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.

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

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.

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.

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℃.

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.

Microstructure and Hardness of T250 Maraging Steel in Heat Affected Zone

Electron-beam （EB） welding was used in T250 maraging steel, microstructures of both base material and heat affected zone （HAZ） were investigated by optical microscopy （OM）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）, and microhardness was tested. The results showed that during EB welding, the HAZ of T250 maraging steel exhibited a continuous gradient structure. The microstrueture of the entire HAZ, from fusion line, could be divided into four zones： fusion zone, overheated zone, transition zone, and hardened zone. The microhardness showed a distinct regularity in each area. The softest region was the fusion zone, whereas the hardest was the hardened zone. In the overheated zone, the hardness increased as the grain size decreased. Furthermore, in the transition zone, the hardness level dropped noticeably. The peak temperature during the thermal cycle had a great influence on the formation of reverted austenite and dissolution of the precipitated particles, which contributed a lot to the microstructure and hardness of this material.

Additive manufacturing of 18%nickel maraging steels:Defect,structure and mechanical properties:A review

This paper reviews the latest research progress in the additive manufacturing(AM)process of 18%nickel maraging steels,which involves laser-based powder bed fusion(L-PBF),laser-based directed energy deposition(L-DED),and wire arc additive manufacturing(WAAM).The emphasis is on the defects,structure,and mechanical properties of the additive manufactured 18%nickel maraging steels.At first,this article outlines the influences of the processing parameters of AM techniques on the defects formed in the fabricated parts,in terms of the L-PBF and WAAM.The macrostructure and microstructure characteristics of as-built and heat-treated 18%nickel maraging steel are then described in detail.Later,the mechanical properties of as-built and heat-treated 18%nickel maraging steel are assessed,such as their tensile,hardness,impact toughness,and fatigue performances.Finally,future directions for work on the AM of 18%nickel maraging steel are provided.

Effect of Tube Spinning With Subsequent Heat-Treatment on Performance and Microstructure Evolution of T250 Maraging Steel

The effects of spinning deformation and subsequent heat treatments on the mechanical properties and microstructure of 18Ni Co-free Maraging steel （T250） tube were evaluated comparatively with the perform. An obvious radial shrinkage is detected in spun tubes after heat treatment and the magnitude of the shrinkage induced by solution treatment is almost the same as that by aging. Plastic deformation during spinning elongated the grains severely in the direction of metal flow. The solution treatment resulted in a drastically refined grain and recrystallised microstructure, removing the effect of plastic deformation, relieving the tangential residual stress and strain and improving hardness. Subsequent aging obtained a tempered microstructure, enhancing hardness values strikingly for precipitation strengthening. XRD （X-ray diffraction） analysis indicated that the reversed austenite formed in a plate-like along the grain boundaries and the volume fraction of austenite in spun tube was more than double that in the preform. These results imply that the residual stress and stain induced by spinning process and reversed austenite forming during aging might have the similar contribution to the radical shrinkage.

A novel laminated metal composite with superior interfacial bonding composed of ultrahigh strength maraging steel and 316L stainless steel

A 5-layer laminated metal composite composed of ultrahigh-strength maraging steel and ductile 316L stainless steel was fabricated by hot pressing in vacuum and post-heat treatment.Microstructure characterization on hierarchical structure of the composite before and after heat treatment was made by optical microscopy,scanning electron microscopy and electron back-scattered diffraction technique,respectively.Meanwhile,the difference of mechanical performance on both sides of the interface was characterized by nano-hardness testing.Uniaxial tensile test showed that superior interfacial bonding was achieved due to the micro-‘bite’structure between the two steels without obvious defects or oxides at the interface and with coordinated deformation of the two components.Thus,a laminated metal composite consisting of two different constituents with extreme difference in strength can be well fabricated.

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.

Evaluation of Mechanical Properties According to Nb Content of High Strength Steel Manufactured in VIM

As the aircraft and aerospace industry and the automobile industry are developed, the demand of reliable materials with high strength and high toughness is steadily increased. The grain miniature method which improves the ductility and the toughness simultaneously under high strength is the miniature of the casting microstructure. In this paper, the Nb which affects the austenite grain miniature is added by 0.00%, 0.03% and 0.06% in this steel and the ingot is manufactured in a vacuum induction melting furnace （VIM）. The casting microstructure and the mechanical property of the maraging steel according to Nb contents are analyzed by conducting the solution annealing and the age hardening after hot rolling. In this result, a specimen containing 0.03% Nb is most miniature. When it does the long time age hardening, the precipitation estimated as the carbide is appeared. The mechanical properties are excellent if a specimen containing 0.06% Nb does the age hardening during 8 h in 482℃.

Effect of Additive Cu-10Sn Alloy on Sintering Behavior of Elemental Powders in Composition of 465 Stainless Steel

The addition of Cu-10Sn alloy for developing the high strength 465 maraging stainless steel from elemental powders was studied. The sintering parameters investigated include the sintering temperature, the sintering time, and the mass percent of Cu-10Sn. For vacuum sintering, effective sintering occurs at temperature between 1 250 ℃ and 1 300 ℃. The maximum sintered density was achieved at 1 300 ℃ for 60 min with 3% （in mass percent） Cu- 10Sn alloy. More than 3% （in mass percent） Cu-10Sn content and temperature above 1 300 ℃ caused slumping of the samples. A maximum density of 7.4 g/cm^3 was achieved with 3% （in mass percent） Cu-10Sn content at a sintering temperature of 1 300 ＂C for 60 rain. A maximum ultimate tensile strength （UTS） of 517 MPa was achieved with 3% （in mass percent） Cu-10Sn content. With content higher than 2% （in mass percent） Cu-10Sn, a maximum increase in the density was observed. The fracture morphologies of the sintered samples are also reported.

Stainless steel binder for the development of novel TiC-reinforced steel cermets

Steel reinforced TiC composites are an attractive choice for wear resistance and corrosion resistance applications. TiC- reinforced 17-4PH maraging stainless matrix composites were processed by conventional powder metallurgy （P/M）. TiC-reinforced maraging stainless steel composites with 〉97% of theoretical density were fabricated. The microstructure, mechanical and wear properties of the composites were evaluated. The microstructure of these composites consisted of spherical and semi-spherical TiC particles. A few microcracks appeared in the composites, showing the presence of tensile stress in the composites produced during sintering. Typical properties, namely, hardness and bend strength were reported for the sintered composites. After heat treatment and aging, the increase of hardness was observed. The increase of hardness was attributed to the aging reaction in the 17-4PH stainless steel. The precipitates appeared in the microstructure and were responsible for the increase in hardness. The specific wear behavior of the composites was strongly dependent on the content of TiC particles, the interparticle spacing, and the presence of hard precipitates in the binder phase.

HOT DIPPING ALUMINIZED COATING AS HYDROGEN PERMEATION BARRIER

The hydrogen permeation experiment of MANET II with hot dipping aluminized coating was performed in temperature range of 573 to 623 K, in gas phase and in liquid Pb-17Li phase. The hydrogen permeation reduction factor (PRF) evaluated in gas phase is 620 at 573 K and 260 at 623 K, and in liquid Pb-17Li phase is 24-45 at 573 K and 12-30 at 623 K. The self-healing of coating is obvious and effective above 673 K. The pressure dependence of permeation flux indicates strong surface contribution. The way of filling hydrogen by continuous flow and/or bubble can increase permeation flux. The result of SEM-EDS shows that the microcrack is on the surface of the wetted part, but not on the not wetted part. The crack is superficial and affects only thin outside layer not penetrate aluminized layer. The surface elemental analysis shows that Al/O atomic ratio changes from 2/3 of not wetted part to about 1 of wetted part. The damage of coating surface seems to be related to the interaction of outside layer with liquid Pb-17Li and thermal stress during heating sample.

Role of Co in formation of Ni-Ti clusters in maraging stainless steel

The effect of Co addition on the formation of Ni-Ti clusters in maraging stainless steel was studied by three dimensional atom probe（3 DAP） and first-principles calculation. The cluster analysis based on the maximum separation approach showed an increase in size but a decrease in density of Ni-Ti clusters with increasing the Co content. The first-principles calculation indicated weaker Co-Ni（Co-Ti） interactions than Co-Ti（Fe-Ti） interactions, which should be the essential reason for the change of distribution characteristics of Ni-Ti clusters in bcc Fe caused by Co addition.