Medium Carbon Martensitic Steel Research Articles

Mechanisms of secondary crack initiation and propagation and closure during the water quenching process in medium-carbon martensitic steel

Mechanisms of secondary crack initiation and propagation and closure during the water quenching process in medium-carbon martensitic steel

Textural index for martensitic and bainitic steels to assess influence of hot rolling mode on parent austenite structure before quenching

To assess a state of parent austenite before the steel quenching, a scalar textural index for martensite and bainite is introduced in terms of EBSD orientation data. Deformed and recrystallized states of the parent phase are discriminated by the sign of this index, whereas its magnitude in each of the two reflects the texture sharpness depending on the hot rolling mode. Accordingly, in a virtual case of randomly distributed orientations the considered parameter vanishes. Performance of the proposed approach is demonstrated on medium carbon martensitic steel hot rolled at laboratory conditions and on industrial rolled plates of low carbon bainitic steel.

Influence of Mo carbides and two-stage tempering methodology on the susceptibility of medium carbon martensitic steel to hydrogen embrittlement

The objective of the present study was to enhance the hydrogen embrittlement (HE) resistance of the quenched and tempered martensitic steel via the interplay of heat treatment variance and precipitation of nanosized carbides. For this purpose, one-stage tempering and two-stage tempering methodologies were implemented, and steel was alloyed with Mo to instigate the precipitation of Mo carbides. The results revealed that two-stage tempered steel exhibited superior resistance to HE, as a result of reduced dislocation density and higher quantity of Mo2C. To discern the role and trapping behaviour of Mo2C carbides, Thermal Desorption Spectroscopy (TDS) combined with electrochemical hydrogen charging was utilized. Precipitated nanosized Mo2C exhibited the ability to trap hydrogen. On the contrary, an increase of dislocations and higher diffusible hydrogen content in one-stage tempered steel promoted deterioration of mechanical properties which was investigated by Slow Strain Rate Test (SSRT) and fracture surface morphology analysis. In addition, the effective diffusion coefficient for one-stage tempered steel was lower, as dislocations served as additional trap sites.

Characteristics of Microstructure and Mechanical Properties in Al-bearing Medium Carbon Martensitic Steels

Characteristics of Microstructure and Mechanical Properties in Al-bearing Medium Carbon Martensitic Steels

Dual grain size-effects on hydrogen-assisted fatigue crack growth in 1 GPa-class medium-carbon martensitic steel

The influence of prior austenite grain (PAG) size ranging from 6 to 90 μm on the fatigue crack growth (FCG) of a tempered 0.41%C lath martensitic steel was investigated in a 90 MPa H2 gas environment at ambient temperature. Under the presence of H, severe FCG accelerations accompanying intergranular (IG) cracking along PAG boundaries as well as quasi-cleavage (QC) fracture along martensite block boundaries and {011} crystallographic planes were systematically found. Smaller PAG mitigated the acceleration at a relatively high-stress intensity via suppressing IG cracking. However, PAG refinement instead escalated the acceleration at a low-stress intensity factor where QC facture prevailed. This inverse grain size effect was discussed in terms of the plasticity criterion in triggering QC and its possible dependence on the microstructural length scale.

The role of cementite on the hydrogen embrittlement mechanism in martensitic medium-carbon steels

Although cementite is the most common carbide found in steel components, its interaction with hydrogen (H) is still under discussion. Most studies investigate the effect of cementite by comparing the as-quenched vs. tempered structure of the same material. However, tempering reduces the dislocation density which plays an important role in several hydrogen embrittlement (HE) mechanisms. Therefore, this study investigates the HE behavior by in-situ hydrogen bending tests of two tempered medium-carbon martensitic steels with a similar dislocation density but a different cementite concentration. Moreover, a comparison between the as-quenched and the tempered samples is also made. This experimental strategy allows to generate important insights on the role of cementite on the susceptibility to HE while the influence of the dislocation density is limited. Thermal desorption spectroscopy and melt extraction results show that cementite acts as an effective H trap and reduces the H diffusion coefficient. When mechanically tested in a H rich environment, the presence of cementite increases the resistance to HE by hindering the H diffusion towards the most critical sites in the microstructure. However, when the samples are fully saturated with H prior to testing, cementite loses its capability to influence the fracture behavior.

Erosion–corrosion resistance of Mo–Ti- and Ni–Cr–Mo-alloyed medium-carbon martensitic steels: a critical analysis of synergistic effect of erosion and corrosion

Erosion–corrosion resistance of Mo–Ti- and Ni–Cr–Mo-alloyed medium-carbon martensitic steels: a critical analysis of synergistic effect of erosion and corrosion

Quantitative evaluation of dislocation density in as-quenched martensite with tetragonality by X-ray line profile analysis in a medium-carbon steel

The crystal structure of as-quenched martensite in steel changes from body-centered cubic to body-centered tetragonal because of the presence of interstitial carbon, and the c/a ratio increases with increasing carbon content. Because the X-ray diffraction peak profiles associated with the c-axis overlap those of the a-axis, the diffraction peak observed in martensite gradually broadens as the solute carbon increases. This may cause an overestimation of the dislocation density in the X-ray line profile analysis (for example, modified Williamson-Hall/Warren-Averbach method). In this study, the effect of the tetragonality of martensite in the X-ray line profile analysis was investigated to evaluate the dislocation density in a medium-carbon martensitic steel. The apparent dislocation density estimated without considering peak broadening due to tetragonality was calculated to be 1.7 × 1016 m−2 in the as-quenched 0.55%C steel. This value is approximately six times higher than that estimated using transmission electron microscopy (TEM), indicating that the dislocation density was overestimated. To eliminate the effect of tetragonality, the specimens were tempered at temperatures below 623 K. The estimation based on the TEM method revealed that the dislocation density only slightly decreased by tempering at 573 K, indicating negligible recovery at this temperature. The dislocation density obtained in the tempered specimen was calculated to be approximately 4.5 × 1015 m−2 using the mWH/WA method. This value was regarded as the true dislocation density of the as-quenched martensite in the 0.55%C steel.

Impact of Dislocation Density and Mobility on Yielding Behavior in Quenched Medium-carbon Martensitic Steel Tempered at Low Temperature

Middle-carbon martensite steels are vital materials for mechanical components and their mechanical properties have attracted significant interest. However, the decrease in the elastic limit of the as-quenched materials is one of the remaining puzzles. Herein, we quantitatively characterized the dislocation density and its structure in the as-quenched and tempered martensite steel by neutron diffraction line profile analysis and discussed their impact on the yield stress. The dislocation density in the as-quenched specimen was the highest at 9.7 × 1015 m−2, while it decreased with an increase in the tempering temperature. In addition, the component ratios of edge and screw dislocations decreased and increased, respectively, depending on the increase in the tempering temperature. The dislocation arrangement parameter (M) varied between the tempering temperatures of 220 and 290°C. Although there was a large difference between the yield stress obtained from the tensile test and that estimated from the dislocation density, the experimental results could be explained by correcting them with the inverse of M value as an index showing the effective dislocation density ratio.

Simultaneously enhancing strength and toughness of medium-carbon martensitic steel via nano precipitates and fine-grained structure

A medium-carbon martensitic mold steel with high strength and toughness is obtained based on the synergistic effect of nano precipitates and fine-grained (FG) structure. Treatment increases the length per unit area of the effective grain boundary by 37.9%. Compared with the untreated samples, the excellent properties of the steel include increases of 39.2% in impact energy and more than 110 Mpa in ultimate yield strength. The FG structure is produced by the interaction between the MC nano precipitates formed by V alloying and the subgrain structure formed by thermomechanical processing (TMP). Additionally, the anisotropy of the microstructure is eliminated through the innovative introduction of austenitization, resulting in the refinement and homogenization of the grain structure. MC nano precipitates not only prevent the growth of subgrain structure but also enhance the thermal stability of FG structure through a pinning mechanism during austenitization.

Simultaneous Enhancement of Strength and Toughness in a Medium-Carbon Martensitic Steel by Ti-Mo Addition

Simultaneous Enhancement of Strength and Toughness in a Medium-Carbon Martensitic Steel by Ti-Mo Addition

Microstructure Evolution and Mechanical Properties of Medium Carbon Martensitic Steel during Warm Rolling and Annealing Process.

The microstructure evolution and mechanical properties of medium carbon martensitic steel during the warm rolling and annealing process were studied by scanning electron microscope (SEM), electron back scattering diffraction (EBSD), and electronic universal testing machine. The results showed that the microstructure of ferrite matrix with mass dispersive cementite particles was obtained by decomposition of martensitic in medium-carbon martensitic steel after warm rolling. The grain size of ferrite was ~0.53 μm, the yield strength and tensile strength were 951 MPa and 968 MPa, respectively, and the total elongation rate was 11.5% after warm rolling at 600 °C. Additionally, after the next 4 h of annealing, the grain size of ferrite and particle size of cementite increased to ~1.35 μm and ~360 nm and the yield strength and tensile strength decreased to 600 MPa and 645 MPa, respectively, with a total elongation increases of 20.9%. The strength of the material increased with increasing strain rate in tension, and the yield-to-tensile strength ratio increased from 0.92 to 0.94 and maintained good plasticity.

Advanced quantification of the carbide spacing and correlation with dimple size in a high-strength medium carbon martensitic steel

Interparticle spacing in materials science is a key parameter controlling ductile fracture. It is generally evaluated as the distance between particle centers of mass. High strength medium carbon martensitic steels contain a high density of carbides, whose size is non-negligible with respect to the distance between them. Current microstructural quantification methods do not take the shape of analyzed particles into account, thus being not necessarily relevant to materials containing a high density of particles that trigger void nucleation and ductile fracture.This work reports on the potency of a simple full field particle quantification method based on a watershed algorithm that takes the shape and spatial distribution of particles into account. Three microstructures were produced by heat treating a hot-rolled 40CrMo4 steel bar. Cementite carbide populations of each microstructure were quantified using carbon extractive replicas, conventional image processing and application of the developed algorithm. A comparison of results from a full field Voronoi tiling algorithm revealed pronounced differences in the shape of distributions. The watershed-based method was shown to allow efficient interpretation of the relationships between carbide distribution and ductile fracture features.

Relationship between Microstructure and Tensile Behavior of FSWed Medium Carbon Martensitic Steel

Relationship between Microstructure and Tensile Behavior of FSWed Medium Carbon Martensitic Steel

Effect of Silicon, Chromium and Molybdenum on Resistance to Temper Softening of Medium-carbon Martensitic Steel

The addition of alloy elements such as silicon, chromium and molybdenum is effective in the restraint of temper softening in martensitic steels. The martensitic steel containing such alloy elements is known to have excellent resistance to temper softening in low and high temperature tempering treatments. However, the effects of the compound addition and its mechanism have not yet been satisfactorily revealed. In this study, the effect silicon, chromium, and molybdenum on resistance to the temper softening of the medium-carbon martensitic steel was investigated. As a result, it is revealed that the temper softening resistance quantity is maximum at the tempering temperature 773 K in the martensitic steel containing silicon, chromium, and molybdenum; this is because of the miniaturization of cementite (θ) and clustering of (Cr,Mo,Mn) 2C. Moreover, it is revealed to form this cluster at a tempering temperature lower than the temperature at which the alloy carbide usually precipitates (823 K-873 K).

Influence of Iron Carbide on Mechanical Properties in High Silicon-added Medium-carbon Martensitic Steels

Influence of Iron Carbide on Mechanical Properties in High Silicon-added Medium-carbon Martensitic Steels

Influence of Iron Carbide on Mechanical Properties in High Silicon-added Medium-carbon Martensitic Steels

Influence of Iron Carbide on Mechanical Properties in High Silicon-added Medium-carbon Martensitic Steels

The Mechanism of High-Strength Quenching-Partitioning-Tempering Martensitic Steel at Elevated Temperatures

High-strength medium-carbon martensitic steel was heat treated through a quenching-partitioning-tempering (Q-P-T) treatment. Both the mechanism for improved ductility and the high temperature stability of austenite were investigated. The Q-P-T martensitic steel showed good products of strength and elongation (PSE) at various deformation temperatures ranging within 25–350 °C. The optimum PSE value (>57,738 MPa%) was achieved at 200 °C. The microstructure of the Q-P-T steel is constituted of laths martensite with dislocations, retained austenite located within lath martensite and small niobium carbides (NbC), and/or transitional ε-carbides that precipitated in the lath martensite. The good ductility can be mainly attributed to the laminar-like austenite that remained within the lath-martensite. The austenite can effectively enhance ductility through the effect of dislocation absorption by the retained austenite and through transformation-induced plasticity. The relationship between the microstructures and mechanical properties was investigated at high deformation temperatures.

Microstructural Characterization of Surface Softening Behavior for Cu-Bearing Martensitic Steels after Laser Surface Heat Treatment

The surface hardening and softening behavior of two types of medium carbon martensitic steel (AISI P20-improved and AISI P21) after laser-assisted heat treatment was quantitatively compared. The laser-assisted heat treatment was performed using a high-power diode laser with in situ temperature and laser power control (two-color pyrometer system). For AISI P20-improved steel, the peak hardness value within the hardening zone was approximately 640 HV after laser-assisted heat treatment at a temperature of 1473 K. In other words, the hardness increased by 120% from the base metal level (290 HV). However, for AISI P21 steel, the hardness within the heat-treated zone did not change from that of the base metal (410 HV), despite being accompanied by martensite transformation. Moreover, it was clearly observed that the hardness dropped below the level of the base metal at the boundary between the heat-treated zone and the base metal region, forming a softening zone. This softening behavior was strongly related to coarsening and a looser distribution of Cu precipitates compared with that of the base metal region, despite the same matrix phase (i.e., tempered martensite) existing in the softening zone and in the base metal region.

Optimization of Thermal and Deformation Effect during Plastic Deformation and Thermal Treatment of Hot Rolled Heavy Wall Pipes Made of Medium Carbon Martensitic Steel

The method of stress relaxation in isothermal aging after deformation is employed to determine recrystallization temperature range (Trec) for 38HN3MFA (35HN3MFA) steel grade. The paper shows that billet final deformation at the temperature lower than Trec followed by annealing including heating and aging at Trec enables austenite grain refinement.