Tempering Behavior of 4.5Cr-2W-0.25V Steel

The tempering behavior of a Cr-W-V steel was investigated in this research.This new alloy with the composition of Fe-4.5Cr-2W-0.25V-0.1C was austenitized at 1000 ℃ for 30 min and tempered at 600 and 700 ℃ for different time up to 100 h.An OM analysis of the microstructure of air cooled and water quenched specimens before tempering showed that although under both conditions fully martensitic matrix formed,finer structure had formed in the water quenched specimens.The XRD and TEM results showed that the most stable carbides formed during tempering of the steel were M23C6 and M7C3,respectively.Other carbides such as M3C and M2C,formed in the first stages of tempering,and stable MC were also observed.The results showed that when the tempering time,temperature and cooling rate were increased,mass percent of extracted precipitates was increased.In addition,the formation rate of the stable carbides such as M23C6 and dissolution rate of the metastable carbides such as M3C and M2C were increased.     

Evolution and Coarsening of Carbides in 2.25Cr-1Mo Steel Weld Metal During High Temperature Tempering

Transformation and coarsening of carbides in 2.25Cr-1Mo steel weld metal during tempering at 700 ℃ for different time intervals ranging from 1 to 150 h were examined by transmission electron microscopy and scanning electron microscopy. M3C carbides were observed in the as-welded specimens and when tempered, the precipitates were mainly composed of M3C, M7C3, and M23C6 carbides. A sequence for corresponding carbide transformation during tempering with initial precipitation of M3C and the subsequent precipitation of M7C3 and M23C6 was proposed. The precipitation of M7C3 with higher chromium content was the main factor contributing to the decrease in coarsening rate of precipitates after prolonged tempering. The decrease in hardness of the tempered specimens agreed well with the prediction of the weakening of precipitation strengthening owing to the coarsening of carbides.     

Effect of Heat Treatment on the Mechanical Properties and the Carbide Characteristics of a High Strength Low Alloy Steel

Effect of heat treatment on the mechanical properties and the corresponding morphology changes of carbides such as (NbTi)C and Mo,Cr rich carbides in a high strength steel was studied in the paper.It was found that the strength depend more on tempering temperature than quenching temperature.While the toughness seemed to be sensitive to both quenching temperature and tempering temperature.Under the as water quenched condition,the toughness gave a peak value at the quenching temperature of 860℃.Under the as tempered condition,the toughness showed a temper brittleness at the temperature range of 230℃ to 450℃.The morphology changes of the carbides in the experimental steel were explained from the view of the thermal dynamic behavior.     

Effect of Initial Microstructure on Warm Deformation Behavior of 45 Steel

 In order to investigate the effect of initial microstructure on warm deformation behavior, some specimens of 45 steel were annealed and some quenched. Then the specimens were isothermally compressed on a Gleeble 3500 machine. The deformation temperature range was 550 to 700 ℃ and the strain rate range was 0.001 to 0.1 s-1. An optical microscope (OM) and a transmission electron microscope (TEM) were used to study the microstructures. The results show that the microstructure of annealed specimens is ferrite and pearlite and that of quenched specimens is martensite. The flow stress of quenched specimens is higher than that of annealed ones at 550 ℃ when strain rates are greater than 0.001 s-1. However, at 600 to 700 ℃ and strain rate of 0.001 s-1, the whole flow curves of quenched specimens are below that of annealed ones. Under the rest conditions, the flow stress of quenched specimens is higher at the beginning of compression and then the opposite is true after the strain is greater than a critical value. The microstructure examination proves that the tempering and dynamic recrystallization easily occur in the specimens with martensite during warm compression, which results in the above phenomena.     

Phase Transformation of a Cold Work Tool Steel during Tempering

The hardness and microstructure evolution of a 8% Cr cold work tool steel during tempering for 40 h were investigated. Transmission electron microscope examinations showed that M_3C carbides precipitated from supersaturated martensite after tempering at 350 ℃. When the tempering temperature was higher than 520 ℃,the M_(23)C_6 carbides precipitated to substitute for M_3C carbides. After ageing at the temperature of 520 ℃ for 40 h,it was observed that very fine and dense secondary Mo_2C precipitates were precipitated. Thus,it can be concluded that the early stage of Mo_2C-carbide precipitation is like to be Gunier-Preston( G-P) zone formed by [Mo-C] segregation group which is responsible for the secondary hardening peak at 520 ℃. Overageing at 700 ℃ resulted in recovery of martensitic microstructure and precipitation of M_(23)C_6 carbides.When ageing at 700 ℃ for more than 20 h,recrystallization occurred,which resulted in a change of the matrix morphology from martensitic plates to equiaxed ferrite. It was noticed that the size of recrystallized grain / subgrain was very fine,which was attributed to the pinning effect of M_(23)C_6 precipitates.     

Characteristics of the rough-cut surface of quenched and tempered martensitic stainless steel using wire electrical discharge machining

This article studies the surface characteristics of quench- and temper-treated AISI 440A martensitic stainless steels, which were rough cut using wire electrical discharge machining (WEDM). The microstructure of the recast layer on the cut surface was investigated using scanning and transmission electron microscopes, and the phase compositions were analyzed with an energy-dispersive X-ray (EDX) spectrometer. Experimental results showed that the thickness of the recast layer varied with the heat-treatment condition of the workpiece, the largest thickness was obtained with a quenched specimen, and the thickness decreased with increasing tempering temperature. Intergranular surface cracks were observed only from the as-quenched specimen, whereas surface cracks were not found in the rough-cut specimens after tempering above 200 °C. It is reckoned that reliefing of the thermal residual stress in the quenched workpiece induced the surface intergranular cracks. Microstructures of the recast layer on the rough-cut surfaces of the 600 °C tempered specimen were examined using cross-sectional transmission electron microscopy (TEM) specimens. An amorphous layer exists at some parts of the outermost cut surface. A high density of wire electrode droplets of spherical shape, approximately 10 to 60 nm in size, was found throughout the porous recast layer. Besides, many high-chromium containing sigma spheres with sizes of approximately 120 to 200 nm were precipitated at the bottom part of the recast layer, and its formation mechanism was proposed. Adjacent to the recast layer was a heat-affected zone (HAZ) with a thickness of about 4 μm, in which temper-induced carbides were fully dissolved. The HAZ comprised basically two distinct regions: the first region adjacent to the recast layer was composed of a lath martensite structure, while the other region was an annealed ferrite structure.     

Influence of rare earths on eutectic carbides in AISI M2 high speed steel

The effect of rare earths on the morphology and microstructure of eutectic carbides in M2 high speed steel was studied. The results showed that rare earths promoted the formation of fishbone-like M6C eutectic carbides, compared to plate-like M2C carbides in ingots without modification. The formation of M6C was expected to be caused by rare earth inclusions which acted effectively as the substrate for nucleation of M6C carbides during solidification. M2C and M6C eutectic carbides exhibited different stability during heating. M2C eutectic carbides were much less stable than M6C carbides, and decomposed at high temperatures, favoring the spheroidization and refinement of carbides in high speed steels.     

Formation of AlN Nano Particles Precipitated in St-14 Low Carbon Steel by Micro and Nanoscopic Observations

In low carbon steels, dissolution and precipitation of the second phases such as carbides and nitrides during annealing cycles can affect the final structure and properties of the materials. The interaction of above processes depends on parameters such as reheating temperature, heating rate, annealing temperature, soaking time and finishing temperature in hot rolling stage before cold rolling. The effects of heating rate and annealing temperature on the microstructure and hardness were investigated. Two heating rates for annealing temperatures of 550, 610 and 720℃ were applied on cold-rolled specimens and St-14 low carbon steel, which were immediately quenched after isothermal annealing. The intercept method was used tO measure average grain sizes. However, resulted microstructures are dif- ferent for the two heating rates. While pancaked structures were observed in specimens annealed with low heating rate, in samples annealed with high heating rate, equiaxed microstructures were observed. Vickers micro-hardness values decreased at all temperatures, which were more significant at higher temperatures. At longer annealing time, signs of increase of hardness values were detected. All results and observations consistently suggest that a precipitati- on process has occurred concurrently with restoration processes during annealing. In addition, the energy dispersive spectroscopy analysis resulted from transmission electron microscopic micrographs have proved that the nano particles precipitated in grain boundaries are AlN.     

A New Approach for Refining Carbide Dimensions in M42 Super Hard High-speed Steel

Obtaining small carbides is crucial but difficult for high-speed steels.A new approach for refining carbide dimensions in M42 super hard high-speed steel by increasing cooling rate and spheroidizing treatment was proposed. The morphologies and properties of eutectic carbides formed at different cooling rates were investigated by means of scanning electron microscopy (SEM),energy dispersive spectroscopy (EDS),X-ray diffraction (XRD),transmis-sion electron microscopy (TEM),electron back-scattered diffraction (EBSD)and differential scanning calorimeter (DSC).The results show that eutectic carbides change from a lamellar shape into a curved-rod shape as cooling rate increases.Despite different morphologies,the two carbides are both of M2 C type with a hexagonal close-packed structure and display a single crystal orientation in one eutectic colony.The morphology of M2 C mainly depends on the growing process of eutectic carbides,which is strongly influenced by cooling rate.Compared with lamellar car-bides,M2 C carbides with curved-rod shapes are less stable,and decompose into M6 C and MC at lower temperatures. They are more inclined to spheroidize during heating,which ultimately and distinguishably refines the carbide dimen-sions.As small carbides are much easier to dissolve into matrices during austenization,the process described herein improves the supersaturation of alloying elements in martensite,which leads to an increment of hardness in M42 steel.     

Microstructural evolution of 405 stainless steel under high temperature

The ferrite-to-austenite phase transformation temperature of SA240-405 stainless steel was measured using the thermodilatometry method and calculated using Thermo-Calc. In addition,the effect of temperature and the soaking time on the microstructural evolution was investigated for various quenching and tempering treatments. The results indicated that the ferrite-to-austenite transformation of this steel started between 795 ℃and 832 ℃ and finished between 910 ℃ and 925 ℃. W hen the specimens were annealed above 1050 ℃,the austenite gradually transformed into ferrite; consequently,the content of as-quenched martensite decreased with increasing temperature. M oreover,when the specimens were quenched between 950 ℃ and 980 ℃,a microstructure of duplex phases comprising ferrite and martensite was obtained. Relatively high B-scale of Rockwell hardness( HRB) values were observed for quenching times of 30-60 minutes; then,the hardness gradually decreased with increasing quenching time. Tempering at 730 ℃ resulted in ferrite and tempered martensite,and no obvious residual austenite was observed. In addition,the hardness gradually decreased with increasing tempering time.     

Evaluation of Microstructural and Mechanical Behaviours of Tempered 2.25Cr–1Mo Steel Through Electromagnetic Characterization

The effect of tempering treatment has been investigated on water quenched P22 steel with the chemical composition of 0.13C, 0.24Si, 0.47Mn, 0.012P, 0.005S, 2.19Cr, 0.93Mo and balance Fe (all in wt%) within the temperature ranges of 650–900 °C. The microstructural, mechanical and magnetic properties of as-quenched and tempered steels have been investigated through optical and scanning electron microscopy, hardness and universal tensile testing, electromagnetic sensor (Magstar), respectively. The water quenched sample consists of fine martensitic structure with a hardness of 381 HV. With the progress of tempering, the martensite becomes coarse till 800 °C, decreasing the hardness of steel samples. The tempering at 700 °C results in martensite coarsening and precipitation of rod and globular shaped carbides; while a fraction of globular carbide is observed to increase in the matrix after 750 °C of tempering. Beyond 800 °C, the ferrite and bainite phases gradually form by replacing martensite, and the ferrite structure is prevalent after 900 °C. Due to microstructural changes, the magnetic properties are also affected as a function of tempering temperature. The coarsening of martensite causes the decrease in coercivity with increasing tempering temperature, leading to magnetic softening.     

Wear and Fatigue Behaviour of Deep Cryogenically Treated H21 Tool Steel

The influence of deep cryotreatment before and after double tempering on hardness, wear rate, surface finish and fatigue limit of AISI H21 tool steel has been examined. In the present work, H21 tool steel has been subjected to heat treatment at 1195 °C, double tempering at 540 °C, deep cryotreatment at ??185 °C and soft tempering at 100 °C. The microstructure of samples has been characterized for the number density of carbides, hardness, wear rate, surface roughness and fatigue limit. The fatigue test has been carried out using a rotating bending fatigue machine to study the fatigue strength of the material. The obtained results show that the HTTC24 specimen reduces the wear rate by?≈?24% and surface roughness by?≈?21% with an increase in fatigue limit by?≈?13% compared to HTT specimens. This has been attributed to the increased number density of carbides and hardness; and approximately the complete conversion of retained austenite content into martensite.

     

Mechanism of Secondary Hardening in Rapid Tempering of Dual-Phase Steel

Dual-phase steel with ferrite-martensite-bainite microstructure exhibited secondary hardening in the subcritical heat affected zone during fiber laser welding. Rapid isothermal tempering conducted in a Gleeble simulator also indicated occurrence of secondary hardening at 773 K (500 °C), as confirmed by plotting the tempered hardness against the Holloman–Jaffe parameter. Isothermally tempered specimens were characterized by analytic transmission electron microscopy and high-angle annular dark-field imaging. The cementite (Fe₃C) and TiC located in the bainite phase of DP steel decomposed upon rapid tempering to form needle-shaped Mo₂C (aspect ratio ranging from 10 to 25) and plate-shaped M₄C₃ carbides giving rise to secondary hardening. Precipitation of these thermodynamically stable and coherent carbides promoted the hardening phenomenon. However, complex carbides were only seen in the tempered bainite and were not detected in the tempered martensite. The martensite phase decomposed into ferrite and spherical Fe₃C, and interlath-retained austenite decomposed into ferrite and elongated carbide.     

Carbide Precipitation During Tempering of a Tool Steel Subjected to Deep Cryogenic Treatment

Using transmission electron microscopy, Mössbauer spectroscopy, and measurements of hardness, the carbide precipitation during tempering of steel X153CrMoV12 containing (mass pct) 1.55C, 11.90Cr, 0.70V, and 0.86Mo is studied after three treatments: quenching at RT and deep cryogenic treatment, DCT, at 77 K or 123 K (?196 °C or ?150 °C). In contrast to some previous studies, no fine carbide precipitation after long-time holding at cryogenic temperatures is detected. After quenching at room temperature, RT, the transient ε(ε′) carbide is precipitated between 373 K and 473 K (100 °C and 200 °C) and transformed to cementite starting from 573 K (300 °C). In case of DCT at 123 K (?150 °C), only fine cementite particles are detected after tempering at 373 K (200 °C) with their delayed coarsening at higher temperatures. Dissolution of cementite and precipitation of alloying element carbides proceed at 773 K (500 °C) after quenching at RT, although some undissolved cementite plates can also be observed. After DCT at 123 K (?150 °C), the transient ε(ε′) carbide is not precipitated during tempering, which is attributed to the intensive isothermal martensitic transformation accompanied by plastic deformation. In this case, cementite is the only carbide phase precipitated in the temperature range of 573 K to 773 K (300 °C to 500 °C). If DCT is carried out at 77 K (?196 °C), the ε(ε′) carbide is found after tempering at 373 K to 473 K (100 °C to 200 °C). Coarse cementite particles and the absence of alloying element carbides constitute a feature of steel subjected to DCT and tempering at 773 K (500 °C). As a result, a decreased secondary hardness is obtained in comparison with the steel quenched at RT. According to Mössbauer studies, the structure after DCT and tempering at 773 K (500 °C) is characterized by the decreased fraction of the retained austenite and clustering of alloying elements in the α solid solution. It is suggested that a competition between the strain-induced transformation of the retained austenite and carbide precipitation during the wear can control the life of steel tools.     

Delamination Fracture Related to Tempering in a High-Strength Low-Alloy Steel

The delamination or splitting of mechanical test specimens of rolled steel plate is a phenomenon that has been studied for many years. In the present study, splitting during fracture of tensile and Charpy V-notch (CVN) test specimens is examined in a high-strength low-alloy plate steel. It is shown that delamination did not occur in test specimens from plate in the as-rolled condition, but was severe in material tempered in the temperature range 500 °C to 650 °C. Minor splitting was seen after heating to 200 °C, 400 °C, and 700 °C. Samples that had been triple quenched and tempered to produce a fine equiaxed grain size also did not exhibit splitting. Microstructural and preferred orientation studies are presented and are discussed as they relate to the splitting phenomenon. It is concluded that the elongated as-rolled grains and grain boundary embrittlement resulting from precipitates (carbides and nitrides) formed during reheating were responsible for the delamination.     

Microstructure and Mechanical Properties of Martensitic Stainless Steel 6Crl5MoV

 Martensitic stainless steel containing Cr of 12% to 18% (mass percent) are common utilized in quenching and tempering processes for knife and cutlery steel. The properties obtained in these materials are significantly influenced by matrix composition after heat treatment, especially as Cr and C content. Comprehensive considered the hardness and corrosion resistance, a new type martensitic stainless steel 6Cr15MoV has been developed. The effect of heat treatment processes on microstructure and mechanical properties of 6Cr15MoV martensitic stainless steel is emphatically researched. Thermo-Calc software has been carried out to thermodynamic calculation; OM, SEM and TEM have been carried out to microstructure observation; hardness and impact toughness test have been carried out to evaluate the mechanical properties. Results show that the equilibrium carbide in 6Cr15MoV steel is M23C6 carbide, and the M23C6 carbides finely distributed in annealed microstructure. 6Cr15MoV martensitic stainless steel has a wider quenching temperature range, the hardness value of steel 6Cr15MoV can reach to HRC 608 to HRC 616 when quenched at 1060 to 1100 ℃. Finely distributed carbides will exist in quenched microstructure, and effectively inhabit the growth of austenite grain. With the increasing of quenching temperature, the volume fraction of undissolved carbides will decrease. The excellent comprehensive mechanical properties can be obtained by quenched at 1060 to 1100 ℃ with tempered at 100 to 150 ℃, and it is mainly due to the high carbon martensite and fine grain size. At these temperature ranges, the hardness will retain about HRC 592 to HRC 616 and the Charpy U-notch impact toughness will retain about 173 to 20 J. A lot of M23C6 carbides precipitated from martensite matrix, at the same time along the boundaries of martensite lathes which leading to the decrease of impact toughness when tempered at 500 to 540 ℃. The M3C precipitants also existed in the martensite matrix of test steel after tempered at 500 ℃, and the mean size of M3C precipitates is bigger than that of M23C6 precipitates.     

喷射成形与传统熔炼高速钢M2的组织与力学性能研究

通过喷射成形和传统熔炼（中频冶炼+电渣重熔）两种工艺生产了高速钢M2（W6Mo5Cr4V2）试样，利用金相显微镜和M-200磨损试验机对同规格同位置的两种试样的退火组织、非金属夹杂物、淬回火硬度、显微组织和力学性能进行了研究。结果表明，喷射成形M2试样的碳化物分布均匀、尺寸细小，传统熔炼M2试样碳化物呈条带状分布；在相同热处理制度和位置下，喷射成形M2试样的回火硬度与传统熔炼M2试样相当；喷射成形M2试样的耐磨性要比传统M2试样提高约41%；喷射成形M2试样中尺寸大于2μm的MC类碳化物数量明显多于传统M2试样，使得在同等硬度下喷射成形M2试样的耐磨性能要优于传统M2试样。由此可知，喷射成形M2试样的组织及力学性能均优于传统熔炼M2试样，喷射成形技术具有工艺先进性。     

M2C precipitates in isothermal tempering of high Co-Ni secondary hardening steel

The effects of isothermal tempering on the coarsening behavior of hexagonal M₂C precipitates and the secondary hardening reaction in ultrahigh-strength AerMet 100 steel were investigated. The tempering temperatures were 468 °C, 482 °C, and 510 °C, and the tempering time spanned the range from 1 to 400 hours. Experimental studies of the coarsening behavior of the carbides were made by utilizing transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffractometry (XRD). The hardness at the secondary hardening peak was about HRc 55. The average length and diameter of M₂C carbides were 4 to 8 nm and 1.5 to 2.5 nm, respectively, at all three tempering temperatures; hence, the aspect ratio was almost 3, an equilibrium value in this case. The size of the M₂C carbides increased monotonically with time, but the growth kinetics did not exactly follow the classical coarsening behavior. The amount of precipitated austenite increased with tempering time and temperature. M₂C precipitates were still relatively fine even after 200 hours of tempering. This feature seemed to be closely related to the high hardness maintained after prolonged tempering.