the model for taking into consideration the effect of prior-loading on the subsequent viscoplastic response has been presented.

Effect of microstructural evolution on the cyclic softening of a 10% Cr martensitic steel under low cycle fatigue at 600 °C

Microstructure evolution and softening during interrupted low cycle fatigue (LCF) tests at 600 °C and constant strain amplitudes of ±0.2% and ±0.6% corresponding to dominance of the elastic and plastic strain components, respectively, was examined in a 10% Cr steel. Cyclic softening attributed to changes in the strengthening from dislocations and lath structure is more pronounced during the first half-life. The Burgers vector analysis of dislocation reveals the role of the single and multiple dislocation slip in retention and transformation of lath structure at cycling with low and high strain amplitude, respectively.

Electron beam welding of oxide dispersion strengthened 9 Cr martensitic steel-an experimental and theoretical perspective

Abstract Advanced nuclear energy systems present significant improvements in economics, safety, reliability and sustainability over currently operating reactor technologies. Proper choices of cladding and structural material with a. high neutron damage resistance b. good mechanical properties at high operating temperatures and c. low activation properties are essential for the safe and reliable operation of the system. The 9-12% Cr transformable steels with lower carbon (∼0.1%) contents and addition of Mo, W, V, Nb present better high temperature strength than austenitic steels. The use of 9%Cr steel is considered as an optimum solution taking into consideration the present day requirements like environment, efficiency, elasticity, experience and economy that dominate considerations of modern power station design, layout, fabrication and operation. The pursuit of improved efficiency/power plant performance resulted in the design of system operating at higher temperature and pressure, which requires materials with appropriate high-temperature mechanical properties and metallurgical stability at operating temperatures. Oxide Dispersion Strengthened (ODS) 9% Cr steels have the potential to advance the high temperature performance of associated components in advanced nuclear energy systems with marginal oxidation resistance. The research focuses on Electron beam welding (EBW) of Hot Isostatically Pressed (HIP) ODS 9 Cr martensitic steel in as-received condition to arrive at the optimized weld process parameters. The weldments were characterized using Optical Microscopy for depth of penetration and width of weldments. The experimental results were theoretically verified using Minitab software

On the microstructural evolution in 12% Cr turbine steel during low cycle fatigue at elevated temperature

In order to better understand the physical process of deformation and cyclic softening a 12% Cr martensitic stainless steel FV566 has been cyclically tested at high temperature in strain control. Increase in temperature was found to increase the cyclic life, softening rate and viscous stress magnitude. An increase in the dwell time led to the acceleration of the material degradation. The microstructure changes and dominating deformation mechanisms were investigated by means of scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The results have revealed a gradual sub-grain coarsening, transformation of lath structure into fine equiaxed subgrains, and misorientation angle development in blocks and packets until material failure. The softening behaviour was attributed to the mutual annihilation of the dislocations and rearrangement of the residual dislocation in the low energy structures.

High-Temperature Creep Tests of Two Creep-Resistant Materials at Constant Stress and Constant Load

Creep is defined as a time dependent component of plastic deformation. Creep tests can be performed either at constant load or at constant applied stress. Engineering creep tests carried out at constant load are aimed at determination of the creep strength or creep fracture strength, i.e. the data needed for design. The constant stress tests are important as a data source for fundamental investigations of creep deformation and fracture mechanisms and for finite element modelling of more complex stress situations. For some materials, the difference between the two type of testing can be very small, while for other materials is large, depending on the creep plasticity of the material under testing. The paper aims to compare the creep results of two different creep-resistant materials: the advanced 9%Cr martensitic steel (ASME Grade P91) and a Zr1%Nb alloy obtained by both testing methods and to clarify the decisive factors causing observed differences in their creep behaviour.