JPH0368741A - Austenitic iron-base alloy - Google Patents

Abstract

PURPOSE:To refine a structure, to prevent the formation of Cr carbide in grain boundaries, and to improve corrosion resistance by reducing C content in an austenitic stainless steel and adding specific small amounts of Zr to the above steel. CONSTITUTION:At the time of producing a member used in a high-temp. pure water environment in a nuclear reactor, a stock prepared by adding 0.02-1.0% Zr to an austenitic stainless steel having a composition consisting of, by weight, 9.0-11.0% Ni, 18.0-20.0% Cr, <0.03% C, <2.0% Mn, <0.005% P, <0.004% S, <0.03% Si, and the balance Fe is used. Since the grain size of the austenitic stainless steel is refined by the addition of Zr, and further, Zr carbide is formed in the grain boundaries and the formation of Cr carbide is prevented, the grain boundaries are free from the occurrence of Cr-deficient phase due to the formation of Zr carbide, and, as a result, the austenitic stainless steel member free from deterioration in corrosion resistance and having superior corrosion resistance can be obtained.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an austenitic iron-based alloy.

In particular, the present invention relates to a high-purity austenitic iron-based alloy with excellent corrosion resistance that is suitable for internal equipment in nuclear reactors.

[Conventional technology] In-reactor equipment such as austenitic stainless steel control rods and in-reactor instrumentation tubes are placed in an environment of high-temperature pure water and are relatively expensive compared to other reactor constituent materials. Being irradiated with neutrons.

On the other hand, austenitic stainless steel in high-temperature pure water
Intergranular stress corrosion cracking (IGSCC) may occur.

The main material-related factors in IGSCC are the formation of grain boundary carbides due to thermal cycles such as welding and the accompanying formation of a chromium-deficient layer near the grain boundaries, that is, weld sensitization. However, it has been reported that even solution-treated austenitic stainless steel, which has not undergone any sensitization, exhibits higher intergranular corrosion cracking susceptibility when irradiated than unirradiated material.

The effects of irradiation on materials are (1) due to irradiation-induced segregation caused by irradiation, Si (silicon), P (phosphorus), etc. segregate and the corrosion resistance of grain boundaries decreases; (2)
It is thought that phase transformation is promoted as a result of promoting diffusion of alloying elements by irradiation, specifically, the formation of grain boundary carbides and the accompanying formation of a chromium-depleted layer, that is, the promotion of sensitization.

High-purity austenitic stainless steel focuses on the fact that the above factors are particularly affected by impurity elements, and by limiting the amount of impurity elements, it has excellent intergranular corrosion cracking resistance even when subjected to high irradiation. It was developed for this purpose.

In addition, as references related to this type of technology, (1)
"Intergranular corrosion of austinato steel" (J, S, Arm1j
o: Intergranular Corrosion
of Non5ensitized Au5teni
tic 5teels, Corrosio0. NACE
, Janu, 1968. p24) and (2) rBWR
and Deformation properties of austenitic stainless steels and Ni-based alloys in PWR cores J (F, Garaza
rolli etal,, Defors+abilit
y of Au5tenitic 5tainless
5teel and N1-base A11oy
in the core of a Boiljung
and a Pressurized Water P
reactor, Proc, Int'1. Symp, En
vironmental degradation of
Materials in Nuclear Sys
tems-Water Reactors, Monto
ry, Ca1. Ll, S, A, 5ept, 1983. p
442).

[Problems to be Solved by the Invention] However, it has become clear that the intergranular corrosion cracking resistance of the above-mentioned high-purity austenitic stainless steel may not necessarily be improved as intended. The causes of this are: (a) By limiting the impurity content in the steel, the segregation of Si and P due to irradiation-induced segregation can be reduced, but it cannot be completely eliminated.

(b) The carbon content in this steel is limited to a smaller amount than that of ordinary austenitic stainless steel, but it is not completely absent. Therefore, since a trace amount of C combines with Cr to form grain boundary carbides such as Cr, C, etc., it is not possible to completely prevent the appearance of a Cr-depleted layer near grain boundaries due to neutron irradiation. It is inferred that.

The present invention was made in view of the above-mentioned circumstances, and improves intergranular cracking resistance by reducing the crystal grain size of the above-mentioned high-purity austenitic stainless steel and preventing the formation of intergranular carbides. [Means for Solving the Problems] The structure of the austenitic iron-based alloy according to the present invention for solving the above problems is as follows: ~11゜0wt%, Cr
18.0-20.0 wt%, CO.

03wt% or less, Mn2.0wt% or less, po.

005wt% or less, S0.004wt% or less, Si0.
In an austenitic iron-based alloy containing Zr of 0.02 to 1.03 wt% or less and the remainder mainly consisting of Fe,
The content is 0 wt%.

[Function] The present invention provides so-called 18-8 austenitic stainless steel with C0.03wt% or less and PO,005wt%.
% or less, S0.004wt% or less, Si0.03wt%
By adding Zr to the high purity austenitic steel limited below.

The crystal grain size is increased to m, and Zr carbide is generated.

By making the crystal grain size finer, the grain boundary area per unit volume increases, and accordingly, the concentration of Si, P, etc. segregated at the grain boundaries decreases, which has the effect of improving corrosion resistance. Furthermore, since Zr has a strong affinity with C and forms grain boundary carbides, it has the effect of preventing the formation of grain boundary carbides of Cr. As a result, it is possible to prevent the formation of Cr-depleted layers near grain boundaries, improving corrosion resistance. can be improved. Furthermore, Zr is a ferrite former, and if Zr is added above a certain level, it becomes a duplex stainless steel. Generally, duplex stainless steel is known to have better corrosion resistance than austenitic stainless steel, and from this point as well, corrosion resistance can be improved. However, the solid solubility limit of Zr is about 1% at most.

[Example] An embodiment of the present invention will be described below with reference to the drawings.

In weight%, Ni9.0-11. O, Cr18゜0~20
．． O, Mn 2.0 or less, G 0.03 or less.

Zr is added in various concentrations (%) from 0.01% to the austenitic stainless steel of the present invention, which is composed of P 0.005 or less, S 0.004 or less, Si 0°03 or less, and the balance Fe, to produce several types of austenitic iron. Base alloys are manufactured, corrosion test pieces for cracking susceptibility are manufactured from each, and the amount of Zr added (
%) on corrosion resistance was investigated.

In the intergranular corrosion test, each of the above specimens is immersed in a 5N boiling nitric acid solution containing hexavalent chromium for 12 hours, and then taken out and the crack length is measured to examine crack susceptibility. It was carried out using the method.

The drawings show the above test results.

The horizontal axis represents the Zr concentration (%) in the austenitic steel, and the vertical axis represents the intergranular corrosion susceptibility (crack length ratio). From this drawing, at Zr0.01%, the sensitivity is 0.
．． 4%, but 0.02% is 0.35, 0.05
%, it is 0.3, and the value is almost the same in the case of 1.0% Zr.

That is, it was demonstrated that when the Zrwt% was within the above range, the intergranular corrosion resistance was significantly improved.

Furthermore, since the solid solubility limit of Zr element in 18-8 series austenitic steel is estimated to be at most 1.0 wt%, the amount of Zr added in this example is preferably 0.02 to 1.0 wt%. It is.

[Effects of the Invention] As explained above, by adding a predetermined amount of zirconium to the high-purity austenitic iron-based alloy of the present invention, the crystal grain size becomes finer and the formation of grain boundary carbides is prevented.
It is possible to provide a material for furnace equipment that is resistant to IGSCC even when subjected to neutron irradiation and has excellent intergranular corrosion cracking resistance.

[Brief explanation of drawings]

The drawing is a diagram showing the relationship between Zr concentration in steel and intergranular corrosion susceptibility. Explanation of symbols>

Claims (1)

[Claims] 1, Ni9.0-11.0wt%, Cr18.0-20
．． 0wt%, C0.03wt% or less, Mn2.0wt%
The following contains P0.005wt% or less, S0.004wt% or less, and Si0.03wt% or less, and the remainder is mainly F.
In the austenitic iron-based alloy consisting of e, Zr is 0
．． An austenitic iron-based alloy characterized by containing 02 to 1.0 wt%.