Investigation of the Microstructural Changes and Hardness Variations of Sub-Zero Treated Cr-V Ledeburitic Tool Steel Due to the Tempering Treatment

The microstructure and tempering response of Cr-V ledeburitic steel Vanadis 6 subjected to sub-zero treatment at ??196 °C for 4 h have been examined with reference to the same steel after conventional heat treatment. The obtained experimental results infer that sub-zero treatment significantly reduces the retained austenite amount, makes an overall refinement of microstructure, and induces a significant increase in the number and population density of small globular carbides with a size 100-500 nm. At low tempering temperatures, the transient M₃C-carbides precipitated, whereas their number was enhanced by sub-zero treatment. The presence of chromium-based M₇C₃ precipitates was evidenced after tempering at the temperature of normal secondary hardening; this phase was detected along with the M₃C. Tempering above 470 °C converts almost all the retained austenite in conventionally quenched specimens while the transformation of retained austenite is rather accelerated in sub-zero treated material. As a result of tempering, a decrease in the population density of small globular carbides was recorded; however, the number of these particles retained much higher in sub-zero treated steel. Elevated hardness of sub-zero treated steel can be referred to more completed martensitic transformation and enhanced number of small globular carbides; this state is retained up to a tempering temperature of around 500 °C in certain extent. Correspondingly, lower as-tempered hardness of sub-zero treated steel tempered above 500 °C is referred to much lower contribution of the transformation of retained austenite, and to an expectedly lower amount of precipitated alloy carbides.     

The Kinetics of Phase Transformations During Tempering in Laser Melted High Chromium Cast Steel

The precipitation of secondary carbides in the laser melted high chromium cast steels during tempering at 300-650?°C for 2?h in air furnace was characterized and the present phases was identified, by using transmission electron microscopy. Laser melted high chromium cast steel consists of austenitic dendrites and interdendritic M₂₃C₆ carbides. The austenite has such a strong tempering stability that it remains unchanged at temperature below 400?°C and the secondary hardening phenomenon starts from 450?°C to the maximum value of 672 HV at 560?°C. After tempering at 450?°C fine M₂₃C₆ carbides precipitate from the supersaturated austenite preferentially. In addition, the dislocation lines and slip bands still exist inside the austenite. While tempering at temperature below 560?°C, the secondary hardening simultaneously results from the martensite phase transformation and the precipitation of carbides as well as dislocation strengthening within a refined microstructure. Moreover, the formation of the ferrite matrix and large quality of coarse lamellar M₃C carbides when the samples were tempered at 650?°C contributes to the decrease of hardness.     

Alloy carbide precipitation in tempered 2.25 Cr–Mo steel under high magnetic field

The influence of a high magnetic field on carbide precipitation during the tempering of a 2.25 Cr–Mo steel was investigated by means of transmission electron microscopy. As-quenched specimens were tempered at 200, 550 and 700 °C for various times in the absence and presence of a 12 T magnetic field. Experimental results indicate that the applied high magnetic field effectively promotes the precipitation of M₂₃C₆ carbides at low temperature (200 °C) and M₇C₃ and M₂₃C₆ carbides at intermediate temperature (550 °C). The increased Fe content in the M₂₃C₆ and M₇C₃ carbide significantly increases the magnetization. The magnetic Gibbs free energy, which influenced the alloy carbide precipitation behavior, was considered to be mainly determined by the intrinsic magnetization energy for M₂₃C₆ and M₇C₃ carbides. With the increase of the tempering temperature (700 °C), there was no pronounced effect of the high magnetic field on the precipitation sequence and the concentration of substitutional solute atoms in paramagnetic carbides. The investigation of alloy carbide precipitation under high magnetic fields could contribute to a better understanding of phase transformation of alloy carbides and to the heat treatment and fabrication of heat-resistant steels.     

Effect of single and double austenitization treatments on the microstructure and mechanical properties of 16Cr-2Ni steel

Double austenitization (DA) treatment is found to yield the best combination of strength and toughness in both low-temperature as well as high-temperature tempered conditions as compared to single austenitization (SA) treatments. Obtaining the advantages of double austenitization (DA) to permit dissolution of alloy carbides without significant grain coarsening was attempted in AISI 431 type martensitic stainless steel. Structure-property correlation after low-temperature tempering (200 °C) as well as high-temperature double tempering (650+600 °C) was carried out for three austenitization treatments through SA at 1000 °C, SA at 1070 °C, and DA at 1070+1000 °C. While the increase in strength after DA treatment and low-temperature tempering at 200 °C is due to the increased amount of carbon in solution as a result of dissolution of alloy carbides during first austenitization, the increased toughness is attributable to the increased quantity of retained austenite. After double tempering (650+600 °C), strength and toughness are mainly found to depend on the precipitation and distribution of carbides in the microstructure and the grain size effect.     

Effect of Austenitizing Temperature on Microstructure and Mechanical Properties of Semi-High-Speed Steel Cold-Forged Rolls

The effect of austenitizing temperature on the microstructure and mechanical properties of semi-high-speed steel (S-HSS) cold-forged rolls was investigated. Low-temperature austenitizing below 1313 K induced carbide coarsening during subsequent tempering at 973 K due to the nucleation effect of undissolved M₇C₃. On the other hand, the heavy dissolution of M₇C₃ above 1353 K caused the fine carbide formation on lath and plate boundaries, which retarded the subgrain growth during tempering. The increase in strength with increasing austenitizing temperature was attributed to the fine carbide distribution and the high dislocation density. Furthermore, as the austenitizing temperature increased, the impact energy markedly reduced, due to the large prior austenite grain size and the high strength. Finally, based on the microstructure and mechanical properties, an optimal austenitizing temperature range between 1313 and 1333 K was determined.     

Microstructure evolution,fracture and hardening mechanisms of quenched and tempered steel for large sized bearing rings at elevated quenching temperatures

Based on 42CrMo steel, a steel with a higher C and Ni content is developed for use in large sized bearing rings. The impact energy and hardness of the quenched and tempered steel increase with the quenching temperature, but then decrease when the temperature is above 925 °C. When the temperature is below 925 °C, some larger M₂₃C₆-type carbides (with average diameters of 255.6 μm) exist in the quenched and tempered microstructure. The number of carbides is reduced as the quenching temperature increases. At the same time, the fracture modes change from microvoid coalescence and quasi-cleavage to microvoid coalescence. The number of round quasi-cleavage fractures, which are formed around the carbides, decrease as the number of carbides decrease. The existence of larger M₂₃C₆-type carbides leads to round quasi-cleavage fractures and decrease the impact energy. The precipitation strengthening of M₂₃C₆-type carbides increases the hardness at a quenching temperature of 925 °C.     

Relative Dimensional Change Evaluation of Vacuum Heat-Treated JIS SKD61 Hot-Work Tool Steels

JIS SKD61 hot-work steel is usually used as precision mold material for die casting; hence, it demands a higher level of dimensional stability during the hardening process, especially for fairly large sections. This study investigates the microstructural evolution and measures the relative dimensional changes in various tempering states. The results show that the retained austenitic contents of all quenched and tempered SKD61 steel specimens were less than 2%. When the tempering temperature reached 500 °C, the retained austenitic content decreased from 1.35 to 0.45%. TEM investigations revealed that a large number of secondary carbides, molybdenum-rich M₂C and vanadium-rich MC carbides, precipitated near the dislocations when the tempering temperature reached 525 °C. A secondary hardening phenomenon and evident expansion phenomenon occurred as the tempering temperature exceeded 500 °C. These phenomena were mainly contributed by the precipitation of secondary carbides in hot-work steels. The reason is that only 0.9% of the retained austenite transformed into martensite as the tempering temperature reached 500 °C, allowing the hardness and dimensional change to be neglected.     

Effect of Tempering and Strain on Decomposition of Metastable Austenite in X210CrW12 Thixo-Cast Steel

Thixoforming of hot rolled X210CrW12tool steel led to the formation of globular austenitic grains (82.4 vol.%) surrounded by eutectic mixture (α-Fe and M₇C₃ carbides). The thixo-cast steel reached compression strength 4.8 GPa at plastic strain 34%. The analysis of pole figures after deformation indicated distinct texturization of microstructure in comparison with undeformed steel. Main texture components for austenite were {101}, 〈010〉, while ferrite did not reveal clearly formed orientation. DSC analysis confirmed that austenitic structure in the X210CrW12 steel was metastable and temperature of decomposition depended on the strain applied at 634 °C for the un-deformed sample and at 599 °C for sample compressed up to 4.8 GPa. Discontinuous transformation of austenite into perlite, that started mainly at grain boundaries and proceeded to the center, was the predominant mechanism responsible for the decomposition of globular grains in thixoformed X210CrW12 steel. The decomposition caused by tempering of supersaturated and severely strained steel led to obtaining characteristic product of transformation of higher hardness in comparison with only tempered sample. In the deformed sample the reaction started on slip bands and twins which revealed high density of defects, promoting precipitation of carbides, followed by local depletion in carbon as a result of α′- Fe formation. In contrast to non-deformed state they covered the area of grains. Two fronts of reaction α-Fe plate +M₃C → mixture of α-Fe and M₇C₃ carbides were also observed.     

Kinetic Parameters of Secondary Carbide Precipitation in High-Cr White Iron Alloyed by Mn-Ni-Mo-V Complex

This study presents kinetics of precipitation of secondary carbides in 14.55%Cr-Mn-Ni-Mo-V white cast iron during the destabilization heat treatment. The as-cast iron was heat treated at temperatures in the range of 800-1100 °C with soaking up to 6 h. Investigation was carried out by optical and electron microscopy, dilatometric analysis, Ms temperature measurement, and bulk hardness evaluation. TTT-curve of precipitation process of secondary carbides (M₇C₃, M₂₃C₆, M₃C₂) has been constructed in this study. It was determined that the precipitation occurs at the maximum rate at 950 °C where the process is started after 10 s and completed within 160 min further. The precipitation leads to significant increase of Ms temperature and bulk hardness; large soaking times at destabilization temperatures cause coarsening of secondary carbides and decrease in particles number, followed by decrease in hardness. The results obtained are discussed in terms of solubility of carbon in the austenite and diffusion activation of Cr atoms. The precipitation was found to consist of two stages with activation energies of 196.5 kJ/g-mole at the first stage and 47.1 kJ/g-mole at the second stage.     

Microstructural Modifications of As-Cast High-Chromium White Iron by Heat Treatment

The initial as-cast microstructure of a high-chromium (2.35% C, 18.23% Cr) white cast iron consisting of primary austenitic dendrites and a eutectic mixture of M₇C₃ carbides/austenite was extensively modified by four different heat treatments: H.T.A: destabilization (970 °C-2.5 h), H.T.B: destabilization/subcritical treatments (970 °C-2.5 h + 600 °C-13 h), H.T.C: subcritical treatment (600 °C-13 h) and H.T.D: subcritical/destabilization treatments (600 °C-13 h + 970 °C-2.5 h). H.T.A leads to martensitic structures that present considerable precipitation of cubic secondary carbide particles of M₂₃C₆ type. H.T.B produces pearlitic structures and causes further carbide precipitation and pre-existent carbide particle shape modifications. H.T.C extensively modifies the initial as-cast structure to more pearlitic morphologies accompanied with spheroidization/degradation of the M₇C₃ primary carbide structure. H.T.D causes extensive formation of secondary carbide particles within the primary austenitic matrix; the latter has been mainly transformed to martensite. The effect of each heat treatment on the hardness of the alloy was correlated with the attained microstructure.     

Optimal heat treatment conditions and properties of bimetal (high Cr cast iron/alloyed steel) hammers

Abstract

This study intended to establish the optimal heat treatment conditions for the desired hardness and wear resistance property for the bimetal hammers developed by the authors. The objective of this study is to attain bimetal hammers that have a tough Cr–Ni alloyed steel shank and a high wear resistant high Cr cast iron head to replace conventional single alloy (high Mn steel) hammers. The results show that the optimal heat treatment condition obtained for the bimetal hammers is: destabilisation: 1000–1050°C for 2 h, quench: FAC and tempering: 480–500°C for 6 h. By employing this optimal heat treatment condition, the highest hardness value can be attained along with the best wear resistance property for the head portion and acceptable toughness for the shank portion. The microstructure of the head portion that corresponds to the optimal properties consists of eutectic M₇C₃ carbides, secondary M₇C₃ carbides, tempered martensite and almost nil retained austenite.     

Influence of various heat treatment stages on evolution of microstructure and grain in H407 steel

Regarding heat treatment as one of the main methods for improving property of die steel, dead annealing, quenching, once tempering, twice tempering, and thrice tempering treatment of H407 die steel were conducted in this thesis. Microstructure conversion and grain size development in various stages of heat treatment were analyzed, and then magnitude, shape, and distribution of secondary phase during heat treatment were investigated to explore the function mechanism of microalloyed elements on evolution of microstructure and grain during heat treatment. The steel achieves homogeneous microstructure and composition after this heat treatment. The final phase constituent is α and γ phase as well as the final microstructure consists of tempered martensite, trace retained austenite and granular carbides. A large number of fine and dispersive MC as well as M₇C₃ type granular carbides containing V, Mo and Cr precipitate in trice tempered microstructure. After this heat treatment grain is finer with grain size of 5.96 μm.     

Effects of hot compression on carbide precipitation behavior of Ni—20Cr—18W—1Mo superalloy

The effect of hot compression on the grain boundary segregation and precipitation behavior of M₆C carbide in the Ni–20Cr–18W–1Mo superalloy was investigated by thermomechanical simulator, scanning electronic microscope (SEM) and X-ray diffraction (XRD). Results indicate that the amount of M₆C carbides obviously increases in the experimental alloy after hot compression. Composition analyses reveal that secondary M₆C carbides at grain boundaries are highly enriched in tungsten. Meanwhile, the secondary carbide size of compressive samples is 3–5 μm in 10% deformation degree, while the carbide size of undeformed specimens is less than 1 μm under aging treatment at 900 and 1000 °C. According to the thermodynamic calculation results, the Gibbs free energy of γ-matrix and carbides decreases with increase of the compression temperature, and the W-rich M₆C carbide is more stable than Cr-rich M₂₃C₆. Compared with the experimental results, it is found that compressive stress accelerates the W segregation rate in grain boundary region, and further rises the rapid growth of W-rich M₆C as compared with the undeformed one.     

Effect of Destabilization Heat Treatments on the Microstructure of High-Chromium Cast Iron: A Microscopy Examination Approach

A 18.22 wt.% Cr white iron has been subjected to various destabilization heat treatments. Destabilization at 800 °C caused gradual precipitation of M₂₃C₆ secondary carbide particles with time leading to a gradual increase in the bulk hardness. At 900, 1000, and 1100 °C, an initial sharp increase in bulk hardness with time occurred, reaching a plateau that was followed by a slightly decreasing trend. The combination of martensite formed, stoichiometry, and morphology of the secondary carbides present (mostly M₇C₃) are responsible for the obtained values of hardness. At 1100 °C, severe dissolution of the secondary carbides and consequent stabilization of the austenitic phase took place. Maximum hardness values were obtained for destabilization at 1000 °C. The correlation between bulk hardness and microstructural features was elaborated.     

The Microstructure Degradation of the IN 713C Nickel-Based Superalloy After the Stress Rupture Tests

The aim of the work was to examine the degradation phenomena taking place in the microstructure of the as-cast IN 713C superalloy after stress rupture tests, performed at T = 980 °C under a tensile stress of 150 MPa. A directional growth of γ′ phase (rafting) and decomposition of the NbC primary carbides accompanied by the precipitation of M₂₃C₆ secondary carbides rich in chromium and of γ′ phase were observed. It was also indicated that the decomposition of the NbC primary carbides may be accompanied by the precipitation of M₃B₂ borides rich in Mo.     

Microstructure and properties of high chromium cast irons: effect of heat treatments and alloying additions

Abstract

Different combinations of critical and subcritical heat treatments variously modify the initial as cast microstructure of high chromium white cast irons leading to secondary carbide precipitation of different extent and nature. Destabilisation (critical heat treatment) of austenite at 970°C for 2·5 h followed by annealing (subcritical heat treatment) at 600°C for 13 h results in massive precipitation of M₂₃C₆ carbide particles along with spheroidised M₇C₃. The reversed order of heat treatments leads to extensive precipitation of M₇C₃ secondary carbide particles. Mo has a favouring effect on the hardness of the microstructures containing pearlite by limiting pearlite formation. The gradual increase in the alloying additions, C and Cr, increases the hardness of the materials at the different treatment states by inducing carbide precipitation. The increase in the Si content leads to the opposite effect by favouring pearlite formation.     

Complete model for effects of silicon in 5%Cr hot work tool steels

Abstract

Recent modifications in chemical composition have been applied commercially to high alloy tool steels to improve toughness and tempering resistance. A common point in all compositions is the reduction of silicon content from the 1·0% used in AISI H11 and H13 down to 0·3% or lower levels. The present work investigates in detail the effect of silicon on tempering sequence and alloy carbide formation, proposing an explanation for the mechanical properties. Laboratory heats with silicon contents between 0·05 and 2·0% were cast and forged under industrial conditions. Mechanical tests were based on impact toughness and hardness measurements, after hardening from 1020°C and tempering at temperatures between 400 and 650°C. Secondary carbides were evaluated through transmission electron microscopy, mainly on extraction replicas, and matrix features were observed in thin foils. High resolution scanning electron microscopy was also applied, especially on fracture surface samples, to correlate toughness results with secondary carbide distributions. The effect of Si on cementite formation was found to be the major factor for the differences observed for the mechanical properties. During the initial tempering stages, cementite formation is delayed or inhibited in high Si steels, anticipating alloy carbide formation with preferred M₇C₃ precipitation on high energy interfaces. After longer tempering, M₇C₃ particles coarsen and may act as preferential cracking routes, explaining the lower toughness of high Si steels. In low Si steels, cementite is stabilised by Cr, Mo and V in solid solution, delaying alloy carbide precipitation and thus increasing tempering resistance.     

Microstructural changes in cast martensitic steel after creep at 620°C

Microstructural changes in the cast steel GX12CrMoWVNbN10-1-1 (Fe–0.11 C–0.31 Si–0.89 Mn–9.57 Cr–0.66 Ni–1.01 Mo–1.00 W–0.21 V–0.06 Nb–0.05 Cu–0.05 N in wt %) have been investigated after tests for long-term strength at a temperature of 620°C in the range of stresses of 120–160 MPa. Upon short-term creep (up to 5000 h), the tempered troostite structure and distribution of particles of proeutectoid constituents change insignificantly, except for the precipitation of particles of the Laves phase ～100 nm in size along boundaries of laths, blocks, packets, and initial austenite grains. Upon long-term creep (to 10000 h), the tempered troostite partially transforms into the subgrain structure, which is accompanied by a decrease in the dislocation density from 6.4 × 10¹⁴ to 3.1 × 10¹³ m^–2 and connected with growth of sizes of M₂₃C₆ carbides of 105–150 nm and particles of the Laves phase to 380 nm, due to the dissolution of these particles located along path boundaries. Upon long-term creep, the average size of V(C,N) particles increases from 45 to 64 nm (while Nb(C,N) particles increase from 48 to 87 nm), and the Nb content in V-enriched carbonitrides and the V content in Nb-enriched M(C,N) particles substantially decrease. No formation of the Z phase has been revealed. The combination of M(C,N) nanoparticles with the presence of W in the solid solution has been found to be responsible for the enhanced high-temperature strength of the steel.     

Experimental investigation of in-situ transformations of the <Emphasis Type="Italic">M</Emphasis><Subscript>7</Subscript>C<Subscript>3</Subscript> carbide in the cast Fe–Cr–Ni alloy

The microstructure and the phase composition of a heat-resistant Fe–Cr–Ni alloy (0. 45C–25Cr–35Ni) has been investigated in the cast state and after annealing at 1150°C for 2–100 h. After a 2-h high-temperature annealing, the fragmentation of the crystal structure of the eutectic M ₇C₃ carbides into domains of ~500 nm in size with a partial transition into M ₂₃C₆ carbides is observed. After a 100-h holding, the complete transition of the hexagonal M ₇C₃ carbides into M ₂₃C₆ with a face-centered cubic structure occurs. The carbide transition M ₇C₃ →M ₂₃ can be considered to be an in situ transformation.     

Transformations of Carbides During Tempering of D3 Tool Steel

The studies were performed on D3 tool steel hardened after austenitizing at 1050 °C during 30 min and tempering at 200-700 °C. Based on the diffraction studies performed from the extraction replicas, using electron microscopy, it was found that after 120-min tempering in the consecutive temperatures, the following types of carbides occur: $$ 200\;^\circ {\text{C}} \to \upvarepsilon + \upchi + {\text{ Fe}}_{ 3} {\text{C}},\quad 3 50\;^\circ {\text{C}} \to \upvarepsilon + \upchi + {\text{ Fe}}_{ 3} {\text{C,}} $$ $$ 500\;^\circ {\text{C}} \to \upchi + {\text{ M}}_{ 3} {\text{C }} + {\text{ M}}_{ 7} {\text{C}}_{ 3} ,\quad 600\;^\circ {\text{C}} \to \upchi + {\text{ M}}_{ 3} {\text{C }} + {\text{ M}}_{ 7} {\text{C}}_{ 3} , $$ $$ 700\;^\circ {\text{C}} \to {\text{M}}_{ 3} {\text{C }} + {\text{ M}}_{ 7} {\text{C}}_{ 3} . $$ Apart from higher mentioned carbides, there are also big primary carbides and fine secondary M₇C₃ carbides occurring, which did not dissolve during austenitizing.