Precipitation hardened martensitic stainless steel, manufacturing method therefor, and turbine moving blade and steam turbine using the same

Abstract

The precipitation hardened martensitic stainless steel is characterized by containing, in percent by weight, 12.25 to 14.25% Cr, 7.5 to 8.5% Ni, 1.0 to 2.5% Mo, 0.05% or less C, 0.2% or less Si, 0.4% or less Mn, 0.03% or less P, 0.005% or less S, 0.008% or less N, 0.90 to 2.25% Al, the balance substantially being Fe, and the total content of Cr and Mo being 14.25 to 16.75%. A turbine moving blade and a steam turbine are manufactured by using this martensitic stainless steel.

Description

TECHNICAL FIELD [0001] The present invention relates to a precipitation hardened martensitic stainless steel having high strength, high toughness, and high resistance to delayed cracking, a manufacturing method therefor, and a turbine moving blade and a steam turbine using the martensitic stainless steel. BACKGROUND ART [0002] In order to improve the thermal efficiency of a steam turbine, it is advantageous to increase the blade length of a low-pressure final-stage moving blade. In order to increase the length of a turbine moving blade, a blade material having high specific strength is needed. However, in a steam turbine of 3600 rpm, at present, a 40-inch class is a limit for a steel-made blade, and hence a titanium alloy is used for a 45-inch class. [0003] As a high-strength steel material for a turbine blade, Japanese Patent Provisional Publication No. 2001-98349 describes a martensitic stainless steel having a composition, in percent by weight, of 0.13 to 0.40% C, 0.5% or less Si...

AI Technical Summary

Benefits of technology

The present invention provides a precipitation hardened martensitic stainless steel with high strength, toughness, and resistance to delayed cracking. The steel has a high Cr content of 14.25 to 16.75% and a high Al content of not less than 1.35%. The total content of Cr and Mo is preferably 15.5 to 16.75%. The steel has a high tensile strength of not less than 1350 MPa and a high toughness of not less than 20 J. The steel can be used for manufacturing turbine moving blades and steam turbines. The invention also provides a manufacturing method for the steel and a steam turbine using the steel.

Problems solved by technology

However, for a tempered martensitic stainless steel, in which the strength is controlled by quench-and-temper, as described in Japanese Patent Provisional Publication No. 2001-98349, if the tensile strength is increased to 1350 MPa or higher, a delayed crack may be generated as described later.

On the other hand, for a precipitation hardened martensitic stainless steel, although high strength, high toughness, and high corrosion resistance are achieved, the precipitate of Cu, Nb or Ti alone of the background art does not provide a sufficient strength as compared with a required value of 1500 MPa or higher.

For the technique described in Japanese Patent No. 3251648, although high strength and high toughness are achieved by decreasing the crystal grain size, it is difficult to obtain a fine grain structure in a thick portion such as a turbine blade root portion, which presents a problem of insufficient strength and toughness.

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