Decrease In Upper Shelf Energy Research Articles

Effects of Si on the Ductile‐to‐Brittle Transition Behavior of As‐Cast Advanced High‐Strength Steels

High Si concentrations in third‐generation advanced high‐strength steels (AHSS) are known to cause cracking in continuous cast slabs during cooling at temperatures below 300 °C. To investigate the mechanism connecting Si to this embrittlement, impact toughness tests are conducted on as‐cast Fe–0.2C–3.0Mn steels with 0.5, 1.5, and 3.0 wt% Si at temperatures between 100 and 400 °C. The propagation path of the cracks through the microstructure of the specimens is examined. The ductile‐to‐brittle transition (DBT) behaviors of the three steels are compared. Higher Si concentrations raise the DBT temperature of the steels. The influence of Si on both solid‐solution strengthening and autotempering during cooling likely contributes to this by increasing the hardness of these as‐cast microstructures. In addition, the precipitation of pro‐eutectoid ferrite (αPE), which is promoted by higher Si concentrations, significantly lowers the upper shelf energy of the DBT curve. The αPE phase also alters the propagation path of brittle fracture in the specimens, potentially contributing to the increase in DBT temperature. The increase in DBT temperature and decrease in upper shelf energy may contribute to the as‐cast AHSS slab embrittlement at low temperatures observed in the industry.

Direct Observation of Cu Clusters and Dislocation Loops by Cs-Corrected STEM in Fe-0.6wt%Cu Alloy Irradiated in BR2

The neutron irradiation of Fe-based fusion and fission reactor materials leads to an increase in ductile-to-brittle transition temperature with a decrease in upper shelf energy. It is well known that Cu content has a strong influence on the embrittlement phenomenon, as Cu-rich clusters (CRPs) are thought to be directly responsible for embrittlement. In contrast, mechanical property studies for steels with different Cu levels exhibit dominant matrix defects in the embrittlement of both low-Cu steels and high-Cu steels at high fluences. To determine the effects of dislocation loops and CRPs on radiation hardening in those steels, neutron irradiation was conducted on Fe-0.6wt%Cu alloy. The neutron irradiation was performed in BR2 at 290 °C up to a dose of 4.1 × 1024 n/m2. After irradiation, the microstructure was observed and analyzed by spherical aberration-corrected transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) combined with X-ray energy-dispersive spectroscopy, using a JEOL ARM200FC. This technique enabled simultaneous observation of ~10 nm CRPs and dislocation loops. Additional high-voltage electron irradiation was performed at room temperature, and the dislocation loops were identified as interstitial-type dislocation loops. Radiation-induced hardening due to neutron irradiation was estimated by measuring the size and density of the dislocation loops and the CRPs. These results suggest that simultaneous observation of dislocation loops and CRPs using the Cs-corrected STEM with EDS analysis is essential for the study of radiation-induced hardening in Fe-based alloys.

Effect of thermal ageing on microstructure, tensile and impact properties of reduced activated ferritic-martensitic steel

Effects of thermal ageing on tensile and impact properties of India-specific Reduced Activated Ferritic-Martensitic (INRAFM) steel (9Cr-1.4W-0.06Ta-0.22V) have been studied by correlating with microstructure. The tempered martensitic steel received in the normalized and tempered (N&T) condition was thermally aged at 873K for 5000h. Thermal ageing was found to decrease hardness and tensile strength and the decrease was accompanied with increase in ductility. The steel displayed plateaus/humps in the temperature variation of yield stress and ultimate tensile strength and dip in ductility over the range 473–673K, which is considered due to the occurrence of dynamic strain ageing (DSA). The prevalence of DSA activity was found more on thermal ageing. Increase in ductile to brittle transition temperature (DBTT) and decrease in upper shelf energy (USE) of the steel were observed on thermal ageing. Detailed microscopic investigation of the steel revealed coarsening of martensitic sub-boundaries and M23C6 carbides along with the precipitation of Laves phase on thermal ageing. Coarsening of microstructural features on thermal ageing is considered to influence the reduction in strength and USE of the steel; whereas coarsening of M23C6 and precipitation of Laves phase is considered to cause the increase in DBTT. SEM fractographic study of the impact specimens revealed three distinct zones as shear lip, fibrous dimple and cleavage in an ordered away from the notch tip. With decrease in test temperature, the widths of the shear lip and fibrous dimple zones deceased whereas width of the cleavage zone increased progressively. Thermal ageing increased the widths of the shear lip and fibrous dimple zones and decreased the cleavage zone in the N&T steel at relatively higher temperatures but at lower test temperatures cleavage zone became more predominant. TEM study of the specimens extracted from the impact fractured samples at the vicinity of fracture surface, revealed substantial activity of dislocations manifested as dislocation debris in N&T steel even for impact testing at −80°C against individual dislocations for the aged steel, implying enhancement of DBTT on thermal ageing.

Influences of crystallography and delamination on anisotropy of Charpy impact toughness in API X100 pipeline steel

The anisotropic behavior of Charpy toughness has been investigated in API X100 pipeline steel, and the combination of crystallographic textures and delamination is considered to be the reason for the observed anisotropy in Charpy toughness. The crystallographic textures can contribute to anisotropy by increasing the probability of {001} cleavage planes parallel to the fracture surfaces of the Charpy impact specimens, and biasing available slip systems for ductile fracture. Thus, the Charpy impact toughness is worst at 45° to the rolling direction, because the grains with {001} cleavage planes parallel to the Charpy specimen fracture surfaces is much more at that orientation. The existence of much more {001} cleavage planes at the 45° orientation can lead to a greater probability of cleavage fracture compared with the other orientations at the same temperature. Delamination related to elongated grain structures and {001} grain planes parallel to the rolling plane also lead to anisotropy in Charpy properties. The deep groove shape cleavage crack induced by delamination in the upper shelf region is responsible for a decrease in upper shelf energy. Delamination as a part of the fracture energy absorption process in the ductile–brittle transition region also entails additional plasticity, resulting in an increase in impact energy. Therefore, delamination can also play a role in the anisotropy of Charpy toughness.

Deformation Behavior in Pipe Production Process for New High Strength Microalloyed Steel

The influence of forming process on mechanical properties and microstructure of high strength microalloyed steel has been studied in this research. All samples selected from 90º and 180º of pipes in transverse and longitudinal orientations after UOE forming. The steel was supplied as a hot rolled plate with accelerated cooling. Microstructure of the micro alloy steel was various combinations of acicular ferrite, granular ferrite and M/A phases. Charpy impact fracture toughness tests were carried out in -50 °C to 0 °C. Minimum and maximum yield strength appears in the 180º and 90º of pipe in longitudinal and transverse orientations respectively. Increasing in the yield strength related to minimum amount was about 5.8 % after UOE forming. Comparison between yield strength after and before forming appears increased about 6.9 % due to forming. With decreasing in the test temperature from -50 °C to 0 °C, fracture energy decrease up to 0.9 % that it’s very little and relinquishment. However for samples that fractured in the temperature of -50 °C difference between highest and lowest energy was about 5.9 %. Charpy test results appear that fine M/A phase not a major factor on decrease of upper shelf energy, if homogenize distributed in the acicular ferrite matrix.

Influence of Anisotropy Behavior in UOE Process for X80 Micro Alloy Steel

The anisotropy of tensile properties and charpy impact fracture toughness and its relationship with UOE pipe forming has been study on X80 micro alloy steel. All samples selected from 90º and 180º of pipes in transverse and longitudinal orientations after UOE forming. The steel was supplied as a hot rolled plate with accelerated cooling. Microstructure of the micro alloy steel was various combinations of acicular ferrite, granular ferrite and M/A phases. Charpy impact fracture toughness tests were carried out in -50 °C to 0 °C. Minimum and maximum yield strength appears in the 180º and 90º of pipe in longitudinal and transverse orientations respectively. Increasing in the yield strength related to minimum amount was about 5.8 % after UOE forming. Comparison between yield strength after and before forming appears increased about 6.9 % due to forming. With decreasing in the test temperature from -50 °C to 0 °C, fracture energy decrease up to 0.9 % that it’s very little and relinquishment. However for samples that fractured in the temperature of -50 °C difference between highest and lowest energy was about 5.9 %. Charpy test results appear that fine M/A phase not a major factor on decrease of upper shelf energy, if homogenize distributed in the acicular ferrite matrix.

Influence of irradiation on the ductile fracture of a reactor pressure vessel steel

The mechanical properties of 15Ch2MFA steel were characterised by tensile and instrumented Charpy tests. The fracture surfaces of Charpy specimens broken in the ductile-to-brittle transition temperature range contain a certain proportion of ductile fracture correlated to fracture energy. Measured ductile crack lengths show the same dependence on fracture deflection and/or fracture energy for irradiated and non-irradiated specimens. The decrease of upper shelf energy with increasing neutron fluence could be explained by an increasing amount of shear fracture.

Atom probe tomography characterization of radiation-sensitive KS-01 weld

The microstructure of a radiation-sensitive KS-01 test weld has been characterized by atom probe tomography. The levels of copper, manganese, nickel and chromium in this weld were amongst the highest of all the steels used in Western reactor pressure vessels. After neutron irradiation to a fluence of 0.8 × 10 23 n m −2 ( E>1 MeV) at a temperature of 288 °C, this weld exhibited a large Charpy T 41J shift of 169 K, a large shift of the fracture toughness transition temperature of 160 K, a decrease in upper shelf energy from 118 to ∼78 J, and an increase in the yield strength from 600 to 826 MPa. However, the mechanical properties data conformed to the master curve. Atom probe tomography revealed a high number density (∼3 × 10 24 m −3) of Cu-, Mn-, Ni-, Si- and P-enriched precipitates and a lower number density (∼1 × 10 23 m −3) of P clusters.

Secondary hardening and fracture behavior in alloy steels containing Mo, W, and Cr

In 4Mo, 6W, 2Mo3W, 2Mo2Cr, and 3W2Cr alloy steels, which contain alloying elements, such as Mo, W and Cr, contributing to the secondary hardening by forming M2C-type carbide, the secondary hardening and fracture behavior were studied. Molybdenum had a strong effect on secondary hardening, while W had a very weak effect on it but delayed the overaging. The MoW steel exhibited both moderately strong hardening and considerable resistance to overaging. On the other hand, the secondary hardening effect was diminished by the Cr addition, because the cementite of M3C type was stabilized at higher temperatures and the formation of M2C carbides was thus inhibited. Although the Cr addition had no merit in the secondary hardening itself, it eliminated the secondary hardening embrittlement (SHE). This was observed as a severe intergranular embrittlement due to the impurity segregation for the Mo and MoW steels and as a decrease in upper shelf energy for W steel, even in the overaged condition.

Service-induced Changes in the Microstructure and Mechanical Properties of a Cr-Mo-Ni-V Turbine Steel.

The effect of service temperature on the degradation of mechanical properties of a low alloy Cr-Mo-Ni-V steel applied for turbine rotor, retired after 15.8 years of service was evaluated and supplemented with the results from microstructural and fractographic examinations. Maximum embrittlement, manifested as decrease in upper shelf energy and increase in ductile-to-brittle transition temperature had occurred at a service temperature of 714 K and the material serviced at 811 K exhibited relatively better toughness. A re-aging treatment at 811 K for 24 h partially recovered the loss in toughness. The dominant carbides identified were M23C6 and M6C, enriched with chromium and iron, and molybdenum and iron, respectively and M2C with molybdenum and vanadium as the major metallic constituents. Coarsening of carbides occurred with increasing temperature through carbide reactions. The embrittled material showed the presence of coarse M23C6 carbides along the prior-austenite grain boundaries. However, material serviced at 811 K contained relatively less coarse grain boundary precipitates. Homogeneous precipitation of fine carbides of M 2 C type also occurred at this temperature through carbide reactions. Re-aging of the embrittled material led to partial dissolution of grain boundary carbides and also to copious precipitation of M2C type of carbide. Observed changes in mechanical properties are partly attributed to the in-service evolution of carbides and partly attributed to segregation of impurities at grain boundary.

Fractographic study of neutron irradiated pressure vessel steel

The presence of phosphorous as a residual element has been correlated with moderate to severe irradiation embrittlement of nuclear reactor pressure vessel steels (RPV's). The degree of radiation embrittlement is usually characterized by a shift in ductile-to-brittle transition temperature and a decrease in upper shelf energy, as obtained by Charpy V-notch impact testing. The mechanism by which phosphorous enhances the radiation sensitivity is unclear, however several possible mechanisms can be proposed. These include 1) radiation induced segregation to interfaces such as grain boundaries and particle/matrix interfaces 2) modification of possible radiation induced changes in the carbide microstructure; 3) radiation induced precipitation of phosphorous and 4) stabilization of defects produced by irradiation.