Ultrahigh Strength Low Alloy Steel Research Articles

Mechanical properties and microstructure evolution of 1800 MPa grade low alloy ultrahigh strength steel during quenching and tempering process

Mechanical properties and microstructure evolution of 1800 MPa grade low alloy ultrahigh strength steel during quenching and tempering process

Correlation between microstructure and fracture mechanism of 960 MPa Grade ultra-high toughness steel

This study investigated the correlation between microstructure and fracture mechanisms in a 960 MPa grade ultra-high toughness high strength low alloy steel produced through controlled rolling and off-line heat treatment. The steel features a low-carbon composition (≤0.1 wt%) and a fine-grained(≤10 μm) bainitic microstructure. Through microstructural characterization and fractographic analysis, extensive quantitative analysis of grain, packet, dimple, and cleavage facet sizes was conducted. Fracture surfaces were examined to identify crack initiation sites. The findings indicate that grain refinement significantly enhances toughness. In fine-grained conditions, bainitic packets that approximate the size of the grains modulate crack propagation pathways. The near-oriented bainite across adjacent grains forms structures comparable to abnormally large grains, crucial for crack nucleation. Moreover, martensite/austenite (M/A) islands, while crucial to the initiation of cracks, require the synergistic interaction with grain boundaries and inclusions to significantly influence fracture initiation. This investigation elucidates the complex interplay between microstructural refinement and fracture mechanisms.

Effect of rolling reduction on microstructure, mechanical properties and fracture features of hot isostatically pressed 30CrMnSiNi2A low alloy ultrahigh strength steel

As a powder metallurgy (PM) defect, existence of prior particle boundaries (PPBs) often makes mechanical properties visibly deteriorated. Hot rolling has been found to be an effective method of destroying PPBs. In present study, five different rolling reductions are applied to explore the effects of rolling reduction on evolution of PPBs in PM 30CrMnSiNi2A steels. Microstructure, mechanical properties and fracture modes of five samples are systematically analyzed and compared. Increase of rolling reduction not only makes Al-rich PPBs refined but also makes continuous PPBs structure gradually transform to dispersed oxides and short chain fragments, which can be attributed to deformation of powder particles. Change on distribution of PPBs leads to the variation of fracture modes. A strength-ductility-toughness balance is achieved when the rolling reduction scales up to 60 %. Although increase of rolling reduction causes grain refinement, work hardening and improvement of PPBs, it promotes formation of edge cracks. Occurrence of edge cracks is strongly associated with PPBs inclusions and makes ductility and toughness deteriorated.

The Fabrication of Ultrahigh-Strength Steel with a Nanolath Structure via Quenching-Partitioning-Tempering.

A novel low-alloy ultrahigh-strength steel featuring excellent mechanical properties and comprising a nanolath structure was fabricated in this work using a quenching-partitioning-tempering (Q-P-T) process. The Q-P-T process comprised direct quenching and an isothermal bainitic transformation for partitioning after thermo-mechanical control processing (online Q&P) and offline tempering (reheating and tempering). The ultrafine nanolath martensite/bainite mixed structure, combined with residual austenite in the form of a thin film between the nanolaths, was formed, thereby conferring excellent mechanical properties to the steel structures. After the Q-P-T process, the yield and tensile strengths of the steels reached 1450 MPa and 1726 MPa, respectively. Furthermore, the Brinell hardness and elongation rate were 543 HB and 11.5%, respectively, with an average impact energy of 20 J at room temperature.

Fatigue strength of an ultra-high strength low alloy steel fabricated via laser powder bed fusion

High strength low alloy (HSLA) steels are important metallic materials especially in the defense and military sectors due to their superior mechanical properties and cost-effectiveness compared to superalloys. This study investigates monotonic tension/compression strength and fatigue strength, including both the low cyclic fatigue (LCF) and high cyclic fatigue (HCF), of a HSLA steel produced using laser powder bed fusion (LPBF) additive manufacturing (AM). The LCF tests were strain-controlled (from 0.45% to 1.2% strain amplitudes) achieving a constant strain rate of 0.0005/s, while the HCF tests were stress-controlled (from 467 MPa to 1007 MPa stress range), all carried out at room temperature. Specimens of the alloy were manufactured in two build orientations to evaluate anisotropy. Moreover, a set of rolled plate and as-cast specimens were tested to facilitate comparisons. The alloy exhibited excellent performance confirming the potential of LPBF AM to manufacture HSLA steels with significant performance gains over as-cast counterparts. Nevertheless, the rolled material exhibited the highest performances. Details pertaining to the processing along with the testing results and characterization of microstructural origins of the high strength are presented and discussed in this paper.

Superior Pitting Corrosion Resistance of Ultra-high Strength Low Alloy Steel Via Co-alloying Al and Cu

Superior Pitting Corrosion Resistance of Ultra-high Strength Low Alloy Steel Via Co-alloying Al and Cu

A phenomenological understanding of the novel design of hierarchical structure for 1 GPa ultrahigh strength and high toughness combination low alloy steel

In the present study we propose a novel design of hierarchical structure with the aim to obtain high strength of 1 GPa and low temperature toughness in a heavy low alloy steel plate by intercritical quenching and tempering (IQ&T). The hierarchical microstructure consisted of bimodal prior austenite grains with complex microstructure obtained via intercritical quenching from 810 °C and tempering at 500 °C. The large size grains with average diameter of ∼22 μm consisted of tempered martensite and fine intercritical ferrite. While the surrounding fine grains with average diameter of ∼12 μm consisted of tempered lower bainite and tempered lath martensite. Ultra-high yield strength of 998 MPa and tensile strength of 1059 MPa were obtained for the novel hierarchical microstructure steel, which was similar to the conventional quenched and tempered (Q&T) steel. Whereas, the low temperature impact energies at both −40 and −80 °C were ∼100% higher for the hierarchical microstructure steel than the conventional Q&T steel. The significant improvement of low temperature toughness is attributed to the synergistical effect of hierarchical microstructure. The fine intercritical ferrite in large grains increased the plastic deformation ability during impact and was beneficial for formation of dimples, and the surrounding fine grains offered high density of large mis-orientation grain boundaries to deflect crack propagation, leading to high impact energy at low temperature.

Deformation Behavior and Plastic Instability of Ultra-High Strength Low Alloy Steel over Wide Temperature and Velocity Range

Deformation Behavior and Plastic Instability of Ultra-High Strength Low Alloy Steel over Wide Temperature and Velocity Range

Surface Topography Model of Ultra-High Strength Steel AF1410 Based on Dynamic Characteristics of Milling System

AF1410 is a low carbon high alloy ultra-high strength steel. It not only has high strength and high toughness, but also has a high stress corrosion resistance. However, due to the characteristics of hard quality and poor thermal conductivity, AF1410 is a difficult material to process. In the process of milling, the geometric factors of process parameters, the flexible deformation of milling cutter and the flutter of the process system all affect the surface roughness, which makes it difficult to predict the surface roughness of milling parts. In order to solve this problem, a prediction model for surface topography of ultrahigh strength steel AF1410 was studied. To solve this problem, this paper studies the formation of milling surface topography, considers the dynamic displacement of the milling system, proposes a modeling method of surface topography based on the dynamic characteristics of the milling system and forms a prediction model. On this basis, the surface topography of ultra-high strength steel is simulated and analyzed, and the accuracy of the model is verified by experiments. The study realizes the prediction of milling surface topography of AF1410 parts and reveals the formation mechanism of milling surface topography from geometric and physical perspectives.

A novel strategy for preparing high-performance powder metallurgical low alloy ultrahigh strength steel

In this study, a combined process of hot isostatic pressing and hot rolling (HIP-rolling) is developed to prepare high-performance powder metallurgical (PM) low alloy ultrahigh strength steel. The HIP-rolled 30CrMnSiNi2A steel achieves ultrahigh ultimate tensile strength of 1823.6 MPa, high elongation of 17.6% and superior impact energy of 29.4 J. The formation of lower bainite/martensite duplex structure, the refinement of prior austenite grains and blocks, and the disruption of prior particle boundaries are the critical factors to achieve the excellent strength-toughness balance. This study provides a novel insight for the development and application of PM low alloy ultrahigh strength steel.

Mechanical properties of welded ultrahigh-strength S960 steel at low and elevated temperatures

New ultrahigh-strength steels have been developed to meet the need for better performance, load bearing capacity, safety and weight saving. However, the design guidelines are incomplete, especially regarding the design of welded ultrahigh-strength steel components in fluctuating operating conditions. This reduces usability and can cause serious safety risks when the welded structures are in use. The heat input and cooling rate have a significant effect on the microstructure and mechanical properties of the ultrahigh-strength steels. Therefore, a model to predict the strength and microstructure, based on welding parameters, is required for designers to create safer solutions in engineering design. In this study, a 6 mm thick S960 low alloy ultrahigh-strength steel was welded using gas metal arc welding (MAG) and laser welding. The effects of welding heat input and operating temperature on the tensile properties, hardness, microstructure, and fracture mechanism of the welded specimens were investigated. The effects of operating temperature on the mechanical properties of welded joints were investigated by performing tensile tests between temperatures of −80 °C and + 400 °C. The unwelded base material was also tested in the same temperature range. The results showed a ductile fracture mechanism in all the samples regardless of the test temperature and welding heat input. However, the tensile strengths and elongations increased when the test temperature drops to −80 °C from room temperature. In addition, mathematical predictions for the strength and elongation properties, and grain sizes in heat-affected zones, as a function of temperature and welding heat input were proposed.

A comparative study on modified and optimized Zerilli-Armstrong and arrhenius-type constitutive models to predict the hot deformation behavior in 30Si2MnCrMoVE steel

To better understand the elevated temperature flow behaviors of 30Si2MnCrMoVE low-alloy ultra-high strength steel, hot compressive deformation tests were performed at a wide range of forming temperatures (1123–1423 K) and strain rates (0.01–1s−1). The constitutive equations of the material were established through the physical-based modified Zerilli-Armstrong (ZA) and phenomenological Arrhenius-type models, along with the optimized ZA model of strain compensation constructed based on the modified ZA model. Suitability of these models were evaluated by comparing the correlation coefficient and absolute average error between the prediction and experimental data, its ability to predict the deformation behavior, and the amount of calculation required for the material constants. Results show that the modified ZA model cannot accurately describe the flow behavior of the 30Si2MnCrMoVE steel in the abovementioned hot working zone. The optimized ZA model and Arrhenius-Type model based on strain compensation require similar computational effort, and both can effectively predict the high temperature flow behavior of 30Si2MnCrMoVE steel. Relatively, the Arrhenius type model has the highest prediction accuracy.

Constitutive Analysis on Deformation Behavior of XF1700 Ultra-high Strength Low Alloy Steel in Perceptive of Adiabatic Temperature Rise and Strain

Constitutive Analysis on Deformation Behavior of XF1700 Ultra-high Strength Low Alloy Steel in Perceptive of Adiabatic Temperature Rise and Strain

Microstructure and Mechanical Properties of Low Alloy Ultra-High Strength Steel Microalloyed with Cerium

This study aims to investigate the influences of rare earth element cerium (Ce) on microstructure and mechanical properties of low alloy ultra-high strength steel. The strength, plasticity, and impact toughness of steels with 0.0367% Ce and without Ce were tested. The influence mechanism of Ce on the microstructure and mechanical properties were investigated by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and electron back scattered diffraction (EBSD). The results showed that the addition of Ce improved the comprehensive mechanical properties of low alloy ultra-high strength steel. In particular, the plasticity and toughness were improved obviously. The addition of Ce increased the elongation from 9.47% to 10.49%, and the low-temperature impact energy from 50J to 58J. The elongation and impact energy increased by 10.77% and 16%, respectively. And the yield strength of all samples remained above 1400 MPa. The rare earth element Ce did not change the matrix composition phase which were martensite. However, the addition of Ce increased the proportion of high-angle grain boundary from 33.2% to 40.2%. In addition, the Ce make the inclusions denatured and hence spherical inclusion with small size can be obtained. The EDS results showed that rare earth and harmful elements P and O formed inclusions, which as a purifier for the molten steel and hindered the formation of the large-size composite inclusions.

Dynamic constitutive behavior investigation of a novel low alloy ultra-high strength steel

The goal of this work is to reveal the dynamic mechanical properties and constitutive relationship of a novel low alloy ultra-high strength DT506 steel under high strain rates. The quasi-static state and dynamic compression behavior of the material at the strain rates of 10−3–103 s−1 were examined using an MTS landmark electro-hydraulic servo universal tester and a Split-Hopkinson bar (SHPB). The results show that DT506 steel is a strain-rate-sensitive material that shows an increase in strength with increasing strain rate. Based on the quasi-static and dynamic compression data, the parameters in the Cowper-Symonds (C-S) and the Johnson-Cook (J-C) models are determined. Since the previously developed models are cannot accurately predict the effect of the strain rate. A new theoretical model is obtained through the optimization of the parameters in the standard J-C model. The optimized model greatly improves the prediction accuracy of the true stress-strain dynamic behavior of materials. This new model is helpful for the verification of the underlying mechanism of the dynamic behavior of the material. Test results may provide basic data for future research on dynamic mechanical properties and constitutive relationship of metal alloys.

Effect of As-Cast Structure and Macrosegregation on Mechanical Properties in Direct-Quenched Low-Alloy Ultrahigh-Strength Steel

The effect of as-cast structure and macrosegregation on the mechanical properties of a direct-quenched low-alloy martensitic ultrahigh-strength aluminum killed and calcium treated steel cast at different superheats was studied. Samples from the castings were laboratory hot rolled with two different finishing rolling temperatures to distinguish the effect of hot rolling. Using optical emission spectrometry, the steel composition was analyzed as a function of slab thickness in order to detect the variations in steel chemistry due to macrosegregation. Further, hardness profiles, prior austenite grain sizes and tensile and impact toughness were determined for the hot-rolled specimens. It was found that interdendritic segregation was more intense at the higher superheat, which led to more pronounced positive segregation in the columnar-to-equiaxed transition (CET) zone, and negative segregation between CET and the centerline. These macrosegregation patterns were inherited by the hot-rolled samples causing local variations in hardness, which followed the variations in carbon content. However, altering the superheat had a minor effect on the nominal transformed microstructures and nominal prior austenite grain sizes. This occurred because of the interdendritic segregation induced composition variations both enlarged and decreased by turns the grain sizes. The CET also reduced measured impact toughness values.

Hot-Rolling and a Subsequent Direct-Quenching Process Enable Superior High-Cycle Fatigue Resistance in Ultra-High Strength Low Alloy Steels.

The current study investigated the effect of hot rolling reduction rate of ultra-high strength low alloy steel manufactured via the direct quenching process on microstructure, tensile and high-cycle fatigue properties of the alloy. In order to control the reduction rate of ultra-high strength steels (UHSSs) differently, the steels were produced with two different thicknesses, 6 mm (46.2%—reduction rate, A) and 15 mm (11.5%—reduction rate, B). Then, the two alloys were directly quenched under the same conditions. Both the UHSSs showed martensite in the near surface region and auto-tempered martensite and bainite in the center region. Tensile results showed that alloy A with higher fraction of finer martensite had higher yield strength by about 180 MPa (1523 MPa) than alloy B. The alloy A was also found to possess a higher tensile strength (~2.1 GPa) than alloy B. In addition, alloy A had higher strength than B, and the elongation of A was about 4% higher than that of alloy B. High-cycle fatigue results showed that the fatigue limits of alloys A and B were 1125 MPa and 1025 MPa, respectively. This means that alloy A is excellent not only in strength but also high-cycle fatigue resistance. Based on the above results, the correlation between the microstructure and deformation behaviors were also discussed.

Surface characterization of plasma rotating electrode atomized 30CrMnSiNi2A steel powder

Characterization of surface oxides on atomized steel powder is essential in order to determine the sintering conditions for providing strong metal bonding between the powder particles during consolidation. In the present study, it was the first time to systematically investigate the surface characterization of plasma rotating electrode atomized steel powder. China ultra-high strength low alloy steel 30CrMnSiNi2A powder produced by plasma rotating electrode process (PREP) was chosen as the model material. X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy were performed to methodically investigate the morphology, size, composition and distribution of surface oxides of the PREP powder. The results indicated that the PREP powder surface was covered by dispersed oxide particulates and homogeneous Fe-oxide layer (about 4.6 nm) between the oxide particulates. MnO and FeO were the main forms of the oxide particulates and the Fe-oxide layer, respectively, and a little Fe2O3 existed on the outermost of the Fe-oxide layer. Besides, a small quantity of Cr2O3 and SiO2 existed in local areas of the oxide particulates and the Fe-oxide layer. At last the model of surface oxides distribution of the PREP steel powder was determined.

Fabrication of a low alloy ultra-high strength (>1500 MPa yield) steel using powder bed fusion additive manufacturing

In this work, a low alloy ultra-high strength steel is fabricated using powder bed fusion (PBF) additive manufacturing (AM). PBF process parameters and scan strategy to manufacture this ferrous alloy from powder form to fully dense part were successfully developed. Specifically, a process map for laser processing of this material was determined by observing the shape and regularity of the melt pool as a function of laser powers and laser scan speeds using laser profilometry. In addition to processing, mechanical testing and metallurgical examinations using 2D optical and electron microscopy along with the 3D examination of the defect (porosity) structure using micro X-ray computed tomography have been carried out. Since the total volume of detectable pores found throughout the material was negligible, the AM fabrication of the steel is regarded as successful. AM samples were found to have a quasi-static yield strength greater than 1500 MPa, an ultimate strength exceeding 1800 MPa, and 20% elongation in the as-built condition. Significantly, these mechanical properties are superior over the same material in the as-cast conditions and are comparable to those of the wrought form of the material. Details pertaining to processing along with the characterization and testing results are presented and discussed in this paper.

Laser Heat Treatment of Low-Alloy Ultrahigh-Strength Steel Laser-Welded Joints

Laser Heat Treatment of Low-Alloy Ultrahigh-Strength Steel Laser-Welded Joints