Cryogenic treatment of martensitic steel with mixed hardening
View Patent ↗The invention relates to a method for producing martensitic steel that comprises a content of other metals such that the steel can be hardened by an intermetallic compound and carbide precipitation, with an Al content of between 0.4% and 3%, comprising the following steps: (a) heating the entirety of the steel above its austenizing temperature, (b) cooling said steel approximately to ambient temperature, (c) placing said steel in a cryogenic medium. The temperature T 1 is substantially lower than the martensitic transformation temperature Mf, and the time t during which said steel is kept in said cryogenic medium at a temperature T 1 from the moment when the hottest part of the steel reaches a temperature lower than the martensitic transformation temperature Mf is at least equal to a non-zero time t 1 , the temperature T 1 (in ° C.) and the time t 1 (in hours) being linked by the equation T 1 =ƒ(t 1 ), the first derivative of the function ƒ relative to t, ƒ′(t), being positive, and the second derivative of ƒ relative to t, ƒ″(t), being negative.
1. A method for producing a martensitic steel, the method comprising:
(a) heating a steel to a first temperature above an austenizing temperature thereof,
(b) subsequently cooling the steel to a second temperature equal to an ambient temperature, and
(c) subsequently placing and keeping the steel in a cryogenic medium at a third temperature T 1 for a period of time t greater than a time t 1 and less than 5 hours,
wherein
the third temperature T 1 is less than a martensitic transformation end temperature M f of the steel, which is below 0° C.,
the period of time t in (c) is determined from a moment when an internal portion of the steel having a highest temperature following said cooling (b) reaches a temperature lower than M f ,
the third temperature T 1 in ° C. with a tolerance of +/− 5° C. and the time t 1 in hours with a tolerance of +/− 5% are related according to an equation T 1 =ƒ(t 1 ), where the function ƒ is given by
ƒ(t)=57.666×(1−1/(t 0.3 −0.14) 1.5 )−97.389
or by a temperature-translated curve relative to ƒ(t), and
the steel comprises Al in a content of from 0.4 wt % to 3 wt % and is capable of being hardened by an intermetallic compound and carbide precipitation.
2. The method of claim 1 , wherein the steel consists of:
0. 18 to 0.3 wt % of C,
5to 7 wt % of Co,
2to 5 wt % of Cr,
1to 2 wt % of Al,
1to 4 wt % of Mo+W/2,
traces to 0.3 wt % of V,
traces to 0.1 wt % of Nb,
traces to 50 ppm of B,
10.5 to 15 wt % of Ni with Ni ≥7+3.5 Al,
traces to 0.4 wt % of Si,
traces to 0.4 wt % of Mn,
traces to 500 ppm of Ca,
traces to 500 ppm of at least one rare earth metal,
traces to 500 ppm of Ti,
traces to 50 ppm of O if developed from molten metal or to 200 ppm of O if developed through powder metallurgy,
traces to 100 ppm of N,
traces to 50 ppm of S,
traces to 1 wt % of Cu,
traces to 200 ppm of P, and
a remainder of Fe.
3. The method of claim 2 , wherein
a content of C is from 0.200 wt % to 0.250 wt %,
a content of Ni is from 12.00 wt % to 14.00 wt %,
a content of Co is from 5.00 wt % to 7.00 wt %,
a content of Cr is from 2.5 wt % to 4.00 wt %,
a content of Al is from 1.30 wt % to 1.70 wt %, and
a content of Mo is from 1.00 wt % to 2.00 wt %.
4. The method of claim 1 , wherein the time t 1 is at least 1 hour.
5. The method of claim 1 , wherein said cooling (b) comprises quenching the steel in a medium with a drasticity of at least a drasticity of air.
6. The method of claim 1 , wherein (c) starts less than 70 hours after a surface temperature of the steel reaches 80° C.
7. A piece made from a martensitic steel obtained by the method of claim 1 , wherein a residual austenite level in the martensitic steel is less than 3wt %.
8. A turbomachine transmission shaft made from a martensitic steel obtained by the method of claim 1 , wherein a residual austenite level in the martensitic steel is less than 3 wt %.
9. A martensitic steel obtained by the method of claim 1 , wherein an average hardness of the martensitic steel is 575 Hv with a statistical minimum of 570 Hv and maximum of 579 Hv.
10. The method of claim 1 , wherein t 1 is greater than 2 hours.
11. The method of claim 1 , wherein t 1 is greater than 3 hours.
12. The method of claim 1 , wherein t 1 is greater than 4 hours.
13. The method of claim 1 , wherein a residual austenite level in the martensitic steel is less than 3 wt %.
14. The method of claim 1 , wherein a residual austenite level in the internal portion of the martensitic steel is less than 3 wt %.
15. The method of claim 14 ,
wherein the martensitic steel has an average hardness of 575 Hv with a statistical minimum of 570 Hv and maximum of 579 Hv.
16. The method of claim 1 , wherein an average hardness of the martensitic steel is 575 Hv with a statistical minimum of 570 Hv and maximum of 579 Hv.
17. The method of claim 1 , wherein the internal portion of the steel during (c) is a central region of the steel.
18. The method of claim 17 , wherein after the internal portion of the steel reaches a temperature lower than M f and before the time t i , a residual austenite level increases from a surface region to the internal portion.