JPS6136582B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to steel for pressure vessels. More specifically, the present invention relates to a base material that can prevent cracking (hereinafter referred to as "peeling") caused by hydrogen at the interface between the base material and the stainless steel overlay metal on the inner surface of a pressure vessel that handles high-temperature, high-pressure hydrogen. . High-temperature, high-pressure hydrogen is handled in desulfurization reactions and hydrocracking reactions in the petroleum refining industry, and low-alloy steels such as carbon steel, molybdenum steel, and chromium-molybdenum steel are used for these reaction vessels. The inner surface is usually overwelded with stainless steel to prevent corrosion. Conventionally, austenitic stainless steels such as type 309, type 347, or type 316 have been selectively used as these corrosion-resistant materials. When overlay welding the austenitic stainless steel mentioned above onto a low alloy steel ferritic base material such as carbon steel, molybdenum steel, and chromium-molybdenum steel, the linear expansion coefficient of the ferritic steel and austenitic steel Strain due to the difference remains at the boundary between the stainless steel and the base metal. For this reason, as shown in the schematic diagram of FIG. 1, when hydrogen in a high temperature and high pressure state inside the reaction vessel dissociates and passes through the initial layer stainless steel overlay metal 2 and diffuses into the reaction vessel body base material 1, the reaction vessel Hydrogen-induced cracking (peeling) 5, as shown by the diagonal lines at the boundaries, sometimes occurred when the system was shut down. This peel is from the reaction vessel.
It also affects safety and has become an important issue for the oil refining industry as a whole. As a result of detailed research on the overlay welding method to prevent "peeling", the present inventors found that peeling can be prevented by selecting a welding material for the first layer, that is, the first layer welded onto the base metal, or by improving the welding method. Regarding finding a method and selecting welding materials, patent application No. 52-138935
No. (Special Publication No. 7792, 1983) ``Overlay welding method for stainless steel that prevents peeling and patent application 1987-
No. 12978 (Japanese Unexamined Patent Publication No. 107453/1983) has been applied for a patent entitled ``Method for preventing peeling of stainless steel overlay on inner surface of reaction vessel''. In addition, regarding the welding method, a patent application was made in 1973.
-13286 (Japanese Unexamined Patent Application Publication No. 117562/1983) has filed a patent application for ``Method of Overlay Welding of Stainless Steel.'' All of the inventions related to these patent applications are methods for obtaining overlay welds with good peeling resistance by changing overlay welding materials or by changing welding conditions. However, the present invention differs from these patent-pending inventions in that it aims at obtaining an overlay weld with excellent peeling resistance by adjusting the components of the base metal. Hereinafter, the present invention will be explained in detail based on diagrams showing its properties and examples. The present invention is based on the results of a detailed study of the causes of flaking that occurs in the boundary layer between stainless steel and the base metal due to hydrogen. was found to be dependent. In other words, the boundary layer between the austenitic stainless steel overlay metal and the base metal after post-heat treatment includes (a) a coarse-grained austenite structure carburized on the weld metal side, (b) a martensitic structure in the fusion zone, and (c) the base metal. Although there is a heterogeneous structure adjacent to the side, such as the coarse grain structure of the decarburized heat-affected zone, when observing the occurrence of flaking, it was found that flaking occurred in the microstructure near the boundary of the overlay weld and in the coarse-grained austenite boundary. As can be seen from Fig. 2, which is a cross-sectional view of an overlay weld showing the peeling situation, the peeling (cracking) 5 occurs in most cases when the coarse grains of the stainless steel overlay metal 2, which are formed parallel to the boundary, are connected to the austenite grain boundaries. It develops and progresses along the carbide layer 4 at the boundary in some parts. Furthermore, the relationship between the number of these coarse-grained austenites and flaking susceptibility was investigated in detail. The base material with the chemical composition shown in Table 1 is
Overlay welding was performed using a 309 series strip electrode.
At that time, a peeling test was conducted by changing the welding heat input and changing the number of austenite crystal grains.
The results are shown in FIG.

[Table] As shown in Figure 3, the smaller the number of coarse-grained austenites, the better the peeling resistance, especially if the number per unit length (1 mm) is 3 or less, the peeling resistance is excellent. It was found that Therefore, the present inventors found that it is effective to refine the crystal grain size of the base metal in order to prevent or reduce the occurrence of coarse-grained austenite, and as a result, overlay welding with excellent peeling resistance can be achieved. It was found that the following results were obtained. Therefore, the present invention has a carbon content of 0.05 to 0.25% by weight.
%, silicon 0.50% or less, manganese 0.30-1.00%,
Nickel 0.50% or less, chromium 1.00-3.50%, molybdenum 0.50-1.50%, and (a) titanium 0.01-0.10%, (b) niobium 0.01-0.10%, vanadium 0.01-0.10
% and at least one of aluminum 0.01~0.10% and titanium 0.01~0.10%, (c) niobium 0.01~0.10% and vanadium 0.01~
0.10% of at least one kind and aluminum 0.01~
A low-alloy steel for high-temperature, high-pressure pressure vessels that has excellent hydrogen stripping resistance and contains 0.10% of hydrogen, with the balance consisting of iron and impurity elements. The steel of the present invention is usually produced by melting and forming an ingot in a basic electric furnace, forging or rolling the obtained steel ingot, and then subjecting it to a heat treatment process. The present invention will be specifically explained below using Examples 1 to 11. Table 2 shows the chemical compositions of the 21/4Cr-1Mo steel base material according to the present invention and the conventional 21/4Cr-1Mo steel base material for comparison. After carrying out normal heat treatment on the base material according to the present invention and the base material of the comparative example, welding strips shown in Table 3 using a commercially available type 309 stainless steel strip electrode (thickness 0.4 mm, width 75 mm) were used. 〓
According to the circumstances, overlay welding was performed, and the number of overlay metal coarse-grained austenite per unit in the cross section was measured, and a peeling test was performed on a part of the test material, and the results shown in Table 4 were obtained.

【table】

【table】

【table】

[Table] 〓〓〓〓
As can be seen from these results, by using the base material according to the present invention, it is possible to suppress the number of coarse-grained austenites to 3 pieces/mm or less,
It can be seen that an overlay weld having excellent peel resistance is obtained. Next, the reason for limiting the component range of the base material used in the present invention will be explained. All percentages in the text are percentages by weight. Carbon is an effective element for increasing the tensile strength and creep rupture strength of low alloy materials, and 0.05% or more is required to achieve this effect, but on the other hand, increased carbon content improves weldability, In particular, it increases cold cracking and SR cracking susceptibility, and also improves impact toughness.
Since it has the effect of reducing creep rupture strength, the upper limit was set at 0.25%. Silicon is used as a deoxidizing agent in the refining process of steel materials, and is an effective element for increasing strength and hardenability. However, when it is contained in an amount exceeding 0.50%, it often exists as nonmetallic inclusions (SiO ₂ ), reducing impact toughness. Like silicon, manganese has the effect of a deoxidizing agent and also improves hardenability, but its effect is 0.30%.
The above is recognized. On the other hand, if the content exceeds 1.00%, it will exist as nonmetallic inclusions such as MnS or MnO, reducing impact toughness. Nickel improves hardenability and low-temperature toughness, but on the other hand, it has the effect of greatly reducing high-temperature creep rupture strength, so it must be kept at 0.50% or less. Chromium is an element necessary to improve oxidation resistance in high-temperature materials, and has the effect of improving hardenability, forming carbides, and increasing creep rupture strength. For this purpose, a chromium content of 1.00% or more is required, but if it exceeds 3.50%, the solid solution carbon content will decrease and carbides will become coarser, conversely decreasing the creep rupture strength.
A range of 1.00% to 3.50% is most suitable. Molybdenum dissolves in the ferrite matrix and increases the creep rupture strength, and also combines with carbon to form carbides, increasing the creep rupture strength.
It is necessary to contain 0.50% or more. but,
This effect of increasing fracture strength due to molybdenum reaches almost saturation when it exceeds 1.50% in low alloy steel.
Also, since molybdenum is expensive, the
%. Titanium has the effect of refining the crystal grains of the base metal, but if it is contained in an amount exceeding 0.10%, secondary oxidation will occur during post-weld heat treatment, resulting in a decrease in ductility and impact toughness. If the content is less than 0.01%, the effect will hardly be obtained. For the above reasons, the content was set at 0.01 to 0.01%. Niobium has the effect of refining the crystal grains of the base material, but if the content is less than 0.01%, this effect will be negligible, while if the content exceeds 0.10%, the impact toughness will decrease. Therefore, the niobium content is 0.01~
The range was set at 0.10%. Vanadium has the effect of refining the crystal grains of the base metal, but if the content is less than 0.01%, this effect will be negligible, while if the content exceeds 0.10%, secondary hardening will occur due to post-weld heat treatment. Especially Cr
-In Mo-based steels, it significantly increases SR cracking susceptibility. For the above reasons, the content was limited to a range of 0.01 to 1.00%. Aluminum has the effect of refining crystal grains, but the effect is small if the content is less than 0.01%. On the other hand, if the content exceeds 0.10%, a large amount of nonmetallic inclusions (Al ₂ O ₃ ) will occur, resulting in a decrease in ductility and impact toughness. The content is 0.01~ for the following reasons.
It was limited to a range of 0.10%. In addition, although the examples described here describe overlay welds made of type 309 stainless steel, type 347 or type 347 stainless steel may be used as the overlay material.
It has been confirmed that similar effects can be obtained when overlay welding is performed using stainless steel such as 316.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the occurrence of peeling in the stainless steel overlay weld on the inner surface of a pressure vessel used in a high-temperature, high-pressure hydrogen environment, and Figure 2 shows the occurrence of peeling and cracking in the overlay weld shown in Figure 1. FIG. 3 is an explanatory diagram showing the relationship between the number of coarse austenite particles per unit length and peeling resistance. In the diagram: 1...Pressure vessel body base material, 2...First layer stainless steel overlay metal, 3...
...Second layer stainless steel overlay metal, 4...Carbide layer, 5...Peeling and cracking. 〓〓〓〓

Claims (1)

[Claims] 1. Carbon 0.05-0.25%, silicon 0.50% by weight
% or less, manganese 0.30-1.00%, nickel 0.50%
Below, chromium 1.00~3.50%, molybdenum 0.50~
1.50%, and (a) titanium 0.01-0.10%, (b) niobium 0.01-0.10%, vanadium 0.01-0.10
% and at least one of aluminum 0.01~0.10% and titanium 0.01~0.10%, (c) niobium 0.01~0.10% and vanadium 0.01~
0.10% of at least one kind and aluminum 0.01~
A low-alloy steel for high-temperature, high-pressure pressure vessels with excellent hydrogen peeling resistance, containing either 0.10% or less, and the balance consisting of iron and impurity elements.