Formation Of Austenite Grains Research Articles

Partitioning temperature-dependent microstructures and mechanical properties of precipitation-hardened medium-Mn steel

This study reports the partitioning temperature-dependent microstructures and mechanical properties of precipitation-hardened medium Mn steel processed by room-temperature quenching and partitioning. The hot-rolled Fe–7Mn-0.3C–3Ni-1.5Si–1Al–1Cu-0.3V (wt.%) steel was processed by austenitization, water quenching to room-temperature, and partitioning treatment at 200, 300, 400, and 500 °C for 600 s. The samples before and after the partitioning process at temperatures below 400 °C show a microstructure consisting of lath martensite and retained austenite, whereas the sample partitioned at 500 °C shows a notably high fraction of retained austenite, owing to the formation of austenite grains during the partitioning process. The C partitioning during the partitioning process led to higher C contents in the samples processed by austenitization, quenching, and partitioning than in the sample processed by austenitization and quenching. The co-precipitation of VC precipitates and dispersed fine particles could exhibit a precipitation-strengthening effect. The sample partitioned at 200 °C shows an excellent strength-ductility combination, i.e., a yield strength of 740.9 MPa, an ultimate tensile strength of 1623.6 MPa, and a total elongation of 23.8%. Film-like retained austenite stabilized by carbon partitioning could exhibit high mechanical stability, leading to a gradual transformation induced plasticity (TRIP) effect and superior mechanical properties. In contrast, the samples partitioned at temperatures above 400 °C show poor ductility with brittle fracture, which could be related to the formation of martensite/retained austenite (M/A) constituents.

Features of formation of austenite grains in 12 % Cr heat-resistant ferritic-martensitic steels

Ferritic-martensitic heat-resistant high-chromium steels (FMHS) with chromium content of 11 – 12 % are quenched to martensite from temperatures of 1050 – 1100 °С. Possible undesirable consequences of heating to such high temperatures are an increase in the size of austenite grains, increase in the amount of delta ferrite in the final structure, and a decrease in mechanical characteristics. In this work, the change of all these factors during heating of FHMS to quenching temperatures in the range of 950 – 1250 °С was studied. Ratios of the contents of martensite (its amount was identified with the proportion of austenite before quenching) and high-temperature delta ferrite on metallographic sections were analyzed. It was found that behavior of structure of the studied FHMS upon heating to temperatures of 1150 °С and above depends on the steels structural class. In steels whose structure at room temperature consists of martensite and delta ferrite, or in which delta ferrite begins to form at heating temperatures of 1200 °С and higher, size of austenite grain decreases with increasing temperature in the range of 1200 – 1250 °С, and the amount of delta ferrite – increases. Such structural transformations can be associated with features of the phase equilibrium diagrams of steels of this class. Such structural transformations can be associated with a change in the position and (or) inclination of boundaries of the high-temperature region of coexistence of austenite and delta-ferrite in the phase equilibrium diagrams of FHMS at a change in heating temperature in this range. Compression tests at 20 °С of 15Cr12Mn3SiMoW2VB steel samples after heat treatment with heating to temperatures for hardening 1000 – 1250 °С showed that formation of an additional amount of delta ferrite at temperatures above 1200 °С is a stronger factor than the refinement of austenite grains. This causes a decrease in yield strength of the samples quenched from these temperatures followed by high tempering.

On the role of carbonitrides in formation of austenite grains during continuous hot rolling of pipe steel

A study of the formation of austenite grain size during rolling of a thick strip on a continuous mill by simulating multi-stage rolling on a Gleeble complex has been performed. It is shown that at low degrees of deformation in the stands of the finishing group of a rolling mill, the formation of a different-grained austenite structure can occur due to selective grain growth without the formation of recrystallization nuclei, despite the precipitation of carbonitrides of microalloying elements. The value of the difference between the deformation strengthening of neighboring grains, sufficient for the grain boundary to overcome the retardation caused by the carbonitride particles, has been determined. It is possible to avoid this negative phenomenon by increasing the degree of deformation in each pass and ensuring the conditions for the formation of recrystallization nuclei. An example of a similar phenomenon observed when rolling beryllium is given.The research was carried out with the financial support of the Ministry of Education and Science of Russia as part of the implementation of the program of the world-class Scientific Center in the direction of "Advanced Digital Technologies" of SPbPU (agreement dated April 20, 2022 No. 075-15-2022-311).

Improvement of the Mechanical Characteristics, Hydrogen Crack Resistance and Durability of Turbine Rotor Steels Welded Joints

This article is devoted to the following issues: calculating the values of temperatures obtained by simulating welding heating and the subsequent implementation of the welding process at the given mode parameters made it possible to obtain a welded joint of the rotor with an improved initial structure and increased mechanical properties, hydrogen resistance and durability by up to 10–15%; simulating welding heating in the areas of fusion, the overheating and normalization of the HAZ and the formation of austenite grains; specified welding heating creates the conditions for the formation of new products of austenite decomposition in the form of sorbitol in the area of the incomplete recrystallization of the HAZ. In air and gaseous hydrogen, the destruction of the combined joints took place on the weld metal, as well as on the fusion areas, the overheating and the incomplete recrystallization of the HAZ of 20H3NMFA steel as the base metal. Structural materials have a relatively low strength and high fracture toughness in air. This is manifested in a significant reduction in the elongation (δ), the area (ψ) and critical stress intensity factor (KIc) of welded joints and the endurance limit of cylindrical smooth rotor steel specimens, which were cut from transverse templates. Welded joints in the whole range of load amplitudes are sensitive to the action of hydrogen.

Special Features of Formation of Austenite Grains in Steel 55

Special Features of Formation of Austenite Grains in Steel 55

A new insight into annealing parameters in tailoring the mechanical properties of a medium Mn steel

Generally, the mechanical properties of medium Mn steels are mainly tailored by adjusting temperature and duration of intercritical annealing (IA). However, in the present study, it is found that the temperature-rise-period in IA also plays a remarkable role in the control of the mechanical properties of the investigated medium Mn steel. A longer temperature-rise-period is conducive to the formation of austenite grains at different temperatures, resulting in more heterogeneous distribution of Mn element within an individual austenite grain and among different austenite grains. This heterogeneous microstructure leads to the difference in the yield strength among austenite laths and a slow strain-induced-martensite-transformation kinetics. Eventually, the yield strength and total elongation of the investigated steel has been significantly increased while high ultimate tensile strength is maintained. This study provides a new insight into comprehensively understanding the role of annealing parameters so as to improve the mechanical properties of medium Mn steels in industrial production.

Grain Refinement of Austenite through Reverse Transformation from Martensite in Fe-15Ni-Co-Mo-Ti Alloys

The effects of 5% Mo, 15% Co, or 2% Ti addition to Fe-15%Ni-10%Co-5%Mo base alloy on grain refinement of reversed austenite subjected to single austenitizing were studied at heating rates between 0.083 and 100°C/s. Grain refinement was promoted by Co or Ti addition and was suppressed by Mo addition. Grain refinement was promoted in the base, (base+15% Go) and (base+2% Ti) alloys at the lower heating rates, whereas it was suppressed in the (base+5% Mo) alloy. Refining processes depended on chemical compositions and heating rates, and were classified into the following two groups in terms of the differences in the initial stage of formation of austenitic grains:(1) After completion of reverse transformation, austenitic grains were formed due to recrystallization of austenite.(2) During reverse transformation, austenitic grains were formed probably by the nucleation and growth of diffusionally transformed austenite.It was shown that the mechanisms of reverse transformation associated with chemical compositions and heating rates played an important role in these refining processes.

A direct electron-microscopic investigation of the processes of austenitizing of 60F1 steel

The formation of austenitic grains in 60F1 steel consists of two stages, recreation of the grains after the previous heating (coarse with overheating) and their refinement as a result of recrystallization. Recrystallization of the austenite of 60F1 steel occurs at temperatures of Ac3+(70–90)°C, i.e., at 870–890°C.

Effect of inheritance in deformed steel 20Kh

1. After deformation at 600–800° the austenite grain size resulting from heating with double phase recrystallization is smaller than in the normalized steel. This indicates that the structure of the deformed steel is resistant and affects the formation of austenite grains during heating. 2. The finest grains of austenite are formed after preliminary deformation at 700°, which leads to considerable pulverization and spheroidization of cementite particles. 3. The higher strength and ductility of the deformed steel in comparison with the normalized steel after heat treatment are evidently due to austenite grain refining and inheritance of defects occurring during phase transformations.

On the Formation of Austenite Grains from Prior Martensitic Structure

On the Formation of Austenite Grains from Prior Martensitic Structure

Some Observations on Formation of Austenite Grains

Some Observations on Formation of Austenite Grains

Phase recrystallization of steels alloyed with molybdenum

1. The initial crystallographically ordered structure produced after martensite or bainite transformation is retained during subsequent heating up to the temperature at which there is formation of grains with high-angle boundaries, detected in the fracture. 2. Numerous intragranular interfaces obtained after initial martensite quenching, on which the dense carbide chains are predominantly found, prevent further disorientation of the austenite forming through advancement of the diffusion front deep into the ferrite plate absorbed by it. Further disorientation of the low-angle austenite plates, produced until the formation of independent grains with high-angle boundaries, is delayed until a higher heating temperature at which there is dissolution of the bulk of the carbide particles deposited on the ferrite (former martensite) plate boundaries. 3. As the amount of molybdenum in the steel is increased, given the same amount of carbon, there is sharp increase in the resistance of carbide particles to dissolution in austenite, on account of the increased molybdenum in them. There is a corresponding increase in the temperature difference between Ac3 and the temperature at which the carbide particles dissolve and grains detectable in the fracture are formed. 4. In steel with an initial crystallographically ordered structure, the heating rate has a substantial effect on the temperature at which there is absorption of the ferrite plates and carbide particle chains creating suitable conditions for the formation of polyhedral grains. The heating rate affects the composition of the ferrite and carbides as well as their size, shape, arrangement, and conditions for dissolving carbides, and, therefore, also influences the effectiveness of phase recrystallization through nonuniform grain growth during slow heating. 5. Dissolution of carbides and formation of austenite grains in initially quenched steels is completed at higher temperatures than in annealed steels through slowing down of the diffusion processes under the influence of intragranular interfaces. 6. The arbitrary process of austenite grain formation occurring in initially quenched steel after the carbide particles have dissolved at temperatures considerably above Ac3 leads to a reduction in the free surface energy of the system.