Hydraulic Descaling

Heat Transfer and Fluid Flow Laboratory studies hydraulic descaling of steel experimentally. Experimental work in the lab is done to optimize descaling process in steel plants. Experimental results are used for development of theory of descaling as well. Hydraulic descaling should be incorporated into the production processes as continuous casting or hot rolling and must be designed to achieve product quality requirements with minimal cost.   

Introduction

Oxides are created on steel surfaces during continuous casting or hot rolling. The oxide scales are generally unwanted because of dissimilar properties to the underlying material, which is often designed to have special characteristics. Scales also act as a thermal barrier for cooling during rolling and have a negative effect on surface quality. Hydraulic descaling is used to remove oxide scales while the surface is impinged by high pressure water that disrupts scales by impact pressure and by thermal shock as well. 

The laboratory work is focused on descaling in relation to the surface quality and also in relation to heat transfer. A test bench for measuring water impact forces is available. The scales on tested surfaces are created in the laboratory with specific parameters provided in advance or during experimentation according to process specifications.


Heat Transfer Test

Heat transfer tests are carried out for test plates without oxide scales. The final heat transfer results (heat transfer coefficient distribution) are compiled using correlations based on additional heat tests of surfaces covered by scales.

The key heat transfer measurements are done on a linear test bench. The test plate is connected to a driving mechanism and to temperature sensors. The sensors record temperatures while running through the descaling section. The experiments are carried out for variable parameters that generally include water pressure, inclination angle, type of nozzles, distance of the nozzle from surface, and sizes of the overlapping areas. The heat transfer coefficient distribution is obtained by using the IHCP (inverse heat conduction problem) as a function of the spray parameters

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Temperature history at sensor position, pressure 5 MPa and 45 MPa

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Averaged HTC depending on water pressure

 
 

Experimental Procedure:

  • An electric furnace heats the test plate to the initial experimental temperature.
  • The plunger water pump is switched on and the water pressure is adjusted.
  • A driving mechanism moves the test plate through the spray. After recovering the temperature field in the plate, the movement of the plate under the spray is repeated.
  • The sensors measure the temperature at a depth of less than 1 mm from the cooled surface.
  • The positions of the test plate and the sensors (in the direction of movement) are recorded together with the temperature values.
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Test plate equipped by sensors during heat transfer test


Quality Test

The goal of quality tests is to evaluate the quality of descaling for given descaling nozzles, spray parameters, speed and chemical content of the plate.

Experimental procedure:

  • An electric furnace heats the test plate to the initial experimental temperature. When required, the examined surface of the test plate is covered by an additional plate to avoid scale formation during furnace heating.
  • A plunger water pump is switched on and the water pressure is adjusted.
  • The test plate is exposed to air in the furnace for required time by removing the additional plate.
  • The test plate is placed on the linear test bench and the driving mechanism moves the test plate through the spray.
  • The test plate cools in a protective atmosphere to avoid additional scale formation.

The final surface quality after hydraulic descaling is studied. The thickness of scale is measured by an ultrasonic thickness gauge or by using an electron microscope. The surface roughness is determined by surface analysis. Image analysis is used for evaluating the percentage of remaining scale.

      

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Electron microscope photo of a primary scale         Electron microscope photo of a secondary scale     

     

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Scales thickness measured by an electromagnetic probe  

         

 

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The roughness of the surface after descaling can be investigated by optical scanning. The surface roughness is expressed by colors in the left picture and by using topology on the diagonal profile in the right picture

    

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Evaluation of the percentage of remaining scale using image analysis.
Two cases of spray parameters - pressure 15/45 MPa, inclination angle 30/0 deg, velocity 3 m/s

     

 When it is necessary to obtain more types of descaled surfaces at one test plate, the entire experimental procedure is done twice, where the test plate is rotated 90˚ around its center before the second descaling. This way, four different types of surfaces are achieved.
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The test plate after two following quality tests; the arrows show first and second descaling directions. Each of the strips was descaled once, the middle of the plate was descaled twice, and the corners are without descaling


Impact Pressure Measurement

Impact pressure is one of the basic nozzle spray parameters, and is especially important for the characterization of descaling nozzles. To obtain the impact pressure distribution on the sprayed surface, a test bench was developed in the Heat Transfer and Fluid Flow Laboratory.

           

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The test bench designed for impact pressure distribution measurement - diagram and photo

           

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The graphical representation of the results obtained by impact pressure distribution measurement

             



Special Cases

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Special cases can be studied. The above photo shows a descaling test with the sub-cooled central part of a test plate

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Great effort is focused on the study of defects related to the overlapping area of descaling nozzles. Overlapping can cause thermal strips and quality defects to appear on the surface

 
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The left side of the plate sprayed by high pressure descaling nozzle without pulsation, the right side with pulsation

               

 

Additional Information

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