OXIDATION RESISTANT, THERMAL CONDUCTIVE, AND ELEVATED TEMPERATURE STRENGTH STEEL

A steel alloy can comprise, consist essentially of, or consist of, in weight percent: 0.022 to 0.257 C; 0.01 to 0.085 N; 0.0 to 1.5 Ni; 0.1 to 0.7 Mn; 2.52 to 5.05 Cu; 8.67 to 14 Cr; 0.1 to 0.96 Si; 0.1 to 0.47 V; 0.4 to 2 Mo; 0 to 1.1 W; 0.0 to 0.5 Nb; 0.0 to 0.03 S; 0.0 to 0.03 P; 0.0 to 2 Co; and, balance Fe.

Skip to: Description  ·  Claims  · Patent History  ·  Patent History
Description
CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. 63/458,965 filed on Apr. 13, 2023, entitled “OXIDATION RESISTANT, THERMAL CONDUCTIVE, AND ELEVATED TEMPERATURE STRENGTH STEEL”, the entire disclosure of which incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates generally to steel alloys, and more particularly to steel alloys for applications where high strength, stability, thermal diffusivity, and oxidation resistance at high temperatures are required.

BACKGROUND OF THE INVENTION

Increasing the peak cylinder pressures and temperatures of heavy-duty diesel engines (HDDE) is the primary method for increasing efficiency of such engines. However, this technical pathway also increases thermal and mechanical loading on the piston and other engine components. An HDDE piston is comprised of two parts, a crown, and a skirt, which are friction welded together. The piston crown may be forged to make the bowl shape (prior to friction welding) and numerous machining operations are also required to finish the bowl shape and fabricate the ring grooves.

Next generation HDDE concepts will result in peak piston metal temperatures of approximately 600 to 700° C. or greater at the bowl rim of the piston crown. However, the primary steels used for piston crown materials in ICEs, AISI 4140 martensitic steel and Micro Alloyed Steel (MAS) grade 38MnSiVS5 or similar, are currently operating near their peak allowable operating temperatures which range from about 500-550° C., respectively. The temperature limitations of 4140 and MAS arise due to significant loss of strength and oxidation resistance of the steels above these temperatures, which can result in premature failure of the piston material. Practically, this means that increasing the severity of combustion conditions cannot occur without causing unacceptable degradation or damage to 4140 or MAS pistons, limiting further increases in engine temperature and efficiency. As such, new piston crown materials are needed with higher elevated temperature strength and oxidation resistance that can operate at targeted temperatures of 600° C. or greater.

To keep piston surface temperatures from exceeding materials limits, pistons are primarily cooled by oil sprayed into the cooling gallery in the underside of the piston, which removes heat from the piston crown. While the cooling is important to keep piston crown surface temperatures below materials limits, the oil spray cooling is also a parasitic loss. A smaller amount of cooling also occurs by heat flow from the piston crown, through the piston ring grooves and rings, to the cylinder liner, and ultimately to the cooling system of the engine. Because of the active oil cooling and also passive cooling through the piston ring, the thermal diffusivity of the piston crown material plays a critical role in managing the surface temperatures, heat transfer, temperature gradients in the piston, and amount of oil that needs to be pumped into the cooling gallery during operation.

A primary metallurgical problem of developing steel piston materials for higher temperature operation is that the elevated temperature strength, oxidation resistance, and thermal diffusivity are often in conflict with each other. Metallurgical optimization of one property can result in significant deterioration of one or both of the other properties. For instance, it is well known that high temperature martensitic stainless steel alloys with substantially higher oxidation resistance and elevated temperature strength relative to 4140 or MAS tend to exhibit substantially lower thermal diffusivity, on the order of 35% at typical piston crown operating temperatures. For example, the thermal diffusivity of commercial martensitic stainless steel 422 is substantially lower than commercial piston alloy 4140. The lower thermal diffusivity of 422 is due in large part to higher alloy content, particularly Cr, which has a strong reducing effect on the thermal diffusivity, but which is also primarily responsible for the high oxidation resistance of stainless steels. Therefore, increasing oxidation resistance through additions of Cr, or other elements like Si, has a strong reducing effect on the thermal diffusivity, causing higher operating temperatures in a piston application, particularly at the bowl rim. Under identical engine operating conditions, the substitution of a 422 steel piston crown for a 4140 piston crown predicted an approximate 100° C. increase in the peak metal temperature at the bowl rim. This tradeoff can partially or completely negate the increased oxidation resistance and elevated temperature strength of an alloy such as 422 or other martensitic stainless steel or steel with high Cr content, which could require lowering operating temperatures of notional 422 steel pistons, either through less intense combustion or by increasing the oil cooling flow rate, both of which can adversely affect engine efficiency. There are currently no commercially available steels that provide an optimum balance of thermal diffusivity and oxidation resistance for HDDE piston applications.

Therefore, a steel alloy which could operate at 600° C. or greater, exhibit greater thermal diffusivity, elevated temperature strength, and/or oxidation resistance over commercially available martensitic stainless steel alloys, yet remain affordable to the HDDE market, would provide numerous advantageous in a piston application of next generation HDDE. The primary advantage of the notional alloys would be to enable significantly greater engine efficiencies. Furthermore, the notional alloys could be beneficial in other applications where an excellent combination of oxidation resistance, thermal diffusivity, and strength are important, such as heat exchanger, energy conversion, tooling applications, etc.

SUMMARY OF THE INVENTION

A steel alloy can comprise, in weight percent:

    • 0.022 to 0.257 C;
    • 0.01 to 0.085 N;
    • 0.0 to 1.5 Ni;
    • 0.1 to 0.7 Mn;
    • 2.52 to 5.05 Cu;
    • 8.67 to 14 Cr;
    • 0.1 to 0.96 Si;
    • 0.1 to 0.47 V;
    • 0.4 to 2 Mo;
    • 0 to 1.1 W;
    • 0.0 to 0.5 Nb;
    • 0.0 to 0.03 S;
    • 0.0 to 0.03 P;
    • 0.0 to 2 Co; and,
    • balance Fe.

A steel alloy can comprise, in weight %:

    • 0.05 to 0.17 C;
    • 0.01 to 0.085 N;
    • 0.28 to 1.25 Ni;
    • 0.2 to 0.6 Mn;
    • 2.52 to 4.02 Cu;
    • 9.28 to 13 Cr;
    • 0.1 to 0.5 Si;
    • 0.1 to 0.47 V;
    • 0.4 to 1 Mo;
    • 0.15 to 0.4 Nb;
    • 0.0 to 0.03 S;
    • 0.0 to 0.03 P; and,
    • balance Fe.

A steel alloy can comprise, in weight %:

    • 0.068 to 0.13 C;
    • 0.01 to 0.085 N;
    • 0.28 to 1.002 Ni;
    • 0.2 to 0.5 Mn;
    • 2.52 to 4.02 Cu;
    • 10.7 to 11.88 Cr;
    • 0.2 to 0.5 Si;
    • 0.2 to 0.4 V;
    • 0.4 to 0.8 Mo;
    • 0.21 to 0.4 Nb;
    • 0.0 to 0.03 S;
    • 0.0 to 0.03 P; and
    • balance Fe.

The steel alloy can have 17≥Cr+Cu≥11.65. The steel alloy can have 17≥Cr+Cu≥12.0.

The steel alloy can have 14.7≥Cu/(Ni+Mn)≥1.6. The steel alloy can have 1.8≥Ni+Mn≥0.1. The steel alloy can have−6≥0.64Cu−0.75Cr−1.0Ni−4.2Si−1.1W−1.3Nb+4.9C−5.3Mn−2.7V≥−14.5. The steel alloy can have 8.3≥Cu/(Ni+Mn)≥1.6. The steel alloy can have 15.9≥Cr+Cu≥13.3. The steel alloy can have 2.85>0.251Cr−9.74C≥1.7. The steel alloy can have 7.02≥Cu/(Ni+Mn)≥1.9.

The steel alloy can have an Ultimate Tensile Strength at 650° C. after quenching and tempering that is greater than 360 MPa. The steel alloy can have an Ultimate Tensile Strength at 650° C. after quenching and tempering that is greater than 435 MPa. The steel alloy can have an Ultimate Tensile Strength (MPa) at 650° C. after thermally soaking at 650° C. for 500 h that is greater than 250 MPa.

The steel alloy can have a thermal diffusivity of the steel at 600° C. that is greater than or equal to 4.4 mm2 s−1 in the quenched and tempered condition.

The steel alloy can have an Ultimate Tensile Strength (MPa) at 650° C. after quenching, tempering, and thermally soaking at 650° C. for 500 h that is greater than 305 MPa.

The steel alloy can have an Ultimate Tensile Strength (MPa) at 650° C. after quenching, tempering, and thermally soaking at 650° C. for 500 h is greater than 320 MPa.

The steel alloy can have an oxidation mass change after 1000 h of cyclic oxidation testing (1 h cycles) at 700° C. in an atmosphere of air+10% water vapor that is less than or equal to 2 mg cm−2 but greater than or equal to −2 mg cm−2. The steel alloy can have an oxidation mass change after 1000 h of cyclic oxidation testing (1 h cycles) at 700° C. in an atmosphere of air+10% water vapor is less than or equal to 1 mg cm−2 but greater than or equal to −1 mg cm−2.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments that are presently preferred it being understood that the invention is not limited to the arrangements and instrumentalities shown, wherein:

FIG. 1 is a scanning electron microscope (SEM) image of the microstructure of an alloy according to the invention.

FIG. 2A is a plot of Oxidation Mass Change (mg cm−2) vs Time (1 h cycles) during cyclic oxidation testing at 700° C. in air plus 10% water vapor for a series of invention alloys 40, 41, 42, 43, 44, and 46 as compared to commercial (reference) alloys 422U and X10CrMoVNb9-1 (designated X10 in the figure); FIG. 2B is an enlarged version of FIG. 2A.

FIG. 3A is a plot of Oxidation Mass Change (mg cm−2) vs Time (1 h cycles) during cyclic oxidation testing at 700° C. in air plus 10% water vapor for a series of invention alloys 66, 61, 64, 65, 63, and 62 as compared to commercial (reference) alloy X22CrMoV12-1 (designated X22in the figure); FIG. 3B is an enlarged version of FIG. 3A.

FIG. 4 is a plot of Oxidation Mass Change (mg cm−2) vs Time (1 h cycles) at 700° C. in air plus 30% water vapor as a function of time to 250 h for invention alloy 62, reference alloy 72, and commercial reference alloy 422C.

FIG. 5 is a plot of Oxidation Mass Change (mg cm−2) after 1000 h (1000 cycles) at 700° C. vs Cr+Cu content (wt. %).

FIG. 6 shows the values of thermal diffusivity at 600° C. for select invention and reference alloys (commercial and 61, 66, 71 and 72) in the tempered martensitic condition (after austenitizing, quenching, double tempering for 2 h at 650° C., air cooling to ambient temperature, tempering again for 2 h at 650° C., then air cooling to ambient temperature).

FIG. 7 shows the thermal diffusivity at 600° C. for select invention alloys and commercial alloys plotted as a function of the oxidation mass gain after 1000 h at 700° C. in an atmosphere of air plus 10% water vapor.

FIG. 8 shows the thermal diffusivity plotted as a function of Cr+Cu (wt. %).

FIG. 9 is a plot of the Thermal Diffusivity Factor, 0.64Cu−0.75Cr−1.0Ni−4.2Si−1.1W−1.3Nb+4.9C−5.3Mn−2.7V (wt. %), plotted as a function of Cr+Cu (wt. %) for select invention alloys and reference alloys (commercial and 61, 66, 71 and 72).

FIG. 10 is a plot of Ultimate Tensile Strength (MPa) vs. Test Temperature and Condition for select invention alloys and commercial reference alloys.

 DETAILED DESCRIPTION OF THE INVENTION

The disclosed high temperature martensitic steel alloys exhibit an exceptional combination of elevated temperature strength, oxidation resistance, and thermal diffusivity that is superior to current state of the art high temperature martensitic steels. The disclosed alloys achieve this exceptional combination of properties due to a novel alloying strategy disclosed herein. Exemplary alloy compositions are shown in Table 1.

TABLE 1 Exemplary alloy compositions in wt. %. Example Example Example Designation Alloy 1 Alloy 2 Alloy 3 Ni 0.0 to 1.5 0.28 to 1.25  0.28 to 1.002 Mn 0.1 to 0.7 0.2 to 0.6 0.2 to 0.5 C 0.022 to 0.257 0.05 to 0.17 0.068 to 0.13  N  0.01 to 0.085  0.01 to 0.085  0.01 to 0.085 Cr 8.67 to 14   9.28 to 13    10.7 to 11.88 Si  0.1 to 0.96 0.1 to 0.5 0.2 to 0.5 V  0.1 to 0.47  0.1 to 0.47 0.2 to 0.4 Mo 0.4 to 2   0.4 to 1   0.4 to 0.8 Cu 2.52 to 5.05 2.52 to 4.02 2.52 to 4.02 Nb 0.0 to 0.5  0.15 to 0.4 Nb 0.21 to 0.4  Co 0.0 to 2   0-1* 0-1* W   0 to 1.1 ≤0.1* ≤0.1* S  0.0 to 0.03  0.0 to 0.03  0.0 to 0.03 Ti* ≤0.025 ≤0.025 ≤0.025 P*  0.0 to 0.03  0.0 to 0.03  0.0 to 0.03 Al*  0.0 to 0.06  0.0 to 0.06  0.0 to 0.06 Sn*  0.0 to 0.025  0.0 to 0.025  0.0 to 0.025 Fe Bal. Bal. Bal. *Represents residual traces/usual impurities

A steel alloy according to the invention comprises, consists essentially of, or consists of, in weight percent:

    • 0.022 to 0.257 C;
    • 0.01 to 0.085 N;
    • 0.0 to 1.5 Ni;
    • 0.1 to 0.7 Mn;
    • 2.52 to 5.05 Cu;
    • 8.67 to 14 Cr;
    • 0.1 to 0.96 Si;
    • 0.1 to 0.47 V;
    • 0.4 to 2 Mo;
    • 0 to 1.1 W;
    • 0.0 to 0.5 Nb;
    • 0.0 to 0.03 S;
    • 0.0 to 0.03 P;
    • 0.0 to 2 Co; and,
    • balance Fe.

The steel alloy can comprise, consist essentially of, or consist of, in weight %:

    • 0.05 to 0.17 C;
    • 0.01 to 0.085 N;
    • 0.28 to 1.25 Ni;
    • 0.2 to 0.6 Mn;
    • 2.52 to 4.02 Cu;
    • 9.28 to 13 Cr;
    • 0.1 to 0.5 Si;
    • 0.1 to 0.47 V;
    • 0.4 to 1 Mo;
    • 0.15 to 0.4 Nb;
    • 0.0 to 0.03 S;
    • 0.0 to 0.03 P; and,
    • balance Fe.

The steel alloy can comprise, consist essentially of, or consist of, in weight %:

    • 0.068 to 0.13 C;
    • 0.01 to 0.085 N;
    • 0.28 to 1.002 Ni;
    • 0.2 to 0.5 Mn;
    • 2.52 to 4.02 Cu;
    • 10.7 to 11.88 Cr;
    • 0.2 to 0.5 Si;
    • 0.2 to 0.4 V;
    • 0.4 to 0.8 Mo;
    • 0.21 to 0.4 Nb;
    • 0.0 to 0.03 S;
    • 0.0 to 0.03 P; and,
    • balance Fe.

The alloy could contain other elements such as Sn, Sb, As, Pb, O, H, Zn but the combined amount of these other elements cannot exceed 1 wt. %

The value in wt. % of C, Cr, Nb, Ni, and Si can be: 0.155≥C≥0.068, 14≥Cr≥10.7, 0.5>Nb≥0.22, 1.002>Ni≥0.28, 0.5≥Si≥0.1. These ranges may result in enhanced ultimate tensile strength at 650° C. after thermal aging at 650° C. for 500 h.

The Ultimate Tensile Strength (UTS) (MPa) at 650° C. after quenching, tempering, and thermally soaking at 650° C. for 500 h can be greater than 320 MPa. The Ultimate Tensile Strength (MPa) at 650° C. after thermally soaking at 650 C for 500 h can be greater than 250 MPa.

The alloy can exhibit an Ultimate Tensile Strength (MPa) at 650° C. after quenching, tempering at 650° C., and thermally soaking at 650° C. for 500 h that is greater than or equal to 305 MPa while also exhibiting thermal diffusivity of the steel at 600° C. that is greater than or equal to 4.4 mm2 s−1 after quenching and tempering at 650° C.

The alloy can exhibit an Ultimate Tensile Strength (MPa) at 650° C. after quenching, tempering at 650° C., and thermally soaking at 650° C. for 500 h that is greater than or equal to 320 MPa while also exhibiting thermal diffusivity of the steel at 600° C. that is greater than or equal to 4.4 mm2 s−1 after quenching and tempering at 650° C.

The thermal diffusivity of the steel at 600° C. can be greater than or equal to 4.4 mm2 s−1 in the quenched and tempered condition.

The oxidation mass change of the steel after 1000 h of cyclic oxidation testing (1 h cycles) at 700° C. in an atmosphere of air+10% water vapor may be less than or equal to 2 mg cm 2 but greater than or equal to −2 mg cm−2. The oxidation mass change of the steel after 1000 h of cyclic oxidation testing (1 h cycles) at 700° C. in an atmosphere of air+10% water vapor may be less than or equal to 1 mg cm−2 but greater than or equal to −1 mg cm−2. The steel alloy can be austenitized at 1150 C for 0.5 h cooled to ambient temperatures, and tempered at 650 for 2 h, cooled to RT, and tempered again at 650 C for 2 h.

The C in weight percent can be from 0.022 to 0.257 wt. %. The C weight % can be 0.022, 0.05, 0.075, 0.100, 0.125, 0.150, 0.175, 0.200, 0.250, 0.275 wt. %. The weight % of C can be within a range of any high value and low value selected from these values.

The N in weight percent can be from 0.023 to 0.085 wt. %. The N weight % can be 0.01, 0.02, 0.023, 0.025, 0.030, 0.035, 0.040, 0.045, 0.050, 0.055, 0.060, 0.065, 0.070, 0.075, 0.080, 0.085 wt. %. The weight % of N can be within a range of any high value and low value selected from these values.

The Ni in weight percent can be from 0.0 to 1.5 wt. %. The Ni weight % can be 0.00, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00, 1.10, 1.20, 1.30, 1.40, 1.50 wt. %. The weight % of Ni can be within a range of any high value and low value selected from these values.

The Mn in weight percent can be from 0.1 to 0.7 wt. %. The Mn weight % can be 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.6, 0.65, and 0.7 wt. %. The weight % of Mn can be within a range of any high value and low value selected from these values.

The Cu in weight percent can be from 2.52 to 5.05 wt. %. The Cu weight % can be 2.52, 2.60, 2.70, 2.80, 2.90, 3.00, 3.10, 3.20, 3.30, 3.40, 3.50, 3.60, 3.70, 3.80, 3.90, 4.00, 4.10, 4.20, 4.30, 4.40, 4.50, 4.60, 4.70, 4.80, 4.90, 5.0, 5.05 wt. %. The weight % of Cu can be within a range of any high value and low value selected from these values.

The Cr in weight percent can be from 8.67 to 14 wt. %. The Cr weight % can be 8.67, 8.70, 8.80, 8.90, 9.00, 9.10, 9.20, 9.30, 9.40, 9.50, 9.60, 9.70, 9.80, 9.90, 10.00, 10.10, 10.20, 10.30, 10.40, 10.50, 10.60, 10.70, 10.80, 10.90, 11.00, 11.10, 11.20, 11.30, 11.40, 11.50, 11.60, 11.70, 11.80, 11.88, 12.00, 12.20, 12.40, 12.60, 12.80, 13.00, 13.20, 13.40, 13.60, 13.80, 14.00 wt. %. The weight % of Cr can be within a range of any high value and low value selected from these values.

The Si in weight percent can be from 0.1 to 0.96 wt. %. The Si weight % can be 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50 wt. %, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, and 0.96 wt. %. The weight % of Si can be within a range of any high value and low value selected from these values.

The V in weight percent can be from 0.1 to 0.47 wt. %. The V weight % can be 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.47 wt. %. The weight % of V can be within a range of any high value and low value selected from these values.

The Mo in weight percent can be from 0.4 to 2.0 wt. %. The Mo weight % can be 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0 wt. %. The weight % of Mo can be within a range of any high value and low value selected from these values.

The W in weight percent can be from 0 to 1.1 wt. %. The W weight % can be 0.0, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, 1.0, 1.10 wt. %. The weight % of W can be within a range of any high value and low value selected from these values.

The Nb in weight percent can be from 0.0 to 0.5 wt. %. The Nb weight % can be 0.0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50 wt. %. The weight % of Nb can be within a range of any high value and low value selected from these values.

The S in weight percent can be from 0.0 to 0.03 wt. %. The S weight % can be 0.0, 0.005, 0.01, 0.015, 0.020, 0.025, 0.030 wt. %. The weight % of S can be within a range of any high value and low value selected from these values.

The P in weight percent can be from 0.0 to 0.03 wt. %. The P weight % can be 0.0, 0.005, 0.010, 0.015, 0.020, 0.025, 0.030 wt. %. The weight % of P can be within a range of any high value and low value selected from these values.

The Co in weight percent can be from 0.0 to 2.0 wt. %. The Co weight % can be 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0 wt. %. The weight % of Co can be within a range of any high value and low value selected from these values.

The value of Ni+Mn can be 1.8>Ni+Mn≥0.1. Ni and Mn are austenite stabilizing elements. This range of Ni+Mn enables sufficient austenite stabilization. The Ni+Mn is limited such that the A1 transformation temperature of the steel does not become too low and cause unwanted phase transformation during elevated temperature exposure. The value of Ni+Mn can be 1.7≥Ni+Mn≥0.48. The value of Ni+Mn can be 1.5>Ni+Mn>0.48. The Ni+Mn in weight percent can be from 0.1 to 1.8 wt. %. The Ni+Mn weight % can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, and 1.8 wt. %. The weight % of Ni+Mn+can be within a range of any high value and low value selected from these values.

The value of Cr+Cu can be 17>Cr+Cu≥12. The value of Cr+Cu can be 16.93>Cr+Cu≥12. This range of Cr+Cu conveys excellent oxidation resistance during cyclic oxidation testing at 700° C. in air and 10% water vapor. The value of Cr+Cu can be 15.9>Cr+Cu≥13.3. The Cr+Cu in weight % can be 12.00, 12.25, 12.5, 12.75, 13.00, 13.25, 13.50, 13.75, 14.00, 14.25, 14.50, 14.75, 15.00, 15.25, 15.50, 15.75, 16.00, 16.25, 16.50, 16.75, or 17.00. The value of Cr+Cu in weight percent can be within a range of any high value and low value selected from these values.

The value of Cu/(Ni+Mn) can be: 14.7≥Cu/(Ni+Mn)≥1.6. Increasing the amount of Cu and decreasing the amount of (Ni+Mn) can increase the thermal diffusivity of the alloys. The value of Cu/(Ni+Mn) can be: 8.3≥Cu/(Ni+Mn)≥1.6. The value of Cu/(Ni+Mn) can be: 7.02≥Cu/(Ni+Mn)≥1.9. The Cu/(Ni+Mn) in weight percent can be from 1.6 to 14.7 wt. %. The Cu/(Ni+Mn) can be 1.6, 1.7, 1.8, 1.9, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, 3.8, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0, 10.5, 11.0, 11.5, 12.0, 12.5, 13.0, 13.5, 14, 14.5, or 14.7. The value of Cu/(Ni+Mn) can be within a range of any high value and low value selected from these values.

The value of −6≥0.64Cu−0.75Cr−1.0Ni−4.2Si−1.1W−1.3Nb+4.9C−5.3Mn−2.7V≥−14.5. The values of this formula are proportional to thermal diffusivity at 600° C. for the developed and reference alloys in the tempered martensitic condition. The value of −6.5≥0.64Cu−0.75Cr−1.0Ni−4.2Si−1.1W−1.3Nb+4.9C−5.3Mn−2.7V≥−14.25. The value of −7≥0.64Cu−0.75Cr−1.0Ni−4.2Si−1.1W−1.3Nb+4.9C−5.3Mn−2.7V≥−14. The value of 0.64Cu−0.75Cr−1.0Ni−4.2Si−1.1W−1.3Nb+4.9C−5.3Mn−2.7V in weight percent can be from −6 to −14.5 wt. %. The 0.64Cu−0.75Cr−1.0Ni−4.2Si−1.1W−1.3Nb+4.9C−5.3Mn−2.7V weight % can be −14.50, −14.25, −14, −13.75, −13.5, −13.25, −13.00, −12.75, −12.5, −12.25, −12.00, −11.75, −11.5, −11.25, −11.00, −10.50, −10.00, −9.50, −9.00, −8.50, −8.00, −7.50, −7.00, −6.50, or −6.00. The value of 0.64Cu−0.75Cr−1.0Ni−4.2Si−1.1W−1.3Nb+4.9C−5.3Mn−2.7V in weight percent can be within a range of any high value and low value selected from these values.

The invention alloys have value of 2.85≥0.251Cr−9.74C≥1.7. Values for the invention alloys which fall within this range may convey superior strength. The values of 0.251Cr−9.74C may be 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, or 2.85, or can be within a range of any high value and low value selected from these values.

The above relationships between the constituents making up the alloys can be applied independently or collectively. For example, the above noted relationships of Cr+Cu and Cu/(Ni+Mn) can be applied independently or collectively to assist in defining an alloy according to the invention, and can be applied to alloys having differing weight % constitutions that are within the defined ranges of the invention.

In one aspect, the technologies described herein provide martensitic steel alloy compositions. In another aspect, the technologies described herein provide methods for making martensitic steel alloy compositions described herein.

To demonstrate the exceptional combination of strength, thermal diffusivity, and oxidation resistance of the alloys, a series of alloys according to the invention were prepared. The measured compositions of the new alloys and reference alloys are shown in Table 2. Commercial 422 steel in this work was tested from two different heats, with significant differences in the Cr content, and thus has suffixes of either “U” or “C”, depending in manufacturer.

TABLE 2 Measured compositions of invention alloys (35-81, excluding alloys 61, 66, 71 and 72 which are reference alloys) and commercial reference alloys 403Cb+, 418, 422C, 422U, X10CrMoVNb9-1, X22CrMoV12-1. Alloy Designation Ni Mn C N Cr Si V Mo Cu W New Alloys 35 0.31 0.27 0.102 0.023 11.88 0.37 0.25 0.95 3.5 1.08 40 0.28 0.26 0.103 0.052 11.55 0.39 0.27 1.03 3.85 1.07 41 0.3 0.26 0.098 0.06 11.44 0.38 0.26 0.51 3.93 0.025 42 0.3 0.27 0.1 0.064 8.99 0.4 0.26 0.54 3.91 0.03 43 0.3 0.27 0.103 0.051 9.11 0.91 0.28 0.53 3.92 0.031 44 0.3 0.28 0.24 0.052 9.11 0.9 0.27 0.53 3.93 0.074 45 0.0047 0.27 0.257 0.052 9.21 0.94 0.47 1.02 3.93 0.037 46 0.94 0.26 0.119 0.049 9.28 0.39 0.24 0.51 3.93 0.028 47 0.005 0.27 0.192 0.071 9.3 0.96 0.47 1 4.02 0.02 50 0.29 0.27 0.097 0.024 11.76 0.36 0.26 0.96 5.05 1.1 62 0.99 0.27 0.083 0.085 11.3 0.36 10.26 0.5 2.97 0.026 63 1.01 0.26 0.124 0.051 10.7 0.37 0.26 0.5 2.99 0.075 64 1 0.26 0.095 0.052 11.6 0.37 0.26 0.49 2.99 0.066 65 0.51 0.27 0.095 0.049 9.08 0.37 0.26 0.48 3 0.069 73 1.01 0.29 0.068 0.015 10.6 0.37 0.27 0.49 3 0.041 74 0.02 0.29 0.086 0.016 10.6 0.37 0.29 0.49 2.99 0.079 80 0.98 0.29 0.022 0.02 10.5 0.37 0.3 0.49 3.01 0.023 81 0.97 0.28 0.019 0.017 9.09 0.37 0.29 2.03 2.99 0.021 Reference Alloys 61 1.02 0.26 0.155 0.048 9.21 0.37 0.4 0.5 2.52 0.083 66 1.02 0.26 0.171 <0.001 8.67 0.37 0.47 0.48 2.98 0.011 71 0.27 0.27 0.064 0.012 11.6 0.37 0.26 0.5 0.004 0.04 72 1.02 0.26 0.083 0.01 10.9 0.37 0.26 0.5 0.003 0.058 Commercial 0.52 0.71 0.17 0.059 10.54 0.39 0.21 0.83 0.05 <0.05 403Cb+ Commercial 2.06 0.4 0.164 0.029 12.39 0.27 0.049 0.083 0.13 2.81 418 steel Commercial 0.79 0.78 0.23 0.048 12.09 0.36 0.22 1.08 0.08 0.96 422(C) Commercial 0.8 0.66 0.21 0.06 11.22 0.3 0.24 0.98 0.09 1 422(U) Commercial 0.325 0.455 0.098 0.045 8.631 0.346 0.207 0.9 0.095 <0.001 X10CrMoVNb9-1 Commercial 0.43 0.56 0.2 — 10.99 0.369 0.259 0.834 0.06 <0.04 X22CrMoV12-1 Alloy Designation Nb Ti S P Co B Fe New Alloys 35 <0.002 <0.0005 — — — — Bal. 40 <0.002 0.0013 0.002 0.014 — — Bal. 41 0.32 0.0011 0.003 0.013 — — Bal. 42 0.31 0.001 0.001 0.013 — — Bal. 43 0.3 0.0011 0.002 0.016 — — Bal. 44 0.29 0.0011 0.003 0.018 — — Bal. 45 0.31 0.0012 0.004 0.017 — — Bal. 46 0.32 0.0011 0.005 0.016 — — Bal. 47 <0.002 0.0038 0.004 0.013 — — Bal. 50 <0.002 <0.0005 — — — — Bal. 62 0.31 0.001 0.001 0.013 0.007 — Bal. 63 0.31 0.001 0.002 0.014 0.007 — Bal. 64 0.31 0.001 0.002 0.014 0.007 — Bal. 65 0.31 0.001 0.002 0.016 0.96 — Bal. 73 0.22 0.001 <0.001 0.012 0.008 0.003 Bal. 74 0.21 0.0009 <0.001 0.011 1.97 0.002 Bal. 80 0.31 0.001 <0.001 0.013 0.008 0.002 Bal. 81 0.31 0.001 <0.001 0.014 0.008 0.002 Bal. Reference Alloys 61 0.31 0.001 0.002 0.013 0.008 — Bal. 66 0.019 0.11 0.001 0.013 0.008 — Bal. 71 0.31 0.001 0.001 0.008 0.007 — Bal. 72 0.32 0.001 0.001 0.01 0.007 — Bal. Commercial 0.36 <0.005 0.0005 0.008 0.05 — Bal. 403Cb+ Commercial 0.002 <0.002 <0.0005 0.016 0.033 <0.0005 Bal. 418 steel Commercial 0.026 0.0016 0.006 0.019 0.022 <0.0005 Bal. 422(C) Commercial <0.002 <0.01 <0.0005 .0016 .022 <0.0005 Bal. 422(U) Commercial 0.06 0.0005 0.0006 0.011 0.015 — Bal. X10CrMoVNb9-1 Commercial <0.002 0.0045 <0.003 <0.005 0.017 — Bal. X22CrMoV12-1

The constituent elements of the alloys were arc melted to form the ingots. Then ingots were then hot rolled, using a start rolling temperature of 1120° C. and multiple passes, then cooled to room temperature. The alloys were then pre-heated and held at 700° C. for 15 minutes to 1 hour, heated to 1070° C. and held for 15 m, then heated to 1150° C. and held for 30 minutes, followed by rapid quenching in oil to form a martensitic microstructure. The alloys were tempered at 650° C. for 2h, air cooled to room temperature, then tempered again at 650° C. for 2h, and air cooled to room temperature. While the aforementioned processing was used to demonstrate the superior properties of the disclosed alloys, those skilled in the art will recognize that the process outlined above for fabricating the disclosed alloys, including how the steel is melted, the rolling parameters, and the heat treatment times and temperatures, etc., may be changed and/or adjusted to achieve specific production volumes, product form (geometry), and/or final properties, as may be desired by the end user. The processing parameters are summarized in Table 3.

TABLE 3 Thermal mechanical processing of invention alloys and reference alloys (commercial and 61, 66, 71, and 72) used for comparison (OQ = oil quench, AC = air cool). Designation Hot Rolling Annealing Hardening Treatment 35 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./0.5 h OQ + 19% reduction per pass, AC h AC 650° C./2 h AC + 650° C./2 h AC 40 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 41 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 42 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 43 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 44 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 45 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 46 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 47 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 50 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./0.5 h OQ + 19% reduction per pass, AC h AC 650° C./2 h AC + 650° C./2 h AC 61 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 62 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 63 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 64 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 65 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 66 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 71 1120° C., 2 passes, 700° C./2 700° C./0.5 h + 1150° C./0.5 h OQ + 19% reduction per pass, AC h AC 650° C./2 h AC + 650° C./2 h AC 72 1120° C., 2 passes, 700° C./2 700° C./0.5 h + 1150° C./0.5 h OQ + 19% reduction per pass, AC h AC 650° C./2 h AC + 650° C./2 h AC 73 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 74 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 80 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC 81 1120° C., 2 passes, 700° C./2 700° C./15 min + 1070° C./15 min + 19% reduction per pass, AC h AC 1150° C./0.5 h OQ + 650° C./2 h AC + 650° C./2 h AC Commercial 1150° C./0.5 h OQ + 650° C./2 h AC + 403Cb+ 650° C./2 h AC Commercial 954° C./0.5 h OQ + 650° C./2 h AC + 418 steel 650° C./2 h AC Commercial 1038° C./1 h OQ + 650° C./2 h AC + 422(C) 650° C./2 h AC Commercial 1038° C./1 h OQ + 650° C./2 h AC + 422(U) 650° C./2 h AC Commercial 1050° C./1 h OQ + 650° C./2 h AC + X10CrMoVNb9-1 650° C./2 h AC Commercial 1030 C./1 h OQ + 650° C./2 h AC + X22CrMoV12-1 650° C./2 h AC

These aforementioned commercial alloys have been identified as having the highest potential for next generation HDDE piston alloys which are commercially available and are widely used in other high temperature applications such as turbine blades and piping systems. Their thermal, mechanical, and oxidation properties have been compared to the invention alloys described herein. The chemical composition of the invention alloys described herein has been uniquely tailored to impart these improved properties relative to the commercially available martensitic steels.

The following summarizes the important contributions of each element in the alloy compositions described herein.

Carbon (C): C contributes to elevated temperature strengthening and wear resistance through precipitation hardening upon tempering. C can also adversely impact weldability. In the present steels, C can form Cr rich M23C6 carbides, Vanadium (V) carbonitrides (V (C/N)), and Niobium carbides (Nb/C), depending on the amount and proportions of carbide forming elements Cr, Mo, V, and Nb and temperature. C may increase thermal diffusivity but too much C can result in loss of strength during long term thermal aging.

Chromium (Cr): Cr is a strong carbide forming element, enhances oxidation, and significantly reduces the thermal diffusivity of the steel. Cr reduces oxidation kinetics and improves oxide adherence, particularly when Cr levels are high enough to preferentially form a CrO2 oxide scale rather than an Fe oxide scale. The alloy compositions described herein exhibit relatively low oxidation mass gain during cyclic oxidation testing in air+10% H2O at 700° C. compared to commercial alloys. Increasing Cr can improve strength retention during long term thermal aging.

Copper (Cu): The Cu content of the alloy compositions described herein is elevated relative to the commercial high temperature steels listed for comparison. Cu can improve the corrosion resistance and machinability of steel, which is highly beneficial for components such as pistons, which require a large amount of machining. If the Cu content of the steel is too high, hot shortness can occur during hot working, depending on the Cu content, the presence of other elements in the steel which can suppress hot shortness, such as Ni, Si, Cr, or Co, and the atmosphere present during hot working. Cu is soluble in austenite but exhibits very low solubility in ferrite, causing the Cu to precipitate out of the ferrite matrix as face centered cubic crystal structure precipitates. In the present invention, the alloys take advantage of the low solubility of Cu in ferrite. The present steels are quenched and tempered, and upon tempering, the Cu precipitates out of the martensite matrix as nearly pure Cu precipitates. The high purity of the Cu precipitates is in part accomplished by optimal alloy composition, including limiting the amounts of elements which are soluble in Cu to relatively low levels. Mn and Ni, both of which have relatively high solubility in Cu, are generally limited to ˜0.7 and 1.5 wt. % each, respectively, in the present steels. By reducing the amount of Ni and Mn, the Cu precipitates have less impurity content and higher thermal diffusivity, so as not to significantly degrade the overall bulk thermal diffusivity of the steel, and in some cases, can increase the bulk thermal diffusivity of the steel. This is illustrated in Table 4, which shows the thermal diffusivity of alloy compositions 41 and 62, which contain 3.93 and 2.97 wt. % Cu, respectively, have slightly larger thermal diffusivities to alloys 71 and 72, which have only trace amounts of Cu. Cu in the present alloys improves the oxidation resistance, as indicated by increased mass gain of alloy 72 (Cu free) relative to alloy 62 during oxidation testing at 700 C in air plus 30% water vapor, as shown in FIG. 4. As such, Cu improves the oxidation resistance and strength of the alloy compositions, and has only a modest or increasing impact on the thermal diffusivity, thus improving the overall balance of elevated temperature strength, oxidation resistance, and thermal diffusivity.

TABLE 4 Thermal diffusivity of invention alloys and reference alloys as well as the value of 0.64Cu—0.75Cr—1.0Ni—4.2Si—1.3Nb + 4.9C—5.3Mn—2.7V. Thermal Diffusivity Thermal Diffusivity Factor Alloy at 600° C. (0.64Cu—0.75Cr—1.0Ni—4.2Si—1.3Nb + Designation (mm2 s−1) 4.9C—5.3Mn—2.7V) New Alloys 65 4.98 −9.10 44 4.91 −9.89 50 4.89 −10.26 46 4.80 −8.91 42 4.75 −8.30 47 4.72 −10.22 63 4.71 −10.63 41 4.70 −10.00 40 4.65 −10.90 43 4.65 −10.55 45 4.64 −10.23 62 4.58 −11.23 64 4.58 −11.43 35 4.54 −11.33 Reference Alloys 66 4.88 −9.02 Commercial 4.77 −10.76 X10CrMoVNb9-1 61 4.76 −10.06 72 4.54 −12.90 71 4.49 −12.79 Commercial 4.38 −14.00 403Cb+ Commercial 4.36 −10.76 X22CrMoV12-1 Commercial 4.26 −14.63 422(U) Commercial 4.20 −15.98 422(C) Commercial 4.00 −16.95 418 steel

Nickel (Ni): Ni is an austenite stabilizer and also improves oxidation resistance in high temperature steels. Ni improves the oxidation resistance while having minimal impact on thermal diffusivity. Adding Ni to steel is also one method of suppressing hot shortness in steels containing elevated levels of Cu. Ni also is soluble in Cu, and is present in the Cu precipitates of the present steels.

Manganese (Mn): Mn is an austenite stabilizer and may provide an important manufacturing function by removing S from solution through the formation of MnS. This may improve fatigue strength and toughness. Mn may also provide hardenability and solid solution strengthening. Too much Mn may cause embrittlement and may excessively lower the martensite start temperature in the present steels which contain elevated Cu levels, resulting in incomplete transformation to martensite during quenching. The amounts of Mn in the disclosed alloys is sufficient to remove S from solution, while not resulting in undue reductions in thermal diffusivity by increasing the amount of Mn in solid solution in the ferrite/martensite matrix and/or by increasing the Mn content in the Cu precipitates, thereby lowering the thermal diffusivity of the precipitates and hence, the bulk material.

Cobalt (Co): Cobalt is an austenite stabilizer, raises the martensite start temperature, and improves resistance to softening at elevated temperature. Co also reduces susceptibility to hot shortness. While Co is a solid solution element in steel, its tendency to reduce the thermal diffusivity of steel is relatively small, making it attractive for applications requiring high elevated temperature strength and thermal diffusivity. Co may be substituted partially for Ni in some applications to improve strength without degrading thermal diffusivity, as is the case for alloy 65 (compared to alloy 61).

Nitrogen (N): Nitrogen is a strengthening element in steel. It typically manifests in nitrides or carbonitrides of V and Cr. Too much N can cause embrittlement and can excessively lower the martensite start temperature in the present steels, resulting in incomplete transformation to martensite during quenching.

Silicon (Si): Si is used as a de-oxidant in steel making and is a relatively inexpensive element. Si may improve oxidation resistance in part by reducing oxidation kinetics. Si additions also improve the machinability of the steel, which is important for piston applications where significant machining of the component is required. Si additions reduce the thermal diffusivity of the steel.

Molybdenum (Mo): Mo additions may promote solid solution strengthening, temper resistance, secondary hardening, as well as oxidation and corrosion resistance. Excessive Mo can result in degraded thermal diffusivity. Having less Mo in solid solution is beneficial to thermal diffusivity. Increasing bulk Mo content (along with V) can negatively impact thermal diffusivity, as indicated by the relatively low thermal diffusivity of alloy 45 compared to other disclosed alloys with nominally 9 wt. % Cr, but less Mo and V).

Vanadium (V): V is a strong carbide, nitride, and carbonitride forming element and contributes to elevated temperature precipitation strengthening. To much V can increase the amount of Cr in solid solution, degrading thermal diffusivity.

Tungsten (W): W substantially increases hardenability. W increases elevated temperature strength through solid solution strengthening and also by reducing the coarsening rates of M23C6 carbides. W also reduces the thermal diffusivity of steel.

Titanium (Ti): Ti is a strong carbide, nitride, and carbonitride forming element. It may provide substantial enhancement to elevated temperature strength and oxidation resistance in martensitic steels with 9 wt. % Cr according to U.S. Pat. No. 8,317,944, incorporated herein fully by reference. Too much Ti may result in large primary Ti carbides which degrade toughness and fatigue strength.

Niobium (Nb): Nb is a strong carbide forming element and may provide enhanced elevated temperature strength in martensitic stainless steels. Nb may also enhance oxidation resistance. Too much Nb can result in large primary carbides and degrade toughness and fatigue strength.

The combinations of elements that exists in the alloy compositions described herein are unique and optimized to provide advantageous properties for piston applications in combustion engines or other applications requiring high oxidation resistance and relatively high thermal diffusivity and strength at elevated temperatures.

Tensile testing was used to demonstrate elevated temperature strength and performed at temperatures ranging from 25 to 650° C. in accordance with ASTM E8/21. Tensile specimens were sub-sized with a 25.4 mm overall length, 7.62 mm gage length, 1.52 mm gage width, and 0.76 mm gage thickness. All tensile samples were cut from bars, with the long axis of the specimen along the rolling direction/extrusion, and or long axis of the bar. The tests were conducted under quasi-static conditions, with an initial engineering strain rate of 0.001 s−1. Ultimate Tensile Strength (UTS) values are averages of two to three tests per condition.

Thermal diffusivity measurements were obtained by the laser flash method in accordance with ASTM E1461-13 and using a flash diffusivity system (Netzsch LFA 457). Thermal diffusivity specimens were 12.7 mm in diameter and 1.5 mm thick and were coated with graphite prior to performing the measurement. Measurements were performed at 50° C. increments on heating up to 700° C., and during cooling at 300° C. and 100° C. to identify any irreversible changes in diffusivity relative to the initial measurements performed on heating. The heating/cooling rate between set points were 3-5° C./min. Typical measurements include three tests on the same specimen. Time spent at each set point was 10-15 minutes. Diffusivity measurements were conducted in ultra-high purity (UHP) argon purge gas flowing at 100 mL/min.

Oxidation test coupons were approximately 20 mm long×10 mm wide×1.5 mm thick. Surfaces of the coupons were ground and polished with SiC paper to remove all cutting damage and surface contamination. A final 600 grit polish on all specimens was used to achieve a consistent surface finish. Exact dimensions of the samples were measured after final polish, and the total surface area and weights of the sample was calculated. Specimen mass change was measured using a Mettler Toledo model XP205 balance with an accuracy of ±0.04 mg.

Cyclic oxidation testing of the samples was performed using automated high-temperature oxidation rigs. Coupons were suspended on alumina rods to facilitate heating and cooling and were attached with a Pt—Rh wire using a small hole drilled in each specimen. Specimens were lowered into a vertical resistively heated tube furnace by an automated arm. The furnace was calibrated to ±5° C. of specified temperatures. A test temperature of 700° C. was used to compare the oxidation resistance of the alloys in the present study. The samples were tested in air+10±1 vol. % water vapor to approximate combustion gas chemistry. Distilled water was added by atomization into the flowing gas stream above its condensation temperature. The tests used flow rates of 500 cc/min air and 2.5 to 3 ml/h water. The specimens were held for 1 h in the furnace in the present study, then removed from the furnace and held in ambient air for 10 minutes to enable cooling to ≤30° C., thereby completing 1 cycle. Samples were tested for 1000 cycles (e.g., 1000 hours of cumulative time in the furnace) unless otherwise noted. Samples were removed from the furnace rig and weighed after 50 cycle intervals (or in some cases, after every 20 cycles at the beginning of testing) to measure specimen mass change (gain or loss) with time. Absolute mass change was then converted to mass change per surface area, and this quantity is reported as a function of time (number of cycles) at peak cycle temperature.

The disclosed alloys in the tempered martensitic condition exhibit significant improvements in the balance of thermal diffusivity, oxidation resistance, and elevated temperature strength over some existing commercial martensitic steels for next generation HDDE pistons or other applications, including, 403Cb+, 422, X10CrMoVNb9-1, 418 and X22CrMoV12-1. These aforementioned commercial alloys have been identified as having the highest potential for next generation HDDE piston alloys which are commercially available. Their thermal, mechanical, and oxidation properties have been compared to the disclosed alloys. The chemical composition of the disclosed alloys has been uniquely tailored to impart these improved properties relative to the commercially available martensitic steels.

An example of the martensitic microstructure of these alloy compositions is shown in FIG. 1. FIG. 1 is a scanning electron microscope (SEM) image of the microstructure after it was pre-heated and held at 700° C. for 1 hour, heated to 1070° C. and held for 15 m, then heated to 1150° C. and held for 30 minutes, followed by rapid quenching in oil, then tempered at 650° C. for 2 h, air cooled to room temperature, then tempered again at 650° C. for 2 h, followed by air cooling.

FIGS. 2A-B and FIGS. 3A-B show the mass change as a function of time during cyclic oxidation testing at 700° C. in air plus 10% water vapor for select invention alloys as compared to commercial (reference) alloys 422U, X10CrMoVNb9-1 (designated X10 in figure), and X22CrMoV12-1 (designated X22 in figure). Commercial alloys 422U, X10CrMoVNb9-1, and X22CrMoV12-1 all exhibit undesirably high mass gains during oxidation testing. In contrast, invention alloys 40, 41, 42, 43, 44, 46, 62, 63, 64, and 65 all exhibit relatively low mass change (<1 mg cm−2) that is consistent with protective oxidation behavior. In the invention alloys, Cr and Cu together in the appropriate proportions, work synergistically to improve the oxidation resistance. This is evidenced by the reduced oxidation mass gain of the developmental alloys containing Cu compared to the commercial alloys for which Cu is present at trace amounts.

FIG. 4 shows additional evidence for the positive influence of Cu on the oxidation behavior of the disclosed alloys by comparing the cyclic oxidation mass gain at 700° C. in air plus 30% water vapor as a function of time to 250 h for new alloy 62 and reference alloy 72 (which is similar in composition to alloy 62 but without intentional addition of Cu), and commercial alloy 422C. Alloy 62 exhibits very limited oxidation mass change and protective oxide behavior, whereas alloys 72 and 422C all exhibit significantly more mass gain (>1.5 mg cm−2). Table 5 shows the oxidation mass gain after 1000 h of cyclic oxidation testing in air+10% water vapor for select disclosed and commercial alloys. The table also lists the value of Cr+Cu (wt. %) for those alloys. The invention alloys possess a value of Cr+Cu that is between 12.0 and 16.8 wt. %. As indicated by Table 5 and FIG. 5, if the Cr+Cu amount is between 12 and 17 wt. %, the oxidation mass change is below 1 mg cm−2 and the oxidation resistance is improved significantly at the tested conditions.

TABLE 5 Mass change after cyclic oxidation testing at 700° C. for 1000 h in an atmosphere of air plus 10% water vapor. Mass Gain After 1000 cycles Alloy (1 h cycles) at 700° C. in Cr + Cu Designation air + 10% water vapor (mg cm−2) (wt. %) Invention Alloys 41 0.08 15.4 44 0.10 13.0 43 0.10 13.0 42 0.11 12.9 46 0.11 13.2 35 0.12 15.4 50 0.15 16.8 63 0.22 13.7 62 0.33 14.3 64 0.44 14.6 65 0.76 12.08 Reference Alloys Commercial 0.58 12.2 422(C) 61 8.20 11.73 Commercial 8.99 11.1 X22CrMoV12-1 Commercial 9.35 11.3 422(U) Commercial 13.48 10.6 403Cb+ 66 21.14 11.65 Commercial 21.71* 8.7 X10CrMoVNb9-1 *Data for X10CrMoVNb9-1 corresponds to 500 h of testing, after which sample was removed from test due to excessive mass gain.

FIG. 6 shows the values of thermal diffusivity at 600° C. for select disclosed and commercial alloys in the tempered martensitic condition after double tempering for 2 h at 650° C., air cooling to ambient temperature, tempering again for 2 h at 650° C., then air cooling to ambient temperature. The thermal diffusivity of invention alloys 35, 64, 45, 62, 43, 40, 41, 63, 42, 46, 50, 44, and 65 is superior to commercial alloys 403Cb+, X22CrMoV12-1, 422U, 422C, and 418. FIG. 7 shows the thermal diffusivity at 600° C. for select disclosed and commercial alloys plotted as a function of the oxidation mass gain after 1000 h at 700° C. in an atmosphere of air plus 10% water vapor. The invention alloys 35, 64, 62, 43, 41, 63, 42, 46, 50, 44, and 65 shown in FIG. 7 exhibit a superior combination of low oxidation mass gain (below 1 mg cm−2) and superior thermal diffusivity at 600° C., with thermal diffusivity ranging from 4.5 to 5 mm2 s−1, relative to commercial alloys 422C, X22CrMoV12-1, 422U, 403Cb+, and X10XrMoVNb9-1. The values of thermal diffusivity at 600° C. are listed in Table 4.

FIG. 8 shows the thermal diffusivity plotted as a function of Cr+Cu. A dashed line outlining example alloys with a range of 17≥Cr+Cu≥12.0 and thermal diffusivity from 5 mm2 s−1>Thermal Diffusivity≥4.4 mm2 s−1. FIG. 9 shows the Thermal Diffusivity Factor, 0.64Cu−0.75Cr−1.0Ni−4.2Si−1.1W−1.3Nb+4.9C−5.3Mn−2.7V (wt. %), plotted as a function of Cr+Cu (wt. %), with a dashed line outlining example alloys with a range of 17≥Cr+Cu≥12 and thermal diffusivity factor from −6≥0.64Cu−0.75Cr−1.0Ni−4.2Si−1.1W−1.3Nb+4.9C−5.3Mn−2.7V≥−14.5.

Table 6 and FIG. 10 show the ultimate tensile strength of select new and reference alloys at 25, 600, and 650° C. in the quenched and tempered condition.

TABLE 6 Tensile Strength of new alloys in the hardened condition at 25° C., 600° C., and 650° C. compared to reference alloys (alloys are grouped with invention alloys at the top and reference alloys at the bottom. Within each group, alloys are listed from highest 650° C. strength to lowest). Ultimate Tensile Strength (MPa) Designation 25° C. 600° C. 650° C. Invention Alloys 41 1102 583 496 42 1074 565 481 62 1055 568 480 65 1048 549 464 63 1043 550 459 64 1043 543 458 45 1134 555 455 43 1072 538 452 46 1060 537 449 44 1151 541 442 40 1040 533 440 47 1107 498 393 35 988 507 407 50 1022 514 406 73 897 440 361 74 945 477 398 80 879 455 81 884 476 379 Reference Alloys Commercial 1077 565 478 403Cb+ Commercial 961 536 449 X10CrMoVNb9-1 61 1034 529 431 Commercial 429 422(C) Commercial 1085 520 410 422(U) Commercial 1069 505 400 X22CrMoV12-1 72 880 430 339 66 951 453 338 Commercial 1009 428 306 418 steel 71 784 367 299

Table 7 and FIG. 10 show the ultimate tensile strength at 600° C. (Figure only) and 650° C. after quenching, tempering, and aging at 650° C. for 500 h to simulate long term thermal exposure. Alloys 62, 41, and 64 exhibit the highest ultimate tensile strengths of all invention or commercial reference alloys at 650° C. after quenching, tempering, and aging at 650° C. for 500 h.

TABLE 7 Tensile Strength at 650° C. of select new and commercial alloys in the hardened + aged condition, where samples were hardened in accordance with Table 2 and then subjected to an aging treatment at 650° C. for 500 h (alloys are grouped with new alloys at the top and commercial alloys at the bottom. Within each group, alloys are listed from highest 650° C. UTS after aging to lowest). Alloy UTS at 650° C. after aging Designation 500 h at 650° C. (MPa) Invention Alloys 62 385 41 383 64 382 46 334 63 330 40 327 65 308 44 294 42 292 43 283 45 277 47 272 Reference Alloys Commercial 373 403Cb+ Commercial 302 422(C) Commercial 288 422(U) Commercial 281 X10CrMoVNb9-1 Commercial 281 X22CrMoV12-1 61 274 66 266 Commercial 265 418 steel

Table 8 lists the values of thermal diffusivity at 600° C., strength at 600° C., and oxidation mass change after 1000 h at 700° C. in air plus 10% water vapor for developmental alloy 35, 41, 44, and 50 as well as reference alloys 403Cb+, X22CrMoV12-1, X10CrMoVNb9-1, 422U, 422C, 61 and 66. Table 9 indicates the performance improvement or decrease (%) in these parameters for all alloys relative to 403Cb+. Invention alloys 41, 44, 62, 64, and 65 all exhibit a 94-99% improvement (reduction in oxidation mass gain) than commercial alloy 403Cb+. Furthermore, despite substantially improved oxidation resistance, which historically has been obtained with a tradeoff in thermal diffusivity, alloy compositions 41, 44, 62, 64, and 65 all exhibit improvements in thermal diffusivity relative to 403Cb+as well, with the magnitude of the improvement ranging from 4-14%. Further illustrating the potential performance improvement of the alloy compositions described herein is the 2-3% improvement in elevated temperature UTS at 650° C. after thermal aging for invention alloys 41, 62, and 64 relative to 403Cb+. The performance of invention alloys 41, 62, and 64 reveals that significant and simultaneous performance improvements in the three criteria (thermal diffusivity, elevated temperature strength, and oxidation resistance) are possible with the alloying strategies disclosed herein . . .

TABLE 8 Thermal diffusivity at 600° C., ultimate Tensile Strength at 650° C. after aging for 500 h at 650° C. (MPa), and oxidation mass change after 1000 h of cyclic oxidation testing at 700° C. in air plus 10% water vapor for select disclosed ORNL alloys (41, 44, 62, 64, and 65) and reference alloys 403Cb+, X22CrMoV12-1, X10CrMoVNb9-1, 422U, 422C, 61, and 66. Ultimate Tensile Oxidation Mass Thermal Strength at 650° C. Change at Alloy Diffusivity after Aging for 500 700° C. after Designation (mm2 s−1) h at 650° C. (MPa) 1000 h (mg cm−2) Invention Alloys 41 4.7 383 0.1 44 4.9 294 0.1 62 4.6 385 0.3 64 4.6 382 0.4 65 5.0 308 0.8 Commercial Reference Alloys Commercial 4.4 373 13.5 403Cb+ Commercial 4.4 281 9.0 X22CrMoV12-1 Commercial 4.8 281 21.8* X10CrMoVNb9-1 Commercial 4.2 302 0.1 422C Commercial 4.3 288 9.4 422U 61 4.8 274 8.2 66 4.9 266 21.1 *Data for X10CrMoVNb9-1 corresponds to 500 h of testing, after which sample was removed from test due to excessive mass gain.

TABLE 9 Increase or decrease (%) in Thermal diffusivity at 600° C., ultimate Tensile Strength at 650° C. after aging for 500 h at 650° C. (MPa), and oxidation mass change after 1000 h of cyclic oxidation testing at 700° C. in air plus 10% water vapor for select disclosed ORNL alloys (41, 44, 62, 64, and 65) and reference alloys 403Cb+, X22CrMoV12-1, X10CrMoVNb9-1, 422U, 422C, 61, and 66. Change (%) in Ultimate Change (%) in Tensile Strength at Oxidation Change (%) in Thermal 650° C. after Aging Mass Change at 700° Diffusivity at 600° C. for 500 h at 650° C. C. after 1000 h Alloy Designation Relative to 403Cb+ Relative to 403Cb+ Relative to 403Cb+ 41 7.3 2.6 −99.4 44 11.9 −21.3 −99.3 62 4.5 3.1 −97.5 64 4.4 2.5 −96.7 65 13.6 −17.6 −94.4 Commercial 0.0 0.0 0.0 403Cb+ Commercial −0.6 −24.8 −33.4 X22CrMoV12-1 Commercial 8.8 −24.6 60.9 X10CrMoVNb9-1 Commercial −4.2 −19.0 −98.9 422C Commercial −2.9 −22.7 −30.6 422U 61 8.6 −26.6 −39.2 66 11.3 −28.8 56.8 *Data for X10CrMoVNb9-1 corresponds to 500 h of testing, after which sample was removed from test due to excessive mass gain.

The invention as shown in the drawings and described in detail herein disclose arrangements of elements of particular construction and configuration for illustrating preferred embodiments of structure and method of operation of the present invention. It is to be understood however, that elements of different construction and configuration and other arrangements thereof, other than those illustrated and described may be employed in accordance with the spirit of the invention, and such changes, alternations and modifications as would occur to those skilled in the art are considered to be within the scope of this invention as broadly defined in the appended claims. In addition, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

Claims

1. A steel alloy, comprising:

0.022 to 0.257 C;
0.01 to 0.085 N;
0.0 to 1.5 Ni;
0.1 to 0.7 Mn;
2.52 to 5.05 Cu;
8.67 to 14 Cr;
0.1 to 0.96 Si;
0.1 to 0.47 V;
0.4 to 2 Mo;
0 to 1.1 W;
0.0 to 0.5 Nb;
0.0 to 0.03 S;
0.0 to 0.03 P;
0.0 to 2 Co; and,
balance Fe.

2. The steel alloy of claim 1, comprising:

0.05 to 0.17 C;
0.01 to 0.085 N;
0.28 to 1.25 Ni;
0.2 to 0.6 Mn;
2.52 to 4.02 Cu;
9.28 to 13 Cr;
0.1 to 0.5 Si;
0.1 to 0.47 V;
0.4 to 1 Mo;
0.15 to 0.4 Nb;
0.0 to 0.03 S;
0.0 to 0.03 P; and,
balance Fe.

3. The steel alloy of claim 1, comprising in weight %:

0.068 to 0.13 C;
0.01 to 0.085 N;
0.28 to 1.002 Ni;
0.2 to 0.5 Mn;
2.52 to 4.02 Cu;
10.7 to 11.88 Cr;
0.2 to 0.5 Si;
0.2 to 0.4 V;
0.4 to 0.8 Mo;
0.21 to 0.4 Nb;
0.0 to 0.03 S;
0.0 to 0.03 P; and
balance Fe.

4. The steel alloy of claim 1, wherein 17≥Cr+Cu≥11.65.

5. The steel alloy of claim 1, wherein 17≥Cr+Cu≥12.0.

6. The steel alloy of claim 1, wherein 14.7≥Cu/(Ni+Mn)≥1.6.

7. The steel alloy of claim 1, wherein 1.8≥Ni+Mn≥0.1.

8. The steel alloy of claim 1, wherein −6≥0.64Cu−0.75Cr−1.0Ni−4.2Si−1.1W−1.3Nb+4.9C−5.3Mn−2.7V≥−14.5.

9. The steel alloy of claim 1, wherein the ultimate tensile strength at 650° C. after quenching and tempering is greater than 360 MPa.

10. The steel alloy of claim 1, wherein the ultimate tensile strength at 650° C. after quenching and tempering is greater than 435 MPa.

11. The steel alloy of claim 1, wherein the Ultimate Tensile Strength (MPa) at 650° C. after thermally soaking at 650° C. for 500 h is greater than 250 MPa.

12. The steel alloy of claim 1, wherein 8.3≥Cu/(Ni+Mn)≥1.6.

13. The steel alloy of claim 1, wherein the Ultimate Tensile Strength (MPa) at 650° C. after quenching, tempering, and thermally soaking at 650° C. for 500 h is greater than 305 MPa.

14. The steel alloy of claim 1, wherein the thermal diffusivity of the steel at 600° C. is greater than or equal to 4.4 mm2 s−1 in the quenched and tempered condition.

15. The steel alloy of claim 1, wherein 15.9≥Cr+Cu≥13.3.

16. The steel alloy of claim 1, wherein 7.02≥Cu/(Ni+Mn)≥1.9.

17. The steel alloy of claim 1, wherein the Ultimate Tensile Strength (MPa) at 650° C. after quenching, tempering, and thermally soaking at 650° C. for 500 h is greater than 320 MPa.

18. The steel alloy of claim 1, wherein the oxidation mass change after 1000h of cyclic oxidation testing (1 h cycles) at 700° C. in an atmosphere of air+10% water vapor is less than or equal to 2 mg cm−2 but greater than or equal to −2 mg cm−2.

19. The steel alloy of claim 1, wherein the oxidation mass change after 1000 h of cyclic oxidation testing (1 h cycles) at 700° C. in an atmosphere of air+10% water vapor is less than or equal to 1 mg cm−2 but greater than or equal to −1 mg cm−2.

20. The steel alloy of claim 1, wherein 2.85≥0.251Cr−9.74C≥1.7.

Patent History
Publication number: 20240344188
Type: Application
Filed: Apr 12, 2024
Publication Date: Oct 17, 2024
Inventors: Dean T. Pierce (Knoxville, TN), Govindarajan Muralidharan (Knoxville, TN)
Application Number: 18/634,803
Classifications
International Classification: C22C 38/54 (20060101); C22C 38/00 (20060101); C22C 38/02 (20060101); C22C 38/04 (20060101); C22C 38/42 (20060101); C22C 38/44 (20060101); C22C 38/46 (20060101); C22C 38/48 (20060101); C22C 38/50 (20060101); C22C 38/52 (20060101);