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.     

Internal friction analysis of lath martensite in press hardened steel

Internal friction (IF) measurements were carried out on a press hardened steel (PHS) after continuous annealing, press hardening and bake hardening. The IF peaks of the PHS with a lath martensite microstructure were analysed by comparison with previously published data. This was supplemented by comparison with the IF spectra of the same steel with a ferrite–pearlite microstructure after deformation at room temperature, and after recrystallisation annealing and quenching. The relation between the IF peaks of PHS, and the γ-peak, Snoek peak and Snoek-Kê-Köster peak observed for ferritic steel is discussed.     

On the microstructure and mechanical properties of high-speed steels

The microstructure of high-speed steels consists of a martensitic matrix with a dispersion of two sets of carbides. These carbides are usually known as primary and secondary carbides. The role of the primary carbides has been reported to be of no importance in strengthening the steels, due to their large size and large interparticle spacing. The present authors have studied the role of the primary carbides on the wear of high-speed steels and found them to be of no importance, and under certain conditions contributing to higher wear rates. It has been shown analytically and experimentally that in quenched and tempered high-speed steels, the precipitation of the secondary hardening carbide (cubic M₂C type) is the main reason for the improved strength and wear resistance. This shows that the secondary hardening phenomenon of high-speed steels is a direct result of the hardening caused by the precipitation of the cubic M_2C-type carbide. The present study has estimated that at peak hardness the volume fraction of secondary hardening carbides is approximately 20%. The measured strength of high-speed steels was found to be lower than the theoretically calculated strength due to non-homogeneous precipitation of the secondary hardening carbides. Areas which were observed to be free from secondary hardening carbides are real and are not artefacts. It has been shown that the strength of high-speed steel in the region of peak hardness depends primarily on the precipitation of the secondary hardening carbide and secondarily on martensitic strengthening.     

Precipitates change of ferritic‐martensitic 11 % chromium steel under short‐term creep

A ferritic‐martensitic (FM) 11 % chromium steel with final heat treatment was subjected to a short‐term creep test at a stress of 150 MPa and 600 °C for 1100 h in order to study the change of precipitates in the steel during the creep test. Except for Nb‐rich metall carbides (MC, M₂₃C₆) and Laves phases, Fe‐W‐Cr‐rich M₆C (based on Fe₃W₃C) carbides forming during the creep test were also identified in the crept steel by electron diffraction and x‐ray diffraction in combination with energy dispersive x‐ray analysis of extraction carbon replicas. The identified M₆C carbides have a fcc crystal structure, a metallic element composition of approximately 44Fe, 32 W, and 20Cr in atomic %, and large sizes ranging from 100 nm to 300 nm in diameter. The M₆C carbides are a dominant phase in the crept steel. M₆X precipitates are generally not easy to form during high temperature creep, even if it is a long‐term creep, in ferritic‐martensitic 9–12 % chromium steels with a final heat treatment. The present work provides the evidence for the M₆C carbides forming during short‐term creep in ferritic‐martensitic high chromium steels. The formation of the M₆C carbides was discussed.     

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.     

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.     

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.     

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.     

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 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     

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.     

HRTEM Study on Precipitates in High Co-Ni Steel

The secondary hardening reaction is accompanied with precipitation of fine carbides in high CoNi ultrahigh strength steel. The crystal structure of the precipitating carbides is unambiguously determined by microbeam diffraction in transmission electron microscopy. It is identified that the needle-shaped carbides are M2C with a hexagonal structure. The concentration of substitutional alloying elements in the carbides quantified by energy dispersive X-ray spectroscopy (EDS) also supports the result above. The spatial structure of M2C is identical with L'3 type. Metal atoms are in a close packed hexagonal structure, the carbon atoms partly distribute with random in the octahedral interstices and the filling probability is less than 1/2. Particular attention was paid to the relationship of needle-shaped carbides/ferrite matrix at secondary hardening peak tempered at 482癈 for 5 h. Observation by high resolution transmission electron microscopy (HRTEM) confirms that carbides with black-white contrast are ful     

Morphology and microstructure of M<Subscript>2</Subscript>C carbide formed at different cooling rates in AISI M2 high speed steel

In as-cast structure of AISI M2 high speed steel, M₂C carbide prevails, the morphology of which has crucial influence on distribution and dimension of carbides in final products. In this study, the morphology and microstructure of M₂C formed at different cooling rates have been investigated by scanning electron microscope, X-ray diffraction, transmission electron microscope, and electron back-scatter diffraction. The results show that the morphology of M₂C carbide changes from the plate-like type to the fibrous one with increasing cooling rates. Surprisingly, the microstructure between plate-like and fibrous M₂C is significantly different. Twining and stacking faults are observed in the plate-like M₂C, which is supported by great misorientations between adjacent carbides. However, no planar faults are identified in fibrous M₂C and the carbides in one colony have almost identical orientation. It is expected that plate-like M₂C grows as a faceted phase, while fibrous M₂C formed at high cooling rates is likely a non-faceted phase. The difference of liquid/solid interface structure is supposed to result in the morphology change of M₂C.     

Experimental investigation on heat treatment of a high‐speed steel for hot rolling roll mill

The authors describe the researching results to optimise the hardening and tempering of the high carbon high‐speed steel for rolls containing 2.38%C, 5.07%V, 6.34%Mo, 5.09%Cr, 1.20%Ni, 1.17%Nb, 0.09%Ti and 0.05%RE by means of light optical microscope (LOM), scanning electron microscope (SEM), backscattered electron image (BSE), X‐ray diffraction (XRD), and hardness, tensile strength, impact toughness and wear testers. The results show that the microstructure of above casting high‐speed steel is given by a tempered martensitic matrix surrounded by eutectic carbides. Casting high‐speed steel has higher hardness quenching at 1280 K–1340 K, and it has higher hardness, tensile strength, impact toughness, and abrasive wear resistance tempering at 793 K–833 K. The comprehensive properties of casting high‐speed steel is the best while air‐cooling quenching about 1340 K and tempering about 813 K.     

Effect of Nb on long-term creep life of JIS SUS304HTB and JIS SUS347HTB steels – heat-to-heat variation and life assessment of stainless steels

Heat-to-heat variation in creep life has been investigated for the 9 heats of JIS SUS 304HTB (18Cr–8Ni steel) and also for the 9 heats of JIS SUS 347HTB (18Cr–12Ni–Nb steel) in the NIMS Creep Data Sheets, mainly taking the effect of Nb into account. The heat-to-heat variation in creep life of 304HTB is mainly caused by the variation in precipitation hardening due to fine NbC carbides at short times, while it is mainly caused by the variation in available nitrogen concentration, defined as the concentration of nitrogen free from AlN and TiN, at long times. The heat-to-heat variation in creep life of 347HTB is mainly explained by the variation of boron concentration, 3–27 ppm, but not by the variation of solution temperature, Nb/C atomic ratio and phosphorus concentration. Boron reduces the coarsening rate of fine M₂₃C₆ carbides along grain boundaries, which enhances the grain boundary precipitation hardening.     

FeCr1 8.2Ni6.9Mo2.5Cl.5合金二次硬化机制的研究

奥氏体 FeCr18.2Ni6.9Mo2.5C1.5合金时效时可产生明显的二次硬化效果。在时效过程中除有M_(23)C_6碳化物沉淀析出外,还有体心相形成。经差热分析、x 射线衍射分析和电子衍射分析证明该体心楣为加热过程中形成的铁素体,但往往会被误认为二次淬火时形成的马氏体。高温硬度试验证明二次硬化效果是由 M_(23)C_6碳化物形成引起的。     

Evolution of microstructure and room temperature tensile properties in aluminum-bearing high strength steel processed by ultrafast cooling

An aluminum-bearing high strength steel with relatively high precipitation hardening was fabricated by lowering coiling temperature to 627 °C. The influence of coiling temperature on microstructure evolution and precipitation behavior was comprehensively investigated by means of scanning electron microscopy, electron back-scatter diffraction and transmission electron microscopy. Both yield strength and tensile strength can be increased by around 100 MPa through decreasing coiling temperature to 627 °C, whereas there is nearly no deterioration in ductility. In addition, The grain boundary precipitation of (Fe, Cr, Mn)_xC_y-type carbides can be effectively suppressed by lowering coiling temperature. Regardless of coiling temperature, both interphase precipitation consisting of curved and planar shape and random precipitation can be observed. Moreover, the sheet spacing can be refined from 29 nm–47 nm to 18 nm–27 nm by lowering coiling temperature from 721 °C to 627 °C. This small sheet spacing provides a greater precipitation hardening. Compared with a coiling temperature of 721 °C, the precipitation hardening can be increased by around 100 MPa for a coiling temperature of 627 °C.     

Effect of K/Na on microstructure of high-speed steel used for rolls

In the present work, the effect of K/Na on microstructure of high-speed steel (HSS) used for rolls was investigated utilizing Hi-scope video microscope (HSVM) and electron probe microanalyser (EPMA). As-cast microstructure of the alloy is mainly composed of pearlite matrix, M₇C₃, M₂C and MC eutectic carbides. The carbides are connected or placed next to each other to form a network along grain boundaries. After K/Na modification, the morphology, size and distribution of carbides change greatly. The carbide network tends to break, and all carbides are refined and distributed homogeneously in the matrix. The mechanism of K/Na modification on microstructure of the alloy is also discussed.     

Precipitation behaviour of a sensitized AISI type 316 austenitic stainless steel in hydrogen

The purpose of this study was to characterize the precipitation behaviour of AISI type 316 steel in hydrogen. The different precipitates (M₂₃C₆, M₆C), the intermetallicχ-phase and the martensitic phase (α′,ε) were determined by using transmission electron microscopy (TEM) and X-ray diffraction techniques. All the specimens were sensitized at 650? C for 24 h. Some samples were carburized up to 2 wt% C. Additions of carbon content decrease the time required for sensitization. Short-term (24 h) exposure of this steel to sensitization temperature results in a complex precipitation reaction of various carbides and intermetallic phases. Hydrogen was introduced by severe cathodic charging at room temperature. This study indicates that by conventional X-ray techniques it is possible to detect those precipitates and their behaviour in a hydrogen environment. The zero shift as observed by X-ray diffraction from the carbides (M₂₃C₆, M₆C) and the intermetallicχ-phase, indicates that those phases absorb far less hydrogen than the austenitic matrix. TEM studies reveal that hydrogen inducesα′ martensite at chromium-depleted grain-boundary zones, near the formation of the carbides.