Microstructural evolution of oxide-dispersion-strengthened Fe–Cr model steels during mechanical milling and subsequent hot pressing

Oxide-dispersion-strengthened (ODS) ferritic steels of Fe–9Cr–0.3Y₂O₃ and Fe–9Cr–0.2Ti–0.3Y₂O₃ (in mass) incorporating nanoscale oxide particles, were produced by mechanical milling (MM) followed by hot pressing (HP). Microstructural evolution of these two types of ODS steels were structurally characterized at each step of the elaboration processes by means of scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), and optical microscope. The observations of structure of the mixed powders and the nanoscale oxide particles in both ODS steels after MM indicate that the initial powders, coupled with the original yttria powders, get fractured by severe plastic deformation and ultrafine bcc grains (~20 nm) of the matrix and Y₂O₃ nanocrystals with irregular edges are formed during MM. The addition of titanium (Ti) promotes the refinement of bcc grains, Y₂O₃ nanocrystals and the formation of amorphous phase of Y₂O₃ during MM. TEM observations of these two Oxide-dispersion-strengthened (ODS) steels exhibit a very fine structure of micrometer-scale grains in which large number of nanoscale oxide particles are distributed after HP process. The observation of some unreinforced domains without the nanoscale oxide particles indicates that there still exist inhomogeneous areas, although the size of those oxide particles reaches nanoscale. Threshold stress of the HPped Fe–9Cr–0.2Ti–0.3Y₂O₃ steel with the relatively homogeneous dispersion was carefully evaluated on the basis of higher magnified images of the nanoscale oxide particles. Different values of threshold stress were obtained due to the various dispersions of the nanoscale oxide particles within different areas. That may be the reason why the threshold stress cannot be clearly obtained by the results of creep tests.     

Preparation and structural investigation of nanostructured oxide dispersed strengthened steels

Although capability of steels has been improved in the past by thermomechanical treatment, utilization of powder metallurgy provides more controlled microstructure, a homogeneous dispersion of nanosized oxide particles in the metal matrix and tailored properties in terms of strength and radiation resistance. This article is summarizing recent results on preparation, structural, and mechanical investigation of oxide dispersed strengthened steel (ODS). Two commercial steel powders, austenitic 17Cr12Ni2.5Mo2.3Si0.1C and martensitic Fe16Cr2Ni0.2C powders have been used as starting materials. Nanosized yttria dispersed martensitic and austenitic sintered steel samples have been realized by powder metallurgical methods. An efficient dispersion of nano-oxides in ODS steels was achieved by employing high efficient attrition milling. A combined wet and dry milling process of fine ceramic and steel particles is proposed. Spark Plasma Sintering (SPS) was applied to realize nanostructured steel compacts. Grains with 100 nm mean size have been observed by SEM in sintered austenitic ODS. In comparison, the sintered martensitic dry milled and martensitic dry and combined milled ODS microstructure consisted of grain size with 100–300 nm in each case. A brittle behavior is shown in all of the cases. The martensitic ODS is two times harder than the austenitic ODS. The bending strength high as 1806.7 MPa was found for the martensitic ODS, whereas 1210.8 MPa was determined for the austenitic ODS. The combined milling assured higher strength and hardness compared to dry milling.     

Preliminary study on the fabrication of 14Cr-ODS FeCrAl alloy by powder forging

A simple powder forging process was presented herein to fabricate an Fe-14Cr-4.5Al-2W-0.4Ti-0.5Y2O3 ODS FeCrAI alloy.The forged alloy exhibits a high density that exceeds 97％of the theoretical density.The ODS alloy was investigated in terms of the residual porosity,morphology and phase structure of oxide nanoparticles,impact toughness and tensile properties.It was found that refined grains were obtained during powder forging.A residual porosity less than 1.1％has no impact on the precipitation of oxide nanoparticles.The average diameter of the oxide particles is 7.99 nm,with a number density of 2.75 x 1022 m-3.Almost all of the oxides are identified as orthorhombic YAlO3 particles.The refined grains and uniformly distributed oxide nanoparticles enable the alloy to show excellent mechanical strength and ductility below 700℃,and enable the ductile-to-brittle transition temperature to be close to room temperature.However,a slight decrease in strength at 1000℃and the Charpy upper shelf energy has been suggested to be due to the residual porosity.These results indicate that powder forging can be used as a promising technique for the fabrication of ODS alloys.     

Structure of oxide nanoparticles in Fe-16Cr MA/ODS ferritic steel

Oxide nanoparticles in Fe-16Cr ODS ferritic steel fabricated by mechanical alloying (MA) method have been examined using high-resolution transmission electron microscopy (HRTEM) techniques. A partial crystallization of oxide nanoparticles was frequently observed in as-fabricated ODS steel. The crystal structure of crystalline oxide particles is identified to be mainly Y₄Al₂O₉ (YAM) with a monoclinic structure. Large nanoparticles with a diameter larger than 20 nm tend to be incoherent and have a nearly spherical shape, whereas small nanoparticles with a diameter smaller than 10 nm tend to be coherent or semi-coherent and have faceted boundaries. The oxide nanoparticles become fully crystallized after prolonged annealing at 900 °C. These results lead us to propose a three-stage formation mechanism of oxide nanoparticles in MA/ODS steels.     

Structure and mechanical properties of high-manganese dual-phase steels

The structure and mechanical properties of 3 wt% Mn ferritic dual-phase steels were examined as a function of heat treatment. Because of the high maganese content, these alloys can be prepared easily even by a simple air cooling and they show a fine dispersion of martensite phase which leads to higher tensile strength and good ductility. The addition of a small amount of silicon gives beneficial effects: the tensile strength further increases without a significant loss of ductility.     

Relation between microstructure and Charpy impact properties of an elemental and pre-alloyed 14Cr ODS ferritic steel powder after hot isostatic pressing

This article describes the microstructure and Charpy impact properties of an Fe–14Cr–2W–0.3Ti–0.3Y₂O₃ oxide dispersion strengthened (ODS)-reduced activation ferritic (RAF) steel, manufactured either from elemental powders or from an Fe–14Cr–2W–0.3Ti pre-alloyed powder. ODS RAF steels have been produced by mechanical alloying of powders with 0.3 wt% Y₂O₃ nanoparticles in either a planetary ball mill or an attritor ball mill, for 45 and 20 h, respectively, followed by hot isostatic pressing (HIPping) at 1,150 °C under a pressure of 200 MPa for 4 h and heat treatment at 850 °C for 1 h. It was found that the elemental ODS steel powder contains smaller particles with a higher specific surface area and a two times higher oxygen amount than the pre-alloyed ODS steel powder. After HIPping both materials exhibit a density higher than 99%. However, the pre-alloyed ODS steel exhibits a slightly better density than the elemental ODS steel, due to the reduced oxygen content in the former material. Charpy impact experiment revealed that the pre-alloyed ODS steel has a much larger ductile-to-brittle transition temperature (DBTT) (about 140 °C) than the elemental ODS steel (about 25 °C). However, no significant difference in the upper shelf energy (about 3.0 J) was measured. TEM and SEM–EBSD analyses revealed that the microstructure of the elemental ODS steel is composed of smaller grains with a larger fraction of high-angle grains (>15°) and a lower dislocation density than the pre-alloyed ODS steel, which explains the lower DBTT value obtained for the elemental ODS steel.     

高碳工具钢半固态流变轧制

探讨了高碳工具钢常规的加工方法和半固态流变轧制技术，结果表明将在凝固过程中进行搅拌后制备的高碳工具钢半固态浆料直接进行流变轧制，可生产出成分均匀、没有偏析的产品，消除了在常规轧制中很难克服的网状碳化物等缺陷；半固态固相颗粒细小而且呈球状可生产出高强、高韧性的工具钢。半固态轧制有可能成为未来高碳工具钢生产的一种新工艺。     

ODS铁素体钢中弥散氧化物的研究进展

ODS 铁素体钢具有优异的高温力学性能和抗辐照能力, 有望成为新一代先进反应堆如超临界水堆包壳管的候选材料以及未来聚变堆的结构材料.ODS铁素体钢的这种优异性能主要源于其内部弥散分布的细小氧化物颗粒.回顾了国内外所涉及的关于ODS铁素体钢中氧化物弥散强化颗粒的研究进展,包括氧化物弥散粒子的选择、检测、分解和析出机理、存在形式等,展望了目前ODS铁素体钢的应用前景并总结了存在的问题.     

The creep and fracture behaviour of tempered martensitic steels

Creep strength enhanced ferritic steels contain 9 to 12% Cr and were developed to exhibit excellent high temperature properties. These should be achieved when the microstructure exhibits a tempered martensitic matrix containing a substructure with a high dislocation density and a uniform dispersion of fine, second phase precipitates. It is interesting to note that when properly processed the typical alloy compositions for these steels provide reasonable strength but can exhibit brittle creep behaviour. The levels of ductility required in engineering applications necessitate proper control of composition (including trace elements), steel making and processing and all heat treatments. The properties needed for modern design methods can only be obtained using validated procedures for both uniaxial and multiaxial testing and documentation to establish the mechanisms controlling deformation and fracture for relevant stress states.     

Impact of friction stir welding on recrystallization of oxide dispersion strengthened ferritic steel

Oxide dispersion strengthened (ODS) steels can be used as the structural materials in the future fusion reactors and the fuel cladding materials in the advanced fission reactors. However, the weldability of ODS steels is a severe problem. In the present study, defect-free joints of the 15Cr-ODS ferritic steel were achieved by friction stir welding at different rotation speeds. The recrystallization, hardness and tensile properties are highly related with the rotation speed of the stir tool. The higher rotation speed results in coarser grains in the top SZ, while the grain size exhibits more complicated relation with the rotation speed in the SZ center. The joint welded at 250 rpm exhibits a maximum tensile strength of 974 MPa that reaches about 84% of that of the base metal.     

新型纳米强化超高强度钢的研究与进展

随着资源、能源和环境压力日益加大,超高强度钢的开发越来越受到世界各国的极大重视。传统的超高强度钢大都是依赖提高碳含量或合金元素含量而获得较高强度的马氏体或贝氏体钢,此种钢存在着焊接性能差、塑韧性低、钢材尺寸受限制和成本昂贵等问题,严重制约了经济的快速发展和现代国防的建设,因此,开发综合性能良好、成本低廉的新型超高强度钢刻不容缓。结合当前纳米科技的发展,介绍了新型纳米强化超高强度钢的设计理念,阐述了以纳米相析出强化为主、多种强化方式结合的强韧化理论,并总结了纳米析出强化超高强度钢在合金设计和工艺优化等方面的初步研究进展,最后探讨了新型纳米强化超高强度钢亟待解决的问题。     

Microstructure and mechanical properties of ODS Fe14Cr model alloy processed by equal channel angular pressing

Abstract

A nanograin sized model oxide dispersion strengthened (ODS) ferritic steel with nominal composition Fe–14Cr–0·3Y₂O₃ (wt-%) was produced by mechanical alloying and consolidated by hot isostatic pressing. The alloy was submitted to severe plastic deformation by equal channel angular pressing (ECAP). Microstructural and mechanical characterisation was performed before and after ECAP. It was found that ECAP decreases and homogenises grain size without altering the nanoparticle dispersion, in addition to enhancing ductility and shifting the strength drop at high temperatures.     

Microstructural studies on nanocrystalline oxide dispersion strengthened austenitic (Fe–18Cr–8Ni–2W–0.25Y<Subscript>2</Subscript>O<Subscript>3</Subscript>) alloy synthesized by high energy ball milling and vacuum hot pressing

In the present work, nanostructured (Fe–18Cr–8Ni–2W) austenitic base and oxide dispersion strengthened (ODS) alloy powders were produced through mechanical alloying and these nano powders were consolidated by vacuum hot pressing. The results showed that initially bcc solid solution formed in both the alloys and this transformed to fcc with continued milling. The bcc solid solution formation and the subsequent transformation to fcc were significantly faster in the ODS alloys when compared to the base alloy. In the ODS alloy, a grain size of ~25 nm is achieved within 5 h of milling. Study of variation of microhardness of mechanically alloyed powder particles with grain size showed linear Hall–Petch kind of behavior. Following vacuum hot pressing of mechanically alloyed powders, nearly fully dense (>99% of theoretical density) compacts were obtained with a grain size of ~80 nm. The bulk hardness of base and ODS alloys are ~530 and ~900 HV, respectively. These are significantly higher than the values reported in the literature so far. The enhanced strength the ODS alloy is due to increased dislocation density and presence of fine dispersoids along with the nanocrystalline grains.     

Recrystallisation of austenite grain when non-isotherm steel working: Effects of thermal kinetics and deformation-based mechanisms

The effects of high temperature deformation on the recrystallisation of austenite grains and hardening occurring during hot forging of steels were studied. Three commercial steels containing various carbon weight percentages were heated beyond the austenitising temperature and free forged up to desired deformation ratios. The specimens were then air-, or oil-cooled. Two zones were distinguished according to the grain-size: a zone with fine grains, associated with highest plastic deformation and, a zone with coarse grains located within the subsurface layers. Unexpectedly, the highest values of microindentation hardness were obtained in the coarse-grain zone. Consequently, the interaction between the grain-size gradient induced by thermal kinetics of cooling and the local hardening governed by dislocation kinetics was studied by means of microindentation hardness inspections. Analysis of stress–strain curves confirmed that while forging enhances mechanical strength, it has a detrimental effect on ductility of steel.     

Microstructure characterisation and tensile properties of 18Cr-4.5Al-0.3Zr- oxide dispersion strengthened steel

The 18Cr–4.5Al–0.3Zr–oxide dispersion strengthened (ODS) steel was fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) technique. A microstructural characterisation was performed on an 18Cr–4.5Al–0.3Zr–ODS steel using high angle annual dark field (HAADF) and synchrotron small angle X-ray scattering (SAXS). HAADF and SAXS results showed that high-density nanoscale oxides are formed in 18Cr–4.5Al–0.3Zr–ODS steel. The oxides in the specimen can be roughly divided into two categories according to their compositions: (1) core/shell structure oxides with Al–O oxide cores and Y shells; (2) nm-scale trigonal-phase Y₄Zr₃O₁₂ oxides. In addition, tensile testing results revealed that the specimen exhibited better tensile strength and ductility as compared with another commercial ODS steels with similar composition.     

Microstructural changes upon annealing in ODS-strengthened ultrafine grained ferritic steel

In this study, the stability of grain size and oxide nanoparticles in the ODS steel upon annealing at high temperature (650–1350 °C) has been evaluated. The ODS Fe–Cr–W–Ti–Y₂O₃ steel has been manufactured by powder metallurgy, consolidated by hot isostatic pressing and processed by hydrostatic extrusion. Such a processing brings about ultrafine grain structure reinforced with oxide nanoparticles (few nm in diameter) and results in superior mechanical properties. The stability of nano-oxides has been analyzed by small angle X-ray scattering together with transmission electron microscopy. The results obtained revealed excellent thermal stability of ultrafine grained ODS ferritic steel, which was attributed to the resistance of oxides against coarsening.     

Diffusion bonding of ferritic oxide dispersion strengthened alloys to austenitic superalloys

The superior high temperature mechanical strength and oxidation resistance of ferritic oxide dispersion strengthened (ODS) tubular alloys are compromised by the difficulties encountered in joining. Conventional fusion welding techniques generate a weld fusion zone which is devoid of the mechanical strength exhibited by the base material. Therefore, more sophisticated solid state joining techniques, such as diffusion bonding, must be employed when joining ODS materials. This paper describes a series of solid state diffusion bonding experiments carried out between two tubular ferritic ODS alloys and two high temperature austenitic alloys. Careful control of bonding conditions produced pressure retaining joints between one of the tubular ODS alloys and both austenitic alloys. The successful joint design was incorporated into the manufacture of a tubular creep component, which enabled a series of internally pressurized creep tests to be carried out. The microstructure developed at the bond interface of each of the four separate material couples is described and the high temperature performance of the pressure retaining joints is discussed.     

New developments in carbon and alloy steels

Recent developments in carbon and alloy steels outlined in this paper include those pertaining to low-carbon mild steels, high-strength low-alloy (HSLA) steels, dual-phase steels, low-carbon bainitic steels, ultrahigh strength steels and ferritic stainless steels. The factors that improve the cold-forming characteristics of low carbon sheets and strips are outlined. The physical metallurgy principles governing the ferrite grain refinement in HSLA steels are discussed, pointing out how it can be achieved by the controlled rolling process. The importance of sulphide shape control in imparting the necessary through-thickness ductility in HSLA steels is discussed and the various methods available for inclusion shape control are outlined. Improved formability coupled with adequate strength characterizes the dualphase steels. Among the ultrahigh strength steels, two recent developments,viz. TRIP steels and maraging steels are outlined. It is pointed out how the improved formability of ferritic stainless steels is making them compete with the more expensive austenitic stainless steels. The scope for future developments in these steels is discussed at the end.     

Phase Transformation Behavior and Microstructural Control of High-Cr Martensitic/Ferritic Heat-resistant Steels for Power and Nuclear Plants: A Review

The martensitic/ferritic steels have been used as boiler and turbine materials in power plants, and also been selected as potential materials for structural materials in nuclear reactors. In this paper, the kinetic analysis of the martensite formation and microstructural control of high-Cr martensitic/ferritic steels are reviewed. A modular approach, incorporating Fisher partitioning nucleation and anisotropic growth for impingement, was proposed to describe the martensite formation kinetics under different cooling rates.The kinetic analysis suggested a thermal-activated growth feature occurring during the martensitic transformation of martensitic steels. The microstructure can be tuned by composition optimization and various combinations of heat treatment parameters(temperature, time, severe and minor deformation).For the application in power plant, the potential of boundary-design, refinement of original austenite grain size and the final martensitic lath, pinning effect of stable carbides, in improving the performances of martensitic/ferritic steels at elevated temperatures should be investigated more thoroughly.Furthermore, efforts should be made to explore the effects of retained austenite on the improvement of high-temperature creep strength. For the application of nuclear plants, attempts should also be made to produce Fe powders with uniformly distributed oxide particles by chemical reactions.     

Towards improved ODS steels: A comparative high‐temperature low‐cycle fatigue study

Within the frame of this work, the mechanical behaviour of a bimodal ferritic 12Cr‐ODS steel as well as of a ferritic‐martensitic 9Cr‐ODS steel under alternating load conditions was investigated. In general, strain‐controlled low‐cycle fatigue tests at 550°C and 650°C revealed similar cyclic stress response. At elevated temperatures, the two steels manifest transitional stages, ie, cyclic softening and/or hardening corresponding to the small fraction of the cyclic life, which is followed by a linear cyclic softening stage that occupies the major fraction of the cyclic life until failure. However, it is clearly seen that the presence of the nano‐sized oxide particles is certainly beneficial, as the degree of cyclic softening is significantly reduced compared with non‐ODS steels. Besides, it is found that both applied strain amplitude and testing temperature show a strong influence on the cyclic stress response. It is observed that the degree of linear cyclic softening in both steels increases with increasing strain amplitude and decreasing test temperature. The effect of temperature on inelastic strain and hence lifetime becomes more pronounced with decreasing applied strain amplitude. When analysing the lifetime behaviour of both ODS steels in terms of inelastic strain energy calculations, it is found that comparable inelastic strain energies lead to similar lifetimes at 550°C. At 650°C, however, the higher inelastic strain energies of 12Cr‐ODS steel result in significant lower lifetimes compared with those of the 9Cr‐ODS steel. The strong degradation of the cyclic properties of the 12Cr‐ODS steel is obviously linked to the fact that the initial hardening response appears significantly more pronounced at 650°C than at 550°C. Finally, the obtained results depict that the 9Cr‐ODS steel offers higher number of cycles to failure at 650°C, compared with other novel ODS steels described in literature.