LOW NI-CONTAINING STEEL ALLOYS WITH HYDROGEN DEGRADATION RESISTANCE

The present invention provides steel alloys with hydrogen degradation resistance comprising controlled amounts of Mn and C, as well as Al, Cr, Cu, Ni and Si. The steel alloys have an austenite microstructure and relatively high stacking fault energies, which avoid the formation of martensitic phases that reduce hydrogen resistance.

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Description
CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application No. 63/336,431 filed Apr. 29, 2022, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to low-Ni steel alloys with favorable resistance to hydrogen degradation during service.

BACKGROUND INFORMATION

Alloys currently used for high pressure hydrogen storage applications include grade 316L austenitic stainless steel that contains nominally 18 weight percent Cr and 13 weight percent Ni in addition to iron and several other elements. However, Cr and Ni additions are relatively expensive, and a lower cost alternative would benefit hydrogen applications such as the use of hydrogen as a fuel for automobiles, trucks and the like.

SUMMARY OF THE INVENTION

The present invention provides steel alloys with hydrogen degradation resistance comprising controlled amounts of Mn and C, as well as Al, Cr, Cu, Ni and Si. The steel alloys have an austenite microstructure and relatively high stacking fault energies, which avoid the formation of martensitic phases that reduce hydrogen resistance.

An aspect of the present invention is to provide a hydrogen degradation resistant steel alloy comprising from 15 to 30 weight percent Mn, from 0.15 to 1 weight percent C, and from 0.05 to 3 weight percent Al. The steel alloy has a microstructure comprising at least 99 percent volume austenite, and possesses a relative reduction in area of no more than 20 percent.

This and other aspects of the present invention will be more apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating that fully austenitic steels with desirable stacking fault energies may be achieved using relatively large amounts of Mn with additions of controlled amounts of C, Al and Ni in accordance with the present invention.

FIG. 2 is a graph illustrating the achievement of suitable stacking fault energies based upon Mn content with further additions of Al, Cr, Cu and Ni in accordance with embodiments of the present invention.

FIGS. 3-6 are graphs of mechanical properties and hydrogen content for hydrogen degradation resistant steel alloys of the present invention in comparison with a standard stainless steel alloy. FIG. 3 illustrates ultimate tensile strengths, FIG. 4 illustrates yield strengths, FIG. 5 illustrates total elongations, and FIG. 6 illustrates relative reductions of area.

FIGS. 7 and 8 are three-dimensional plots for steel alloys of different compositions having varying amounts of Mn and C, and the resultant effect on relative reduction of area for the steel alloys.

FIGS. 9-14 are photomicrographs of fracture surfaces of a hydrogen degradation resistant steel alloy of the present invention before and after electrochemically hydrogen charging taken at different locations of the sample, illustrating ductile fracture features.

 DETAILED DESCRIPTION

The present steel alloys may be used for hydrogen service due to their relatively high stacking fault energies, e.g., greater than 20 mJ/m2, to avoid formation of martensitic phases which greatly reduce hydrogen resistance. Alloy compositions that stabilize austentite and avoid martensite formation may be selected in accordance with the present invention.

The hydrogen-resistant steel alloys of the present invention may typically comprise at least 15 weight percent Mn, for example, at least 18 weight percent, or at least 20 weight percent, or at least 20.5 weight percent, or at least 21 weight percent, or at least 22 weight percent. The Mn may comprise up to 30 weight percent, or up to 25 weight percent, or up to 24 weight percent. In certain embodiments, the Mn may comprise from 15 to 30 weight percent, or from 18 to 25 weight percent, or from 20 to 24 weight percent. The relatively large amount of Mn may provide similar qualities as stainless steel, including resistance to hydrogen degradation during service. The Mn content leads to fully austenitic steels that resist degradation effects of hydrogen such as ductility loss or embrittlement.

The hydrogen-resistant steel alloys may typically comprise at least 0.18 weight percent C, for example, at least 0.25 weight percent, at least 0.3 weight percent, or at least 0.4 weight percent. The C may comprise up to 1 weight percent, or up to 0.9 weight percent, or up to 0.8 weight percent, or up to 0.6 weight percent. In certain embodiments, the C may comprise from 0.18 to 1 weight percent, or from 0.25 to 0.9 weight percent, or from 0.3 to 0.8 weight percent, or from 0.4 to 0.6 weight percent.

The hydrogen-resistant steel alloys may typically comprise at least 0.05 weight percent Al, for example, at least 0.1 weight percent, or at least 0.5 weight percent, or at least 0.8 weight percent, or at least 1.0 weight percent, or at least 1.2 weight percent. The Al may comprise up to 2.5 weight percent, or up to 2.2 weight percent, or up to 2 weight percent, or up to 1.8 weight percent. In certain embodiments, the Al may comprise from 0.05 to 2.5 weight percent, or from 0.8 to 2.2 weight percent, or from 1 to 2 weight percent, or from 1.4 to 1.8 weight percent.

The hydrogen-resistant steel alloys may typically comprise at least 0.5 weight percent Si, for example, at least 1 weight percent, or at least 2 weight percent, or at least 2.5 weight percent. The Si may comprise up to 4 weight percent, or up to 3.5 weight percent, or up to 3.2 weight percent, or up to 3 weight percent. In certain embodiments, the Si may comprise from 1 to 4 weight percent, or from 1.5 to 3.5 weight percent, or from 2 to 3.2 weight percent, or from 2.5 to 3 weight percent. In certain embodiments, the steel alloys may be substantially free of Si.

The hydrogen-resistant steel alloys may typically comprise at least 0.8 weight percent Ni, for example, at least 1 weight percent, or at least 1.2 weight percent. The Ni may comprise up to 2.5 weight percent, or up to 2 weight percent, or up to 1.5 weight percent. In certain embodiments, the Ni may comprise from 0.8 to 2.5 weight percent, or from 1 to 2 weight percent, or from 1.2 to 1.5 weight percent. In certain embodiments, the steel alloys may be substantially free of Ni.

The hydrogen-resistant steel alloys may typically comprise at least 0.2 weight percent Cu, for example, at least 0.4 weight percent Cu, or at least 0.6 weight percent Cu. The Cu may comprise up to 2 weight percent, or up to 1.5 weight percent, or up to 1.2 weight percent. In certain embodiments, the Cu may comprise from 0.2 to 2 weight percent, or from 0.4 to 1.5 weight percent, or from 0.6 to 1.2 weight percent. In certain embodiments, the steel alloys may be substantially free of Cu.

The hydrogen-resistant steel alloys may typically comprise at least 1 weight percent Cr, for example, at least 1.5 weight percent Cr, at least 2 weight percent Cr, or at least 2.2 weight percent Cr. The Cr may comprise up to 3.5 weight percent, or up to 3.2 weight percent, or up to 3 weight percent, or up to 2.8 weight percent. In certain embodiments, the Cr may comprise from 1.5 to 3.5 weight percent, or from 2 to 3.2 weight percent, or from 2 to 3 weight percent, or from 2.2 to 2.8 weight percent. Alternatively, the Cr may be less than 1.5 weight percent, or less than 1 weight percent, or less than 0.5 weight percent, or less than 0.2 weight percent. In certain embodiments, the steel alloys may be substantially free of Cr.

The hydrogen-resistant steel alloys may typically comprise at least 0.01 weight percent Ti, for example, at least 0.05 weight percent, or at least 0.08 weight percent. The Ti may comprise up to 0.5 weight percent, or up to 0.3 weight percent, or up to 0.2 weight percent. In certain embodiments, the Ti may comprise from 0.01 to 0.5 weight percent, or from 0.02 to 0.3 weight percent, or from 0.08 to 0.2 weight percent. In certain embodiments, the steel alloys may be substantially free of Ti.

As used herein, the term “substantially free” when referring to alloying additions, means that a particular element or material is not purposefully added to the alloy, and is only present, if at all, in minor amounts as an impurity. For example, in amounts of less than 0.05 weight percent, or less than 0.01 weight percent.

The hydrogen degradation resistant steel alloys have an austenitic microstructure in which austenite comprises at least 95 volume percent, or at least 98 volume percent, or at least 99 volume percent, or at least 99.5 volume percent. Other than austenite, the hydrogen degradation resistant steel alloys may be substantially free of other phases such as ferrite and martensite. For example, such phases, if present, are less than 1 volume percent, or less than 0.5 volume percent, or less than 0.1 volume percent, or zero volume percent.

FIGS. 1 and 2 illustrate the design concept for the present invention. FIG. 1 indicates that fully austenitic steels with the target SFE range may be achieved using relatively high amounts of Mn, e.g., 22 weight percent and 15 weight percent, with additions of suitable amounts of C, e.g., 0.45 weight percent C plus Al, Cu and Ni. An alloy with only 0.18 weight percent C and 15 weight percent Mn may not meet the design goal for SFE. FIG. 2 further shows that for the carbon and manganese contents studied, SFE falls into the desired range with further additions of Al, Cr, Cu and Ni.

Laboratory scale heats of each composition listed in Table 1 are melted, hot rolled and prepared for electrochemical charging to form nascent hydrogen at the sample surface. The electrochemical charging technique was performed by electrochemically charging the test samples in a solution of 20 g/L Na2SO4 for 48 hours at 70 C with additions of 2 g/L NH4SCN to prevent recombination of the nascent hydrogen. A current density of 70 A/m2 was used for the test. During the electrochemical charging, the nascent atomic hydrogen diffuses into the test samples.

Hydrogen charged samples may be tested for hydrogen resistance by performing standard tensile tests and comparing ductility with samples that are not charged. Reduction in Area (RA) may be used to measure ductility. A target for Relative Reduction in Area (RRA) of 20% is considered to be competitive with 316L stainless steel. Thus, if RA degrades no more than 20% then alloys of the present invention are considered to be competitive with 316L stainless from a hydrogen resistance standpoint.

The following examples are intended to illustrate various aspects of the present invention, and are not intended to limit the scope of the invention.

Examples

Laboratory melts were made in a vacuum induction furnace with the actual chemistries shown in Table 1.

TABLE 1 Melt Chemistries Alloy Compositions (values in weight percent) Al loy C Mn Al Cr Cu Ni Si Ti 2 Aim 0.18 22 1.5 2.5 0.8 1.3 2.7 — 2 Actual 0.18 20.5 1.43 3.2 0.645 0.994 3.20 3 Aim 0.45 22 1.5 2.5 0.8 1.3 2.7 — 3 Actual 0.478 24.0 1.36 2.44 0.811 1.33 2.60 4 Aim 0.45 22 1.5 2.5 0.8 1.3 2.7 0.1 4 Actual 0.48 24.0 1.38 2.44 0.834 1.35 2.61 0.0988 5 Aim 0.18 15 1.5 2.5 0.8 1.3 2.7 — 5 Actual 0.198 16.5 1.44 2.49 0.828 1.37 2.70 6 Aim 0.45 15 1.5 2.5 0.8 1.3 2.7 0.1 6 Actual 0.478 16.7 1.45 2.51 0.859 1.39 2.65 0.122

The chemistries were measured either by a LECO C/N/O/S Analyzer or by Inductively Couple Plasma Optical Emission Spectroscopy (ICP-OES). Titanium was added to some of the melts for microalloying to improve yield strength, in addition to possibly reducing the kinetics of twin formation. Low levels of phosphorus, 0.015 weight percent, and sulfur, 0.005 weight percent, were added to each alloy to simulate residual phosphorus and sulfur in a steel melt. The material was hot rolled from a 7-inch-thick ingot to a 1.25-inch-thick slab in the laboratory and air cooled. All testing was completed on the hot rolled slabs.

The samples were measured with a Metis MSAT 30 instrument to determine the percentage of austenite in the material. The results are listed in Table 2 and compared against the 316L stainless steel material used in this study. A fully austenitic structure is desirable to prevent hydrogen embrittlement. Since the samples will be stored in liquid nitrogen to prevent de-absorption of the hydrogen, the samples were also tested after a 24-hour storage in liquid nitrogen. There were no indications of microstructural changes after storage in liquid nitrogen, except the percent of austenite in Alloys 4 and 6 reduced by 0.1 volume percent. The alloys were almost fully austenitic microstructures.

TABLE 2 Percent of Austenite in the Microstructure Alloy % Austenite % Austenite After Liquid Nitrogen 316L SS 99.6 99.6 2 99.7 99.7 3 99.9 99.9 4 99.9 99.8 5 99.8 99.8 6 99.9 99.8

ASTM E8-22 Specimen 2 round tensile samples in the longitudinal direction, parallel to the rolling direction, were tested according to the ASTM E8-22 standard before and after electrochemically charging for hydrogen. The extensometer range was exceeded during testing, so the total elongation was manually measured for all the samples. The tensile samples along with a hydrogen analysis test sample were electrochemically charged in a solution of 20 g/L Na2SO4 for 48 hours at 70 C with a current density of 70 A/m2. Additions of 2 g/L NH4SCN was added to prevent recombination of the nascent hydrogen. The hydrogen test samples were selected from the same melt and near the same location as the tensile sample to minimize parameters that could affect quantities of hydrogen adsorption, such as grain size. Immediately after electrochemically charging for hydrogen, the tensile samples were stored in liquid nitrogen to await tensile testing. There was an 8 to 12 minute delay for the tensile sample temperature to stabilize to room temperature prior to testing. The mechanical properties before and after electrochemically charging were compared, along with the concentration of diffusible hydrogen from the hydrogen test sample that was measured with a Bruker hydrogen analyzer mass spectrometer at 300 C.

The mechanical property results before and after electrochemically charging, along with the hydrogen concentrations are shown in Table 4 and FIGS. 3-6, compared against 316L stainless steel material. It is suspected that the higher hydrogen concentration in 316L stainless steel may be due to microstructural differences, such as grain size and the percentage of ferrite, compared to the hot rolled alloys in this study. The mechanical properties were all comparable or higher in strength and total elongation than the 316L stainless steel material. Alloys 2, 3, 4 and 6 have relative reduction of areas (RRA) less than 20% with Alloy 3 performing the best. The RRA of Alloy 5, which had both lower carbon and manganese levels of 0.2 weight percent and 16.5 weight percent, respectively, was 23.2%. Increasing the manganese levels to 20.5 weight percent as in Alloy 2 improves the average RRA to a value of 11.8% and increasing the carbon levels to 0.48 weight percent as in Alloy 6 improves the RRA to an average value of 14.1%. Increasing both the carbon and the manganese levels to 0.48 weight percent carbon and 24.0 weight percent manganese further improves the RRA to an average value of 2.6% for Alloy 3 and 11.3% for Alloy 4.

FIGS. 7 and 8 visually show the effects on Mn and C contributions on the RRA. The data in FIGS. 7 and 8 show a decrease in RRA as the C and Mn concentrations increase, and that both Mn and C contribute to the RRA individually. The addition of titanium in Alloy 4, compared against Alloy 3, increased the tensile and yield strength but also increased the RRA value.

TABLE 3 Mechanical properties before and after hydrogen charging and measured hydrogen concentrations of test samples Before After Before Before Charging After After Charging Charging Charging Avg TE % Before Charging Charging TE % After Hydrogen Avg 0.2% Avg UTS (manually Charging 0.2% OYS UTS (manually Charging RRA 300 C Melt OYS (MPa) (MPa) measured) Avg RA % (MPa) (MPa) measured) RA % (%) (ppm) 316L SS 386 648 48.8 74.4 322.0 623.0 54.0 70.0 5.9 5.04 Alloy 2 370 765.5 59.4 73.6 352.0 745.0 59.0 64.4 12.5 1.08 Alloy 2 370 765.5 59.4 73.6 345.0 724.0 59.0 65.4 11.2 0.79 Alloy 3 400 848 67.85 63.16 386.0 807.0 65.9 61.2 3.1 1.82 Alloy 3 400 848 67.85 63.16 379.2 806.7 63.4 62.2 1.5 1.38 Alloy 3 400 848 67.85 63.16 406.8 820.5 64.0 60.7 3.9 1.51 Alloy 3 400 848 67.85 63.16 393.0 813.6 66.1 62.1 1.7 1.48 Alloy 4 437.5 886 61.45 59.79 427.5 861.8 59.9 53.7 10.2 1.05 Alloy 4 437.5 886 61.45 59.79 448.2 882.5 58.5 53.6 10.4 1.91 Alloy 4 437.5 886 61.45 59.79 420.6 848.1 59.0 51.8 13.4 1.43 Alloy 5 358.5 823.5 57.3 61.18 344.7 779.1 53.9 47.0 23.2 0.93 Alloy 6 444.5 899.5 61.45 62.74 441.3 882.5 59.8 52.6 16.2 0.98 Alloy 6 444.5 899.5 61.45 62.74 434.4 875.6 55.5 55.2 12.0 0.89

Hydrogen induced cracking (HIC) and Sulfide Stress Cracking (SSC) tests were completed on Alloy 4 according to NACE TM0284-2016, and NACE TM0177-2016—Method A, respectively. The applied stress during the SSC test was 85% of the actual yield stress to simulate a higher hydrogen pressure environment. No cracks were present after each test. Some pitting corrosion was observed in the SSC test.

TABLE 4 Sulfide Stress Cracking (SSC) Testing Applied Temper- Test Specimen % Stress ature Period Initial Final ID AYS (psi) (° C.) (hours) pH pH Result 865 85 54,400 24 720 2.63 3.10 Pass* Specification: NACE TM0177-2016 - Method A Test Environment: NACE TM0177 Solution A, pH = 2.7, 100 mol. % H = S Test Specimen: Type: Standard Orientation Longitudinal Finishing Process: Ground Loading Method: Proof Ring *Indications were noted on gauge length. The cross section was examined, and no SSC was observed.

The microstructures of the electrochemically charged tensile fracture exhibited ductile fracture features in both the uncharged and hydrogen charged samples throughout the entire fractured surface. Microstructures of the fractured surface of Alloy 6 before and after electrochemically hydrogen charging were taken at the edge (FIGS. 9 and 10), quarter (FIGS. 11 and 12) and center (FIGS. 13 and 14) of the round tensile sample.

The low-Ni austenitic grade design proposed in this study with carbon levels between 0.18 weight percent and 0.5 weight percent and manganese levels between 16.5 to 24.5 weight percent, along with the Al, Cr, Cu, Ni, and Si levels proved to be resistant to hydrogen embrittlement, producing results less than 20 weight percent RRA after electrochemically charging for ingress of nascent hydrogen atoms. Based on this study and literature review, materials with carbon levels between 0.18 to 0.6 weight percent, manganese levels between 16 to 30 weight percent, chromium levels between 2.0 and 3.5 weight percent, copper levels between 0.6 to 2 weight percent, nickel levels greater than 0.9 weight percent with an aim of 1.3 weight percent for cost reduction purposes, silicon levels between 2.0 weight percent and 4.0 weight percent, and aluminum levels between 0.04 weight percent to 2 weight percent show to be suitable affordable low-Ni austenite substitutes to 316L stainless steel in resisting hydrogen degradation. In this study, the grade with both low carbon and low Mn was more susceptible to hydrogen embrittlement. Titanium may be used to increase mechanical properties, but titanium may also increase the material's susceptibility to hydrogen embrittlement.

As used herein, “including,” “containing” and like terms are understood in the context of this application to be synonymous with “comprising” and are therefore open-ended and do not exclude the presence of additional undescribed or unrecited elements, materials, phases or method steps. As used herein, “consisting of” is understood in the context of this application to exclude the presence of any unspecified element, material, phase or method step. As used herein, “consisting essentially of” is understood in the context of this application to include the specified elements, materials, phases, or method steps, where applicable, and to also include any unspecified elements, materials, phases, or method steps that do not materially affect the basic or novel characteristics of the invention.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.

Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. In addition, in this application, the use of “or” means “and/or” unless specifically stated otherwise, even though “and/or” may be explicitly used in certain instances. In this application and the appended claims, the articles “a,” “an,” and “the” include plural referents unless expressly and unequivocally limited to one referent.

Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.

Claims

1. A hydrogen degradation resistant steel alloy comprising from 15 to 30 weight percent Mn, from 0.15 to 1 weight percent C, and from 0.05 to 3 weight percent Al, wherein the steel alloy has a microstructure comprising at least 99 percent volume austenite, and possesses a relative reduction in area of no more than 20 percent.

2. The steel alloy of claim 1, wherein the C is greater than 0.2 weight percent.

3. The steel alloy of claim 1, wherein the Mn is greater than 18 weight percent.

4. The steel alloy of claim 1, wherein when the Mn is less than 18 weight percent, the C is greater than 0.2 weight percent.

5. The steel alloy of claim 1, wherein when the C is less than 0.3 weight percent, the Mn is greater than 18 weight percent.

6. The steel alloy of claim 1, wherein the Mn comprises from 18 to 25 weight percent, and the C comprises from 0.3 to 1 weight percent.

7. The steel alloy of claim 1, wherein the Mn comprises from 20 to 24 weight percent, and the C comprises from 0.4 to 0.6 weight percent.

8. The steel alloy of claim 1, further comprising from 0.8 to 2.5 weight percent Ni.

9. The steel alloy of claim 1, further comprising at least 0.2 weight percent Cu.

10. The steel alloy of claim 1, further comprising from 0.8 to 2.5 weight percent Ni, and from 0.2 to 2 weight percent Cu.

11. The steel alloy of claim 10, further comprising at least 0.5 weight percent Si.

12. The steel alloy of claim 10, further comprising at least 1 weight percent Cr.

13. The steel alloy of claim 10, further comprising from 0.5 to 4 weight percent Si, and from 1 to 3.5 weight percent Cr.

14. The steel alloy of claim 1, further comprising at least 0.02 weight percent Ti.

15. The steel alloy of claim 1, wherein the Mn comprises from 20 to 24 weight percent, and the C comprises from 0.3 to 0.6 weight percent.

16. The steel alloy of claim 15, further comprising from 0.8 to 2.5 weight percent Ni, and from 0.2 to 2 weight percent Cu.

17. The steel alloy of claim 16, further comprising from 0.5 to 4 weight percent Si, and from 1 to 3.5 weight percent Cr.

18. The steel alloy of claim 17, wherein the Al comprises from 1.4 to 1.8 weight percent, the Ni comprises from 1.2 to 1.5 weight percent, the Cu comprises from 0.6 to 1.2 weight percent, the Si comprises from 2 to 3.2 weight percent, and the Cr comprises from 2 to 3.2 weight percent.

19. The steel alloy of claim 18, further comprising from 0.08 to 0.2 weight percent Ti.

20. The steel alloy of claim 1, wherein the microstructure comprises at least 99.5 volume percent austenite.

21. The steel alloy of claim 1, wherein the relative reduction in area is less than 15 percent.

22. The steel alloy of claim 1, wherein the steel alloy possesses an ultimate tensile strength of greater than 700 MPa, and a total elongation of greater than 50 percent.

23. A method of producing the steel alloy of claim 1, comprising hot rolling the steel alloy to form a slab and cooling the slab.

24. The method of claim 23, further comprising subjecting the steel alloy slab to electrochemical charging to generate nascent atomic hydrogen.

Patent History
Publication number: 20230349031
Type: Application
Filed: Apr 28, 2023
Publication Date: Nov 2, 2023
Applicant: United States Steel Corporation (Pittsburgh, PA)
Inventors: Kelly Allison Cetin (Gibsonia, PA), Dennis Michael Haezebrouck (Murrysville, PA)
Application Number: 18/141,052
Classifications
International Classification: C22C 38/58 (20060101); C22C 38/42 (20060101); C22C 38/50 (20060101); C22C 38/06 (20060101); C22C 38/02 (20060101); C25B 1/02 (20060101);