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

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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. (XγCγ)1/2 where Xγ is the volume fraction of austenite and Cγ 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.

Introduction

Austempered ductile cast iron (ADI) is an alloyed and heat treated ductile or nodular cast iron. It has emerged as an important engineering material in recent years because of its excellent mechanical properties. ADI has been used as structural components in wide diverse fields such as gears, crankshafts, locomotive wheels, agricultural equipment etc. The combination of high strength with good ductility [1], [2], [3], good wear resistance [4], [5], [6] and good fatigue properties [7], [8], [9], [10], [11] and fracture toughness [12], [13], [14], [15], [16], [17], [18] achieved by ADI suggests that the engineering applications of this material will continue to expand in the coming years.

The starting material for ADI is ductile or nodular cast iron. It is subjected to an isothermal heat treatment process called austempering. Austempering process involves austenitizing the alloy in the temperature range of 871 to 982°C (1600–1800°F) (for 1–2 h) and then quenching it to an intermediate temperature range of 260–400°C (500–750°F) and holding there for sufficient time (usually between 2 and 4 h). This results in an unique microstructure in which the matrix consists of a mixture of ferrite and high carbon austenite. This microstructure is often referred to as ausferrite [19]. Besides this, the microstructure also consists of graphite nodules dispersed in it. The microstructure of ADI is different from austempered steels where the microstructure primarily consists of ferrite and carbide. The austenite produced in ADI as a result of austempering reaction is rich in carbon making it highly stable. This unique combination of ferrite and high carbon and stable austenite is the most desirable microstructure in ADI and gives ADI its excellent properties. Depending on the heat treatment, the relative amounts of these two phases, i.e. ferrite and austenite can be varied. Thus it is possible to obtain a wide range of properties in ADI by adjusting the austempering heat treatment cycle, i.e. both austenitizing and austempering temperature and time. ADI can have tensile strengths up to 1600 MPa with about 1% elongation and high hardness for wear application when austempered at lower temperatures (e.g. 260°C (500°F)). For applications where ductility is important, ADI with lower strengths of the order of 1000 MPa and elongation upto 14% can also be produced by austempering it in the upper bainitic temperature range (e.g. 316°C (600°F) and above). During the austempering of ductile or nodular cast iron, ADI undergoes a two stage transformation process. In the first stage or stage I reaction, the austenite decomposes into ferrite and high carbon or carbon saturated austenite. If the austempering reaction is continued for a very long duration at this temperature, the high carbon austenite further decomposes into ferrite and carbide. This is known as stage II reaction. This stage II reaction is undesirable because the carbide is a brittle phase [1], [2] and reduces ductility and fracture toughness of ADI.

Alloying elements such as nickel, molybdenum and copper are usually added to alter the transformation behavior by increasing the process window [1], [2] for ADI. The process window refers to the time interval during which austempering can be done successfully, i.e. the time period between completion of the first reaction but before the onset of the second or the embrittling reaction. Some alloying elements like silicon and manganese are inherently present in ADI. These elements also play an important role by affecting the mechanical properties of ADI. With growing manganese addition, the mechanical properties like yield strength, elongation and tensile strength are all found to decrease because of the formation of intercellular embrittling regions due to the stronger segregation effect of Mn [20], [21], [22]. Alloying elements like molybdenum and manganese segregate strongly in the intercellular region encouraging carbide formation and thus delaying the austempering reactions in these areas. Hence it may be possible to improve the mechanical properties including fracture toughness of ADI by eliminating molybdenum and keeping the manganese content as low as possible.

Low manganese ADI is therefore a viable material. Previous studies by this investigator [15] and other researchers [22] have shown that unalloyed ADI has higher fracture toughness than alloyed ADI. However, very little information is currently available in literature [23], [24] on the mechanical properties of unalloyed ADI with low manganese content. Specifically, the influence of microstructure on fracture toughness of unalloyed ADI with low manganese content is not clearly established. Moreover, most of the studies on ADI have been carried out on ductile iron with a predominantly pearlitic microstructure. However, cooling rate and section size of casting can significantly affect the as cast (solidified) structure of ductile iron. No information is currently available in literature on the influence of austempering process on the resultant microstructure and mechanical properties of unalloyed ADI with low manganese content and with an as cast ferritic structure. The present investigation was therefore undertaken to examine the influence of austempering process (i.e. austenitizing time and temperature, austempering time and temperature) on the microstructure and mechanical properties of this material. In a previous publication by this author [25] the influence of austempering temperature on the mechanical properties and fracture toughness of unalloyed low manganese ADI (with ferritic as cast structure) has been discussed. The present investigation is a continuation of the above study where the effect of austenitizing temperature on the resultant microstructure and mechanical properties including fracture toughness has been examined.

Section snippets

Material

The material used for the present investigation is a low manganese ductile iron with only trace amounts of the alloying elements. The chemical composition of the material in weight percent is reported in Table 1. The material did not have any conventional alloying elements like nickel and Mo and the manganese content was also kept low intentionally. The microstructure of the as cast material is shown in Fig. 1. The as cast structure was predominantly ferritic in nature. The material was cast in

Microstructure

The microstructure of the as-cast material is shown in Fig. 1. The microstructure shows a predominantly ferritic matrix (about 80%) with the graphite nodules dispersed in it. Since the matrix is predominantly ferritic in nature, compared to the pearlitic matrix, it can dissolve very low amounts of carbon.

Fig. 2(a–e) show the variation in the microstructures as the austenitizing temperature was increased from 871°C (1600°F) to 982°C (1800°F) with the austempering temperature and time remaining

Conclusions

No significant effect of austenitizing temperature on fracture toughness was observed till 954°C (1750°F) in low manganese ADI with an as cast ferritic structure. However, there is a significant drop in fracture toughness when austenitized at 982°C (1800°F).

The yield, tensile strength and hardness decreased significantly for the samples austenitized at 982°C (1800°F). This was due to the excessive grain coarsening at this temperature and increase in mean free path of dislocation motion.

The

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