JPS6123260B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a new casing for a steam turbine, and particularly to a casing body, a main steam valve casing, and a control valve casing of a high-efficiency steam turbine exposed to temperatures of 550 to 600°C.
Cr―Mo― exhibits excellent properties with low cracking susceptibility
This invention relates to a steam turbine casing using V-cast steel. Conventional steam turbines have a maximum steam temperature of 566°C and a maximum steam pressure of 246 atg, and the casing body 1 and control valve casing 2 shown in Fig. 1, and the main steam valve casing material shown in Fig. 2 are made of Cr-Mo-V.
Cast steel is used. Recently, the cost of fossil fuels such as oil and coal continues to rise, and the power generation efficiency of thermal power plants that use these fossil fuels has become important. In order to increase power generation efficiency, it is necessary to increase the steam temperature or pressure of the steam turbine. Current turbine materials lack strength as materials for these high-efficiency turbines, and materials with higher strength are required. The inventors used Cr-Mo as the above-mentioned casing material.
-The basic composition is V cast steel, and the application of boron-containing Cr-Mo-V cast steel with a trace amount of boron added was investigated. It was discovered that increasing boron content increases hardenability and high-temperature strength, but at the same time reduces weldability and significantly increases SR cracking susceptibility. Therefore, there was a need to add boron to casing materials used for repair welding and joint welding to improve strength, improve weldability, and prevent SR cracking. The object of the present invention is to have high creep rupture strength at 550 to 600℃, and to have SR resistance in the welding process.
An object of the present invention is to provide a steam turbine casing made of extremely good cast steel with low cracking susceptibility. In the present invention, by weight, C0.05-0.2%, Mn2% or less, Si1% or less, Cr0.5-2%, Mo0.5-2%,
V0.05~0.5%, Ni0.5% or less, B0.0002~0.005%,
Al0.1% or less, Ti0.1% or less and Ca0.0002~0.2%
The casing for a steam turbine is characterized in that the casing is made of steel mainly having a bainite structure, with the remainder being substantially Fe. It has been experimentally determined that the present invention significantly reduces SR cracking susceptibility in the welding process and increases high-temperature strength. C is an element necessary to obtain creep rupture strength when the amount is 0.05% or more, but when the amount is 0.2%
If it exceeds this amount, the structure becomes unstable when exposed to high temperatures for a long time, lowering the long-term creep rupture strength, and further increasing the cracking susceptibility of the welded part during the welding process, so it must be set at 0.05 to 0.2%. In particular, 0.1 to 0.15% is preferable. Si and Mn are added as deoxidizing agents, and a small amount of addition can achieve sufficient effects. Si and
Mn is an element that increases hardenability, but on the other hand, adding a large amount increases susceptibility to tempering embrittlement. Therefore, Si and Mn must be kept at 1% or less and 2% or less, respectively. In particular, Si is 0.2-0.6% and Mn is
0.6-1.0% is preferred. While Ni is very effective in increasing toughness, addition of more than 0.5% lowers creep rupture strength, so the content must be 0.5% or less. In particular, 0.05 to 0.3% is preferable. Cr increases high-temperature strength and acid resistance, and is an indispensable element for high-temperature materials. For this purpose, 0.5% or more is necessary, but 2
If the Cr content exceeds 0.5 to 2%, the precipitated carbides will become coarser and the creep rupture strength will decrease.
% range. In particular, 0.9~
1.5% is preferred. Mo is an element that improves creep strength through solid solution strengthening and precipitation hardening effects, and also prevents temper embrittlement, but if it is less than 0.5%, the effect is insufficient, and if it exceeds 2%, it will not be effective. It becomes saturated with no further effect. Therefore, a range of 0.5 to 2% is effective for Mo. In particular, 0.9-1.5% is most effective. V combines with carbon to form carbide and increase creep rupture strength. However, the effect is insufficient if it is less than 0.05%, and conversely, if it exceeds 0.5%, it increases the SR cracking susceptibility in the welding process and reduces creep rupture ductility.
Must be within 0.5%. In particular, 0.1~
0.35% is the most effective. B improves hardenability and increases creep rupture strength. However, the effect is insufficient if it is less than 0.0001%, and if it exceeds 0.005%, it increases weldability, especially SR cracking susceptibility. Therefore, 0.0001 to 0.005
Must be in the range of %. In particular, from 0.0008
0.003% is the most effective. A nitride-forming element having a higher nitride-forming ability than B is added for the purpose of fixing N. N
combines with B and reduces the original effect of B. Therefore, it is necessary to add an element having a higher nitride-forming ability than B in combination with B to fix N and bring out the original effect of B. This effect alone
If the content exceeds 0.2% in a composite of 0.1% or more, the creep rupture ductility will be significantly reduced. As this element,
Significant effects can be obtained by adding Al and Ti in combination. Al 0.01~0.05% and Ti 0.01~0.08% are the most effective. Addition of 0.0002% or more of Ca as a sulfide-forming element with greater sulfide-forming ability than Mn acts as desulfurization and dephosphorization during steelmaking, and
This fixes S' in the steel, suppresses segregation of S to grain boundaries in the weld heat affected zone, and prevents SR cracking. SR cracks are cracks at grain boundaries in the weld heat affected zone, and among impurity elements such as S and P, the more S in particular segregates at grain boundaries, the more likely it is to occur. However, addition of more than 0.2% has no further effect on SR cracking resistance. Ca reduces the S concentration at grain boundaries in the weld heat affected zone and improves SR cracking resistance. In particular, in the present invention, B is added to increase the creep strength, but the addition of B has the opposite effect.
There is a risk of SR cracking. Therefore, Ca addition is
It is a necessary element in terms of preventing SR cracking. The amount added is
Below 0.0002%, there is no effect on SR cracking resistance;
% or more, the effect is saturated. As an appropriate amount
0.005-0.05% is effective. The steam turbine casing of the present invention must primarily have a bainitic structure.
The bainite structure is preferably a tempered bainite structure, which has high strength at high temperatures. Depending on the chemical composition and heat treatment, a ferrite structure may occur, so
It is most effective to have an entirely bainite structure so that substantially no ferrite structure is precipitated. Example A cast steel ingot was produced using a high frequency induction melting furnace.
Table 1 shows the chemical composition of these representative samples.

【table】

[Table] After holding all steel ingots at 1,050℃ for 15 hours,
After normalizing at ℃/h, No.1
Hold at 710℃ for 15 hours and 720℃ for No. 2, 3, and 4.
After holding for 15 hours, a furnace cooling tempering process was performed. The samples are 1Cr-1Mo-1/4V, Al, Ti, B and Ca.
The amount was varied. No. 1 is a comparative material without B and Ca additives. No. 2 has 0.0015% B added and Ca
Additive-free materials, Nos. 3 and 4 are materials of the present invention with addition of B and Ca. All samples had an all-bainitic structure. The SR cracking susceptibility test was carried out in accordance with JIS Z3158 by processing the sample into a diagonal Y-shaped welded cracking test piece (plate thickness 30mm). Welding was carried out using a commercially available coated arc welding rod (4φ) for Cr--Mo steel under the conditions shown in Table 2.

【table】

[Table] Table 3 shows the results of various mechanical tests. Although the impact properties of the B-additive material are slightly lower than that of the non-additive material, the tensile strength is significantly higher. On the other hand, the material of the present invention has a creep rupture strength of 9.9 to 10.2 Kg/ ^mm2 at 600℃ for 10 ⁵ hours, which is extremely effective as a casing material for high-efficiency steam turbines and is significantly higher than that of comparative material No. 1. This was confirmed.

[Table] Figure 3 is a graph showing the SR cracking test results. In the present invention and comparative materials, No. 1 material without B and Ca does not cause SR cracking, but
In No. 2 with 0.0015% addition, the SR cracking rate was 61%.
SR cracking occurred. This indicates that the addition of B promotes SR cracking. On the other hand, compared to No. 2, No. 2 of the present invention had the same amount of B but added Ca.
3, no SR cracking occurs. Furthermore, the amount of B
No. 4 of the present invention increased to 0.002% and added Ca
SR cracking did not occur in both cases. From the above results, it is clear that the present invention has extremely low SR cracking sensitivity. As mentioned above, the steel of the present invention has high high-temperature strength and toughness, and also has extremely good SR cracking susceptibility.
The high-temperature creep rupture strength up to 600℃ is extremely high, and it is clear that it fully satisfies the strength required for a casing for a high-efficiency steam turbine, making it suitable as a casing for a high-efficiency steam turbine at temperatures of 550 to 600℃. Therefore, the effect of the present invention contributing to the development of industry is extremely large.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional diagram of the steam turbine casing body and steam control valve casing, Figure 2 is a cross-sectional diagram of the main steam valve casing, and Figure 3 is after welding.
It is a graph showing the cracking rate due to SR treatment. 1...Casing body, 2...Adjustment valve casing, 3...Main steam valve casing.

Claims (1)

[Claims] 1 By weight: C0.05-0.2%, Mn2% or less, Si1%
Below, Cr0.5~2%, Mo0.5~2%, V0.05~0.5
%, Ni 0.5% or less, B 0.0001 0.005%, Al 0.1% or less, Ti 0.1% or less, and Ca 0.0002 to 0.2%, the balance essentially consists of Fe, and mainly has a bainitic structure. A steam turbine casing characterized by being made of cast steel.