0. Process Mathematical Simulation of the Rebar Quenching System Towards the optimization of the Temperature Profile M. Al-Harbi, T. Mehmood, S. Al-Motham, R. Kumar, H. Al-Qahtani March 2016

1. Objective
Towards the optimization of rebar quenching system, it is essential to understand the heat flux mechanism and its influence on the temperature distribution in rebar cross section.
A CFD mathematical model was developed to identify the key quenching process parameters on the rebar temperature profile, thus influencing its macrostructure.
The mathematical model would help to understand further the “on-line” heat treatment system and its influence on the production quality, and then this would lead to optimize the process further.

2. REBAR ON-LINE HEAT TREATMENT CONCEPT
Schematic illustration of temperature profile and microstructure evolution in heat treated rebars (A: austenite; M: martensite; TM: tempered martensite; F: ferrite;P: pearlite)*.
*M. Economopoulos, Y. Respen, G. Steffes, Application of the Tempcore Process to the Fabrication of High Yield Strength Concrete-Reinforcing Bars,CRM, vol. 45,1975

3. MATHEMATICAL MODEL: ASSUMPTIONS
One 1/8 of the system was simulated due to symmetry.
The transient state solution approach was used to run the simulation.
The additional heat content generated by the phase changes from austenite to martensite and austenite to ferrite/pearlite was not accounted in this model.
(a) Side view for quenching system single stage model. (b) Close view of the water inlet nozzles and rebar.

4. MATHEMATICAL MODEL: CHALLANGES
During this process, the development of a steam jacket surrounding the rebar would be possible. However, the tube geometry should maintain a water flow with hammering effect on the rebar surface.
The continuous breaking of the generated steam layer would result in developing a water steam mixture as a domain material. Therefore, a
(a) Tilted inlet water flow. (b) Tube zigzag design to generate steam breaking points.
corrected water properties were used as a domain material to capture the effect of the developed mixture on the cooling process.

5. MATHEMATICAL MODEL: DEVELOPMENT
In this Model, the rebar is defined as a set of coherent steel layers with different sizes. These layers would help monitor precisely the temperature evolution at different depth of the rebar.
Layer
Thickness (mm)
Radius size (mm) A
0.5 B
1.0
11-12 C
10-11 D
9-10 E
8-9 F
7-8 G
6-7 H
core
0-6

6. MATHEMATICAL MODEL: RESULTS

7. MATHEMATICAL MODEL: COOLING STAGE
Equalization Model to estimate the rebar temperature homogenization and natural cooling on the cooling bed after quenching process.

8. MATHEMATICAL MODEL: RESULTS
Rebar temperature evolution over 30 seconds during the cooling process for indicated distances from the bar center

9. MATHEMATICAL MODEL: RESULTS
Rebar temperature curves set on the related steel chemistry CCT diagram.

10. Hardness test results of rebar size 25mm.
MATHEMATICAL MODEL: RESULTS
Hardness test results of rebar size 25mm.

11. CONCLUSION
Results indicate that the martensite formation would occur within the rebar shallow shell of about 2 mm thickness. Later this would lead to develop the target rims of the tempered martensite. This proposal fits reasonably with the literature discussion about the aimed microstructure characteristics.
The CFD simulation is powerful tool that can be utilized to understand the rebar quenching practice and the influence of its operational parameters. Use of this model analysis leads to possible improvement.

12. Thank