航空轴锻件用Maraging250钢热挤压成形数值模拟及组织性能

采用DEFORM-2D软件对航空轴锻件用Maraging250钢的热挤压过程进行数值模拟,分析了其在挤压过程中温度场、等效应变场和应力场等的分布规律,并对其挤压后的微观组织和力学性能进行了分析。结果表明:热挤压成形后,锻件的温度沿径向向外表面呈升高的趋势;锻件主体区域等效应变为1.0~1.5,温度在1135~1174℃之间,锻件的温度分布和应变分布较均匀;锻件各部位的组织和力学性能具有良好的均匀性,其主体的平均晶粒尺寸为41.42~61.07μm。

Failure Analysis of Electron Beam Weld Joints for 18Ni Co-free Maraging Steels

Microstructure of two different 18Ni Co-free maraging specimens and their electron beam weld joints were investigated comparatively by optical microscopy and SEM. It is showing that both of the steels are typical lath martensite, however, one grain size is about three times as another one, and XRD reveals that the amount of the retained austenitic phase in the former is less then the latter. The austenite distributes in plate form along granular and lath boundaries while some in fine particle within the matrix. The microstructural difference between two specimens led to diverse behaviors in electron beam welding. The first specimen is weldable well but the second shows obvious welding defects of pits and burn-through holes in weld face. The welding microstructure exhibits a typical dendritic morphology, and the grains in the heat-affected zone recrystallized and grew up obviously for high temperature heated by welding electron beam. The weldablity is relative to the thermal conduction performance of the base materials,which is contributed greatly for grain size and austenite content.

Metallurgical analysis of a failed maraging steel shear screw used in the band separation system of a satellite launch vehicle

Maraging steels have excellent combination of strength and toughness and are extensively used for a variety of aerospace applications. In one such critical application, this steel was used to fabricate shear screws of a stage separation system in a satellite launch vehicle. During assembly preparations, one of the shear screws which connected the separation band and band end block has failed at the first thread. Microstructural analysis revealed that the crack originated from the root of the thread and propagated in an intergranular mode. The failure is attributed to combined effect of stress and corrosion leading to stress corrosion cracking.

Effect of Aging on Microstructure and Mechanical Property of 1900 MPa Grade Maraging Stainless Steel

The 18%Ni alloy steels provide high strength and toughness, while age-hardenable or PH stainless steels also have good corrosion resistance. This paper focuses on an investigation of the heat treatment, mechanical properties and microstructural development of a new maraging stainless steel. It is reported that the heat treatment process should consist of solution treatment and cryogenic cooling to attain a fully martensitic structure, followed by aging at 813 K. This heat treatment resulted in an ultimate tensile strength of over 1900 MPa combined with good impact toughness. Transmission electron microscopy is used to show that, for the peak-aged condition （813 K/4 h）, nano-sized precipitates, e.g. Ni3Mo and/or R-phase, and a high density of dislocations were uniformly dispersed in the lath martensite matrix. The calculated yield strength, based on a revised Orowan mechanism, is in good agreement with the test data. The steel studied has an ultimate tensile strength over 1900 MPa, excellent fracture toughness, and good resistance against over-aging and relatively good corrosion resistance as well.

Microstructure and mechanical properties of similar and dissimilar metal gas tungsten constricted arc welds:Maraging steel to 13-8 Mo stainless steel

Maraging steel (250) and 13-8 Mo stainless steel plates were joined by gas tungsten constricted arc welding(GTCAW) process in similar and dissimilar metal combinations using 13-8 Mo stainless steel filler wire. The similar and dissimilar metal welds made in solutionized condition were subjected to standard post weld hardening treatments direct ageing at 485 ℃, soaking for 31/2 hours followed by air cooling(ageing treatment of maraging steel) and direct ageing at 510 ℃, soaking for 4 h followed by air cooling(ageing treatment of 13-8 Mo stainless steel). The joint characterization studies include microstructure examination, microhardness survey across the weldments and transverse weld tensile test.Similar and dissimilar metal weldments responded to both the post weld ageing treatment. After post weld aging, increase in yield strength, UTS and slight reduction in % elongation of similar and dissimilar metal were observed. The observed tensile properties were correlated with microstructure and hardness distribution across the welds.

Microstructure, properties and hot workability of M300 grade maraging steel

This paper presents isothermal uniaxial compression test results of M300 grade maraging steel over a wide range of temperatures(900 e1200℃) and strain rates(0.001 e100 s^(-1)) to examine hot deformability and concurrent microstructural evolution. Processing map is generated and indicated the optimum processing parameters in the temperature range of 1125℃-1200℃ and strain rate range of 0.001 e0.1 s^(-1). High values of the efficiency of power dissipation, microstructural observations and EBSD results indicate softening mechanism to be the occurrence of dynamic recrystallisation. Material constants in a constitutive relation are evaluated from the flow stress data useful in computer modelling.

Microstructure and corrosion behaviour of gas tungsten arc welds of maraging steel

Superior properties of maraging steels make them suitable for the fabrication of components used for military applications like missile covering, rocket motor casing and ship hulls. Welding is the main process for fabrication of these components, while the maraging steels can be fusion welded using gas tungsten arc welding(GTAW) process. All these fabricated components require longer storage life and a major problem in welds is susceptible to stress corrosion cracking(SCC). The present study is aimed at studying the SCC behaviour of MDN 250(18% Ni) steel and its welds with respect to microstructural changes. In the present study, 5.2 mm thick sheets made of MDN 250 steel in the solution annealed condition was welded using GTAW process. Post-weld heat treatments of direct ageing(480 C for 3 h), solutionizing(815 C for 1 h) followed by ageing and homogenizing(1150 C for 1 h) followed by ageing were carried out. A mixture of martensite and austenite was observed in the microstructure of the fusion zone of solutionized and direct aged welds and only martensite in as-welded condition. Homogenization and ageing treatment have eliminated reverted austenite and elemental segregation. Homogenized welds also exhibited a marginal improvement in the corrosion resistance compared to those in the as-welded, solutionized and aged condition. Constant load SCC test data clearly revealed that the failure time of homogenized weld is much longer compared to other post weld treatments, and the homogenization treatment is recommended to improve the SCC life of GTA welds of MDN 250 Maraging steel.

Effects of cold rolling on the microstructure and mechanical properties of Fe-Ni-Mn-Mo-Ti-Cr maraging steels

Effects of cold rolling on the microstructure and mechanical properties of Fe-Ni-Mn-Mo-Ti-Cr maraging steels were studied.To investigate the microstructure and mechanical properties,optical microscopy,scanning electron microscopy,X-ray diffraction,tensile test,and hardness test were used.The results show that the solution-annealing treatment in the cold-rolled steel redounds to the formation of sub-microcrystalline Fe2（Mo,Ti） Laves phase particles,which are stable at high temperatures.These secondary Laves phase particles prevent from recrystallization at high temperatures and correspond to semi-brittle fracture in the subsequent aging treatment.

RECRYSTALLIZATION-INDUCED PLASTICITY IN 18Ni MARAGING STEEL UNDER ELASTIC STRESS

An approach to study the recrystallization-induced plasticity(RIP)phenomenon in the 18Ni maraging steel under the elastic stress has been made by the analysis of dynamic behaviour at elevated temperatures,the phenomenological analysis of dynamic strain feature curves,the confirmation of phase transformation point and TEM observation of RIP processing itself.

CRYOGENIC MECHANICAL BEHAVIOUR OF MARAGING STEEL CONTAINING HYDROGEN

Within the range of 77 to 295 K,the strength and plastieity of 18Ni maraging steel increase with decreasing temperature and decrease with increasing hydrogen content.The susceptibility to hydrogen embrittlement of the maraged specimens is superior to solid solution ones.The dislocations after plastically deforming will tangle into dislocation cells which reduce thermselves in size as the temperature decreases,and coarsen as the hydrogen content in- creases.No cryogenie deformation twin was found.The tensile fracture surfaces of the solu- tion and maraged specimens containing 5.90 ppm hydrogen under 295 to 223 K are revealed as quasi-cleavage and intergranular features respectively and as transgranular ones under other experimental conditions.The influence of hydrogen on the mechanical behaviour of steel and the mechanism of hydrogen induced deformation and fracture are discussed. Research Assistant, Institute of Metal Research,Academia Sinica, Shenyang 110015,China

TRANSFORMATION INDUCED SUPERPLASTICITY IN 18Ni MARAGING STEEL

After exploring the influence of stress,cold-rolling reduction and cooling rate on transforma- tion induced strain,the transformation induced superplasticity forming technique of the 18Ni maraging steel(2450 MPa)was developed.Through 14 times γα′ cyclic transformation,the limited elongation of 320% can be obtained in 60% cold-rolled specimen.TEM observation revealed that the morphology of stress induced martensite is massive.

Hydrogen Trapping Phenomenon at Grain Boundaries in a 18Ni Maraging Steel

An investigation of the phenomenon of hydrogen trapping at grain boundaries in 18 Ni maraging steel has been carried out by the thermal hydrogen evolution technique.Grain boundary on- ly acts as a trapping site of hydrogen at low temper- ature region,and the peak of its hydrogen evolution from it is observed at 405K with 3.00K/rain heat- ing rate and with specimen of 0.55mm thick.The trap activation energy of hydrogen escaped from grain boundary is estimated as 14.2kJ/mol. Hydrogen trapping at grain boundary is mainly as sociated with segregated hydrogen by impurities, and its behaviour is primarily the interaction be- tween hydrogen and the hydrostatic stress field of the grain boundary.

Modulated Structures in 10Ni and 18Ni Maraging Steels

1.IntroductionSince the theory of spinodal decompo-sition in a supersaturated solid solution wasformulated by Hillert[1]and Cahn[2],thephenomena of spinodal decomposition havebeen found in many alloys.Tamura[3]haspointed out that spinodal decompositioncould also occur in maraging steels.A mod-ulated structure in 8Ni maraging steel was

STUDY ON BEHAVIOR OF TRAPPED HYDROGEN IN 18Ni MARAGING STEEL BY THERMAL HYDROGEN EVOLUTION TECHNIQUE

The interaction of hydrogen with interface between the precipitates and the martensitie matrix in 18Ni maraging steel has been studied by means of thermal evolution hydrogen technique us- ing gas chromatograph as hydrogen detector.An evolution rate peak has been observed at 451 K.The height of the peak relates to the amount and distribution of the precipitates.The activation energy for hydrogen escaping from the trap sites is 23.2 kJ/mol.

Manufacturing of T300 cobalt-free maraging steel and its tensile property

The 18Ni 300 grade cobalt-free maraging steel T300 was successfully manufactured on an industrial scale by application of vacuum induction melting （VIM） and vacuum arc remelting （VAR） process at Baosteel. With the content of O and N less than 15 ppm and 10 ppm respectively （ 1 ppm = 10 -6 ） ,the T300 steel produced by Baosteel obtained a high degree of purity and a good combination of ultra-high strength and fracture toughness. The effects of solution temperature and ageing temperature on tensile property of solution-treated and aged T300 steel were investigated. The results show that the solution temperature slightly influences the strength of the solution-treated steel, indicating that the solution strengthening rather than the grain boundary strengthening is more important to the solution-treated steel. For the solution-treated T300, the elongation and reduction of area don＇t change much with a rising solution temperature, and remain above 17% and 70% respectively at the solution temperature ranging from 800℃ to 1200℃. For the aged T300 steel,the strength and elongation are reduced with an increasing solution temperature, and the solution temperature has a greater effect on the reduction of area than that on the elongation. The reduction of area of the aged steel reaches a high peak when the solution temperature reaches 920℃. The experimental data indicates that precipitation strengthening makes a contribution of l 100 MPa to the tensile strength of the aged steel. The ageing temperature exerts a greater effect on the strength than on the elongation and reduction of area. The steel achieves a better combination of strength and toughness at the peak ageing temperature of 500℃.

Effect of Chloride Concentration on the Corrosion Rate of Maraging Steel

The corrosion behavior of Maraging steel has been studied by using different techniques, including open circuit potential and polarization measurements in addition to microstructure examination such as optical microscopy and XRD (X-Ray Diffraction) investigation. The corrosion behavior of Maraging steel has been examined in sodium chloride solutions with different concentrations from 0.1 M to 2 M. It was found that the corrosion resistance of Maraging steel is inversely proportional with the concentration of sodium chloride solution. The corrosion resistance is directly proportional to the Mo and Ti content in the Maraging steel. Heat treatment of the Maraging steel improved its mechanical properties with no effect on the corrosion behavior as the precipitation of inter-metallic compounds leading to some galvanic action. However, sample IV having lower Mo content than sample V showed after heat treatment an improvement in the corrosion resistance.

Investigation of Sintered Maraging Steels

Processing of five grades of sintered maraging steels containing 13% Cr, 9% Ni, 1% Ti and different quantities of Mo, Co is described. Variations of ageing temperatures and ageing holding times had resulted in choosing optimum regime of ageing f T=550℃, T=2.5 h. It allowed to receive mechanical properties of these steels like to them for compact steels; UTS=1200 MPa, EL=5.3%, RA=7%, and Charpy impact =1445 kJ/m2. X-ray analysis had shown phases Fe7Mo6, Fe2Mo in steels containing Co and decreasing of the period 'of or-phase lattice in these steels after ageing.

A Study of Neutron and Gamma-Ray Interaction Properties with Cobalt-Free Highly Chromium Maraging Steel

Cobalt-free maraging steels of different compositions have been prepared by electro-slag remelting technique using titanium and chromium instead of cobalt. Neutron removal cross-sections have been calculated, also mass attenuation coefficients and effective electron densities have been determined for the prepared samples in the photon energy range up to 2.8 MeV. Other steel alloys and lead samples have also been investigated for the sake of comparison. The results prove the superiority of cobalt-free maraging steels compared with the other steel types to be used as a proper shielding material in the nuclear field. Among the investigated steels, the steel “0.045%C- 13.35%Ni-2.05%Cr-4.5%Mo-0.06%Ti” has the best attenuation properties.

Significance of Alloying Element Levels in Realizing the Specified Tensile Properties in 18 Wt % Nickel Maraging Steel

Among the various grades of commercially available 18 wt. % nickel maraging steels, the one with nominal 0.2% proof strength in the range 1700-1750 MPa is the most commonly used and is distinguished by an excellent combination of high strength and high fracture toughness. The main alloying elements are nickel, cobalt, molybdenum and titanium. The first three of these are present at relatively high concentrations in the chemical composition. The high cost of these metals leads to a high cost of production and this becomes a deterrent to extensive use of the steel. In the present study, an attempt was made to produce the steel by pegging the levels of these alloying elements in the lower half of the specified range. The objective was to save on the raw material cost, while still conforming to the specification. The steel so produced could not, however, attain the specified tensile properties after final heat treatment. The observed behavior is explained based on the role played by the different alloying elements in driving the precipitation hardening reaction.

连续应变分布Maraging250钢的动态再结晶行为

通过设计高通量双锥试样,在单个热压缩试样的不同区域产生梯度应变,研究了1020~1150℃变形温度下Maraging250钢的动态再结晶(DRX)行为。采用有限元数值模拟、光学显微镜和电子背散射衍射技术,定量分析了各双锥变形试样垂直截面中心线附近的显微组织,确定了DRX的含量,建立了不同变形温度下Maraging250钢的动力学曲线。结果表明:将双锥试样热压缩后,等效应变沿垂直截面中心线呈连续对称分布,应变从边缘到心部逐渐增大至1.7,内部变形温度基本稳定在972~985℃。在热压缩过程中,Maraging250钢发生了连续应变DRX,其动力学曲线表现为典型的“S”形特征,DRX体积分数在变形中期增长速率最高,在变形初期及末期增速缓慢。随着变形温度的升高,DRX形核过程所需的应变有所降低,DRX体积分数最大增长率明显升高,但各变形温度下完全DRX所需的应变量变化较小。