Austempered ductile iron (ADI) is a type of graphite cast iron produced by an isothermal heat treatment process. It is an alternative to traditional steel castings and forgings and even aluminum due to the exceptional mechanical properties such as high strength, good toughness and excellent machinability. In the matrix, the unique ausferritic structure consists of graphite nodules uniformly surrounded by acicular ferrite and carbon saturated austenite. In the automotive industry, ADI has been commonly applied in the manufacturing of camshafts, crankshafts, gears and engine valves. These components are frequently subjected to surface contact with relative motion and external load. Therefore, it is necessary to understand the wear behavior of ADI in order to solve the tribological issues in existing ADI applications and utilize ADI into future designs appropriately. This paper was aimed at reviewing the fabrication processes of single-step and dual-step ADIs. The corresponding microstructure and mechanical properties were also briefly discussed. The studies on wear performance of ADI and potential mechanisms have been systemically reviewed.

A review: phase transformation and wear mechanisms of single-step and dual-step austempered ductile irons

Austempered ductile iron has been considered as a substitution to replace conventional steel castings, steel forgings and even aluminum in many applications. Its excellent mechanical properties are mainly attributed to its unique ausferritic structure containing acicular ferrite and carbon saturated austenite. Some ADI components such as camshaft and engine valve commonly experience surface contact and relative motion under external load and pressure. Outstanding wear resistance of ADI is required to ensure the satisfactory working performance and service life of these key components. In order to meet the advanced requirements of mechanical properties and wear behavior of ADI, a dual-step austempering process and additional surface hardening treatments are suggested to be utilized. In this review, conventional single-step austempering process and corresponding ausferritic structure are introduced. Second, the positive effects of a novel dual-step austempering process on microstructure and mechanical properties of ADI are provided. Next, wear behavior of single-step and dual-step ADIs and wear mechanisms are discussed. Then, studies regarding the applications of surface hardening treatments on wear resistance of ADI are reviewed. Finally, a summary and expectations are discussed. The information presented in this paper would be beneficial to future studies and applications for any grade of ADI.

Microstructure, wear behavior and surface hardening of austempered ductile iron

Ultrasonic surface rolling (USR) treatment with different rolling loads (600, 800, 1000 and 1200 N) was introduced to the 17Cr2Ni2MoVNb steel. The microstructure and rolling contact fatigue (RCF) behavior after USR treatment were investigated. Results pointed out that grain refinement existed in the surface-modified layer of the material after USR treatment. The treatment with rolling load at 1200 N could produce a best surface enhancement effect, compared to the untreated sample, the microhardness of USR treated sample (25 kHz, 1200 N) was increased from 795 HV0.2 to 905 HV0.2, with an increase by 13.8%. The surface residual compressive stress was increased from −334 MPa to −527 MPa, with an increase by 57.8%. However, the samples treated at 1200 N increased the surface roughness and caused some surface cracks on the rolled surface, which was an important factor to reduce the fatigue life. The results from RCF tests showed that USR treatment with rolling load at 1000 N exhibited the maximum mean life of 3.71 × 106 cycles.

Effect of ultrasonic surface rolling on microstructure and rolling contact fatigue behavior of 17Cr2Ni2MoVNb steel

Tribological behavior of austempered ductile iron (ADI) obtained at different austempering temperatures

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.

Characteristics of tempering response of austempered ductile iron

Rolling contact fatigue resistance of austempered ductile iron processed at various austempering holding times

Study of Ausferrite Transformation Kinetics for Austempered Ductile Irons with and without Ni

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Rolling contact fatigue resistance of austempered ductile iron processed at various austempering holding times

Study of Ausferrite Transformation Kinetics for Austempered Ductile Irons with and without Ni