S. Yazdani

Strengthening and mechanical stability mechanisms in nanostructured bainite

Understanding the main relationships between the microstructure parameters controlling the strength and ductility of low temperature bainitic microstructures is of considerable importance for further development of these grades. Although the microstructure essentially consists of solely two phases, bainitic ferrite and retained austenite, the complexity of the different microstructural characteristics, the natural consequence of its unique transformation mechanisms, might not provide with one unique answer, but a set of several parameters interdependent among them. This paper will deal with some of these relationships’ microstructure properties, strength, and ductility, with special emphasis in the mechanical stability (TRIP effect) of retained austenite.

Characterisation of microstructure and mechanical properties in two different nanostructured bainitic steels

Abstract Recently, valuable combinations of mechanical properties with strength of 1.9 GPa accompanied by very decent ductility of 19% and toughness of 31 J, have been achieved in a set of nanostructured bainitic steels. However, it is necessary to elucidate the significance of various microstructural features responsible of that extraordinary mechanical response in more detail. Thus, using two steels, with different Mn, Ni and V contents, and changing the nanostructured bainite isothermal transformation temperatures (200–300°C), has led to a plethora of subtle and essential microstructural variations, necessary to explain how the mechanical response of nanostructured bainite is attained.

Fabrication and characterisation of nickel coated Ni-NCZ (nickel coated ZrO2) composite coating

Abstract For the first time, electroless nickel plated ZrO2 (Ni-NCZ) particles were used for the co-electrodeposition of Ni-NCZ composite coating. Optical microscope, SEM and X-ray diffraction studies showed that Ni-NCZ has a rough surface with a smaller Ni crystallite size than Ni-ZrO2 due to the conductive properties of NCZ particles, which provide more nucleation sites for Ni clusters. The energy dispersive X-ray spectroscopy measurement results showed that Ni-NCZ has more co-electrodeposited particles than Ni-ZrO2. This is due to the more positive zeta potential of NCZ particles compared to that of ZrO2. The microhardness measurements demonstrated that the Ni-NCZ composite coating has a higher microhardness than Ni-ZrO2 due to the higher amounts of co-electrodeposited particles and lower crystallite size. Electrochemical impedance spectroscopy and potentiodynamic polarisation test showed that the corrosion resistance of Ni-NCZ is higher than that of Ni-ZrO2.

Investigations on corrosion proceeding path and EIS of Ni–ZrO2 composite coating

Abstract Electrochemical impedance spectroscopy of Ni and Ni–ZrO2 composite coatings was studied. Investigation of corroded surfaces showed that the cluster boundaries in pure Ni and weak bonds between Ni matrix and ZrO2 particles in Ni–ZrO2 composite coating are the appropriate paths for corrosion to proceed. An equivalent circuit diagram based on blocked and partially corroded surface characterisations was proposed, and good agreement was observed between theoretical impedance spectra obtained on the basis of the equivalent circuit and spectra recorded during the measurements. Changes of microstructure and corrosion proceeding paths were recognised as the reasons for the higher corrosion resistance of Ni–ZrO2 with respect to pure Ni.

Manufacturing of functionally graded Ni–ZrO2 composite coating controlled by stirring rate of the electroplating bath

For the first time, functionally ZrO2 content graded Ni–ZrO2 composite coating has been successfully co-electrodeposited from a bath with gradually increasing stirring rate. For this, different composite coatings were electroplated in the same bath with different stirring rates to find the optimum stirring rate in which the maximum content with uniform distribution of ZrO2 particles in the coating can be achieved. To produce ZrO2 content graded Ni–ZrO2 composite coating, the stirring rate was continuously increased from 0 to optimum value. The results showed that functionally graded Ni–ZrO2 composite coating has been successfully electroplated with continuous gradient distribution of Zirconia content in the coating using controlling of bath stirring rate. With increasing of ZrO2 particles content, the microhardness increases from the interface towards the surface of the coating. This could provide a high wear resistance with high adhesion of the coating to the substrate.

Wear Behavior of an Austempered Ductile Iron Containing Mo-Ni-Cu

The aim of the this investigation is to study the influence of Ni on tribiological behavior of an austempered ductile iron (ADI) containing Mo, Ni and Cu. Ductile irons with chemical composition Fe-3.56%C– 2.67%Si –0.25%Mo–0.5%Cu and Ni contents of 0.8 and 1.5% were cast into standard Y-blocks. Wear test samples were machined off from the bottom section of blocks. Austenitizing heat treatment was carried out at 870°C temperature followed by austempering at 270, 320, and 370°C for 5-1140 minutes. The wear test was carried out by using block-on-ring test machine. Sliding dry wear behavior was studied under applied loads of 50, 100 and 150N. The results show that wear resistance is independent of austempering temperature with an applied load of 50N, but there is a strong dependence at higher austempering temperatures with applied loads of 100 and 150N. Wear mechanism is described as being due to subsurface fatigue, with cracks nucleated at plastically, deformed graphite interfaces. The wear controlling mechanism is the crack growth when wear shows a dependence on applied load and austempering temperature.

Effect of Austempering Temperature on High Cycle Fatigue Behavior of an Austempered Ductile Iron (ADI)

Ductile irons with chemical compositions of Fe-3.6%C-2.6%Si-0.50%Cu-0.51%Ni were cast into standard keel blocks. Austenitizing heat treatment was carried out on test pieces at 875°C followed by austempering at 320, 365 and 400°C for times within the austempering processing window in a salt bath furnace. Rotating bending fatigue tests were performed with Roell Amsler UBM 200™ equipment at 3500 rpm at room temperature. Metallography and X-ray diffraction techniques were used to evaluate the fatigue life. Results indicate an increase of 10, 20 and 24% in fatigue life for specimens austempered at temperatures of 320, 365 and 400°C respectively, compared to that of as cast samples. According to the XRD test results; there is an increase in volume fraction of high carbon austenite by increasing the austempering temperature.

Co-electrodeposition of Functionally Graded Ni-NCZ (Nickel Coated ZrO2) Composite Coating

In this study, functionally NCZ (electroless nickel plated ZrO2) content graded Ni-NCZ composite coating has been successfully co-electrodeposited from a bath with gradually increasing of stirring rate. For this, different composite coatings were electroplated in the same bath with different stirring rates to find the optimum condition. SEM, XRD, EDX and electrochemical studies showed that co-electrodeposition in a bath with stirring rate of 250 rpm results in the maximum co-electrodeposited particle content and the best particle distribution and corrosion resistance. Also, this sample had the highest wear resistance with respect to the other samples. To produce NCZ content graded Ni-NCZ composite coating, the stirring rate was continuously increased from 0 to 250 rpm. The electroplated coating had a continuous gradient increasing of co-electrodeposited NCZ content from substrate toward the surface. This distribution of NCZ particles results in a gradient increasing of the microhardness in the cross section of the coating. Bend test revealed that the functionally graded composite coating shows better adhesion to the substrate compared with the uniformly distributed Ni-NCZ on the same substrate. This result has been attributed to lower mechanical mismatch between coating and substrate in the functionally graded composite coating with respect to the uniformly distributed one.

Wear Resistance of Two Nanostructural Bainitic Steels with Different Amounts of Mn and Ni

Extremely valuable mechanical properties in combination with acceptable wear resistance can make nanostructural bainitic steels to be used extensively in different engineering and tribological applications. However, it is critical to characterize the contributed factors to investigate the wear response of these high-strength materials. This work aims to study the wear behavior of two nanostructural bainitic steels with different amount of austenite stabilizer elements Mn and Ni. For this purpose, wear resistances of the materials were evaluated using the pin-on-disk method. The results indicated that the hardness of the sample is a critical factor affecting the tribological behavior, and the volume fraction and morphology of high-carbon retained austenite are also of considerable importance. It has also been demonstrated that transformation-induced plasticity effect during the wear test and oxide formation at worn surfaces are critical factors.

Effect of austempering temperature on machinability of Cu-Ni-Mo alloyed austempered ductile iron for heavy section parts

Abstract The machinability of an austempered ductile iron with a suitable chemical composition for heavy sections has been assessed. Austempering heat treatment was carried out at three temperatures, 300, 340 and 375°C, after austenitising at 870°C for 100 min. Drilling tests, tool wear and surface roughness measurements were used to evaluate the machinability. Drilling operation failure, severe tool wear and the poorer surface roughness of the specimens austempered at lower temperatures indicated that austempering at higher temperatures clearly resulted in better machinability. The machinability of testpieces austempered at 375°C, which contained higher fractions of retained austenite, was superior to that of testpieces autenitised at lower temperatures, indicating that hardness is an important factor in assessing machinability in addition to high carbon austenite content.