JPH03162517A - Solution heat treatment of super-austenitic stainless steel - Google Patents

Abstract

PURPOSE:To allow sigma phase in a casting of a super-austenitic stainless steel to perfectly enter into solid soln. in the matrix and to improve the corrosion resistance of the casting by heating the casting at a specified temp. for a specified time to the thickness of the casting when the casting is subjected to soln. heat treatment. CONSTITUTION:A casting of a super-austenitic stainless steel consisting of, by weight, <0.03% C, <0.5% Si, <1.0% Mn, 24-27% Ni, 19-23% Cr, 5-7% Mo, 0.5-2.0% Cu and the balance Fe is heated at 1,150-1,250 deg.C for >=2hr per 25mm thickness of the casting to allow sigma phase precipitated at the time of casting to enter into solid soln. in the austenite matrix and then the casting is water- cooled. Soln. heat treatment is perfectly carried out and the corrosion resistance of the casting is improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an improvement in a method for heat treatment of austenitic stainless steel, and more specifically, to solution treatment of so-called super austenitic stainless steel containing a large amount of Mo. Concerning improvements in methods.

[Prior Art and Problems] Since piping for crude oil and natural gas production, seawater pumps, etc. are used in corrosive environments, their materials must have excellent corrosion resistance. Until now, as this kind of material, S
US316L (JIS), duplex stainless steel, etc. have been widely used. However, as the usage environment has become more severe in recent years, materials with even better corrosion resistance are required, and M
Super austenitic stainless steel containing a large amount of O has been developed and is in practical use.

However, since this super austenitic stainless steel contains approximately 6% Mo, conventional heating conditions (105
Even when solution treatment is performed at temperatures between 0 and 1100°C, the precipitates such as σ phase generated during the casting process do not dissolve sufficiently in the austenite phase of the matrix, which is the property of super austenitic stainless steel. It was found that the corrosion resistance was not sufficiently exhibited.

[Problems to be Solved by the Invention] To provide a solution treatment method for super austenitic stainless steel, which can sufficiently dissolve precipitates such as σ phase generated during the casting stage into the quasi-austenite phase (base). The purpose is to exhibit extremely excellent corrosion resistance when used as the aforementioned piping material, etc. in a corrosive environment.

The alloy composition of the super austenitic stainless steel targeted by the present invention is C': 0.03 in weight%.
% or less, Si: 0.5% or less, Mn: 1.0% or less, N
i: 24.0 to 27.0%, Cr: 19. O ~
23.0%, Mo: 5.0 to 7.0%, Cu: 0
．． 5 to 2.0%, the remainder consisting of Fe, PSS and other unavoidable impurities.

[Means for solving the problem] In terms of weight %, C: 0.03% or less, Si: 0.5% or less, Mn: 1. O% or less, Ni: 24.0 to 27.0%,
Cr: 19.0 to 23.0%, Mo:
50-7.0%, Cu: 0.5-2.0%, balance Fe
In a method for solution treatment of super austenitic stainless steel containing unavoidable impurities,
After heating at a temperature of 250°C for 2 hours or more per wall thickness of 25 mm, it is cooled with water.

[Effect of the invention] The corrosion loss after solution treatment of super austenitic stainless steel was measured in accordance with the intergranular corrosion test based on ASTM A262 Practice C (Huey test method, 65% nitric acid boiling test method). 0.2g/
m”·hr or less, and extremely good corrosion resistance can be obtained.

[Example] A test tube (with an outer diameter of 1
70 mm, wall thickness 25 mm, length 6000 mm), and a sample tube 200 mm in length was made from this test tube.

Sakii Itsu C: 0.023%, S i: 0.26%, M n
: 0.71%, P: 0.015%, S: 0.007
%, N i :26.01%, C r :20.95%
, Mo: 5.16%, Cu: 1.56%, Fe: remaining Each of the prepared samples was subjected to solution treatment at various heating temperatures, and the microstructure was examined and intergranular corrosion tests were conducted. . These results are shown in Table 1. Note that the intergranular corrosion test is performed according to ASTM A262 Bractis C (
Corrosion loss was investigated using the Huey test method and 65% nitric acid boiling test method.

Table 1 1050℃ x 3hr (water cooling) Precipitate remaining 0.4
01100°C x 3hr (water cooling) Precipitate remains 0.
351150℃X3hr (water cooling) No precipitate 0,
l81200℃ x 3hr (water cooling) No precipitates 0
．． 151250℃ x 3hr (water cooling) No precipitates
0. l2 As is clear from the above test results, 1150℃
It has been shown that if the solution treatment is carried out at the above temperature, the precipitates formed during the casting stage will completely dissolve into the austenite phase of the matrix. As described above, when the precipitates dissolve into the base phase, the corrosion loss becomes extremely small at 0.18 g/m2·hr or less, and excellent corrosion resistance can be provided.

Claims (1)

[Claims] (1) In weight%, C: 0.03% or less, Si: 0.5
% or less, Mn: 1.0% or less, Ni: 24.0 to 27.
0%, Cr: 19.0-23.0%, Mo: 5.0-7
．． Super austenitic stainless steel consisting of 0% Cu, 0.5 to 2.0%, balance Fe and unavoidable impurities is heated at a temperature of 1150 to 1250°C for 2 hours or more per wall thickness of 25 mm, and then cooled with water. A method for solution treatment of super austenitic stainless steel.