Non-martensitic transformation products: Bainite

In this column, I will discuss the non-martensitic transformation product called bainite.

Introduction

The decomposition of austenite, when occurring at a fast enough rate to exceed the critical cooling rate (based on the Time-Temperature-Transformation diagram), creates martensite. The critical rate depends on the chemistry and hardenability of the steel. At slow cooling rates, pearlite and ferrite are formed. At intermediate cooling rates, bainite is formed. This is shown in Figure 1.

During the late 1920s, during studies of isothermal transformation of austenite, at temperatures above martensite formation but below martensite transformation, Davenport and Bain [1] discovered a microstructure that etched differently than martensite or pearlite. This microstructure was later named “bainite” in honor of E.C. Bain [2].

Bainite

The microstructure of bainite was unique in that it formed under continuous cooling or under isothermal conditions, with a structure that was like both martensite and pearlite. Pearlite contains a mixture of ferrite and iron carbide, and is diffusion controlled. Martensite is non-lamellar and not diffusion controlled. Bainite showed both the characteristics of pearlite and martensite, depending on the temperature of formation. Two different morphologies of bainite were observed: upper bainite that formed just below pearlite, and lower bainite which forms at just above the Ms temperature. It has a similar appearance to tempered martensite, with a feathery appearance. It is very similar to tempered martensite in terms of hardness and toughness. Because of the difficulty in controlling the formation of bainite, it generally has limited applicability, except for austempering. This is a process using molten salts where bainite is formed on isothermal holding at approximately 325°C (620°F). This process is used when a hard and tough structure is needed, in thin sections, such as lawn mower blades. Figure 2 shows the different etching behavior of bainite and martensite.

Upper and lower bainite are plates of ferrite, separated by untransformed austenite, martensite, or cementite [4]. The shape of the groups of bainite is a wedge-shaped plate. The nucleation of bainitic ferrite is typically associated with the formation of small, needle-like precipitates of carbon-rich iron carbides, known as “sheaves.” The individual plates within a sheave are called sub-units and have a lens-shaped or lath morphology. The thickest part of the plate usually forms at an austenite grain boundary.

In general, the formation of bainite is suppressed by many alloying elements. However, carbon has the strongest effect. An empirical equation for the start temperature for the formation of bainite (Bs) is [5]:

Note that all alloy concentrations are in weight percent.

Bainitic microstructures will both be harder and tougher than pearlite, but lower than martensite. Generally, the properties improve with lower transformation temperatures. “Lower” bainite will have properties approaching that of tempered martensite at the same hardness. It is this property of bainite that is the basis of the process of austempering.

Applications

The application of bainitic microstructures is growing. The advantages of the elimination of tempering after quenching are of economic benefit in reduction of cost and cycle time. Advances in controlled cooling during rolling have shown benefits in creating bainitic structures in plate and rail. This plate is used in the creation of pipe for natural gas transportation, enabling easier welding and improved strength and toughness over traditional pipe. Austempered ductile iron, to create a structure of bainite, is used to manufacture automotive brake components. Automotive structures use bainitic steels for crash reinforcement to allow energy to be expended during a crash to protect the occupants. Additional applications are the use of bainitic steels in engine components such as cam shafts. This is accomplished by using controlled cooling from the die forging temperatures.

With the increased weight and speed of trains, there has been extensive interest in the use of bainitic steels for rails. Rail steels must be designed to resist plastic deformation and rolling contact fatigue. The use of bainitic steels for rails increases fatigue resistance and improves strength over traditional pearlitic steels [6].

Conclusions

Bainite is becoming increasingly important in many industries because of its unique set of properties of good strength and toughness. It is being used in many areas, including automotive, agricultural, and rail transport. One major benefit is the elimination of tempering, and the ability to direct quench by controlled cooling.

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References

1. E. S. Davenport and E. C. Bain, “Transformation of austenite at constant subcritical temperatures,” Trans. Met. Soc. AIME, vol. 90, pp. 117-154, 1930.
2. C. S. Smith, A History of Metallography, Chicago, IL: University of Chicago Press, 1960, p. 225.
3. ASM International, Metallography and Microstructure, vol. 9, G. F. Vander Voort, Ed., Materials Park, OH: ASM International, 2004.
4. H. K. D. H. Bhadeshia, Bainite in Steels, London: IOM Communications, 2001.
5. W. Steven and A. G. Haynes, J. of Iron and Steel Institute, vol. 183, p. 349, 1956.
6. I. Hiavaty, M. Sigmund, L. Krejci and P. Mohyia, “The bainitic steels for rails applications,” Materials Engineering, vol. 16, no. 4, pp. 44-50, 2009.