Metal treatment – Stock – Carburized or nitrided
Reexamination Certificate
2001-02-26
2004-03-02
Ip, Sikyin (Department: 1742)
Metal treatment
Stock
Carburized or nitrided
C148S233000
Reexamination Certificate
active
06699333
ABSTRACT:
The present invention relates to a carburizing steel composition, to parts formed from said steel, and to a process for producing parts formed from said steel.
Carburizing is a thermochemical surface treatment which generally produces parts combining good core ductility with a “case-hardened” carburized surface that is hard and resistant to wear.
Many applications require a steel with a good resistance to softening at working temperatures. Examples that can be cited are gear wheels, bearings and transmission shafts for helicopters or for vehicles for motor racing, gear wheels, camshafts and other parts used in engine distribution systems, fuel injectors and compressors.
The following particular carburizing steels are routinely used for such applications: 17CrNiMo6, 16NiCr6, 14NiCr12, 10NiCrMo13, 16NiCrMo13 or 17NiCrMo17. Such steels can be used up to working temperatures of close to 130° C., but the carburized layer has neither a resistance to softening nor an elevated temperature hardness sufficient for working temperatures exceeding 190° C.
U.S. Pat. No. 3,713,905, granted to T. V. Philip and R. L. Vedder on Jan. 30
th
, 1973, describes the properties obtained for a steel with the following chemical composition as a percentage by weight:
0.07%-0.8% of C;
at most 1% of Mn;
0.5%-2% of Si;
0.5%-1.5% of Cr;
2%-5% of Ni;
0.65%-4% of Cu;
0.25%-1.5% of Mo;
at most 0.5% of V;
the complement being iron.
The tensile strength and the impact strength obtained with that steel are compatible with the envisaged applications, but the tempering properties and the elevated temperature hardness of the carburized layer are insufficient for the applications cited above and for working temperatures of up to 220° C.
U.S. Pat. No. 4,157,258, granted to T. V. Philip and R. L. Vedder on Jun. 5
th
1979, describes a steel with the following chemical composition as a percentage by weight:
0.06%-0.16% of C;
0.2%-0.7% of Mn;
0.5%-1.5% of Si;
0.5%-1.5% of Cr;
1.5%-3% of Ni;
1%-4% of Cu;
2.5%-4% of Mo;
≦0.4% of V;
≦0.05% of P;
≦0.05% of S;
≦0.03% of N;
≦0.25% of Al;
≦0.25% of Nb;
≦0.25% of Ti;
≦0.25% of Zr;
≦0.25% of Ca
the complement being iron.
The compromise between tensile strength and impact strength for that steel is good. The carburized layer allows a tempering temperature of up to about 260° C. The maximum working temperature is about 230° C.
However, none of the prior art carburizing steel compositions can allow a tempering temperature for the carburized layer of up to 350° C. to be used, nor do they provide good elevated temperature hardness for working temperatures of up to 280° C. while preserving satisfactory core characteristics.
There is currently a need for such steels in a number of fields. As an example, regarding the manufacture of gear parts for helicopters, regulations require that a helicopter must be capable of functioning for thirty minutes after losing oil from its transmission following an incident. That requirement assumes that the materials used to manufacture the gears have been tempered at a minimum temperature of about 280° C.
In the field of engines, designers tend to increase the working temperature of engine parts and its connected equipment such as gearboxes, in order to increase yields and/or to simplify heat extraction circuits. Depending on the location of the parts in this equipment, working temperatures can reach 280° C., imposing a minimum tempering temperature of 330° C. to guarantee that properties are stable during use.
The present invention aims to provide a carburizing steel composition that has all of the characteristics mentioned above.
In a first aspect, the invention provides a carburizing steel composition comprising, by weight:
0.06% to 0.18% of C;
0.5% to 1.5% of Si;
0.2% to 1.5% of Cr;
1% to 3.5% of Ni;
1.1% to 3.5% of Mo;
and, if appropriate:
at most 1.6% of Mn; and/or
at most 0.4% of V; and/or
at most 2% of Cu; and/or
at most 4% of Co;
the complement being constituted by iron and residual impurities;
the weight contents of Ni, Mn, Cu, Co, Cr, Mo and V in said composition, expressed by weight, satisfying the following relationships:
2.5 ≦Ni+Mn+1.5Cu+0.5Co≦5 (1)
2.4≦Cr+Mo+V≦3.7 (2).
Preferably, the sulfur content is limited to 0.010% and the phosphorous content is limited to 0.020% by weight, for applications in the upper part of the range, but higher contents are acceptable for other applications, provided that they do not cause a reduction in the ductility, toughness and fatigue strength properties of the steel.
The amount of elements such as aluminum, cerium, titanium, zirconium, calcium or niobium, which act either to deoxidize or to refine grain size, is preferably limited to 0.1% by weight each.
Regarding the principal elements of the composition, in general it has been shown that low carbon, silicon, molybdenum, chromium, and vanadium contents, and high manganese, nickel, cobalt, and copper contents can improve the ductility and toughness of the steel.
In contrast, high carbon, silicon, molybdenum, chromium, and vanadium contents and low manganese, nickel, cobalt, and copper contents can improve the tempering strength of the steel.
The essential role of carbon is to contribute to producing hardness, tensile strength, and hardenability. For carbon contents of less than 0.06% by weight, the hardness and tensile strength obtained in the core of carburized and treated parts are insufficient.
In practice, the desired minimum tensile strength is about 1000 MPa, i.e., about 320 VH (Vickers hardness). The higher the carbon content, the greater the hardness, tensile strength and hardenability but, at the same time, the impact strength and toughness decrease. For this reason, the carbon content is limited to a maximum of 0.18% by weight.
The most important range for the compromise between tensile strength and toughness is 0.09%-0.16% by weight of carbon. However, the ranges 0.06%-0.12% and 0.12%-0.18% are also of interest for applications requiring different core hardnesses.
Silicon provides a major contribution to the tempering strength of this steel and its minimum content is 0.5% by weight. In order to avoid the formation of delta ferrite and to retain sufficient toughness, the silicon content is limited to a maximum of 1.5% by weight. The optimum range is 0.7%-1.3% by weight, but the range 1.3%-1.5% is also of interest.
Chromium contributes to core hardenability and to good tempering strength of the carburized layer, and its minimum content is 0.2% by weight. To avoid embrittlement of the carburized layer by an excess of interlaced carbides, the chromium content must be limited to a maximum of 1.5% by weight. The optimum range is 0.5%-1.2%, but ranges of 0.2%-0.8% and 0.8%-1.5% are also of interest.
The role of molybdenum is identical to that of chromium, and it can keep the elevated temperature hardness high, in particular by forming intragranular carbides in the carburized layer. Its minimum content is 1.1% by weight. However, its embrittling effect on this steel limits its maximum content to 3.5% by weight. The optimum range is 1.5%-2.5%, but ranges of 1.1%-2.3% and 2.3%-3.5% are also of interest.
Vanadium contributes to limiting enlargement of the grain during the carburizing cycles and treatment cycles used. Because of its embrittling effect and its influence on ferrite formation, its content must be limited to a maximum value of 0.4% by weight. The optimum range is 0.15%-0.35%, but ranges of 0.05%-0.25% and 0.25%-0.4% are also of interest.
Manganese, nickel and copper are gamma-forming elements necessary for equilibrating the chemical composition, avoiding ferrite formation and limiting the temperature of the &agr;/&ggr; transformation points. They also provide a major contribution to increasing hardenability, impact strength and toughness but in too high a content, they deteriorate the tempering strength, the elevated temperature hardness and the wear resistance and increase the quantity of residual austenite in the carburized layer.
For these reasons, t
Amster Rothstein & Ebenstein
Aubert & Duval
Ip Sikyin
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