Effect of Deformation on Microstructure and Mechanical Properties of Medium Carbon Steel During Heat Treatment Process

The current research of the Q-P and Q-P-T process has been focused on controlling the heating temperature and holding time, or adding alloy elements into the steel to induce precipitation strengthening and improve the strength and plasticity of the steel. In this article, based on a quenching-partitioning-tempering (Q-P-T) process combined with a hot deformation technology, a deforming-quenching-partitioning-tempering (D-Q-P-T) process was applied to medium carbon steel. The effect of the heat treatment parameters on the microstructure and mechanical properties of experimental steel under deformation was studied. Through use of a scanning electron microscope (SEM), transmission electron microscopy (TEM) and tensile tests, the optimal heat treatment conditions for realizing high strength and plasticity that meet the safety requirements were obtained. The mechanism for the D-Q-P-T process to improve the strength and plasticity of experimental steel was discussed. A multiphase composite structure of lath martensite and retained austenite was obtained. Compared with the Q-P-T process, use of the D-Q-P-T process can increase the strength of steel by 57.77 MPa and the elongation by 5%. This study proposes a method to improve the strength and plasticity of steel.

Deformation Mechanism Investigation on Low Density 18Mn Steels under Different Solid Solution Treatments

To meet the demand of the 10% weight reduction goal for automotive steel, the microstructure and mechanical properties of Fe-18Mn-Al-C steel with different carbon and aluminum contents were investigated under different solid solution treatments, and the deformation mechanisms of the experimental steels were elucidated. Aided by thermodynamic calculation, transmission electron microscopy (TEM) and in situ scanning electron microscope (SEM) analysis, it was shown that for the 18Mn-1.5Al experimental steel with about 20 mJ/m2 stacking fault energy (SFE), the twinning-induced plasticity (TWIP) effect always dominated in this steel after different solid solution treatments under tensile deformation. With the 7 wt% aluminum addition, the SFE of austenite was affected by temperature and the range of SFE was between 60 and 65 mJ/m2. The existence of δ-ferrite obviously inhibited the TWIP effect. With the increase in the solution treatment temperature, δ-ferrite gradually transformed into the austenite, and the n-value remained low and stable in a large strain range, which were caused by the local hardening during the tensile deformation. Due to the difference in the deformability of the austenite and δ-ferrite structure as well as the inconsistent extension of the slip band, the micro-cracks were easily initiated in the 18Mn-7Al experimental steel; then, it exhibited lower plasticity.

Corrosion Behavior of Corrosion-Resistant Spring Steel Used in High Speed Railway

Corrosion behavior of 60Si2Mn-A and 60Si2Mn-B in simulated industrial atmospheric environment was investigated by alternate immersion corrosion test and electrochemical method. The phase, morphology, characteristics of corrosion products, and the distribution of Cr, Cu, and Ni in the corrosion products of experimental steel were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalyzer (EPMA). The results show that the phase of rust layer is Fe3O4 and γ-FeOOH in the early stage and then changes to α-FeOOH and γ-FeOOH in the later stage; the size of the rust layer with corrosion resistance of 60Si2Mn is less than 60Si2Mn; the Cr element accumulates in the rust layer of the experimental steel in the early stage of corrosion resistance; and Cu, Ni, and Cr in the corrosion resistance 60Si2Mn are concentrated in the rust layer near the substrate In the later stage of corrosion. As the corrosion cycle is prolonged, the corrosion potential and the resistance of the rust layer of the experimental steel increases, and the corrosion current decreases; in the same corrosion cycle, the corrosion potential and corrosion resistance of 60Si2Mn-B are greater than 60Si2Mn, and the corrosion current is less than 60Si2Mn.

Microstructure Evolution and Mechanical Properties of 67GPa•% Grade Medium Manganese Steel

In this study, a new Fe-6Mn-4Al-0.4C high strength medium manganese hot rolled steel sheet was designed. The influence mechanism of the intercritical annealing (IA) temperature on microstructure evolution and mechanical properties of experimental steel were studied by SEM and XRD. The experimental steel was held for 30 minutes at 640°C, 680°C, 720°C, 760°C, 800°C, respectively. When the annealing temperature was 640°C, cementite particles precipitated between the austenite and ferrite phase boundary. As the annealing temperature increased, the cementite gradually dissolved and disappeared, the fraction of lamellar austenite increased significantly. When the annealing temperature is 800°C, the coarse equiaxed austenite and ferrite appeared. The yield strength (YS) decreased, the product of strength and elongation (PSE) and total elongation (TE) both increased first and then decreased, while the ultimate tensile strength (UTS) showed the opposite trend. The experimental steel exhibited excellent comprehensive mechanical properties after held at 760°C for 30 min. The UTS was 870 MPa, the YS was 703 MPa, and the TE was 77 %, the PSE was 67 GPa·%.

Study of corrosion resistance of pump-compressor pipes made of ferrite-pearlite steel under the action of carbon dioxide mediums

To ensure required complex of steel strength properties, pipes heat treatment is widely used as a quenching for martensite followed by tempering. It is accepted, that metal microstructure formed at that, has a little effect on a steel corrosion resistance in carbon dioxide mediums. Studies were accomplished of steel microstructure effect of the carbon dioxide corrosion speed. In particular, a low-alloyed structural steel, used for production of oil field pipes was selected. As the object of the study, seamless pumpcompressor pipes (PCP), made of medium-carbon steels of several grades with chrome content about 1% (mass.), subjected to two methods of heat treatment: quenching with tempering or normalization with tempering. Corrosion resistance of the steels were accomplished under laboratory conditions of JSC “RusNITI” by application of autoclave facility imitating operation conditions parameters of a working medium at oil extraction. The speed of general metal corrosion was measured as samples mass loss during tests (from 360 to 70 hours) by gravimetric method. By the results of laboratory tests the steel 35ХГФА was recommended to the industrial production of PCP of K72 and N80 strength groups due to GOST 31446 with normalization and tempering. It was established that pipes of steel 35ХГФА with ferrite-pearlite microstructure have higher corrosion resistance in mineralized water mediums at carbon dioxide partial pressure up to 0.20 atm, comparing with regular pipes, manufactured with quenching and tempering. A complex of field tests of PCP was carried out, the pipes made of experimental steel 35ХГФА at an oil deposit. The result of operation of CPC of 88.9×6.45 mm made of steel 35ХГФА having K72 group of strength (after normalization with tempering) during 550 days under conditions of high-mineralized water phase at СО2 partial pressure up to 0.27 atm was found positive. After an intermediate inspection (550 days) the column of PCP was assembled and is in operation at present, the current running time exceeding 1000 days. Based on the results of the complex comparative tests carried out in the corrosion laboratory and under field oil extraction conditions it was established that usage of experimental steel 35ХГФА containing 1% of Cr, after application of normalization technology with tempering resulted in increasing of operating reliability of CPC.

Microstructure, Hardness, and Tensile Properties of Vacuum Carburizing Gear Steel

We investigated the effects of the austenitizing temperature on the microstructure, hardness, and tensile properties of case-carburized steel after vacuum carburization at 930 °C and then re-austenitization at 820–900 °C followed by oil quenching and tempering. The results show that fractures occurred early with the increase in the austenitizing temperature, although all the carburized specimens showed a similar case hardness of 800 HV0.2 and case depth of 1.2 mm. The highest fracture stress of 1919 MPa was obtained for the experimental steel when the austenitizing temperature was 840 °C due to its fine microstructure and relatively high percentage of retained austenite transformed into martensite during the tensile tests. We also found that the stress–strain behavior of case-carburized specimens could be described by the area-weighted curves of the carburized case and the core in combination. The strain hardening exponent was about 0.4 and did not vary with the increase in the austenitizing temperature. We concluded that the optimum austenitizing temperature was around 840 °C for the experimental steel.

The effect of M (M = Ce, Zr, Ce–Zr) on rolling microstructure and mechanical properties of FH40

Ce, Zr and Ce–Zr composite experimentl steel were prepared by vacuum induction furnace and 550 twin-roll reversible rolling mill. Optical microscope (OM), scanning electronic microscopy (SEM) and energy dispersive spectrometer (EDS) were used to observe the rolling microstructure of the experimental steel. The mechanical properties of the experimental steel were tested and analyzed. The effect of cerium zirconium oxide inclusions on nucleation, tensile and impact fracture mechanism of intragranular acicular ferrite (IAF) was investigated. The results show that the rolling microstructure of steel containing 0.0052% Ce and the steel under composite treatment containing 0.0053% Ce and 0.0055% Zr is refined. IAF generation can be induced by Al–Ce–O inclusion of the size of 4 µm or induced by Al–Ce–Zr–O + MnS inclusion of the size of 3 µm. The yield strength and tensile strength of the steel treated by Ce–Zr are 428 and 590 MPa, respectively, the elongation is 23.55%, the longitudinal impact energy at −60°C is 189 J, which are 31, 45, 46 J and 6.25%, respectively, higher than those of the matrix steel. The dimple of the experimental steel at the fracture surface is larger and deeper than that of the matrix steel. The small inclusions in uniform distribution contribute to the high tensile strength of the experimental steel.

Hot Deformation and Dynamic Recrystallization Behavior of Fe-8Mn-6Al-0.2C Steel

Hot deformation and dynamic recrystallization (DRX) behaviors of Fe-8Mn-6Al-0.2C steel were investigated by means of single-pass compression experiments at temperature ranging from 850 °C to 1150 °C and different strain rate of 0.01~10 s-1. The flow strain curves were analysed and showed that the higher temperature and lower strain rate led to the smaller critical stress, which meant softening mechanism was easier to occur; The constitutive equation of experimental steel was established and the relationship between peak strain and Z parameter was figured out; A hot processing map of experimental steel was developed over a power dissipation map for the typical strain of 0.9, indicating the best region of high workability in the study conditions. DRX was promoted with increase of deformation temperature and decrease of strain rate, and the banded grains had been broken and grew up obviously.

Microstructure and Mechanical Properties of X70 Pipeline Steel with High H2S Resistance

Anti-H2S X70 pipeline steel was developed. The microstructure of X70 pipeline steel was studied by the analysis of OM, SEM and TEM. The precipitation behavior was discussed. The comprehensive mechanical properties, HIC and SCC performance were systematically studied. The results indicated that the microstructure of the experimental steel was mainly acicular ferrite and granular bainite. The second phase precipitates dispersedly distributed in the matrix. The experimental steel possessed excellent strength, plasticity, low temperature toughness and low yield ratio. And therefore, the X70 pipeline steel in the study is suitable for sour service with the high strength, excellent toughness and low HIC&SSC susceptibility.