JPH07209162A - Method for evaluating heat resistance shock property of graphite - Google Patents

Abstract

PURPOSE:To provide a method for evaluating the heat shock resistance of a graphite electrode quantitatively and rationally. CONSTITUTION:The method for evaluating heat shock resistance of graphite features arc discharge heating at an arbitrary part of a cylindrical end face of a cylindrical graphite test piece for a specific thermal diffusion time, measurement of a limit power W for generating crack at the cylindrical end face edge, and the evaluation of heat resistance shock breakdown resistance parameter Re of graphite by the expression, where beta indicates a heating efficiency, sigmathetathetamax indicates a non-dimensional maximum tensile thermal stress in the circumferential direction as a function of radius ratio of the non-dimensional thermal diffusion time heating region of the cylindrical end face, an eccentricity position, and heat transfer rate of the cylindrical circle edge, R indicates the radius of a column, and a indicates the radius at a heated part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating thermal shock resistance of graphite, and more particularly to a method for evaluating thermal shock resistance of graphite electrodes for steelmaking or melting arc furnaces.

[0002]

2. Description of the Related Art A graphite electrode is used in a large amount to produce an arc from its tip and melt a steel material or the like to produce an alloy steel. Therefore, the tip of the electrode is 2,000 ℃
It becomes a high temperature that exceeds, and a temperature gradient occurs in its axial direction,
Remarkably large thermal stress and thermal shock occur during its practical use.

In the production of graphite electrodes, in order to improve their thermal shock resistance, a raw material is selected that has a high tensile strength σt and a high thermal conductivity λt, and a low longitudinal elastic modulus E and a low thermal expansion coefficient α. Manufactured. The performance of the graphite electrode is basically a parameter of the above thermal shock strength.

[0004]

[Formula 2] [σt · λt · (1-ν) / E · α] is manufactured (ν is Poisson's ratio).

This performance of the graphite electrode is confirmed only when it is put into practical use. Therefore, it is inconvenient to change the raw materials and the manufacturing conditions at the earliest time of 2-3 months and evaluate the result. Therefore, a simple and rational evaluation of the thermal shock resistance in the laboratory of the performance of the graphite electrode is highly desired from the viewpoint of the raw material preparation of graphite and the manufacturing technology.

[0006]

The inventors of the present invention first analyzed the three-dimensional steady-state thermal stress distribution using the results of the practical temperature distribution of the graphite electrode, and investigated the practical failure pattern of the graphite electrode. Was able to explain well (Sato et al., Carbo
n, vol. 12, No5 (1974) pp. 555-
572).

An arc is generated from the tip surface of the graphite electrode, which often fluctuates unstablely on the end surface. Moreover, it is difficult to easily reproduce the temperature distribution of the electrode experimentally. Therefore, for the rational evaluation of the thermal shock resistance parameter of the graphite electrode, the initial conditions can be set clearly and the end face unsteady heating conditions are given to obtain the precise arc discharge energy that causes thermal shock breakdown. It is necessary to make various measurement evaluations.

The present invention provides a direct measurement method of thermal shock parameters by a simulation experiment based on an analysis of unsteady thermal stress due to arc discharge heating at an eccentric location on an end face of a cylindrical model graphite similar to the shape of a graphite electrode. To aim.

[0009]

[Means for Solving the Problems] And, the purpose is to perform arc discharge heating at an arbitrary position on the end face of a cylinder of a cylindrical graphite test piece for a predetermined heat diffusion time, and to limit the formation of cracks at the edge of the end face of the cylinder. Is measured and the thermal shock rupture resistance parameter Re of graphite is evaluated by the following equation.

[0010]

[Equation 3] Where β is the heating efficiency, and σθθ _max is the dimensionless maximum thermal stress in the circumferential direction as a function of the radius ratio of the dimensionless heat diffusion time heating zone of the end face of the cylinder, the eccentric position and the heat transfer coefficient of the cylinder edge. , R is the radius of the cylinder, and a is the radius of the heated portion.

The present invention will be described in detail below. First, as a basis of the present invention, a general three-dimensional unsteady thermal stress in which an eccentric local heat generation corresponding to an arc discharge acts on an electrode tip from an arbitrary position on an end face of a cylinder was analyzed. In this solution, in a semi-infinite solid cylinder, a heat source of intensity Q0 acts on an arbitrary point on the end face of the semi-infinite solid cylinder, and at a time t = 0 as an initial condition, a uniform reference temperature is taken as the temperature T. = 0, the temperature distribution was obtained by the variable separation method when the heat transfer was from the end surface and the side surface of the cylinder as the boundary condition. Based on this, the mathematical analysis was carried out by introducing the thermoelastic displacement potential, Mitchell's function, and Boucine's function.

FIG. 1 shows a coordinate system when a heat source Q (= W / π · a ² ) having a radius a acts at a position e from the center on the end face of a cylinder. FIG. 2 shows the heating radius ratio ρa = a / R = 0.2,
Eccentricity ρe = e / R = 0.5, dimensionless temperature on the end face of the cylinder, that is, at the axial distance ζ = 0, when the Biot numbers related to heat transfer on the side surface and end surface of the cylinder are H1 and H2, respectively.

[0013]

[Equation 4] With the heat diffusion time τ (= κ · t / R ² , κ is the thermal diffusivity, t is the time) after the start of heating, which made the distribution on the diameter ρ (= r / R) of Indicated. According to this, when τ> 0.5 or more, the temperature distribution becomes constant. Therefore, the distribution of thermal stress also converges uniformly.

FIGS. 3 and 4 show the stress in the radial direction under the same conditions as in FIG.

[Table 1] And circumferential stress

[Table 2] The distribution is shown as dimensionless. Dimensionless stress here

[Table 3] Is expressed by the following equation for the actual stress σij.

[0015]

[Equation 5] z)

According to the figure

[Table 4] Are all compressive stresses and produce the maximum compressive stress at the center of heating.

[Table 5] Has the same compressive stress in the heating region, but on the outside it turns into tensile stress, and the maximum tensile stress occurs at the adjacent outer peripheral edge. When τ = 0.5 or more, these thermal stresses converge to a maximum value and become a constant value. But in reality τ = 0.2
At 5, the maximum value is reached. It should be noted that if the Biot numbers of the side surface and the end surface of the cylinder are set to be equal to H0, the influence of the Biot number H0 tends to decrease the thermal stress as H0 increases. In an experiment in a laboratory without air flow, which was described later, it was considered that H0 was about 0.5 from the observation of the Sirilen image, but H0 was set to 1 in consideration of air flow fluctuation due to arc. The tensile strength σt of the graphite electrode is about 1/3 of the compressive strength σc. Therefore, looking at the distribution of the thermal stress, it is most likely that the maximum tensile stress σθθ _{max at the} outer peripheral edge at σθθ causes fracture.

Therefore, if a crack is generated at the outer peripheral edge of the cylinder at a certain limit value of the local arc discharge heating power W on the end surface of the cylinder, the dimensionless thermal stress is given.

[Table 6] From the value of, it is considered that fracture occurs when the actual stress σθθ reaches the tensile strength σt.

[0018]

[Equation 6] However, rather than substituting various physical properties of the sample to evaluate σt, these physical properties are collectively used as the thermal shock resistance parameter Re.

[0019]

[Equation 7] It is actually more useful to evaluate.

According to the above equation, the thermal shock resistance parameter can be determined by measuring the critical power W at which cracks are generated from the circumferential edge of the end face of the cylinder which is close to the heating zone corresponding to the test condition. In the present invention, the cylindrical graphite test piece,
For example, it is preferable to take a boring from socket portions at both ends for connecting a nipple of a graphite electrode, but the present invention is not limited thereto, and a graphite test piece can be separately prepared by extrusion molding or the like. When the length of the cylinder is ρ = z / R> 3, the stress in the axial direction of thermal stress

[Table 7] Since the calculation result of is almost unchanged, it is sufficient if the diameter is at least twice the diameter.

FIG. 5 is a conceptual view of the situation for arc discharge heating during the test. In FIG. 5, (1) is an arc heating graphite electrode having a heating radius of 2a, which is screwed to a water-cooled upper copper block (4). (2) is a test piece (graphite cylinder), (3) is a lower copper table, (4 ')
It is screwed to the water-cooled lower copper block. The connections (2) and (3) and (3) and (4 ') are adhered by a conductive paste. The upper copper block (4) is connected to a power source for arc generation and is supported by a steel leaf spring so as to make instantaneous contact with the test piece (2) when an arc occurs. The gap between (1) and (2) is previously set to a constant value using a thickness gauge.

In addition to the thermal diffusivity κ, the heating duration t * is
The time corresponding to τ = 0.25 is defined by the following equation.

[0023]

(8) t * = 0.25R ² / κ

For example, if the radius of the graphite cylinder is R = 15 mm and the thermal diffusivity κ = 0.20 m ² / h, t * is 1.0 second. It should be noted that the thermal stress near τ = 0.25 hardly changes due to some error in t *, and there is an advantage that the effect of some error in κ is small. The unit area of the end face of the cylinder and the heating amount Q per unit time are 1/2 of the electric energy I ² R2 of the arc portion.
The heating efficiency β is determined by the following equation.

[0025]

## EQU9 ## β = (W-W ') / 2W

Where W is the total power between the voltage measuring terminals, W '
Is the electric power when the electric pole, the test piece cylinder and the conductive silver paste are closely connected. The voltage measurement terminals are set on the upper copper block and the lower copper base. The electrical resistance in the meantime is given by the following equation, if it is composed of the resistance R1 of the electric pole, the resistance R2 of the arc portion, the resistance R3 of the test piece cylinder and the contact resistance R4 of the cylinder and the lower copper base.

[0027]

## EQU10 ## W = I ² (R1 + R2 + R3 + R4) W '= I ² (R1 + R3 + R4) The relation curve of I and β is obtained by the least square method from the experiment for various values of the current I of W and W'. Determine β from the figure. The value of β is about 0.25.

[0028]

【Example】

Example-1 Eccentricity e on the end face of a cylinder of quasi-isotropic graphite made of mesophase pitch carbon with a diameter of 20 mm and a length of 50 mm.
An arc discharge heating experiment was performed with /R=0.5 and a heating radius ratio a / R = 0.2. In this condition

[Table 8] It is 0.026 at H0 = 0.1 and τ = 0.25.

As the theory shows, the location where the destruction occurred as a result of eight thermal shock experiments was the circumferential edge near the heating location,
The direction was radial due to circumferential stress. The limit power at which destruction occurred is the minimum power at which destruction occurred 5.15.
kw is 5.93 kw, which is the middle of the maximum electric power of 6.70 kw that did not cause destruction. β is the result of the test 0.2
7

[0030]

[Equation 11]

Poisson's ratio ν = 0.2 with respect to the value of Re
The value of thermal shock resistance σt · λt / E · α is calculated to be 8.29 w / mm. This value is the thermal shock resistance (σt / λt / E · α) due to the central heating of a graphite disk of the same type.
It agrees well with 7.5 to 8.8 w / mm. As described above, according to the evaluation method of the present invention, the thermal shock fracture resistance parameter Re including the Poisson's ratio of the cylindrical electrode model
Can be quantitatively evaluated.

[0032]

According to the present invention, the thermal shock resistance of graphite can be quantitatively and rationally evaluated in a laboratory, and the conventional arc steelmaking furnace must be qualitatively evaluated on site. The performance evaluation of the graphite electrode can be carried out easily and finely, which has a great industrial effect.

[Brief description of drawings]

FIG. 1 shows a coordinate system when a heat source acts on an arbitrary circular local region on an end face of a cylinder.

FIG. 2 shows a dimensionless temperature distribution over a diameter in which a heat source is present.

FIG. 3 shows radial and circumferential stresses in the dimensionless stress distribution, respectively.

FIG. 4 shows radial and circumferential stresses in the dimensionless stress distribution.

FIG. 5 is a conceptual view of a situation when an eccentric local portion of an end surface of a cylindrical test piece is heated by arc discharge.

Claims (1)

[Claims] 1. A graphite graphite test piece is subjected to arc discharge heating at an arbitrary position on the end face of a cylinder for a predetermined thermal diffusion time, and a limit electric power W at which cracks are generated at the end face of the cylinder is measured to determine the heat resistance of graphite. A method for evaluating the thermal shock resistance of graphite, characterized in that the impact fracture resistance parameter Re is evaluated by the following formula. [Equation 1] (Where β is the heating efficiency, σθθ _max is the dimensionless maximum heat in the circumferential direction as a function of the radius ratio of the dimensionless heat diffusion time heating zone of the end face of the cylinder, the eccentric position and the heat transfer coefficient of the cylinder edge. Stress, R is the radius of the cylinder, and a is the radius of the heating point.)