Effect of Particular Combinations of Quenching, Tempering and Carburization on Abrasive Wear of Low-Carbon Manganese Steels with Metastable Austenite

The effect of quenching from 900°C (20 min exposure) and different tempering in the 250-650°C (for 1 hour) interval, as well as additionally preliminary carburization for 8 hours at 930°C, followed by a similar heat treatment on abrasive and shock-abrasive wear of low-carbon manganese (10-24%Mn) steels, phase composition and mechanical properties was studied. It was confirmed that an increase in the manganese reduces the abrasive wear resistance and increases the impact-abrasive wear resistance. The expediency of carburization of low-carbon manganese steels is shown in order to obtain the residual austenite in the structure which amount and stability must be optimized in relation to specific abrasive impact characterized by the dynamic ratio with taking into account the chemical composition.

Tests on SENT and CT Specimens to Study Geometry Effects in the Brittle to Ductile Transition

The accurate prediction of ductile fracture behaviour plays an important role in structural integrity assessments of critical engineering structures under fully plastic regime, including nuclear reactors and piping systems. Many structural steels and aluminium alloys generally exhibit significant increases in fracture toughness, characterized by the J-integral, over the first few mm of stable crack extension (Δa), often accompanied by large increases in background plastic deformation. Conventional testing programs to measure crack growth resistance (J–Δa) curves employ three-point bend, SEN(B), or compact, CT. However, laboratory testing of fracture specimens to measure resistance curves (J–Δa) consistently reveals a marked effect of absolute specimen size, geometry, relative crack size (a/W ratio) and loading mode (tension vs. bending) on R-curves. These effects observed in R-curves have enormous practical implications in defect assessments and repair decisions of in-service structures under low constraint conditions. Structural components falling into this category include pressurized piping systems with surface flaws that form during fabrication or during in-service operation. A research program was launched by EDF R&D to study geometry effects (e.g. triaxiality effects) in the brittle to ductile transition of carbon-manganese steels using Single-edge notch tension (SENT) specimens, by comparing the results obtained on these specimens with the results obtained on CT specimens. This paper presents the results of the tests conducted between −40°C and −100°C on a large number of specimens of both types. The toughness values of the SENT specimens appear to be included in the scatter of the CT12.5 ones, so the geometry effect between CT and SENT specimens in the brittle to ductile region is not significant. Moreover, the results of the CT12.5 cut in the L-S direction are not very different of those of the specimens cut in the T-S direction. The Master Curve methodology fits rather well the CT12.5 results, whereas the SENT results are not well covered by this methodology. The energetic approach called GP has been applied to the analysis of some tests. This approach shows that the geometry effect between both types of specimens is limited, in agreement with the experimental observations.

Dislocations Gliding Study by IR Thermography in C-Mn Steels with Different Solute Atoms Content in the Gigacycle Fatigue Domain

Tests were performed on two Carbon-Manganese steels (A42 and A48 steels, French standard) in the gigacycle fatigue domain thanks to a piezoelectric fatigue machine working at 20000Hz. During the tests, temperature recordings were achieved by an infrared camera for various stress amplitudes. The main difference between the two steels compositions was the aluminum content (0.045% for the A42 steel and 0.004% for the A48 steel), and the carbon content (0.140% for the A 42 steel and 0.198% for the A48 steel). In the A48 steel, the few aluminum content induces a higher free content of solute nitrogen in the lattice. Mechanical spectroscopy tests were performed and gave qualitative results on the solute contents repartition in the lattice. The temperature increase recorded during the fatigue tests for the two steels are different at the beginning of the tests. The differences can be explained by the different repartition of the solute atoms which induces a different dislocation gliding between the two materials. At the end of the tests, the thermal recordings are similar and attributed to the evolution of the solute atoms repartition and the dislocation structure.

Influence of Cooling Path on the Dynamic Transformation of Austenite-to-Ferrite in Low Carbon Manganese Steels

For an effective thermomechanical process control, knowledge of the start and finish temperatures of the austenite transformation is critical. Continuous Cooling uniaxial-Tension (CCT*) is a useful way to measure these values. Therefore, the dynamic transformation of austenite to ferrite (γ → α) was investigated in C-Mn steels to understand the hot ductility behaviour of these steels after varying the cooling paths i.e. simulating “hard” and “soft” cooling rates of the strand during continuous casting. Results show that hard cooling into the dual phase (γ + α) region significantly increases the dynamic transformation temperatures due to a higher driving force owing to double transformation and precipitation of AlN during the process. A comparison of dynamic versus static transformation (Ar3 and Ar1) temperatures and equilibrium transformation temperatures gives a better understanding of the contribution of strain to transformation during casting or hot deformation.