Journal of Materials Research and Technology Journal of Materials Research and Technology
Short Communication
Characteristics of tempering response of austempered ductile iron
Bingxu Wanga,, , Gary C. Barbera, Chuanlin Taoa, Xichen Sunb, Xu Ranc
a Oakland University School of Engineering and Computer Science, Rochester, MI, United States
b Fiat Chrysler Automobiles (FCA) North America LLC, Auburn Hills, MI, United States
c Changchun University of Technology, Changchun, China
Received 23 April 2017, Accepted 15 August 2017
Abstract

Austempered ductile iron (ADI) is produced by an isothermal heat treatment. Tempering is an effective method to increase the toughness and decrease the hardness of ADI. In the present research, the transformation of ADI was investigated after applying various tempering temperatures. The hardness of ADI samples with and without tempering was measured and the microstructure of ADI samples was analyzed by using metallographic optical microscopy. It was found that the ausferrite decomposed into dispersive cementite particles above a tempering temperature of 538°C. Thus, the tempering process for ADI must be carefully selected so that the excellent properties of ADI are not degraded.

Keywords
ADI, Tempering process, Hardness measurement, Phase transformation, Microstructure
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1Introduction

Austempered ductile iron (ADI) is ductile iron which undergoes an isothermal heat treatment. The optimal microstructure of ADI consists of graphite nodules surrounded by acicular ferrite and high carbon content austenite, which is called ausferrite [1–3]. The excellent mechanical properties of ADI have led it to becoming a good alternative to steel castings and forgings and even aluminum in diverse applications, especially in the automotive area [4–6]. The production of ADI requires stringent control because the final ausferritic morphology can be influenced by chemical composition, holding time of heat treatment, cooling rate, etc. [7,8]. Standard ADI grades have been specified in terms of material properties [9].

The ADI heat treatment is comprised of two controlled steps: austenitizing and austempering. The austempering step can be subdivided into two continuous reaction stages depending on holding time. The unique ausferritic matrix can be achieved only within the first stage. In the second stage, the high carbon content austenite will be decomposed into ferrite and carbon will be precipitated in the form of carbide. In this case, some of the mechanical properties of ADI will be degraded with the formation of bainite in the matrix [2,8].

In industry, tempering has been utilized as an effective heat treatment to reduce brittleness and relieve the internal stress of quenched and normalized materials [10–12]. The ductility and toughness of materials can be improved by tempering along with a decrease in hardness. The resulting material properties are dependent on the tempering temperature and time and alloying elements and their percentages. However, the hardness may not be changed by the tempering process for some materials containing Mo and S, and high speed steel after tempering is also an exception, which becomes harder due to completed martensite formation [13].

Kshemendranath et al. [14] reported that the hardness of austempered low carbon equivalent ductile iron decreased with increasing tempering temperature. Putatunda et al. [15] found that the microstructure of austempered ductile cast iron had an austenite-free ferritic matrix after being tempered at 484°C for 2h. The hardness of tempered ADI samples could be increased or decreased, dependent on the original microstructure of non-tempered ADI.

Even though the metallurgy and material properties of ADI have been studied extensively, there is a lack of information with respect to how the morphology of the ADI matrix responds after applying various tempering temperatures. It is desirable to confirm if the ausferritic structure will remain in the matrix or will transform into some other phases during tempering. This is also important in the understanding of microstructural changes which may occur in case hardening processes such as nitriding and nitrocarburizing. Hence, the objective of this research was to explore ausferritic transformation of ADI material processed using various tempering temperatures.

This objective was carried out by performing heat treatment on samples including austenitizing, austempering and tempering processes. Then, hardness tests and observation using metallographic optical microscopy were carried out to analyze the characteristics of tempered ADI samples.

2Experimental procedure2.1Chemical composition

The chemical composition of the experimental ductile iron was 3.87% C, 0.49% Mn, 0.023% P, 0.007% S, 2.17% Si, 0.04% Cr, 0.02% Ni, 0.02% Mo, 0.73% Cu, 0.007% Al, 0.005% V, 0.015% Nb, 0.01% Ti, 0.003% Co, 0.028% Sn, 0.052% Mg and 0.001% W.

2.2Heat treatment procedure

The original samples were cut from bars into a 60 degree sector shape with a diameter of 50mm and thickness of 8mm. The original microstructure of the as-cast ductile iron has graphite nodules uniformly surrounded by pearlite (see Fig. 1). An austenitizing temperature of 900°C was selected based on previous research [16]. The ductile iron samples were austenitized in a salt bath furnace for 25min. Then, samples were rapidly transferred to another pre-heated salt bath furnace for an austempering process at 276°C, 321°C or 373°C for 30min, and then cooled in air to room temperature. The ADI samples were then subjected to a tempering process at various temperatures for 60min in an electric furnace and cooled by water quenching to room temperature. A schematic of the heat treatment processes utilized in this research is shown in Fig. 2.

Fig. 1.
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Original microstructure of as-cast ductile iron (500×).

Fig. 2.
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(a) Austenitizing and austempering processes. (b) Tempering processes carried out for 60min on ADI samples: (1) austempering (276°C), (2) austempering (321°C), (3) austempering (373°C).

3Results3.1Hardness measurement (Rockwell C)

A Rockwell hardness tester was used to measure the hardness of the ADI samples. In Fig. 3, it can be seen that the hardness of the ADI samples decreases with increasing austempering temperature at the same holding time. This may be due to a decrease in martensite in the matrix and more austenite being transformed into ausferrite. In addition, the hardness also decreases with increasing tempering temperature for each austempering condition.

Fig. 3.
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Hardness measurements of ADI samples for each tempering temperature.

3.2Microstructure analysis

Figs. 4–6 show the microstructure of the ductile iron samples obtained from different austempering and tempering temperatures. The ADI samples austempered at 373°C have thick feather-like ausferrite, but thin needle-like ferrite is formed for 276°C and 321°C austempering temperatures. This is because the diffusion rate of carbon atoms is faster under higher austempering temperatures and the growth rate of nucleated ferrite platelets increases.

Fig. 4.
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Microstructure photos of samples at various tempering temperatures: (a) austempered (276°C, without tempering), (b) austempered (276°C, with 316°C tempering), (c) austempered (276°C, with 371°C tempering), (d) austempered (276°C, with 427°C tempering), (e) austempered (276°C, with 482°C tempering), (f) austempered (276°C, with 538°C tempering), (g) austempered (276°C, with 593°C tempering) (500×).

Fig. 5.
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Microstructure photos of samples at various tempering temperatures: (a) austempered (321°C, without tempering), (b) austempered (321°C, with 371°C tempering), (c) austempered (321°C, with 427°C tempering), (d) austempered (321°C, with 482°C tempering), (e) austempered (321°C, with 538°C tempering), (f) austempered (321°C, with 593°C tempering) (500×).

Fig. 6.
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Microstructure photos of samples at various tempering temperatures: (a) austempered (373°C, without tempering), (b) austempered (373°C, with 427°C tempering), (c) austempered (373°C, with 482°C tempering), (d) austempered (373°C, with 538°C tempering), (e) austempered (373°C, with 593°C tempering), (500×).

After applying the tempering process, the ferrite and high carbon content austenite gradually vanish, and particles are formed in the matrix. These particles are likely cementite. The main microstructural constituents of the ductile iron samples for each heat treatment condition are reported in Table 1. For the samples austempered at 276°C and 321°C, the characteristic ausferritic structure still exists along with tempered martensite when the tempering temperatures are lower than 538°C. The formation of cementite particles becomes obvious for 538°C or higher tempering temperatures. For the samples austempered at 373°C, the ausferritic structure is no longer present after tempering at 482°C or higher temperatures. Wen et al. [17] suggested that a tempering temperature of 200°C had insignificant effects on ausferrite structure, but was effective in producing tempered martensite. As a result, the uniform ADI matrix with martensite has been transformed into tempered martensite and cementite. Moreover, the microstructure of samples after being austempered at 276°C with a tempering process of 427°C displays similar ausferritic morphology to those produced at an austempering process of 321°C without tempering.

Table 1.

Main components in microstructure at different austempering temperatures and various tempering temperatures.

Austemper  Temper
  W/O tempering  W/316°C (600°F)  W/371°C (700°F)  W/427°C (800°F)  W/482°C (900°F)  W/538°C (1000°F)  W/593°C (1100°F) 
276°C (530°F)/30min  ADI+martensite  ADI+tempered martensite  ADI+tempered martensite  ADI+tempered martensite  ADI+tempered martensite  Ferrite+cementite  Ferrite+cementite 
321°C (610°F)/30min  ADI+martensite  ADI+tempered martensite  ADI+tempered martensite  ADI+tempered martensite  Ferrite+cementite  Ferrite+cementite 
373°C (700°F)/30min  ADI+martensite  ADI+tempered martensite  Ferrite+cementite  Ferrite+cementite  Ferrite+cementite 
4Summary and conclusions

In this research, several tempering temperatures were applied on ADI samples produced by three different austempering temperatures. After investigating the hardness and microstructure, several conclusions can be drawn:

  • -

    The hardness of ADI samples decreases with increasing austempering temperature at the same holding time, and also decreases with increasing tempering temperature for each austempering condition.

  • -

    The ADI samples austempered at 373°C have thick feather-like ausferrite, but thin needle-like ausferrite is formed for 276°C and 321°C austempering temperatures.

  • -

    The ausferritic structure is gradually decomposed into dispersive cementite particles with higher tempering temperatures. There is very little needle-like component which still exists at and above 538°C tempering temperature for ductile iron samples austempered at 276°C and 321°C. Also, there is only a small amount of feather-like component existing at and above 482°C tempering temperature for ductile iron samples austempered at 373°C.

  • -

    The above results related to hardness and microstructure are important for selecting proper tempering temperatures and should also be considered when applying case hardening processes, such as nitriding treatment (500–600°C) and nitrocarburizing treatment (530–600°C).

Conflicts of interest

The authors declare no conflicts of interest.

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Corresponding author. (Bingxu Wang bwang@oakland.edu)
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