Showing papers on "Austenite published in 2018"

Correlation between process parameters, microstructure and properties of 316 L stainless steel processed by selective laser melting

Abstract: High-density 316 L specimens were fabricated by selective laser melting (SLM). Different processing parameters, including laser power (100, 200 W) and scanning strategies (alternating stripes without and with re-melting after every layer) were employed to evaluate their impact on microstructure and texture of the specimens. Microstructures of the specimens in as-built condition were characterised by columnar grains of austenite with intercellular segregation of Mo, Cr and Si, resulting in creation of non-equilibrium eutectic ferrite. It was found that laser energy density and scanning strategy strongly affect cellular substructure of austenite and amount of ferrite, as well as kind and degree of texture. Specific microstructure of austenite in as-built condition is the cause of almost double increase of yield strength accompanied by much smaller improvement of ultimate tensile strength and 1.4 times reduction of elongation at fracture in comparison of properties of hot-rolled SS316L sheet. Moreover, features of this substructure determine kind of the changes occurring during stress relieving at 800 °C for 5 h (among others, precipitation of sigma-phase strongly activated by presence of ferrite and residual stresses), demonstrated by decreased yield strength value with no significant changes of ultimate tensile strength and elongation. At the same time, an attempt was made to explain some unclearly interpreted observations in the literature related to a correlation between process parameters, microstructure and properties of SLM-processed steel 316 L.

New thermomechanically treated NiTi alloys - a review.

Abstract: In the past 10 years, several proprietary processing procedures for nickel titanium (NiTi) alloy were developed to improve the mechanical properties of NiTi endodontic instruments. Beside specific thermal and mechanical treatments, manufacturers introduced several machining procedures (e.g. twisting, electrical discharge machining), as well as techniques for final surface finishing. NiTi alloys used for endodontic instruments can be subdivided into instruments that mainly contain the austenite phase (austenitic: conventional NiTi, M-Wire, R-Phase) and those mainly containing the martensite phase (martensitic: CM Wire, Gold and Blue heat-treated NiTi). Thermomechanically treated NiTi alloys have been reported to be more flexible with improved cyclic fatigue resistance and greater angle of deflection at failure when compared to conventional NiTi. These enhanced properties may be attributed to a modified phase composition containing varying amounts of R-phase and martensite. Endodontic instruments made of austenitic alloys possess superelastic properties because of stress-induced martensite transformation and consequently tend to spring-back to their original form after deformation. In contrast, the martensitic instruments can easily be deformed due to the reorientation of the martensite variants and show a shape memory effect when heated. The use of martensitic alloy results in more flexible instruments, with an increased cyclic fatigue resistance compared with austenitic alloy.

Effect of morphologies of martensite–austenite constituents on impact toughness in intercritically reheated coarse-grained heat-affected zone of HSLA steel

Abstract: By using the gleeble-3500 simulator, three different two-pass weld thermal cycles were applied to base metal of a high strength low alloy (HSLA) steel with the purpose of obtaining martensite-austenite (MA) constituents of different morphologies in intercritically reheated coarse-grained heat-affected zones (ICGHAZ). Morphology of each MA constituent was characterized by maximum length, maximum width and aspect ratio (maximum length/ maximum width). Toughness of thermal simulated specimens was examined by using an instrumented Charpy impact tester. Behaviour of cracks was present by Charpy impact curve and typical data. Correlation between behaviour of cracks and morphologies of MA constituents was further analysed by observing crack propagation and microstructure of MA constituent. Fracture modes of MA constituents with slender and massive shape were proposed. Results show that slender MA constituents are more harmful to toughness compared with massive ones.

Fatigue crack growth anisotropy, texture and residual stress in austenitic steel made by wire and arc additive manufacturing

Abstract: Wire based additive manufacturing of metals is a novel and cost-effective method for the production of large-scale metallic parts in a wide range of engineering applications. While these methods display excellent tensile properties, relatively little is known of the microstructure-to-property relationships of wire-based additively manufactured stainless steel builds as they relate to fatigue and fracture behavior. Stainless steel alloy 304L walls were fabricated using wire and arc additive manufacturing and subjected to mechanical tests to characterize location and orientation dependant properties and microstructural features affecting crack growth. Fatigue crack growth rate analysis in the high cycle fatigue regime was undertaken on horizontally- and vertically-oriented single-edge notch bend specimens extracted at several positions from the wall. The R ratio was 0.1 and the test frequency was 10 Hz. Paris Law behavior similar to that observed wrought steel alloys has been achieved with vertical orientations showing the greatest crack growth resistance. In conjunction with mechanical testing, scanning electron microscopy and electron backscatter detection were used to assess microstructural effects on crack growth within the build.

A study of thermal expansion coefficients and microstructure during selective laser melting of Invar 36 and stainless steel 316L

Abstract: This paper presents an experimental study on the metallurgical issues associated with selective laser melting of Invar 36 and stainless steel 316 L and the resulting coefficient of thermal expansion. Invar 36 has been used in aircraft control systems, electronic devices, optical instruments, and medical instruments that are exposed to significant temperature changes. Stainless steel 316 L is commonly used for applications that require high corrosion resistance in the aerospace, medical, and nuclear industries. Both Invar 36 and stainless steel 316 L are weldable austenitic face-centered cubic crystal structures, but stainless steel 316 L may experience chromium evaporation and Invar 36 may experience weld cracking during the welding process. Various laser process parameters were tested based on a full factorial design of experiments. The microstructure, material composition, coefficient of thermal expansion, and magnetic dipole moment were measured for both materials. It was found that there exists a critical laser energy density for each material, EC, for which selective laser melting process is optimal for material properties. The critical laser energy density provides enough energy to induce stable melting, homogeneous microstructure and chemical composition, resulting in thermal expansion and magnetic properties in line with that expected for the wrought material. Below the critical energy, a lack of fusion due to insufficient melt tracks and discontinuous beads was observed. The melt track was also unstable above the critical energy due to vaporization and microsegregation of alloying elements. Both cases can generate stress risers and part flaws during manufacturing. These flaws could be avoided by finding the critical laser energy needed for each material. The critical laser energy density was determined to be 86.8 J/mm3 for Invar 36 and 104.2 J/mm3 for stainless steel 316 L.

Impact of composition on the heat treatment response of additively manufactured 17–4 PH grade stainless steel

Abstract: Precipitation hardened (PH) martensitic grade stainless steels are commonly used in additive manufacturing (AM) processes, but the heat treatment response can vary depending upon the powder feedstock composition. As-built AM 17–4 PH grade stainless steel fabricated using argon and nitrogen atomized feedstocks in separate powder bed fusion systems responded differently to standard heat treatment cycles. Materials fabricated from argon atomized feedstocks, containing low levels of nitrogen (0.01 wt%) and retained austenite (

Effect of Ni content on stainless steel fabricated by laser melting deposition

Abstract: The novel stainless steel + x wt.% Ni (x = 0, 3.05, 6.10, 9.15) specimens were successfully fabricated by laser melting deposition, aiming at investigating the influence of Ni content on stainless steel structure and property. The effects of Ni content on phase compositions, microstructure, microhardness, wear and electrochemical corrosion resistance of as-deposited stainless steel were studied systematically using XRD, OM, SEM, microhardness tester, friction-wear tester and potentiodynamic polarization measurement, respectively. Experimental results showed that with the increase of Ni content, the constituent phase of the as-deposited specimen changed from ferrite phase (specimen for x = 0) to austenite phase (specimen for x = 9.15). The microstructure growth followed the principle of dendrite growth. However, the dominant microstructure varied from equiaxed dendrite to columnar dendrite with increasing Ni content. Phase transition from ferrite phase to austenite phase with the addition of Ni content resulted in the decrease of microhardness value from 643HV to 289HV. Meanwhile, the wear resistance of as-deposited specimens decreased gradually with the increasing of Ni content, which might be attributed to the fact that the wear resistance is proportional to microhardness according to Archard's law. It was noted that corrosion resistance of as-deposited stainless steel was extremely improved with the increase of Ni content. The higher Ni content specimen (specimen for x = 9.15) exhibited the best corrosion resistance among the tested specimens based on corrosion rate, which was one order of magnitude lower than that of the lower Ni content specimens (specimens for x = 0, 3.05).

Ultrastrong nanocrystalline steel with exceptional thermal stability and radiation tolerance.

Abstract: Nanocrystalline (NC) metals are stronger and more radiation-tolerant than their coarse-grained (CG) counterparts, but they often suffer from poor thermal stability as nanograins coarsen significantly when heated to 0.3 to 0.5 of their melting temperature (Tm). Here, we report an NC austenitic stainless steel (NC-SS) containing 1 at% lanthanum with an average grain size of 45 nm and an ultrahigh yield strength of ~2.5 GPa that exhibits exceptional thermal stability up to 1000 °C (0.75 Tm). In-situ irradiation to 40 dpa at 450 °C and ex-situ irradiation to 108 dpa at 600 °C produce neither significant grain growth nor void swelling, in contrast to significant void swelling of CG-SS at similar doses. This thermal stability is due to segregation of elemental lanthanum and (La, O, Si)-rich nanoprecipitates at grain boundaries. Microstructure dependent cluster dynamics show grain boundary sinks effectively reduce steady-state vacancy concentrations to suppress void swelling upon irradiation. Weaker ferritic/matensitic steels rather than stronger austenitic steels are usually candidates for nuclear reactors since they do not easily swell under irradiation. Here, the authors make an ultrastrong lanthanum-doped nanocrystalline austenitic steel that is thermally stable and radiation-tolerant.

Microstructural characteristics and tensile behavior of medium manganese steels with different manganese additions

Abstract: Manganese is normally considered as the most important alloying element in medium Mn steels. However, the influence of Mn additions on the microstructural characteristics and the ensuring mechanical properties has not been much studied. This was addressed in the present study by investigating two variants of medium Mn steels with compositions of 0.2C-7/10Mn-3Al (in wt%), subjected to intercritical annealing (IA) and tensile testing. Results showed that a higher Mn addition can effectively increase the fraction of retained austenite (RA) at ambient temperature. However, the mechanical stability of RA was reduced, due to the lower C concentration partitioned into austenite during IA treatments. The higher fraction and lower mechanical stability of RA in the 10Mn steel gave rise to an enhanced transformation-induced plasticity (TRIP) effect, which was the main contributor to its higher strain hardening rate and improved mechanical properties, confirmed by a dislocation density-based strain hardening model. The yielding behavior and the Portevin-Le Chatelier (PLC) effect of the investigated steels were also found to be altered with different austenite mechanical stability as a consequence of different Mn additions and IA temperatures. Higher deformation-induced martensite transformation (DIMT) kinetics, i.e. lower mechanical stability of austenite, resulted in a lower yield point elongation, and eventually changed the yielding to a continuous manner due to the formation of stress-induced martensite. The PLC effect only occurred in the intermediate austenite stability range, and the critical strain for the onset of jerky flow showed a first decrease and then an increasing trend with higher DIMT kinetics. The observed beneficial effect of Mn additions on mechanical property improvement in medium Mn steels offers a significant aspect for further alloy design of such steels.

An Investigation of the Microstructure and Fatigue Behavior of Additively Manufactured AISI 316L Stainless Steel with Regard to the Influence of Heat Treatment

Abstract: To exploit the whole potential of Additive Manufacturing, it is essential to investigate the complex relationships between Additive Manufacturing processes, the resulting microstructure, and mechanical properties of the materials and components. In the present work, Selective Laser Melted (SLM) (process category: powder bed fusion), Laser Deposition Welded (LDW) (process category: direct energy deposition) and, for comparison, Continuous Casted and then hot and cold drawn (CC) austenitic stainless steel AISI 316L blanks were investigated with regard to their microstructure and mechanical properties. To exclude the influence of surface topography and focus the investigation on the volume microstructure, the blanks were turned into final geometry of specimens. The additively manufactured (AM-) blanks were manufactured in both the horizontal and vertical building directions. In the horizontally built specimens, the layer planes are perpendicular and in vertical building direction, they are parallel to the load axis of the specimens. The materials from different manufacturing processes exhibit different chemical composition and hence, austenite stability. Additionally, all types of blanks were heat treated (2 h, 1070 °C, H2O) and the influence of the heat treatment on the properties of differently manufactured materials were investigated. From the cyclic deformation curves obtained in the load increase tests, the anisotropic fatigue behavior of the AM-specimens could be detected with only one specimen in each building direction for the different Additive Manufacturing processes, which could be confirmed by constant amplitude tests. The results showed higher fatigue strength for horizontally built specimens compared to the vertical building direction. Furthermore, the constant amplitude tests show that the austenite stability influences the fatigue behavior of differently manufactured 316L. Using load increase tests as an efficient rating method of the anisotropic fatigue behavior, the influence of the heat treatment on anisotropy could be determined with a small number of specimens. These investigations showed no significant influence of the heat treatment on the anisotropic behavior of the AM-specimens.

Effects of atomizing media and post processing on mechanical properties of 17-4 PH stainless steel manufactured via selective laser melting

Abstract: Water-atomized and gas-atomized 17-4 PH stainless steel powder were used as feedstock in selective laser melting process. Gas atomized powder revealed single martensitic phase after printing and heat treatment independent of energy density. As-printed water atomized powder contained dual martensitic and austenitic phase regardless of energy density. The H900 heat treatment cycle was not effective in enhancing mechanical properties of the water-atomized powder after laser melting. However, after solutionizing at 1315oC and aging at 482 °C fully martensitic structure was observed with hardness (40.2 HRC), yield strength (1000 MPa) and ultimate tensile strength (1261 MPa) comparable to those of gas atomized (42.7 HRC, 1254 MPa and 1300 MPa) and wrought alloy (39 HRC, 1170 MPa and 1310 MPa), respectively. Improved mechanical properties in water-atomized powder was found to be related to presence of finer martensite and higher volume fraction of fine Cu-enriched precipitates. Our results imply that water-atomized powder is a promising cheaper feedstock alternative to gas-atomized powder.

Microstructure and mechanical properties of stainless steel CX manufactured by Direct Metal Laser Sintering

Abstract: In the present paper, the microstructural evolution and tensile properties of additively manufactured stainless steel CX were investigated. Using scanning electron microscope (SEM), several powder particle morphologies were identified in the stainless s steel CX feedstock powder where the spherical morphology was found to be the dominant one. In addition, X-ray diffraction (XRD) technique detected austenite and martensite phases in both stainless steel CX powder and as-built sample, whereas no carbide peak appeared on the XRD patterns. Moreover, lath or needle-like martensite phase was observed in the microstructure of the as-built sample. The level of porosity was very low in the as-built sample, indicating the manufacturing of a nearly fully dense sample. Furthermore, a high ultimate tensile strength together with a good elongation to fracture was obtained for the horizontally-built stainless steel CX sample. Finally, examination of the fracture surfaces after tensile tests confirmed the ductile failure mode of the samples, in which the pull-out of the scan tracks and coalescence of the voids resulted in the tear and final rupture. This study demonstrates the successful additive manufacturing of stainless steel CX with outstanding tensile properties.

Crystallographic Features of Microstructure in Maraging Steel Fabricated by Selective Laser Melting

Abstract: This study characterizes the microstructure and its associated crystallographic features of bulk maraging steels fabricated by selective laser melting (SLM) combined with a powder bed technique. The fabricated sample exhibited characteristic melt pools in which the regions had locally melted and rapidly solidified. A major part of these melt pools corresponded with the ferrite (α) matrix, which exhibited a lath martensite structure with a high density of dislocations. A number of fine retained austenite (γ) with a orientation along the build direction was often localized around the melt pool boundaries. The orientation relationship of these fine γ grains with respect to the adjacent α grains in the martensite structure was (111)γ//(011)α and [-101]γ//[-1-11]α (Kurdjumov–Sachs orientation relationship). Using the obtained results, we inferred the microstructure development of maraging steels during the SLM process. The results depict that new and diverse high-strength materials can be used to develop industrial molds and dies.