FM Steel Research Articles

Dependence of fretting wear resistance on microstructural features of alloyed steels

Abstract Systematic experimental observations concerning the response of microstructural features on fretting wear of steel grades were conducted. Five steel grades with different typical microstructural features were selected. The effect of various microstructures on fretting wear behaviour and the resulting wear mechanism were evaluated. Results show that microstructural features play a significant effect on fretting wear resistance depending on the applied loads. The DP and Q&P steel having low hardness show comparable or even better fretting wear resistance than the FM steel of the highest hardness regardless of loads, which are all better than TWIP and IF steel. The result suggests that the combination of soft ferrite and hard martensite with/without retained austenite having low hardness displays superior fretting wear resistance.

Ratcheting behavior of Eurofer97 at 550 °C

The components in reactor are usually exposed to cyclic loading caused by frequent startups, shutdown and load fluctuations which lead to ratcheting effect. Eurofer97 is a structural material candidate for in-vessel components of future fusion reactor. To investigate the ratcheting behavior of Eurofer97, uniaxial stress-controlled cyclic loading tests are performed at 550 °C. Ratcheting rates under various loading conditions, including various peak stresses, stress ratios and stress rates are determined to build the database of Eurofer97 for future application in the blankets of fusion reactor. An already developed unified visco-plastic constitutive model for mod.9Cr-1Mo FM steels is further applied to adapt the ratcheting behavior of Eurofer97. The developed model describes both cyclic softening in strain-controlled LCF tests and its impact on the ratcheting behavior of mod.9Cr-1Mo FM steel fairly good, at both room temperature and 550 °C. Verifying whether the current model can also describe the ratcheting behavior of Eurofer97 is a further aim of the work reported here.

Abrasion resistance characterization of low alloy construction steels: A comparison between three different scratch test protocols

In the present work, three different scratch tests are compared on their ability to rank the abrasion resistance of low alloy steels for industrial applications where the abrasion play a key role, e.g., in earthmoving, agricultural and mining equipment. The first test involves single pass scratching of pristine surfaces with a relatively large rigid indenter. The second test involves multi-pass scratching along a fixed track using the same large indenter. The third test involves the creation of a multi-pass scratch track using the same large indenter followed by final scratching of the abrasion track with a sharp indenter, i.e., Multi-Pass Dual Indenter (MPDI) scratch test. The three test protocols activate different abrasion mechanisms. Five low alloy construction steel grades with different strain hardening capabilities, i.e., Interstitial-free Ferritic steel (IF steel), Fully Martensitic steel (FM steel), Dual Phase steel (DP steel), Quench Partitioning steel (Q&P steel) and TWining Induced Plasticity steel (TWIP steel), are used. The results show that for both single (large) indenter scratch test protocols, the scratch depths always increase with indenter load and the failure mechanism is pure ploughing except for the IF steel due to the nature of softness. While for the dual-indenter scratch test, the scratch depth is a more complex function of the load applied during creation of the work hardened surface layer. The conventional scratch test protocols cannot well reveal micro-cracks and defect in (sub) surface and cannot well reflect the real response of strain hardening of steels. However, the MPDI test can distinguish between different steels with different initial hardness and different strain hardening behaviour and reveal the damage and, equally important, can indicate how average loading conditions may affect the relative abrasion resistance of construction steels during steady state conditions.

Void swelling in high dose ion-irradiated reduced activation ferritic–martensitic steels

To determine the void swelling resistance of reduced-activation ferritic–martensitic steels CNS I and CNS II at high doses, ion irradiation was performed up to 188dpa (4.6×1017ion/cm2) at 460°C using 5MeV Fe++ ions. Helium was pre-implanted at levels of 10 and 100 appm at room temperature to investigate the role of helium on void swelling. Commercial FM steel T91 was also irradiated in this condition and the swelling results are of included in this paper as a reference. Voids were observed in all conditions. The 9Cr CNS I samples implanted with 10 appm helium exhibited lower swelling than 9Cr T91 irradiated at the same condition. The 12Cr CNS II with 10 and 100 appm helium showed significantly lower swelling than CNS I and T91. The swelling rate for CNS I and CNS II were determined to be 0.02%/dpa and 0.003%/dpa respectively. Increasing the helium content from 10 to 100appm shortened the incubation region and increased the void density but had no effect on the swelling rates.

A novel multi-pass dual-indenter scratch test to unravel abrasion damage formation in construction steels

A proper understanding of abrasion resistance and associated damage mechanisms is of vital importance in the design of improved abrasion resistant construction steels. The conventional scratch test, sliding a rigid indenter under a controlled load and speed against a smooth surface, mimics the nature of the abrasion process and can be used to evaluate the abrasion resistance of various microstructures. However, scratch tests are mostly done on the initial surface, which can be very different from that formed during the abrasion process and hence do not truly reflect its abrasion resistant response. In the present work, a new scratch test methodology is developed to approach the real abrasion condition by carrying out a multi-pass dual indenter scratch tests, in which a small indenter scratches a pre-scratched surface produced by a large indenter. Five steel grades with different work hardening capacities, i.e. Interstitial-Free Ferritic steel (IF steel), Fully Martensitic steel (FM steel), Dual Phase steel (DP steel), Quench Partitioning steel (Q&P steel) and TWining Induced Plasticity steel (TWIP steel) were selected. Systematic scratch resistance experiments were performed to investigate the damage mechanisms, the work hardening behavior and the development of subsurface deformation layers. Results suggest that the work hardening layer formed beneath the abraded surface plays the dominant role in determining the abrasion resistance. Steels grades of DP, Q&P and TWIP display superior scratch and abrasion resistances, notwithstanding their relative low hardness compared to that of a corresponding steel with a fully martensitic microstructure.

Stability of MX-type strengthening nanoprecipitates in ferritic steels under thermal aging, stress and ion irradiation

The stability of MX-type precipitates is critical to retain mechanical properties of both reduced activation ferritic–martensitic (RAFM) and conventional FM steels at elevated temperatures above ∼500°C. The stability of TaC, TaN and VN nanoprecipitates under thermal aging (600 and 700°C), creep (600°C) and ion irradiation (Fe ion, 500°C) conditions was systematically studied in this work. The statistical particle evolution in density and size was characterized using transmission electron microscopy. Nanoprecipitate stability under the studied conditions manifested differently through either dissolution, reprecipitation, growth or fragmentation, with TaC exhibiting the greatest stability followed by VN and TaN in sequence. Nanoprecipitate evolution phenomena and mechanisms and the apparent disagreement of this interpretation with published literature on the subject are discussed. These findings not only help understanding the degradation mechanisms of RAFM and conventional FM steels at elevated temperatures and under stress and irradiation, but should also prove beneficial to the development of advanced RAFM steels.

Stability of the strengthening nanoprecipitates in reduced activation ferritic steels under Fe2+ ion irradiation

The stability of MX-type precipitates is critical to retain mechanical properties of both reduced activation ferritic–martensitic (RAFM) and conventional FM steels at elevated temperatures. Radiation resistance of TaC, TaN, and VN nanoprecipitates irradiated up to ∼49dpa at 500°C using Fe2+ is investigated in this work. Transmission electron microscopy (TEM) utilized in standard and scanning mode (STEM) reveals the non-stoichiometric nature of the nanoprecipitates. Irradiation did not alter their crystalline nature. The radiation resistance of these precipitates, in an order of reduced resistance, is TaC, VN, and TaN. Particle dissolution, growth, and reprecipitation were the modes of irradiation-induced instability. Irradiation also facilitated formation of Fe2W type Laves phase limited to the VN and TaN bearing alloys. This result suggests that nitrogen level should be controlled to a minimal level in alloys to gain greater radiation resistance of the MX-type precipitates at similar temperatures as well as postpone the formation and subsequent coarsening of Laves phase.

Effects of alloying elements and thermomechanical treatment on 9Cr Reduced Activation Ferritic–Martensitic (RAFM) steels

RAFM steels are one of the candidate structural materials for fusion reactors, in which tantalum (Ta) and tungsten (W) are alloyed to replace niobium (Nb) and molybdenum (Mo) in conventional FM steels, respectively. This paper, using three RAFM heats, presents the effects of Ta and the primary austenite stabilizer carbon (C) on microstructure and strength. Thermomechanical treatment (TMT) was also applied to the heats, leading to significant increases in strength, attributable to the TMT-refined sub-grains and precipitates. The Ta-alloying favored the formation of (V,Ta)(N,C) and (Ta,V)C and exhibited greater strength. Fractographs also revealed the beneficial effects of TMT and Ta-alloying. However, extra C content, favoring a larger amount of M23C6 precipitates, did not show strengthening effect.

R&D OF SICF/SIC COMPOSITE FOR FUSION REACTOR MATERIAL

During the operation of a fusion reactor, a certain type of transients could induce rapid cooldown of the reactor with relatively high or increasing system pressure. This induces a high tensile stress at the inner surface of the blanket, which is called the thermal shock. Test Blanket Module (TBM) has been designed to account for the performance of DEMO-relevant blanket. But, the TBM uses helium as coolant and tritium as purge gas, and FM steel, SiC f / SiC composites as a structural material. The SiC f / SiC composites with a SiC sub-layer/ PyC interlayers were therefore fabricated in order to improve the oxidation resistance. In the refractory field SiC f / SiC composites need to investigate residual stress and thermal shock behavior when high chemical stability and corrosion resistance are necessary. In this study, SiC f / SiC composite were fabricated by NITE process and the chemical vapor infiltration (CVI) method. SiC f / SiC composites have been examined and the detail analyses of the microstructure and the mechanical strength were conducted. Various pyrolytic carbons ( PyC ) with SiC interlayer coatings were applied to the composites for improvement of the mechanical properties at the interface.

Study on the cold working process for FM steel cladding tubes

A cold working process for FM steel cladding tubes of the sodium-cooled fast reactor was studied. Hot-rolled 9Cr–2W steel plates were heat-treated to dissolve the M 23C 6 and V-rich MX precipitates into the matrix, followed by several cold rollings and intermediate/final heat treatments with the purpose of nucleating the dissolved precipitates on the dislocations formed through the cold rolling. As a result, it was possible to obtain the FM steels with very fine and uniform precipitates which were more promising for superior tensile properties.

Performance of Al‐rich oxidation resistant coatings for Fe‐base alloys

AbstractThe oxidation resistance of Al‐rich coatings made by chemical vapor deposition and pack cementation was examined on representative ferritic‐martensitic (FM, e.g. Grade 91, Fe‐9Cr‐1Mo) and austenitic steel substrates at 650°‐800 °C. To evaluate the potential benefits and problems with these alumina‐forming coatings, oxidation exposures were conducted in a humid air environment where the uncoated substrates experience rapid oxidation, similar to steam. Exposure temperatures were increased to accelerate failure by oxidation and interdiffusion of Al into the substrate. The difference in the coefficient of thermal expansion (CTE) between coating and substrate was found to cause cracking and coating failure during rapid thermal cycling on thicker coatings with Fe‐Al intermetallic phases. Therefore, thinner coatings with less Al and a ferritic Fe(Al) structure were evaluated more extensively and tested to failure at 700° and 800 °C on FM steels. The remaining Al content at failure was measured and used to improve a previously developed coating lifetime model. At 700° and 800 °C, thin coated austenitic specimens continue to exhibit protective behavior at more than double the lifetime of a similar coating on FM steel. The longer lifetime was attributed to the ferritic coating‐austenitic substrate phase boundary inhibiting Al interdiffusion.

R&D activities on helium cooled solid breeder TBM in Korea

R&D activities currently being undertaken for HCSB TBM include joining technologies of structural material, breeder and reflector pebble material development, the effect of TBM ferritic–martensitic steel on the ripple of toroidal magnetic field, and ceramic coating on graphite pebble. The HIP joining performance of FM steel is evaluated. Lithium ceramic breeder and graphite reflector pebble fabrication methods are under development using special fabrication process, and the initial characteristics of the pebbles are assessed. Silicon carbide coating on graphite pebble is also investigated and its preliminary results are mentioned. Finally, an accurate evaluation of the effect of TBM and ferromagnetic inserts on magnetic field are implemented. The current results of these R&D issues are addressed in this paper.

Formation of Nano M<sub>2</sub>X Particles by a Tempering in High Cr Ferritic/Martensitic Steel

The precipitation of nano Cr2N particles in high Cr FM steels has been studied. The nitrogen content of the FM steels was changed to form stable Cr2N particles. Tempering temperature was also changed from 500 oC to 800 oC to study the precipitation behavior of the Cr2N particles with the tempering temperature. The Cr2N particles remained as a stable phase at a higher tempering temperature by increasing the nitrogen content. The shape of these particle was a fine needle type which was very similar to V(C,N) particles. The size of some Cr2N particles was increased as the nitrogen content increased. But these precipitates were not dissolved or largely coarsened during a creep deformation at 600°C. So it seems that they may act as an effective obstacle against a dislocation glide during a creep deformation, thus contribute to an increase of the creep rupture strength in high Cr FM steels.

Change of Mechanical Properties of FM Steels after Thermal Aging

The effects of thermal aging on microstructural evolution and mechanical properties are important in the understanding of the in-service behavior of ferritic/martensitic steels in advanced nuclear power system. Ferritic/martensitic steels have been aged at 600oC for times up to 20,000 hrs. The change of mechanical properties has been examined for these aged materials. The strength and hardness was hardly changed after the thermal aging at 600oC for 20,000 hrs in all specimens. The impact absorbed energy decreased with the aging time. But the decrease of the impact absorbed energy was larger at the early stage of aging in tungsten added steels. This is attributed to the formation of Laves phase. Nitrogen which is known to increase the creep rupture strength had no effect on the degradation of the microstructure and mechanical properties during thermal aging.

Fatigue behavior of 1500 MPa bainite/martensite duplex-phase high strength steel

The fatigue properties of C–Si–Mn–Cr low alloy steel can be improved at the tensile strength of 1500 MPa or higher level by the following heat treatment: austenitizing at 900 °C for 20 min followed by air cooling and then tempering at 280 and 370 °C for 2 h. The steel had a duplex-phase of carbide-free bainite and martensite containing retained austenite of 8 vol% and higher (designated as CFB/M steel). The results obtained for the CFB/M steel were compared with those of the fully martensitic steel which was oil-quenched and then tempered at 280 and 370 °C for 2 h after austenitization at 900 °C for 20 min (designated as FM steel). The fatigue strength was determined by a method of axial fatigue test, and the fatigue crack propagation was studied by using compact-tension specimens. Compared to the FM steel, the CFB/M steel increased the fatigue strength and fatigue crack threshold (Δ K th), and then lowered crack propagation rate. In particular, the CFB/M steel had a better combination of strength, toughness and fatigue properties when tempered at 370 °C for 2 h.