JPS63203750A - Fe-ni-cr alloy having superior resistance to intergranular corrosion - Google Patents

Abstract

PURPOSE:To manufacture an Fe-Ni-Cr alloy having superior resistance to intergranular corrosion by adding specified percentages of C, Si, Mn, P, S, Ni, Cr, Al and Ti to Fe. CONSTITUTION:An Fe-Ni-Cr alloy consisting of, by weight, <=0.03% C, <=1.0% Si, <=1.5% Mn, <=0.02% P, >0.015-0.085% S, 30-35% Ni, 19-23% Cr, <=0.060% Al, 0.15-0.60% Ti and the balance Fe with inevitable impurities is manufactured. The alloy has characteristics comparable to those of conventional low-C alloy 800 and also has such superior resistance to intergranular corrosion that the alloy hardly undergoes intergranular corrosion even by strain relief annealing after welding. The alloy is useful for the piping of a nuclear powder plant or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention primarily relates to an Fe-Ni-Cr alloy with excellent intergranular corrosion resistance that is used in environments where intergranular corrosion is likely to occur.

[Conventional technology]

Corrosion-resistant alloys are currently used in pressure vessels, heat exchangers, steam generation pipes, and other piping in nuclear power plants, as well as in chemical plant alloys.

As one of the alloys used in environments where intergranular corrosion or intergranular stress corrosion is a problem, approximately
Aloy 800 is an Fe-based alloy containing 5% Ni and about 20% Or, and this alloy is less susceptible to intergranular corrosion under normal solution treatment conditions.

However, it is generally known that even alloys that are difficult to undergo intergranular corrosion under solution treatment become more susceptible to intergranular corrosion when sensitized by post-weld strain relief annealing, etc.
oy800 is no exception.

Here, sensitization means that Cr carbides are continuously precipitated at grain boundaries, resulting in a low Cr concentration along the grain boundaries.
This is a phenomenon in which an r-depleted layer is formed, and intergranular corrosion is caused by this Cr
It is said that this occurs due to preferential corrosion of the deficient layer.

Therefore, in order to prevent intergranular corrosion, it is considered that it is sufficient to suppress sensitization, and one way to do this is f −<+1− F
a'ik ah -he cho'IL l= jG 34
1-1 fr L, %) A4 It is considered effective to lower the amount of C in the entire city.

However, in order to prevent sensitization only by lowering C, it is necessary to keep C quite low. On the other hand, lowering C lowers the strength, so there is a problem that C cannot be kept very low.

Furthermore, Si, P%S, etc. are generally kept low as impurity elements that adversely affect corrosion resistance and other properties.

[Problem that the invention seeks to solve]

In view of this point, the present invention provides a conventional low C level A11o.
It has the same characteristics as Y800, and does not suffer from intergranular corrosion even after stress relief annealing after welding (F with excellent intergranular corrosion resistance).
The present invention provides an e-Ni-Cr alloy.

[Means for solving problems]

In order to solve this problem, the inventor developed A11oy800.
As a result of experiments conducted on a considerable number of Fe-Ni-Cr alloys, it was found that by actively adding S, which is generally kept low as an impurity element, above the normal level, the effect of S on the alloy after strain relief annealing due to its interaction with Ti. The present invention was developed based on the new discovery that the intergranular corrosion resistance of
i 1.0 or less, Mn 1.5 or less, po.

02 or less, over 80,015, 0.085 or less, Ni
30-35, Cr19-23, AI! 0.60 or less,
It is a Fe-Ni-Cr alloy with excellent intergranular corrosion resistance characterized by consisting of TiO, 15 to 0.60, the balance Fe and unavoidable impurities.

The reason for limiting the range of components in the present invention is as follows.

C: If C exceeds 0.03%, not only will the intergranular corrosion resistance deteriorate significantly but also the effect of S addition will disappear.
It was set to 0.03% or less.

Si: Si is added as a deoxidizing agent, but if it is more than 1.0%, intergranular corrosion resistance deteriorates, so the content is limited to 1.0% or less.

Mn: Mn is added as a deoxidizing agent and desulfurizing agent, but 1.
If it exceeds 5%, intergranular corrosion resistance deteriorates, so the content is set to 1.5% or less.

P: If P is more than 0.02%, intergranular corrosion resistance deteriorates, so it is set to 0.02% or less.

s: S makes the precipitation of C carbides discontinuous at the grain boundaries through interaction with Ti, and greatly improves intergranular corrosion resistance, but below 0.015%, it has no effect, and 0
．． If it exceeds 0.085%, hot workability will deteriorate; o i s% and 0.085% or less.

Ni: Ni not only stabilizes austenite but also has the effect of improving stress corrosion cracking resistance.
If it is less than 35%, a sufficient effect cannot be obtained, and even if it exceeds 35%, no further improvement effect can be obtained.
It was set at 0 to 35%.

Cr: Cr is an element necessary to obtain corrosion resistance, and if it is less than 19%, a sufficient effect cannot be obtained;
Since it becomes easier to precipitate the σ phase if 3% is determined, 19
~23%.

However, even if it exceeds 0.6%, no further improvement effect can be obtained, so it was set to 0.6% or less.

Ti: Ti is not only effective as a deoxidizing agent, but also
Intergranular corrosion resistance is greatly improved by interaction with S, but 0
．． If it is less than 15%, the effect will not be obtained, and if it is less than 15%, the effect will not be obtained.
Even if it exceeds 60%, no further improvement effect can be obtained, so it was set at 0.15 to 0.60%.

〔Example〕

The present invention will be explained below with reference to Examples.

An alloy having the composition shown in Table WS1 was melted in a vacuum induction melting furnace.
After melting 105 kg, it is cast into ingots and further 105
After heating to 0°C, it was forged and finished into a 30+m square bar. The test piece taken from this was subjected to solution treatment at 1050°C for 1 hour, and then cooled in oil.

Furthermore, as a sensitization treatment equivalent to stress relief annealing after welding,
500"/' after heating and holding at 625℃ for 40 hours!
Y 9 A Shakaiha Sekikyo IZ eclipse 9 粁 か - ト inclination It was sold cold.

Thereafter, it was finished into a test piece measuring 3 m + * x 15 mm x 50 cm, which was subjected to an intergranular corrosion resistance test.

A modified striker test was used as the intergranular corrosion test, but this method is similar to the standard striker test (ASTM
G28) using a test solution with twice the amount of ferric sulfate
This is a 4-hour immersion test in boiling liquid.

After the test, the corrosion loss was measured, and the test piece was placed in a radius of 50 m.
There was whitening in the arc shape of -, and the maximum crack depth was determined. Intergranular corrosion resistance was evaluated based on this corrosion weight loss and maximum crack depth.

The results are shown in Table 2.

As is clear from the results in Table 2 of Table 11P12, Comparative Alloy 3 is the normal A11oy 800, and although it has poor intergranular corrosion resistance even if the Cfi is reduced to this extent, it is as good as Invention Alloy 1.2. It can be seen that intergranular corrosion resistance is significantly improved by adding a specified amount of S so as to improve the intergranular corrosion resistance.

In addition, the effect of S is small when the amount of C is high, as seen in comparative alloy 4.5 and h1 Shiny: RLra Yaku + - L eye - Seal Sabahinjin 6.7.

Therefore, the effect of S addition is that the amount of C is low to some extent, and
It has been found that it is effective when Ti is contained to some extent, and if this condition is met, intergranular corrosion resistance is significantly improved.

〔Effect of the invention〕

As explained above, the alloy of the present invention has a low C level of A11
By adding S to oy800, intergranular corrosion resistance has been greatly improved, and if this alloy is used in various corrosion-resistant parts such as piping in nuclear power plants,
Even when a structure is assembled by welding, intergranular corrosion resistance is not impaired (this has a significant industrial effect).

Claims (1)

[Claims] C 0.03 or less, Si 1.0 or less, Mn 1.0 in weight%.
5 or less, P0.02 or less, S0.015 or more 0.08
5 or less, Ni30-35, Cr19-23, Al0.6
An Fe-Ni-Cr alloy with excellent intergranular corrosion resistance, characterized by comprising: 0.0 or less, Ti: 0.15 to 0.60, the balance being Fe and unavoidable impurities.