V-rich MC Research Articles

Hydrogen embrittlement of a V+Nb-microalloyed medium-carbon bolt steel subjected to different tempering temperatures

The effect of different tempering temperatures on the hydrogen embrittlement (HE) resistance of a V + Nb-microalloyed medium-carbon bolt steel was studied by slow strain rate tensile testing and hydrogen thermal analysis. The HE resistance represented by notch tensile strength σbN of hydrogen-charged specimens increases with increasing tempering temperature Ttem, and the σbN of the in situ hydrogen-charged sample is considerably lower than that of the pre-hydrogen-charged sample. Both the diffusible hydrogen content Cdiff and hydrogen-trapping capability of the precipitates show a significant increase-decrease change trend with Ttemp. The ratio of Cdiff primarily trapped by nanoscale V-rich MC precipitates increases linearly with increasing Ttemp. The control of V-rich MC through suitable increasing Ttemp is vital for obtaining the best HE resistance although at the cost of decreased strength. It is thus suggested that a compromise between strength and HE resistance should be considered for the practical application of this type of steel.

Effect of direct-quenching and tempering on hydrogen-induced delayed fracture resistance of high-strength bolt steel

This investigation attempts to explore the potential significance of controlled forging, i.e., direct-quenching at two different finish-forging temperatures of ∼930 °C (B8H sample) and ∼780 °C (B8L sample), and tempering at 600 °C in improving the hydrogen-induced delayed fracture (HIDF) resistance of a 1300-MPa-grade high-strength bolt steel by constant load tensile testing using circumferentially notched round bar specimens and hydrogen thermal analysis. The microstructural characteristics of the tested samples were analyzed and their influences on HIDF were discussed. The results showed that the B8L sample demonstrated a considerably increased HIDF resistance expressed by critical notch delayed fracture stress of 1825 MPa and a decreased hydrogen embrittled index of as low as 21% compared with those of 1675 MPa and 27% for the B8H sample, although the former exhibited slightly higher strength and absorbed hydrogen content. Scanning electron microscopy observation of the fracture surface demonstrated that the B8L sample prevented the brittle intergranular fracture with smaller quasi-cleavage facet and decreased area fraction of the brittle crack initiation zone compared with the B8H sample. The increased resistance to HIDF of the B8L sample is primarily because of its fine microstructure with increased number of finely distributed V-rich nanoscale MC precipitates and decreased dislocation density. It is proposed that controlled forging coupled with V-microalloying is a feasible and economical way to further increase the HIDF resistance of high-strength bolt steel.

Role of W addition in reducing heat checking and enhancing the mechanical properties of hot work tool steel

The influence of carbide precipitation on the mechanical properties of hot work tool steels was investigated in this study. Compared to the commercial H13 hot work tool steel, the KMoW and RW steels containing W exhibited a reduced high-temperature softening and an enhanced thermal fatigue resistance during long-term aging. These improvements can be attributed to the increased structural and chemical stability of V-rich MC carbides. The increased (W + Mo)/(V + Fe) atomic ratios at MC metallic sites increased their coherency as well as reduced the driving force required for the precipitation of Cr-rich carbides. These findings suggest that the addition of W effectively improves the heat-checking resistance and mechanical properties of hot work tool steels.

Supersolidus Liquid Phase Sintering and Heat Treatment on Atomic Diffusion Additive Manufacturing Produced Ledeburitic Cold Work Tool Steel*

Abstract In this work, the possibility of manufacturing complex-shaped components from a carbon-martensitic hardenable cold-work steel (1.2379; X153CrMoV12; D2) is investigated. For this purpose, cube-shaped samples with an edge length of 10 mm were produced using the fused-filament fabrication process, which were post-compacted after solvent debinding by supersolidus liquid-phase sintering. Using the knowledge of liquid phase volume content as a function of temperature, supersolidus liquid phase sintering experiments were performed. The microstructure formation process was characterized by electron microscopy and X-ray diffraction. The microstructure and hardness of the processed samples were compared in the heat-treated condition with the properties of the same steel 1.2379 (X153CrMoV12; D2) in the as-cast, deformed and heat-treated condition. The results demonstrate effective post-densificationc close to theoretical density of cold-work tool steel samples fabricated by fused-filamet fabrication using supersolidus liquid-phase sintering at 1280 °C. The defect-free microstructure in the heat-treated state is characterized by a martensitic matrix and eutectic Cr-rich M7 C3 and small amounts of V-rich MC carbides. The hardness of the annealed Supersolidus liquid phase sintering samples are 681 ± 5 HV10, which is above the level of the reference material 1.2379 (629 ± 7 HV10) in the as-cast, formed and heat-treated condition.

Transformation behavior of primary [formula omitted] and [formula omitted] carbides in Cr4Mo4V steel

The transformation of primary MC and M2C carbides, during high-temperature treatment, was investigated by quasi in-situ observation of Cr4Mo4V steel. It was found that the V-rich rod-like MC carbides have great thermal stability, and disintegrating and coarsening are their dominant transformation mechanisms. The M2C carbides mainly possess rod-like (M2C-R) and lamellar (M2C-L) morphologies. The M2C-R carbides are Mo-enriched, and their transformation follows the paths: the separate precipitation of MC and M2C-R + γ→MC + M6C. The V- and Fe-rich MC carbides nucleates randomly in the interior of M2C-R before the nucleation of Mo- and Fe-rich M6C carbides, which is controlled by the diffusion of V. Soaking for longer durations can cause spheroidization and coarsening of the MC carbides, along with the dissolution of M6C carbides. The evolution of Fe- and Mo-rich M2C-L carbides consists of two stages: the partial dissolution of M2C-L into the matrix and the transformation of M2C-L→MC + γ. These novel findings contribute towards the understanding of how primary carbides evolve during heat treatment and provide a potential method for carbides refinement in Cr4Mo4V steel.

Mechanical Properties and Microstructures of Mo<sub>2</sub>C Strengthened Vanadis 4 Extra Alloy Steel by Powder Metallurgy and Heat Treatments

In this work, different ratios of molybdenum carbide (1, 3 and 5 mass% Mo2C) powders were added to Vanadis 4 extra alloy steel powders and then mixed by ball milling for 6 h. The composite powders underwent vacuum sintering at 1200, 1220, 1240, 1260 and 1280°C for 1 h, respectively. The results showed that the optimal sintering temperature for the addition of 5 mass% Mo2C powders was 1220°C. It also represented that the apparent porosity was 0.18%, and that a transverse rupture strength (TRS) value of 2281.3 MPa and a hardness value of 79.2 HRA were obtained, respectively. Additionally, the TRS value was obviously enhanced to 2437.6 and 2491.4 MPa by the addition of 5 mass% Mo2C powders after heat treatment and sub-zero plus heat treatments, respectively. Meanwhile, the hardness value also increased to 80.6 and 81.3 HRA, respectively, whereas the Mo-rich M6C carbides distributed in the grain boundaries, and V-rich MC carbides appeared in the grain and grain boundaries after sub-zero plus heat treatments. Significantly, a series of heat treatment processes is effective in improving the microstructure and strengthening the mechanical properties of the sintered Vanadis 4 extra composites.

Chemical composition and structural identification of primary carbides in as-cast H13 steel

The aim of this study was to characterize the primary carbides formed in as-cast H13 steel. The composition, morphology, type, and size of primary carbides in as-cast H13 steel were investigated by optical microscope (OM), field emission scanning electron microscopy (FE-SEM), electron back-scattered diffraction (EBSD), and X-ray diffraction (XRD) analysis. The number of primary carbides was investigated by ASPEX automated inclusion analysis system. The results indicated that primary carbides in as-cast H13 steel are mainly composed of Cr, Mo, V, and Ti, and there exist four kinds of primary carbides in the interdendritic zones of H13 steel, which are stripy Mo–Cr-rich M2C, eutectic Mo–Cr-rich M2C, V-rich MC, and V-rich MC with Ti and N. Thermodynamic calculation indicated that M2C precipitates in liquid phase at solid fractions larger than 0.99, while MC precipitates in liquid phase at solid fractions larger than 0.96. Statistical results indicated that the number of M2C is much greater than the number of other kinds of primary carbides. Most primary carbides are blocky, with lengths of no more than 10 μm and a length/width ratio of no more than 3. The large primary carbides in as-cast H13 steel are mainly M2C.

Evolution of carbides on surface of carburized M50NiL bearing steel

The dissolution and precipitation behaviors of the carbides in carburized M50NiL steel were derived from different solution and tempering treatments. Totally four kinds of carbides of (V, Cr)-rich MC, (Mo, Fe)-rich M2C, Fe-rich M3C and (Fe, Cr)-rich M7C3 were obtained from the carburized M50NiL steel after different heat treatments. The key carbides of carburized M50NiL steel were proved to be tough V-rich MC and Cr-rich M7C3. The highest hardness (634 HV) and the optimal surface structure with 1.0% volume fraction of uniformly distributed MC carbides were obtained after the carburized M50NiL steel was solution-treated at 1150 °C and tempered at 500 °C. The quantitative statistics show that 63% of the MC carbides were less than 200 nm under that heat treatment. The variety of carbides changed with solution and tempering conditions. When the solution temperature increased from 1050 to 1150 °C, the undissolved carbides were proved to be Fe-rich M7C3, Mo-rich MC and (Mo, Fe)-rich M2C. Besides, the equivalent content of V-rich MC was found increased when the tempering temperature changed from 500 to 550 °C. The combination of high-temperature solution and low-temperature tempering is recommendable heat treatment for the high hardness as well as the tiny and uniformly distributed carbides.

A Novel Method to Calculate the Carbides Fraction from Dilatometric Measurements During Cooling in Hot-Work Tool Steel

Dilatometry is a useful technique to obtain experimental data concerning transformation. In this paper, a dilation conversional model was established to calculate carbides fraction in AISI H13 hot-work tool steel based on the measured length changes. After carbides precipitation, the alloy contents in the matrix changed. In the usual models, the content of carbon atoms after precipitation is considered as the only element that affects the lattice constant and the content of the alloy elements such as Cr, Mo, Mn, V are often ignored. In the model introduced in this paper, the alloying elements (Cr, Mo, Mn, V) changes caused by carbides precipitation are incorporated. The carbides were identified using scanning electron microscope and transmission electron microscope. The relationship between lattice constant of carbides and temperature are measured by high-temperature X-ray diffraction. The results indicate that the carbides observed in all specimens cooled at different rates are V-rich MC and Cr-rich M23C6, and most of them are V-rich MC, only very few are Cr-rich M23C6. The model including the effects of substitutional alloying elements shows a good improvement on carbides fraction predictions. In addition, lower cooling rate advances the carbides precipitation for AISI H13 specimens. The results between experiments and mathematical model agree well.

Precipitate Phase Evolution in a 11Cr Ferritic/Martensitic Steel During Tempering

The precipitate phases of a 11Cr F/M steel normalized at 1323 K (1050 °C) for 0.5 h and tempered for 1.5 h at temperatures ranging from 573 K to 1053 K (300 °C to 780 °C) were investigated using transmission electron microscopy and energy-dispersive X-ray spectroscopy. In the normalized condition, Fe-rich M3C, Fe-rich M7C3, and large MC carbides were identified in the steel. In the relatively low tempering temperature ranging from 573 K to 773 K (300 °C to 500 °C), Fe-rich M2C, Fe-rich M3C, and large-sized Nb-rich MC carbides were observed in the steel. In the high tempering temperature ranging from 873 K to 1053 K (600 °C to 780 °C), Cr-rich M2C, Cr-rich M3C2, Cr-rich M23C6, and Fe-rich M5C2 carbides and MX particles including Nb-rich, Ta-Nb-rich, and V-rich MC carbides were detected in steel besides large MX particles. The process of precipitate phase evolution in the steel during tempering is discussed.

Influence of Tool Rotational Speed and Post-Weld Heat Treatments on Friction Stir Welded Reduced Activation Ferritic-Martensitic Steel

The effects of tool rotational speed (200 and 700 rpm) on evolving microstructure during friction stir welding (FSW) of a reduced activation ferritic-martensitic steel (RAFMS) in the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ) have been explored in detail. The influence of post-weld direct tempering (PWDT: 1033 K (760 °C)/ 90 minutes + air cooling) and post-weld normalizing and tempering (PWNT: 1253 K (980 °C)/30 minutes + air cooling + tempering 1033 K (760 °C)/90 minutes + air cooling) treatments on microstructure and mechanical properties has also been assessed. The base metal (BM) microstructure was tempered martensite comprising Cr-rich M23C6 on prior austenite grain and lath boundaries with intra-lath precipitation of V- and Ta-rich MC precipitates. The tool rotational speed exerted profound influence on evolving microstructure in SZ, TMAZ, and HAZ in the as-welded and post-weld heat-treated states. Very high proportion of prior austenitic grains and martensite lath boundaries in SZ and TMAZ in the as-welded state showed lack of strengthening precipitates, though very high hardness was recorded in SZ irrespective of the tool speed. Very fine-needle-like Fe3C precipitates were found at both the rotational speeds in SZ. The Fe3C was dissolved and fresh precipitation of strengthening precipitates occurred on both prior austenite grain and sub-grain boundaries in SZ during PWNT and PWDT. The post-weld direct tempering caused coarsening and coalescence of strengthening precipitates, in both matrix and grain boundary regions of TMAZ and HAZ, which led to inhomogeneous distribution of hardness across the weld joint. The PWNT heat treatment has shown fresh precipitation of M23C6 on lath and grain boundaries and very fine V-rich MC precipitates in the intragranular regions, which is very much similar to that prevailed in BM prior to FSW. Both the PWDT and PWNT treatments caused considerable reduction in the hardness of SZ. In the as-welded state, the 200 rpm joints have shown room temperature impact toughness close to that of BM, whereas 700 rpm joints exhibited very poor impact toughness. The best combination of microstructure and mechanical properties could be obtained by employing low rotational speed of 200 rpm followed by PWNT cycle. The type and size of various precipitates, grain size, and evolving dislocation substructure have been presented and comprehensively discussed.

Oxidative wear behaviour of laser clad High Speed Steel thick deposits: Influence of sliding speed, carbide type and morphology

The oxidative wear behaviour of four different High Speed Steel (HSS) thick coatings (one cast material and three laser clad deposits with varying Mo, V and W contents) was investigated using a pin-on-disc tribometer at two different sliding speeds of 10cm/s and 50cm/s. Microstructural characterization (before and after the wear tests) was carried out by SEM and wear debris was analysed by XRD. For all four materials, the oxide layer was formed of hard and brittle haematite-type α-Fe2O3, prone to break and release debris that acted as a third body, thus increasing sample wear. The laser clad HSS materials exhibited a higher wear resistance than their conventional cast counterpart, thanks to their finer microstructures. In particular, the coarser MC and M2C carbides present in the cast material were sensitive to cracking during the wear tests, releasing debris that contributed to increased third body abrasion together with oxide fragments. A detailed comparison of the wear behaviour of the three laser clad deposits, in correlation with their different microstructures, further demonstrated that harder V-rich MC carbides offered better wear resistance compared to the softer W-rich M2C carbides. The morphology of the carbides also played a role in determining the wear resistance at the higher sliding speed of 50cm/s. Clover-shaped primary MC carbides resisted wear better than angular ones due to their better geometric anchoring. Similarly, the geometric anchoring of eutectic M2C carbides, forming a quasi-continuous network at the grain boundaries of the matrix, proved beneficial at higher sliding speed.

Microstructural Evolution and Carbide Precipitation in a Heat-Treated H13 Hot Work Mold Steel

The microstructure and carbide precipitation for a H13 hot work mold steel after heat treatment and stabilization treatment at 620 °C for up to 20 h were investigated by optical microscopy, electron backscatter diffraction, and transmission electron microscopy. After solutionization at 1020 °C for 40 min followed by oiling cooling and double tempering at 610 °C for 2 h, lathlike martensitic structure was obtained with various alloyed carbides including Cr-rich M23C6 carbide located at prior austenite grain boundaries or lath boundaries and V-rich MC and Mo-rich M2C carbides distributed in grain interiors. The heat-treated sample had high strength surpassing 1000 MPa at room temperature and 500 MPa at 650 °C. After stabilization treatment at 620 °C for 20 h, the carbides underwent limited coarsening, particularly for the MC and M2C carbides, which resulted in excellent resistance to tempering softening for the microstructure as the hardness surpassed 300 HV.

Investigation of microstructural damage to eutectic carbides from scratch tests of a heat-treated Fe–Cr–W–Mo–V–C alloy

The characteristics of abrasive wear damage to the carbides and eutectic phases in a Fe–Cr–W–Mo–V–C alloy were systematically investigated using point scratch testing on heat-treated specimens with preserved eutectic structures from a characteristic eutectic carbide formation temperature of 1240°C. A deep-etching method is applied to study the detailed morphologies of the eutectic carbides by optical microscopy (OM) and field emission scanning electron microscopy (FESEM). The types of the eutectic carbides were identified by X-ray diffraction (XRD) as V-rich eutectic MC and Mo-rich eutectic M2C. Single-pass scratch tests were carried out on polished and deep-etched surfaces of the specimens to investigate the micro-scale wear behavior of the eutectic carbides with and without protection by the matrix phase. It was found that, when a surrounding matrix is present, the micro-cracks initiate from the eutectic region during the scratch tests and slip-lines appear in scratched edges of the matrix phase. Without matrix protection as revealed in the deep-etched specimens, the main failure form is carbide dendrites break-off forming wear debris. The detailed differences in response and deformation mode of carbide microstructures with/without matrix support to single-point abrasion behavior are schematically depicted for main eutectic MC and M2C carbides under different loading conditions. Potential improvement on wear performance through the composition design is also discussed based on the different fracture and wear behavior of V-rich eutectic MC and Mo-rich eutectic M2C identified.

Preparation and Characterization of Spray Formed 2060 High Speed Steel

Spray forming has attracted considerable attention for the production of high speed steels due to its potential and priority in the microstructure refining and cost saving. In this study, high-quality large billets of 2060 high speed steel were successfully produced by spray forming process using a twin-atomizer facility. As-deposited billet was subsequently processed by hot forging, quenching in oil at 1180 °C and a triple tempering in the temperature range of 500-580 °C. The microstructures and hardness of the deposit and their subsequent development resulting from hot forging and heat treatment were investigated. This paper was designed to provide insight and have a better understanding of such a system for the steel. The results showed that the as-deposited microstructure was composed of the fine equiaxed grains with V-rich MC and W-Mo-rich M2C carbides non-uniformly distributed along the grain boundaries and inside the grains. M2C presented rod-like or unconnected net-shaped morphologies in the as-deposited microstructure. Following hot forging, metastable M2C carbides were completely decomposed into refined MC and M6C nearly spherical carbides uniformly distributed throughout the microstructure. A hardness value of 31HRC was attained for the spray deposited and hot forged samples. With increasing the tempering temperature, hardness was increased firstly and then decreased. Secondary hardening peak appeared at 540 °C for spray formed 2060 steel austenitized at 1180 °C, and the corresponding peak hardness reached 71HRC.

Very high cycle fatigue properties of Cr–Mo low alloy steel containing V-rich MC type carbides

The present study was attempted to investigate the very high cycle fatigue (VHCF) properties of Cr–Mo low alloy steel containing V-rich MC carbides by using ultrasonic fatigue testing machine. All the specimens were quenched and tempered at 500°C and 600°C and part of them were electrochemical hydrogen charged. Mainly owing to the precipitation of fine V-rich MC particles at 600°C, the introduced hydrogen content of the 600°C tempered specimen is much higher than that of the 500°C tempered specimen. Both the hydrogen-charged and uncharged specimens revealed internal fractures in the long-life region and no notable fracture surface difference were observed. Both fatigue strength and fatigue strength ratio increase with increasing tempering temperature from 500°C to 600°C for both the hydrogen-charged and uncharged specimens, though the 600°C tempered specimen possessed higher hydrogen content. Increasing hydrogen content was definitely detrimental to the fatigue properties. However, compared to other high strength steels investigated previously, the detrimental effect of hydrogen on fatigue properties of the tested steel was comparatively small, which was ascribed mainly to the presence of fine V-rich MC carbides acting as hydrogen trapping sites. In is concluded that appropriate controlling of fine V-rich MC carbides is a promising way to alleviate the detrimental effect of hydrogen on VHCF properties of high strength steels.

Effects of cryogenic treatment on mechanical property and microstructure of JIS SKH51 high-speed steel

When SKH 51 high-speed steel was first cryogenically treated and then tempered at 540 °C, the surface hardness of the SKH 51 steel increased from HRC 64 to HRC 65. This result indicates the reduction of retained austenite as well as the secondary precipitation of tiny carbides. TEM analyses indicated that cryogenic treatment can induce high-density dislocations and stacking faults within the martensite matrix. In the subsequent tempering process, conducted at 540 °C, tiny secondary carbides precipitated nearby the existing dislocations and stacking faults, resulting in the hardening of SKH 51 high-speed steel. These secondary carbides mainly consisted of V-rich MC carbides and (Mo, W)-rich M2C carbides, which were identified by STEM/EDS techniques. After being cryogenically treated and then tempered at 590°C, both the surface hardness and wear resistance of SKH 51 high-speed steel were weakened below acceptable ranges. This result indicates the promotion of rapid decomposition and growth of tempered martensite. Therefore, to achieve the best surface hardness, SKH 51 high-speed steel should first be cryogenically treated, and then tempered at a temperature below 590°C.

Precipitate phases in normalized and tempered ferritic/martensitic steel P92

Ferritic/martensitic steel P92 is a promising candidate for cladding and duct applications in Sodium-Cooled Fast Reactor. The precipitate phases of the P92 steel normalized at 1323 K (1050 °C) for 30 min and tempered at 1038 K (765 °C) for 1 h have been investigated using transmission electron microscopes. Four types of phases consisting of M23C6, MX, M2X and sigma-FeCr were identified in the steel. MX phases consist of Nb-rich M(C,N) carbonitride, Nb-rich MC carbide, V-rich M(C,N) carbonitride, V-rich MC carbide, V-rich MN nitride, and complex MC carbides with Nb-rich MC core and V-rich MC wings. M2X phases consist of Cr-rich M2(C,N) carbonitride, Cr-rich M2C carbide and M2N nitride. Sigma-FeCr has a simple tetragonal lattice and a typical chemical formula of Fe0.45Cr0.45W0.1. M23C6 and MX are the dominant phases, while the sigma-FeCr has the lowest content. The formation of sigma-FeCr and M2X phases in the steel is also discussed.

The effects of annealing temperature and cooling rate on carbide precipitation behavior in H13 hot-work tool steel

The precipitation behavior of H13 hot-work tool steel was investigated as a function of both annealing temperature and cooling rate through thermodynamic calculations and microstructural analyses using transmission and scanning electron microscope and a dilatometer. The V-rich MC carbide and Cr-rich M7C3 and M23C6 carbides were observed in all annealed specimens regardless of annealing and cooling conditions, as expected from an equilibrium phase diagram of the steel used. However, Mo-rich M2C and M6C carbides were unexpectedly precipitated at a temperature between 675°C and 700°C during slow cooling at a rate of below 0.01°C/s from the annealing temperatures of 830°C and below. The solubility of Mo in both M7C3 and ferrite reduces with decreasing temperature during cooling. Mo atoms diffuse out of both M7C3 and ferrite, and accumulate locally at the interface between M7C3 and ferrite. Mo carbides were form at the interface of M7C3 carbides during the transition of Cr-rich M7C3 to stable M23C6.

M5C2 Carbides in a High-Chromium Ferritic/Martensitic Steel

The precipitate phases in an 11 pct Cr ferritic/martensitic steel normalized at 1323 K (1050 °C) for 0.5 hour and tempered at 1053 K (780 °C) for 1.5 hours have been investigated. Except for dominant phases, Cr-rich M23C6 carbide and Nb-rich, Ta-Nb-rich, and V-rich MC carbides, needle-like precipitates with a typical size of 70 to 310 and 10 to 30 nm for the length of the long and short axis of the needles, respectively, were also observed on the extraction carbon replica of the steel. The typical metallic element composition of the needle-like precipitates is about 53-82Fe, 14-26Cr, 0.5-18Ta, 1-6W, and 2-5Co in atomic pct. Through energy dispersive X-ray analysis and electron diffraction along with calculations regarding lattice parameter and interplanar spacing, the needle-like precipitates were identified as a Fe-rich M5C2 carbide, which is not known to have been reported previously in high-chromium steels. The M5C2 carbide has a base-centered monoclinic crystal structure with the approximate lattice parameters a/b/c = 1.142/0.5186/0.5383 nm and β = 104.68 deg. The formation of the Fe-rich M5C2 carbides in the steel has been discussed. The effect of chromium content in matrix and boron addition on the precipitate phases in ferritic/martensitic steels has also been discussed.