18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká Republika NUMERICAL SIMULATION OF THE HIGH PRESSURE HYDRAULIC DESCALING
Jozef HRABOVSKÝ A, Jaroslav HORSKÝ B
a,b
Heat Transfer and Fluid Flow Laboratory , Faculty of Mechanical Engineering, VUT Brno, Technická 2,
619 69 Brno, Czech Republic, yhrabo01@stud.fme.vutbr.cz
Abstract
The process of descaling is a necessary part of forming processes, in which the clean surface before the
contact with the tool is required. Hydraulic or mechanical methods are the most commonly used methods for
descaling. Hydraulic high-pressure descaling is currently the most exploited process. Scales can be very
diverse in terms of chemical bonds and mechanical properties. In principle, they can be divided into primary
that are formed in specific conditions in the furnace, and secondary, arising in air. This article is focused on
simulation of the hydraulic descaling of the layers of the secondary scale.
To simulate the high-pressure spray, numerical analysis by Ansys was chosen, through which the influence
of thermal and mechanical stress effects on the layer scale was classified. Numerical simulation of highpressure spray was carried out in two steps. The first step was the simulation from the macro point of view,
when the high-pressure spraying was simulated as a continuous function of heat transfer. This function has
been experimentally measured in the test of the descaling nozzles. The second step was the simulation from
the micro perspective. Very short but intense pulse of heat transfer on small areas was simulated. This
approach represents a high-speed impact of a single drop of water. By studying these two aspects, it is
possible to compare the impact of individual components, mechanical and thermal, on the stress in the layer
scale. Such comparison reveals which of these components has a greater influence on the process and it is
possible to design a strategy for hydraulic removal of scale to ensure process efficiency and quality of the
surface.
Keywords: oxide scales, descaling, numerical simulation, stress, deformation
1.
INTRODUCTION
Increased demand for the products with good quality surface layers without residual and rolled oxides leads
the steel industry to search options and methods to fulfill these claims. To achieve these goals we need to
examine the mechanism of the emergence of oxides as well as the mechanism of its removal. The
emergence of oxides is a very uneasy process, which is accompanied by both a deterioration of the quality
of surface and its degradation in the form of micro cracks or cores. There are many mechanisms for their
deletion. Chemical or mechanical methods are used for tertiary descaling. For secondary descaling, the
mechanical and mainly hydraulic methods are used. This article is focused on the hydraulic mechanism of
the secondary oxides descaling. Secondary oxides are produced in phase after extraction of the steel from
furnace on the current atmosphere, on the air. Secondary oxides are usually composed of three chemical
layers. The basic layers are FeO (wustit), Hematite (Fe2O3) and Fe3O4 (magnetite). These layers are formed
at different temperatures. Magnetite and Hematite are formed if the oxidation is under 570° C and wustit
rises if the temperature gets over 570° C [1]. The thickness of the secondary oxides is not too large due to
the short oxidation time. Values of thickness are between 20-50 µm. Hydraulic descaling can be split in two
components. The first component is thermal. It is represented by temperature gradient between the heavily
cooled, thin upper layer and the remaining massive hot parts of steel. The second is the mechanical
component, which is caused by a high pressure water jet. The aim of this article is to determine the share of
18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká Republika these two components on the final quality of the surface during the descaling process. To achieve relevant
results, the numerical simulation - method of FEM - was selected. Monitoring of each component was taken
from two aspects. From macro-perspective, when the hydraulic descaling was regarded as the ray of the
water jet with a defined heat transfer coefficient, and impact pressure. From micro-perspective, when the
water jet of the hydraulic descaling was decomposed into each drops of water. Water drop had defined value
of heat transfer and impact pressure as well.
2.
FORMULATION OF THE PROBLEM AND ITS COMPREHENSIVE ANALYSIS
Comparison of the effects of heat and mechanical components at the hydraulic descaling from the surface of
steel taken from the macro and micro-perspective through design modeling.
To achieve the formulated goals, numerical simulation by ANSYS program was selected. As already
mentioned in the introduction, the mechanism of descaling was decomposed on thermal and mechanical
part. For the macro aspect, heat transfer function and impact pressure of the water jet were identified as
significant factors. Both of these have been taken from real experiment (see chapter 3.1). Determination of
the essential factors for micro aspect quantitatively matches the preceding factors listed for macro aspect.
They are qualitatively different, because they come from design modeling referred to in article (see chapter
3.2). They represent the values of the heat transfer and impact pressure of one water drop.
3.
INPUT DATA
This thesis is based on the input data coming from real experiment, but also on data taken from articles or
literature, which are based on calculations and numerical simulation.
3.1
Experimental data
The measurement, from which the input data for this analysis come from, was conducted in The Heat
Transfer and Fluid Flow Laboratory. Two types of measurements were made. In the first case, the heat
transfer coefficient (HTC) was measured, which characterizes the intensity of the water jet cooling. In the
second case, the impact pressure was measured, which represents the pressure force of the water jet.
Measurements were not carried out within the framework of this work, they are mentioned to document the
origin of the input data used in this article [2].
3.2
Inputs from literature
These data have been especially used in the calculation from the micro aspect, in which it is the value of the
heat transfer coefficient and impact pressure from one water drop. Data representing the variables were
calculated or partly measured. Procedures, by which the data have been obtained, are listed in the relevant
publications [3], [4]. This type of data also includes material properties of the steel [5], [6] and oxide scales
[7], [8], which were used as well. The specific values are stated in chapter 4.3.
4.
CALCULATION METHODOLOGY
To solve the problem (see chapter 2), the FE method in ANSYS 12 was chosen. As already mentioned in the
introduction, the issue was examined from two aspects. The macro and micro aspects, while both can be
divided into two groups. The first group is the thermal-stress analysis, where temperature boundary
conditions (B.C.) appear, which is the temperature of the sample and defined function of heat transfer
coefficient. The second group is the structural analysis, where the function of impact pressure appears.
4.1
18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká Republika Decision procedure
The decision procedure was selected on the basis of established goals. Numerical simulation, which was
chosen as a method of solution, was carried out from macro and micro aspect. Both approaches have
qualitatively the same results that could be reconciled. For the macro aspect, experimentally measured B. C.
have been used (see chapter 4.4.1) and the corresponding model of geometry has been created (see
chapter 0). For the micro aspect, the boundary conditions (see chapter 4.4.2) were represented by the data
from literature and a model of geometry has been prepared as well. For both approaches, the same model of
material data, have been used. After creating all of the needed models, the transient thermal analysis was
made. The result of this analysis was the distribution of the temperature field in the model. On the basis of
these data, the stress analysis was performed. Structural analysis has been the other calculation made on
the model. This procedure was used for both macro and micro aspect.
4.2
Topology and geometry model
The Model of geometry was prepared on the basis of previous calculations with regard to the geometry of
the samples used in the Heat Transfer and Fluid Flow Laboratory, at descaling experiments or heat transfer
measuring. The Model of geometry is composed of two parts. The first robust part is constituted by base
material (steel) and the second part consists of a thin layer of oxide scales. Oxide scale layer is not more
structured and is contiguous. This structure is the same for both the aspects, but it differs in geometric
dimensions (see Fig. 1).
Fig. 1 The Geometry
model from macro and
micro aspect
4.3
Model
Material model
geometry
is
composed of two types of materials therefore it was needful to enter the material properties for both of them.
Due to the combination of temperature and structural analysis, material properties depending on temperature
were used. Material properties of steel correspond with the values for steel S235 [5], [6]. Material properties
of oxide scales correspond with the values for wustit (FeO) [7], [8]. The values of material properties for both
materials are listed below (see Tab. 1). To affect the plastic behavior, the bilinear material model with
isotropic hardening has been used.
4.4
Boundary conditions and loading
This chapter is divided into two parts, B.C., for macro aspect and B.C. for micro aspect. Both of these
sections are further divided into B.C. and loads for thermal and structural analysis.
Temperature
Specific
heat
Thermal
conductivity
18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká Republika Thermal
diffusivity
Density
Young´s
modulus
Poisson
ratio
[m2/s]
15.8e-6
16.5e-6
19.1e-6
19.4e-6
19.6e-6
19.8e-6
[kg/m3]
7800
7780
7570
7540
7510
7480
[Pa]
2.1e11
2.1e11
2.7e10
1.9e10
1.4e10
9.5e9
[-]
0.3
0.3
0.3
0.3
0.3
0.3
2.4e11
2.3e11
1.6e11
1.5e11
1.4e11
1.3e11
0.36
0.36
0.36
0.36
0.36
0.36
Material properties of structural steel
[°C]
[J/kg.K]
20
513
100
514
700
580
800
593
900
605
1000
616
Material properties of FeO
20
100
700
883
800
913
900
942
1000
972
[W/m.K]
12.6
14.0
24.7
26.4
27.8
29.2
1.2e-5
1.2e-5
1.7
1.2e-5
1.8
1.2e-5
1.9
1.2e-5
2.0
1.2e-5
Tab. 1 Material properties
5700
5700
5700
5700
5700
5700
4.4.1 B.C. and loading for macro aspect
Thermal B.C., were determined by a constant value of model temperature which was 1000°C. Structural
B.C., were applied on the geometry model. It has been chosen to simulate real behavior by heating and to
burden the pattern mechanically. Therefore, to the relevant areas were assigned B.C. that prevent or
authorize the displacements in the corresponding direction. Input data for loading are the data received from
an experimental analysis (see chapter 3.1). The course of HTC function based (see Fig. 3) was used for the
temperature task. Load entering the structural role was represented by the course of function of impact
pressure of water jet on the position (see
Fig. 2 HTC function of water jet
Fig. 3)
Fig. 3 Impact pressure of water jet
4.4.2 B. C. and loading used for micro aspect
As in the previous case, the thermal B. C. were fixed by constant value of the temperature of the model,
which was 1000° C. Real behavior of thermally and mechanically loaded sample was simulated equally as in
the previous case. Corresponding shifts were authorized or precluded to relevant areas. Loads are based on
the values from the literature (see chapter 3.2). For the thermal role, a linear course of HTC with
dependence on time (see Fig. 4) was used. Load entering the structural role was represented by the
linearized progress of an impact pressure of water jet on time (see Fig. 5). Both the loads for the temperature
and the structural role are very fast actions and appear in µs.
Fig. 4 HTC of water drop
5.
18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká Republika Fig. 5 Impact pressure of water drop
RESULTS
To maintain initial structure, the results are presented successively for macro aspect and micro aspect. Each
section is further divided into the results of the thermal and the structural analysis.
5.1 Macro aspect
Thermal analysis:
Fig. 6 Temperature distribution
5.2
Structural analysis:
Fig.
7
Equivalent
stress
Micro aspect
Thermal analysis:
Fig. 8 Temperature distribution
Fig. 10 Stress results of the steel parts
Structural analysis:
Fig. 9 SY - compression stress
Fig. 11 Stress results of the oxide scales
6.
18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká Republika CONCLUSION
The aim of this work was to compare the influence of the thermal and mechanical component of the
hydraulic descaling from two perspectives. The first was the simulation of oxide removal from macro aspect,
where the full hydraulic beam has been considered. In this approach the values of cooling of the sample
surface due to the heat transfer and the subsequent values of equivalent, pressure and shear stress from
both the temperature and pressure load have been observed. These values have been observed for both the
steel part of the sample, and for the part of the scale. Illustration of results are given in chapter 5.1 and
shown on Fig. 6 and Fig. 7. In the second approach, the descaling simulation has been studied from micro
aspect, when a single drop of hydraulic radius has been considered. In this case, the procedure has been
the same as in the previous method. The surface temperature, pressure and shear stress have been the
observed variables. The results of this analysis are given in chapter 5.2 and individual values are presented
in Fig. 8 and Fig. 9. Final results are divided in two parts. First part shows the results of the steel (see
Fig.10), second part shows the results of the oxide scale surface (see Fig. 11). Presented results for macro
aspect suggest that the influence of the impact pressure is larger in both the stress in the surface of the
scale layer and on the steel structure. Received values of stress at the steel part of the sample can be used
to assess stress at the interface of the scale and steel. Results for micro aspect however show that in this
case the thermal load has a greater influence upon stress. But it is not as clear-cut as at the first approach.
These conclusions, however, generally can´t claim to be universal. For this to be done, it would be
necessary to make a great amount of calculations of stress influence upon different types of nozzles, under
various pressures, thickness of scales, etc. However, this analysis reveals the effects of individual
components upon stress and points out the sensitivity of parameters closely linked with the hydraulic
descaling.
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