US3925109A

US3925109A - Precise carbon control of fabricated stainless steel

Info

Publication number: US3925109A
Application number: US438148*A
Authority: US
Inventors: Roy J Nilsen
Original assignee: US Department of Energy
Current assignee: US Department of Energy; Energy Research and Development Administration ERDA
Priority date: 1974-01-29
Filing date: 1974-01-29
Publication date: 1975-12-09
Anticipated expiration: 1992-12-09
Also published as: SE7500183L; JPS50131812A; DE2503634A1; FR2259154A1

Abstract

A process for controlling the carbon content of fabricated stainless steel components including the steps of heat treating the component in hydrogen atmospheres of varying dewpoints and carbon potentials.

Description

States Patent [191 Nilsen Dec. 9, 1975 PRECISE CARBON CONTROL OF FABRICATED STAINLESS STEEL Roy J. Nilsen, Pittsburgh, Pa.

Assignee: The United States of America as represented by the United States Energy Research and Development Administration, Washington, DC.

Filed: Jan. 29, 1974 Appl. No.: 438,148

Inventor:

US. Cl. 148/6.35; 148/16; 148/165;

176/78; 176/88 Int. Cl. C23C 11/12; G21C 3/34 Field of Search 148/6.35, 12.1, 16, 16.7

References Cited UNITED STATES PATENTS 5/1948 Uhlig 148/16 X 3,277,149 10/1966 Brickner et a1. [48/16 X FOREIGN PATENTS OR APPLICATIONS 1,186,485 2/1965 Germany 148/16 118,285 2/1945 Australia 148/16 Primary ExaminerC. Lovell Attorney, Agent, or Firm.lohn A. Horan; Richard A. Lambert 1 Claim, N0 Drawings PRECISE CARBON CONTROL OF FABRICATED STAINLESS STEEL BACKGROUND OF THE INVENTION This invention was made in the course of, or under, a

contract with the United States Atomic Energy Commission.

My invention relates to a process whereby the carbon content of fabricated components, particularly nuclear reactor fuel rod grids, can be precisely and uniformly controlled to any desired level. The process was conceived and experimentally proven feasible during development of techniques for reducing the carbon content of AM-350 stainless steel. AM-350 is a semi-auste nitic iron-base alloy nominally containing 17 percent chronium, 4 percent nickel, 3 percent molybdenum, and 0.10 percent carbon. AM-350 sheet, annealed at 1900F. (Condition H) is austenitic with small amounts of delta ferrite and is ductile and can readily be formed into intricate components. The AM-350 composition is carefully balanced between austenite and ferrite formers in such a fashion that the solution of carbon in the austenite-ferrite matrix during the H anneal yields a stable austenite-ferrite structure at room temperature. If the material is given a lower temperature L anneal (1,710F.), part of the carbon in solution precipitates. The decrease of solid solution carbon content decreases austenite stability and results in a martensitic transformation on cooling to room temperature. AM- 350 processed in this fashion possesses all the desired nuclear and mechanical properties for LWBR fuel rod support systems with the exception of the degree of corrosion resistance required for core materials. This lack of corrosion resistance is caused by grain boundary carbide precipitates formed during the L anneal and during cooling through critical carbide precipitation ranges. The solution to this problem is to simply remove carbon in excess of that which produces a deleterious precipitate. This is most easily done during melting. However, reduced carbon AM-350 is unstable, and cannot be cold rolled or formed. Essentially any amount of deformation will cause the ductile austenitic structure to transform a brittle martensite.

For this reason, the alloy must have a high enough carbon content for formability and after fabrication the carbon content must be reduced to impart corrosion resistance to heat treated components. Additionally, the carbon content must not be reduced excessively or required strength levels will not be attainable, and the carbon content must be maintained at an extremely uniform level to reduce nonuniform transformation induced distortion. Carbon gradients will result in nonuniform Ms temperatures causing warpage as local areas transform before others.

Decarburization of ferrous sheet is an extremely common commercial process used, e.g., in the preparation of iron plus silicon alloys for transformer cores. However, all commercial decarburization processes are designed to reduce the carbon content of low alloy transformer steels to the lowest possible level to opti- C+2H2 CH,

2 The second process requires exposing components to extremely wet hydrogen gas (5,000 to 10,000 ppmv H O) at temperatures in the range of 1,500F. to 1,800 F. Carbon is removed by the water gas reaction Neither of these processes is strictly applicable to the decarburization of intricate precision components formed from chromium bearing stainless steel. The first process for carbon removal by the methane reaction requires temperature far in excess of what can be tolerated from a distortion standpoint. The second process can be used but with significantly reduced water contents (approximately 200 ppmv H O at 1,900F.), to prevent chromium oxidization With reduced water contents, the reaction is sluggish and tends to be nonuniform due to water depletion at localized areas. Precision carbon control is virtually impossible. For these reasons, a new process for controlled carbon reduction was developed.

It was desired to reduce the as-received carbon content of an AM-350 grid to precisely 0.05 weight percent (w/o). The grid is an open cellular array assembled from 0.015 inch thick components. The assembly is approximately 20 inches in diameter, 2 inches high, and weighs 7 pounds.

SUMMARY OF THE INVENTION It is an object of my invention to provide a method of controlling the carbon content of fabricated stainless steel components.

It is another object of my invention to provide a method of reducing the carbon content of fabricated stainless steel components.

It is another object to provide a method of control-' ling the carbon content of nuclear reactor fuel rod grids to a precise, preselected value.

Other objects of my invention will become apparent from the following descriptions and the attached claims.

In accordance with my invention I have provided a method of controlling the carbon content of fabricated stainless steel components comprising: (.a) decarburizing the component in two steps;

1. heating the component to an oxidizing wet hydrogen atmosphere to form chromium oxide on the surface of the component,

2. replacing the wet hydrogen with dry hydrogen to prevent further formation of chromium oxide and to permit the reduction of chromium oxide by carbon (autodecarburization); and optionally b. carburizing the decarburized component to the desired level by equilibrating in reducing C-O-H mixtures.

These steps are explained in detail below, with particular reference to AM-350 stainless steel fuel rod grids.

DETAILED DESCRIPTION AND EXAMPLE A. Decarburization is achieved by exposing the fabricated AM-350 stainless steel component to an oxidizing atmosphere of wet hydrogen having a dew point of from 23F. to -7F. at a temperature of from l,880F. to l920F. for about 1 hour. Preferably the component is exposed to a hydrogen atmosphere containing approximately 2,000 ppmv water for 28 to 40 minutes at a temperature of about 1,900F. (The actual water concentration is not critical, provided it is in excess of 200 ppmv at 1,900F.) Three reactions occur and are listed As indicated, some decarburization occurs during this step as a result of the water gas reaction (2) and the formation of hydrocarbons (3). However, the most significant reaction is the formation of Cr O on the AM- 350 surface.

It was mentioned earlier that a similar process is used to decarburize low alloy steel, but that it was not suitable for stainless steel because chromium was oxidized. My invention, however, actually takes advantage of chromium oxide formation in the following autodecarburization step.

B. Purge the oxidizing H O H mixture with dry hydrogen (dew point 80F. or less) to prevent further formation of Cr O and maintain the component at a temperature of 1880F. to 1920F. Preferably the component is maintained at a temperature of 1900F. for to minutes. Two reactions occur and are listed below in order of significance:

2 3 surface 3 m: mhnlnn 2 Cr+ 3 CO mllll xulullun 2 4 This step of the process results in a thorough decarburization of the charge. Decarburization (autodecarburization) is effected by the reduction of Cr O by carbon in solid solution in the AM-35O base metal. Some Cr O will be reduced by hydrogen; however, the kinetics of this reaction are slow compared to Cr O reduction by carbon. Reduction of Cr O by hydrogen produces water which may contribute to some further decarburization by the water-gas reaction (2). Additionally, some carbon is lost by the formation of hydrocarbons (5). Carbon levels as low as 0.05 w/o have been obtained with my method.

The carbon content of the component has now been reduced to a very low level and can be used in applications where corrosion resistance is paramount and strength is of little importance. However, generally it is desirable to raise the carbon content to a level great enough to impart adequate strength to the component. The following carburizing step will also be explained with particular reference to AM-35O stainless steel.

The final step of the process requires equilibrating the charge in reducing C O H mixtures exhibiting a carbon activity equal to an equilibrium AM-350 carbon concentration of 0.05 w/o at 1,900F.

Virtually any gas mixture of the desired carbon potential can be used for equilibration; however, CH and H and CO, H 0, and H mixtures have been used experimentally with good success. Atmospheres containing nitrogen should be avoided since nitrogen pickup can have deleterious effects on stainless steel mechanical properties. At equilibrium the following reversible reactions occur for CH and H and CO, H 0, and H mixtures, respectively.

Rewriting these equations in terms of equilibrium constants yields:

4 Inserting carbon activities for carbon steel, and the equilibrium constants for equations 6.A and 7.A from Composition of Atmospheres Inert to Heated Carbon Steel R. W. Gurry, Trans Aime Vol. 188, 4-1950, p. 671, yields:

.018 CO 7.C .126

Equation 6.C when solved indicates that a hydrogen atmosphere containing 31 ppmv CH has a carburizing potential of 0.05 w/o for the AM-350 composition at l,900F. Equation 7.C cannot be solved directly without first selecting a moisture content. At 1900F. the equilibrium partial pressure of water for the oxidization of chromium, Equation 1, is approximately 200 ppmv. It was found experimentally, that the carburization reaction occurs most rapidly at this moisture concentration, and is the basis for selecting this concentration. Equation 7.C indicates that a hydrogen atmosphere containing 105 ppmv CO and 200 ppmv H O also has a carburizin g potential of 0.05 w/o for the AM-350 composition at 1,900F.

One variation of the process which may substantially reduce the length of the cycle would be to combine the purging and equilibration steps. Rather than first reducing the overall carbon level to less than 0.05 w/o and then increasing the level to 0.05 w/o one could establish a floor, e.g., 0.05 w/o below which the carbon content could not be reduced. This can be accomplished by exposing the charge after oxidation to a purging and equilibrating hydrogen mixture of the same CO/I-I O ratio (105/200 but containing less than 200 ppmv H O. This gas mixture could be, e.g., 52.5 ppmv CO, ppmv H 0, and hydrogen.

Theoretically, if an AM-350 structure was exposed to either a hydrogen atmosphere containing 31 ppmv CH or ppmv CO and 200 ppmv H O, for long times, the resulting carbon content would be precisely 0.05 w/o. In practice, however, this was found not to be the case. In order to obtain an equilibrium carbon concentration of 0.05 w/o using H CH it was necessary to use approximately 300 ppmv CH This discrepancy can be attributed to flow meter errors, and to impurities in the hydrogen gas which could oxidize the methane prior to exposure to the AM-350 as follows:

8.2CH4+O22CO+4H2 9. CH H2O C0 H For this reason the H CH gas mixture while it can be used, is not recommended.

However, much better experimental agreement with theoretical calculations was found when using H CO H O mixtures. For example, hydrogen atmospheres containing 350 ppmv CO and 200 ppmv H O will consistently yield an average carbon content of 0.054 w/o, after equilibration at 1900F. for 1 hour. Errors causing this discrepancy must include uncertainties in water content determination, and flow meter calibrations. and uncertainties in factors used to compute carbon activity in AM-350.

As mentioned, the initial oxidizing treatment, decarburization deoxidizing treatment (autodecarburization) and equilibration treatments were carried out each for 1 hour at 1,900F. These times and temperatures are conservative and were chosen based on calculations that the diffusion time for a carbon atom to traverse one-half the 0.015 inch AM-3 50 thickness at 1900F. is approximately 1 hour. Times and temperatures could be adjusted for different materials and thicknesses, and to increase or decrease the reaction rates. The carbon content of the entire cross section of every component treated in this fashion can be con trolled to 0.05 w/o or virtually any other carbon level desired. Increasing the desired carbon level would simply require increasing the CO content of the hydrogen gas as necessary; the opposite would be true for lower carbon contents. Thicker gauge components would require longer times at temperature or increased temperatures. Additionally, Steps A and B alone can be used to decarburize chromium bearing stainless steels.

EXAMPLE It was desired to reduce the as-received carbon content (about 0.10 w/o) of an AM-350 grid to about 0.05 w/o. The grid is an open cellular array assembled from 0.015 inch thick components. The assembly is approximately 20 inches in diameter, 2 inches high, and weighs 7 pounds.

The grid was exposed to an oxidizing H O H mixture containing approximately 2,000 parts per million 6 by volume (ppmv) for 1 hour at 1,900F. Some decarburization occurred during this step as a result of the water gas reaction and the formation of hydrocarbons, however, the most significant reaction was the formula tion of Cr O on the AM-35O surface.

The oxidizing H O H mixture was purged with dry hydrogen (less than 200 ppmv H 0) and the grid was maintained in this atmosphere for about 10 minutes at a temperature of l,900F. The grid was found to have a carbon content of about 0.005 w/o.

The dry hydrogen was then replaced with a H CO H O mixture containing 350 ppmv CO and 200 ppmv H O. The grid was maintained in this atmosphere for 1 hour at l,900F. After this treatment the grid was found to have a carbon content of 0.054 w/o.

I claim:

1. A method for controlling the carbon content to a level of about 0.05 weight percent in a fabricated AM- 350 stainless steel nuclear reactor fuel rod grid-com prising:

a. exposing the said nuclear reactor fuel rod grid to an oxidizing atmosphere of wet hydrogen having a dew point of from 23F. to 7F. at a temperature of from 1,880F. to 1,920F. for a period of 28 to 40 minutes;

b. replacing said oxidizing atmosphere with dry hydrogen having a dew point of F. or less and maintaining said nuclear reactor fuel rod grid in said dry hydrogen for a period of 10 to 15 minutes at a temperature of 1,880F. to 1,920F.; and

. c. replacing said dry hydrogen atmosphere with a reducing carbon/oxygen/hydrogen atmosphere containing about 350 ppmv CO and about 200 ppmv H 0 and exhibiting a carbon activity equal to an equilibrium AM-3 50 carbon concentration of about 0.05 weight percent at about 1,900F and maintaining said grid at said temperature for about 1 hour.

Claims

1. A METHOD FOR CONTROLLING THE CARBON CONTENT TO A LEVEL OF ABOUT 0.05 WEIGHT PERCENT IN A FABRICATED AM-350 STAINLESS STEEL NUCLEAR REACTOR FUEL ROD GRID COMPRISING: A. EXPOSING THE SAID NUCLEAR REACTOR FUEL ROD GRID TO AN OXIZING ATMOSPHERE OF WET HYDROGEN HAVING A DEW POINT OF FROM :23*F TO -7*F. AT A TEMPERATURE OF FROM 1,880*F. TO 1,920*F. FOR A PERIOD OF 28 TO 40 MINUTES, B. REPLACING SAID OXIDIZING ATMOSPHERE WITH DRY HYDROGEN HAVING A DEW POINT OF -80*F. OR LESS AND MAINTAINING SAID NUCLEAR RACTOR FUEL ROD GRID IN SAID DRY HYDROGEN FOR A PERIOD OF 10 TO 15 MINUTES AT A TEMPERATURE OF 1,880*F. TO 2,920*F, AND C. REPLACING SAID DRY HYDROGEN ATOMPHERE WITH A REDUCING CARBON/OXYGEN/HYDROGEN ATMOSPHRE CONTAINING ABOUT 350 PPMV CO AND ABOUT 200 PPMV H2O AND EXHIBITING A CARBON ACTIVITY EQUAL TO AN EQUILIBRIUM AM-350 CARBON CONENTRATION OF ABOUT 0.05 WEIGHT PERCENT AT ABOUT 1,900*F AND MAINTAINING SAID GRID AT SAID TEMPERATURE FOR ABOUT 1 HOUR.