Carburized parts, method for producing same and carburizing system
AssigneeKomatsu Ltd.
InventorsTakemori Takayama, Naoji Hamasaka
Life of this patent
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Abstract
A carburizing method is described which is capable of dispersing cementite grains in the surface of steel uniformly and finely as not to affect fatigue strength and capable of fining the crystal grains of austenite. To prevent cementite precipitation during high temperature carburization at 980° C. or more, steel contains Al in an amount in the range of 0.05≦[Al wt %]≦2.0 and Cr in an amount in the range of 0.3≦[Cr wt %]≦4.0, and the composition of the steel satisfies the requirement represented by 1.9≧−5.6[Si wt %]−7.2[Al wt %]+1.1[Mn wt %]+2.1[Cr wt %]−0.9[Ni wt %]+1.1[Mo wt %]+0.6[W wt %]+4.3[V wt %].
Claims17
A carburized part produced by carburizing a steel workpiece in an atmosphere having a carbon potential adjusted so that the surface carbon content of the workpiece becomes 1.2 to 2.0 wt %, preventing cementite precipitation at a surface layer of the workpiece during the carburization, and then cooling the workpiece to a temperature equal to or lower than Al transformation temperature and reheating the workpiece so that 5 to 20% by volume cementite having an average grain size of 1 μm or less disperses and precipitates in the carburized surface layer and the grains of said dispersed, precipitated cementite make austenite crystal grains present in the carburized layer finer, so that said austenite crystal grains have a grain size equal to or higher then ASTM grain size #9,
- the steel workpiece containing 0.2 to 2.0 wt % Al and having the composition satisfying the requirement described by 1.9≧−5.61(Si wt %)−7.2(Al wt %)+1.1(Mn wt %)+2.1(Cr wt %)−0.9(Ni wt %)+1.1(Mo wt %)+0.6(W wt %)+4.3(V wt %), when the amount of Cr is 3.5 wt % or less.
A carburized part by carburizing a steel workpiece in an atmosphere having a carbon potential of 1.2 wt % or more so as to disperse and precipitate, in a surface layer of the workpiece, 35% by volume or less special carbides Cr7C3 and V4C3 having an average grain size of 1 μm or less, cooling the workpiece to a temperature equal to or lower than Al transformation temperature and reheating the workpiece, and carbo-nitriding and/or nitriding the workpiece during the reheating process so that one or more kinds of fine nitrides/fine carbon nitrides containing at least AlN having an average grain size of 0.5 μm or less are dispersed and precipitated in addition to fine cementite and/or special carbides dispersed and precipitated in addition to fine cementite and/or special carbides dispersed and precipitated in addition to fine cementite and/or special carbides Cr7C3 and V4C3 austenite crystal grains in the surface layer of the workpiece are fined to have a grain size equal to or higher then ASTM grain size #9,
- the steel workpiece comprising, at least Al in an amount in the range of 0.2≦(Al wt %)≦2.0;
- Cr in an amount in the range of 3.5<(Cr wt %)≦15;
- and Si in an amount which satisfies the requirement represented by 1.0≦(Si wt %+Al wt %)≦2.5.
Dependencies are inferred from the claim text.