Characteristics and Kinetics of Bainite Transformation Behaviour in a High-Silicon Medium-Carbon Steel above and below the Ms Temperature

This work deals with the kinetic aspects of bainite formation during isothermal holding above and below the martensite start (Ms~275 °C) temperature using a low-alloy, high-silicon DIN 1.5025 steel in a range suitable for achieving ultrafine/nanostructured bainite. Dilatation measurements were conducted to study transformation behaviour and kinetics, while the microstructural features were examined using laser scanning confocal microscopy and electron backscatter diffraction (EBSD) techniques combined with hardness measurements. The results showed that for isothermal holding above the Ms temperature, the maximum bainitic transformation rate decreased with the decrease in isothermal holding temperature between 450 and 300 °C. On the other hand, for isothermal holding below the Ms temperature at 250 and 200 °C, the maximum rate of transformation was achieved corresponding to region I due to the partitioning of carbon and also possibly because of the ledged growth of isothermal martensite soon after the start of isothermal holding. In addition, a second peak was obvious at about 100 and 500 s, respectively, during holding at 250 and 200 °C due to the occurrence of bainitic transformation, marking the beginning of region II.

The strain variation during the isothermal B2→B19’ martensitic transformation in the quenched or annealed Ni51Ti49 alloy

The strain variation during the isothermal holding under constant stress was studied in the quenched or annealed Ni51Ti49 alloy samples. The isothermal strain variation was found in both samples and this strain was completely recovered on subsequent unloading and heating. This allowed to conclude that the strain variation on holding was caused by the isothermal martensitic transformation. It was found that the maximum value of isothermal strain depended on the alloy heat treatment. This value was equal to 0.5 % in annealed sample and it was equal to 6 % in quenched sample. It was assumed that the formation of the Ni4Ti3 phase during annealing led to a decrease in concentration of substitutional Ni atoms in NiTi phase that were responsible for the isothermal transformation. As a result, the less volume fraction of the martensite formed during holding that supresses the strain variation in annealed samples.

Influence of Austempering of As-Cast Medium Carbon High-Silicon Steel on Wear Resistance

The aim of this study was to evaluate the effect of austempering compared to quenching and low-temperature tempering on wear resistance of an as-cast medium carbon high-silicon steel intended for rock breaking. Austempering was done by isothermal holding at 270, 300 and 350 °C in molten salt baths, while quenching was done in water. The austempering treatments resulted in microstructural combinations of bainite and martensite. The isothermal holding at 270 °C resulted in bainite and self-tempered martensite, while isothermal holdings at 300 and 350 °C resulted in bainite and untempered martensite. The two quench and temper treatments resulted in tempered martensite. In general austempering resulted in lower hardness values when compared to quenching and tempering but higher impact toughness. The wear resistance was best for quenching and low temperature tempering, followed by austempering at 270 °C, but at slightly lower hardness and 25% higher impact toughness. The other two austempering treatments resulted in worse wear resistance.

Laboratory- and Semi-Industrial-Scale Thermomechanical Processing of TRIP-Aided Steel with Acicular Ferrite

The modification of the deformation and cooling methods resulting in the obtainment of acicular ferrite promotes an increase in the proportion of retained austenite (RA) and a corresponding increase in mechanical properties in Si-Al TRIP-aided steel. The effect of controlled thermomechanical processing in laboratory- and semi-industrial scales on the possibility of obtaining acicular ferrite and a high fraction of retained austenite was investigated. The steel was hot deformed in three steps: at 1050, 900 and 750 ˚C to introduce dislocations into the hot-deformed pancake austenite. Next, slow cooling in a ferritic transformation region was performed, followed by isothermal holding of steel at 450 ˚C. The interrupted tensile tests at the strain levels of 5, 10 and 15% were performed to investigate the mechanical properties response and the stability of the obtained retained austenite. Light and scanning electron microscopy, XRD and EBSD analyses were performed to assess microstructural features. The produced material showed a multiphase microstructure containing acicular ferrite and 10% of retained austenite. The microstructures obtained in both production methods were slightly different due to high temperature inertia in the semi-industrial process.

Development of a Prototype Plant for the Heat Treatment of Workpieces with Preferably Bainitic Properties

In vehicle construction, components with high tensile strengths are used, especially in the chassis area. At the same time, these components must have high toughness and be insensitive to cracking. For this purpose, hardened and tempered but also salt-bainitized components are used. The associated usual process chain after steel production consists of forming processes with subsequent cooling of the forging blanks and subsequent heat treatment with renewed heating to set the required material properties. From an energy point of view, heat treatment from the forging heat is desirable, which in addition to shortening the process chain is also associated with a reduction in CO2 emissions. A prototype system for controlled bainitization has been developed, which implements the heat treatment immediately after hot forming by utilizing the still existing forming temperature. Here, a controlled spray field generates both a quenching and an isothermal holding phase. Various sensors generate input variables to cool the workpieces in a controlled manner. This paper gives an overview of the system technology, realized cooling curves and the resulting hardness.

Effects of Microwave-Assisted Heat Treatments on the Nanostructural Evolution of Amorphous Silicon Oxycarbide Thin Films

This study investigated the effects of heat treatment on changes in the nanostructure of amorphous silicon oxycarbide thin films. Hydrogenated amorphous silicon oxycarbide (a-Si0.6C0.3O0.1:H) thin films were prepared via plasma-enhanced chemical vapor deposition. The films were subjected to post-deposition heat treatments via microwave-assisted heating, which resulted in the formation of nanocrystals of SiC and Si in the a-Si0.6C0.3O0.1:H matrix at temperatures as low as ~800 °C. The crystallization activation energies of SiC and Si were determined to be 1.32 and 1.04 eV, respectively lower than those obtained when the sample was heat-treated via conventional heating (CH). Microwaves can be used to fabricate nanocrystals at a temperature approximately ~300 °C lower than that required for CH. The optical and nanostructural evolutions after post-deposition heat treatments were examined using photoluminescence (PL) and X-ray diffraction. The position of the PL peaks of the nanocrystals varied from ~425 to ~510 nm as the annealing temperature was increased from 800 to 1000 °C. In this study the optical band gap of SiC and Si varied from ~2.92 to ~2.40 eV and from ~2.00 to ~1.79 eV, as the size of the SiC and Si nanocrystals varied with respect to the heating temperature and isothermal holding time, respectively.

Cast steel 4Kh4N5M4F2 for hot pressing mold of copper M1 and aluminum alloy AK7ch

The mode of quenching and tempering of the investigated 4Kh4N5M4F2 steel with controlled austenitic transformation during operation is developed. The optimal temperature regime of hardening of the investigated steel is 1100 ± 5 °C and with increasing hardening temperature (above 1100 °C) the size of austenitic grain increases and the recrystallization process is intensive, which leads to a decrease in strength. It is recommended to carry out low-temperature tempering at temperatures of 190 ± 10 °C of hardened steel to reduce internal stresses. It has been established that tempering of hardened steel is necessary by cooling in the air. The optimum temperature mode of steel tempering is 590 ± 5 °C. The analysis of the structural state of the investigated steel after hardening and tempering (isothermal holding for two and four hours) is carried out. It was established that the maximum operating temperature of the die for hot pressing of copper (grade M1) can reach up to 650 °C. It is shown that the die of the studied steel is able to work at (extreme) temperature operating conditions of 625–650 °C. Heat resistance decrease (below 40 HRC) and softening occur in steel above the higher operating temperature (>650 °C). A pilot test was carried out on a die tool made of 4Kh4N5M4F2 steel (non-forging technology) for hot pressing of an aluminum alloy of the AK7ch grade, which showed the same service properties at the level of 4Kh5MF1S steel (grade H13, USA), which was subjected to ingot hot deformation (forging) with working surface nitride hardening to a depth of 300 microns. Keywords: die steel, thermal treatment, hot deformation, hardness, toughness.