JPS61246328A - Heat treatment of metal material - Google Patents

Abstract

PURPOSE:To prevent effectively stress corrosion cracking and fatigue fracture, by induction heating the inner part of metal material while cooling the surface thereof at the side where induction heating coil is provided, in heat treatment of metal material by the coil. CONSTITUTION:A heating coil 4 connected to a transformer 3 for induction heating is provided to inner surface of a welded zone 2 for connecting tubes 1 and 1a, the zone 2 is positioned almost at the center of the coil 4, and a coolant 5 is jetted to the inner surface of the tube at the vicinity of the part 2 from the coil 4 itself. AC having a suitable frequency is passed to the coil 4 through the transformer 3, to generate heat at the inner surface and the inner part of the tubes 1, 1a in which the coil 4 is set. In this case, even if inner surface is cooled, the outer surface is heated by heat generation of inner part, becomes to higher temp. than inner surface and the tensile stress is generated at the inner surface. After the stress exceeds the yield stress of tube material, the electricity conduction is stopped and the cooling of tube inner surface is stopped if the tube is cooled thoroughly.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an induction heating treatment method for a metal body, and particularly to a heat treatment method suitable for relieving welding residual stress on the inner and outer surfaces or either surface of a pipe weld. .

[Background of the invention]

There is Japanese Patent No. 95732 as a conventional induction heating treatment for relieving residual stress in pipe welding. This performs induction heating while cooling the surface on the side where no heating coil is provided. According to this, the side where the heating coil is not installed,
That is, although the residual stress on the cooling side is relaxed, the residual stress on the side where the heating coil is provided cannot be relaxed at all.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an induction heating treatment method for alleviating tensile residual stress existing in welded parts and the like.

[Summary of the invention]

Generally, in order to cause compressive stress to remain on a metal surface, the surface may be mechanically or thermally deformed by tensile plasticity while the interior remains as elastic as possible. The present invention has devised a method of efficiently achieving this by inductively heating the inside of the metal body while constantly cooling the entire surface on which the induction heating coil is provided. This invention focuses on the fact that induction heating is not performed by heat transfer on the metal surface, but by internal heat generation of the metal itself.

[Embodiments of the invention]

As an example of the present invention, a case will be described in which welding residual stress is applied to the inner surface of a pipe.

In Fig. 1, a heating coil 4 is connected to an induction heating transformer 3 on the inner surface of a welded part 2 connecting the pipes 1 and 1a.
is provided. This heating coil 4 is capable of ejecting a coolant 5 for cooling the inner surface of the tube from itself.

A current supply cable 6 for generating an induced current and a heating position control rod 7 are connected to the transformer 3, and the cable 6 is connected to a power source 8. 9 is a coolant supply tube. Temperature monitoring sensors 10 are installed on the inner and outer surfaces of the tube. toa is installed. 7 is connected to a heating positioning device 11. Tube 9 is communicatively connected to coolant delivery device 12 .

Furthermore, the inner surface of the pipe and the transformer 3. In order to optimize the gap between the coils 4, guide rollers 13 and 138 are provided on the transformer 3 and the coil 4.

First, positioning is performed using the positioning device 11 and the rod 7 so that the welded part 2 is located approximately at the center of the coil 4, the valve 14 is opened, the coolant feeding device 12 is started, and the coolant 5 is placed in the gold welded part 2. It is ejected onto the inner surface of the tube near the. The cable 6 and tube 9 need to be flexible and have slack in order to position the transformer 3 and coil 4.

After determining the position and starting cooling, the coil 4 is fully energized with alternating current by the power source 8 and transformer 3.

When energized, the part of the tube 1.1 where the coil 4 gold is set
An induced current is induced in a, and since the tube has electrical resistance (induced current) x (air resistance) heat is generated. If it is high, and this is 'eIc+, then the current 1 at t, which enters the tube thickness by X from the inner surface, is I x = I cB
e-"δ. Therefore, if the surface heat generation amount is Qcs, the heat generation amount Q at X is expressed as follows.

Qx=Qcse-"/J--" (1) Here, δ is said to be the heating penetration depth, and is expressed by the following equation.

δ= 5oaov'W Ishishita...Town□(2)
Here, μ and ρ are the relative permeability and specific resistance of the tube.

f is the frequency of the current introduced in 3. For stainless steel etc., it is about μ~1 ρ~10-4Ωm, so δ-,
50.3f".

According to equations (1) and (2), when f is high, that is, at high frequency current, Q8 decreases rapidly as X increases, and only the so-called inner surface layer generates heat. For example f=10
When stainless steel is heated at KHz, at a point 5 cm inside from the surface, only about 15 cm of heat is generated from the surface. In such a case, most of the generated heat is taken away by the coolant 5,
The temperature of the tube is difficult to rise. But for example f -200
If it is Hz, even a point inside x=10m generates about 60 inches of the heat generated by the surface. In such cases, even if the inner surface is completely cooled, the temperature will rise due to internal heat generation, and the outer surface will reach a high temperature even though it is not forced to cool. That is, a temperature distribution occurs in which the inner surface is low temperature and the outer surface is high temperature. This generates tensile strain on the inner surface of the tube. The current is continued until this tensile strain exceeds the yield strain of the tube material. It is possible to judge whether the tensile strain on the inner surface of the tube has exceeded the yield strain by measuring all the temperatures on the inner and outer surfaces of the tube.Temperature monitoring sensor 10°10a
This is done by monitoring the detected temperature. When a temperature difference between the inner and outer surfaces is sufficient to cause a full tensile yield strain on the inner surface, the current supply is stopped. However, cooling of the inner surface of the tube continues, and when the tube is completely cooled, the coolant supply device 12 is stopped and the valve 14 is closed. Next, when the transformer 3 and the coil 4 are removed from the heating and positioning device 11, the process ends.

The principle by which the residual stress on the inner surface of the tube is alleviated by this treatment is described below.

FIG. 2 shows the temperature Tc of the tube surface on the side where the heating coil 4 is set (hereinafter referred to as the coil side surface) and the temperature T of the opposite side surface (hereinafter referred to as the anti-swivel side surface) during this process. and shows the relationship between energization, cooling, and time. However, To rises rapidly with the start of energization.

Tc increases less than To due to forced cooling.

If the temperature distribution inside the thickness of the tube is a linear distribution connecting To and Tcfr, then the temperature difference ΔT required to cause a tensile yield strain ε, 1- to occur on the inner surface of the tube and to make the same compressive stress as the yield stress remain is.

becomes. Here, α is the coefficient of linear expansion of the tube, and ν is Poisson's ratio. For example, ΔT for stainless steel.

When calculating, α~17X10'''1/C1ν=0.3,
If εy~1.5X10", then ΔT is ~212C. When the temperature difference reaches ΔT, the energization is completely stopped.
Cooling is stopped when ΔT=0. However, although it is not necessary to perform the cooling to the point where it is completely cooled, it is preferable to perform the cooling at ΔT=θ″11 for safety reasons.

On the other hand, corresponding to the temperature behavior shown in FIG. 2, the stress strain behavior becomes as shown in FIG. 3. At the start of this process, tensile stress and strain occur on the surface of the heating coil, and the yield point σ
When reaching A (point A), stress does not increase and only strain increases. This is because the surface of the coil side, which is forcibly cooled, is stretched due to expansion due to the rise in temperature on the opposite side of the coil. If the heating is continued even after the yield point is reached, the strain increases as shown in A-H. Heating is stopped when the strain at point B becomes twice the strain εa at the yield point. When heating is stopped, the temperature decreases and the stress and strain decrease accordingly. However, B-) A- instead of returning to the origin, it falls elastically from point B, i.e. parallel to O → A,
When completely cooled, the strain becomes O and the stress reaches 0 point, which is equal to the compressive yield stress. On the other hand, compressive stress and compressive strain occur on the anti-coil side surface, and the order becomes O→A1→B1→C1. After all, when this process is finished, there is a compressive residual stress (0 point) on the surface of the coil side, and a tensile stress (point 0) on the surface of the anti-coil side.
Point C1) will remain.

In Fig. 3, the stress state before heat treatment is shown by assuming that the initial residual stress is O and all starting points are from the origin. However, the residual stress on the inner and outer surfaces of the weld is often about the same as the tensile yield point. FIG. 4 shows such a case. The initial state is at point P (corresponding to tensile yield), and all thermal strain on the coil side surface is plastic strain.Heating until ε = 26A and cooling from there results in P-4A-4B. →C, and when this process ends with ξ=0, a compressive stress equal to the yield point remains. The initial tensile residual stress (point P) is relaxed to compressive residual stress (point 0).

On the other hand, on the anti-coil side, the compressive strain is C starting from point P.
=-ε Stress = 0 at point A (point A1), ε=-2ε
When heating is stopped at point A (point B1), since no compressive plastic strain has occurred, the material passes through Ate and returns to the original point P.

That is, in this case, the residual stress on the coil-side surface is relaxed from point P to zero point, but on the surface on the opposite coil side, the residual stress remains the same because it returns to point P through the same route in both directions.

As can be seen from Figure 3.4, the coil side surface has compressive residual stress regardless of whether there is initial residual stress.

By reversing the total residual stress from tension to compression, fatigue failure, corrosion damage, corrosion fatigue failure, stress corrosion cracking, etc. can be easily prevented. In particular, problems caused by stress corrosion cracking have actually occurred in the welded parts of austenitic stainless steel M5U8304 piping in nuclear power plants, and good results are expected when used in these areas. In the above embodiments, the entire coolant is injected, but the coolant may also be flowed into the pipes.

Next, FIG. 5 shows another embodiment. The difference from the example in Fig. 1 is that the cooler 1 can spray 5ai of coolant also on the outer surface of the tube.
5 is being offered. In this example, coolants 5 and 5a
t-It is characterized by induction heating while simultaneously injecting. FIG. 6 shows the heat generation distribution (H curve) and temperature distribution (1 curve) within the wall thickness of the tube. Since heat generation is higher on the heating coil side, cooling on the heating coil side must be performed more efficiently than cooling on the reflective heating coil side. To this end, it is preferable to provide a difference in heat transfer without making the flow rates and coolants of the coolants 5 and 58 the same. The temperature-time curves of Tc and To in Fig. 6 are the same as T c = t in Fig. 2, and that of Tp is the same as rf + o, ... (in Fig. 2). The stress-strain behavior of is 0→A in Figure 3.
→ B → C, and the center of the board thickness is O → A1 → B
It becomes like l-+CI.

That is, according to this example, compressive residual stress can be applied to both the inner and outer surfaces. Therefore, it becomes a strength improvement measure when both the inner and outer surfaces are exposed to a corrosive environment, or when both the inner and outer surfaces are subjected to repeated loads.

As a modification of the above example, instead of using a tube, as shown in FIG. 7, a heating coil 4 is installed on one side of the plate 16, and a cooler 1 is installed on the other side to cool and heat from both sides. The effect is similar to the example shown in FIG.

Another modification is shown in FIG. In this example, heating coils are placed on the inner and outer surfaces of the tube (on both sides of the plate). Provide coolant 5
．． It is dangerous to carry out induction heating while cooling in step 5a. In this case, the heat distribution by each coil is H in Figure 9.
As shown in the curve of t and Hz, the overall result is Ht
The sum of Hz and Hz, that is, the curve H, which is symmetrical about the center of the plate thickness, makes it easy to obtain a temperature suitable for improving residual stress. Another advantage is that the temperature distribution can be adjusted by changing the heating conditions inside and outside.

The reason why it is possible to heat while cooling is that the present invention focuses on the internal heat generation of an object. In addition to induced current, there are heat generation methods that use direct current as applicable internal heat generation methods. FIG. 10 shows an embodiment using this. The current-carrying electrode 17° 17aIi is attached directly to the pipe 1.1a, sandwiching the welded part 2 where the total residual stress is to be improved.
The electrodes 17.degree. 17a are connected to the energizing heating equipment 19 by energizing cables 18,183. In this configuration,
First, 5.5 ak of coolant is injected from the coolers 15a, 151) from the inner and outer surfaces of the tube, and then electricity is supplied. The current distribution within the tube thickness due to energization becomes a uniform distribution. Therefore, the heat generation distribution becomes a uniform distribution H as shown in FIG.

However, since the surface is cooled, the temperature is low on the inner and outer surfaces.
A temperature distribution T occurs in which the temperature is high at the center of the pipe thickness. If the difference between the center temperature and the surface temperature satisfies equation (3), the residual stress on the inner and outer surfaces can be improved to compressive stress. The feature of this invention is that the heat generation distribution is uniform, and temperature differences are likely to occur.

〔Effect of the invention〕

According to the present invention, the tensile residual stress on the side where the heating coil is installed can be reduced, so stress corrosion cracking and fatigue fracture can be effectively prevented.

In particular, it can be used for T-pipes that have been assembled, or for piping that is in service but with limited installation of heating coils and coolers, and has a wide range of applications.

[Brief explanation of drawings]

Figure 1 is a basic configuration diagram of the present invention, Figure 2 is a temperature-time curve diagram during implementation of the invention, Figure 3 is a stress-strain behavior diagram during implementation of the invention, and Figure 4 supplements Figure 3. Fig. 5 is a diagram showing the configuration of an application example of the present invention, Fig. 6 is a diagram showing the heat generation distribution and temperature distribution of an application example of the invention, and Fig. 7 is a diagram showing another application example of the invention. 8 is a configuration diagram showing a modified example of the present invention, FIG. 9 is a diagram showing heat generation distribution and temperature distribution of a modified example of the present invention, and FIG. 10 is a diagram showing still another modified example of the present invention. The configuration diagram, FIG. 11, is a diagram showing the heat generation distribution and temperature distribution in the example of FIG. 10. 1.1a...Pipe, 2...Welded part, 3...Induction heating transformer, 4, +48...Heating coil, 5,5
a...coolant, 15...cooler. 1st Snake Koi lL 4 Daiji 11 Hitan, Koi 1 Shi 4 Daiji no -a Month [(4
[Behavior cylinder 4 narrow XpIl=gl, Y length T ~ saving; force is ゛me 1 sound 1st S factor 1α 5 1 factor

Claims (1)

[Claims] 1. In heat treatment of metal materials using an induction heating coil,
While the surface of the metal material on the side where the heating coil is provided is cooled with either a stationary refrigerant or a flowing refrigerant, the heating coil is simultaneously energized to create a tensile force greater than the tensile yield strain on the surface of the metal material on the heating coil side. A method for heat treatment of metal materials, characterized in that only energization is stopped after strain is generated, and cooling is continued until the metal materials are completely cooled. 2. A method for heat treatment of a metal material according to claim 1, characterized in that a cooler is provided on the side where the heating coil is not provided to cool the surface of the metal material in the same way as on the side where the heating coil is provided. . 3. A method for heat treatment of a metal material according to claim 1, characterized in that heating coils are provided on both sides of the metal material and heating is performed while cooling both surfaces.