Carbonitriding low manganese medium carbon steel
View Patent ↗A method for processing a low manganese steel is more cost effective and improves residual stress, bending fatigue, and surface characteristics for driveline components. The low manganese steel comprises in combination, by weight, about 0.30-0.75% carbon (C) and 0.15-0.40% manganese (Mn), with the balance being essentially iron (Fe). The method for processing the low manganese steel includes carbonitriding the low manganese steel at temperatures between 1600° F. to 1750° F. for a time period of about three to six hours. The low manganese steel is subsequently quenched in a water based solution that is kept at room temperature. The process provides the low manganese steel with an irregular case profile with a core hardness of no more than 50 Rockwell C and a surface hardness of approximately 58-63 Rockwell C. Further, the process provides the low manganese steel with little or no intergranular oxidation or surface high temperature transformation product.
1. A method for processing low manganese steel comprising the steps of:
carbonitriding the low manganese steel having a composition comprising about 0.3-0.75 wt % of carbon, about 0.15-0.40 wt % of manganese, and a balance of iron at temperatures from about 1600° F. to 1750° F. for a time period of about three hours to six hours.
2. The method according to claim 1 including carbonitriding in an atmosphere having about 0.75 to 1.1% carbon (C) potential and 4 to 8% ammonia (NH 3 ).
3. The method according to claim 2 including carbonitriding in an atmosphere having about 0.8% carbon (C) and 5% ammonia (NH 3 ).
4. The method according to claim 1 including subsequently quenching the low manganese steel in a water based solution.
5. The method according to claim 4 wherein the water based solution is at room temperature.
6. The method according to claim 1 including carbonitriding the low manganese steel at a temperature of about 1600° F. for a time period of about three hours to six hours in an atmosphere having about 0.75 to 1.1% carbon (C) potential and 4 to 8% ammonia (NH 3 ).
7. The method according to claim 1 including carburizing the low manganese steel at a temperature of about 1750° F. for a time period of about two to four hours in a first atmosphere having about 0.75 to 1.1% carbon (C) potential and subsequently carbonitriding the low manganese steel at a temperature of about 1600° F. for a time period of about one to three hours in a second atmosphere having about 0.75 to 1.1% carbon (C) potential and 4 to 8% ammonia (NH 3 ).
8. The method according to claim 7 wherein the first atmosphere has about 1% carbon (C) and the second atmosphere has about 0.8% carbon (C) and about 5% ammonia (NH 3 ).
9. The method according to claim 1 including processing the low manganese steel to have an irregular case profile.
10. The method according to claim 1 including processing the low manganese steel to have an effective case depth of about 0.45 to 0.80 inches at a gear tooth root.
11. The method according to claim 1 including processing the low manganese steel to have a core hardness of about 50 Rockwell C.
12. The method according to claim 11 including processing the low manganese steel to have a surface hardness within about 58 to about 63 Rockwell C.
13. The method according to claim 1 wherein the alloy composition has about 0.3 to 0.5% carbon (C).
14. The method according to claim 13 wherein the alloy composition has no more than about 0.04% aluminum (Al).
15. The method according to claim 13 wherein the alloy composition has no more than about 0.035% phosphorous (P).
16. The method according to claim 13 wherein the alloy composition has no more than about 0.025% sulfur (S).
17. The method according to claim 13 wherein the alloy composition has no more than about 0.15% chromium (Cr).
18. The method according to claim 13 wherein the alloy composition has no more than about 0.08% molybdenum (Mo).
19. The method according to claim 13 wherein the alloy composition has no more than about 0.18% silicon (Si).
20. The method according to claim 13 wherein the alloy composition has no more than about 0.04% aluminum (Al), no more than about 0.035% phosphorous (P), no more than about 0.025% sulfur (S), no more than about 0.15% chromium (Cr), no more than about 0.18% silicon (Si), and no more than about 0.08% molybdenum (Mo).
21. The method according to claim 13 including forming the alloy composition into a driveline component for a vehicle prior to carbonitriding.
22. The method according to claim 21 wherein the driveline component is a gear.
23. The method according to claim 21 wherein the driveline component is a shaft.
24. The method according to claim 1 wherein the alloy composition has about 0.32 to 0.40% carbon (C).
25. The method according to claim 24 wherein the alloy composition has no more than about 0.04% aluminum (Al).
26. The method according to claim 25 wherein the alloy composition has no more than about 0.035% phosphorous (P), no more than about 0.025% sulfur (S), no more than about 0.15% chromium (Cr), no more than about 0.18% silicon (Si), and no more than about 0.08% molybdenum (Mo).
27. The method according to claim 26 including forming the alloy composition as a gear for a vehicle driveline component prior to carbonitriding.
28. The method according to claim 27 including providing the gear with a root case depth of about 0.045 to 0.080 inches.
29. The method according to claim 27 having about 0.38% carbon (C), 0.23% manganese (Mn), 0.012% phosphorous (P), 0.010% sulfur (S), 0.04% silicon (Si), 0.07% chromium (Cr), 0.02% molybdenum (Mo), 0.20% copper (Cu), and 0.025% aluminum (Al), the balance being essentially iron (Fe).
30. The method according to claim 1 wherein the alloy composition has about 0.45 to 0.50% carbon (C).
31. The method according to claim 30 wherein the alloy composition has no more than about 0.04% aluminum (Al).
32. The method according to claim 31 wherein the alloy composition has no more than about 0.035% phosphorous (P), no more than about 0.025% sulfur (S), no more than about 0.15% chromium (Cr), no more than about 0.18% silicon (Si), and no more than about 0.08% molybdenum (Mo).
33. The method according to claim 32 including forming the alloy composition as a shaft for a vehicle driveline prior to carbonitriding.
34. The method according to claim 33 including providing the shaft with a surface hardness within about 58 to about 63 Rockwell C.
35. The method according to claim 33 wherein the alloy composition has about 0.46% carbon (C), 0.28% manganese (Mn, 0.020% phosphorous (P), 0.010% sulfur (S), 0.10% silicon (Si), 0.08% chromium (Cr), 0.02% molybdenum (Mo), 0.20% copper (Cu), and 0.025% aluminum (Al), the balance being essentially iron (Fe).
36. A driveline component comprising:
a core having a steel composition including about 0.3-0.75 wt % of carbon, about 0.15-0.40 wt % of manganese, and a balance of iron; and
a case at least partially surrounding the core, the case having a non-uniform case depth extending between an outer surface of the case and the core.
37. The driveline component as recited in claim 36 , wherein the core includes a core hardness of about 50 Rockwell C and the case includes a case hardness of about 58-63 Rockwell C.
38. The driveline component as recited in claim 36 , wherein the case includes a carbon concentration gradient and a nitrogen concentration gradient between the outer surface and the core.