JPH0670257B2 - Heat treatment method for condensing heat exchanger body made of low carbon austenitic stainless steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a heat treatment method for a condensing heat exchanger body made of austenitic stainless steel for condensing water vapor in a gas, and particularly to a gas in which chlorine ions are mixed. The present invention relates to a heat treatment method for reducing welding residual stress in order to prevent intergranular stress corrosion cracking from occurring in a high residual stress portion of a welded portion of a condensation heat exchanger body.

[Prior Art] As an example of a condensing heat exchanger, in the treatment of exhaust gas of a nuclear reactor, oxygen and hydrogen in the exhaust gas generated due to the decomposition of water by radiation in the nuclear reactor are recombined to form steam. ,
Then, in order to reduce the exhaust gas volume, the water vapor is condensed by passing through a condensation heat exchanger. Conventionally, it is generally considered that such a condensation heat exchanger does not cause stress corrosion cracking, and therefore, it is a relatively inexpensive SU that is normally used in equipment where stress corrosion cracking does not occur.
S304 stainless steel is used, and measures for reducing welding residual stress have not been implemented.

[Problems to be Solved by the Invention] However, some chlorine ions may be mixed in the gas sent to the condensation heat exchanger. The steam condensing zone of the condensing heat exchanger is repeatedly dried and wet, and when chlorine ions flow into this zone, the chlorine ions are concentrated and stress corrosion cracking easily occurs in the condensing heat exchanger. They discovered.

By the way, conventionally, for austenitic stainless steel members of uniform thickness and simple thickness such as rolled steel plates and pipes, there is a solution heat treatment of quenching from 1000 to 1100 ° C as a method to prevent intergranular corrosion, but this is condensed heat exchange. When applied to a welded structure having a complicated wall-like structure and various wall thicknesses, it is very difficult to control the temperature and has a drawback of causing deformation. Deformation is caused by gravity during high temperature heating and by the difference in shrinkage during rapid cooling, both of which impair the function of the heat exchanger.

Also, the use of an ultra-low carbon stainless steel having a C content of 0.03% or less (for example, SUS316L) brings about better corrosion resistance than SUS304, but was not a decisive means for completely preventing stress corrosion cracking. In particular, it has been confirmed that cracking occurs in a chloride ion concentrated environment.

As described above, as a measure for preventing the stress corrosion cracking of the condensation heat exchanger, the above-mentioned conventional method is not a satisfactory solution.

Therefore, in view of the above points, it is an object of the present invention to provide an effective and simple heat treatment method for preventing transgranular stress corrosion cracking of a condensing heat exchanger so as to reduce residual stress without deformation. .

[Means for Solving the Problems] According to the present invention, after the austenitic stainless steel condensing heat exchanger body having a reduced carbon content is maintained at 1000 to 1100 ° C., 100
Provided is a heat treatment method for a low carbon austenitic stainless steel condensing heat exchanger body, characterized by gradually cooling at a cooling rate of ℃ / Hr or less.

[Operation] As described above, the present inventors newly discovered the phenomenon and mechanism in which the condensation heat exchanger is easily corroded in terms of its use environment and function. The environment of the steam condensation zone on the inner surface of the body of the condensing heat exchanger is at a temperature of about 102 ° C. and a humidity of 100%, and the upper part of the body has a small number of condensed droplets, so that it is constantly dried and wet. When chloride ions flow into this zone, there is a phenomenon that chloride ions are concentrated during the drying process, and eventually a high chloride ion concentration environment is created. Thus, it was newly confirmed that the condensation heat exchanger functionally has a high corrosion potential when chlorine ions are brought in.

However, based on the heat treatment method of the present invention, when the condensing heat exchanger after welding is heated and maintained at 1000 to 1100 ° C., the material has a reduced deformation resistance, and work hardening, or the effect of sensitization of the temperature rising process is eliminated. Away, the residual stress is reduced, and the precipitated δ ferrite disappears as a solid solution in the austenite matrix.
The weld toughness is improved. In addition, controlling at a cooling rate of 100 ° C./Hr or less has the effect that the Cr carbonide does not cause grain boundary corrosion by precipitating at the crystal grain boundaries, and because the entire structure can be cooled uniformly, It has a function of preventing tensile stress or deformation caused by the difference in cooling rate of the portions having a difference in wall thickness. However, at other heating temperatures, the residual stress does not fall below the allowable stress level of the material below 700 ° C, and
Embrittlement occurs in austenitic weld metal in the temperature range of around 00 ° C to 700 ° C. In addition, high-speed rapid cooling of 100 ℃ / Hr or more,
This causes deformation of the welded structure and causes danger. Thus, the desired effect cannot be obtained except under the conditions according to the method of the present invention. According to the conditions according to the method of the present invention, a residual stress lowering effect satisfying the strength characteristics can be obtained without the problem of deformation, and transgranular stress corrosion cracking can be effectively prevented.

[Examples] Examples of the present invention will be described below. FIG. 1 is a schematic side view of a condensing heat exchanger body, and FIG. 2 is an AA cross sectional view thereof. The body, which is an integrally welded structure in which a drain pot (condensed water collecting place) 2, a foot plate 3, a flange 4, and various nozzles 5 are attached to the body plate 1, controls the carbon content to a low level of 0.03% or less. Stainless steel (eg SUS3
16L, SUS304L, SUS316, etc. with a C content of 0.03% or less) and a plate thickness of several mm to several tens of mm. A pipe group (not shown) is provided therein, and cooling water that has entered from one of the nozzles 5 flows through the pipe group and flows out from the other nozzles 5, while
Exhaust gas entering from the entrance of the end plate 6 (may contain some chlorine ions) flows out from one of the other nozzles,
During that time, the water vapor contained in the exhaust gas is cooled, condensed, and collected in the drain pot 2 to be taken out. By the way, in manufacturing the above-mentioned body, a peripheral welding line is formed on the end plate 6 portion, a longitudinal welding line is formed on the body plate 1, and a fillet welding line is formed on the leg plate 3 portion, and high residual stress exists therein. Therefore, according to the present invention, the heat treatment of the body is performed before the tube group is inserted therein.
Is inserted inside the body, and is clamped from the outside with the outer peripheral restraint band 8 to restrain the body. The condensing heat exchanger body to which the above restraint jig is attached is heat-treated under the heat-treatment conditions shown in FIG. For temperature control, the actual temperature is detected with a thermocouple, the heating rate is set to 100 ° C / Hr or less, and 1H at 1000 ° C to 1100 ° C.
Hold at r or less, and then slowly cool at a cooling rate of 100 ° C / hr or less. After the heat treatment, the restraint jig 7,
8 is taken out.

Next, the effects obtained in the above-described embodiment will be described.

1) Effect of reducing residual stress Fig. 4 shows the results of measuring the residual stress by heat-treating the small test simulated body that was welded by restraint at various heat treatment temperatures after welding. Of about 35 kgf / mm ² about the residual stress in what remains of the welding, it can be seen that a decrease of about 10 kgf / mm ² or less according to this example.

2) Recovery of ductility and toughness of weld metal Figure 5 shows the effect of post-weld heat treatment on the ductility and toughness of weld metal. Impact test / tensile test at various heat treatment temperatures
As a result of microscopic observation, both ductility and toughness decrease at a heat treatment temperature near 800 ° C, but recover and further improve at 1000 to 1100 ° C in the examples.

3) Effect on grain boundary type stress corrosion cracking The members of this example were subjected to the improved Strauss test on two cases, one that was subjected to the low temperature sensitization treatment and the other that was not subjected to the low temperature sensitization treatment. It was confirmed to have field-type stress corrosion cracking performance.

4) Effect on Transgranular Stress Corrosion Cracking A test was carried out by immersing the heat-treated body of this example in boiling MgCl ₂ . With the as-welded product, cracks were produced in all of the longitudinal welded portion, the peripheral welded portion and the fillet portion of the plate, but no cracks were produced with the heat-treated product of this example.

5) Measurement of deformation amount Before and after this heat treatment, the dimensions of the condensation heat exchanger body were measured and the amount of deformation was examined, but the amount of deformation was within the tolerance of 2 mm for manufacturing the heat exchanger, and the nozzle stub did not deform at all.

As described above, according to the present embodiment, the deformation of the body is prevented, the residual stress in the welded portion is reduced, and the corrosion resistance is improved.

[Advantages of the Invention] According to the present invention, deformation of the condensing heat exchanger body is suppressed, residual stress in the welded portion is reduced, and transgranular stress corrosion cracking is prevented. As a result, the maintainability and reliability of the low carbon stainless steel heat exchanger body are improved, the frequency of overhaul and inspection is reduced, and the product life is extended.

[Brief description of drawings]

FIG. 1 is a side view of a condensing heat exchanger body according to an embodiment of the present invention, FIG. 2 is a sectional view taken along line AA of FIG. 3, FIG. 3 is a view showing heat treatment conditions, and FIG. FIG. 5 is a diagram showing the influence of the post heat treatment, and FIG. 5 is a diagram showing the influence on the ductility and toughness of the weld metal. 1 ... Body plate, 2 ... Drain pot 3 ... Leg plate, 4 ... Flange 5 ... Various nozzles, 6 ... End plate 7 ... Restraint ring, 8 ... Outer restraint band

Front page continued (72) Inventor Wataru Sagawa 3-1-1, Saiwaicho, Hitachi City, Ibaraki Hitachi Ltd. Hitachi factory (72) Inventor Akira Kogane 3-1-1, Saiwaicho, Hitachi City, Ibaraki Prefecture Hitachi Ltd., Hitachi Plant (72) Inventor Shigeo Hattori 3-1-1, Saiwaicho, Hitachi City, Ibaraki Prefecture Hitachi Ltd., Hitachi Plant

Claims (2)

[Claims] 1. A condensing heat exchanger body made of austenitic stainless steel having a carbon content reduced to 0.03% or less is maintained at 1000 to 1100 ° C., and then gradually cooled at a cooling rate of 100 ° C./Hr or less. Heat treatment method for condensing heat exchanger body made of low carbon austenitic stainless steel. 2. The heat treatment method for a low carbon austenitic stainless steel condensing heat exchanger body according to claim 1, wherein the heat treatment is performed while the condensing heat exchanger body is constrained by a circular jig for preventing thermal deformation.