EFFECT OF MICROSTRUCTURE ON HYDROGEN DAMAGE OFJBK-75 PRECIPITATE-STRENGTHENED AUSTENITIC STEEL

Effects of γ， η precipitates and grain size on hydrogen damage (HD) of JBK-75 precipitate-strengthened austenitic steel were examined using the method of high-pressure thermal hydrogen charging and tensile tests. The hydrogen content in the charged specimens was 25.2 wppm. At the condition of η phase-free precipitating, the HD of the steel increased with increasing γ size, fine spherical γ dispersively dis-tributed was beneficial to improve the hydrogen-resistant property. η-phae precipitated at grain boundary increased HD tendency, while intragranular cellular η had less effect on it. Fine austenitic gruin Aize was beneficial to decrease the HD. At the condition of η phase-free precipitating, both hydrogen changed and uncharyed specimens of the steel fractured in ductile transgranular mode, whilst hydrogen promoted the intergranular fracture when obvious η precipitates were found.

Precipitation behavior of Ti in high strength steels

Precipitation behavior of Ti in high strength steels was investigated by means of the equilibrium solid solubility theory. The contributions of Ti content to yield strength were calculated. The calculated results were verified by the hot rolling experiment for C–Mn steel and C–Mn–Ti micro alloyed steel, respectively. The research results show that the precipitates are mainly Ti N at the higher temperature. With the decreasing temperature, the proportion of Ti C in precipitates increases gradually. When the temperature drops to 800 °C, Ti C will become predominant for the precipitation of Ti. When Ti content is less than 0.014%(mass fraction), Ti has little influence on the yield strength. When Ti content is in the range of 0.014%–0.03%(mass fraction), the yield strength of Ti micro alloyed steel is greatly increased, which leads to instability of the mechanical properties of the steel. Therefore, the design of Ti content in high strength steels should avoid this Ti content range. When Ti content is higher than 0.03%, the yield strength increases stably. In this experiment, when added Ti content was controlled in the range of 0.03%–0.05%, the contribution to the yield strength of Ti micro alloyed steel can reach about 92.44 MPa.

Nanoscale precipitates and comprehensive strengthening mechanism in AISI H13 steel

The effects of heat treatment on the precipitates and strengthening mechanism in AISI H13 steel were investigated. The results showed that the presence of nanoscale precipitates favorably affected grain refinement and improved the yield strength. The volume fraction of precipitates increased from 1.05% to 2.85% during tempering, whereas the average precipitate size first decreased then increased during tempering. Contributions to the yield strength arising from the various mechanisms were calculated quantificationally, and the results demonstrated that grain refinement and dislocation density most strongly influenced the yield strength. In addition, under the interaction of average size and volume fraction, precipitates＇ contribution to the yield strength ranged from 247.9 to 378.5 MPa. Finally, a root-mean-square summation law of σ = σg ＋ σs ＋（σd^2 ＋ σp^2）^1/2, where σg, σs, σd, and σp represent the contributions of fine-grain strengthening, solid-solution strengthening, dislocation strengthening, and precipitation strengthening, respectively, was confirmed as the most applicable for AISI H13 steel, which indicates a strong link between precipitates and dislocations in AISI H13 steel.

Effect of austenitizing condition on mechanical properties,microstructure and precipitation behavior of AISI H13 steel

The effects of austenitizing temperature(1223,1303,and 1373 K)and holding time(1-1500 s)on the microstructure,mechanical properties,and precipitation behavior of the H13 hot work die steel were investigated.The results indicate a softening phenomenon when H13 steel is austenitized at 1303 K beyond 900 s and 1373 K beyond 600 s,respectively.For the sample held for 1200 s,the tensile strength is found capable of reaching up to 2.2 GPa when quenched from a temperature above 1303 K.Meanwhile,prior-austenite grain size increases with the increase in austenitizing temperature.The kinetic behavior of the precipitates(mainly MC-type carbides)in H13 steel could be elaborated through the principles set forth by the Arrhenius and Avrami equations.Finally,the comprehensive strengthening of the H13 steel was discussed in detail.The results show that the activation energy of the transformed fraction of carbides is higher than that of the diffusion process for common alloying elements(Cr,V,Mo,and Ni)found in the austenite.This suggests that it would be difficult for precipitates to dissolve into the matrix when H13 steel is austenitized at high temperatures.With the increasing austenitizing temperature,the precipitation fraction decreases,and the dislocation density increases.The dislocation strengthening is regarded as the dominant strengthening contributed to yield strength in as-quenched H13 steel.

Mechanical properties and characteristics of nanometer-sized precipitates in hot-rolled low-carbon ferritic steel

The microstructures and properties of hot-rolled low-carbon ferritic steel have been investigated by optical microscopy, field-emission scanning electron microscopy, transmission electron microscopy, and tensile tests after isothermal transformation from 600°C to 700°C for 60 min. It is found that the strength of the steel decreases with the increment of isothermal temperature, whereas the hole expansion ratio and the fraction of high-angle grain boundaries increase. A large amount of nanometer-sized carbides were homogeneously distributed throughout the material, and fine（Ti, Mo）C precipitates have a significant precipitation strengthening effect on the ferrite phase because of their high density. The nanometer-sized carbides have a lattice parameter of 0.411-0.431 nm. After isothermal transformation at 650°C for 60 min, the ferrite phase can be strengthened above 300 MPa by precipitation strengthening according to the Ashby-Orowan mechanism.

Strengthening mechanisms of reduced activation ferritic/martensitic steels:A review

This review summarizes the strengthening mechanisms of reduced activation ferritic/martensitic(RAFM)steels.High-angle grain boundaries,subgrain boundaries,nano-sized M_(23)C_(6),and MX carbide precipitates effectively hinder dislocation motion and increase high-temperature strength.M23C6 carbides are easily coarsened under high temperatures,thereby weakening their ability to block dislocations.Creep properties are improved through the reduction of M23C6 carbides.Thus,the loss of strength must be compensated by other strengthening mechanisms.This review also outlines the recent progress in the development of RAFM steels.Oxide dispersion-strengthened steels prevent M23C6 precipitation by reducing C content to increase creep life and introduce a high density of nano-sized oxide precipitates to offset the reduced strength.Severe plastic deformation methods can substantially refine subgrains and MX carbides in the steel.The thermal deformation strengthening of RAFM steels mainly relies on thermo-mechanical treatment to increase the MX carbide and subgrain boundaries.This procedure increases the creep life of TMT(thermo-mechanical treatment)9Cr-1W-0.06Ta steel by~20 times compared with those of F82H and Eurofer 97 steels under 550℃/260 MPa.

Investigation on the strengthening mechanism of S30432 austenitic heat-resistant steel

From the viewpoint of energy-saving and environment protection,it is necessary to develop Ultra Super Critical(USC) fossil-fired power plants.In order to ensure the reliable operation of power plants under high steam conditions,good mechanical properties(particularly high creep strength),corrosion resistance and fabricability are generally required for the heat resistant steels used in USC boilers.Among these heat-resistant steels,S30432 austenitic heat-resistant steels are of interest due to high creep strength,excellent oxidation and corrosion resistance at temperatures up to 650 -700℃.In this paper,the strengthening mechanism of S30432 austenitic heat-resistant steel was investigated based on the precipitation behavior of S30432 during aging and creep at 650℃.Results show that the microstructure of as-supplied S30432 steel is austenite,the main precipitation consists of only Nb(C,N).After aged for 10 000 h or crept for 10 712 h,there is a slight increase in the size of fine Nb(C,N),but the transformation from Nb(C,N) to NbCrN does not occur.Aging and creep results in the precipitation ofε-Cu and M_(23)C_6.The coarsening velocity ofε-Cu particles diminishes greatly and they are still very fine in the long-term creep range.With the increase of aging and creep time M_(23)C_6 carbides tend to coarsen gradually.The size of M_(23)C_6 is larger and the coarsening is easier in contrast toε-Cu and Nb(C,N).Nb(C,N) precipitates in the as-supplied microstructure,while aging and creep result in the precipitation ofε-Cu and M_(23)C_6.High creep rupture strength of S30432 steel is attributed to the precipitation hardening ofε-Cu,Nb(C,N) and M_(23)C_6.Extremely,ε-Cu plays an important role in improving the creep rupture strength of S30432,and at least 61%of the creep rupture strength of S30432 at 650℃results from the precipitation hardening ofε-Cu particles.

Microstructure and Mechanical Properties of Precipitation Strengthened Fire Resistant Steel Containing High Nb and Low Mo

Through the thermo-mechanical control process （TMCP）, a high Nb low Mo fire resistant steel with the yield strength （YS） of 521 MPa at room temperature （RT） and 360 MPa at elevated temperature （ET） of 600 ℃ was developed based on MX （M=Nb, V, Mo; X=C,N） precipitation strengthening. A series of tensile and con- stant load tests were conducted to study the mechanical properties at ET. The dynamic continuous cooling transfor- mation （CCT） as well as precipitation behavior of microalloy carbonitride was investigated by means of thermal sim- ulator and electron microscopy approaches. Results showed that the failure temperature of tested steel was deter- mined as 653 ℃, and the granular bainite was obtained when the cooling rate was higher than 10 ℃/s. In the rolled state, a certain amount of M/A islands was observed. During heating from RT to ET, M/A islands disappeared, and cementites and high dense compound precipitates （Nb, Mo, V）C with size of less than 10 nm precipitated in ferrite at ET （600 ℃）, which resulted in precipitation strengthening at ET.

3 GPa超高强度马氏体时效钢组织性能

航空航天系统的小型化、轻量化发展趋势对动力轴材料的强塑性提出了更高的要求。为了开发3 GPa级的马氏体时效钢,设计一种高Co、Ni、Mo的马氏体时效钢,其成分为14Ni-15Co-9Mo-0.86Ti-0.35Al-Fe。通过锻比大于10的高温大塑性变形尽可能细化晶粒,并结合预拉伸变形及深冷+时效的热处理工艺调控,实验钢抗拉强度达到3.076 GPa,断后伸长率5.5%,表现出了优异的强塑性。通过对其显微组织进行分析表征,发现其基体组织为高位错密度的板条马氏体结构,平均晶粒尺寸为0.47μm。透射电镜及3DAP结果表明,基体中分布着大量的Ni3(Mo,Ti),析出相平均直径为6~7 nm。析出强化、细晶强化及位错强化是其主要的强化机制,保证了合金超过3 GPa的超高强度,同时极细的亚微米级晶粒保证了材料良好的塑性。

新型压力容器用钢25CrMo3NiTiVNbZr的析出相特征和强化机制

为了进一步提高25Cr3Mo3NiNbZr钢的室温力学性能,对其进行成分优化,设计制备了25CrMo3NiTiVNbZr钢。利用场发射扫描电镜(SEM)、透射电镜(TEM)、电子背散射衍射(EBSD)技术以及拉伸试验机等研究了两种试验钢经1050℃淬火及640℃回火2 h后的室温力学性能、析出相特征和强化机理。结果表明:25Cr3Mo3NiNbZr钢主要析出相为纺锤状富Mo的M_(2)C相,其平均直径为18 nm,25CrMo3NiTiVNbZr钢中析出了更加细小的M_(2)C和MC碳化物,其平均直径为7.93 nm。25CrMo3NiTiVNbZr钢的室温抗拉强度达到1412 MPa,屈服强度达到1279 MPa,相比25Cr3Mo3NiNbZr钢分别提高了283和336 MPa。通过理论计算,两种试验钢室温下的强化机制均以析出强化、细晶强化和位错强化为主,其中25CrMo3NiTiVNbZr钢的位错强化增量和析出强化增量较25Cr3Mo3NiNbZr钢明显增加,分别增加了174和205 MPa。

V对S30403奥氏体不锈钢微观组织和高温力学性能的影响

研究了3种不同V含量(0 mass%、0.097 mass%和0.19 mass%)的S30403奥氏体不锈钢在500~900℃高温下的拉伸行为。结果表明:在500~900℃范围内,3种实验钢的抗拉强度都随着拉伸温度的升高而降低。V含量为0.097 mass%的实验钢在600、700和800℃的极限抗拉强度是3种钢中最高的,分别为362.9、271.4和156.1 MPa,这是不同温度下动态应变时效、动态再结晶和析出强化的协同作用的结果。含V钢在500~900℃范围内拉伸后的断面收缩率高于不含V钢,当V含量为0.19 mass%时,实验钢的断面收缩率最高,这是由于基体中固溶的V元素促进了实验钢的动态再结晶。

薄规格超高强热轧花纹钢带的开发

介绍了薄规格抗拉强度≥700 MPa热轧超高强花纹钢带低成本、高轧制稳定性成分设计思路和薄规格稳定轧制控制实践。薄规格超高强热轧花纹钢带利用钛沉淀析出强化机制和F7机架动态再结晶轧制,有效降低了薄规格超高强热轧花纹钢带的轧制变形抗力,提高了轧制稳定性,拓展了轧机极限规格,利用Cr迟珠光体和贝氏体转变,细化了组织和高温抗氧化性作用,促进了薄而致密的氧化铁皮结构的形成,有利于后续加工处理。

Development of Nb-V-Ti Hot-Rolled High Strength Steel With Fine Ferrite and Precipitation Strengthening

A hot-rolled steel with high yield strength of 700 MPa, good elongation of about 20% and low ductile-brittle transition temperature （DBTT） lower than -70℃ has been developed in laboratory. The results show that adopting finishing rolling temperature of around 800℃ is rational, and coiling temperature is between 400 and 500℃ The strength of developed 700 MPa hot-rolled high strength steel is derived from the cumulative contribution of fine grain size, dislocation hardening and precipitation hardening. The fine grain strengthening and precipitation hardening are the dominant factors responsible for such high strength, and good elongation and toughness are predominantly due to fine grain ferrite.

Precipitation Behavior and Its Strengthening Effect of X100 Pipeline Steel

Using TEM （transmisson electron microscopy）, electron diffraction, EDX （energy dispersive X-ray） analysis and physicochemical phase analysis, the morphology, crystal structure, size distribution and chemical composition of precipitates in the microstructure of high strength Nb-microalloyed Xl00 pipeline steel were investigated, and the strengthening effect of precipitation was quantitatively calculated with Ashhy-Orowan correction model. The precipitates obtained in X100 pipeline steel can be divided into two kinds： ＂complex＂ and ＂single＂ particles by morphology. The EDX analysis of ＂single＂ precipitates reveals that the chemical composition matches well with particle dimensions, especially the Nb to Ti ratio regularly decreases with the increase of particle size. The yield strength increments in the way of precipitation strengthening of X100 pipeline steel reached about 52 MPa, suggesting that the precipitation strengthening is not the dominative strengthening mechanism for X100 pipeline steel.

Strengthening effect of Cu-rich phase precipitation in 18Cr9Ni3CuNbN austenitic heat-resisting steel

The Cu-containing austenitic heat-resistant steel 18Cr9Ni3CuNbN, which is being used as superheater and reheater tube material for modern ultra-super-critical （USC） power plants all over the world, has been investigated at 650 ℃ long time aging till 10 000 h. SEM, TEM and 3DAP （three dimensional atom probe） have been used to follow microstructural changes with mechanical property variations. Experimental results show that Cu-rich phase and MX precipitate in the grains as well as M 23 C 6 precipitates at grain boundaries are the main precipitation strengthening phases in this steel. Among them Cu-rich phase is the most important strengthening phase. Homogeneous distribution of very fine nano-size Cu-rich phase has been formed at very early stage of 650 ℃ aging （less than 1 h）. Cu atoms gradually concentrate to Cu-rich particles and the other elements （such as Fe, Cr, Ni etc） diffuse away from Curich particles to γ-matrix with the increasing of aging time at 650 ？ C. The growth rate of Cu-rich phase at 650 ℃ long time aging is very slow and the average diameters of Cu-rich phase have been determined by TEM method. Cu-rich phase keeps in about 30 nm till 650 ℃ aging for 10 000 h. It shows that nano-size Cu-rich phase precipitation strengthening can be kept for long time aging at 650 ℃ because of its excellent stability at high temperatures. According to structure stability study and mechanical properties determination results the Cu-rich phase precipitation sequence and its strengthening mechanism model have been suggested and discussed.

Precipitation Strengthening by Nanometer-sized Carbides in Hot-rolled Ferritic Steels

The mechanical properties of the hot-rolled plates of Ti steel and Ti-Mo steel after isothermal transformation in a temperature range of 600 700 ℃ for 60 min have been tested, and the microstructures of the matrix and the characteristics of precipitated nanometer-sized carbides have also been examined by scanning electron microscopy and transmission electron microscopy. The precipitation regularity of nanometer-sized carbides has been studied by thermodynamic method and the contributions of corresponding strengthening mechanisms to the total yield strength have been calculated. The tensile strength of hot-rolled Ti-Mo ferritie steel can achieve 780 MPa with an elongation of 20.0% after being isothermally treated at 600 ℃ for 60 rain, and the tensile strength of Ti steel is 605 MPa with an elongation of 22.7%, according to the results of tensile tests. The critical nucleation size of （Ti,Mo）C is smaller than that of TiC at a given isothermal temperature, but the nucleation rate of （Ti, Mo）C is larger than that of TiC. The grainrefinement strengthening and precipitation strengthening contribute the main amount of the total yield strength. The major increase in yield strength with the decrease of isothermal temperature results from the contribution of precipi tation strengthening. The contribution of precipitation strengthening to the yield strength of the steels has been esti mated. The ferrite phase can be strengthened by about 400 MPa through precipitation strengthening in Ti-Mo steel isothermally treated at 600 ℃ for 60 rain, which is about 200 MPa higher than that of Ti steel under the same conditions.

Precipitation strengthening of nano-scale TiC in a duplex low-density steel under near-rapid solidification

Precipitation strengthening of nano-scale TiC is a promising method to improve mechanical properties of Fe–16Mn–9Al–0.8C (wt.%) low-density steel. This work attempted to introduce nano-scale TiC precipitates by adding 1 wt.% Ti element. The experimental results show that these precipitates with the total fraction of about 2 vol.% were formed and no coarse precipitates were observed despite the high Ti addition. It was interesting that the polygonal and needle-shaped TiC precipitates were observed in γ-austenite and δ-ferrite, respectively. Ti addition also decreased the volume fraction of γ-austenite significantly. Correspondingly, the yield strength was increased, but the elongation was significantly decreased due to the significant decrease of γ-austenite. Comparing with the Ti-free steel, the formation of TiC precipitates was the main reason for the increase in yield strength of Ti-bearing steel, and TiC precipitates also led to a higher strain hardening index at the first deformation stage. TiC precipitates promoted the Orowan strengthening, resulting in a higher strain hardening capability than Ti-free steel reinforced by shearable κ-carbide.

Strengthening of Ultrafine Lamellar-Structured Martensite Steel via Tempering-Induced Nanoprecipitation

An ultrafine lamellar-structured martensite steel fabricated by heavy warm rolling(HWR)has shown an excellent combination of strength and ductility.By appending tempering at 400℃to HWR,we show that the comprehensive mechanical property of a lamellar-structured low-carbon martensite steel can be further improved to reach a yield strength of~1.8 GPa,an ultimate tensile strength of~2.0 GPa and a total elongation of~9.3%.This is achieved by tempering the HWR steel from 300 to 750℃,and the optimum tempering temperature is thus obtained.We find that the tempered ultrafine lamellar martensite contains high-density nanoprecipitates dispersed within the aligned martensite laths with reduced crystallographic variations.The ultrahigh strength of the steel is rationalized as mainly the result of grain boundary strengthening and precipitation strengthening,which contribute to yield stress by 610 MPa and 440 MPa,respectively.The good ductility is believed to be closely related to the capacity of the tempered grains to accommodate dense dislocations upon plastic deformation.The present thermomechanical processing provides a feasible routine for producing steels with ultrahigh-strength and good-ductility.

Influence of Tungsten and Cobalt Contents on the Microstructure Changes and Fracture Behavior of New Carbon-Free Steel-alloy Composites

The ever increasing demand for steel materials that have good combinations between strength and toughness urged all researchers working in the field of material science to find new alloys that can approach that requirement.Unfortunately strength and toughness of materials are always counter acting properties.However,carbon contents in the steel define to a great extent its strength and toughness.In this research an effort is paid to produce steel alloy composites that can give higher strength together with good toughness without alloying with carbon.The mechanism of strengthening in Iron-Cobalt-Tungsten composite alloys with variations in Co and W contents is investigated.The fracture toughness and hardness,are measured for all alloy composites under investigation.The changes in microstructures after heat treatment are emphasized using metallurgical microscopy and SEM-aided with EDX analyzing unit.

新能源汽车驱动电机用高强无取向硅钢力、磁性能调控研究进展

新能源汽车能够有效缓解传统汽车行业对化石燃料的严重依赖和全球所面临的环境问题,是未来发展的必然趋势.驱动电机作为新能源汽车的动力核心,不仅需要具有优异的磁性能提高能源转换效率,同时需要具有高强度来抵抗高速运转时的离心力.然而,无取向硅钢的强度和磁性能难以兼顾,因此无取向硅钢力、磁性能的协同调控是新能源汽车驱动电机发展过程中的一个关键科学问题.本文综述了国内外有关高强无取向硅钢力学性能和磁性能调控的相关研究现状,分析了不同强化方式对无取向硅钢磁性能的影响,指出了新能源汽车驱动电机用高强无取向硅钢力学性能和磁性能协同调控的未来发展趋势,即多种强化方式共同作用或利用细小弥散的纳米共格析出相实现高强无取向硅钢力、磁性能的最佳匹配,为新能源汽车驱动电机用高强无取向硅钢的发展提供借鉴.