Properties Of Ultra-high Strength Steel Research Articles

Investigation of Microstructure and Properties of Ultra-High Strength Steel in Aero-Engine Components following Heat Treatment and Deformation Processes

C250 steel, renowned for its remarkable strength and toughness, is extensively utilized in the aerospace industry for manufacturing critical components. This research investigates the microstructure and properties of forgings produced through different heat treatment temperatures, aging durations, and thermal cycling intervals. The results demonstrate that the samples were compressed at 1050 °C followed by air-cooling using the conventional maraging treatment. For the cycle heat treatment, temperature was maintained at 1050 °C and cycled 1–2 times, with a heat preservation period of 1 h and subsequent water-cooling. Solution heat treatment at a temperature of 1050 °C, aging for 5 h, and then air hardening were performed to achieve the best forging hardness. Interestingly, the solution time under age hardening conditions had no significant effect on the grain size but had a significant effect on the hardness of martensitic aged steel.

The effect of cutting fluid on high strain rate dynamic mechanical property and cutting force of ultra-high-strength steel

Due to the different composition of cutting fluid will affect the failure stress of the material in the shear deformation area, so the rational use of cutting fluid can optimize the cutting force and the surface integrity of parts. Four kinds of hat-shaped samples with shear band widths were designed. The effects of different cutting fluids on mechanical properties of ultrahigh strength steel at different shear strain rates were studied in this paper by SHPB, and verified by cutting experiments. The stress–strain curve, shear failure stress, fracture morphology and cutting force were studied systematically. Experimental results show that the stress remains stable for a period of time with the increases of strain after strain hardening at γs= 10.03 × 104 1/s. All shear fracture is ductile fracture and at γs<1 0.03 ×104 1/s, the dimple will be elongated with the increase of the strain rate. When the shear bandwidth is reduced from 100 μm to 50 μm, size effect occurs, and the failure stress is increased by nearly 30%. The Rehbinder effect of TRIM E709 is more significant, the shear failure stress is the smallest, Fx, Fy and Fz decreases by 23.93%, 33.08% and 50.29% compared with HY-103. This research can guide the high-quality machining of ultra-high strength steel and may also evaluate and develop new cutting fluids to improve cutting performance.

Phase transformation and mechanical properties of ultrahigh strength steels under continuous cooling conditions

The present work deals with phase transformation mechanical behavior and of newly developed ultrahigh strength steels. Bainitic and martensitic phase transformations have been studied in detail with respect to their transformation temperature range and microstructural evidences observed under various continuous cooling conditions. The various critical temperatures of Ac1, Ac3, Bs, Ms, and TNR of the investigated steels were initially calculated using numerical equations and validated with experimental results. The dilatometric tests consisted under different cooling rates from 0.1 to 100 °C/s and the respective continuous cooling transformation (CCT) diagrams have been determined. The microstructural features were revealed using optical and different electron microscopy techniques. A strong dependency of cooling rate on microstructure and mechanical properties has been found. The present steels have exhibited ultrahigh strength (1400–2000 MPa) with reasonable ductility (6–13% total elongation), which is attributed to combination of mixed microstructure comprising fine bainite and lath martensite along with thin film-like retained austenite. Fine precipitates of (Ti, Nb)C, Mo2C and/or Cu contribute to strengthening effect by increasing the hardness and strength values for a particular cooling rate. It has been witnessed that the hardness, yield strength and ultimate tensile strength have been increased with the rise of both cooling rate and carbon contents.

Effect of Material Properties of Ultra-High Strength Steels on Crashwothiness of M-Shape Hat-Square Part

Effect of Material Properties of Ultra-High Strength Steels on Crashwothiness of M-Shape Hat-Square Part

Effect of the Welding Thermal Cycles Based on Simulated Heat Affected Zone of S1300 Ultrahigh Strength Steel

In the automotive industry there is an increasing demand for the wider application of high strength steels due to their favourable mechanical properties. The steel producers continuously developing new generations of high strength steels to insure higher strength and toughness properties. Since in most cases these steels are joined in welded structures, great attention must be taken to their weldability. The weldability of high strength steels has still challenges which are as follows: cold cracking sensitivity; reduction of strength and toughness of heat affected zone (HAZ); filler metal selection. Because the mechanical properties of ultrahigh strength steels are provided by using various alloying elements, micro alloys, and by different metallurgical methods, the steels may lose their outstanding properties during welding. In real welded joints the critical parts of the HAZ have small extent so their properties can be limitedly analysed by conventional material testing methods. With the help of physical simulators, the different parts of the heat affected zone can be produced in an adequate size for subsequent tests. In our research work the weldability, especially the HAZ properties of an ultrahigh strength structural steel (Rp0.2 = 1300 MPa) were investigated on thermal simulated samples with the help of Gleeble 3500 physical simulator. Three relevant technological variants for gas metal arc welding (GMAW), t8/5 = 5 s, 15 s and 30 s were applied during the HAZ simulations in the selected coarse-grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse-grained (ICCGHAZ) zones. Both the microstructure was studied by optical microscope and the mechanical properties were analysed by Vickers hardness tests and Charpy V-notch impact tests at -40 °C. According to the results the investigated ultrahigh strength steel was softened on account of the welding heat cycles, besides that the strength of the investigated ultrahigh strength steel can be better with the application of shorter t8/5 cooling time.

Effect of hot stamping and quenching & partitioning process on microstructure and mechanical properties of ultra-high strength steel

In order to better achieve the purpose of automobile lightweight, hot stamping and quenching & partitioning (HS-Q&P) process has become a hot research focus. This process includes a hot stamping followed by a controlled quenching and partitioning process. In this paper, the effect of different austenitizing temperatures and time, as well as different holding pressure and partitioning temperatures and time on the microstructure evolution and mechanical properties of the tested steel was systematically studied. The resulting microstructure was characterized by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD), and the volume fraction of retained austenite and carbon concentration in austenite were measured by x-ray diffraction (XRD). The results show that, the microstructure of HS-Q&P steel was finer and smoother than that of Q&P steel. During the pressure holding and partitioning process, the carbon atoms diffuse from martensite to austenite, thereby improving the stability of the retained austenite. Because both retained austenite and ferrite can improve the product of strength and elongation (PSE), the PSE of two phase zone austenitized HS-Q&P steel is better than that of completely austenitized HS-Q&P steel. When the austenitizing temperature was 780 °C and the pressure holding time was 90 s, the HS-Q&P steel reached the best mechanical properties. Its tensile strength was 1350 MPa, elongation was 14.24%, and PSE was 19.22 GPa%.

Mechanical properties and microstructural evaluation of the heat-affected zone in ultra-high strength steels

This paper presents an investigation into the mechanical properties of ultra-high strength steels (UHSSs) (with nominal yield stress of 960 and 1100 MPa) after welding. Seven weld thermal cycles were simulated using a Gleeble 3800 thermal-mechanical machine. These cycles represented the temperature-time history of the joint at various distances from the weld fusion line (FL) in a typical gas metal arc welding (GMAW) process. The mechanical properties such as Vickers surface hardness, uniaxial tensile behavior and Charpy impact toughness were examined. Microstructural evaluation using field emission scanning electron microscopy (FESEM) was also conducted. According to the results, the S960 which was a direct-quenched type of steel, showed a considerable reduction in its hardness and tensile strength up to 29 and 32%, respectively. On the other hand, the S1100 which was manufactured via quenched and tempered process, showed only a minor degree of softening at far distances from the weld FL (up to 4%) followed by 2% tensile strength reduction, and hardening close to the FL (up to 13%). Microstructure analysis has been carried out to provide insight to the change of mechanical properties in UHSS after welding.

Effects of plastic deformation on austenite transformation in Fe-1.93Mn-0.07Ni-1.96Cr-0.35Mo ultra-high strength steel during continuous cooling

Dynamic continuous cooling-transformation (D-CCT) diagram is an essential research method to predict mechanical properties of ultra-high strength steel (UHSS) before material processing. In this study, D-CCT diagrams of Fe-1.93Mn-0.07Ni-1.96Cr-0.35Mo at different cooling rates of 0.5–15 °C s−1 were plotted by a combined method of dilatometry and metallography. Influences of deformation on microstructure, transformation behavior, and carbon diffusion were elucidated. Proeutectoid ferrite transformation behavior was discussed in detail based on strain induced nucleation kinetics. When the cooling rate increased above 1 °C s−1, the initial austenite grains were divided into blocks by newly forming complex microstructures, which were mainly consisted of martensite (M), bainite (B), deformation induced ferrite (DIF) and retained austenite (RA). Moreover, the decrease of proeutectoid ferrite and the increase of granular bainite (GB) resulted in the strength increased linearly at the cooling rate of 0.25–2 °C s−1. Increasing cooling rate has a remarkable effect on the tensile strength of the UHSS up to 1 °C s−1. Further increase in the cooling rate from 1 to 15 °C s−1, the results showed a relatively small effect on the intensity value. Besides, carbon diffusion from α-phase (face-centered-cubic microstructure) in DIF can enhance the stability of γ-phase (body-centered cubic microstructure) and delay the transformation of γ-phase to GB. Furthermore, attributed to the diffusion path of carbon decreases, the proeutectoid ferrite transformation was restrained, and γ-phases were transformed into GB as raising the cooling rate.

Effects of TMCP and QT on Microstructure and Properties of Ultrahigh Strength Steel

Abstract In the present study, new ultrahigh strength micro-alloyed steel has been developed for automotive and structural applications through thermo-mechanical controlled processing (TMCP) to compare mechanical properties with quenching and tempering (QT) heat treatment. The investigated steel shows the mixed structure of bainite and martensite in case of TMCP process and martensite and tempered martensite in case of QT process. The specimen processed at TMCP followed by air cooling has shown the best combination of mechanical properties, with more than 1650 MPa along with total elongation of >15%..

Analysis of fracture toughness in high Co–Ni secondary hardening steel using FEM

The microstructure and mechanical properties of ultrahigh strength steels (300M, Aermet100 and M54) were analyzed. The experiment results showed that the steels with austenite layers at the boundary of martensite laths had higher KIC than those without austenite layers. Finite element method (FEM) was used to establish models based on the actual microstructure of the steels. Extended finite element method (XFEM) was used to study the crack path and crack growth rate. Contour integral method (CIM) was used to study the best thickness of austenite layer, the effect of crack propagation direction and the value of KIC. According to the simulation results, 10–15nm was the best thickness of austenite layer for the fracture toughness of the steels with martensite as matrix. The simulation results of normalized value of KIC were close to the experimental results.

A Study on the Relationship between Hardness and Magnetic Properties of Ultra-High Strength Steel

Magnetic nondestructive testing methods including eddy current testing, magnetic memory, magnetic Barkhausen noise and so on are widely used in testing mechanical properties of ferromagnetic parts. Experiments were done to study the relationship between hardness and magnetic properties of ultra-high strength steel (UHSS), which plays an important role in automobile industry. The magnetic properties of UHSS specimens with different hardness were studied. The result shows that there is a strong connection between hardness and magnetic properties. Coercive force of UHSS is proportional to the hardness. This can be used to explain certain mechanism of magnetic nondestructive testing method such as connection between hardness and Barkhausen noise.

Ultimate Limit Load in Welded Joints and in Net Sections of High Strength Steels with Yield Stress 960 MPa

Ultra-high strength steels are becoming more common, particularly in lifting machinery and mobile equipment. However, elon- gation is a problem in ultra-high strength steels, which has been reported to be low in traditional material tests. Although in these tests, ultra-high strength steels have exhibited good local strain properties, which suggests that traditional material test procedures are inappropriate.In this paper, notched specimens were used to develop material models and to define failure criteria. The results were verified using welded components and tensile tests. The failure point and capacity levels were determined in welded and net section tests. We conclude that the strain properties of ultra-high strength steels are good, where the full capacity of the net section stress was reached in the tensile tests. The results also indicate that the current elongation requirements in different codes require adjustments or a new formulation.

Microstructures and Properties of Ultra-high Strength Steel by Laser Welding

Based on the experiments of laser welding of ultra-high strength steel (UHSS), joint morphology, microstructure, hardness and tensile-shear properties are investigated. The results show that the defects such as gas holes and cracks do not appear in the weld metal if the optimized parameters are employed. Microstructures of the weld metal and HAZ are mainly martensite, the widths of martensite lath aren't sensitive to the welding speed. However, the welding speed affects the grain sizes of prior autensite of the weld metal. When changing the welding speed from 20 mm/s to 35 mm/s, the mean widths of prior austenite grains decrease from 10.67 μm to 7.68 μm. In the HAZ, the hardness decreases apparently for the soft ferrite appears. The test results also show that the joint tensile-shear strength can satisfy the demand of production. Consequently, the laser welding quality of UHSS is reliable for manufacturing.

Determination of Thermal and Mechanical Material Properties of Ultra-High Strength Steels for Hot Stamping

Determination of Thermal and Mechanical Material Properties of Ultra-High Strength Steels for Hot Stamping

Current situation and properties of ultra-high strength steel for automotive use in Japan

Ultra-high strength steels with a tensile strength of 1,000 MPa to 1,200 MPa have been used for bumper and door beam since 15 years in Japan. Recently, it has been tried to apply these steels to reinforcements such as B-pillars inner and so on. The application of these types of steel tends to spread. Their formability, weldability and fatigue properties are described. Formability of ultra-high strength steel is different from that of mild steel, and it is governed by local elongation or stretch flangeability that can be evaluated by bendability. Local elongation or stretch flangeability has a negative correlation with total elongation. When strength exceeds 1,000 MPa, delayed fracture tends to occur. However, when measures to avoid delayed fracture are taken, there are no application examples of ultra-high strength steel sheet with 1,500 MPa tensile strength restricted by the danger of delayed fracture.