Heat Treatment

Typical task for the Heat Transfer and Fluid Flow laboratory in the field of heat treatment is to design cooling sections which ensure demanded cooling intensity during whole cooling process. Typically there are cooling regimes within cooling process in need of special attention to achieve required material structure of cooled product. These regimes must be necessarily studied in great detail and need to be very well controlled to reach high standard of production. 

Introduction

Microstructure and nature of grains, grain size and composition determine the overall mechanical behavior of steel. Heat treatment provides an efficient way to manipulate the properties of steel by controlling the cooling rate.

In hot rolling plants in-line heat treatment of rolled materials has become frequently used. In-line heat treatment is characterized by running of hot material through the cooling section; the required material structure is achieved without the necessity of reheating. Such a procedure is typically used for the cooling of tubes, rails, long products and plates.

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Cooling of rail with photo of perlite microstructure of the rail head at place "2"

Specific heat treatment in terms of final material structure is known for some types of steel. However, the practice has shown that the results obtained using small samples are different from the results achieved when using real products of a large cross-section. Therefore any continuous heat treatment process needs to be designed with respect to the shape and size of treated product and needs to vary the cooling intensity with time.

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Study of nozzle positioning for rail cooling


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Study of nozzle type influence for plate cooling

Design Procedure

Design procedure of cooling sections for obtaining the demanded structure and mechanical properties is iterative research involving several important steps. The first phase is to find the proper cooling regime for the selected material. The Continuous Cooling Transformation (CCT) diagram can be used as a first step if available, but it needs to be taken into an account that the CCT diagram is done for constant cooling rate and for different size of sample than in real designed cooling section. The cooling tests, studies of material structure and numerical simulations of cooling follow to find appropriate cooling intensity and its duration. Knowing the desired cooling intensity the nozzle configurations and cooling parameters are determined for the new cooling section. The final stage of the design is a laboratory test of a new cooling section with full size sample and evaluation of the sample structure.



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Experimental work for investigation of cooling parameters for cooling of long plates (left) and rails (right)

Preliminary Experiment

A test bench was built for the purpose of finding the optimal cooling strategy for a given material, which is used for subsequent numerical simulation. The suitable regime is obtained with respect to temperature, surface structure and mechanical properties.

Experimental procedure:

  • A steel sample embedded with thermocouples is heated to an initial start temperature in an electric furnace.
  • The heated sample is moved into a cooling position under the nozzle.
  • The deflector controlled by computer is repeatably opened and closed to simulate sample movement under the spray.
  • The temperatures recorded by thermocouples in the steel sample are saved to data logger and used to evaluate cooling intensity by inverse heat conduction algorithm.
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Example of sample (rail head) for preliminary experiment
               

The test bench is able to cool the sample in a wide range of regimes. The sample is cut after cooling experiment and then the hardness and structure of the cooled sample is investigated. This cooling regime can be modified until optimal structure of the material is achieved in dependency on distance from sample surface.



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Laboratory test bench for heat treatment study



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Measured temperature histories in a rail head in two depths and measured micro-hardness in rail head after heat treatment

Numerical Simulation

The experimental data are used as input for the inverse heat conduction algorithm to determine the boundary conditions (heat transfer coefficient history). Having boundary conditions the numerical simulation of cooling of products of various material properties and of various thicknesses can be performed. By repeating the boundary conditions it is possible to simulate long cooling section with more rows of cooling nozzles. The results are drawn in CCT diagram however the  cooling rate in cooling section in industrial application is far away from constant values. Therefore the final structure has to be verified by full-scale experimental measurement.

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Simulation of cooling in CCT diagram (Continuous cooling transformation diagram)

Cooling Intesity and Its Controllability

In order to design cooling section, knowledge of the cooling intensity (specified by the heat transfer coefficient) is required for group of nozzles and nozzle headers. Exact knowledge of the heat transfer coefficient as a function of spray parameters and surface temperature is the key problem for any design work. The cooling intensity is a function of several parameters, mainly nozzle types, chosen pressures and flow rates, surface temperature of a material, and velocity of a material movement whilst under the spray. There is no function available which describes cooling intensity using all the mentioned parameters. This is the reason why measurement and then the inverse heat conduction task calculation are absolutely necessary.

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Heat transfer coefficient distribution perpendicular to sample movement ( position "0" is in nozzle axis) for group of four nozzles with a nozzle pitch of 100 mm, spray angle of 60° along the sample movement axis, spray height of 100 mm, total flow rate 100 l/min at 5 bar and sample velocity of 1 m/s



Another important parameter is controllability of the cooling intensity. For need of soft cooling it is recommended to use the water-air mist or water full cone nozzles, for hard cooling flat jet nozzles at small spray heights. In some cases solid jet nozzles are used for hard cooling but there are often two major problems: large amount of water generate water layer on the product and spray spot is small which causes non homogenous cooling.

         

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Controllability of water-air mist nozzle for surface temperatures of 1000°C
Distribution of HTC under spray for high pressure flat jet nozzles for surface temperature of 1000°C
 
 
 
 
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Comparison of water distribution on flat surface for various nozzle types: flat‑jet, full‑cone, and solid‑jet nozzle

Verification at Pilot Test Bench

It is important to understand that cooling rate in cooling section in industrial application is far away from constant values for which CCT diagrams are obtained. Verification of a newly designed cooling system by further full-scale cooling tests prior to its plant implementation is essential. Pieces of tube, rail, wire or plate of real dimensions with implemented thermocouples are tested at the heat transfer test bench simulating designed cooling section.

There are two heat transfer test benches developed in Heat Transfer and Fluid Flow Laboratory. The first test bench moves the tested sample horizontally, the second vertically. Both of them allow the sample to move several times under the cooling sections. This cooling process is controlled by computer to simulate running under the long cooling section used in the plant. Nozzles, pressures, and header configurations are tested. The design of the cooling and the pressures used are modified until the demanded temperature regime. The full-scale material samples are then cut for the tests of material properties and structure.

Experimental procedure:

  • A steel sample is heated to an initial start temperature in an electric furnace.
  • The heated sample placed on the trolley is turned to spraying position.
  • The pump for the water gets going and the trolley runs several times through the cooling section under given conditions.
  • The temperatures recorded by thermocouples in the steel sample are saved to data logger together with the trolley position.          
 
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Full scale test of rail cooling – initial part of experiment at the horziontal test bench
            
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Full scale test of tube cooling at the horizontal test bench
          

      


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Heat transfer tests at the vertical test bench




Additional information

can be found in Publications of the Heat Transfer and Fluid Laboratory.