Reaustenisation in Steel Weld Deposits

Proceedings of an International Conference on Welding Metallurgy of Structural Steels, The Metallurgical Society of the AIME, Warrendale, Pennsylvania. Edited by J. Y. Koo, 1987, pp. 594-563. By J. R. Yang and H.K.D.H. Bhadeshia

The process of reaustenitisation in weld deposits, beginning with a microstructure of acicular ferrite and austenite, has been studied in order to enable the prediction of the reheated microstructure of welds. The transformation mechanism by which the original acicular ferrite formed is found to strongly influence the reaustenitisation process. The reverse transformation from ferrite does not occur immediately when the temperature is raised, even though the alloy may be in the ferrite plus austenite phase field. Reaustenitisation only begins when the carbon concentration of the residual austenite exceeds its equilibrium carbon concentration. This is a direct consequence of the fact that the acicular ferrite transformation ceases before the lever rule is satisfied. A theory has been developed, which explains the experimental data, including the fact that the degree of reaustenitisation varies with temperature above the Ae3 curve.

This research paper investigates the reaustenitisation process within steel weld deposits, specifically focusing on how the initial acicular ferrite microstructure influences this reversal. The authors used dilatometry and electron microscopy to observe that the transformation back to austenite is not immediate upon heating, even when thermodynamically favoured.

Their findings reveal that the reaction only commences once the carbon concentration in the remaining austenite exceeds equilibrium levels, a delay caused by the unique way acicular ferrite originally forms. By analysing these isothermal experiments, the study proposes a new theoretical model that accurately predicts the temperature at which the transformation begins and how it progresses.

Ultimately, the work clarifies that alloy composition and thermodynamic stability dictate the extent of microstructural changes during the reheating cycles typical of multi-pass welding.

Part 1: Short-Answer Quiz

Instructions: Answer the following questions using 2–3 sentences.

What are the two main classifications of the microstructure in the fusion zone of a multipass weld?
Why does the reverse α → γ transformation not occur immediately upon raising the temperature, even if the alloy is in the α + γ phase field?
What experimental tool was used to record rapid transformation rates?
Describe the relationship between acicular ferrite and classical bainite.
What specific condition must be met regarding carbon concentration for reaustenitisation to begin?
Under what circumstances does reaustenitisation occur without requiring the nucleation of new austenite?
What is the significance of the T₀' curve in the context of acicular ferrite formation?
How do the temperatures T_γ1 and T_γ2 define the stages of the reaustenitisation process?
How do substitutional alloying elements like Manganese (Mn) and Nickel (Ni) behave during reaustenitisation?
What is "paraequilibrium", and how does it differ from local equilibrium?

Part 2: Quiz Answer Key

1. Microstructure Classifications The microstructure is classified into the primary microstructure (formed during cooling from the liquidus, containing allotriomorphic ferrite, Widmanstätten ferrite, and acicular ferrite) and the secondary (reheated) microstructure. The secondary microstructure results from the thermal cycles of subsequent weld passes, which can cause annealing, recrystallisation, or reaustenitisation. 2. Delayed Transformation The transformation is delayed because the acicular ferrite transformation ceases before the lever rule is satisfied, leaving the residual austenite with a specific carbon concentration. Reaustenitisation only commences when the temperature is high enough that this residual carbon concentration exceeds the equilibrium concentration (the Ae3 curve). 3. Experimental Instrumentation A high-speed dilatometer from Theta Industries was used, featuring a water-cooled radio-frequency furnace with zero thermal mass to allow for programmed thermal cycles. It was interfaced with a microcomputer to record length, time, and temperature data at microsecond intervals, essential for capturing very rapid transformation rates. 4. Acicular Ferrite vs. Bainite Acicular ferrite and bainite share the same transformation mechanism: they grow via a diffusionless displactive mechanism followed by the partitioning of carbon into the residual austenite. The primary difference is that acicular ferrite nucleates intragranularly at point sites (typically inclusions), whereas bainite sheaves nucleate at austenite grain boundaries. 5. Carbon Concentration Condition Reaustenitisation begins only when the carbon concentration of the residual austenite (x_γ') exceeds its equilibrium carbon concentration (x_Ae3). This requirement ensures that there is a thermodynamic driving force for the austenite to grow and consume the surrounding ferrite. 6. Growth Without Nucleation In these experiments, the starting microstructure already contains substantial amounts of retained austenite in a matrix of acicular ferrite. Because austenite is already present, the process involves the growth of existing austenite rather than the nucleation of new grains, provided the up-quench rate is sufficiently high to avoid diffusional decomposition. 7. Significance of the T₀' Curve The T₀' curve represents the temperature and carbon concentration where austenite and ferrite of the same composition have equal free energy, accounting for the strain energy (approximately 400 J/mol). Diffusionless transformation of ferrite is only thermodynamically possible if the carbon concentration of the austenite is less than the concentration specified by the T₀' curve. 8. T_γ1 and T_γ2 Stages T_γ1 (identified as 680°C in this study) is the minimum temperature at which reaustenitisation commences. T_γ2 (approximately 760°C) is the temperature at which the alloy becomes fully austenitic; between these two points, the degree of reaustenitisation increases with temperature. 9. Behaviour of Mn and Ni At low reaustenitisation temperatures, substitutional alloying elements redistribute between the phases, deviating from a partition coefficient of unity. However, as the temperature or driving force increases, the transformation tends toward paraequilibrium or negligible-partitioning-local-equilibrium (NPLE), where the redistribution of these elements is minimised. 10. Paraequilibrium vs. Local Equilibrium Paraequilibrium is a state where the substitutional lattice is configurationally frozen, meaning the ratio of iron to substitutional alloying elements remains constant. In contrast, local equilibrium allows for the chemical potentials of all species to be maintained at the interface, though the actual redistribution of substitutional elements may be negligible in NPLE conditions.

Part 3: Essay Questions

The Impact of Multipass Welding on Microstructure: Discuss how thermal cycles create a "secondary microstructure". Explain the various metallurgical changes (annealing, recrystallisation, and reaustenitisation) and how they depend on thermodynamics and kinetics.
Mechanisms of Acicular Ferrite Formation: Compare acicular ferrite with allotriomorphic and Widmanstätten ferrite. Detail the role of inclusions and grain boundaries in determining morphology.
The Thermodynamics of the Incomplete Reaction: Explain the "incomplete reaction phenomenon" in acicular ferrite. Why does transformation stop before reaching lever-rule equilibrium, and how does this affect reheating?
Kinetics of Isothermal Reaustenitisation: Analyse the results of the dilatometric experiments. Discuss why the transformation rate is initially rapid and then decreases.
Solute Redistribution and Phase Equilibrium: Evaluate the role of Manganese and Nickel. Compare local equilibrium, paraequilibrium, and NPLE based on microanalytical data.

Term	Definition
Acicular Ferrite	A microstructure of ferrite characterised by needle-like shapes that nucleate intragranularly on inclusions.
Reaustenitisation	The process by which a previously cooled steel microstructure transforms back into austenite upon reheating.
Ac₁ Temperature	The temperature at which austenite begins to form during heating.
Ae3 Curve	The equilibrium curve representing the boundary between the γ and α + γ phases.
Incomplete Reaction	A condition where a phase transformation stops before the residual phase reaches its equilibrium composition.
Paraequilibrium	A state where only carbon redistributes, while the substitutional alloying elements (e.g. Mn, Ni) remain immobile.
Displacive Mechanism	A transformation involving a coordinated shift of atoms rather than individual atomic diffusion.
Up-quench	A rapid increase in temperature to reach an isothermal holding point while minimising intervening reactions.

Reaustenisation in Steel Weld Deposits

Study Guide: Reaustenitisation in Steel Weld Deposits

Part 1: Short-Answer Quiz

Part 2: Quiz Answer Key

Part 3: Essay Questions

Part 4: Glossary of Key Terms