Carbide Precipitation Research Articles

Impact of carbon segregation on transition carbides and cementite precipitation during tempering of low carbon steels: Experiments and modeling

Tempering of low-carbon steels (less than about 0.3 wt.% C) may differ from the traditional precipitation sequence, which includes transition carbide precipitation, retained austenite decomposition and cementite precipitation. The main difference is the partial or total absence of transition carbides. In the present work, the effect of the carbon segregation on the mitigation of the precipitation of the latter is analyzed by combining multi-scale experimental investigations with a new precipitation model accounting for the carbon heterogeneities induced by the segregation. The full precipitation sequence is considered in order to study the impact of the segregation not only on the transition carbides, but also on the cementite which forms afterwards. The experimental work includes APT characterization of carbon segregation, in-situ HEXRD experiments to reveal precipitation kinetics, and TEM observations for carbide size measurements.The here-in developed precipitation mean-field and physics-based model combines two previous ones dedicated to the nucleation and growth of transition carbides and cementite and to the segregation of carbon at dislocations. It is shown that, even after water-quench, carbon atoms are already segregated on dislocations. The mitigation of transition carbide precipitation is caused by the presence of such segregations, which decrease the driving force for the transition carbide nucleation and enhance cementite precipitation. In agreement with previous experiments, the model also demonstrates that inside a martensitic microstructure, the precipitation sequence is different between the first (formed close to Ms temperature) and the last martensite (formed at room temperature) formed upon cooling, because of the difference in dislocation density, which influences the intensity of the segregation phenomenon.

High-throughput first-principles study on the effect of Ni segregation on the formation of Cr-depleted zone and corrosion resistance of S32205 duplex stainless steels

Recent studies have found that in the absence of Cr carbide precipitation, co-segregation of Cr and other elements at the interface between α-Fe and γ-Fe can also lead to the formation of Cr-depleted zones, leading to intergranular corrosion. The study employed three-dimensional atomic probes, focused ion beams, and high-throughput first-principles calculation techniques to investigate the impact of Ni atoms on the α-Fe/γ-Fe interface of UNS S32205 duplex stainless steels. The goal was to analyze the formation of Cr-depleted zones and interface corrosion resistance. The results indicate that both Ni and Cr atoms are segregated at the interface between α-Fe and γ-Fe phases; The strong interaction between Ni and Cr atoms hinders Cr segregation at the interface and slows down the formation of Cr-depleted zones; co-segregation of Ni and Cr can improve the stability of the α-Fe/γ-Fe phase interface, reduce the generation of microcracks, and reduce the stability of Cl atom adsorption in the cracks. This establishes the theoretical basis for further suppressing the formation of Cr-depleted zones and preventing intergranular corrosion.

The effect of laser shock peening with different power density on the microstructure evolution and mechanical properties of MAR-M247 nickel-base alloy

This study investigates the effects of laser shock peening (LSP) on the microstructural evolution and surface hardening of MAR-M247 nickel-based alloy. Experimental and numerical simulation methods are employed to analyze the influence of pulse power density and spot size. The results demonstrate that LSP significantly enhances surface hardness and yield strength. As the pulse power density increases, the magnitude and depth of plastic deformation increase. Higher power densities can implant deeper and greater compressive residual stress (CRS), but the corresponding tensile residual stress (TRS) also increases. Reducing both power density and spot size decreases the amount of plastic deformation of the surface. However, decreasing the spot size results in a decrease in the depth of the CRS. On the other side, based on the analysis of the microstructure, during the process of LSP, many dislocations accumulate in the γ matrix and evolve into subgrain boundaries, resulting in grain refinement. Additionally, LSP induces the precipitation of nanoscale carbides. This study tries to establish the influence law and action mechanism between LSP process-material property-microscopic evolution, which provides theoretical and applied data support for engineering applications.

Effect of annealing temperature on carbide precipitation, microstructure and mechanical properties of Fe-Mn-C- (Cr, Mo, W, Ni, Nb, Ti, V) multi-alloying TWIP steel

The effect of annealing temperature on the microstructure, carbide precipitation and mechanical properties of a cold-rolled multi-alloying TWIP steel were investigated by means of Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) analysis, tensile and impact tests. As the temperature increases from 400 °C to 800 °C, the amount of carbides climbed to the peak at 700 °C and then decreased, having a strong opposite relationship with the impact energy, which was minimum 26 J at 700 °C. The main distribution of carbides underwent a transition from intragranular, intragranular + intergranular to intergranular, and the composition of carbides changed from (Nb, Ti and W) to (Cr, Mo and W). Compared with the cold-rolled sample, Fe21W2C6 precipitated out at low annealing temperature of 400 °C, when the tensile and yield strengths and elongation were all simultaneously improved due to the precipitation recovery annealing. And the 600 °C annealing sample exhibited identical yield and tensile strengths. Subsequently, the yield strengths decreased but the tensile strengths and elongation increased at 650 °C and 700 °C attributed to the interaction of carbide precipitation and recrystallization. The recrystallization started at 650 °C and finished at 750 °C. And Fe3Mo3C was identified at 750 °C.

Influence of vanadium-carbide precipitation on the deformation behavior of Fe-30Mn-10Co-10Cr-2.5 V-1.5C multi-principal element alloy

Multi-principal element alloys represent a compelling avenue for the development of innovative alloys that have the potential to surpass conventional alloys in various applications, owing to the extensive compositional space they offer. For structural application though, this potential has not yet been realized, mostly because of hardly superior mechanical properties and elevated alloying costs. To overcome this hurdle, efforts have been made to increase mechanical properties using individual strengthening mechanisms, such as grain refinement, prior plastic deformation, precipitation hardening, and solid solution strengthening. In this study, we aim to combine multiple of these mechanisms into a singular alloy by adding V and C to the Fe30Mn10Co10Cr (at.-%) alloy. Using a custom calphad database, thermodynamic calculations were carried out to determine a suitable chemical composition which was subsequently cast and thermo-mechanically processed. Aging treatments above 700 °C for 4–24 h led to vanadium carbide precipitation, increasing the yield strength by up to 450 MPa to nearly 1GPa. Extensive aging times were necessary to facilitate precipitation, thereby indicating slow diffusion processes. A transition from twinning-induced to transformation-induced plasticity by vanadium carbide precipitation was observed and attributed to the reduction of C-content and stacking fault energy in the matrix as well as twin-precipitate interactions.

The effect of Hafnium on the microstructure and tensile property in a Ni-based superalloy

The effects of Hafnium on the microstructure and tensile property of a Ni-based superalloy after solution heat treatment and long-term thermal exposure were investigated with field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), X-ray diffractometer (XRD). The results revealed reveal that Hf intensified intensifies the element segregation and promoted promotes the precipitation of MC carbide and honeycomb-like Ni5Hf phase in the as-cast alloy. The solid solution strengthening and second phase (MC) strengthening effects are both enhanced in the alloy doping 0.5 wt% Hf after solution heat treatment. Therefore, the tensile yield strength at 700 °C of the alloy with 0.5 wt% Hf is higher than that in the Hf-free alloy. Otherwise, a large amount of the low melting point Ni5Hf phases appear in the alloy with 0.8 wt% Hf. Ni5Hf phase is prone to become the crack initiation, which decreases diminishes the yield strength in the alloy with 0.8 wt% Hf. The addition of 0.5 wt% Hf can inhibit the formation of needle-like δ phase at grain boundary due to the decrease of grain boundary energy and diffusion rate at grain boundary. Therefore, the 700 °C tensile strength and ductility increase simultaneously in the alloy with 0.5 wt% Hf after thermal exposure at 700 °C.

Short-term creep approach to redefining the role of 17-4PH stainless steel for high-temperature applications

The creep response of the 17-4PH martensitic age-hardening steel in H1150 state was investigated at 427 and 482 °C. Hardness measurements of the heads of the creep samples demonstrated that the material underwent additional age hardening during the high-temperature exposure. Microstructural investigations confirmed that the additional precipitation of carbides and the G-phase occurred at the lowest temperature. A set of constitutive equations previously developed to describe the creep response of particle-strengthened alloys was successfully used to obtain a comprehensive description of the experimental data. The value of the particle strengthening term was obtained from the hardness measurements and corresponded to the Orowan stress. The model accurately described the observed minimum creep rate dependence on the applied stress and explained the occurrence of lower values of the minimum strain rate observed during variable-load experiments.

On the role of chemical heterogeneity in carbon diffusion during quenching and partitioning

Carbon partitioning is the core design of quenching and partitioning (Q&P) process, which significantly improves the stability of retained austenite (RA). However, the inevitable precipitation of transition carbides leads to a substantial consumption of carbon atoms. Conversely, in the present study, we have realized the full inhibition of transition carbides in martensitic lath and, in turn, improved the carbon utilization efficiency to stabilize austenite. During fast austenitization from Mn-partitioned pearlite, Mn-heterogeneous high-temperature austenite is produced and more carbon atoms are trapped in the Mn-enriched region. Following Q&P, the alternative film RA and lath martensite in nanoscale is obtained, in which RA is enriched with Mn and C while the lath martensite is depleted with Mn and C. On one hand, the depletion of Mn and C in lath martensite strongly reduces the driving force and nucleation rate for carbide precipitation. On the other hand, the nanoscale microstructure and heterogeneous Mn distribution effectively accelerate carbon diffusion from lath martensite into austenite. Therefore, carbide precipitation is substantially inhibited in lath martensite due to the kinetic mismatch between fast carbon diffusion and sluggish carbide precipitation. This study demonstrates that chemical heterogeneity provides a novel pathway to enhance carbon partitioning efficiency and tensile properties in Q&P steels.

Tailoring the Microstructure using Quenching and Partitioning Processing in a Commercial Mn-Si-Cr Spring Steel to Improve Tensile Properties

This study explores a novel approach to quenching and partitioning (Q&P) heat treatment applied to AISI 9260 spring steel, comprising Fe-0.65C-1.58Mn-1.05Si-0.41Cr (wt.%). Our research focuses on balancing strength and ductility through optimized Q&P pathways, leading to a diverse microstructure that includes martensite, bainite, carbide, and retained austenite. Advanced X-ray diffraction and scanning electron microscopy techniques were employed to analyze the complexities of this microstructure. A key aspect of this study is the precise control of partitioning temperature and time, crucial for modulating lattice distortion and dislocation density within martensitic and bainitic structures. Optimal partitioning temperature promotes carbon distribution into austenite, tempering lattice distortions, and dislocation densities. Concurrently, carbide precipitation and segregation contribute to the refinement of the bainite phase. The sample quenched at 125 °C and partitioned at 350 °C (Q&P-125/350) demonstrates notable mechanical properties: a yield strength of 950 ± 15 MPa, an ultimate tensile strength of 1710 ± 15 MPa, and an elongation of approximately 9.7%. These results are partly attributed to the effect of silicon in preventing cementite coarsening and the effective distribution of carbide. Our findings highlight the potential of Q&P heat treatment in developing tailored microstructures with enhanced mechanical properties in steel, without relying on costly alloying elements. This approach presents new avenues for the design and application of high-performance materials.

Influence of prior austenite grain size on martensite-austenite islands morphology and decomposition characteristics during two-step tempering in Mn-Ni-Mo steel

Martensite-austenite (M/A) islands are typical in the quenched microstructure of Mn-Ni-Mo steel, frequently utilized for fabricating reactor pressure vessels (RPV). These islands can decompose into harmful long rod-shaped Fe3C carbides during tempering above 600°C, quantifying the negative effect of M/A island decomposition on mechanical properties and indicating a potential issue in RPV fabrication. To address this issue, the present study investigates a two-step tempering process on three different prior austenite grain size (PAGS) specimens (5 µm, 47 µm, and 119 µm). The effect of PAGS on M/A island decomposition characteristics during sub-600°C tempering and their impact on precipitated carbide morphology is investigated. A preliminary pre-tempering treatment at 200°C, 350°C, and 500°C for 2 hours was followed by a 3-hour final tempering step at 650°C. The findings revealed unique temperature-dependent decomposition characteristics of M/A islands. For example, at 200°C, partial M/A island decomposition took place, resulting in long rod-shaped carbides during final tempering. Meanwhile, the 350°C pre-tempering specimen revealed aggregated carbide precipitates of varying sizes in former M/A island locations. In contrast, the 500°C pre-tempering specimen showed coarse globular carbides and the absence of carbide precipitate clustering. This result can be attributed to the decrease in surface energy associated with the carbide-matrix interface as size increases. After tempering, all specimens produced three types of precipitates: rod-shaped Fe3C carbides, rhombus-shaped Mn7C3, and globular-shaped Mn23C6 carbides. The formation of long rod-shaped carbides during M/A island decomposition reduces the critical cleavage stress for micro-crack initiation, jeopardizing steel mechanical properties. Aggregated carbides cause the simultaneous formation of numerous micro-voids in close proximity, which eventually merge to form a large crack, resulting in material failure. When compared to other pre-tempered and directly tempered specimens, pre-tempering at 500°C produced the most favorable microstructure in terms of impact properties.

Improved Thermodynamic Descriptions of Carbides in Ni-Based Superalloys

The Ni-based superalloy René 41 has sparked recent interest for applications in next-generation aircraft engines due to its high-temperature strength that is superior to all similar grades. These desirable properties are achieved by careful control of the microstructure evolution during thermomechanical processing, and this is commonly informed by simulations. In particular, the grain boundary carbides M6C and M23C6 play an essential role in controlling the grain size and strength of the final product. Therefore, a solid understanding of the thermodynamic stability and thermokinetic evolution of these carbides is essential. However, thermokinetic simulations using existing thermodynamic databases have been demonstrated to have discrepancies between thermodynamic stabilities and experimental observations. Here, we collected a new experimental time–temperature–precipitation diagram. In conjunction with improved crystallographic descriptions, these experimental results are used to modify a CALPHAD database for M6C and M23C6. The modified database correctly identifies temperature regions with rapid carbide precipitation kinetics. Further, kinetic simulations and strengthening models successfully predict the hardness increase due to γ′ precipitation. The modified database has been applied to Udimet 700, Waspaloy, and Haynes 282, demonstrating improved results. These updates will facilitate more accurate simulations of the microstructure evolution during thermomechanical processing of advanced Ni-based superalloys for aerospace and other applications.

Effect of C content and annealing on dendrite structure evolution and corrosion resistance of CoCrFeMnNi alloys

In this study, a series of CoCrFeMnNiCx (0–0.8 at% C) high-entropy alloys (HEAs) were prepared by vacuum arc melting and subsequently vacuum annealed at 800°C or 1000°C for 120 h. The aim was to investigate the microstructure evolution and corrosion behavior of the as-cast and annealed CoCrFeMnNiCx alloys in a neutral corrosion solution. Experimental results revealed that the addition of C can refine the as-cast dendrite structure, with an optimal C content of 0.4 at%. Annealing treatment led to a decrease in C solubility in the face-centered cubic (FCC) phase, resulting in the precipitation of carbides. In a 3.5 wt% NaCl neutral solution, the as-cast alloy containing 0.4 at% C exhibited the highest corrosion resistance. After annealing at 1000°C, the alloy's pitting potential and corrosion resistance were further improved. The corrosion behavior of both the as-cast and annealed CoCrFeMnNiCx alloys was primarily composed of pitting and intergranular corrosion, although the dominant mechanism varied among these alloys. This study establishes a correlation between microstructure evolution and corrosion behavior, providing insights for the design of high-entropy alloys with superior corrosion resistance.

Effects of Ti addition and wall thickness on microstructure, hardness, and wear properties of spheroidal graphite cast irons

Spheroidal graphite cast irons (SGCIs) have been commonly used in automotive and energy sectors where superior mechanical strength and wear resistance are required. This paper investigated the performance of the SGCIs under severe wear conditions as a function of Ti content and cooling rate. SGCIs alloyed with various Ti content (0, 0.1, 0.2, 0.3, and 0.5 wt%) were produced by green sand-casting process in different section sizes (10, 30, and 50 mm). Ball-on-disk type wear equipment with an Al2O3 ball was used to determine the wear rates of the SGCI samples. With increasing Ti content, up to 45% of nodular graphite turned into vermicular graphite, depending on the Ti addition and section sizes. Carbide precipitation was observed in the matrix due to the carbide-forming effect of Ti. The hardness and wear resistance of the SGCIs were improved by adding Ti owing to the combined effects of increasing pearlite and TiC particles precipitated in the matrix. Thinner-wall castings provided a faster cooling rate, which improved hardness and wear resistance. Alloying with 0.5 wt% Ti decreased the wear rates up to 35%.

The role of Si in enhancing the stability of residual austenite and mechanical properties of a medium carbon bainitic steel

In this work, two bainitic steels with medium carbon content were developed by adding different Si. The mechanical properties of the two test steels and the stability of their retained austenite were studied, and the influence of Si in the steels was discussed. The results show that the film-like retained austenite of the high Si bainitic steel is greatly increased compared with that of the low Si bainitic steel and that the C content is increased, effectively preventing crack propagation and significantly improving the comprehensive mechanical properties. Due to the different Si content in the test steels, the orientation relationship of austenite in the steels has changed, and when the Si content is increased from 0.7 wt% to 1.4 wt%, the variant selection of bainite becomes weaker. Moreover, the increase of Si content not only inhibits the precipitation of carbides but also promotes the accumulation of C in the austenite, which improves the stability of the retained austenite.

Numerical Simulation of Reversed Austenite Evolution during Intercritical Tempering of Low-Carbon Martensitic Stainless Steel.

Since the formation of reversed austenite during critical tempering treatment is an important factor affecting the mechanical properties of 13Cr4Ni martensitic stainless steel, a detailed study of the content and morphology of reversed austenite in heat treatment is needed. In this study, the variation curves of a reversed austenite volume fraction with holding time at different tempering temperatures were measured by in situ X-ray diffraction (XRD), and the reversed austenite and carbides of each process were evaluated by transmission electron microscopy (TEM). The austenite content shows a parabolic change with the increase in the tempering temperature; the maximum can reach a peak of about 6.8% at 610 °C, and drops to 0% at 660 °C. It also shows a parabolic change with the extension of the holding time, reaching a maximum of about 9.2% at 5 h of holding time, and a decreasing trend at 10 h of holding time, about 6.8%. The results show that the precipitation of carbides in the microstructure causes elemental segregation at grain boundaries and inside, which is one of the main factors affecting the thermal stability of reversed austenite formation. The kinetic process of reversed austenite during the tempering process was simulated using the JMAK model and the KM model, which can describe the trend of reversed austenite content during the tempering process. Combining the two models, a mathematical model for the room-temperature reversed austenite content under different processes was obtained, and this can predict the room-temperature austenite content.

Revealing the evolution behavior of multiple carbides precipitation and mechanical response in M54 secondary hardening steel

As a new type of secondary hardening ultra-high strength steel, the evolution behavior of multiple carbides and its effect on the strength–toughness of M54 steel have been studied by subjecting to tempering duration (2– 10 h) at 520 °C. Within tempering of 2– 4 h, there is incompletely dissolved cementite with intra-granular distribution; however, it does not deteriorate the impact toughness. Without the observation of cementite, it shows the largest precipitation density of M2C carbides after 8 h-tempering, with the maximum density of 650 μm−2 and minimum inter-particle distance of 12.3 nm. Prolonged tempering of 10 h does not lead to coarsening due to the low diffusivity of constituent W element and coherent relationship of M2C/matrix interface. However, there is obvious segregation of MC carbides along the long-axis of M2C carbides in growth. The favorable precipitation of M2C carbides mostly contributes to the highest yield strength of 1747 MPa and ultimate tensile strength of 2037 MPa in case of 8 h-tempering. The stable ultrafined martensite blocks (∼0.30 μm) and high ratio of high-angle grain boundaries (∼50%) guarantee the desirable V-notched impact absorbed energy of ∼30 J for the 2– 8 h-tempering. Unfortunately, due to the insufficient ability of arresting propagating cracks, the possible preferential MC carbides-distributed sub-grain boundaries leads to an obvious degradation of impact toughness (∼18 J) in the 10 h-tempered samples.

The abnormal carbon redistribution in lath martensite during tempering in Mn-patterned steels

Martensite tempering is a critical step to achieve high strength with decent ductility. It is well-known that, during tempering, the carbon is sequentially arranged as segregation, transition carbide and cementite. Abnormally, this study shows the absence of carbide precipitation in Mn-depleted martensite during tempering when Mn distribution is heterogeneous between austenite and martensite. After fast heating and short austenitization from Mn-depleted ferrite and Mn-enriched cementite, the high-temperature austenite with Mn heterogeneous distribution has formed, resulting in the formation of alternating Mn-depleted lath martensite and Mn-enriched film retained austenite (RA) after quenching. In comparison with the carbide precipitation in conventional lath martensite with Mn homogeneous distribution during tempering at 150–500°C, carbide formation is completely inhibited in Mn-depleted lath martensite. Instead, carbon atoms diffuse from Mn-depleted martensite to its neighboring Mn-enriched film RA, leading to an increased carbon content in RA. This is mainly attributed to the strong interaction between Mn and carbon, further assisted by high dislocation density and small width of lath martensite. Additionally, this RA can promote carbon diffusion in the interior due to its gradient Mn distribution.

Superior strength and ductility in a C-containing CoCrFeNiMn high-entropy alloy with heterogeneous microstructure

The present study is focused on the beneficial effects of adding few amounts of carbon (0.76 at. %) and adopting microstructure engineering by a simple thermomechanical treatment to form a heterogeneous microstructure in a CoCrFeNiMn high-entropy alloy. The results demonstrate that the addition of carbon results in significantly higher strength and improved thermal stability when compared to carbon-free alloy under identical conditions. In fact, the addition of carbon results in strengthening through the solute drag effect and solid solution strengthening. Thermodynamic predictions and XRD patterns indicate that carbon addition enhances the tendency of carbide precipitation in the alloy instead of forming an undesirable σ phase. The microstructural observation reveals that the deformed microstructure remains nearly stable during annealing at temperatures up to 750 °C and complete recrystallization occurs after annealing at 1000 °C, resulting in the formation of a fine-grained microstructure. Post-deformation annealing at 800 and 900 °C leads to the formation of heterogeneous microstructures comprising regions with high dislocation density grains alongside recrystallized fine-grained areas. The results suggest a superior synergy between strength and ductility in the annealed sample at 800 and 900 °C, exhibiting the ultimate-tensile strength of ∼1000 and ∼835 MPa. Additionally, these samples demonstrate decent uniform elongation values of ∼10 and 20%, respectively.

Review on Cellular Automata for Microstructure Simulation of Metallic Materials.

The cellular automata (CA) method has played an important role in the research and development of metallic materials. CA can interpret the microstructure changes of materials and obtain more abundant, accurate and intuitive information of microstructure evolution than conventional methods. CA can visually represent the process of grain formation, growth, development and change to us in a graphical way, which can assist us in analysis, thinking and solving problems. In the last five years, the application of CA in materials research has been rapidly developed, and CA has begun to occupy an increasingly important position in the simulation research of metallic materials. After introducing the advantages and limitations of CA compared to other widely used simulation methods, the purpose of this paper is to review the recent application progress on the microstructure simulation of metallic materials using CA, such as solidification, recrystallization, phase transformation and carbide precipitation occurring during forming and heat treatment. Specifically, recent research advances on microstructure simulation by CA in the fields of additive manufacturing, welding, asymmetrical rolling, corrosion prevention, etc., are also elaborated in this paper. Furthermore, this paper points out the future work direction of CA simulation in the research of metallic materials, especially in the simulation of the crystal structure, the prediction of mechanical properties, CA simulation software and rule systems, etc. These are expected to attract wide attention of researchers in the field of metallic materials and promote the development of CA in materials research.

Insights into microstructure evolution and fracture mechanisms of welded joints of high strength steel patchwork components

The hot roll bending patchwork components of B1500HS uncoated ultra-high strength steel processed at industrial scale were welded with filler wire. The microstructure, grain orientation, mechanical properties and fracture behavior of the joint were investigated. The results showed that the weld metal (WM) was mainly composed of lath martensite and lower bainite (LB). Various microstructure appeared in different positions of the HAZ, among which martensite-austenite (M-A) constituents in the incompletely quenched zone and tempered martensite (TM) in the tempered zone were the key factors determining the performance of the joint. The welded joint exhibited complex structure and anisotropic mechanical response due to the decarburization and rolling texture of the original base material (BM). In the tempered zone, lath merge and boundaries disappear lead to the annihilation of dislocations, carbide precipitation leads to reduction of mobile dislocations, and recrystallization and recovery eliminate work hardening. The reduction of the effect of dislocation strengthening results in the formation of a softening belt at 6 mm from the center of the weld, with a minimum hardness of 266 HV. The tensile strength of the joint in the rolling direction (RD) was 983 MPa, and the fracture occurred in the softening belt. Both brittle and ductile fracture features were observed. The reason why the fracture occurs in the high temperature tempered zone instead of the incompletely quenched zone was the difference in the effect of “constraint strengthening”. Fracture occurred through delamination cracking along the “weak interface” owing to precipitated high-hardness cementite particles and the unfavorably oriented ferrite cleavage planes.