In the present study a Fe-8.8Mn-5.1Al-0.31C (wt pct) medium-Mn lightweight steel with martensitic matrix was newly designed, and the effect of tempering on its microstructure and tensile properties was investigated. When the medium-Mn lightweight steel specimen was quenched from annealing temperature of 1373 K (1100 °C), it revealed a triple-phase microstructure consisting of 64.6 pct fresh martensite, 20.2 pct ferrite, and 15.2 pct retained austenite. Tempering the as-quenched specimen at temperatures less than 573 K (300 °C) induced the precipitation of thin cementite or κ-carbide platelets in tempered martensite. With the increasing tempering temperature, the type of precipitate changed from cementite to κ-carbide, and thickening of platelets with the longer interspacing was observed by means of transmission electron microscopy and atom probe tomography. While the as-quenched specimen revealed high strain-hardening rate (SHR) and poor elongation (  16.8 pct) mainly due to gradual transformation-induced plasticity in retained austenite stabilized by tempering. In particular, the 373 K (100 °C)-tempered specimen with tempered martensite embedded with thin cementite platelets revealed ultrahigh strengths (yield strength of 945 MPa and ultimate tensile strength of ~ 1.56 GPa) with moderate elongation of ~ 16.8 pct.

Effect of Tempering on the Microstructure and Tensile Properties of a Martensitic Medium-Mn Lightweight Steel

The microstructures and mechanical properties of ferrite-based lightweight steel with different compositions were investigated by tensile test, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermodynamic calculation (TC). It was shown that the ferrite-based lightweight steels with 5 wt. % or 8 wt. % Al were basically composed of ferrite, austenite and κ-carbide. As the annealing temperature increased, the content of the austenite in the steel gradually increased, while the κ-carbide gradually decomposed and finally disappeared. The mechanical properties of the steel with 5 wt. % Al and 2 wt. % Cr, composed of ferrite and Cr7 C3 carbide at different annealing temperatures, were significantly inferior to those of others. The steel containing 5 wt. % Al, annealed at 820 °C for 50 s then rapidly cooled to 400 °C and held for 180 s, can obtain the best product of strength and elongation (PSE) of 31242 MPa · %. The austenite stability of the steel is better, and its PSE is higher. In addition, the steel with higher PSE has a more stable instantaneous strain hardening exponent (n value), which is mainly caused by the effect of transformation induced plasticity (TRIP). When the κ-carbide or Cr7C3 carbide existed in the microstructure of the steel, there was an obvious yield plateau in the tensile curve, while its PSE decreased significantly.

Microstructures and mechanical properties of ferrite-based lightweight steel with different compositions

The relevance of the research is caused by the necessity to determine the residual resource of the equipment with a sufficient degree of accuracy and in various circumstances. The development of mathematical models for assessing the mechanisms of damage accumulation and the actual state of the equipment will allow predicting the development of changes in the state of the object, considering the established patterns of damage mechanisms, and determining the moment of occurrence of the limit state. The aim of the research is to propose a method for assessing the residual resource of heat exchangers, considering expert intuitive-logical analysis of the problem with a quantitative assessment of judgments and processing of results. Objects: heat exchangers, expert conclusions on heat exchangers used in oil refining. Methods: Kullback–Leibler informative model, showing the degree of informativeness of individual parameters to the degree of discrepancy between the two classes of objects, diagnosing residual resource for the development of corrosion damage, diagnosing residual resource of equipment related to changed parameters of its technical condition. Results. The paper sets the task to assess the service life of the heat exchanging equipment after diagnosing on the allowed technical parameters depending on the total effect of the parameters of the technical condition of the equipment for oil and gas processing and petrochemistry. 10 technical and technological indicators of heat exchangers were selected, and the evaluation was performed by using such an indicator as the residual service life of heat exchangers. The authors developed a mathematical model, calculated diagnostic coefficients and informativeness of the indicator for each period accepted by the expert. An algorithm for estimating the residual service life of heat exchangers based on the Kullback informativeness measure is proposed. Estimates of the residual resource, out of the constructed model are shown. Conclusions about the rationality of using the proposed algorithm for solving problems of this type are made.

/pdf/vybor-opredeliaiushchikh-parametrov-tekhnicheskogo-xflz2x6fhb.pdf

Выбор определяющих параметров технического состояния, оказывающих влияние на остаточный срок службы теплообменного оборудования

Retained austenite (RA) transformation and its role in the mechanical properties of three low-carbon micro-alloyed quenching and partitioning (Q&P) steels was investigated utilizing in situ tensile tests and electron microscopy. Meanwhile, RA’s strain-induced martensite transformation (SIMT) was analyzed and discussed in terms of the strengthening mechanism. The results show that the ductility of the Q&P steels relies on the size and morphology of RA. In addition, both affect RA’s mechanical or thermostability. Dislocation density and carbon trapping should be considered in estimating the yield strength in the two-step Q&P process. V and Nb-Ti elements promote the formation of blocky RA. Ti accelerates the formation of film-like RA. For experimental Q&P steels with different processes and compositions, the true stress always keeps a linear relationship with the amount of transformed martensite, i.e., 30.38~46.37 MPa per vol. 1% transformed martensite, during the in situ tensile deformation.

/pdf/in-situ-observation-of-retained-austenite-transformation-in-35s4urh1.pdf

In Situ Observation of Retained Austenite Transformation in Low-Carbon Micro-Alloyed Q&P Steels

For its application development, the medium manganese lightweight steel with 3 wt.% and 10 wt.% Mn contents was galvanized in continuous hot dip galvanizing (HDG) simulator and the process parameters on the production line were adopted. Combined with the experimental analysis and thermodynamic calculation, the effect of dew point and alloy composition on the reactive wetting of the steel was investigated. It was shown that MnO existed as a stable oxide for the medium Mn steel with 5 wt.% Al as long as Mn content exceeded 5.1 wt.%. The galvanizability of the steel with 10 wt.% Mn was deteriorated resulting from the formation of a thick and continuous external MnO layer, which had adverse effects on the wettability. MnO particles in the form of unstable phase can be found at the surface of 3Mn steel galvanized at dew point +10 °C. It distributed sparsely and the reactive wettability can be obtained by “bridging connection”, which mitigated the damage of external oxidation. Moreover, the lower dew point, the less tendency to form external oxide. Although the decrease of dew point to −30 °C had a certain benefit for coating quality, the galvanizing quality of 10Mn steel could not be improved due to the formation of a thick MnO layer. Therefore, the Mn content played a stronger role than dew point on the reactive wetting of hot dip galvanized medium manganese lightweight steel.

https://www.mdpi.com/2079-6412/10/1/37/pdf

Effect of Dew Point and Alloy Composition on Reactive Wetting of Hot Dip Galvanized Medium Manganese Lightweight Steel

Microstructures and mechanical properties of Ti-V micro-alloyed TRIP (transformation-induced plasticity) steel with different compositions were investigated by tensile test, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermodynamic calculation (TC). The results indicated that the steel exhibited high ultimate tensile strength (1079 MPa), sufficient ductility (28%) and the highest product of strength and ductility (30212 MPa · %) heat treated after intercritical annealing at 800 °C for 3 min and bainitic annealing at 430 °C for 5 min. In addition, the change of volume fraction of retained austenite (VF-RA) versus tensile strain was measured using in-situ analysis by X-ray stress apparatus and micro-electronic universal testing machine. It was concluded that a-value could be used to evaluate the stability of retained austenite (S-RA) in the investigated Ti-V micro-alloyed TRIP steel. The smaller a-value indicated the higher stability of retained austenite (S-RA) and the higher mechanical properties of Ti-V micro-alloyed TRIP steel.

In-situ analysis of retained austenite transformation in high-performance micro-alloyed TRIP steel

A novel two-step cooling experiment was established to simulate the slow cooling process of continuous annealing production line for transformation-induced plasticity (TRIP) steel. The microstructures and mechanical properties of TRIP steel soaked at 700 °C for different time were investigated by tensile test, scanning electron microscopy, X-ray diffraction, and thermodynamic and kinetic calculation. It is shown that the steel soaked for 15 s exhibits the optimal product of strength and elongation (PSE > 30,000 MPa%) due to the transformation of austenite to proeutectoid ferrite, which delays the bainite transformation and improves the stability of retained austenite. In addition, the mechanical properties of TRIP steel soaked over 30 s are much lower, resulting from the precipitation of cementite, which decreases the stability of retained austenite and weakens the TRIP effect.

Effect of two-step cooling process on mechanical properties of TRIP steel with high performance

Introducing fine precipitates is an important way to enhance the properties of transformation-induced plasticity (TRIP) steels. In present work, two V-Ti microalloyed TRIP steels containing aluminum with different content were compared. The average size, size distribution and numbers of vanadium-titanium carbides in samples cold rolled, quenched after being held at 800°C and quenched after intercritical annealing at 800°C and being held at bainitic isothermal transformation temperature of 400°C were investigated by using the technique of carbon extraction replica, twin jet chemical polishing thinning and transmission electron microscopy. The carbides were identified to be (Ti,V)C precipitates in steel A and VC in steel B respectively, precipitated mainly from ferrites grains. The average equivalent radius was 3~6nm. Comparison of the experimental results in A and B steel revealed low carbon diffusion rate caused by aluminum inhibited the coarsening of vanadium-titanium carbides. The experimental results also showed that VC carbides dissolution occurred during the intercritical annealing at 800°C.

/pdf/quantitative-analysis-on-carbide-precipitation-in-v-ti-571gxo124l.pdf

Quantitative Analysis on Carbide Precipitation in V-Ti Microalloyed TRIP Steel Containing Aluminum

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Microstructures and mechanical properties of ferrite-based lightweight steel with different compositions

In-situ analysis of retained austenite transformation in high-performance micro-alloyed TRIP steel

Effect of two-step cooling process on mechanical properties of TRIP steel with high performance

Quantitative Analysis on Carbide Precipitation in V-Ti Microalloyed TRIP Steel Containing Aluminum