JP2006009052A - Corrosion prevention method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a corrosion prevention method for preventing pitting corrosion, intergranular corrosion and stress corrosion cracking from occurring in a heat-affected zone of martensitic stainless steel and austenitic stainless steel, which are hard-faced with stellite. <P>SOLUTION: This corrosion prevention method comprises: hard-facing the tip of a valve rod 3 of a safety valve 10, with the stellite 13 of a cobalt-based alloy; further hard-facing a hard-faced boundary 14 formed between the valve rod 3 and the stellite 13 so as to cover it, with a corrosion resistant alloy 15, wherein the corrosion resistant alloy 15 has a chemical composition containing, by wt., 0.10% or less carbon, 17.0-26.0% chromium, 40.0% or more nickel, 25.0% or less iron, 2.5% or less tungsten, 6.5% or less silicon, 3.5% or less manganese, 10.0% or less molybdenum, 5.5% or less niobium and tantalum in total, 1.15% or less titanium, 0.8% or less aluminum and 1.0% or less boron. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an anticorrosion method, and more particularly, to an anticorrosion method for members used in valve stems, valve seats (valve seats with valve boxes), and valve bodies (valve seats with valve bodies).

Since stellite has characteristics such as high strength and good slidability, it is used as a hardened material on the surface of parts that require sliding parts of valves and high compressive strength.
As for the conventional hardening of stellite, as shown in FIG. 5, for example, as shown in FIG. 5, carbon is added to the tip of the valve stem 41 of the martensitic stainless steel safety valve 41 to 0.9 to 3.0% by weight. The stellite 42 containing 26.0 to 33.0% by weight of chromium and 3.0 to 14.0% by weight of tungsten is hardened to withstand a high compressive load. A hardfacing boundary 43 is formed between the valve stem 41 and the stellite 42.
Further, as shown in FIG. 6, a valve seat with valve body 52 is formed by hardening and hardening stellite having the same composition as the stellite 42 of FIG. 5 on the valve body 51 of the austenitic stainless steel gate valve. The surface is kept slidable. A hardened boundary portion 53 is formed between the valve body 51 and the valve seat with valve body 52.

When the base material of the valve part is martensitic stainless steel or austenitic stainless steel, when hardening the stellite on the base material, as shown in FIG. A part (hereinafter referred to as a heat-affected zone of stainless steel) becomes susceptible to corrosion. FIG. 7 is a photograph of the austenitic stainless steel, but a martensitic stainless steel has a similar structure.
This is because the carbon content of stellite is high, and carbon enrichment (carburization) occurs in the heat-affected zone of stainless steel, promoting the precipitation of chromium carbides that are prone to pitting. In addition, when the stellite is hardened, the heat affected zone of the stainless steel is heated to a high temperature, resulting in coarsening of the crystal grains and precipitation of chromium carbide that easily causes intergranular corrosion and stress corrosion cracking at the crystal grain boundaries. Because it does.

  As an example of a conventional corrosion prevention method, Patent Document 1 discloses a method in which nickel plating is applied to a hardfacing boundary between stainless steel and stellite.

JP-A-6-254678

  However, this method has a problem that the nickel plating portion may corrode in an environment containing chloride, SOx, or NOx.

  The present invention was made to solve such problems, and pitting corrosion, intergranular corrosion and stress generated in the heat-affected zone of martensitic stainless steel and austenitic stainless steel hardened by stellite. An object of the present invention is to provide an anticorrosion method for preventing corrosion cracking.

  The anticorrosion method for a member in which stellite is built up on martensitic stainless steel or austenitic stainless steel according to the present invention is corrosion resistant so as to cover the hardened built-up boundary between martensitic stainless steel or austenitic stainless steel and stellite. The alloy is built up and the chemical composition of the corrosion-resistant alloy is 0.10% by weight or less for carbon, 17.0 to 26.0% by weight for chromium, 40.0% by weight or more for nickel, and 25.0% by weight for iron. Below, tungsten is 2.5 wt% or less, silicon is 6.5 wt% or less, manganese is 3.5 wt% or less, molybdenum is 10.0 wt% or less, and the total of niobium and tantalum is 5.5 wt% or less. Titanium is 1.15% by weight or less, aluminum is 0.8% by weight or less, and boron is 1.0% by weight or less.

  According to the present invention, since the corrosion-resistant alloy is built up so as to cover the hardened built-up boundary between martensitic stainless steel or austenitic stainless steel and stellite, the heat-affected zone of martensitic stainless steel or austenitic stainless steel Corrosion of can be prevented.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
Embodiment 1 FIG.
As shown in FIG. 1, the safety valve 10 subjected to the anticorrosion method according to the present invention has a high-pressure fluid passage formed in a valve box 5 attached to a high-pressure vessel (not shown). The martensitic stainless steel valve rod 3 that holds the valve body 4 closing the passage is pressed downward by the coil spring 6, and a force is constantly applied so that the valve body 4 is closed. Is shut off.
On the other hand, when the pressure in the high-pressure vessel (not shown) to which the safety valve 10 is attached rises and exceeds the set pressure, the valve body 4 and the valve stem 3 are lifted upward against the spring force of the coil spring 6, and the safety valve Ten fluids at the valve box inlet 11 pass through the valve box 5 and are discharged from the valve box outlet 12.

In such control of the flow of fluid in and out by the safety valve 10, a large compressive force is applied to the tip of the valve stem 3. Therefore, as shown in FIG. 2, the stellite 13, which is a cobalt-based alloy, is hardened on the tip of the valve stem 3 of the safety valve 10. A hardened boundary portion 14 is formed between the valve stem 3 and the stellite 13.
Further, when the stellite 13 is hardened, the hardened boundary part 14 is prevented from being caused by carbon contained in the stellite to cause the heat-affected zone 16 to be enriched and corroded. The corrosion resistant alloy 15 is built up so as to cover the surface.
In addition, about the hardening buildup of the stellite 13, it performs by gas welding, TIG welding, PTA welding, etc., and buildup of the corrosion-resistant alloy 15 performs by TIG welding, covering arc welding, etc.

  The chemical composition of stellite 13 is 0.9 to 3.0% by weight of carbon, 26.0 to 33.0% by weight of chromium, and 3.0 to 14.0% by weight of tungsten. Since the carbon content of martensitic stainless steel is 0.20% by weight or less, it has a higher carbon content than the valve stem 3, and forms and hardens chromium and tungsten carbides. The strength can be maintained.

The chemical composition of the corrosion resistant alloy 15 is as follows: carbon is 0.10% by weight or less, chromium is 17.0 to 26.0% by weight, nickel is 40.0% by weight or more, iron is 25.0% by weight or less, tungsten Is 2.5 wt% or less, silicon is 6.5 wt% or less, manganese is 3.5 wt% or less, molybdenum is 10.0 wt% or less, the total of niobium and tantalum is 5.5 wt% or less, titanium is 1.15% by weight or less, aluminum is 0.8% by weight or less, and boron is 1.0% by weight or less.
Since this corrosion resistant alloy 15 has a higher chromium content than the valve stem 3, it has corrosion resistance. Moreover, since carbon content is 0.10 weight% or less, it becomes difficult to produce | generate the chromium carbide which is easy to receive the corrosion in the heat affected zone 16 of the valve stem 3 by overlaying. Furthermore, since the nickel content is high, it can be prevented from forming a harmful metal structure such as martensite by fusing with iron, chromium, or the like during building.

  Thus, since the corrosion-resistant alloy 15 is built up so as to cover the hardened built-up boundary 14 between the valve rod 3 and the stellite 13 made of martensitic stainless steel, corrosion of the heat affected zone 16 can be prevented. it can.

Embodiment 2. FIG.
Next, the anticorrosion method according to Embodiment 2 of the present invention will be described with reference to FIG. This anticorrosion method is applied to the valve body of the gate valve instead of the valve stem of the safety valve in the first embodiment as an object to be anticorrosive.
The valve body 21 of the gate valve shown in FIG. 3 (a) has a substantially disc shape, but is formed on each outer peripheral portion of both surfaces of the austenitic stainless steel valve body 21 having a carbon content of 0.08% by weight or less. The stellite having the same composition as that of the stellite 13 of the first embodiment is hardened and formed in a ring shape to form the valve body valve seat portion 22. Further, as shown in FIG. 3B, a hardfacing boundary portion 23 is formed between the valve body 21 and the valve body valve seat portion 22. The corrosion-resistant alloy 15 having the same composition as that of the first embodiment is built up so as to cover the cured build-up boundary 23.
The build-up of stellite (valve valve seat 22) is performed by gas welding, TIG welding, PTA welding or the like, and the build-up of the corrosion-resistant alloy 15 is performed by TIG welding, covered arc welding or the like.

  Thus, also in the valve body 21 of austenitic stainless steel, the same anticorrosion effect as Embodiment 1 can be acquired by building up the corrosion-resistant alloy 15 so that the hardening buildup boundary part 23 may be covered. .

Embodiment 3 FIG.
Next, the anticorrosion method according to Embodiment 3 of the present invention will be described with reference to FIG. This anticorrosion method is applied to a valve box of a ball valve instead of the valve stem of the safety valve in the first embodiment as an object to be anticorrosive.
As shown in FIG. 4, a stellite 13 having the same composition as that of the first embodiment is hardened on a valve box 31 of an austenitic stainless steel ball valve having a carbon content of 0.08% by weight or less, and the valve A valve seat portion 32 with a box is formed. Further, a hardfacing boundary portion 33 is formed between the valve box 31 and the valve seat portion 32 with a valve box. The corrosion-resistant alloy 15 having the same composition as that of the first embodiment is built up so as to cover the hardfacing boundary portion 33.
The build-up of stellite (valve seat portion 32 with valve box) is performed by gas welding, TIG welding, PTA welding, and the like, and the build-up of the corrosion resistant alloy 15 is performed by TIG welding, covered arc welding, or the like.
Thus, the effect similar to Embodiment 1 and 2 can be acquired also by applying the anticorrosion method which concerns on this invention also to the valve box 31 of a ball-shaped valve.

  In the first to third embodiments, the case where the anticorrosion method according to the present invention is applied to the valve stem 3 of the safety valve 10, the valve body 21 of the gate valve, and the valve box 31 of the ball valve is described, but the present invention is limited to these. The same effect can be obtained by applying this anticorrosion method to valve members that are different from each other and have different valve types and parts shapes for controlling the flow of fluid.

<Examples 1-8>
For the test piece, one end of a martensitic stainless steel or austenitic stainless steel material having a diameter of 52 mm and a length of 150 mm was machined into a truncated cone shape having a diameter of 38 mm. Next, after hardening the stellite on the end face by gas welding, the tip portion was processed into a substantially conical shape to prepare a test piece. Furthermore, the corrosion-resistant alloy was built up by TIG welding so as to cover the hardfacing boundary between the base material and stellite.
<Comparative Examples 1-3>
For Examples 1 to 8, test pieces were prepared when there was no buildup of the corrosion resistant alloy.
In Examples 1 to 6 and Comparative Examples 1 and 2, the base material is martensitic stainless steel. Table 1 shows the chemical compositions of the base material, stellite, and the corrosion-resistant alloy for each of these test pieces. On the other hand, in Examples 7 and 8 and Comparative Example 3, the base material is austenitic stainless steel. Table 2 shows the chemical compositions of the base material, stellite, and the corrosion-resistant alloy for each of these test pieces.

<Accelerated corrosion test method>
Each test piece under the conditions shown in Table 1 was immersed in a 3% saline solution at room temperature, a sulfuric acid solution at pH 4 and a nitric acid solution at pH 4 for 1000 hours. After immersion, whether or not corrosion holes were generated at the hardfacing boundary portion of each test piece was confirmed by visual inspection, liquid penetration inspection, and observation of a microscopic structure of a cross-section for representative ones.

Table 3 shows the results of the accelerated corrosion test.
In Examples 1 to 4, four types of corrosion-resistant alloys having different chemical compositions are built up on a martensitic stainless steel hardened by stellite so as to cover the hardened built-up boundary. Comparative Example 1 is a martensitic stainless steel that has been hardened with stellite having the same composition as in Examples 1 to 4, which does not build up a corrosion-resistant alloy at the hardfacing boundary. For Examples 1 to 4, no corrosion pores were observed when immersed in any of 3% saline, pH 4 sulfuric acid solution and pH 4 nitric acid solution, whereas for Comparative Example 1, 3% sodium chloride was used. When immersed in water and a sulfuric acid solution of pH 4, the generation of corrosion pores was confirmed.
Moreover, Example 5 and 6 is what changed the chemical composition of stellite with respect to Examples 1-4, and builds up two types of corrosion-resistant alloys from which a chemical composition differs so that a hardfacing boundary part may be covered. It is a thing. Comparative Example 2 is a martensitic stainless steel that is hardened and hardened with stellite having the same composition as in Examples 5 and 6 and does not build up a corrosion-resistant alloy at the hardfacing boundary with respect to Examples 5 and 6. In Examples 5 and 6, no corrosion pores were observed when immersed in any of 3% saline, pH 4 sulfuric acid solution, and pH 4 nitric acid solution, whereas in Comparative Example 2, 3% sodium chloride was used. When immersed in water and a sulfuric acid solution of pH 4, the generation of corrosion pores was confirmed.

  On the other hand, in Examples 7 and 8, two types of corrosion-resistant alloys having different chemical compositions were built up on the austenitic stainless steel hardened by stellite so as to cover the hardened built-up boundary. Comparative Example 3 is an austenitic stainless steel in which stellite having the same composition as Examples 7 and 8 is hardfaced and hardened with respect to Examples 7 and 8 and does not build up a corrosion-resistant alloy at the hardfacing boundary. In Examples 7 and 8, no corrosive pores were observed even when immersed in a 3% saline solution and a pH 4 sulfuric acid solution, whereas in Comparative Example 3, a 3% saline solution and a pH 4 sulfuric acid solution were used. When immersed, the occurrence of corrosion holes was confirmed.

  From the above results, in the anticorrosion method according to the present invention, even in an environment containing chloride, SOx or NOx, which is a problem in the conventional anticorrosion method, the corrosion resistance has been built up so as to cover the hardened built-up boundary. It was confirmed that the anticorrosion effect can be obtained without the alloy being corroded.

It is sectional drawing of the safety valve which gave the corrosion prevention method which concerns on Embodiment 1 of this invention. It is a figure of the anticorrosion method concerning Embodiment 1. FIG. It is a figure of the anticorrosion method which concerns on Embodiment 2. FIG. It is a figure of the anticorrosion method which concerns on Embodiment 3. FIG. It is the figure which hardened the stellite at the front-end | tip part of the valve rod made from a martensitic stainless steel by the conventional method. It is the figure which hardened the stellite on the valve body of the austenitic stainless steel gate valve by the conventional method. It is the photograph of the corrosion which generate | occur | produced in the heat affected zone of stainless steel.

Explanation of symbols

  13 Stellite, 14, 23, 33 Hardfacing boundary, 15 Corrosion resistant alloy, 16 Heat affected zone.

Claims (1)

A martensitic stainless steel or austenitic stainless steel is a method for preventing corrosion of a member formed by hardening stellite,
Overlaying a corrosion-resistant alloy so as to cover the hardfacing boundary between the martensitic stainless steel or the austenitic stainless steel and the stellite,
The chemical composition of the corrosion resistant alloy is:
Carbon is 0.10% by weight or less,
17.0 to 26.0% by weight of chromium,
40.0% by weight or more of nickel,
Iron is 25.0% by weight or less,
2.5% by weight or less of tungsten,
6.5% by weight or less of silicon,
Manganese is not more than 3.5% by weight,
Molybdenum is 10.0% by weight or less,
The total of niobium and tantalum is 5.5% by weight or less,
1.15% by weight or less of titanium,
0.8% by weight or less of aluminum,
An anticorrosion method, wherein boron is 1.0% by weight or less.

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