Homogenisation of austenite during non-equilibrium heating in hypereutectoid steels

ABSTRACT

The homogenisation (Stage III) of austenite during non-equilibrium heating of hypereutectoid steels has been characterised for the first time through dilatometric strain measurements and electron microscopic techniques. It has been found that the homogenisation temperature (A_ch) is significantly higher than the cementite dissolution temperature (A_cm). Though there is a complete conversion of low-temperature constituents to austenite above the A_cm, a significant segregation of carbon in the austenite is observed below the A_ch. Performing heat treatment with an incomplete austenitisation between the A_cm and the A_ch temperature can affect the subsequent on-cooling phase transformations. It is proposed that the A_ch temperature should be used as the full austenitisation temperature in hypereutectoid steels.

KEYWORDS:

• Hypereutectoid steel
• austenitisation
• homogenisation
• dilatometry
• non-equilibrium heating

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Rangasayee Kannan http://orcid.org/0000-0002-5792-5739

Yiyu Wang http://orcid.org/0000-0001-7248-0646

Leijun Li http://orcid.org/0000-0003-2411-3846

Notes

1 The temperature at which the homogenisation is complete is designated as A_ch in this paper.

2 Martensitic transformation is promoted in the samples so that the high temperature austenite transforms in a displacive manner. This helps in studying the composition gradients in the high temperature austenite using the room temperature martensitic microstructure.

3 Though a change in slope of derivative is seen, as will be discussed for the 1173 K (900°C) and beyond samples, this slope is likely an instrumental artefact and has nothing to do with phase transformations.

4 The molar Gibb’s free energy change of the transformation becomes positive (Figure 15a) and hence a higher temperature is needed to render the transformation thermodynamically feasible.