JPS6326336A - Austenite iron, nickel or cobalt base alloy - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention provides 12-50% by weight Cr substantially free of chromium carbide and 0.001-0.
．． Iron, Nickel, and Kobal I containing 2% carbon by weight
・Regarding base alloys.

[Conventional technology and problems]

Particularly in the petroleum and nuclear industries, it is necessary for alloys to withstand sensitization caused by chromium carbide precipitation at high temperatures.

Sensitization is a term commonly used to describe the precipitation of chromium carbide at the grain boundaries of the alloy and the depletion of chromium carbide in the matrix near the grain boundaries. In the prior art, depletion of chromium from the matrix causes inter-granular corrosion to the alloy.
n) and stress corrosion cracking.

These phenomena occur when the alloy is exposed to an aqueous medium following sensitization. In such environments, conventional alloys such as stainless steel often deteriorate due to intergrain corrosion due to precipitation of chromium carbide at grain boundaries, stress corrosion cracking, or both. Precipitation of chromium carbide at grain boundaries is from about 425°C to about 75°C.
This is particularly problematic in the temperature range of 0°C. This temperature is common in many commercial applications. Chromium carbide is also formed during processing of the alloy, especially during welding. These chromium carbide precipitations generally occur at grain boundaries and are always associated with chromium deficiency at and near grain boundaries. Below about 425°C, the kinetic energy of the process is detrimental to the formation of chromium carbide;
On the other hand, at temperatures above about 750° C., chromium diffuses from inside the grains to the grain boundaries, thereby replenishing the grain boundaries with chromium. Chromium deficiency accelerates crack propagation and causes premature deterioration of the alloy.

Considerable effort has been expended in the metallurgical industry in an attempt to develop alloys that are substantially free of chromium carbide in use. One such effort has been directed to methods of producing alloys with concentrations below that at which chromium carbide occurs.

This concentration is generally less than about 0.001% by weight, which is too low to be commercially viable.

Other efforts are directed toward producing alloys containing elements such as niobium and titanium combined with carbon, such as niobium carbide and titanium carbide.

Once formed, such carbides are not found at grain boundaries where chromium carbide is present. Niobium carbide and titanium carbide are more stable than chromium carbide in equilibrium, but at temperatures between about 425°C and 750°C, chromium carbide precipitates at grain boundaries.
- There is still enough carbon left in the lix.

None of these studies have been successful in substantially eliminating chromium carbide precipitation. As a result, there remains a need to develop alloys that are substantially free of chromium carbide and resist intermediate grain corrosion and stress corrosion cracking during service.

[Means for solving problems]

According to the present invention, the above problems can be solved by using 12-50 wt% Cr, 0
．． 0OL - an austenitic iron, nickel or cobalt based metal containing 0.2% by weight C and at least one carbide-forming element more stable than chromium carbide, the alloy forming chromium carbide at temperatures between 425°C and 750°C. The solution is an austenitic iron, nickel, or cobalt-based alloy characterized by having an insufficient solid solution concentration of carbon to form.

Furthermore, according to the present invention, the above-mentioned problem can be solved by using 12-50% by weight of Cr.
, 3% by weight or less Mo, 3% by weight or less Mn, 2% by weight
AI below, Zr below 0.75% by weight, 0.001
- an austenitic iron, nickel, or cobalt base metal containing 0.2% by weight C and at least one carbide-forming element more stable than chromium carbide, wherein the alloy
The solution is an austenitic iron-, nickel-, or cobalt-based alloy characterized by having an insufficient solid solution concentration of carbon to form chromium carbide at a temperature of -750°C.

Margin below Alloy of the invention 12-25 wt% Cr 5-35 weight Ni 0-3% by weight M.

O-0.5wt%Si 0.001-0.2wt%C1 Concentration of carbon (C), oxygen (0) and nitrogen (N) is wt%
From about 10 (C+O+N) to 30 (C+0+
It is preferable to consist of Hf and/or Ta in an amount of N) and the balance Fe.

Another preferred embodiment of the present invention is Mr of 3% by weight or less, 0.
Preferably, up to 2% by weight Al and up to 0.5% by weight Si are present.

According to another preferred embodiment of the invention, the alloy is 15-
25 wt% Cr, 70-80 wt% N + , 0.0
01-0, 2% by weight C1 and C901 and N are expressed in weight%, approximately 10 (C+O+N> to 30 (CfO+N)
contains an amount of Hf.

The precipitation of chromium carbide is detrimental to the intermediate grain corrosion and stress corrosion cracking resistance of austenitic alloys because it eliminates the chromium grain boundaries and their nearby regions. In embodiments of the present invention, the formation of chromium carbide is such that the carbide is more stable than chromium carbide and has insufficient carbon concentration in the matrix to form chromium carbide at temperatures of about 425° C. to 750° C. at equilibrium. This can be avoided by adding one or more carbide-forming elements.

Preferred carbide forming elements suitable for use in the present invention are hafnium for iron, nickel and cobalt based alloys and tantalum for iron based alloys. Such carbide-forming elements are present in sufficient quantities such that the concentration of carbon in the 7 trix is too low to cause chromium carbide precipitation.

This amount, commonly expressed in weight percent, is 1.0 (C+O
+N), but greater than, but less than or equal to, preferably about 1.5 (C+O+N).

For economic reasons, it is preferable to use low cost Hf-Zr as opposed to high elemental Hf. Its I4
f-Zr alloy is a by-product of the manufacturing process for making Zr fuel rods in the nuclear industry and is available for a fraction of the cost of Hf. If Hl-Zr is used in the example of the present invention, it is necessary to use Mf-Zr such that less than 0.75% by weight of Zr is present in the finishing alloy. Use of carbide-forming elements in such amounts will result in a good alloy that substantially prevents precipitation of chromium carbide at grain boundaries and prevents intermediate grain corrosion and stress corrosion cracking. The above criteria for the addition of hafnium and tantalum are met considering the concentration of carbon alone, since the concentrations of oxygen and nitrogen are usually small relative to carbon. As the concentrations of oxygen and nitrogen are usually high, all three elements should be considered equally to meet the above criteria. Hf
When less than about 10 (C + N) and Ta are used, the alloy contains sufficient carbon to form chromium carbide in the matrix at equilibrium. If Mf and Ta exceed 30 (C10+N), metal in phase will be formed and the physical and mechanical properties of the alloy will deteriorate.

The elements comprising the alloys of the present invention and their concentrations are important as the combination results in a type of alloy that has unexpectedly good resistance to intermediate grain corrosion and stress corrosion cracking.

Chromium is important in the alloy of the present invention to increase the corrosion resistance of the alloy. It is noted that increasing amounts of chromium lead to the formation of sigma or other metal homophases.

The amount of chromium required to provide corrosion resistance in the alloys of the present invention is at least about 12% by weight, while less than about 50% by weight chromium is required in more severe corrosive environments and/or higher temperatures.

Other preferred alloys of the invention are about 19 to 23% by weight C.
r, about 30 to 35% by weight Ni, about 1.5% by weight Mn
, about 0.06 to 0.1% by weight C5 about 0.06 to 0.1% by weight Al, about 0.5% by weight S;
It consists of Hf in an amount of C10+N) and the balance Fe.

Even more preferred alloys of the present invention contain about 17 to 19% Cr, about 9 to 12% Ni, about 1.5 to 2% by weight.
．． 5 wt% Mn, about 0.5 wt% or less Si, about 0.0
It consists of Hf in an amount of about C20 (C10+N) less than 8% by weight and the balance Fe.

The use of hafnium and/or tantalum in the iron-based alloys of the present invention and the use of iron, nickel, and cobalt-based alloys allows the formation of stable hafnium and/or tantalum carbides to reduce the carbon concentration of the matrix and for chromium carbide precipitation. to reduce the carbon concentration to very low levels. Other known carbide-forming elements such as niobium and titanium have not been found to have the same beneficial effects as hafnium and tantalum in the alloys of the present invention. That is, niobium and titanium also lower carbon from the strix to form stable niobium carbide and titanium carbide, but still enough carbon remains in equilibrium at temperatures between about 425°C and 750°C to form chromium carbide. invite Hf is a preferred carbide-forming element in the present invention.

The alloys of this invention each contain less than about 0.01% B by weight unless the properties of the alloy are adversely affected.

It may contain inevitable impurities such as Sn, Pb, Zn, Bi, etc.

〔Example〕

The following examples fully describe methods of carrying out the invention and set forth the best mode contemplated for carrying out various aspects of the invention. It is understood that these examples are presented for illustrative purposes and not as limitations on the true scope of the invention.

The experimental alloys used here were prepared from substantially pure elemental raw materials. Each element has approximately 50 bonds (2,2568k
The weight was measured so that the weight was > g. - Melt the alternating alloying elements and make the molten metal 2172 inches (63,5n+m) in diameter.
cast into an iron mold. The solidified casting was removed from the mold, homogenized at 1200'C, and hot rolled at 1000'C to produce a 172 inch (12.7 mm) thick plate.

Table 1 below shows all alloy components used in this example.

Below, Yoji No. 1 - Unalloyed Fe Cr Ni Mn CTi N
b Hf Note A Bat 18.3
9.9 2.2. 0B7. 27 --- ---
Ti/C=48 Bal 18” 10”
2”, 07 --- 0.6 --- .28
7aCBal 18.3 9.9 1.8. 073
．． 4 --- ． 28 Hf/C near, 8D
Bal 17.3 10.22.3 . 07. 4B
---, 54Hf/C=7.7E Bat
18.9 10.2 1.9. 073. 4 ---
- 1.05 If/C = 14.4 *Normal (unanalyzed) components Alloys A and B are commercially available alloy components of Ti-containing alloy and Nb and Ta-containing alloy B, respectively. Alloy B is believed to be the best commercially available conventional alloy to resist polythionic acid stress corrosion cracking. Alloys C, D and E are alloys of the present invention. These alloys have Hf/C ratios ranging from 3.8 for Alloy C to 14.4 for Alloy E.

Alloys C, D, and E contain Ti, but they contain sufficient Mf based on their carbon content to prevent the formation of chromium carbide.

Each of the above alloy test pieces was solution-treated at 1040° C. for 1 hour and cooled in water. One specimen of each alloy was held under slick solution treatment conditions for microscopic observation.

The next specimen of each alloy was placed at 590°C for 100 hours and the third alloy specimen was placed at 590°C for 1000 hours. testing the heat-treated specimens by scanning and transmission electron microscopy;
The type of second phase formed during aging was determined. The results of these studies are shown in Table 2 below.

The following is the rest of the day - λ - Shishi Tsui 琲 4 wealth Φ ■ light r - A TiCTiC, CrzaC6T+C, CrHC
sB (Nb, Ding a) C (Nb, Ta) C
(Nb,Ta)C,Cr23CJe2NbCTiC,1
lfc Tie, t(re, Cr2:+Ca
TiC, tlfc, Crz+CsD HfC
, TiCHfC, TiCIIfc, TiC, Cr2. C
6E HfCHfC, Fe2 (Hf, Ti)
The data in Phase Table 2, in which the amounts of HfC, Fe2 (Hf, Ti) are arranged in the order of cast U, shows that all of the alloys are primary carbides (T) before aging.
i, Nb, Ta, Hf-carbide), and the type and amount thereof vary depending on the alloy. Commercial alloys A and B are each Ti
C and (Nb,Ta)'C, whereas the alloy of the present invention contains T
Contains iC and HfC. The carbide type of the alloy of the present invention is Hf
Depends on ZC ratio. That is, in an alloy with Hf/C<15, the primary carbide is a mixture of TiC and 1(f C, but H
When f/C-=15, the primary carbide is mostly HfC.

After aging for 100 hours, the specimens were investigated by transmission electron spectroscopy and several phases were characterized by X-ray microanalysis and electron diffraction analysis. The results are shown in Table 2. Commercial alloys A and C formed chromium carbides, while alloy B and alloys did not form Cr carbides or other precipitates.

Alloy E formed a small amount of F C2 (Hf, Ti) intermetallic precipitates in the grains and at the grain boundaries. This is usually caused by the undesirable presence of titanium in the alloy.

After 1000 hours of aging, both commercial alloys (A and B) and experimental alloys (C and D) formed carbide precipitates. In the alloys that formed chromium carbide after 100 hours (alloys A and C), the density of chromium carbide increased after 1000 hours. Both Alloy B and Alloy A formed chromium carbide. The only alloy that did not form chromium carbide was Alloy E.

FIG. 1 shows a micrograph of the alloy after aging for 1000 hours. In these photographs the formation of chromium carbide is found as holes in the grain boundaries. These holes are the result of preferential attack of the chromium-depleted areas around the carbides by 10% oxalic acid, which was used to view the alloy structure. Alloys A, B and D were etched for 15 seconds and Alloy E was etched for 45 seconds. Alloy E was etched for 45 seconds as no signs of erosion at the grain boundaries were observed for 15 seconds. The absence of chromium carbide in Alloy E is a result of the addition of Hf in an amount sufficient to combine with all or most of the carbon so that insufficient carbon escapes and forms harmful chromium carbide during aging. At temperatures between 425°C and 750°C, the equilibrium reduced carbon levels are below the level at which chromium carbide can form. Traditionally used carbide-forming elements such as titanium and niobium are unable to achieve this, and it is unexpected that strong carbide-forming elements such as hafnium and tantalum can bring molten carbon to the required low levels. discovered. The data also show that TiC or other primary carbides such as NbC are unable to bind all the carbon in the alloy.

[Brief explanation of drawings]

The figure shows heating at 1040℃ for 1 hourttt, 590℃toooo
1 is a micrograph of the example alloys A, B, D, and E after time exposure. O Figure Procedural amendment (method) % formula % 1. Indication of the case Patent Application No. 117190 of 1988 2. Name of the invention Austenitic iron, nickel or cobalt base 3. Person making the amendment Relationship to the case Patent application Person name: Exxon Research and Engineering Company 4, Agent address: 8-1-5 Toranomon-chome, Minato-ku, Tokyo 105
Date of amendment order: July 28, 1985 (shipment date) 6. Column 7, "Brief explanation of drawings" of the specification subject to amendment, Contents of amendment: Brief explanation of drawings, page 17, line 19 of the specification. From the “Microscope Group” of the eye, page 2
In the 0th line, "This is a photograph of a textile." is corrected to "This is a photograph that shows the metallographic structure and replaces a drawing."

Claims (1)

[Claims] 1. About 12-50% by weight Cr, about 3% by weight or less Mo,
Mn up to about 3% by weight, Al up to about 2% by weight, about 0.
Up to 75 wt% Zr, about 0.001-0.2 wt% C
and an austenitic iron, nickel, or cobalt-based alloy containing at least one carbide-forming element more stable than chromium carbide, the alloy containing insufficient carbon to form chromium carbide at temperatures between 425°C and 750°C. Austenitic iron, nickel, characterized by having a solid solution concentration
or cobalt-based alloys. 2. The alloy of claim 1, wherein the Cr concentration is about 15-25% by weight. 3. The alloy of claim 1, wherein said carbide-forming element selected from Hf and Ta is in a concentration of about 10 (C+O+N) to about 30 (C+O+N). 4. The alloy according to claim 3, wherein the Hf or Ta is at a concentration of about 15 (C+O+N). 5, about 17-19 wt% Cr, about 9-12 wt% Ni,
About 1.5-2.5 wt% Mn, about 0.08 wt% C
3. An alloy according to claim 2, characterized in that the alloy consists of Hf such that , O, and N are expressed in weight percent as Hf=20 (C+D+N), with the balance being iron. 6, about 19-23 wt% Cr, about 30-35 wt% Ni
, about 1.5-2.0 wt% Mn, about 0.05-0.1 wt% Al, about 0.5 wt% Si, about 0.05-0.1 wt% C, C, O, and N is shown in weight% and Hf=20 (
The alloy according to claim 2, characterized in that the alloy comprises Hf in an amount such that C+O+N) and the balance is iron. 7, 15-25 wt% Cr, 70-80 wt% Ni, 0
．． 001 - 0.2% by weight C, and C, O, and N expressed in weight% from about 10 (C+O+N) to 30 (C+O+
3. An alloy according to claim 2, characterized in that it contains Hf in an amount of N). 8, 12-50 wt% Cr, 0.001-0.2 wt%
an austenitic iron, nickel, or cobalt-based alloy containing C and at least one carbide-forming element more stable than chromium carbide, the alloy having insufficient carbon to form chromium carbide at temperatures between 425°C and 750°C; an austenitic iron, nickel, or cobalt-based alloy characterized by having a solid solution concentration of