Showing papers on "Maraging steel published in 2010"

Microstructure and mechanical properties of maraging steel 300 after Selective Laser Melting

Abstract: Selective laser melting (SLM) is an additive manufacturing process for the direct fabrication of prototypes, tools and functional parts. The process uses a high intensity laser beam to selectively fuse fine metal powder particles together in a layer-wise manner by scanning cross-sections generated from a three-dimensional CAD model. The SLM process is capable of producing near fully dense functional products without almost any geometrical limitation and having mechanical properties comparable to those produced by conventional manufacturing techniques. There is a wide range of materials that are suitable to be processed by SLM including various steels, Ti, Al and CoCr alloys. Being one of these materials, maraging steel 300 (18Ni-300) is an iron-nickel steel alloy which is often used in applications where high fracture toughness and strength are required or where dimensional changes have to remain at a minimal level, e.g. aircraft and aerospace industries for rocket motor castings and landing gear or tooling applications. To achieve its superior strength and hardness, maraging steel, of which the name is derived from ‘martensite aging’, should be treated with an aging heat treatment. In this study, the effect of the SLM parameters (scan speed and layer thickness) on the obtained density, surface quality and hardness of maraging steel 300 parts is investigated. Moreover, various aging heat treatments (different combinations of duration and maximum temperature) are applied on the SLM parts to achieve high hardness values. The mechanical testing of maraging steel 300 specimens produced by SLM and treated with an appropriate aging treatment is accomplished by impact toughness and tensile tests and compared to the results obtained using conventional production techniques. Additionally, the microstructures of as-built and heat treated parts are investigated. Introduction Maraging steels are well known for their high strength, high fracture toughness, good weldability and dimensional stability during aging. Due to this unique combination of several attractive features, maraging steels find extensive use in high performance industrial and engineering parts such as aerospace and motor racing applications. Some of the applications can be listed as rocket motor castings, drill chucks, tools for punching, extrusion, plastics injection moulds and metal casting dies [1]. Maraging steels offer an attractive alternative to the medium to high carbon tool steels since they do not suffer some problems like high carbon content promoting corrosion and quench cracking which may only become evident during service and result in unexpected failure. The low carbon content of maraging steels reduces the risk for quench cracking, while the high nickel content and absence of carbides provides a good corrosion resistance [2]. Selective Laser Melting (SLM) is a promising technology due to almost unlimited geometrical freedom especially for tooling applications. It is possible to produce complex geometries with internal cavities by SLM such as conformal cooling channels. Conformal cooling channels are generally considered to be almost impossible to be built by conventional machining techniques but allow major reduction in injection moulding cycle time [2]. Therefore, SLM of maraging steels can offer new opportunities in tooling applications by producing parts with almost full density and high freedom in geometrical complexity. In this study, SLM of maraging steel 300 is taken under investigation regarding many aspects. In the scope of this study, the change of part density and surface quality was determined while varying the SLM process parameters (scan speed, layer thickness) from the recommended values by the SLM machine vendor. Micro and macro hardness measurements were conducted on the produced specimens to test the effect of those parameters on hardness of as-built parts. Since maraging steels obtain their superior mechanical properties, i.e. high strength and toughness, after an aging heat treatment, different aging conditions were tested to find the relationship between aging conditions (duration and maximum re-heating temperature) and obtained hardness. Before testing this material mechanically (by tensile and Charpy impact testing), the influence of laser re-melting after every layer is studied to reduce the porosity formed during SLM. Experimentation During the experiments, maraging steel 300 from Concept Laser GmbH (commercially named as CL50WS) was used. The test specimens were built from gas atomized powder of specification and size fraction listed in Table 1. Figure 1 provides typical morphology of the tested powders before processing showing that the majority of powder particles bear spherical and near-spherical morphology. This morphology without sharp edges and corners ensures free flow of the powder during layer deposition thereby increasing process efficiency [3]. Table 1: Specification and size ranges of gas atomized maraging steel powder Material DIN No. Material name Size ranges (μm) d50 (μm) bulk density (g/cm 3 ) CL50WS 1.2709 X3NiCoMoTi 18-9-5 25-63 43.7 8.1 Table 2: Experimental processing parameters Material Scan speed (mm/s) Scan spacing (μm) Layer thickness (μm) CL50WS 120-600 125 (a1 = 70% of the spot size % 99 φ ) 30, 40, 50, 60 CL50WS powder was processed on a Concept Laser M3 Linear SLM machine [4] which is equipped with a 100 W Nd:YAG laser and has a laser beam diameter ( % 99 φ ) of about 180 μm at the powder bed surface. Experiments were carried out using the maximum available laser power (~105 W). A wide range of scan speed at various layer thicknesses was examined as given in Table 2. In order to minimize the effect of thermal gradients and thermal induced stresses in the component, island scanning (a patented scan pattern from Concept Laser) was used [5]. In this scanning strategy, part surface at each slice is divided to small square islands of 5 mm × 5 mm. The islands are scanned in a random way while scanning direction is altering a right angle with respect to the neighboring islands. The surface roughness of the as-processed samples was measured on horizontal top surfaces using a contact surface profilometer, Talysurf 120L from Taylor Hobson Ltd. Density was measured according to the Archimedes’ method by weighing the samples in air and subsequently in ethanol. The Archimedes’ method is a simple and fast method to evaluate different processing conditions. However, it is not very suitable to compare the densities of SLM parts having very close results due to pores being sometimes filled with un-melted powder particles. A coating with lacquer was also applied to avoid ethanol absorption in open pores at lower densities (<90%). Density results are presented as the arithmetic means of three measurements at each processing condition and are expressed as relative density by taking materials’ bulk density as 8.1 g/cm 3

Influence of reverted austenite on static and dynamic mechanical properties of a PH 13-8 Mo maraging steel

Abstract: Maraging steels are martensitic hardenable steels exhibiting an excellent combination of high strength and adequate toughness. Beside the intermetallic precipitates, reverted austenite, formed during aging, is a decisive microstructural constituent. Static tensile tests showed a pronounced influence of its phase fraction on mechanical properties. Reverted austenite also exhibits a distinctive effect on dynamic properties which were investigated on a split-Hopkinson-pressure-bar. However, reverted austenite is not mechanically stable during external loading and transforms back to martensite. This behavior was investigated by in situ X-ray diffraction using synchrotron radiation. Here, a decreasing phase fraction of austenite with increasing strain was observed. Furthermore, a change of the stability of revered austenite with aging time was noticed.

Real-time martensitic transformation kinetics in maraging steel under high magnetic fields

Abstract: We have monitored the isothermal transformation kinetics of the austenite phase into the martensite phase in a metastable austenitic maraging steel by time-dependent magnetization measurements for temperatures from 4 to 298 K and continuous applied magnetic fields up to 30 T. The transformation kinetics is shown to be accelerated by several orders of magnitude when high magnetic fields are applied. Analyzing the transformation rate as a function of magnetic field and temperature provides direct insight into the martensite nucleation process.

3D shear-mode fatigue crack growth in maraging steel and Ti-6Al-4V

Abstract: Fatigue crack growth tests in mixed-mode II + III were performed on maraging steel and Ti-6Al- 4V. The 3D evolutions of the crack fronts -measured by SEM after interrupted tests- were analyzed, taking into account the reduction in effective crack driving force by the interlocking and friction of the asperities of the crack surface. Under small-scale yielding conditions, the mixed-mode crack growth rates were found to correlate best with Keff2 II + 1.2Keff2 III in maraging steel, while for Ti-6Al-4V, Keff2 II + 0.9Keff2 III appeared suitable. For extended plasticity, a crack growth prediction method is proposed and validated for Ti-6Al-4V. This method is based on elastic-plastic F.E. computations and application, ahead of each node of the crack front, of a shear-dominated fatigue criterion.

Studies on Corrosion of Annealed and Aged 18 Ni 250 Grade Maraging Steel in Sulphuric Acid Medium

Abstract: The corrosion behavior of aged and annealed sample of 18 Ni 250 grade maraging steel was investigated in varied conditions of concentration and temperature of sulphuric acid medium, using electrochemical techniques like Tafel polarization and electrochemical impedance spectroscopy (EIS). The results obtained from both the techniques are in good agreement. These results have shown increase in corrosion rate of aged specimen with increase in concentration and temperature of sulphuric acid. With increase in concentration of sulphuric acid from 0.025 M to 0.25 M the corrosion rate of annealed sample was found to increase, but there after in 0.5 M, 0.75 M and 1 M, the rate of corrosion decreases, indicating passivation of alloy surface at higher concentration of sulphuric acid. The corrosion rate of aged specimen was found to be higher than that of the annealed specimen. Similar observations are revealed by SEM images. The thermodynamic parameters like activation energy, enthalpy of activation and entropy of activation were calculated.

Investigation into Microstructures of Maraging Steel 250 Weldments and Effect of Post-Weld Heat Treatments

Abstract: This study was undertaken to gain a better understanding of microstructures obtained by multipass gas tungsten arc welding in maraging steel grade 250. Metallography and microhardness measurements were carried out on sheet and welded joints in as-welded and post-weld aged conditions. It was found that there was a significant amount of reverted austenite formed on cell boundaries of weld metal after aging at 758-823 K for 3-5 h, and was stable at room temperature. Aging at higher temperatures led to an increase in the continuous network of patchy austenite along the cell boundaries. The reason for the above, in our opinion, is the concentrational heterogeneity which characterizes the microstructure of maraging steel welds. No reverted austenite was observed in as-welded specimens. Solution annealing at 1093 K for 1 h did not completely eliminate the chemical heterogeneity associated with weld structures. However, homogenizing at 1373 K produced homogenous structure that on subsequent aging produces austenite-free lath martensitic structure.

Microstructural characterization of M250 grade maraging steel using nonlinear ultrasonic technique

Abstract: Nonlinear ultrasonic (NLU) technique is used for characterization of microstructures in M250 grade maraging steel subjected to solution annealing at 1093 K for 1 h followed by ageing at 755 K for various ageing durations in the range from 0.25 to 100 h. Using pulse inversion technique, feeble second harmonic is extracted to determine nonlinear ultrasonic parameter, β, and the relative β parameter (RBP) which is the ratio of β parameter of the precipitation hardened specimen to that of the solution annealed specimen. Normalized mean square strain, volume fraction of reverted austenite and hardness have been measured and transmission electron microscopy (TEM) has been carried out to understand the microstructural changes that occur during ageing and to study the correlation between these measured parameters. Hardness and normalized mean square strain are found to increase during initial stages of ageing due to precipitation of intermetallics and decrease at longer durations due to formation of reverted austenite and coarsening of precipitates. This study establishes that NLU technique can be used for non-destructive characterization of ageing behaviour of M250 grade maraging steel.

Influence of post-weld heat treatments on microstructure and mechanical properties of gas tungsten arc maraging steel weldments

Abstract: The response to post-weld heat treatment of an 18%Ni (250 grade) gas tungsten arc weld metal has been investigated. The post-weld heat treatments are (a) direct aging at 480°C/3 h/air cooling, (b) solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling and (c) homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling. Metallographic characterisation of fusion zone revealed pronounced segregation of titanium and molybdenum along the interdendritic and intercellular boundaries. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Solutionised treatment at 815°C does not remove the segregation. Homogenisation treatment (1150°C/1 h/air cooling) succeeded in making the composition become homogenised. Mechanical properties including tensile, hardness and impact toughness were evaluated. Tensile test results showed that directly aged weldments exhibited lower strength bu...

Study of tensile deformation behaviour of M250 grade maraging steel using acoustic emission

Abstract: Tensile testing of solution annealed and thermally aged (755 K for various durations in the range of 0.25–100 h) specimens of M250 grade maraging steel has been carried out along with acoustic emission (AE) monitoring. Results have shown that strength increases and ductility decreases upon ageing up to 10 h and this has been attributed primarily to the precipitation of Ni3Ti. Continued increase in strength up to 40 h of ageing has been attributed primarily to the precipitation of Fe2Mo in addition to Ni3Ti. Increase in ductility for 10–40 h of ageing has been attributed to dissolution of needle like Ni3Ti precipitates and formation of fine spherical Fe2Mo. Ageing beyond 40 h decreases strength and increases ductility due to the reversion of martensite to austenite and coarsening of the precipitates. The AE generated during tensile deformation depends on the ageing time. Increased occurrence of shearing of the precipitates by dislocations and increased brittleness of the matrix up to 10 h ageing increases the AE. The decrease in the AE beyond 10 h of ageing is due to the occurrence of deformation by Orowan looping, dissolution of Ni3Ti precipitates and austenite reversion. The scanning electron microscopy (SEM) of the fracture surfaces has shown ductile fracture characterized by dimples and changes in the size and shape of the dimples with ageing time.

Phase transformation in a nanostructured M300 maraging steel obtained by SPS of mechanically alloyed powders

Abstract: A Maraging M300 steel was produced by ball milling of elemental powders and Spark Plasma Sintering (SPS) consolidation at two different temperatures (950 and 1050 °C). Two types of nanostructured steels have been obtained. Thermal behaviors of these steels were investigated by means of Differential Scanning Calorimetry (DSC) and Dilatometry. Data provided by the two different techniques were seen to be in good agreement. A difference between the behaviors of the steel sintered at 950 °C and that sintered at 1050 °C was observed, due to the material sintered at lower temperature being more reactive to the intermetallics precipitation and austenite reversion on heating. On cooling, it shows a single martensite start temperature (Ms), whereas the steel sintered at 1050 °C shows a double peak for Ms.

Effect of Cooling Rate on Morphology and Crystallography of Lath Martensite in Fe-Ni Alloys

Abstract: The development of blocks and subblocks in the lath martensite was observed with SEM/EBSD and TEM/Kikuchi diffraction patterns analyses. Morphology of lath martensite formed below Ms temperature was observed using step quenching method in 18 mass% Ni maraging steel. The development of lath martensite structure can be described as follow; lath groups with single K-S variant start to form at the austenite grain boundary. Next, other sub-blocks appear adjacent to the lath groups formed first and these lath groups form macroscopic blocks observed in optical microscopy. The morphologies of the lath martensite in Fe -23 mass% Ni alloy cooled at different cooling rates after austenitization were also observed. The packet size and block thickness decreases with increasing cooling rate, although the sub-block thickness do not change. A packet locally contains small packets with different relationship of close packed planes, and the amount of small packets increases with increasing the cooling rate.

Corrosion of Aged and Annealed 18 Ni 250 Grade Maraging Steel in Phosphoric Acid Medium

Abstract: The corrosion behavior of aged and annealed sample of 18 Ni 250 grade maraging steel was investigated in phosphoric acid over a range of acid concentration and solution temperature by using electrochemical techniques like Tafel extrapolation and electrochemical impedance spectroscopy. The studies have revealed that the corrosion rate of both aged and annealed samples increase with increase in temperature and increase in concentration of phosphoric acid in the medium. The corrosion rate of annealed sample is found to be less than that of aged sample. The thermodynamic parameters like activation energy, enthalpy of activation and entropy of activation for the corrosion process were calculated.

Transformation from continuous-to-isothermal aging applied on a maraging steel

Abstract: The advantage of continuous Differential Scanning Calorimetry (DSC) experiments is the possibility of the precise determination of a reaction's enthalpy, the corresponding transition temperatures as well as a material's heat capacity. Additionally, continuous experiments are usually less time-consuming than isothermal procedures. On the contrary, in industry steels are subjected to isothermal aging treatments in order to obtain the desired mechanical properties by the precipitation of nanometer-sized particles. The application of continuous-to-isothermal transformation schemes, which have been developed mostly for precipitation hardened aluminium alloys, combines the benefits of continuous DSC experiments with industrial interests. The premise for the application of such transformation schemes on steels is an appropriate baseline construction strategy for the evaluation of the DSC experiments. In the present investigation, the applicability of a transformation scheme based on a Johnson–Mehl–Avrami–Kolmogorov-approach is tested successfully on a maraging steel hardened by copper precipitates. The results are verified by hardness tests on continuously and isothermally aged samples. Three-dimensional atom probe tomography is carried out in order to interpret the differences in the precipitation behavior between these two types of aging treatments.

Effect of Cyclic Aging on Mechanical Properties and Microstructure of Maraging Steel 250

Abstract: The effects of thermal cyclic aging on mechanical properties and microstructure of maraging steel 250 were studied using hardness tester, tensile testing machine, impact tester, optical, scanning electron, and stereo microscopy. Samples were solution annealed at 1093 K for 1 h followed by air cooling to form bcc martensite. Cyclic aging treatments were carried out at 753 and 773 K for varying time periods. Increase in hardness and strength with corresponding decrease in ductility and impact strength was observed with increasing aging cycles. Reverted austenite was detected by x-ray diffraction technique formed as a result of cyclic aging. The presence of reverted γ was also confirmed by EDX-SEM analysis and attributed to the formation of Mo- and Ni-rich regions which transformed to γ on cooling. Heterogeneity in composition and amount of reverted γ was found to increase with increase in aging cycles and aging time. Fractography reveals the change in fracture mode from ductile dimple-like to brittle cleavage with increase in hardness and strength due to cyclic aging.

Microstructure, hardness and residual stress distribution of dissimilar metal electron beam welds: maraging steel and high strength low alloy steel

Abstract: The present investigation reports on a study that has been taken up to develop an understanding of the electron beam welding characteristics of similar and dissimilar combination of maraging steel and high strength low alloy steel, which are in the hardened condition, i.e. maraging steel, in a solution that was in treated and aged condition, whereas high strength low alloy steel in a quenched and tempered condition before welding. The joint characterisation studies include microstructural examination, microhardness survey across the weldment and measurement of residual stresses. Maraging steel weld metal is under compressive stress rather than tensile stress as observed in low alloy steel welds because the martensite transformation occurs at a relatively low temperature. It has been observed that, in dissimilar metal welds, tensile stress is observed at the fusion boundary of low alloy steel and weld metal, whereas compressive stress is obtained at the location between weld and maraging steel fusi...

Maraging steel strip

Abstract: Provided is a maraging steel strip which has such a composition that can reduce the content of TiN acting as the starting point of fatigue fracture in a high-cycle region, and the bending fatigue strength of which has been improved by the precipitation hardening effect yielded by precipitating coherent nitrides in the nitrided structure. A maraging steel strip produced by nitriding a managing steel which contains by mass, C: 0.01% or less, Si: 0.1% or less, Mn: 0.1% or less, P: 0.01% or less, S: 0.005% or less, Ni: 8.0 to 22.0%, Cr: 0.1 to 8.0%, Mo: 2.0 to 10.0%, Co: 2.0 to 20.0%, Ti: 0.1% or less, Al: 2.5% or less, N: 0.03% or less, and O: 0.005% or less, with the balance being Fe and unavoidable impurities, wherein Baker-Nutting orientation relationship with an orientation difference within 10° exists between the Cr nitride precipitated in the nitrided layer and the matrix martensite.

The high alloy precipitation hardening martensitic steels and their suitability for welding

Abstract: Purpose: Present work was aimed at developing welding technology applicable for hot and cold rolled strips made of high alloy martensitic steels. Electron beam (EB) and TIG welding processes parameters were worked out for strips of the 2N11K13H3M, N19M4T, H11N10M2T and PH13-8 steel grades (unconventional maraging and precipitation hardening stainless steel grades). Design/methodology/approach: The heats of the steels were laboratory vacuum melted, forged and then hot and cold rolled. Welding process parameters were experimentally selected. Quality of the welds was evaluated by several non-destructive techniques and metallographic examination. Hardness distribution and tensile properties of welded strips were analyzed, and post weld heat treatment was proposed. Findings: All four steel grades studied were suitable for the electron beam welding and optimum process parameters were worked out for the 3 mm strips. The suitability of the steels to welding using the TIG process was tested with 1.5 mm strips. Apart from the high cobalt maraging steel, which was not available in the form of thin strips, the other steels were welded successfully in TIG process. After welding, strength of the steels could be largely restored by full heat treatment. Research limitations/implications: Further research is needed on weld microstructure evolution, affected by welding processes, to improve fracture properties of welds. Practical implications: Welding technology of the hot and cold rolled strips of the four steel grades, high alloy martensitic grade, was developed, which along with very high mechanical properties and ductile fracture modes, make the steels suitable for advanced structural applications. Originality/value: Progress achieved in welding technology development is of practical value for manufacture of the structural elements, made of high alloy martensitic steels in the form of hot and cold rolled strips.

Ultra-low carbon high-purity industrial pure iron and manufacturing method thereof

Abstract: The invention discloses an ultra-low-carbon (ULC) high-purity industrial pure iron and a production method thereof. The invention is mainly characterized in that: the ULC high-purity industrial pure iron contains chemical elements in weight percentage as follows: C is less than or equal to 0.008 percent, Si is less than or equal to 0.05 percent, Mn is less than or equal to 0.05 percent, S is lessthan or equal to 0.002 percent, P is less than or equal to 0.005 percent, and Al is less than or equal to 0.055 percent. The materials with the proportions of the above chemical compositions are smelted for dephosphorization and desulfurization in an arc furnace, blown with oxygen for decarbonization by AOD (argon-oxygen decarburization), and desulfurized under high temperature and high alkalinity, and finally cast into steel ingots; therefore, the purity of the steel ingots produced by the method can reach 99.5 to 99.7 percent. The maraging steel trial produced by the pure iron of the invention can reach the best level of the steels in China; the pure iron produced by the method of the invention can also improve the performance of other steels such as 00Ni12Cr5Mo3AlTiVA, and push the research and development of high-purity steels.

Microstructures and Tensile Properties of Maraging Steel Processed by Equal-Channel Angular Pressing

Abstract: An 18Ni (C-250) maraging steel was successfully processed by equal channel angular pressing (ECAP) for a single pass at room temperature. Microstructural observations showed that the martensite laths of 18Ni maraging steel were elongated to more narrow bands with a width of 100-200 nm after ECAP deformation. After ageing treatment, many nano-sized precipitates distributed uniformly within the refined martensite lathes. In comparison with the tensile strength (1940 MPa) of general used steel (solution + aging treatment), the tensile strength of the sample processed by ECAP and subsequent aging treatment was enhanced for more than 100 MPa (above 2050 MPa). The enhancement of tensile properties was attributed to microstructural refinement and uniformly distributed nano-precipitates.

Optimization of Ageing Parameters of a Low Nickel Maraging Steel

Abstract: In the present work the age hardening parameters of a 14Ni (200) maraging steel are studied in order to optimize mechanical properties of the steel. The initial characterization of the as received solution annealed steel has been carried out by optical and scanning electron microscopy (LOM and SEM), and hardness measurements. To identify the structural changes during ageing, differential scanning calorimetry (DSC) tests were performed. Different time-temperature combinations were considered for the precipitation hardening treatment of as-quenched material samples. After hardness measurements, three of these treatments were selected for an in depth study. The obtained microstructure at the maximum hardness peaks then analysed (LOM, SEM and TEM) and mechanical behaviour (strength, toughness and wear resistance) was studied for the final selection of the age hardening conditions.