JPH041058B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to martensitic stainless steel for pumps that handle high-temperature, high-purity water used in boilers or BWRs. [Prior Art] Conventional martensitic stainless steels have had the disadvantage of causing environmental embrittlement cracking caused by pitting corrosion along non-metallic inclusions. As a preventive measure against these problems, chemical components,
Care must be taken in heat treatment. [Objective of the Invention] The present invention provides a martensitic stainless steel for high-temperature water pumps that has excellent environmental embrittlement cracking resistance by reducing the density of pitting corrosion and preventing the continuous progression of pitting corrosion. be. [Summary of the Invention] The present invention provides C: 0.03 to 0.15%, Si:
0.2-1.0%, Mn: 0.2-1.0%, Cr: 11.5-13.5%,
Ni: 0.01~4.5%, Mo: 0.01~1.0%, S: 0.003
A martensitic stainless steel consisting of ~0.015%, Cu: 0.01~0.4%, balance Fe and unavoidable impurities, with an average major axis of sulfide inclusions in any cross section of 5 μm or less, between sulfide inclusions. The average distance parallel to the forged surface is 100 μm or more, and the final material hardness is 220≦Hv≦250, and the pitting corrosion resistance or environmental embrittlement cracking resistance in high-temperature water is improved. 13Cr martensitic stainless steel is widely used as a material for pumps that handle high-temperature water because of its excellent mechanical properties, corrosion resistance, resistance to environmental embrittlement cracking, and cost. Here, after long-term use, the above-mentioned pump parts break due to environmental embrittlement cracking starting from pitting corrosion that occurs in stress concentration areas. Therefore, as a result of detailed observation of a large number of broken parts of the pump, the following facts were discovered. (a) Pitting corrosion occurs at structural gaps or at locations where foreign matter (such as rust) adheres to the free surface, but deep pitting corrosion, which is the starting point of cracks, progresses only along sulfide-based inclusions. (b) When the S content is 0.015 to 0.028% and the sulfide inclusions are oblong, or the average distance in the rolling direction between the sulfide inclusions is small (inclusions exist continuously). ), deep pitting corrosion occurs, which is the cause of breakage. (c) Satisfies (a) and (b), and the material hardness is Hv
>250, breakage occurs more frequently. Next, the reason why the contents of the constituent elements of the martensitic stainless steel for high temperature water pumps of the present invention are limited will be explained. C: 0.03 to 0.15% C improves hardenability, tensile strength and yield strength, but below 0.03%, the degree of hardening of the induced martensitic phase, which is effective for wear resistance during processing, is small and the results are small. . Cast steel with excellent cavitation resistance and erosion resistance cannot be obtained. If it exceeds 0.15%, it is difficult to soften by tempering, hydrogen embrittlement cracking is likely to occur, and intergranular corrosion and intergranular corrosion cracking are also likely to occur, making it impractical. Therefore, the C content was limited to a range of 0.03 to 0.15%. Si: 0.2 to 1.0% Si is a necessary element as a deoxidizing agent, and if it is less than 0.2%, the castability of steel decreases. Also, if the amount added is too large, it may cause embrittlement. Therefore, Si is 0.2
It was limited to a range of ~1.0%. Mn: 0.2-1.0% Mn is a deoxidizing agent like Si and prevents hot cracking. Adding too much will cause embrittlement. Therefore, Mn was set in the range of 0.2 to 1.0%. Cr: 11.5-13.5% Cr is an important element for improving corrosion resistance, but if it is less than 11.5%, the corrosion resistance will be significantly reduced and it is not practical. If the content exceeds 13.5%, the weldability, castability, and other processability of the steel will deteriorate, and it will not soften even when tempered at high temperatures, leading to a decrease in impact value or the risk of hydrogen embrittlement cracking. increase Therefore,
Cr was limited to a range of 11.5 to 13.5%. Ni: 0.01-4.5% Ni is an effective element for improving hardenability and wear resistance, but if it exceeds 4.5%, the Ac ₁ point of the steel will drop to the practical tempering temperature or below.
seriously impeding the tempering conditions. Furthermore, the Ni equivalent becomes too large, making it difficult to ensure cavitation resistance and erosion resistance. The lower limit of 0.01% is a practical value considering the handling of scrap in steelmaking. Therefore, Ni was set in the range of 0.01 to 4.5%. Mo: 0.03-1.0% Mo coexists with C and Cr and is effective in improving high-temperature strength and suppressing high-temperature brittleness and tempering brittleness. If the amount added is too large, the required tempered structure cannot be obtained because Mo is a ferrite-forming element. The lower limit of 0.03% is a practical value considering the handling of scrap in steelmaking. Therefore Mo
was set in the range of 0.03 to 1.0%. S: 0.003 to 0.015% S produces sulfurized inclusions and is not preferred from the viewpoint of corrosion resistance. However, considering the use of scrap in steelmaking, the practical value is 0.003%. In addition, when the content exceeds 0.015%, sulfide inclusions increase,
Deteriorates corrosion resistance and environmental embrittlement cracking resistance. Therefore, S was limited to a range of 0.003 to 0.015%. Cu: 0.01-0.4% Cu is an important element in the present invention, and is thought to combine with S to produce CuS, which is chemically stable in high-temperature water. At 0.01%, the modification effect of sulfide-based inclusions is small, and when it exceeds 0.4%, the modification effect is saturated and weld cracking is likely to occur.
Therefore, Cu was limited to a range of 0.01 to 0.4%. [Embodiments of the invention] Example 1 A lot with a lower S content than commercially available 13Cr-Ni stainless steel was selected, and 50 kg of this was used as the base material.
After remelting using a high-frequency melting furnace and adding FeS and metallic copper in predetermined amounts, the molten metal was poured into a Y block mold to create a sample. The chemical composition of the sample is shown in Table 1. Next, cut out a 50 x 50 x 250 square piece from the bottom of the mold and hot forge it (10~25) x (10~
25) After creating the bar material, heat at 1050~1080℃×2hr→
Slow cooling, 970-1000℃→oil-cooled quenching, 550-700℃
→Given oil-cooled tempering. After that, many samples of 10 x 10 x 2 tons were prepared with the part perpendicular to the stepped surface as the sample surface. Next, the sample surface was polished with #600 emery paper, degreased, and immersed in high-temperature high-purity water at 180°C for 500 hours. During immersion, the thickness of the sample was supported by four 1φ stainless steel rods to prevent the sample surface from coming into contact with foreign materials and forming gaps. Next, the sample after immersion was heated to 90℃ and 10% to remove surface rust.
It was immersed in a mixture of NaOH + 3% KMnO ₄ and subjected to ultrasonic cleaning for 2 hours. And washing with water,

[Table] After drying, the number of pitting corrosion and pitting depth that occurred on the sample surface were measured using an optical microscope. The results are shown in the figure. From the figure, it can be seen that as the S content increases within the range of this test material, the number of pitting corrosion and the maximum pitting depth tend to increase. This tendency is particularly noticeable when S≧0.015%. On the other hand, when copper is added, although no clear change is observed in the number of pitting corrosion, the maximum pitting depth tends to decrease. In particular, this tendency is remarkable in the range of Cu: 0.01 to 0.4%. The above shows that the occurrence of pitting corrosion is closely related to sulfide inclusions, as described above. Also,
This is thought to be because by adding Cu and tempering, some of the sulfide inclusions become chemically stable CuS in high-temperature water. Next, of the above samples, B-2 and B-3 were 17×17
A clear difference in the maximum pitting depth was observed for the 17×17 forged material, which was hot-forged into a 10×10 bar and then subjected to the same heat treatment, same sample, and same test as the above sample. 10×10
In the case of forged materials, it increased significantly by 2.5 times and 5.8 times compared to B-2 and B-3, respectively. here,
No.A-1~A-4, B-1~B-3, D-1~D
In both samples -3, the average major axis of the sulfide inclusions was
The average distance parallel to the forging surface between spherical and sulfide inclusions was 5 μm or less and was 100 μm or more. on the other hand,
In the 17×17 forged material, the average major axis was 5 μm, and the distance between inclusions was 100 μm. In addition, the 10 × 10 forged materials of B-2 and B-3 have an average major axis of 5 to 15 μm,
The average distance parallel to the forging surface between sulfide inclusions is 50
It was ~80 μm. From the above, it is understood that the distribution state of sulfide-based inclusions in steel is an important factor in the maximum pitting corrosion depth. Example 2 A tensile test piece with a parallel portion of 3 x 35 x 2 tons was cut out from the sample shown in Table 1 and subjected to a low strain rate tensile stress corrosion cracking test. The experiment was conducted in high-temperature, high-purity water at 180°C for 48 hours at a strain rate of 1.5×10 ^-7 mm/
Tested in mm·s. The fracture surface was then observed using a scanning electron microscope to determine the presence or absence of stress corrosion cracking. Here, in the heat treatment of each sample, the tempering temperature was varied and the hardness level was set to Hv=220 to 300. The results are shown in Table 2. As shown in the figure, materials with high material hardness generally crack due to stress corrosion cracking, but when the S content is low or copper coexists, the critical hardness for cracking increases slightly. Since the stress corrosion cracking of this test piece all originates from minute pitting corrosion, it can be concluded that the characteristics in Table 2 are a reflection of the pitting corrosion progression behavior shown in the figure. , the effectiveness of S content and Cu addition in improving stress corrosion cracking resistance is understood. Furthermore, if we compare the stress corrosion cracking test conditions described above with the stress application conditions in high-temperature water pumps, in the latter case the material is exposed to the environment for a much longer time, but on the other hand,
The applied stress is sufficiently small. This fact indicates that the occurrence and progression of pitting corrosion should play an important role in the material's destruction process, and the role of pitting corrosion should be even more pronounced in the characteristics shown in the figure. Considering this, the high temperature water pump components mentioned at the beginning

【table】

〔Effect of the invention〕

According to the present invention, it is possible to provide a martensitic stainless steel that is effective in preventing environmental embrittlement cracking in high-temperature water pumps. As a result, not only can this greatly contribute to improved reliability, but also an increase in cost can be prevented since it is not necessary to use high-grade steel (eg, Inconel alloy, ultra-high-grade stainless steel).

[Brief explanation of drawings]

The drawing is a diagram showing the stress corrosion cracking test results of the martensitic stainless steel of the present invention.

Claims (1)

[Claims] 1 Weight ratio: C: 0.03-0.15%, Si: 0.2-1.0
%, Mn: 0.2-1.0%, Cr: 11.5-13.5%, Ni:
0.01~4.5%, Mo: 0.01~1.0%, S: 0.003~
0.015%, Cu: 0.01-0.4%, balance Fe and unavoidable impurities, the average major axis of sulfide inclusions in any cross section is 5 μm or less, the average distance between sulfide inclusions parallel to the forging surface Martensitic stainless steel for high-temperature water pumps, characterized by having a hardness of 100 μm or more, and a final material hardness of 220≦Hv≦250.