JPS58133353A - Casing for steam turbine - Google Patents

Abstract

PURPOSE:To provide the titled casing high in creep breaking strength and low SR crack sensitivity, containing C, Mn, Si, Cr, Mo, V, Ni, B and a nitride forming element having nitride forming ability higher than that B respectively in a predetermined ratio and comprising cast steel having bainite structure mainly. CONSTITUTION:The titled casing comprises a cast steel material containing, on the basis of wt%, 0.05-0.2% C, 2% or less Mn, 1% or less Si, 0.5-2% Cr, 0.5- 2% Mo, 0.05-0.5% V, 0.5% or less Ni, 0.0002-0.005% B and 0.1% or less nitride forming element alone or 0.2% or less in composite and comprising the remainder Fe. This nitride forming element has nitride forming ability larger than that of B and this cast steel has a bainite structure mainly. The titled casing comprising this cast steel has high creep breaking strength at 550-650 deg.C and shows excellent characteristics low in SR crack sensitivity during a welding process.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel casing for steam turbines, particularly 550-60 (1' This invention relates to a caking for a steam turbine using Cr-MO-V cast steel which exhibits excellent properties such as low breaking strength and low susceptibility to 8R cracking.

Conventional steam turbines have a maximum steam temperature of 566C.

The maximum steam pressure is 246 atg, and Cr-MO-V cast steel is used for the casing body 1 and regulating valve casing 2 shown in FIG. 1, and the main steam valve casing material shown in FIG. 2.

Recently, as the cost of fossil fuels such as petroleum 9 and coal continues to rise, the power generation efficiency of thermal power plants that use fossil fuels has become important. It is necessary to increase the temperature or pressure.

As materials for these high-efficiency turbines, currently used turbine materials lack strength, so materials with even higher strength are needed.

The inventors used boron-containing b, which has a basic composition of Cr-MO-V cast steel and added a small amount of boron, as the above-mentioned casing material.
The application of cr-MO-vs steel was studied. However, it has been found that increasing boron content increases hardenability and temperature-induced strength, but at the same time reduces weldability and significantly increases 8R cracking susceptibility. Therefore, it is necessary to improve the strength, further improve weldability, and prevent 88 cracks by adding boron to the casing material used for repair #* and joint welding.

An object of the present invention is to provide a steam turbine casing made of extremely good cast steel that has high creep rupture strength at 550 to 600 t:' and low susceptibility to 8B cracking in the #l welding process.

Bottom, SM 1% or less, Crα 5-2%, MOα 5-2%.

Vo, 05~0.5%, Niα5% or less, BQ, 00
02 to 0.005%, and the nitride-forming elements, which have higher nitride-forming ability than B, are α1% or less alone or 0.2% in combination.
The casing for a steam turbine is characterized in that it includes the following, the remainder being substantially KFe-free and mainly made of steel having a bainitic structure.

Furthermore, the present invention has the above-mentioned (DC, Si, Mfl, Cr
, Ni.

The cr-MO-V-based low alloy cast steel containing V, B, and life-forming elements contains α0002 to α2% of sulfide-forming elements that have a higher sulfide-forming ability than Mn.

It has been experimentally determined that the present invention significantly reduces susceptibility to 88 cracking in the welding process and increases high-temperature strength.

The nitride-forming elements include At, Ti, Nb and 7. r
The sulfide-forming element is preferably at least one of Ca and Mg.

C is an element necessary to obtain creep rupture strength at α05% or more, but if its amount exceeds 0.2%, the structure becomes unstable when exposed to high temperatures for a long time. Since it reduces the creep rupture strength and further increases the cracking susceptibility of the welded part during the welding process, O,OS~α2%
It should be 0%, preferably 0.1-0.15%.

Bt and Mn are added as deoxidizing agents, and sufficient effects can be achieved by adding a small amount. S Todoroki and RJI are elements that increase hardenability, but on the other hand, when added in large amounts, they increase the susceptibility to tempering embrittlement.

Therefore, S1 must be 1% and Ml) must be 2% or less, respectively. In particular, Si is 0.2-0.6% and M
n is preferably 0.6 to 1.0%.

While N1 is very effective in increasing toughness, α5%
Addition of more than
α must be less than 5%. In particular, α05~0.3
% is preferred.

Cr increases high-temperature strength and acid resistance, and is an indispensable element for high-temperature materials.

To achieve this, the Cr content must be 15% or more, but if it exceeds 2%, the precipitated carbides will become coarser and the creep rupture strength will decrease, so the Cr content should be in the range of 0.5 to 2%. It is necessary. In particular, α9 to 1.5% is preferable.

MO is an element that improves creep strength through solid solution strengthening and precipitation hardening effects, and also prevents tempering embrittlement.
If it is less than 0.5%, the effect is insufficient, and if it exceeds 2%, there is no further effect and the effect is saturated. Therefore, it is effective for MO to have α in the range of 5 to 2%. Especially α9~1.5%
is the most effective.

(2) combines with carbon to form carbide and increase creep rupture strength. However, if the content is less than 0.05%, the effect is insufficient (To must be within the range. In particular, 0.1 to 0.35% of moths is also 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 α005%, weldability and susceptibility to SR cracking will increase. Therefore, it must be in the range of 00001 to α005%. In particular, α0008 to α003% 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,
B Reduce the original effect. Therefore, it is necessary to add an element having a greater ability to form nitrides than B in combination with B, to fix N and to exert the original effect of B. When this effect exceeds 0.1% alone or 0.2% in combination, the creep rupture ductility is significantly reduced. As this element, kt.

'l'1. Nb and 7. r is most preferred. Addition of these elements in combination is more effective. At0.01~0.
05%.

TiO, 01-0.08%, NbO, 01-08%, and zro, 01-0.08% are the most effective.

0 of the sulfide-forming elements, which has a greater sulfide-forming ability than Mn.
．． Addition of 0002% or more acts as desulfurization and dephosphorization during steelmaking, fixes 8 in the steel, suppresses segregation of S to the grain boundaries of the weld heat affected zone, and prevents SR cracking. It is.

SR cracking is a crack at grain boundaries in the heat affected zone of welding, and S-
? Among impurity elements such as P, S in particular is more likely to occur as it segregates to grain boundaries. However, addition of more than 0.62% has no further effect on SR cracking resistance.
Ca and Mg are elements with greater sulfide forming ability.
is valid. These elements reduce the Sa degree of the grain boundaries in the weld heat affected zone and improve the S'fL cracking resistance. In particular, in the present invention, B is added to increase the creep retention, but the addition of B# may actually cause SR cracking.

Therefore, addition of Cal is a necessary cost in terms of preventing SR cracking. If the amount added is less than 0.0002%, it has no effect on SR cracking resistance (and if it is more than 0.2%, the effect is saturated. As an appropriate amount, α005 to 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. Since a ferrite structure may occur depending on the chemical composition and heat treatment, it is most effective to form the entire bainite structure so that the ferrite structure does not substantially precipitate.

Example A cast steel ingot was produced using a high-frequency Wani induction melting furnace.

Table 1 shows the chemical composition of these representative samples.

+7) Steel jJL41,050C, after 15 hours of normalization treatment at 400C/h, then 710 tl' for A1, held for 15 hours, and 7 for 162, 3.4.
After holding at 20C for 15 hours, furnace-cooling tempering #&bottle was performed, respectively.

The samples were ICr-IMO-1/4V, with varying amounts of At, T, B, and C. A1 is a comparative material without B and cao additives. No. 42 is a material with 0.0015% B added and no Ca added, and &3 and 4 are materials of the present invention with B and ca added.

All samples had an entirely bainite structure.

The SR cracking susceptibility test was carried out in accordance with JIS Z3158 by processing the sample into a diagonal Y-shaped weld cracking test piece (plate thickness: 3°). Welding was performed using a commercially available coated arc S* rod (4φ) for Or-Mo steel under the conditions shown in Table 2.

Table 3 shows the results of various mechanical tests.

Although the B-additive material has slightly lower impact properties than the non-additive material, the tensile strength is significantly higher.

On the other hand, the material of the present invention has a 600C, 10' hour creep rupture strength of 9.9 to 10.2Kt/■8, which is extremely effective as a casing material for high-efficiency steam turbines.
It was confirmed that comparative material 41 was also significantly higher.

FIG. 3 is a graph showing the results of the SR cracking test.

In the present invention and comparative materials, Al without adding B and Cl
Although SR cracking did not occur in the material, SR cracking occurred in A2 with 0.0015% B addition, with an sFL cracking rate of 61%. This indicates that the addition of B promotes SR cracking. On the other hand, for I62, Bt is the same and C
In No. 43 of the present invention in which I was added, SR cracking did not occur. Furthermore, SR cracking did not occur in Sample No. 14 of the present invention in which the KB amount was increased to 0.002% and Ca was added. From the above results, it is clear that the present invention has extremely low susceptibility to SR cracking.

As mentioned above, the steel of the present invention has high high temperature strength and toughness, and also has high SR resistance.
It has extremely good cracking susceptibility, and in particular, the high-temperature creep rupture strength up to 600C is extremely high, clearly satisfying the strength required for high-efficiency steam turbine casings. It is suitable as a casing for a high-efficiency steam turbine, and the effect of the present invention contributing to the development of industry is extremely large.

[Brief explanation of the drawings] 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 shows cracks caused by SR treatment after welding. It is a graph showing the rate. 1...Casing body, 2...Adjustment valve casing,
3Figure 7

Claims (1)

[Claims] 1. By weight, C0.05 to 0.2%, Mn 2% or less,
811% or less, Cr (L5~2%, Mo 0.
5 to 2%, vo, os to 0.5%, N1CL 5% or less, B O,0001 to a00 and the nitride-forming element that has a large p-type oxide forming ability as B alone is 0.1% or less, or The composite contains 0.2% or less, and the remainder is substantially p.
A casing for a steam turbine, characterized in that it is made of cast steel having mainly a bainitic structure. 2. The steam turbine casing according to claim 1, wherein the nitride-forming elements are ht, l, Nb, and 7, r. 3. By weight, Cα05-0.2%, Mn 2% or less, 81
1% or less, cro, s~2%, MOo, 5~2%, vo
, os~α5%, NiO, 5% or less, and 80.0002
~0.005%, and scissors B has a large nitride-forming ability.The nitride-forming element alone is 1% or less, or in combination is 0.2% or less, and the above-mentioned Mr1) has a large sulfide-forming ability. 4! Contains 0.0002 to α2% of elements, and the remainder is essentially composed of cast steel with a bainitic structure. A steam turbine casing with a distinctive feature. 4. The sulfide forming element is at least one of CI and Mg.
The casing for a steam turbine according to claim 3, which is a seed.