JPH02250945A - Manufacture of corrosion-resistant cupro nickel tube for heat exchanger - Google Patents

Abstract

PURPOSE:To reduce the amt. of a harmful carbon film residue on the internal surface of the cupro nickel tube by specifying carbon residue in a lubricating oil used in a reduction stage, and after the tube-manufacturing by reduction, annealing it in a reducing atmosphere contg. specified amt. of oxygen. CONSTITUTION:In a method for manufacturing a tube material for a heat exchanger from an ingot of cupro nickel through a tube-manufacturing stage and a heat-treatment stage, after the tube-manufacturing by reduction with a water-soluble lubricating oil contg. 0.001 to 0.3% carbon residue, the tube is annealed in a reducing atmosphere contg. 50 to 150ppm O2. At this time, though the amt. of carbon residue in the water-soluble lubricating oil is preferably small, the frictional resistance between the surface in the tube and a tool at the time of the tube-manufacturing is made high to reduce the lubricating effect for the lower limit or less, and a carbon film remained on the surface in the pipe is not reduced to cause pitting corrosion for the upper limit or more. Furthermore, the amt. of O2 in a furnace at the time of annealing is required for burning the carbon residue in the lubricating oil, but the carbon residue forms a film after annealing for the lower limit or less, and the surface of the tube is oxidized and discolored into brown at the time of final annealing for the upper limit or more.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Fields] The present invention is directed to cupronickel (Cu/Ni
) This invention relates to a method of manufacturing an alloy tube, and in particular to a method of manufacturing a cupronickel tube that prevents the initial pitting corrosion that occurs in cupronickel.

[Prior Art] Tubes made of copper, brass, titanium, cupronickel, etc. are widely used for heat exchanger tubes in condensers of power generation plants and heat exchangers in distillation seawater desalination plants. In heat transfer tubes, the cooling fluid flows inside the tube and exchanges heat with the high temperature fluid outside the tube.
Various problems are caused by the cooling water flowing through the pipes.

Cu-N1-F e -Mn alloy (so-called cupronickel), which has excellent corrosion resistance in seawater, is sometimes used for the heat exchanger tubes of heat exchangers that use seawater as cooling water. When Ni is included, sand chloride is also added, so cupronickel containing 30% Ni is used.

As a corrosion prevention method for these heat exchanger tubes, in the case of brass tubes, a well-known corrosion prevention method involves injecting iron ions into the seawater used to form a film of ferric hydroxide on the inner surface. In the case of cupronickel, the applicant has developed a corrosion prevention method in which a high-temperature oxide film formed on the inner surface of the cupronickel tube is removed during the manufacturing process of the pipe, and then seawater containing iron ions is passed through to form a film. proposed. (Unexamined Japanese Patent Publication No. 82-25018
In addition, it is believed that oxide films formed during pipe manufacturing are involved in corrosion resistance, and pitting corrosion is known to occur especially when steel pipes are used for water supply piping that uses hard water. It is being The cause of this is that during the final annealing of the steel pipe, the lubricating oil used in the drawing process in the previous step is incompletely combusted and remains as a carbon film. There is a theory that it is involved in carbon film damage, and this is known as the "carbon film harmful theory." However, in the case of aluminum brass pipes that use seawater as cooling water, carbon films and high-temperature oxide films that occur during pipe manufacturing are
(Sato et al., Journal of the Copper Rolling Technology Study Group, Vol. 17 (1978), P2S5), and the reality is that special treatments such as sandblasting and pickling are not generally performed during pipe manufacturing. It is.

[Problems to be Solved by the Invention] These methods for preventing corrosion of heat exchanger tubes include circulating seawater or the like containing iron ions while the tubes are manufactured or assembled in a heat exchanger.
The purpose is to form an anti-corrosion film.

However, in the case of cupronickel tubes, removing the high-temperature oxide film on the inner surface of the tube requires physical treatment such as sandblasting or sponge ball, and chemical treatment such as acid cleaning, which is difficult to incorporate into the equipment. This has the disadvantage that it becomes complicated later and also involves a significant increase in cost.

Conventionally, in manufacturing plants, the type of lubricating oil used in the drawing process has been selected solely from the viewpoints of surface lubricity, price, and availability. The product concerned (inner diameter 10-3 (L+
am, length 3 to 201 m) is final annealed in a reducing DX gas furnace after drawing, but the inner surface condition of the tube may vary depending on the product dimensions, type of lubricating oil, oxygen partial pressure in the furnace, etc. are significantly different.

In order to suppress the formation of a carbon film, adjusting the lubricating oil may impair the properties of other oils or increase costs.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method for manufacturing a cupronickel tube at a low cost in which the amount of harmful residual carbon film on the inner surface of the tube is reduced by adjusting manufacturing conditions for the cupronickel tube.

[Means for Solving the Problems] In order to solve the above problems, the present inventors have investigated the causes of pitting corrosion and material factors, such as impurities in components, segregation of major elements, metal structure, surface condition, etc. As a result of investigating the relationship between the two, it was discovered that residual carbon formed during pipe manufacturing was involved, and the present invention was completed.

The gist of the present invention is to produce pipe materials for heat exchangers from cupronickel ingots through a pipe making process and a heat treatment process. After the tube, add 50 to 150 ppff of oxygen.
This is a method for manufacturing a pitting corrosion-resistant cupronickel tube for a heat exchanger, which is annealed in a reducing atmosphere containing i to reduce residual carbon on the inner surface of the tube.

The amount of residual carbon in the lubricating oil was determined by measuring the amount of carbonized residue after thermally decomposing the lubricating oil in a steamed state based on JIS K2270 (Conradson method), and expressed as a weight ratio of the amount before steaming.

In addition, although there is no established method for measuring the amount of residual carbon on the tube surface, here we will use Auger spectroscopy to measure the temporal changes in carbon and copper detected when sputtering argon ions, as shown in Figure 2. We decided to calculate it as follows, and calculate it based on the length of time it takes for each value to become constant.

The atmospheric gas in the annealing furnace is D, which is made by burning and refining propane.
X gas is suitable, CO2: 12.5%, CO71.5%
, H2:0.8%, balance N2, and the oxygen concentration was adjusted by replenishing pure oxygen while keeping nj constant using the method described below.

The oxygen concentration was measured using a galvanic cell oxygen analyzer.

[Function] Regarding the amount of residual carbon contained in water-soluble drawing oil: The amount of residual carbon contained in water-soluble drawing oil is small because it becomes a residual carbon film that is burned in the annealing process after drawing and promotes the occurrence of pitting corrosion. is preferable. However, if it is less than 0°001%, the frictional resistance between the inner surface of the tube and the tool (plug) during tube manufacturing becomes large, and the lubricating effect is weakened. Furthermore, if the content is 0.3% or more, even if the oxygen concentration in the furnace is adjusted during final annealing of the tube, the carbon film remaining on the inner surface of the tube will not be reduced and pitting corrosion will be induced.

Regarding the amount of oxygen in the furnace during annealing; The amount of oxygen in the furnace during annealing is necessary to burn the residual carbon content in the lubricating oil, but if it is less than 50 ppm, the amount of residual carbon contained in the water-soluble drawing oil will be reduced. Even if there is a small amount, it will remain on the surface after annealing and form a film, and if it is 150 ppm or more, the surface of the tube will oxidize and discolor to brown during final annealing, and its commercial value will be lost.

Regarding carbon remaining on the inner surface of the tube: Since carbon remaining on the inner surface of the tube causes initial pitting corrosion, it is preferable to have less carbon. However, although it cannot be completely eliminated, it has been found that in practice, if the Auger rise is about 50, it will not affect the initial pitting corrosion.

[Example] Example I Cu-29, 8% Ni-1, 9% Fe-1,
An 8% Mn alloy was melted and cast into an ingot with a diameter of 10 inches and a length of 435 mm. After cutting the surface, extrusion and drawing were performed to form a seamless pipe with an outer diameter of 19.05 n+m and a wall thickness of 1.219 au. The lubricating oil used during drawing was the JISK
2270 (Conradson method), and those shown in Table 1 were used. In addition, the DX gas during annealing was CO2: 12.5%, CO: 1.5%, H2: 0.
The oxygen content in the furnace was varied as shown in Table 1. The obtained test material was subjected to chlorine sterilization treatment (chlorine added dropwise to 1 pI + 111 pI adjustment) and natural seawater was passed through it to examine the occurrence of pitting corrosion, and the results are shown in Table 1.

In all of the invention examples Nos. 1 to 3, the amount of carbon detected on the tube surface was small, and no pitting corrosion was observed.

However, although the amount of residual carbon in the lubricating oil for N014 was 0.21%, the oxygen concentration during annealing was as low as 50 ppm, and pitting corrosion was observed. The amount was as high as 0.40 and 0.60%, and pitting corrosion was observed even if the oxygen concentration was adjusted during annealing.

Table 1 Inventive examples Nos. 1 to 4 all have the residual carbon content of the lubricating oil adjusted to 0.21% or less, and the oxygen concentration during annealing is 150%.
It was adjusted to below ppm, and no pitting corrosion was observed. However, in Nos. 5 and 6, the residual carbon content of the lubricating oil was 0.40.0.68, and pitting corrosion was observed in all of them.

Table 2 Example 2 Cu -10, 1% Ni -1, 40% Fe -0
, 87% Mn alloy was made into a tube in the same manner as in Example 1,
A similar test was conducted. The results are shown in the m2 table.

Example 3 (, u −30,9%N i −0,80%F e −
A 0.80% Mn alloy was made into a tube in the same manner as in Example 1, and the same tests were conducted. The results are shown in Table 3.

In this material, pitting corrosion was not observed when the residual carbon m of the lubricating oil was set to [1.21%], but pitting corrosion was observed when the residual carbon m was set to 0.40%.

From Examples 1 to 3 in Table 3 and above, the relationship between the occurrence of pitting corrosion, the amount of residual carbon in the lubricating oil, and the oxygen concentration during annealing is shown in Figure 1, and the amount of residual carbon is 0.3%. No pitting corrosion was observed in the oxygen concentration range of 50 to 150 ppm.

[Effects of the Invention] According to the present invention, a cupronickel tube with a reduced amount of harmful residual carbon film on the inner surface of the tube can be manufactured at low cost.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the occurrence of pitting corrosion, the amount of residual carbon in the lubricating oil, and the oxygen concentration during annealing, and FIG. 2 is a graph showing the relationship between the amount of residual carbon and copper and time. Drawn coal in the mountains (mystery)

Claims (1)

[Claims] In a method of manufacturing heat exchanger tube material from a cupronickel ingot through a tube making process and a heat treatment process, the tube is drawn with lubricating oil containing 0.001 to 0.3% of residual carbon, and then oxygen is added to 50 to 50% of oxygen. A method for manufacturing a pitting corrosion-resistant cupronickel tube for a heat exchanger, the method comprising annealing in a reducing atmosphere containing 150 ppm to reduce residual carbon on the tube surface.