WO2025187525A1 - Arc welding method - Google Patents

Abstract

According to the present invention, an arc period includes a first period and a second period that comes after the first period. In the first period, a welding current is changed such that the welding current becomes larger than a virtual straight line connecting a first change point P1 and a second change point P2. In the second period, the welding current is changed in a straight line connecting the second change point P2 and a third change point P3.

Description

Arc welding method

The present invention relates to an arc welding method.

Patent Document 1 discloses an arc welding machine that, after a predetermined time has elapsed since the short circuit was released, applies constant current control for a predetermined period at a current value higher than the current value output under conventional constant voltage control, thereby suppressing the occurrence of a short circuit immediately after the short circuit is released and suppressing the generation of spatter.

Japanese Patent Application Publication No. 10-109163

In arc welding, the amount of molten metal needs to be increased to ensure sufficient bead width and penetration.

However, if the welding current is increased at a steep rate until it reaches a peak current in order to increase the amount of molten metal, there is a problem in that spatter is more likely to occur.

The aspects of the present disclosure were made in consideration of these points, and their purpose is to reduce the generation of spatter by appropriately changing the welding current during the arc period.

The first aspect is an arc welding method that performs welding by cyclically repeating forward and reverse feed of a welding wire, which is a consumable electrode, and alternating between short-circuit periods in a short-circuit state and arc periods in an arc state, wherein the arc period includes a first period and a second period that follows the first period, and the point at which the welding current starts to change in the first period is defined as a first change point, the point at which the welding current starts to change in the second period is defined as a second change point, and the point at which the welding current reaches a peak current in the second period is defined as a third change point, and the method includes a first step of changing the welding current in the first period so that the welding current is larger than an imaginary line connecting the first change point and the second change point, and a second step of changing the welding current in the second period along a line connecting the second change point and the third change point.

In the first aspect, the heat input to the welding wire is ensured by changing the welding current during the first period so that it is greater than the imaginary line connecting the first change point and the second change point. This allows droplets to be stably formed at the tip of the welding wire in the initial state of the arc period.

Furthermore, during the second period, the welding current is changed in a linear fashion connecting the second change point and the third change point, thereby ensuring that the heat input to the welding wire is increased at a constant rate.

This allows droplets, from the initial droplet formation stage through to their growth, to be constantly maintained at the tip of the welding wire. Furthermore, by ensuring stable droplet growth until the peak current is reached, droplets of the desired size can be formed.

Furthermore, the molten droplets held at the tip of the welding wire can be transferred stably toward the molten pool. As a result, it becomes easier to form a wide bead and the generation of spatter due to micro-short circuits can be suppressed.

In a second aspect, in the arc welding method of the first aspect, in the first step, the welding current is changed along an arc-shaped curve in which the welding current is greater than the virtual straight line.

In the second aspect, the welding current is changed along an arc-shaped curve during the first period to ensure the heat input to the welding wire. This allows for stable formation of droplets at the tip of the welding wire during the initial state of the arc period.

In a third aspect, in the arc welding method of the first aspect, a predetermined point between the first change point and the second change point is designated as a fourth change point, and in the first step, the welding current is changed in a linear fashion connecting the first change point and the fourth change point, and then the welding current is changed in a linear fashion connecting the fourth change point and the second change point.

In the third aspect, during the first period, the welding current is changed in a linear fashion connecting the first change point and the fourth change point, and then the welding current is changed in a linear fashion connecting the fourth change point and the second change point, thereby ensuring the heat input to the welding wire. This allows droplets to be stably formed at the tip of the welding wire in the initial state of the arc period.

A fourth aspect is an arc welding method according to any one of the first to third aspects, wherein the slope of the line connecting the second change point and the third change point is 200 A/msec or more and 1500 A/msec or less.

In the fourth aspect, by appropriately setting the gradient of the line connecting the second change point and the third change point, it is possible to ensure the heat input to the welding wire while suppressing the generation of spatter.

Specifically, if the slope of the line connecting the second and third change points is less than 200 A/msec, the heat input to the welding wire is small, and droplet growth at the tip of the welding wire becomes unstable. As a result, the periodicity between the short-circuit period and the arc period is disrupted, which may result in unstable welding.

On the other hand, if the slope of the line connecting the second and third change points is greater than 1500 A/msec, the heat input to the welding wire will be excessive, increasing the arc pressure applied to the molten pool. This increased arc pressure will then cause the molten pool to vibrate, which could result in spatter. Furthermore, if the heat input to the welding wire is excessive, the tip of the welding wire will rise significantly from the droplet, which could cause unstable droplet transfer to the molten pool and unstable bead formation.

Therefore, in this aspect of the present disclosure, the slope of the line connecting the second change point and the third change point is set to satisfy the above-mentioned condition.

In a fifth aspect, in the arc welding method of the fourth aspect, the slope of the line connecting the second change point and the third change point is 1200 A/msec.

In the fifth aspect, by appropriately setting the gradient of the line connecting the second change point and the third change point, it is possible to ensure the heat input to the welding wire while suppressing the generation of spatter.

According to this aspect of the disclosure, the occurrence of spatter can be suppressed by appropriately changing the welding current during the arc period.

FIG. 1 is a diagram showing a schematic configuration of an arc welding apparatus according to the first embodiment. FIG. 2 is a graph showing the time waveforms of the welding wire feed speed and the welding current. FIG. 3 is a graph showing the time waveforms of the welding wire feed rate and the welding current in Comparative Example 1. FIG. 4 is a graph showing the time waveforms of the welding wire feed rate and the welding current in Comparative Example 2. FIG. 5 is a graph showing the time waveforms of the welding wire feed rate and the welding current in Comparative Example 3. FIG. 6 is a graph showing the time waveforms of the welding wire feed rate and the welding current in Comparative Example 4. FIG. 7 is a graph showing the time waveforms of the welding wire feed rate and the welding current in Comparative Example 5. FIG. 8 is a graph showing the time waveforms of the welding wire feed rate and the welding current in Comparative Example 6. FIG. 9 is a graph showing the time waveforms of the welding wire feed rate and the welding current in Comparative Example 7. FIG. 10 is a graph showing the time waveforms of the welding wire feed rate and the welding current in Comparative Example 8. FIG. 11 is a graph showing the time waveforms of the welding wire feed speed and the welding current according to the second embodiment. FIG. 12 is a graph showing the time waveforms of the welding wire feed rate and the welding current according to the third embodiment.

Embodiments of the present invention will now be described with reference to the drawings. Note that the following description of the preferred embodiment is merely exemplary in nature and is not intended to limit the present invention, its applications, or its uses.

First Embodiment
As shown in FIG. 1 , the arc welding device 1 periodically repeats forward and reverse feed of the welding wire 15, and alternates between a short-circuit period in a short-circuit state and an arc period in an arc state between the welding wire 15 and the workpiece W to generate an arc 16, thereby welding the workpiece W.

The arc welding device 1 has a welding unit 10 and a control unit 30. The welding unit 10 has a welding torch 11, a feed motor 12, and a power conversion unit 20. The feed motor 12 feeds the welding wire 15 to the welding torch 11 at a predetermined feed speed.

The power conversion unit 20 includes a primary side rectifier 21, a switching unit 22, a main transformer 23, a secondary side rectifier 24, a reactor 25, a voltage detection unit 26, and a current detection unit 27.

The primary side rectifier 21 rectifies and outputs the output of the input power source 5. The switching unit 22 converts the DC output from the primary side rectifier 21 into AC. The switching unit 22 controls the welding output, which consists of the welding current and welding voltage.

The main transformer 23 converts the AC voltage output by the switching unit 22. The output of the main transformer 23 is output as a welding output via the secondary side rectifier 24 and reactor 25. The secondary side rectifier 24 rectifies the secondary side output of the main transformer 23. The voltage detection unit 26 detects the welding voltage. The current detection unit 27 detects the welding current.

The control unit 30 has a drive unit 31, a state detection unit 32, a short-circuit control unit 33, an arc control unit 34, a set current setting unit 35, a basic frequency setting unit 36, a basic speed amplitude setting unit 37, and an average feed speed setting unit 38. The drive unit 31 controls the switching unit 22.

The state detection unit 32 detects whether the state is a short circuit state or an arc state based on the detection result of the voltage detection unit 26. Specifically, the state detection unit 32 determines whether the welding output voltage is above or below a certain value based on the signal from the voltage detection unit 26. Based on this determination result, it determines whether the state is a short circuit state in which the welding wire 15 is in contact with the workpiece W and short-circuited, or an arc state in which a welding arc is generated in a non-contact state. A determination signal indicating the determination result of the state detection unit 32 is output to the short circuit control unit 33 and the arc control unit 34.

The short-circuit control unit 33 receives a short-circuit signal from the state detection unit 32 and controls the short-circuit current during the short-circuit period. Based on the set current, the short-circuit control unit 33 outputs a command to the drive unit 31 so that the short-circuit current has a predetermined waveform.

The arc control unit 34 receives an arc signal from the state detection unit 32 and controls the arc voltage during the arc period when the arc is in an arc state.

The arc control unit 34 outputs a command to the drive unit 31 to output peak current IP and base current IB for a predetermined time during the arc period. In this embodiment, the transition period from peak current IP to base current IB during the arc period is controlled by current control.

The set current setting unit 35 sets the set current. A signal indicating the set current is output to the short circuit control unit 33 and the arc control unit 34.

The basic frequency setting unit 36 determines the wire feed frequency appropriate for each set current based on the set current. Specifically, the basic frequency setting unit 36 is provided with a table or relational expression that correlates the set current with the frequency, and the frequency is determined from this table or the like based on the set current. A signal indicating the frequency is output to the feed motor 12.

The speed amplitude basic setting unit 37 determines the wire feed speed amplitude appropriate for each set current based on the set current. Specifically, the speed amplitude basic setting unit 37 is provided with a table or relational expression that correlates the set current with the speed amplitude, and the speed amplitude is determined from this table or the like based on the set current. A signal indicating the speed amplitude is output to the feed motor 12.

The average feed speed setting unit 38 determines the welding wire feed speed, which is the average wire feed speed appropriate for each set current, based on the set current. Specifically, the average feed speed setting unit 38 is provided with a table or relational expression that correlates the set current with the average feed speed, and the average feed speed is determined from this table or the like based on the set current. A signal indicating the welding wire feed speed is output to the feed motor 12.

The feed motor 12 controls the feeding of the welding wire 15 in a repeated, cyclical manner between forward and reverse feed based on the frequency, speed amplitude, and welding wire feed speed. The wire feed speed command is a command that repeats sinusoidal forward and reverse feed using the frequency and speed amplitude in accordance with the average wire feed speed appropriate for the set current value.

<Welding current time waveform>
2, time t1 indicates the time when the short circuit starts. During the short circuit period from time t1 to time t2, an initial short circuit current is output for a predetermined period from time t1, and then the welding current is gradually increased.

Between time t1 and time t2, a constriction of the droplet between the molten pool and the tip of the welding wire 15 is detected. When the constriction of the droplet is detected, the welding current is instantly shifted to a low current, and the short-circuit state ends.

Time t2 indicates the time when the short-circuit state ends and an arc state occurs. The period from time t2 to time t3 is an arc period in which an arc 16 is generated between the welding wire 15 and the workpiece W. During this arc period, an arc 16 is generated between the welding wire 15 and the workpiece W, and the heat of the arc 16 forms a molten droplet at the tip of the welding wire 15 and melts part of the workpiece W.

During the arc period from time t2 to time t3, the welding current is increased from base current IB to peak current IP. If the welding current is increased at a steep slope from base current IB to peak current IP, spatter is more likely to occur.

In this embodiment, the welding current is appropriately changed during the arc period to suppress the generation of spatter.

Specifically, the arc period includes a first period and a second period that follows the first period.

The point at which the welding current begins to change during the first period is the first change point P1, the point at which the welding current begins to change during the second period is the second change point P2, and the point at which the welding current reaches the peak current IP during the second period is the third change point P3.

During the first period, the welding current is changed so that it is greater than the imaginary line connecting the first change point P1 and the second change point P2. In the example shown in Figure 2, the welding current is changed along an arc-shaped curve where the welding current is greater than the imaginary line.

This ensures the heat input to the welding wire 15, allowing droplets to be stably formed at the tip of the welding wire 15 in the initial state of the arc period.

Next, during the second period, the welding current is changed in a linear fashion connecting the second change point P2 and the third change point P3.

This ensures that the heat input to the welding wire 15 increases at a constant rate, allowing droplet growth to occur stably until the peak current IP is reached, thereby forming droplets of the desired size.

Furthermore, the molten droplet held at the tip of the welding wire 15 can be stably transferred toward the molten pool. As a result, it becomes easier to form a wide bead and the generation of spatter due to micro-short circuits can be suppressed.

Here, if the slope of the line connecting the second change point P2 and the third change point P3 is less than 200 A/msec, the heat input to the welding wire 15 is small, and droplet growth at the tip of the welding wire 15 becomes unstable. As a result, the periodicity between the short-circuit period and the arc period is disrupted, which may result in unstable welding.

On the other hand, if the slope of the line connecting the second change point P2 and the third change point P3 is greater than 1500 A/msec, the heat input to the welding wire 15 will be excessive, and the arc pressure applied to the molten pool will increase. This increased arc pressure will then cause the molten pool to vibrate, potentially resulting in spatter.

Furthermore, if the heat input to the welding wire 15 is excessive, the tip of the welding wire 15 will rise significantly from the droplet, which may cause unstable droplet transfer to the molten pool and unstable bead formation.

Therefore, in this embodiment, the slope of the line connecting the second change point P2 and the third change point P3 is set to be greater than or equal to 200 A/msec and less than or equal to 1500 A/msec. Preferably, the slope of the line connecting the second change point P2 and the third change point P3 is set to be 1200 A/msec.

In this way, by appropriately setting the gradient of the line connecting the second change point P2 and the third change point P3, it is possible to ensure the heat input to the welding wire 15 while suppressing the generation of spatter.

During the arc period from time t2 to time t3, the distance between the tip of the welding wire 15 and the molten pool of the workpiece W transitions from T1 (first arc growth), to T2 (second arc growth), to T3 (third arc growth), to T4 (securing arc length). Here, T1 < T2 < T3 < T4.

This allows droplets, from the initial droplet formation stage through to their growth, to be constantly held at the tip of the welding wire 15. Furthermore, by stably growing the droplets until the peak current IP is reached, droplets of the desired size can be formed.

Time t3 indicates the time when the next short circuit occurs, and is in the same state as time t1. When the welding wire 15 and workpiece W come into contact and short-circuit, the molten metal droplet formed at the tip of the welding wire 15 during the arc period is transferred to the workpiece W by short-circuiting, forming a molten pool and performing short-circuit welding.

In the example shown in Figure 2, welding wire feed control is performed with a predetermined frequency and a predetermined speed amplitude, with forward and reverse feed alternately repeated multiple times in a sinusoidal waveform based on this basic waveform. At the peak of the forward feed, a short circuit occurs around time t1, and at the peak of the reverse feed, an arc 16 occurs around time t2. At the peak of the forward feed after time t2, another short circuit occurs around time t3.

As described above, the period from time t1 to time t3 constitutes one control cycle, and welding is performed by repeating this cycle.

Comparative Example 1
The following describes time waveforms when the welding current is changed during the arc period by a method different from that of the arc welding method according to the present embodiment. In the figures illustrating the comparative examples below, the change in the welding current according to the present embodiment is shown by a virtual line to facilitate comparison with the change in the welding current according to the comparative example.

In Comparative Example 1 shown in Figure 3, during the first period, the welding current is changed in a linear fashion connecting the first change point P1 and the second change point P2. Then, during the second period, the welding current is changed in a linear fashion connecting the second change point P2 and the third change point P3.

In this case, during the period from the first change point P1 to the second change point P2, the welding current applied to the welding wire 15 is small, resulting in insufficient heat input, and the growth of the droplet at the tip of the welding wire 15 becomes relatively small. As a result, there is a risk that the bead width will become narrow.

Comparative Example 2
4 , in a first period, the welding current is changed along an arc-shaped curve where the welding current is greater than the imaginary line connecting the first change point P1 and the second change point P2. Then, in a second period, the welding current is changed along an arc-shaped curve where the welding current is greater than the imaginary line connecting the second change point P2 and the third change point P3.

In this case, during the period from the second change point P2 to the third change point P3, as the welding current approaches the peak current IP, the growth of the droplets slows down, and the droplets of the welding wire 15 reach the peak current IP while still small. As a result, the total heat input is insufficient, and the droplets transfer to the molten pool in a relatively small state, which may result in a narrower weld bead.

Comparative Example 3
In Comparative Example 3 shown in Figure 5, during the first period, the welding current is changed along an arc-shaped curve that increases the welding current more than the imaginary line connecting the first change point P1 and the second change point P2. Then, during the second period, the welding current is changed along a straight line connecting the second change point P2 and the third change point P3'. Here, the third change point P3' is later than the third change point P3 in the above embodiment.

As a result, the slope of the line connecting the second change point P2 and the third change point P3' is gentler than the slope of the line connecting the second change point P2 and the third change point P3 in the above embodiment.

In this case, the amount of heat input to the welding wire 15 decreases during the period from the second change point P2 to the third change point P3', causing droplet growth at the tip of the welding wire 15 to become unstable, disrupting the periodicity between the short-circuit period and the arc period, which could result in unstable welding.

Comparative Example 4
In Comparative Example 4 shown in Figure 6, the welding current is changed from the first period to the second period along an arc-shaped curve in which the welding current is greater than the imaginary line connecting the first change point P1' and the third change point P3'. Here, the first change point P1' is later than the first change point P1 in the above embodiment. Furthermore, the wire feed speed is slower than the wire feed speed in the above embodiment, and the peak current IP' at the third change point P3' is smaller than the peak current IP at the third change point P3 in the above embodiment.

In this case, during the period from the first change point P1' to the third change point P3', the welding current applied to the welding wire 15 is small, resulting in insufficient heat input, and therefore the growth of the droplet at the tip of the welding wire 15 becomes relatively small. As a result, there is a risk that the bead width will become narrower.

Comparative Example 5
7, the welding current is changed along an arc-shaped curve from the first period to the second period, where the welding current is greater than the imaginary line connecting the first change point P1′ and the third change point P3. Here, the first change point P1′ is later than the first change point P1 in the above embodiment.

In this case, the change in welding current becomes steeper overall during the period from the first change point P1' to the third change point P3, and the welding current also increases when rising. As a result, the arc pressure applied to the molten pool increases, causing the molten pool to vibrate, which may cause spatter to fly off during the second arc growth, as shown in the figure.

Comparative Example 6
In Comparative Example 6 shown in Figure 8, the welding current is changed from the first period to the second period along an arc-shaped curve in which the welding current is greater than the virtual line connecting the first change point P1' and the third change point P3. Here, the first change point P1' is a point in time after the first change point P1 in the above embodiment. Note that Comparative Example 6 and Comparative Example 5 have the same change in welding current, but the cause of the welding defect in Comparative Example 6 will be described, which is different from that in Comparative Example 5.

In this case, during the period from the first change point P1' to the third change point P3, excessive heat input to the welding wire 15 causes droplets that have grown at the tip of the welding wire 15 to migrate to the side of the welding wire 15, disrupting the periodicity between the short circuit period and the arc period and making the bead appearance unstable, which may also lead to spatter scattering due to micro-short circuits.

Comparative Example 7
In Comparative Example 7 shown in Figure 9, the welding current is changed linearly from the first period to the second period, connecting the first change point P1' and the third change point P3'. Here, the first change point P1' is later than the first change point P1 in the above embodiment. The wire feed speed is slower than the wire feed speed in the above embodiment, and the peak current IP' at the third change point P3' is smaller than the peak current IP at the third change point P3 in the above embodiment.

In this case, during the period from the first change point P1' to the third change point P3', the welding current applied to the welding wire 15 is small, resulting in insufficient heat input, and therefore the growth of the droplet at the tip of the welding wire 15 becomes relatively small. As a result, there is a risk that the bead width will become narrower.

Comparative Example 8
10, the welding current is changed linearly from the first period to the second period, connecting the first change point P1′ and the third change point P3. Here, the first change point P1′ is a point in time later than the first change point P1 in the above embodiment.

In this case, the change in welding current becomes steeper overall during the period from the first change point P1' to the third change point P3, and the welding current also increases when rising. As a result, the arc pressure applied to the molten pool increases, causing the molten pool to vibrate, which may cause spatter to fly off during the second arc growth, as shown in the figure.

-Effects of the first embodiment-
As described above, according to the arc welding method of the first embodiment, the heat input to the welding wire 15 can be ensured by changing the welding current in the first period so that the welding current is greater than the imaginary line connecting the first change point P1 and the second change point P2.

Furthermore, during the second period, by changing the welding current in a linear fashion connecting the second change point P2 and the third change point P3, the heat input to the welding wire 15 can be ensured at a constant rate of increase.

This allows the molten droplet held at the tip of the welding wire 15 to be transferred stably toward the molten pool. As a result, it becomes easier to form a wide bead and the generation of spatter due to micro-short circuits can be suppressed.

Furthermore, even if the welding wire 15 feed speed is increased, welding can be stabilized, thereby shortening the tact time.

For example, when welding two overlapping workpieces W each having a thickness of 1.6 to 6.0 mm, the welding speed can be set to a range of 0.6 to 1.5 m/min. Furthermore, productivity can be improved by improving the gap tolerance.

Second Embodiment
Hereinafter, the same parts as those in the first embodiment will be denoted by the same reference numerals, and only the differences will be described.

As shown in FIG. 11, the arc period includes a first period and a second period that follows the first period.

The point at which the welding current begins to change in the first period is the first change point P1, the point at which the welding current begins to change in the second period is the second change point P2, the point at which the welding current reaches the peak current IP in the second period is the third change point P3, and a predetermined point between the first change point P1 and the second change point P2 is the fourth change point P4.

During the first period, the welding current is changed so that it is greater than the imaginary line connecting the first change point P1 and the second change point P2. In the example shown in Figure 11, the welding current is changed along a line connecting the first change point P1 and the fourth change point P4, and then the welding current is changed along a line connecting the fourth change point P4 and the second change point P2. After that, the welding current is changed along a line connecting the second change point P2 and the third change point P3.

As described above, the arc welding method according to the second embodiment allows droplets to be stably formed at the tip of the welding wire 15 in the initial state of the arc period.

Third Embodiment
As shown in FIG. 12, the arc period includes a first period and a second period that follows the first period.

The point in time when the welding current begins to change during the first period is the first change point P1, the point in time when the welding current begins to change during the second period is the second change point P2, the point in time when the welding current reaches the peak current IP during the second period is the third change point P3, a predetermined point in time between the first change point P1 and the second change point P2 is the fourth change point P4, and a predetermined point in time between the first change point P1 and the fourth change point P4 is the fifth change point P5.

During the first period, the welding current is changed so that it is greater than the imaginary line connecting the first change point P1 and the second change point P2. In the example shown in Figure 12, the welding current is changed along a line connecting the first change point P1 and the fifth change point P5, then along a line connecting the fifth change point P5 and the fourth change point P4, and then along a line connecting the fourth change point P4 and the second change point P2. Thereafter, the welding current is changed along a line connecting the second change point P2 and the third change point P3.

As described above, the arc welding method according to the third embodiment allows stable formation of droplets at the tip of the welding wire in the initial state of the arc period.

As explained above, the present invention has the highly practical effect of suppressing the generation of spatter by appropriately changing the welding current during the arc period, making it extremely useful and highly applicable industrially.

1 Arc welding device 15 Welding wire IP Peak current P1 First change point P2 Second change point P3 Third change point P4 Fourth change point

Claims (5)

An arc welding method in which welding is performed by periodically repeating forward and reverse feeding of a welding wire, which is a consumable electrode, and alternately repeating a short circuit period in a short circuit state and an arc period in an arc state,
the arc period includes a first period and a second period subsequent to the first period,
a first change point is a point in time when the welding current starts to change in the first period, a second change point is a point in time when the welding current starts to change in the second period, and a third change point is a point in time when the welding current reaches a peak current in the second period;
a first step of changing the welding current so that the welding current is greater than a virtual line connecting the first change point and the second change point during the first period;
a second step of varying the welding current in a linear fashion connecting the second change point and the third change point during the second period. The arc welding method of claim 1,
In the first step, the welding current is changed along an arc-shaped curve in which the welding current is greater than the virtual straight line. The arc welding method of claim 1,
a predetermined point between the first change point and the second change point is set as a fourth change point;
In the first step, the welding current is changed in a linear fashion connecting the first change point and the fourth change point, and then the welding current is changed in a linear fashion connecting the fourth change point and the second change point. In the arc welding method according to any one of claims 1 to 3,
an arc welding method, wherein a gradient of a straight line connecting the second change point and the third change point is 200 A/msec or more and 1500 A/msec or less; The arc welding method of claim 4,
An arc welding method, wherein a gradient of a line connecting the second change point and the third change point is 1200 A/msec.