High Strength Martensitic Steel Research Articles

Microstructure evolution, crack initiation and early growth of high-strength martensitic steels subjected to fatigue loading

Fatigue loading induced microstructure evolution featured by Fine Granular Area (FGA) in high-strength martensitic steels have close ties to the fatigue crack initiation and early growth, which are worthy of being investigated. On this issue, we conducted Very High Cycle Fatigue (VHCF), High Cycle Fatigue (HCF) and Low Cycle Fatigue (LCF) tests on three different types of specimens. Combined with post-mortem/quasi in-situ Scanning Electron Microscope (SEM) and Electron Back-Scattered Diffraction (EBSD) observations, as well as further Transmission Kikuchi Diffraction (TKD) and Transmission Electron Microscope (TEM) analyses, we captured two different types of microstructure evolution behaviors during the fatigue loading. One is the “dynamic precipitation”, a kind of phase transformation from martensitic matrix (BCC) to Nb- and Mo-rich small carbides (MC, M7C3) accelerated by repeated slight plastic deformation, whose formation does not rely on the stress concentration effect but also has no ties to the plastic strain localization and fatigue crack initiation. Another is the “local grain refinement” around those stress singularities (crack tip, or interior inclusion) generating substantial amounts of fine sub-grains in all the VHCF, HCF and LCF regimes, which can then promote fatigue crack initiation and early growth along sub-grain boundaries, fine/coarse grain boundaries and martensitic packet boundaries.

Effect of carbon segregation at prior austenite grain boundary on hydrogen-related crack propagation behavior in 3Mn-0.2C martensitic steels

The present study aimed at strengthening prior austenite grain boundary (PAGB) cohesive energy using carbon segregation and investigated the effect of carbon segregation at PAGB on the microscopic crack propagation behavior of hydrogen-related intergranular fractures in high-strength martensitic steels. At the low hydrogen content (below 0.2 wt. ppm), the fracture initiation toughness (JIC) and tearing modulus (TR), corresponding to crack growth resistance, were significantly improved by carbon segregation. In contrast, JIC and TR did not change by carbon segregation at the high hydrogen content (above 0.5 wt. ppm). Considering the non-linear relationship between the toughness properties and the PAGB cohesive energy, the experimentally evaluated toughness properties (JIC and TR) and the GB cohesive energy previously calculated by first-principles calculations were semi-quantitatively consistent even at the high hydrogen content. The microstructure observation confirmed that the plastic deformation associated with crack propagation, such as the local ductile fracture of uncracked ligaments and the formation of dislocation cell structures / nano-voids, played an important role in the non-linear relationship between the toughness properties and PAGB cohesive energy.

Coupling between phase transition and spallation in hierarchically structured high-strength martensitic steels under shock loading

This work presents a comprehensive and in-depth investigation of spall damage behaviors of bainitic and martensitic steels involving phase transitions. Plate-impact experiments, postmortem characterizations, molecular dynamics simulations considering realistic microstructures of bainitic and martensitic steels, and one-dimensional hydrodynamics simulations incorporating phase transition and spall damage are conducted and investigated. Origins of the multiple spall planes and their characteristics within steels are elucidated. Both steels exhibit a quasi-cleavage fracture mode involving phase transitions, with damage morphologies dependent on the microstructures and validated by molecular dynamics simulations. One-dimensional hydrodynamics simulations successfully replicate the dynamic spallation evolutions in steels considering phase transitions.

Study of Quenching and Partitioning (Q&P) and Ultrasonic Surface Rolling (USR) Process on Microstructure and Mechanical Property of a High-Strength Martensitic Steel.

Steel with a combination of strength and plasticity is prevalently demanded for lightweight design and emission reductions in manufacturing. In this study, a high-strength Cr-Ni-Mo martensitic steel treated by quenching and partitioning (Q&P) and ultrasonic surface rolling (USR) processes was studied for both strength and plasticity enhancement. Specimens were austenitized at 850 °C and then quenched to 240 °C via cooling by water, oil, and normalization in quenching. This was followed by partitioning, in which two groups of specimens were heated to 370 °C and 350 °C for 45 min, respectively. At last, all the specimens were quenched to room temperature with the same methods of quenching. The highest tensile strength increased from 681.73 MPa to 1389.76 MPa when compared to as-received (AR) steel after the Q&P process. The USR process with a static force of 800 N further improved the tensile strength of specimens with high tensile strength after the Q&P process, which improved from 1389.76 MPa to 1586.62 MPa and the product's strength and elongation (PSE) increased from 15.76 GPa% to 15.9 GPa%, while the total elongation showed a mitigatory decrease from 11.34% to 10.02%. Tensile fractures were also studied and verified using a combination of strength and plasticity after a combined process of Q&P and USR.

Effect of heating-assisted ultrasonic surface rolling on the microstructure evolution and wear behavior of martensitic steel

In this study, a plastic deformation layer and a nano-gradient structure were prepared on the surface of high-strength martensitic steel using the Heating-Assisted Ultrasonic Surface Rolling (HUSR) technique. The microstructure, texture evolution, and wear performance of the plastic deformation layer were characterized and analyzed using techniques such as Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). The influence of HUSR treatment on the friction and wear behavior of martensitic steel was discussed. The results indicate significant changes in the texture components, dislocation structures, and grain distribution in the plastic deformation layer prepared by HUSR. The predominant textures in the plastic deformation layer are Rotated Cube {001}<110>, Cube {001}<100>, F {111}<112>, and E {111}<110>. HUSR treatment induces the formation of abundant deformation twins and stacking faults (SF) on the surface of sample. This ultimately results in a 28.3 % increase in the surface hardness of the HUSR sample and the introduction of residual compressive stress of −746.2 MPa. The friction coefficient and wear volume of the HUSR sample are significantly reduced. After HUSR treatment, the martensitic steel exhibits excellent wear resistance.

Multi-scale damage mechanism of hierarchically structured high-strength martensitic steels under shock loading

The multi-scale damage behaviors and underlying mechanisms of hierarchically structured high-strength martensitic steels are investigated via plate-impact recovery experiments, postmortem characterizations. Plate-impact experiments were conducted at compressive peak stresses ranging from approximately 5.5 to 11.0 GPa, and the tensile strain rates are ∼105 s −1. Significant improvement in the spall strength of the heat-treated martensitic steel is achieved (5.4 GPa), about 80% higher compared to the as-received bainite steel. The spall failure of the investigated martensitic steel exhibited a mixed-mode combination of brittle (micro-cracks) and ductile (voids) characteristics, known as quasi-cleavage spallation. As the impact velocity increased, voids nucleated successively at different types of boundaries as follows: boundaries between enriched and depleted zones, prior austenite grain boundaries, packet boundaries, block boundaries and lath boundaries. Shear cracking was found to be an important mechanism to accommodate the severe plastic deformation during spallation, giving the strict geometric constraints due to void nucleation tending to occur at grain boundaries. Additionally, molecular dynamics simulations considering realistic hierarchical structure of martensitic steel have been developed to validate the experimental results. These findings and simulation methods contribute a better understanding of the spall behavior of martensitic steels, as well as guidance for future design of high-strength martensitic steels with exceptional reliability and safety under extreme conditions.

Microstructure, mechanical properties, and corrosion behaviour of wire arc additive manufactured martensitic stainless steel 410 for pressure vessel applications

High-strength martensitic stainless steel 410 (MSS 410) was successfully fabricated without defects via wire arc additive manufacturing (WAAM). The microstructural features, mechanical properties, and corrosion performance of WAAM-processed MSS 410 were examined. Microstructural analysis revealed the existence of equiaxed and coarse columnar dendrites and the formation of residual delta-ferrite and retained-austenite (RA) was confirmed within the martensitic matrix. The fraction of residual ferrite and RA varied along the building direction due to complex thermal cycles. Uni-axial tensile results exhibited anisotropic behavior and are related to layered microstructure. Microhardness values varied from bottom to top (384–460 HV). Ductile mode of failure was noticed, and the existence of RA influences the ductility. The corrosion behaviour of the WAAM 410 specimens in 3.5% NaCl solution was considerably acceptable, and the corrosion rate ranged between 0.05 and 1.37 mpy. This shall be attributed to the absence of defects, microstructural features, and ferrite fraction in the WAAM 410 samples. The mechanical properties and corrosion behaviour of the WAAM 410 were comparable to the wrought grade and are suitable for pressure vessel applications with adequate corrosion resistance.

Ultrasonic Impact Treatment (UIT) combined with powder bed fusion (PBF) process for precipitation hardened martensitic steels

High-strength martensitic stainless steels such as 17–4 PH (SS 17–4 PH) generally exhibit poor ductility and strain-hardening rates. In this study, an Ultrasonic Impact Treatment (UIT) is combined with a powder bed fusion (PBF) manufacturing process with the objective of enhancing the mechanical properties of SS 17–4 PH. UIT is introduced as a surface peening step at regular intervals during the PBF process after depositing a set of predetermined number of layers. The coupon specimens extracted from the build are then subjected to a post-build heat treatment. Then the microstructural and the mechanical properties of these specimens are characterized. The characterizations reveal that the heavy plastic deformation induced by UIT drives microstructural relaxation and recrystallization under heat treatment, reducing defects extensively. The heavy plastic deformation also drives diffusional reversion of the austenite phase during deposition and an inhibition of the martensitic phase transformation during the post-build heat treatment. As a result, the UIT specimens subjected to post-build heat treatment exhibit remarkable enhancement in ductility along with high strength and strain-hardening rate compared to the non-UIT PBF specimens under as-built condition.

In-situ neutron imaging of delayed crack propagation of high strength martensitic steel under hydrogen embrittlement conditions

This paper presents an in-situ observation, using neutron imaging, of delayed crack propagation in a high-strength martensitic steel specimen. Delayed cracking is believed to be caused by hydrogen embrittlement occurring due to the slow diffusion and accumulation of hydrogen ahead of a crack front, causing decreased ductility and eventual cracking under constant load. The experiment involved mechanical loading of a single-edge-notch bend specimen while submerged in an electrolyte solution (H2O + 3.5% NaCl) under cathodic polarization to facilitate hydrogen ingress. Intermittent crack propagation was observed for 12 h after the environment had been removed. The stress state at each crack configuration was extracted from a three-dimensional elastic–plastic finite element simulation, which was tailored to match the quantitative information acquired from the neutron radiographs of the fracture process. To gain insight into the evolution of hydrogen concentration with crack propagation, a modeling scheme for stress-assisted hydrogen diffusion was also employed.

Change in Mechanical Properties of High-Strength Martensitic Steel by the Combination of Pre-strain and Deformation Temperature

Change in Mechanical Properties of High-Strength Martensitic Steel by the Combination of Pre-strain and Deformation Temperature

Hydrogen content dependence of the contribution of dislocation-slip stability and carbide precipitation morphology to the hydrogen embrittlement property of high-strength martensitic steels

Hydrogen content dependence of the contribution of dislocation-slip stability and carbide precipitation morphology to the hydrogen embrittlement property of high-strength martensitic steels

Effects of Pre-Strain and Tempering on Mechanical Properties in High-Strength Martensitic Steels

Effects of pre-strain and tempering on mechanical properties in high-strength martensitic steels were investigated. In this study, strain tempering (ST) and quenching and tempering (QT) martensitic steels were prepared, and their mechanical properties were studied. In the tensile tests at the deformation temperatures between 296 and 573 K, the ST sample increased both of tensile strength (TS) and uniform elongation (U.El) from 473 to 523 K whereas the QT sample increased U.El with little change of TS. From the in situ neutron diffraction experiments, stress partitioning to the bcc phase increased with an increase in the deformation temperature from 296 to 523 K. The difference of phase stress between the bcc and cementite phases decreased with increasing the temperature because of a decrease in the cementite strength. In the ST sample, Pre-straining of 0.5% increased YS at 296 K with slight work hardening. The initial value of dislocation density (ρ) decreased at 523 K but ρ increased significantly after yielding, leading to better combination of TS and U.El. The combinations of pre-strain, tempering, and deformation temperatures have changed ρ before deformation and the increase of ρ after yielding of the martensitic steels.

Design of high-strength martensitic steels by novel mixed-metal nanoprecipitates for high toughness and suppressed hydrogen embrittlement

To obtain a fundamental understanding of mechanisms of hydrogen embrittlement (HE) and its prevention in advanced high-strength steels containing novel nanoscale mixed-metal precipitates, it is necessary to study local microstructure, H trapping, and crack path with new multiscale experimental and simulation approach. Spatially resolved hydrogen mapping via SKPFM is used together with investigation of the crack path using high-resolution EBSD and HMPT, and global trapping behavior of the alloys by TDS. These results are combined with newly introduced method to elucidate real-time distribution of hydrogen in the alloy using high-energy synchrotron X-ray diffraction (HES-XRD). Mixed-metal precipitates improves HE resistance of the alloy, due to nature of the trapping sites, e.g. irreversible H-trapping by carbon vacancies inside novel nanoprecipitates and high total length of PAGBs. This is because of lower possibility of build-up of critical local hydrogen content at PAGBs for intergranular hydrogen-assisted cracking due to hydrogen-enhanced decohesion mechanism. Less weakly trapped hydrogen also reduces frequency of dislocation activation and enhanced dislocation slip in {011} slip plane due to hydrogen-enhanced localized plasticity in regions with affinity for transgranular hydrogen-assisted cracking at lower local hydrogen content. Direct evidence of carbon vacancies in novel nanoprecipitates is observed for the first time via HAADF-STEM.

Fatigue Crack Propagation in Pearlitic Steel under Pressurized Gaseous Hydrogen: Influences of Microstructure Size and Strength Level

For the wall-thickness reduction of the components destined for pressurized gaseous hydrogen, widespread use of high-strength martensitic steels has long been desired. However, their strong susceptibility to hydrogen-assisted fatigue crack growth (HA-FCG) is still limiting their proactive applications. Here, we instead focused on pearlite as another potential reinforcing agent for the development of new hydrogen-compatible steels with acceptable cost performance. Fatigue crack growth (FCG) behavior of three eutectoid steels with different microstructure sizes (i.e., ferrite/cementite interlamellar spacing, colony and block sizes) and strength levels was investigated in a 90 MPa hydrogen gas, an essential evaluation when attempting to perform a defect tolerant design of the components used for high-pressure gases.

Micro-Segregation induced strain inhomogeneity in >900 MPa UTS martensitic hot rolled advanced high strength steel

High strength martensitic strip steels used in the yellow goods industry require good ductility and high strength. This paper examines how local compositional inhomogeneity of alloying elements (Mn, Mo, and Cr), caused by interdendritic microsegregation during casting and subsequent deformation, affect local straining behaviour in a >900 MPa autotempered martensite hot rolled strip steel. The alloy rich and alloy depleted bands have been characterised using micro-XRF and SEM-EDX through thickness for a 10 mm thick strip. Tensile samples were selectively machined from areas with a range of microsegregation severity in the longitudinal and transverse orientations. Digital Image Correlation (DIC) during tensile testing was used to examine local straining behaviour and shear band formation. Differential straining of up to 2.5x between solute enriched (lower strain) and solute depleted (high strain) bands was found in the strongly microsegregated transverse samples due to the lower hardness of the solute depleted bands (reduced solid solution strengthening). The high local strain in solute depleted bands in the strongly segregated sample led to premature failure (at lower global applied strain) compared to the less segregated samples. No significant strain partitioning was observed for the longitudinal oriented samples regardless of segregation level.

An experimental fracture mechanics study of the combined effect of hydrogen embrittlement and loss of constraint

This work presents a systematic investigation of the combined effect of hydrogen embrittlement and loss of constraint. The fracture mechanics experiments are performed on an advanced martensitic high strength steel using a single-edge-notch bend specimen, with different crack over height ratio, subjected to electrochemical in-situ hydrogen charging at various loading rates. It is found that the environmentally driven ductile-to-brittle transition region in fracture toughness is obtained for both the high and low constraint specimen configurations. This region is characterized by a change from transgranular dimple rupture to an intergranular mode of fracture. The transition region for the low constraint specimen is shifted towards longer hydrogen exposure times, which is an effect of the reduced hydrostatic stress ahead of the crack front compared to the high constraint specimen. The low constraint specimen exhibits significant plastic straining, which is reflected in a significant decrease in the fracture toughness due to hydrogen assisted transgranular dimple rupture.

Thermo-mechanics Driven Dynamic Recrystallization Behavior and Mechanism in High Strength Martensitic Stainless Steel

Thermo-mechanics Driven Dynamic Recrystallization Behavior and Mechanism in High Strength Martensitic Stainless Steel

Three-dimensional propagation behavior of hydrogen-related intergranular cracks in high-strength martensitic steel

Using X-ray computed tomography and focused ion beam-scanning electron microscopy (FIB-SEM) serial sectioning, this study investigated the three-dimensional propagation behavior of hydrogen-related intergranular cracks in martensitic steel with a tensile strength of 1.2 GPa, focusing on segments of prior austenite grain boundaries (PAGB segments). X-ray computed tomography revealed that the crack morphology was more continuous in the hydrogen-charged specimens. Through FIB-SEM serial sectioning, we found that crack-tip blunting and ductile rupture of un-cracked ligaments were associated with specific PAGB segments in the uncharged specimen. In the case of hydrogen-related intergranular crack propagation, even very fine, low-angle PAGB segments (sub-micrometer size) can act as obstacles for crack propagation. Based on these results, we propose that the misorientation of each PAGB segment has a large influence on the local arrestability of intergranular crack propagation.

Effect of Temperature and Deformation on the Stability of Retained Austenite in Closed-die Forgings from High-strength Martensitic Manganese-silicon Steels

Effect of Temperature and Deformation on the Stability of Retained Austenite in Closed-die Forgings from High-strength Martensitic Manganese-silicon Steels

Newly developed high-strength martensitic stainless steel for the sniper rifle barrel with high shooting accuracy

With the increasing requirements for the shooting accuracy of the sniper rifle, steel combined with excellent high-temperature strength, low-temperature toughness, and extraordinary corrosion resistance is desirable for the barrel material of the sniper rifle with high accuracy and a long lifespan. In this study, martensitic stainless steel 2Cr11Ni2Mo2WV (MPS700A) was developed by introducing Mo, W, and V into the currently used 2Cr13 steel. The 600 °C tensile tests demonstrated that the ultimate tensile strength of MPS700A reached 767 MPa, which was 46.4 and 97.2 % higher than those of 2Cr13 and 30SiMn2MoV, respectively. Meanwhile, the −40 °C impact toughness of MPS700A reached 88 ± 4 J/cm2, which was 20.5% higher than that of 2Cr13 and comparable to that of 30SiMn2MoV. The intralath precipitation of ellipsoidal M2C nano carbides was observed in MPS700A after tempering, which followed a Dyson orientation relationship (OR) of (01¯12)M2C//(200)α with the martensitic matrix. After deformation at 600 °C, such a Dyson orientation relationship (OR) was maintained, and the M2C in MPS700A exhibited higher thermal stability than the M3C in 2Cr13, which hinders the dislocation movement and contributes to the higher strength of MPS700A at 600 °C. The fine martensitic laths and homogeneously dispersed M2C nano carbides also contributed to the higher impact toughness of MPS700A than that of 2Cr13. Additionally, the salt spray test indicated that the mass loss of MPS700A was 1.72 ± 0.24 g m−2 at 72 h, which was 95.6% lower than that of Cr-plated 30SiMn2MoV and comparable to that of 2Cr13. Based on these results, we demonstrate that MPS700A exhibits high strength up to 600 °C, excellent impact toughness at low temperatures, and outstanding corrosion resistance, making it a promising gun barrel material for sniper rifles with long lifespans and high shooting accuracy.