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.     

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     

Development of microstructure during heat treatment of high carbon Cr–Mo steel

Abstract

Stainless steels containing enhanced chromium and carbon contents are particularly attractive for applications requiring improved wear and corrosion resistance. The as cast microstructure of such steels is composed mainly of ferritic matrix along with a network of interdendritic primary carbides. It has been shown that heat treatment of these steels results in microstructures that contain more than one type of carbide. A selective dissolution technique has been employed to isolate carbides from the matrix. Scanning electron microscope and X-ray diffraction studies of the as cast steels have shown that the primary carbides are essentially of M₇C₃ type, whereas in heat treated specimens both M₇C₃ (primary) and M₂₃C₆ (secondary) type carbides have been observed. The relative amounts of these carbides are found to be dependent on the heat treatment temperature. In addition, nucleation of austenite occurs above 950°C and at ～1250°C the matrix transforms entirely to austenite, which is retained completely on quenching to room temperature.     

Computer-aided design of transformation toughened blast resistant naval hull steels: Part I

A systematic approach to computer-aided materials design has formulated a new class of ultratough, weldable secondary hardened plate steels combining new levels of strength and toughness while meeting processability requirements. A theoretical design concept integrated the mechanism of precipitated nickel-stabilized dispersed austenite for transformation toughening in an alloy strengthened by combined precipitation of M₂C carbides and BCC copper both at an optimal ∼3 nm particle size for efficient strengthening. This concept was adapted to plate steel design by employing a mixed bainitic/martensitic matrix microstructure produced by air-cooling after solution-treatment and constraining the composition to low carbon content for weldability. With optimized levels of copper and M₂C carbide formers based on a quantitative strength model, a required alloy nickel content of 6.5 wt% was predicted for optimal austenite stability for transformation toughening at the desired strength level of 160 ksi (1,100 MPa) yield strength. A relatively high Cu level of 3.65 wt% was employed to allow a carbon limit of 0.05 wt% for good weldability, without causing excessive solidification microsegregation.     

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.     

高耐磨钢回火过程中碳化物沉淀的透射电镜研究

用透射电镜研究了三种高耐磨钢在二次硬化峰附近回火时碳化物的沉淀，结果表明：耐磨钢产生二次硬化的特殊碳化物为ＭＣ＋Ｍ２Ｃ，它们和基体间满足如下取向关系：〔１１１〕ＭＣ／／〔０１１〕α，（１１０）ＭＣ／／（１００）α；〔００１〕ＭＣ／／〔０１１〕α，（２００）ＭＣ／／（２００）α；〔０１１１〕Ｍ２Ｃ／／〔００１〕α，（２１１０）Ｍ２Ｃ／／（２００）α。在５４０℃回火时，Ｓｉ能抑制耐磨钢中Ｍ３Ｃ碳化物的     

Characterization of precipitates in a 7.9Cr–1.65Mo–1.25Si–1.2V steel during tempering

In this paper, the precipitates formed during the tempering after quenching from temperature 1150 °C for 7.90Cr–1.65Mo–1.25Si–1.2V steels are investigated using an analytical transmission electron microscope (A-TEM).The study of this tempering is carried out in isothermal and anisothermal conditions, by comparing the results given by dilatometry and hot hardness.Tempering is performed in the range of 300–700 °C. Coarse primary carbides retained after heat treatment are V-rich MC and Cr–Mo-rich M₇C₃ types. In turn, it gives a significant influence on the precipitation of fine secondary carbides, that is, secondary hardening during tempering. The major secondary carbides are Cr–Mo–V-rich M′C (and/or) Cr–Mo-rich M₂C type. The peak hardness is observed in the tempering range of 450–500 °C. In the end, we observe between 600 and 700 °C, that this impoverished changes the phase. At these high temperatures of tempering, we observe that there is a carbide formation of the types M₆C developing at the expense of the fine M₇C₃ carbides previously formed.     

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     

Development of de-alloyed zones during oxidation: effects on microstructure and spallation behaviour

Abstract

The development of de-alloyed zones during oxidation of martensitic and austenitic steels and Ni based superalloys has been reviewed and the influence of de-alloying on local creep strength has been assessed. The de-alloyed zones in martensitic steels have similar, possibly higher, strength than the bulk material, whereas in Ni based superalloys the de-alloyed zone is significantly weaker than the bulk alloy. The effect in austenitic steels varies according to the strengthening phases present in the alloys: the de-alloyed zone is weaker in alloys strengthened by chromium carbides and/or γ′ but has similar strength in alloys strengthened by niobium carbide.     

Influence of nickel on secondary hardening of a modified AISI H13 hot work die steel

This paper focuses on the effects of nickel on secondary hardening of a modified H13 hot work die steel. Both the non‐nickel steel and the nickel‐added steel get a secondary hardening peak at 520 °C, and the secondary hardening peak trends to increase in the nickel‐added steel. On the basis of scanning electron microscope and transmission electron microscope observation, the rise of the secondary hardening peak is in connection with the precipitation of M₃C type carbides. More strip‐shaped and needle‐shaped M₃C type carbides precipitated from matrix. By means of internal friction, the result suggests that nickel does not affect the position of the Snoek‐Kê‐Köster peak, but the height of Snoek‐Kê‐Köster peak of the nickel‐added steel is higher, which indicates nickel enhances the interaction between dislocations and interstitial atoms, promoting the precipitation of carbides.     

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.     

Carbide precipitation in some secondary hardened steels

The precipitation of alloy carbides in three commercial quenched and tempered martensitic steels has been studied using phase stability calculations, microscopy, microanalysis and dilatometry. The sequence of carbide stability as estimated using phase stability calculations is found to be consistent with the microstructural observations, if it is assumed that the equilibrium M₂₃C₆ phase is suppressed by kinetic considerations. One interesting result is that the molybdenum-rich phase M₂C is neither predicted nor observed in any of the alloys, all of which contain substantial quantities of molybdenum. Other results include dilatometric experiments which enabled the measurement of transformation temperatures, which are compared against theoretical values. This revised version was published online in November 2006 with corrections to the Cover Date.     

The influence of vanadium on fracture toughness and abrasion resistance in high chromium white cast irons

The influence of vanadium on wear resistance under low-stress conditions and on the dynamic fracture toughness of high chromium white cast iron was examined in both the ascast condition and after heat treatment at 500 °C. A vanadium content varying from 0.12 to 4.73% was added to a basic Fe-C-Cr alloy containing 2.9 or 19% Cr. By increasing the content of vanadium in the alloy, the structure became finer, i.e. the spacing between austenite dendrite arms and the size of massive M₇C₃ carbides was reduced. The distance between carbide particles was also reduced, while the volume fraction of eutectic M₇C₃ and V₆C₅ carbides increased. The morphology of eutectic colonies also changed. In addition, the amount of very fine M₂₃C₆ carbide particles precipitated in austenite and the degree of martensitic transformation depended on the content of vanadium in the alloy. Because this strong carbide-forming element changed the microstructure characteristics of high chromium white iron, it was expected to influence wear resistance and fracture toughness. By adding 1.19% vanadium, toughness was expected to improve by approximately 20% and wear resistance by 10%. The higher fracture toughness was attributed to strain-induced strengthening during fracture, and thereby an additional increment of energy, since very fine secondary carbide particles were present in a mainly austenitic matrix. An Fe-C-Cr-V alloy containing 3.28% V showed the highest abrasion resistance, 27% higher than a basic Fe-C-Cr alloy. A higher carbide phase volume fraction, a finer and more uniform structure, a smaller distance between M₇C₃ carbide particles and a change in the morphology of eutectic colonies were primarily responsible for improving wear resistance.     

Das Kriechverhalten von Warmarbeitsstählen mit 5% Chrom

Creep Behaviour of Hot-Work Tool Steels Comparison of four steels with varying hardening and annealing treatment. Creep test of up to 200 h duration under constant stress at 550°C. Investigation of carbides by x-ray diffraction of electrochemical residues and electron diffraction of extraction replicas. Marked strengthening at the beginning of creep due to carbide precipitation followed by a decrease in strength due to carbide growth and formation of cells. Up to 1000 h the steel X 40 CrMo V 51 seems to offer the best creep strenght when hardened from 1100°C.     

Nano-sized precipitates in 11 % Cr ferritic-martensitic steel after thermomechanical treatment

9 %–12 % Cr ferritic/martensitic steels with a good long-term creep strength at temperatures up to 650 °C and higher are being developed in order to increase steam temperature of coal-fired power plants.Thermomechanical treatment can effectively enhance the mechanical properties of high-Cr ferritic/martensitic steels mainly due to plenty of nano-sized precipitates produced by thermomechanical treatment. Nano-sized precipitates in an 11 % Cr ferritic/martensitic steel produced by a thermomechanical treatment, including warm rolling at 650 °C plus tempering at 650 °C for 1 h, were investigated by transmission electron microscopy. The average size of precipitates in the steel after the thermomechanical treatment was determined to be about 30 nm in diameter, which is only one-third of the average size of precipitates in the steel with the normalized and tempered condition. A large number of Cr-rich precipitates having an average diameter of about 25 nm in the steel produced by the thermomechanical treatment were identified as Cr-rich M₂C carbide with a hexagonal crystal structure, rather than M₂₃C₆ or MX phase. The plenty of nano-sized Cr-rich M₂C carbides were dominant phase in the steel after the thermomechanical treatment. The reason why prior precipitate phase formed in the steel during the thermomechanical treatment was Cr-rich M₂C carbide is also discussed.     

Effect of tempering temperature on the microstructure and properties of ultrahigh-strength stainless steel

The microstructure, precipitation and mechanical properties of Ferrium S53 steel, a secondary hardening ultrahigh-strength stainless steel with 10% Cr developed by QuesTek Innovations LLC, upon tempering were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile and impact tests. Based on these results, the influence of the tempering temperature on the microstructure and properties was discussed. The results show that decomposition occurred when the retained austenite was tempered above 440 °C and that the hardening peak at 482 °C was caused by the joint strengthening of the precipitates and martensite transformation. Due to the high Cr content, the trigonal M₇C₃ carbide precipitated when the steel was tempered at 400 °C, and M₇C₃ and M₂C (5–10 nm in size) coexisted when it was tempered at 482 °C. When the steel was tempered at 630 °C, M₂C and M₂₃C₆ carbides precipitated, and the sizes were greater than 50 nm and 500 nm, respectively, but no M₇C₃ carbide formed. When the tempering temperature was above 540 °C, austenitization and large-size precipitates were the main factors affecting the strength and toughness.     

TEM investigation of the tempering behaviour of the maraging PH 17.4 Mo stainless steel

The PH 17-4 Mo steel (Z6 CND 17.04.02), used in the steam generator of nuclear reactors, was investigated in order to determine the structural evolution occurring during tempering carried out under various conditions of duration and temperature. The formation and growth of different types of carbides such as Mo₂C, M₂₃C₆ and M₇C₃ and of Fe₂Mo intermetallic compound were studied and also of reversed austenite. A small secondary hardening peak was observed for tempering close to 400' C which is related to the Mo₂C carbide precipitation; beyond this temperature, softening occurs.     

Microstructural characterisation of vacuum sintered T42 powder metallurgy high-speed steel after heat treatments

High-speed steel powders (T42 grade) have been uniaxially cold-pressed and vacuum sintered to full density. Subsequently, the material was heat treated following an austenitising + quenching + multitempering route or alternatively austenitising + isothermal annealing. The isothermal annealing route was designed in order to attain a hardness value of ～50 Rockwell C (HRC) (adequate for structural applications) while the multitempering parameters were selected to obtain this value and also the maximum hardening of the material (～66 HRC). Microstructural characterisation has been carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microstructure consists of a ferrous (martensitic or ferritic) matrix with a distribution of second phase particles corresponding to nanometric and submicrometric secondary carbides precipitated during heat treatment together with primary carbides. The identification of those secondary precipitates (mainly M₃C, M₆C and M₂₃C₆ carbides) has allowed understanding the microstructural evolution of T42 high-speed steel under different processing conditions.     

Precipitate design for creep strengthening of 9% Cr tempered martensitic steel for ultra-supercritical power plants

Abstract

It is crucial for the carbon concentration of 9% Cr steel to be reduced to a very low level, so as to promote the formation of MX nitrides rich in vanadium as very fine and thermally stable particles to enable prolonged periods of exposure at elevated temperatures and also to eliminate Cr-rich carbides M₂₃C₆. Sub-boundary hardening, which is inversely proportional to the width of laths and blocks, is shown to be the most important strengthening mechanism for creep and is enhanced by the fine dispersion of precipitates along boundaries. The suppression of particle coarsening during creep and the maintenance of a homogeneous distribution of M₂₃C₆ carbides near prior austenite grain boundaries, which precipitate during tempering and are less fine, are effective for preventing the long-term degradation of creep strength and for improving long-term creep strength. This can be achieved by the addition of boron. The steels considered in this paper exhibit higher creep strength at 650 °C than existing high-strength steels used for thick section boiler components.