JPS63143218A - Heating treatment for chromium-molybdenum steel having site welding part - Google Patents

Abstract

PURPOSE:To execute heating treatment, so that heat-affected zone by welding of Cr-Mo steel is not softened by normalizing and tempering treatments to the Cr-Mo steel having site welding part in the each specific temp. range, and executing heat-treatment after processing at higher than Ac3 transformation point of the site welding part. CONSTITUTION:For example, in main steam piping 1 composing of 9 Cr-1Mo series, where necessary bending working or welding has already been, executed, in order to execute site assembling, the end part of piping 2 is welded at T7 temp. which is higher than the Ac3 transformation point. Subsequently, heat treatment is executed by winding induction heating type electric heater at the position to be post-heat treated, including the site welding part. Then, the post heat-treatment temp. T6 shall be 760 deg.C. Further, this position is the part, which has been kept outside the furnace in the factory. On the other hand, at the time of executing the finishing heat treatment in the factory, by holding the tempering near the position of site welding at >=680 deg.C, the softening at the heat-affected zone developed by the site welding is prevented.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for heat treatment of chromium-molybdenum (Cr-Mo), particularly when hot-processing or welding heat-processed Cr-MO steel materials on-site. The present invention relates to a heat treatment method.

(Prior art) For equipment operated under high temperature and high pressure conditions, such as equipment for thermal power plants and equipment for chemical plants, C
Cr-Mo steel with addition of r and Mo is used.

This is because the strength at high temperatures is improved by precipitation strengthening and solid solution strengthening by Cr and Mo, and the oxidation resistance is improved by the formation of dense Cr oxides.

However, since cr-Mom is inferior in strength and oxidation resistance at high temperatures compared to austenitic stainless steel, it is necessary to further increase the Cr content and to improve tempered marten by normalizing and tempering treatments. Cr-M'o&
I was developed, and some of them have been put into practical use as a substitute for austenitic stainless steel. This Cr-Mo
To explain the reason why high-temperature strength of steel improves, firstly, it is solid solution strengthening due to an increase in the amount of solid solution Cr in the base iron due to an increase in the amount of Cr added. Secondly, by strengthening by heat treatment, Cr and Mo are uniformly dissolved in the matrix by normalizing treatment, and at the same time, a high-strength martensite or bainitic structure is formed. Since the ductility and toughness are low in this state, further tempering treatment is performed to soften the martensite and bainite, but at that point Cr and Mo dissolved in the matrix combine with C etc. and precipitate. The precipitate strengthens the strength.

If the tempering temperature during the tempering treatment becomes high, martensite or boehni j ~ will become significantly softened, and the precipitates will also become coarser, making it impossible to contribute to improving the strength. Therefore, in order to ensure appropriate strength, ductility, and toughness, various properties are examined to determine an appropriate tempering temperature range.

FIG. 1 is a diagram illustrating the principles of a conventional heat treatment method and a heat treatment method of the present invention, and the solid line indicates the conventional method.

The vertical axis of the figure shows the heating temperature, and the horizontal axis shows the time. Curve A in the figure is the normalizing temperature curve, curve B is the tempering temperature curve, curve C is the heating temperature curve associated with processing, and the curve is after. The heat treatment temperature curve, straight lines E and F indicate the Ac1 transformation point and the AC3 transformation point.

When manufacturing equipment using Cr-Mo steel, Cr-Mo
As shown in curve A, the heat treatment of m is performed at a normalizing temperature (T1) higher than the transformation point, and then as shown in curve B, A c is lower than 1 transformation point and 1 lower than Act.
Tempering is performed at a tempering temperature (T2) higher than 00℃ lower temperature, then processing is performed at a processing temperature (T3) higher than the A c 3 transformation point as shown by curve C, and then the curved line is shown. Thus, the post-heat treatment was performed at a post-heat treatment temperature (T4) that was 100° C. or more lower than the A c 1 transformation point.

When we reproduced such heat treatment and examined various properties, we found that Cr-Mo II
It has been found that by heating the material, a softening phenomenon occurs in the area around the processed part heated to a specific temperature range, resulting in a decrease in strength. This softening phenomenon will be explained using FIG.

In FIG. 3, the straight line ESF is the same as that in FIG. 1, with 1 indicating the base material original zone, 2 indicating the fine-grained heat-affected zone, and 3 indicating the coarse-grained heat-affected zone. The left side of Fig. 3 is the base metal original l kept at room temperature, and the center is the AC1 transformation point E to AC3.
Fine grain heat affected zone 2 heated to transformation point F, right side is A c
3 shows a cross section of a coarse-grained heat-affected zone 3 heated to a transformation point F or higher.

As is clear from this figure, in the coarse-grained heat-affected zone 3 heated above the A c 3 transformation point F, the transformation to austenite has been completed and the crystal grains have become coarse;
In part 2, it is heated to a temperature between the A c 1 transformation point E and the A c 3 transformation point F, and the crystal grains are fine in the middle of transformation. It can be seen that in the base material original part 1, the region heated to a temperature below the AC1 transformation point E is not affected by heat.

The softening phenomenon occurs in the fine-grained heat-affected zone 2, and fracture occurred in this region in the high-temperature tensile test and the creep rupture test. In particular, in the creep rupture test, the rupture time may be about 115 times or less than the rupture time of the base material original part 1.

As a result of metallographic investigation of this softening phenomenon, we found that it is largely influenced by the remarkable refinement of crystal grains in this region and the coarsening of Cr and M□ precipitates compared to other regions. It became clear.

Now, when manufacturing plant equipment with these Cr-M Ogl, there are A
c3 Heating above the transformation point F is unavoidable, and in consideration of this phenomenon, the design allows a considerable margin for the required plate thickness, but in equipment that has been operated for a long time, In some cases, creep cracks occurred in this area, leading to accidents. From the above viewpoint, it is necessary to establish countermeasures against this softening phenomenon in order to expand the usable range of tempered Cr-Mail.

The present inventors invented an invention to solve this problem in June 1986.
The application was filed as Japanese Patent Application No. 134948/1986 on May 12th. Since the content of the present invention has the same basic idea as that of the above-mentioned Japanese Patent Application No. 134948/1982, the content of the above-mentioned earlier application will be explained first. The main points of the heat treatment method of the prior invention will be explained using Table 1.

Table 1 Unit ("C) Normalizing is necessary to uniformly dissolve additive elements such as Cr and Mo in the matrix and to adjust the crystal grains.
The temperature is set to the normalizing temperature T1, which is equal to or higher than the A c 3 transformation point F, as in the conventional heat treatment method.

The tempering treatment is the key point of the heat treatment in the prior invention, and the inventors' various studies revealed that it is necessary to set the tempering temperature TS at least 50°C lower than the minimum necessary tempering temperature T2 of the conventional heat treatment method. I found out something. By performing the tempering treatment at such a tempering temperature T5, precipitates of Cr and MO during the tempering treatment become smaller than in the case of conventional heat treatment methods. Therefore, even in the surrounding area heated to near the A C1 transformation point E by heating during subsequent processing, some of these precipitates are dissolved in solid solution and do not become an obstacle to the growth of austenite grains. , significant grain refinement is reduced. From the viewpoint of preventing softening, it is natural that the tempering temperature should be as low as possible, but as mentioned above, it is necessary to restore ductility and toughness, and the lower limit is conventionally lower than the Ac1 transformation point E by less than 100°C. It is preferable to set the tempering temperature T5 to be 50 to 100°C lower than the tempering temperature T2. After tempering 2 processing During processing such as welding and hot bending, the post-heat treatment temperature after heating at a heating temperature T above the transformation point F is the minimum tempering temperature required by the conventional heat treatment method. T4 (100℃ above Ac1 transformation point
The post-heat treatment temperature T6 was set higher than the lower temperature). Preferably, a temperature between the Ac1 point and A C1 to 100°C is employed.

Through this heat treatment, the ductility and toughness, which were not fully recovered by tempering alone, are restored to the required values, and the mechanical properties necessary for the structure can be guaranteed.

Next, the invention of the prior application will be explained in detail. The material is 9Cr-I
Mo system (Ac1 transformation point is 830-840℃1, Ac
3) whose transformation point is 870 to 880°C.

In the heat treatment according to the conventional heat treatment method, the normalizing temperature T1 is 1050°C, the minimum tempering temperature T2 is 730°C, and the tempering process is actually performed at 760°C.

In the heat treatment of the prior invention, the normalizing temperature T1 is 1050°C.
1. The tempering temperature T was set lower than the conventional tempering temperature T2 (670° C.). The thus heat-treated material was welded at a large heat amount of 25 KJ/(to), and then post-heat-treated at a tempering temperature T and a higher post-heat treatment temperature Tg (760° C.). In the conventional method, post-heat treatment is performed at 740°C.

(Problems to be Solved by the Invention) The invention of Japanese Patent Application No. 134948/1989, which is related to the earlier application by the inventors, is very effective as a method for preventing softening caused by local heating caused by welding or hot bending. Although it is effective, when further processing such as on-site welding is performed on a part of the Cr-Mo5jl material treated with this method in the factory,
Further efforts are needed. In other words, in the method of the prior invention, the post-heat treatment after welding or hot bending is not only performed on the welded part or the hot bent part (it is assumed that the entire base material is heat treated to ensure the performance of the base material, , the temperature T6, which corresponds to the minimum necessary tempering temperature for the parts to be welded on-site.
is heated to.

This temperature corresponds to the conventional tempering temperature T2 after normalizing, and after welding on site,
If this portion is subjected to post-heat treatment, the aforementioned fine-grained heat-affected zone, ie, a softened zone, will occur and the strength will decrease.

The purpose of the present invention is to prevent softening of the weld heat-affected zone when performing on-site welding necessary to make the final structure after partially welding a product made of cr-Mo steel in a factory or the like. The goal is to provide a way to do so.

(Means for Solving the Problems) The present invention has been made to solve the above problems.
Cr-M aill with on-site processing part
After performing normalizing treatment by heating to the C.3 transformation point or higher, further tempering treatment at a temperature lower than the necessary minimum tempering temperature of less than 100 °C lower than the A C1 transformation point by 50 °C or more, c After processing at or above the transformation point, the on-site processed part is kept at a temperature 50°C or more lower than the above-mentioned minimum required tempering temperature, while other parts are subjected to post-heat treatment at a temperature higher than the required minimum tempering temperature. Cr- having an on-site processed part where the on-site processed part is processed at a temperature higher than the A c 3 transformation point, and then the part is subjected to post-heat treatment at a temperature higher than the minimum necessary tempering temperature.
A method for heat treatment of Mo&H is provided.

(Example) Embodiments of the present invention will be described in detail using FIGS. 1 and 2.

Figure 2 shows post-heat treatment being carried out in a factory on main steam piping used in boiler equipment installed in thermal power plants and the like. The material of the main steam pipe 1 is 9Cr-IMo (Ac1 transformation point is 830-840°C, AC3 transformation point is 870-880°C), and the necessary hot bending and welding have already been carried out. . The tempering temperature T of this material is 680°C, and
No. 48 heat treatment has been passed. The post-heat treatment temperature T after hot bending and welding work is set at 760°C, but a hole 12 is drilled in the door 11 of the electric furnace 3 according to the dimensions of the pipe, and on-site welding is performed. The tube end 2 is exposed outside the furnace. Therefore, the pipe temperature near the pipe end is 680°C (T
5) Maintained below.

The range in which the heating temperature is maintained at 680° C. or lower only needs to include the hot forming portion that occurs during subsequent on-site welding. In order to assemble the main steam pipe post-heat-treated in this way on-site, the pipe end 2 is heated to a temperature T7 above the AC3 transformation point.
Weld with. This on-site welding was performed using SMAW using a welding material with the same composition as the base metal. After that, a partial post-heat treatment is performed, and the method is to wrap an induction electric heater around the area to be post-heat-treated, including this on-site welding part. After this, the heat treatment temperature T8 is 760℃
The part to be subjected to post-heat treatment is the part that was taken out of the furnace during the final heat treatment at the factory.

As described above, softening of the heat-affected zone that occurs during on-site welding can be prevented by keeping the area near the on-site welding at 680° C. or lower during the final heat treatment at the factory.

In order to confirm this, a uniaxial creep rupture test was performed on the on-site weld in this example, and it was found that the rupture strength was equivalent to that of the base metal, and moreover, in the test, the weld broke at the base metal.

The principle of the present invention is the same as that of the earlier application, Japanese Patent Application No. 61-134948. In other words, the on-site welding area must be kept at a temperature that is at least 50°C lower than the required minimum urine tempering temperature (60°C).
80℃), and for this reason Cr and Mo
The coarsening of the precipitates was delayed, and subsequent on-site welding caused A.
In the part heated to a temperature slightly higher than the c3 transformation point, some of these fine precipitates are dissolved in solid solution and do not become an obstacle to the growth of austenite grains due to transformation.
Significant refinement of crystal grains is reduced and softening can be prevented.

In addition, in the post-processing heat treatment shown in Fig. 2 in the factory, if the tube end 2 to be welded on site cannot be taken out of the furnace 3, the cooling coil should be placed near the tube end. You can accomplish this by wrapping it around it and running cooling water around it.

(Effects of the Invention) According to the present invention, it is possible to prevent the softening phenomenon of tempered cr-M O6M due to local heating, thereby improving the reliability of agricultural equipment conventionally manufactured using this steel type. can be done. In addition, the range of application of high Cr-containing Cr-Mo&H, which has been developed as a substitute for expensive austenitic stainless steel tA, can be expanded by the heat treatment method, and its industrial value is great.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the principle of the heat treatment method according to the present invention, and FIG.
The figure shows an example in which the part to be processed on-site is taken out of the furnace and the remaining part is post-heat-treated in the present invention, and Figure 3 is a cross-sectional view and heating temperature characteristic curve diagram of the material treated by the conventional heat treatment method. be. T1... Normalizing temperature, T2... Conventional tempering temperature, T3... Processing temperature, T4... Conventional post-heat treatment temperature, T5... Tempering temperature of the present invention, T6... - Post-heat treatment temperature of the present invention, T7...on-site processing temperature of the present invention,
T8: Post-on-site heat treatment temperature of the present invention. Agent Patent Attorney Takenaga Kawakita Figure 1 Figure 3?

Claims (1)

[Claims] (1) Ac_3 Cr-Mo steel with on-site processing parts
After being heated above the transformation point and subjected to normalizing treatment, A
After performing tempering treatment at a temperature 50°C or more lower than the necessary minimum tempering temperature, which is less than 100°C lower than the c_1 transformation point, and processing at a temperature higher than the Ac_3 transformation point, the on-site processing area, including the surrounding area, shall be heated to the above-mentioned minimum temperature. 50 below the limited tempering temperature
While maintaining the temperature at a temperature lower than °C, other parts are subjected to post-heat treatment above the required minimum tempering temperature, and after that, the on-site processing part is processed at a temperature of Ac_3 transformation point or higher, and then that part, including the surrounding area, is processed. A method for heat treatment of Cr-Mo steel having an on-site processed part in which post-heat treatment is performed at a temperature higher than the minimum required tempering temperature.