Papers by Keyword: Cementite

Abstract: The effect of tempering temperature on the impact toughness of 0.3 mass% carbon martensitic steels with prior austenite grain (PAG) size of about 6 μm and 30 μm were investigated. Instrumented Charpy impact test (ICIT) method was used to evaluate the impact toughness. The tempering temperature of 723K gives the largest difference in the Charpy impact energy at room temperature (RT) between the specimens with two different PAG sizes. Investigation of the test temperature dependence of Charpy impact energy in the 723K tempered steels shows a steep transition at around 200 K for the 6 μm PAG specimen, while it shows a continuous slow transition in a wide range of temperature for the 60 μm PAG specimen. ICIT waveform analysis shows that the fracture propagation energy in stead of the fracture initiation energy mainly controls the temperature dependence of the impact energy. The carbide size distribution in these two specimens was investigated by SEM and TEM. The 60 μm PAG specimen shows the distribution of coarser carbides than does the 6 μm PAG specimen, which seems to be the main reason for the observed difference between them in the Charpy impact energy and the other properties of impact fracture.

Abstract: The brittle-to-ductile transition in fully pearlitic steels was investigated. The temperature dependence of the absorbed impact energy was measured with the blade speed of 0.0033 m/s. The absorbed energy per unit area increased with two steps as the test temperature increased: the first jump of around 10 kJm^-2 at 130 K and the second jump of around 22 kJm^-2 at 273 K. The twice jumps of the absorbed energy suggests a two-step brittle-to-ductile transition in fully pearlitic steels. SEM images of side surfaces exhibited that micro-cracks propagated through cementite lamellae at temperatures below only those of the second jump of the absorbed energy, suggesting that the deformability of cementite controls the second jump.

Abstract: Despite numerous investigations in the past, mechanism of cementite dissolution has still remained a matter of debate. The present work investigates cementite dissolution during cold wire drawing of pearlitic steel (~ 0.8wt% carbon) at medium drawing strain (up to true strain 1.4) and the role of dislocations in the ferrite matrix on the dissolution process. Quantitative phase analysis using x-ray diffraction (XRD) confirms more than 50% dissolution of cementite phase at drawing strain ~ 1.4. Detail analysis of the broadening of ferrite diffraction lines confirms presence of strain anisotropy in ferrite due to high density of dislocations (~ 1015m-2) at drawing strain 1.4. The results of the analysis shows that the screw dislocations near the ferrite-cementite interface are predominantly responsible for pulling the carbon atoms out of the cementite phase leading to its dissolution.

Abstract: Energy dispersive synchrotron diffraction has been carried out on cold drawn pearlitic steel wires. In this paper the observed cementite peaks are analysed. For a broad range of true drawing strains sin²(Ψ) curves have been measured. The residual stress in the cementite is found to saturate after reaching a maximum at a strain of about 1.6. No indication of significant texture development in the cementite could be observed. An explanation is given in terms of possible physical mechanisms. Peak broadening was observed at the early stages of deformation.

Abstract: nfluences of various heating processes on the amount of retained austenite after an intercritical annealing were investigated experimentally in a Mn-alloyed TRIP steel. The heating with a higher rate or an interruption at lower temperature can lead to the more rapid formation of austenite during intercritical annealing and hence more retained austenite in final microstructure. Such an enhanced austenitization kinetics is attributed to the rapid dissolution of fine carbides having previously precipitated out at low temperature during the heating, which is also confirmed by the numerical simulations.

Abstract: Iron ore reduction and carbon deposition in pure CO was investigated by using thermogravimetric (TG) method over the temperature range of 0-1200°C. The results of the work may be summarized as follows: in CO stream, carbon deposition occurred below 900°C, no carbon deposition was found above 1000°C. X-Ray analysis of the reacted sample indicated that the carbon deposition occurred with the iron was reduced. The iron reduction process and carbon deposition occurred simultaneously. The rate of carbon deposition changed with the transformation of iron oxides. The specific surface area and pore structure of reduced samples were analyzed. The specific surface area changed with the amount of carbon deposition.

Abstract: Effect of the laying head temperature and controlled cooling process on microstructure and mechanical properties of 72LXA wire rod were investigated．The results show that under the same cooling process，with the raising laying temperature and increasing sorbitizing rate and decreasing proeutectoid ferrite，the steel rod strength is improving，proeutectoid ferrite and sorbitizing rate are the critical impact factors on steel rod properties；indentifying cooling after perlite forming can restrain the dissolve of lamellar cementite；the mechanical properties of whole rod coil are improved by the proper rolling rate and air cooling．The high strength of 1050 MPa of steel rod was obtained，that shows the defined hot rolling process can conform to the steel rod properties requirement．

Abstract: Recent observations regarding the transformation of deformed austenite are reviewed. It is shown that superequilibrium ferrite and pearlite can be formed at temperatures well above the Ae₃ and Ae₁, respectively. The role of the stored energy associated with the introduction of the dislocations introduced by the deformation is discussed. It is shown that the forward dynamic transformation into ferrite and pearlite is several orders of magnitude faster than the reverse static transformation back into austenite. The retarding effect of alloying additions such as niobium is also outlined. The results are interpreted in terms of the effect of deformation on the modified phase diagrams pertaining to the transformation of deformed austenite.

Abstract: Recent observations regarding the transformation of deformed austenite are reviewed. It is shown that superequilibrium ferrite and pearlite can be formed at temperatures well above the Ae₃ and Ae₁, respectively. The role of the stored energy associated with the introduction of the dislocations introduced by the deformation is discussed. It is shown that the forward dynamic transformation into ferrite and pearlite is several orders of magnitude faster than the reverse static transformation back into austenite. The retarding effect of alloying additions such as niobium is also outlined. The results are interpreted in terms of the effect of deformation on the modified phase diagrams pertaining to the transformation of deformed austenite.

Abstract: No clear-cut information is available with regard to the effect of foreign atoms on the solubility limit of C in b.c.c. iron despite many previous studies. Against this backdrop, the influence of substitutional atoms (Mn, Cr, P, Si, Al) on the solubility limit of C in b.c.c. iron in equilibrium with cementite was investigated in low-carbon steels at a temperature of 700°C. In detail, the C solubility limit was determined from internal friction measurements combined with infrared analysis of C using a high-frequency combustion technique. It has been clarified that Mn, Cr, and Al hardly change the C solubility limit, whereas P and Si increase it. The thermodynamical calculation indicates that, under para equilibrium Si increases the C solubility limit and Mn hardly changes it, while under ortho equilibrium Mn and Si decrease it. However, the present experimental condition was verified to be close to ortho form. The discrepancy between the experiments and the calculations seems to come from the fact that: 1) single solute C atoms and the C atoms combined as Substitute-C complex are not distinguished experimentally, and 2) in the regular solution model, the non-uniform distribution of C atoms around alloying atoms is not introduced into the entropy term, which is something that should be studied further in the future.