Susil K. Putatunda

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.

Influence of microstructure on high-cycle fatigue behavior of austempered ductile cast iron

Abstract An investigation was carried out to examine the influence of austempering heat treatments and the resultant microstructure of austempered ductile cast iron, on the fatigue crack growth rate, fatigue threshold, and high-cycle fatigue strength of the material. Two different approaches were used to study the fatigue behavior of this relatively new material, that is, a traditional S-N curve approach for determination of fatigue strength and a fracture mechanics-based approach for determination of the fatigue threshold. Compact tension and cylindrical specimens prepared from alloyed nodular ductile cast iron were given three different austempering heat treatments to produce three different microstructures. The fatigue threshold and high-cycle fatigue behavior of these specimens were studied in room temperature ambient atmosphere. The results of the present investigation demonstrate that the fatigue threshold of the material increases with increase in volume fraction of carbon-saturated austenite. The fatigue strength of the material, on the other hand, was found to increase with decrease in austenitic grain size. The crack growth process in the material was a combination of ductile striations and microvoid coalescence, and crack propagation by connecting the graphite nodules along its path.

Influence of austempering temperature on microstructure and fracture toughness of a high-carbon, high-silicon and high-manganese cast steel

Abstract An investigation was carried out to examine the influence of austempering temperature on the microstructure and mechanical properties of a high-carbon (1.00%), high-silicon (3.00%) and high-manganese (2.00%) cast steel. Cylindrical tensile specimens and compact tension specimens were prepared from this cast steel according to ASTM standards and were given four different austempering heat treatments to produce different microstructures. The tensile properties and fracture toughness of these materials were studied at room temperature in ambient atmosphere. Test results indicate that maximum fracture toughness is obtained in this steel when the microstructure contains very high austenitic carbon ( X γ C γ ).

Fracture toughness of a high carbon and high silicon steel

Abstract A high carbon and high silicon (HCHS) steel containing about 1% carbon and 2.5% silicon has been developed. This steel has been synthesized using concepts from austempered ductile cast iron (ADI) technology. The influence of austempering temperature on the microstructure and the mechanical properties of this steel in room temperature and ambient atmosphere was examined. The influence of microstructure on the plane strain fracture toughness of this HCHS steel was also investigated. Compact tension and round cylindrical specimens were prepared from this steel. These specimens were then austenitized at 927°C for 2 h and then austempered at several temperatures between 260 and 399°C for a fixed time period of 2 h to produce different microstructures. The microstructures were characterized by X-ray diffraction and optical metallography and correlated to the mechanical properties. The test results showed that the maximum fracture toughness is obtained in this steel with a upper bainitic microstructure when the microstructure contains about 35% austenite and the carbon content in the austenite is about 2%. The retained austenite and its carbon contents increased with austempering temperature, reaching a peak value at 385°C and then retained austenite decreased with increasing temperature. The carbon content of the austenite also showed a similar behavior. The fracture toughness was found to depend on the parameter (XγCγ/d)1/2 where Xγ is the volume fraction of the austenite, Cγ is the carbon content of austenite and d is the mean free path of dislocation motion in ferrite.

Investigations on the fracture toughness of austempered ductile irons austenitized at different temperatures

Ductile cast iron was austenitized at four different temperatures and subsequently austempered at six different temperatures. Plane strain fracture toughness was evaluated under all the heat treatment conditions and correlated with the microstructural features such as the austenite content and the carbon content of the austenite. Fracture mechanism was studied by scanning electron microscopy. It was found that the optimum austempering temperature for maximum fracture toughness decreased with increasing austenitizing temperature. This could be interpreted in terms of the microstructural features. A study of the fracture mechanism revealed that good fracture toughness is unlikely to be obtained when austempering temperature is less than half of the austenitizing temperature on the absolute scale.

Tensile behavior of a new single-crystal nickel-based superalloy (CMSX-4) at room and elevated temperatures

Tensile behavior of a new single-crystal nickel-based superalloy with rhenium (CMSX-4) was studied at both room and elevated temperatures. The investigation also examined the influence of γ′ precipitates (size and distribution) on the tensile behavior of the material. Tensile specimens were prepared from single-crystal CMSX-4 in [001] orientation. The test specimens had the [001] growth direction parallel to the loading axis in tension. These specimens were given three different heat treatments to produce three different γ′ precipitate sizes and distributions. Tensile testing was carried out at both room and elevated temperatures. The results of the present investigation indicate that yield strength and ultimate tensile strength of this material initially increases with temperature, reaches a peak at around 800 °C, and then starts rapidly decreasing with rise in temperature. Both yield and tensile strength increased with increase in average γ′ precipitate size. Yield strength and temperature correlated very well by an Arrhenius type of relationship. Rate-controlling process for yielding at very high temperature (T ≥ 800 °C) was found to be the dislocation climb for all three differently heat-treated materials. Thermally activated hardening occurs below 800 °C whereas above 800 °C thermally activated softening occurs in this material.

Influence of austenitizing temperature on fracture toughness of a low manganese austempered ductile iron (ADI) with ferritic as cast structure

Abstract An investigation was carried out to examine the influence of austenitizing temperature on the resultant microstructure and mechanical properties of an unalloyed and low manganese ADI and with an as cast (solidified) ferritic structure. The investigation also examined the influence of austenitizing temperature on the fracture toughness of this material. Compact tension and round cylindrical tensile specimens were prepared from a nodular cast iron without any alloying elements (e.g. nickel, molybdenum or copper) and with very low manganese content and with an as cast (solidified) ferritic structure. These were then austenitized at several temperatures ranging from 871°C (1600°F) to 982°C (1800°F) and then austempered at a constant austempering temperature of 302°C (575°F) for a fixed time period of 2 h. Microstructure was characterized through optical microscopy and X-ray diffraction. Tensile properties and plane strain fracture toughness of all these materials were determined and correlated with the microstructure. Fracture surfaces were examined under scanning electron microscope to determine the fracture mode. The results of this investigation indicate that the austenitizing temperature above 982°C (1800°F) has a detrimental effect on the fracture toughness of this material. Both volume fraction of austenite and its carbon content increased with austenitizing temperature. The strain hardening exponent of this material was found to increase with increase in the austenitic carbon content i.e. ( X γ C γ ) 1/2 where X γ is the volume fraction of austenite and C γ is the carbon content of austenite. A Hall–Petch type relationship was found to exist between yield strength and mean free path of dislocation motion, d in ferrite. A model for fracture toughness of ADI has been developed. Present test results indicate good agreement with the model.

Fracture Toughness of Unalloyed Austempered Ductile Cast Iron (ADI)

The influence of microstructure on the tensile properties and the plane strain fracture toughness (KIc) of unalloyed austempered ductile cast iron (ADI) was examined in room temperature ambient atmosphere. The crack growth mechanism during fracture toughness tests was determined through detailed fractographic studies. Compact tension and round cylindrical specimens were prepared from unalloyed ADI and were given four different austempering heat treatments to produce four different microstructures. Tensile properties and fracture toughness of these four differently heat-treated materials as well as cast materials were determined as per relevant ASTM standards. The results of the present investigation demonstrate that the fracture toughness of unalloyed ADI increases with increase in volume fraction of ferrite in the matrix and reaches a peak when the ferrite content of the matrix is around 65%. Both yield and ultimate tensile strength of the unalloyed ADI was found to increase with increase in volume fraction of ferrite in the matrix. The ductility of ADI, on the other hand, was found to increase with increase in volume fraction of austenite in the matrix. The crack growth mechanism was found to be predominantly by the microvoid coalescence. The crack path appears to connect the graphite nodules along the way.

The influence of chromium on mechanical properties of austempered ductile cast iron

An investigation was carried out to examine the influence of microstructure and chromium on the tensile properties and plane strain fracture toughness of austempered ductile cast iron (ADI). The investigation also examined the growth kinetics of ferrite in these alloys. Compact tension and round cylindrical tensile specimens were prepared from ductile cast iron with Cr as well as without Cr. These specimens were then given four different heat treatments to produce four different microstructures. Tensile tests and fracture toughness tests were carried out as per ASTM standards E-8 and E-399. The crack growth mechanism during fracture toughness tests was also determined.The test results indicate that yield strength, tensile strength, and fracture toughness of ADI increases with an increase in the volume fractions of ferrite, and the fracture toughness reaches a peak when the volume fractions of the ferrite are approximately 60% in these alloys. The Cr addition was found to reduce the fracture toughness of ADI at lower hardness levels (<40 HRC); at higher hardness levels (≥40 HRC), the effect of chromium on the fracture toughness was negligible. The crack growth mechanism was found to be a combination of quasi-cleavage and microvoid coalescences, and the crack trajectories connect the graphite nodules along the way.

INFLUENCE OF AUSTEMPERING TEMPERATURE ON FRACTURE TOUGHNESS OF A LOW MANGANESE AUSTEMPERED DUCTILE IRON (ADI)

The influence of microstructure on the plane strain fracture toughness of an unalloyed, austempered ductile cast iron (ADI) with low manganese content (<0.15 wt %) and with predominantly as-cast (solidified) ferritic structure was studied. Test specimens were austenitized at 927°C (1700°F) for 2 hr and then austempered over a range of temperatures to produce different microstructures. The microstructures were characterized through optical microscopy and X-ray diffraction. Plane strain fracture toughness of all these materials was determined and correlated with the microstructure. The results of the present investigation indicate that the alloy (with an initially ferritic as-cast microstructure) has higher fracture toughness with an upper ausferritic structure, i.e., when austempered in the upper bainitic temperature range (above 316°C [600°F]). This behavior was markedly different from conventional ADI with a pearlitic as-cast microstructure because the pearlitic structure shows higher fracture toughness with a lower ausferritic structure, i.e., when austempered in the lower bainitic temperature range (<316°C [600°F]). The fracture toughness was found to increase with the increase in total austenitic carbon, i.e., XγCγ, where Xγ is the volume fraction of austenite and Cγ is the carbon content of austenite.