Two-step Austempering Process Research Articles

Fracture toughness characterization of austempered ductile iron produced using both conventional and two-step austempering processes

The aim of the present study is to characterize mainly fracture toughness as well as the other mechanical properties of austempered ductile iron produced using both single-step and two-step austempering processes. The effect of alloying with Ni and Mo has been investigated. Austempering heat treatment was conducted at temperatures between 260 °C and 390 °C. Plane strain fracture toughness was evaluated for each material and heat treatment condition. It was found that two-step austempering process resulted in improving the fracture toughness of the material, while maintaining reasonable levels of strength. Alloyed samples showed higher fracture toughness than un-alloyed ones.

Abrasion resistance of ductile iron austempered by the two-step process

The present work focuses in the study of the effect that a new two-step austempering heat treatment process, developed by Putatunda, has on the mechanical properties with emphasis on the response to the abrasive wear. A ductile cast iron melt was prepared in an industrial facility obtaining both, Y-blocks (ASTM A 897) to be used for the laboratory tests samples, and bucket tips to perform field tests by using a wheel loader. The results show that the two-step austempering process promotes an increase in the amount of retained austenite which in turn improves most of the mechanical properties, such as ultimate stress, yield stress, hardness and impact toughness. Two different abrasion tests were carried out in order to evaluate the material response to different tribosystems. The dry sand-rubber wheel abrasion test (ASTM G 65) was used at the lab, where the results show better wear resistance for the two-stepped ADI in comparison with conventional ADI. On the other hand, the bucket tips tested in a more severe environment, showed an opposite tendency.

Influence of Magnetic Field on the Mechanical Properties of Austempered Ductile Iron (ADI)

A novel concept of two-step austempering in a magnetic field has been conceived by this investigator. This twostep process involves first quenching the alloy to a lower temperature after austenitizing and then immediately rising the temperature of the quenching media to a higher temperature and the whole austempering process is carried out in the presence of a high magnetic field. In this investigation, Austempered Ductile Iron (ADI) was processed by this novel two-step austempering process in a high magnetic field of 20 Tesla. The microstructure and the mechanical properties of the ADI processed in a magnetic field has been characterized and compared with ADI processed by conventional process. The results of this investigation indicate significant improvement in the mechanical properties of ADI when Austempered in a high magnetic field. Both yield and tensile strengths were higher in the samples processed in the presence of a magnetic field. Very fine grain ferrite and austenite was obtained in the microstructure. The ferrite content was also significantly higher.

Effect of microstructure on abrasion wear behavior of austempered ductile cast iron (ADI) processed by a novel two-step austempering process

An investigation was carried out to examine the influence of a novel two-step austempering process on microstructural parameters and the abrasion wear resistance of austempered ductile cast iron (ADI). Two batches of cylindrical pin specimens were prepared from an alloyed nodular ductile cast iron and were initially austenitized at 927 °C (1700 °F) for 2 h. The first batch of samples was austempered by the conventional single-step austempering process at five different temperatures, e.g., 288 °C (550 °F), 316 °C (600 °F), 343 °C (650 °F), 371 °C (700 °F), and 385 °C (725 °F) for 2 h, whereas the second batch of samples were processed by the two-step austempering process. These samples were initially quenched in a salt bath maintained at 260 °C (500 °F) and then the temperature of the salt bath was raised to interfacial austempering temperatures316 °C, 343 °C, 371 °C and 385 °C. These samples were austempered at these temperatures for 2 h. The test results show that this two-step austempering process has resulted in significant improvement in microstructural parameters (such as higher volume fraction of austenite, X γ, higher carbon content in austenite, C γ, finer ferritic cell size, d, as well as higher total carbon in the matrix, X γ C γ). Two-step process has also resulted in significant improvement in abrasion wear resistance in ADI, compared to the conventional single-step austempering process.

Near threshold fatigue crack growth behavior of austempered ductile cast iron (ADI) processed by a novel two-step austempering process

The influence of a novel two-step austempering process on the microstructure and the near threshold fatigue crack growth behavior of austempered ductile cast iron (ADI) were investigated. Cylindrical tensile and compact tension (CT) specimens (for fatigue threshold tests) were prepared from an alloyed nodular ductile cast iron as per ASTM standards and were austempered by both the conventional single-step and the novel two-step austempering processes at four different temperatures. The near threshold fatigue crack growth behavior of these samples was examined in room temperature and ambient atmosphere. Tests results indicate this two-step austempering process has resulted in higher hardness, higher yield and tensile strengths for ADI but higher near threshold fatigue crack growth rate and lower fatigue threshold, as compared to the conventional single-step austempering process. Results also demonstrate that fatigue crack growth behavior of ADI in the near threshold region is influenced by microstructural parameters, such as volume fraction of austenite ( X γ), carbon content in austenite ( C γ), ferritic cell size ( d) as well as total austenitic carbon, X γ C γ. SEM fractographs in all samples exhibit a combination of mechanisms, i.e. ductile striation with quasi-cleavage facet around graphite nodules.

Influence of a novel two-step austempering process on the strain-hardening behavior of austempered ductile cast iron (ADI)

An investigation was carried out to examine the influence of a novel two-step austempering process on the strain-hardening behavior of austempered ductile cast iron (ADI). Strain-hardening exponent ( n value) of specimens austempered by conventional single-step austempering process as well as the novel two-step process were determined over the entire plastic deformation regions of the stress–strain curves. Optical microscopy and X-ray diffraction analysis were performed to examine mechanisms of strain-hardening behavior in ADI under monotonic (tensile) loading. Test results show that this novel two-step process has resulted in improved microstructural variables in the ADI matrix, and higher hardness, yield strength and tensile strengths, but lower ductility and strain-hardening exponent values compared to the conventional single-step austempering process. Test results also indicate that strain-hardening exponent of ADI is a function of amount and morphology of microstructural constituents and interaction intensities between carbon atoms and dislocations in the matrix.

Improvement in strength and toughness of austempered ductile cast iron by a novel two-step austempering process

Austempered ductile cast iron (ADI) has emerged as an important engineering material in recent years because of its excellent mechanical properties. These include high strength with good ductility, good wear resistance, fatigue strength and fracture toughness. It is therefore considered as an economical substitute for wrought or forged steel in several structural applications especially in the automotive industry. In this investigation, a nodular or ductile cast iron with predominantly pearlitic as-cast structure was processed by a novel two-step austempering process. Two batches of samples were prepared. All the specimens were initially austenitized at 927 °C (1700 °F) for 2 h. The first batch of samples were processed by conventional single-step austempering process at several temperatures such as 260 °C (500 °F), 273 °C (525 °F), 288 °C (550 °F), 302 °C (575 °F), 316 °C (600 °F), 330 °C (625 °F), 343 °C (650 °F), 357 °C (675 °F), 371 °C (700 °F), 385 °C (725 °F) and 400 °C (750 °F) for 2 h, whereas the second batch of samples were processed by the two-step austempering process. These samples were initially quenched for 5 min in a salt bath maintained at 260 °C (500 °F) and then austempered for 2 h at several austempering temperatures. These temperatures were 288 °C (550 °F), 302 °C (575 °F), 316 °C (600 °F), 330 °C (625 °F), 343 °C (650 °F), 357 °C (675 °F), 371 °C (700 °F), 385 °C (725 °F) and 400 °C (750 °F). Influence of this two-step austempering process on microstructure and mechanical properties of ADI was examined. Test results show that this two-step austempering process has resulted in significant improvement in yield and tensile strengths and fracture toughness of the material over the conventional single-step austempering process.

Influence of stepped austempering process on the fracture toughness of austempered ductile iron

This research studied the effect of a two-step austempering process on the fracture toughness of ductile iron and compared it to that of the conventional upper- and lower-ausferrite austempered ductile irons (ADIs). The results showed that such a two-step austempering heat-treatment process yielded a fracture-toughness value equivalent to that of the upper-ausferrite ADI, while the hardness was maintained at the level of lower-ausferrite ADI. This provided a unique combination of high toughness with good hardness (strength) properties for the ADI with a two-step austempering. Optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural features.

Development of austempered ductile cast iron (ADI) with simultaneous high yield strength and fracture toughness by a novel two-step austempering process

Austempered ductile cast iron (ADI) has emerged as a major engineering material in recent years because of its excellent mechanical properties. These include high strength with good ductility, good wear resistance and fatigue strength. It is therefore considered as an economical substitute for wrought or forged steel in several structural applications especially in the automotive industry. In this investigation, a low-manganese nodular cast iron with a predominantly pearlitic as-cast structure was processed by a novel two-step austempering process. Two batches of samples were prepared. All the specimens were initially austenitized at 927°C (1700°F) for 2 h. The first batch of samples were processed by conventional single step austempering process at several temperatures such as 260°C (500°F), 273°C (525°F), 288°C (550°F), 316°C (600°F), 330°C (625°F), 343°C (650°F), 357°C (675°F), 371°C (700°F), 385°C (725°F) and 400°C (750°F) for 2 h, whereas the second batch of samples were processed by two-step austempering process. These samples were initially quenched to the following austempering temperatures, i.e. 260°C (500°F), 273°C (525°F) 288°C (550°F), 316°C (600°F), 330°C (625°F), 343°C (650°F), 357°C (675°F) and 371°C (700°F), and while being kept at these temperatures in a salt bath, the temperature of the salt bath was raised by 14°C (25°F) per hour for 2 h. The effect of this two-step austempering heat treatment on the microstructure and mechanical properties of the material was examined and compared with the samples processed by conventional single step austempering process. Test results show a significant improvement in mechanical properties and fracture toughness of the material as a result of the two-step austempering process.