Welding wire for modified 9Cr-1Mo steel, and submerged-arc welding material

Abstract

A welding wire for a modified 9Cr-1Mo steel is provided which comprises C: 0.070 to 0.150% by mass, Si: more than 0.15% by mass, but not more than 0.30% by mass, Mn: not less than 0.30% by mass, but less than 0.85% by mass, Ni: 0.30 to 1.20% by mass, Cr: 8.00 to 13.00% by mass, Mo: 0.30 to 1.40% by mass, V: 0.03 to 0.40% by mass, Nb: 0.01 to 0.15% by mass, N: 0.016 to 0.055% by mass, P: not more than 0.010% by mass, S: not more than 0.010% by mass, Cu: less than 0.50% by mass, Ti: not more than 0.010% by mass, Al: less than 0.10% by mass, B: less than 0.0010% by mass, W: less than 0.10% by mass, Co: less than 1.00% by mass, and O: not more than 0.03% by mass, wherein the total amount of Mn and Ni being not more than 1.50%. The welding wire provides good toughness without degradation of creep rupture strength even at the PWHT temperature of 760° C. or above.

Description

FIELD OF THE INVENTION [0001] The present invention relates to welding wires for welding modified 9Cr-1Mo steels, which are used for various types of heat-resistant and pressure-resistant piping, including a boiler for power generation, a turbine, and the like. More particularly, the invention relates to welding wires for modified 9Cr-1Mo steels, which are used for performing a submerged arc welding (SAW) process of the modified 9Cr-1Mo steel, and / or a tungsten inert gas (TIG) welding process thereof, and to welding materials comprising the combination of the welding wire and a flux. BACKGROUND OF THE INVENTION [0002] A modified 9Cr-1Mo steel (hereinafter referred to as “Mod.9Cr-1Mo steel”) is made of a 9Cr-1Mo steel with Nb and V added thereto. For example, the modified 9Cr-1Mo steel is SA387Gr.91, or SA213Gr.T91, which is specified in the American Society for Testing and Materials (ASTM) Specification / American Society of Mechanical Engineers (ASME) Specification, X10CrMoVNb9-1, wh...

AI Technical Summary

Benefits of technology

[0013] The present invention has been accomplished in view of those problems encountered with the prior art, and it is an object of the invention to provide a welding wire for a modified 9Cr-1Mo steel that provides good toughness without degradation of creep rupture strength even at the PWHT temperature of 760° C. or above.

[0015] In the present invention, the total content of Mn and Ni is restricted to not more than 1.50% by mass, and the Co content is also restricted to less than 1.00% by mass, so that the creep rupture strength is not degraded even at the PWHT temperature of 760° C. or above. The contents of Mn, Ni, Si, Cr, Mo, V and Nb, each of which might affect the toughness, are optimized, while the contents of Al, W, Ti, B, C, and O, each of which might degrade the toughness, are restricted, resulting in the good toughness.

[0016] Preferably, in the welding wire, the Ni content may be 0.40 to 1.00% by mass, the Mo content 0.80 to 1.10% by mass, the Cu content not more than 0.10% by mass, and the Al content less than 0.05% by mass. This improves the toughness and the creep rupture strength.

Problems solved by technology

The above-mentioned prior art, however, has the following problems.

Thus, in some welding processes in other countries, the PWHT temperature is intended to be set to 760° C. for the purpose of improving the creep rupture strength and toughness, and the PWHT temperature is often increased until the actual temperature reaches 780° C. In this case, a problem of the Acl transformation temperature for a deposited metal arises.

Concretely, when the PWHT is carried out at a temperature above the Acl transformation temperature of the deposited metal, phase transformation occurs in the deposited metal, resulting in a danger that the creep rupture strength may be significantly degraded.

Furthermore, as disclosed in the above-mentioned cited documents, since Mn and Ni each have effects of ensuring and improving toughness, just restriction of the total content of Mn and Ni in the welding material under the high PWHT temperature condition disadvantageously leads to failure in improvement of the toughness.

The above-mentioned documents do not take into consideration the welding material whose PWHT temperature is not less than 760° C. In the welding materials disclosed in these documents, when the PWHT temperature exceeds the Acl transformation temperature of the deposited metal, phase transformation might occur in the deposited metal, resulting in significantly degraded creep rupture strength.