Effect of heat treatment on microstructure and mechanical properties of Cr–Ni–Mo–Nb steel

Abstract

The microstructure and mechanical properties of a medium carbon Cr–Ni–Mo–Nb steel in quenched and tempered conditions were investigated using transmission electron microscopy (TEM), X-ray analysis, and tensile and impact tests. Results showed that increasing austenitisation temperature gave rise to an increase in the tensile strength due to more complete dissolution of primary carbides during austenitisation at high temperatures. The austenite grains were fine when the austenitisation temperature was <1373 K owing to the pinning effect of undissolved Nb(C,N) particles. A tensile strength of 1600 MPa was kept at tempering temperatures up to 848 K, while the peak impact toughness was attained at 913 K tempering, as a result of the replacement of coarse Fe rich M₃C carbides by fine Mo rich M₂C carbides. Austenitisation at 1323 K followed by 913 K tempering could result in a combination of high strength and good toughness for the Cr–Ni–Mo–Nb steel.     

Hardness improvement of surface alloyed materials fabricated with Fe based metamorphic powders by high energy electron beam irradiation

Abstract

In the present study, surface alloyed materials were fabricated with Fe based metamorphic powders by the high energy electron beam irradiation. A surface alloyed layer was fabricated by depositing metamorphic powders on a carbon steel substrate and by irradiating with high energy electron beam. On top of this surface alloyed layer, metamorphic powders were deposited once again and irradiated with electron beam to fabricate a two layered surface alloyed material. In the surface alloyed layers, 53–64 vol.-%Cr_1·65Fe_0·35B_0·96 borides were densely precipitated in the martensitic matrix. A large amount of hard borides improved the hardness of the surface alloyed layers 2–3 times greater than that of the substrate. In addition, they showed high hardness of ～500 VHN even at 450°C, because borides were thermally stable at high temperatures. These results suggested potentials of the fabricated surface alloyed materials for applications requiring good wear and thermal resistance.     

Microstructural characteristics of gas tungsten arc synthesised Fe-Cr-Si-C coating

Abstract

The gas tungsten arc (GTA) method was used to synthesise Fe-Cr-Si-C alloy coatings, and processing effects on the coating were investigated experimentally. Coatings were developed on an AISI type 1040 steel substrate. Four different regions were obtained in the surface coating; and in these regions either a hypoeutectic or a hypereutectic microstructure was found. The hypoeutectic microstructure consisted of primary dendrites of austenite (γ) phase and eutectic M₇C₃ (M=Cr,Fe) carbides. On the other hand, the hypereutectic microstructure consisted of M₇C₃ primary carbides and eutectic. A hypoeutectic or hypereutectic microstructure was determined by the combination of particularly carbon concentration, solidification rate, and extent of substrate melting. The higher hardness of the hypereutectic microstructure is attributed especially to the formation of M₇C₃ primary carbides. The lower hardness of the hypoeutectic microstructure is related to three effective parameters: first, the presence of γ phase in the primary dendrites; second, excessive dilution from the base material; and third, relatively low concentrations of chromium and carbon.     

Microstructural study and wear behavior of ductile iron surface alloyed by Inconel 617

In this research, microstructure and wear behavior of Ni-based alloy is discussed in detail. Using tungsten inert gas welding process, coating of nearly 1–2 mm thickness was deposited on ductile iron. Optical and scanning electron microscopy, as well as X-ray diffraction analysis and electron probe microanalysis were used to characterize the microstructure of the surface alloyed layer. Micro-hardness and wear resistance of the alloyed layer was also studied. Results showed that the microstructure of the alloyed layer consisted of M₂₃C₆ carbides embedded in Ni-rich solid solution dendrites. The partial melted zone (PMZ) had eutectic ledeburit plus martensite microstructure, while the heat affected zone (HAZ) had only a martensite structure. It was also noticed that hardness and wear resistance of the alloyed layer was considerably higher than that of the substrate. Improvement of wear resistance is attributed to the solution strengthening effect of alloying elements and also the presence of hard carbides such as M₂₃C₆. Based on worn surface analysis, the dominant wear mechanisms of alloyed layer were found to be oxidation and delamination.     

Microstructural study and densification analysis of hot work tool steel matrix composites reinforced with TiB2 particles

Hot work tool steels are characterized by good toughness and high hot hardness but are less wear resistant than other tooling materials, such as high speed steel. Metal matrix composites show improved tribological behavior, but not much work has been done in the field of hot work tool steels. In this paper TiB₂-reinforced hot work tool steel matrix composites were produced by spark plasma sintering (SPS). Mechanical alloying (MA) was proposed as a suited process to improve the composite microstructure. Density measurements and microstructure confirmed that MA promotes sintering and produces a fine and homogeneous dispersion of reinforcing particles. X-ray diffraction patterns of the sintered composites highlighted the formation of equilibrium Fe₂B and TiC, as predicted by thermodynamic calculations using Thermo-Calc® software. Scanning electron microscopy as well as scanning Kelvin probe force microscopy highlighted the reaction of the steel matrix with TiB₂ particles, showing the formation of a reaction layer at the TiB₂-steel interface. Phase investigations pointed out that TiB₂ is not chemically stable in steel matrix because of the presence of carbon even during short time SPS.     

Schwingungsverschleiß an TiB2-ZrO2 und TiC-ZrO2 oberflächenlegierter Al2O3-Keramik unter Variation der Luftfeuchte

Oscillating sliding wear of TiB₂-ZrO₂ and TiC-ZrO₂ surface alloyed Al₂O₃ ceramic at different humidity A commerical alumina ceramic was surface-alloyed by adding TiB₂ and TiC in addition to ZrO₂ using infrared CO₂ laser radiation. Aside from the type of hard particles their volume fraction was varied between 6 and 31%. The average thickness of the alloyed surface layer was about 150 ¨?m. Tribological tests were carried out unlubricated under oscillating sliding contact against alumina counterbodies at relative humidities of 3 and 50% as well as in distilled water. Ceramographic studies showed that multiphase structures containing hard particles of TiC or TiB₂ embedded in the Al₂O₃ matrix were obtained by alloying. In addition Al-Zr-Ti-O complex phases were analyzed by using X-ray diffraction technique. Compared to the commercially available alumina ceramic A123, laser alloying reduced the friction coefficient and improved substantially the wear resistance under the applied conditions of tribological testing. It was found that tribological behaviour was strongly influenced by environmental humidity in addition to the effect of the type and volume fraction of the phases produced by alloying. The unalloyed alumina ceramic depended more sensitively on humidity than the alloyed ceramic.     

Influence of carbon content on microstructure and tempering behaviour of 2 1/4 Cr 1 Mo steel

Transmission electron microscopic studies aimed at elucidating the effect of carbon level on the tempering behaviour of 2 1/4 Cr 1 Mo steels have been carried out. Specimens with two different carbon levels (0.06% and 0.11 %) were cooled in flowing argon gas (AC) from an austenitization temperature of 1323 K and tempered at 823, 923 and 1023 K for times ranging from 2 to 50 h. The tempering behaviour at these temperatures for the two carbon levels is found to differ in the nature of secondary hardening at lower temperatures, variation in the time to peak hardness and the saturation level of hardness at long tempering times. Based on a detailed study, using analytical electron microscopy, on the morphology, crystallography and microchemistry of secondary phases, the factors governing the observed variations in tempering behaviour are related to the difference in the dissolution rate of bainite, nucleation of acicular M₂C carbides and transformation rate of primary carbides into secondary alloy carbides. The carbides which promote softening were identified as M₇C₃, M₂₃C₆ and M₆C, whereas hardening is mainly imparted by M₂C.     

Synthesis and characterisation of TiC reinforced CoCrFeNi composite by hot-pressing sintering

ABSTRACT

Ti, Co, Cr, Fe, Ni and graphite powders were used to fabricate TiC reinforced CoCrFeNi composite by mechanical alloying and consequently hot pressing sintering at 1200°C for 1?h. Results indicated that Co, Cr, Fe and Ni powders were deformed, cold welded and crushed repeatedly during milling and an face-centred cubic-structured solid solution was obtained after milled for more than 10?h. Nano-sized TiC and micron-sized Cr₇C₃ type carbides were formed and embedded in the CoCrFeNi matrix dispersedly after sintering. The hardness and compressive fracture strength of the sintered composite reached 501 HV and 2.55?GPa, respectively, which could be ascribed to the presence of large amount of in-situ formed TiC and Cr₇C₃ type carbides in the composite.     

Microstructure and wear resistance of laser clad TiC particle reinforced coating

Abstract

A TiC–Ni alloy composite coating was clad to 1045 steel substrate using a 2 kW CO₂ laser. The microstructural constituents of the clad layer arefound to be γ-Ni and TiC_p in the dendrites, and afine eutectic of γ-Ni plus (Fe,Cr)₂₃C₆ in the interdendritic areas. Partial dissolution and aggregation of the original TiC particles during melting of the Ni alloy and their growth during resolidification on cooling are observed. The TiC particles offer significantly enhanced wear resistance, the degree of wear depending primarily on the debonding removal of the particles.

MST/3018     

Influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steel

The influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steels was analyzed by means of scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The microstructure of laser melted high chromium steel is composed of austenite with supersaturated carbon and alloy elements and granular interdendritic carbides of type M₂₃C₆. Secondary hardening of the laser melted layer begins at 450 °C after tempering, and the hardness reaches a peak of 672HV at 560 °C and then decreases gradually. After tempering at 560 °C, a large amount of lamellar martensite was formed in the laser melted layer with a small quantity of thin lamellar M₃C cementite due to the martensitic decomposition. The stripy carbides precipitating at the grain boundaries were determined to be complex hexagonal M₇C₃ carbides and face centered cubic M₂₃C₆ carbides. In addition, the granular M₂₃C₆ carbides and fine rod-like shaped M₇C₃ carbides coexisted within the dendrites. As a result, the combined effects of martensitic transformation, ultrafine carbide precipitations, and dislocation strengthening result in the secondary hardening of the laser melted layer when the samples were tempered at 560 °C.     

Development of Fe<Subscript>3</Subscript>C,SiC and Al<Subscript>4</Subscript>C<Subscript>3</Subscript> compounds during mechanical alloying

Mechanical alloying process, as a solid-state technique, is a very useful method for fabrication of high melting point compounds like metal carbides and nitrides, which additionally have nanocrystalline structure with improved properties. In this work the development of several carbides including iron, aluminium and silicon carbides by the mechanical alloying process and the effect of subsequent heat treatment were investigated. Mixtures of elemental powders of Fe–C, Si–C and Al–C were mechanically alloyed, nominally at room temperature using a laboratory planetary ball mill. Structural changes of samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the aluminium carbide (Al₄C₃) could not be synthesized by mechanical alloying process alone, even after long milling times. A suitable subsequent heat treatment was required to allow Al–C reaction to take place kinetically. In contrast mechanical alloying of Fe–C as well as Si–C systems directly led to the formation of Fe₃C and SiC carbides after sufficient milling time. In all cases the end product had a nanosized structure.     

Laser surface alloying of case hardening steel with tungsten carbide and carbon

Abstract

The laser surface alloying process was used to introduce two different alloying materials, tungsten carbide (WC/Co) and carbon, into the molten surface of a case hardening steel (16MnCrS5), to improve its hardness and wear resistance. The chemical composition and the resulting microstructure in the alloyed layers were of particular interest in this investigation, because the strengthening mechanism was strongly dependent upon the type and amount of the alloy material. For laser alloying with carbon the increase in hardness and wear resistance was based on the martensitic transformation in the composition range concerned. For alloying with tungsten carbide it was necessary to consider two different strengthening mechanisms, namely, martensitic transformation and precipitation of carbides. In both cases the grain refinement in the laser affected zone had an additional effect. Resistance to dry abrasive sliding wear was measured using a conventional pin-on-disc wear testing machine. For both alloy materials the wear rate was substantially lower than that of a substrate that had been laser remelted without alloying additions.

MST/1556     

Microstructural evolution in bainite,martensite, and δ ferrite of low activation Cr–2W ferritic steels

Abstract

The microstructural evolution in (2–15)Cr–2W–0·1C (wt-%) firritic steels after quenching, tempering, and subsequent prolonged aging was investigated, using mainly transmission electron microscopy. The steels examined were low induced radioactivation ferritic steels for fusion reactor structures. With increasing Cr concentration, the matrix phase changed from bainite to martensite and a dual phase of martensite and δ ferrite. During tempering, homogeneous precipitation of fine W₂C rich carbides occurred in bainite and martensite, causing secondary hardening between 673 and 823 K. With increasing tempering temperature, dislocation density decreased and carbides had a tendency to precipitate preferentially along interfaces such as bainite or martensite subgrain boundaries. During aging at high temperature, carbides increased in size and carbide reaction from W₂C and M₆C to stable M₂₃C₆ occurred. No carbide formed in δ ferrite. The precipitation sequence of carbides was analogous to that in conventional Cr–Mo steels.

MST/1049     

Microstructure and wear properties of Fe–TiC surface composite coating by laser cladding

AISI 1045 steel surface was alloyed with pre-placed ferrotitanium and graphite powders by using a 5-kW CO₂ laser. In situ TiC particles reinforced Fe-based surface composite coating was fabricated. The microstructure and wear properties were investigated by means of scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, as well as dry sliding wear test. The results showed that TiC carbides with cubic or flower-like dendritic form were synthesized via in situ reaction between ferrotitanium and graphite in the molten pool during laser cladding process. The TiC carbides were distributed uniformly in the composite coating. The TiC/matrix interface was found to be free from cracks and deleterious phase. The coatings reinforced by TiC particles revealed higher wear resistance than that of the substrate.     

TiB2 and TiC stainless steel matrix composites

Stainless steel matrix composites reinforced with TiB₂ or TiC particulates have been in situ produced through the reactive sintering of Ti, C and FeB. X-ray diffraction analysis confirmed the completion of reaction. The TiB₂, TiC and steel were detected by X-ray diffraction analysis. No other reaction product or boride was found, indicating the stability of TiB₂ and TiC in steel matrix. The SEM micrographs revealed the morphology and distribution of in situ synthesized TiB₂ and TiC reinforcements in steel matrix. During sintering the reinforcements TiB₂ and TiC grew in different shapes. TiB₂ grew in hexagonal prismatic and rectangular shape and TiC in spherical shape.     

Strengthening effects in precipitation and dispersion hardened powder metallurgy copper alloys

The hardening of copper and copper alloy matrix using powder metallurgy (PM) techniques and different ways for dispersoids formation, as well as analysis of their single and combined effects on the strength of obtained material at room and elevated temperatures, have been presented and discussed. Gas atomized Cu–3.8 wt.%Ti and Cu–0.6 wt.%Ti–2.5 wt.%TiB₂ (Cu–Ti–TiB₂) powders and mechanically alloyed powder Cu–4 wt.%TiB₂ were used as starting materials. The powders were consolidated by hot isostatic pressing (HIP) and hot pressing (HP). Optical, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDAX), as well as transmission electron microscope (TEM) were used for microstructure characterization of the compacts. High strengthening of the Cu–Ti compacts was achieved by thermal treatment (aging) as a consequence of the development of modular structure and precipitation of metastable Cu₄Ti_(m). Hardening in the Cu–Ti–TiB₂ compacts is due to simultaneous influence of the following factors: the development of modular structure, precipitation of metastable Cu₄Ti_(m), and the presence of TiB₂ dispersoid nanoparticles. In case of Cu–TiB₂ compacts, high starting values of hardness and hardness on the elevated temperatures result from the presence of finely distributed TiB₂ particles in copper matrix obtained by mechanical alloying. Cu–Ti–TiB₂ composite yields much higher hardness values compared with the binary Cu–Ti alloys, owing to primary TiB₂ dispersions formed during atomization. Separation of metastable Cu₄Ti precipitate and the presence of significantly finer TiB₂ particles in the copper matrix are the reason for higher hardness values at peak temperatures (400–500 °C) in multiple-hardened copper alloy compared to the dispersion-hardened.     

Hartstoffauflösung und Gefügeausbildung bei der konvektionsarmen Erzeugung von Hartstoff-Dispersionsschichten auf Stahl mittels Laserstrahlung

Hard Particle Dissolution and Structure of Coarsegrained Dispersion Layers on Steel Produced by Convection-reduced Laser Melting With the aim to produce hard and wear-resistant dispersion layers coarse grained TiC-and TiB₂-particles were incorporated into the surface layers of steels by means of a two-step laser melting process with reduced melt bath convection. The influence of the carbon conent of the steel and content of alloying elements and hard particles in the screen printed layer on solidification structures and particle dissolution was investigated by optical and electron microscopy and x-ray microanalysis. The produced layers were also characterized by hardness measurements.     

Microstructures and high temperature properties of spray formed niobium‐containing M3 high speed steel

The billets of M3 high speed steel (HSS) with or without niobium addition were prepared via spray forming and forging, and the corresponding microstructures, properties were characterized and analysed. Finer and uniformly‐distributed grains without macrosegregation appear in the as‐deposited high speed steel that are different to the as‐cast high speed steel, and the primary austenite grain size can be decreased with 2% niobium addition. Niobium appears in primary MC‐type carbides to form Nb₆C₅ in MN2 high speed steel, whereas it contributes less to the creation of eutectic M₆C‐type carbides. With same treatments to forged MN2 high speed steel and M3 high speed steel, it is found that the peak hardness of these two steels are almost the same, but the temper‐softening resistance of the former is better. With higher high‐temperature hardness of the forged MN2 high speed steel, its temper softening above 600 °C tends to slow down, which is related to the precipitation of the secondary carbides after tempering. A satisfactory solid solubility of Vanadium and Molybdenum can be obtained by Nb substitution, precipitation strengthening induced by larger numbers of nano‐scaled MC and M₂C secondary carbides accounts for the primary role of determining higher hardness of MN2 high speed steel. The results of the wear tests show that the abrasive and adhesive wear resistance of MN2 high speed steel can be improved by the grain refinement, existence of harder niobium‐containing MC carbides, as well as solute strengthening by more solute atoms. The oxidational wear behavior of MN2 high speed steel can be markedly influenced by the presence of the high hardness and stabilization of primary niobium‐containing MC‐type carbides embedded in the matrix tested at 500 °C or increased loads. The primary MC carbides with much finer sizes and uniform distribution induced by the combined effects of niobium addition and atomization/deposition would be greatly responsible for the good friction performance of the forged MN2 high speed steel.     

Effect of vanadium on cast carbide in high speed steels

Abstract

The effect of vanadium (0–4%) on the morphology and amount of eutectic and eutectoid carbides in high speed steels has been investigated using scanning electron microscopy and image analysis. It was found that vanadium promotes the formation of MC carbide and M₂C carbide, but inhibits the formation of M₆C carbide. In the vanadium free steels, the eutectic carbide consists solely of skeletal M₆C. For each steel composition, there is a critical vanadium content at which the skeletal eutectic changes to lamellar eutectic and the critical value decreases as the molybdenum content of steel increases. The effect of vanadium on the total amount of eutectic carbide differs in tungsten alloyed and molybdenum alloyed high speed steels. The δ eutectoid has a rodlike morphology in tungsten high speed steels; δ eutectoid is not present in Mo–W or molybdenum high speed steels. Increasing the vanadium content leads to an increase in the size of eutectic and eutectoid carbides.

MST/1264     

Effects of co-addition of titanium and boron on microstructure of superplastic Cr–Mo steels

Abstract

The effects of titanium and boron on the microstructure of a low alloyed Cr–Mo steel with 0·6 wt-%C have been investigated by comparison with a steel containing only titanium and a steel free from both titanium and boron. Each of the steels was subjected to thermomechanical treatment and annealed at 700°C, resulting in small grains of size a few micrometres. The steel containing both titanium and boron possessed the smallest ferrite grains and M₃C carbides of the three examined. This is attributed to a fine dispersion of borides (TiB₂ ) and borocarbides (Ti(C,B)) of size 10 nm in the ferrite matrix through the pinning effect. At the grain boundaries small carbide particles were present which were effective in inhibiting grain boundary migration. The extremely fine borides and/or borocarbides were useful in suppressing intragranular deformation of ferrite grains due to precipitation hardening. This may have assisted in promoting grain boundary sliding, resulting in superior superplastic elongation.