Patent Application
20170081738
The present invention concerns a method for surface hardening at least one part of a surface of a metal component. The present invention also concerns a metal component that has been subjected to such a method.
Carburizing, carbonitriding and induction hardening are surface hardening treatments that may be used to produce a hard, wear-resistant layer (case) on the surface of a metal component.
Carburizing is a heat treatment process in which iron or steel is heated in the presence of another solid, liquid or gas material, which liberates carbon as it decomposes. The surface or case will have higher carbon content than the original material. When the iron or steel is cooled rapidly by quenching, the high carbon content surface becomes hard, while the core remains soft (i.e. ductile) and tough.
Carbonitriding is a metallurgical surface modification technique in which atoms of carbon and nitrogen diffuse interstitially into the metal, creating barriers to slip and increasing the hardness near the surface, typically in a layer that is 0.1 to 0.3 mm thick. Carbonitiriding can also be used to create carbides or nitrides, primarily to avoid or reduce grain growth and to reduce abrasive wear. Carbonitriding is usually carried out a temperature of 850-860° C.
Induction hardening is a heat treatment in which a metal component is heated to the ferrite/austenite transformation temperature or higher by induction heating and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the surface of the metal component. Induction hardening may be used to selectively harden areas of a mechanical component without affecting the properties of the component as a whole.
An object of the invention is to provide an improved method for surface hardening at least one part of a surface of a metal component.
This object is achieved by a method that provides the steps of a) enriching the at least one part of a surface of a metal component with carbon and/or nitrogen, and b) induction hardening the at least one part of the surface of the metal component.
It has been found that this combination of surface enrichment (step a)) and induction hardening (step b)) provides a metal component having increased surface hardness and increased compressive residual stresses, and thereby improved fatigue properties compared to the surface hardness of a metal component that has been subjected only to surface enrichment (only step a)) or only induction hardening (only step b)). Additionally, the method according to the present invention is faster than a surface hardening method using only case carburizing when hardening deep hardening depths, i.e. depths greater than 2 mm from the surface of a metal component.
A metal component that has been subjected to a method according to an embodiment of the present invention may contain a region that has only been induction hardened, but which has not been subjected to surface enrichment, and which may therefore have a lower carbon content than in a case carburized sample with the same hardening depth which results in reduced brittleness in this region.
It should be noted that the induction hardening step b) is preferably carried out (directly or indirectly) after the surface enrichment step a) since the re-hardening of the case that takes place during induction hardening results in reduced grain size and thereby improved fatigue properties.
According to an embodiment of the invention step a) includes either case carburizing or carbonitriding the at least one part of the surface of the metal component.
According to another embodiment of the invention the method provides the step of tempering the at least one part of the surface of the metal component in between the surface enrichment step a) and the induction hardening step b). Such intermediate tempering has been found to result in increased compressive residual stresses which increase the metal component's fatigue strength and service life since it is more difficult for cracks to initiate or propagate in a compressively stressed zone. Compressive stresses are namely beneficial in increasing resistance to fatigue failures, corrosion fatigue, stress corrosion cracking, hydrogen assisted cracking, fretting, galling and erosion caused by cavitation. Tempering after induction hardening can thereby counteract brittleness caused by the surface enrichment step.
According to a further embodiment of the invention the method provides the step of tempering the at least one part of the surface of the metal component after both of the steps a) and b) have been carried out, preferably directly or indirectly after the induction hardening step b). Such a final tempering step has been found to result in a decreased risk of cracking, a reduced amount of austenite, lower surface hardness and reduced compressive residual stresses.
According to an embodiment of the invention the method provides the step of deep cooling the at least one part of the surface of the metal component to below −20° C. after both of the steps a) and b) have been carried out, preferably after the induction hardening step b). Such deep cooling has been found to result in reduced retained austenite levels, increase compressive residual stresses and increased surface hardness.
According to another embodiment of the invention the surface enrichment step a) is followed by martensitic or bainitic quenching or cooling.
According to a further embodiment of the invention the induction hardening step b) is followed by martensitic or bainitic quenching.
According to an embodiment of the invention the metal component constitutes at least part of one of the following: a ball bearing, a roller bearing, a needle bearing, a tapered roller bearing, a spherical roller bearing, a toroidal roller bearing, a ball thrust bearing, a roller thrust bearing, a tapered roller thrust bearing, a wheel bearing, a hub bearing unit, a slewing bearing, a ball screw, or a component for an application in which it is subjected to alternating Hertzian stresses, such as rolling contact or combined rolling and sliding and/or an application that requires high wear resistance and/or increased fatigue and tensile strength. The metal component may include or constitute gear teeth, a cam, shaft, bearing, fastener, pin, automotive clutch plate, tool, or a die. The metal component may be used in automotive wind, marine, metal producing or other machine applications which require high wear resistance and/or increased fatigue and/or tensile strength.
According to another embodiment of the invention the metal component provides steel containing 0.5-5.0 weight-% Cr, 0.1-5.0 weight-% Mo and 0.1-1.1 weight-% C, the remainder being Fe and optionally any one or more of the following Si, Mn, Ni, and/or V, and normally occurring impurities.
According to a further embodiment of the invention the metal component provides one of the following steels: C56E2, 42CrMo4, 50CrMo4, 20NiCrMo7, 16MnCr5, 18NiCrMo14-6, 18NiCrMo7-6 a high carbon bearing steel grade, such as 100Cr6.
According to an embodiment of the invention the metal component has a case depth (i.e. a case hardening or carbonitriding depth) up to 1+Dw/30 mm where Dw is the maximum transverse dimension of the metal component in millimeters, a surface carbon content of 0.5-2.5 weight-% and/or a surface nitrogen content of 0-1 weight-%, and an induction hardening depth of up to 1.3*(1+Dw/30) mm after being subjected to the method.
According to another embodiment of the invention the metal component has residual stresses lower than −300 MPa at a depth of 0-0.5 mm below its surface after being subjected to the method.
The present invention also concerns a metal component that has a case depth up to 1+Dw/30 mm where Dw is the maximum transverse dimension of the metal component in millimeters, a surface carbon content of 0.5-2.5 weight-% and/or a surface nitrogen content of 0-1 weight-%, and an induction hardening depth of up to 1.3*(1+Dw/30) mm. Such a metal component may be provided using a method according to any of the embodiments of the invention.
According to an embodiment of the invention the metal component has residual stresses lower than −300 MPa at a depth of 0-0.5 mm below its surface after being subjected to the method.
According to another embodiment of the invention the metal component provides steel containing 0.5-5.0 weight-% Cr, 0.1-5.0 weight-% Mo and 0.1-1.1 weight-% C, the remainder being Fe and optionally any one or more of the following Si, Mn, Ni, and/or V, and normally occurring impurities.
According to a further embodiment of the invention the metal component provides one of the following steels: C56E2, 42CrMo4, 50CrMo4, 20NiCrMo7, 16MnCr5, 18NiCrMo14-6, 18NiCrMo7-6 a high carbon bearing steel grade, such as 100Cr6.
According to an embodiment of the invention the metal component constitutes at least part of one of the following: a ball bearing, a roller bearing, a needle bearing, a tapered roller bearing, a spherical roller bearing, a toroidal roller bearing, a ball thrust bearing, a roller thrust bearing, a tapered roller thrust bearing, a wheel bearing, a hub bearing unit, a slewing bearing, a ball screw, or a component for an application in which it is subjected to alternating Hertzian stresses, such as rolling contact or combined rolling and sliding and/or an application that requires high wear resistance and/or increased fatigue and tensile strength. The metal component may include or constitute gear teeth, a cam, shaft, bearing, fastener, pin, automotive clutch plate, tool, or a die.
The metal component may be used in automotive, wind, marine, metal producing or other machine applications which require high wear resistance and/or increased fatigue and/or tensile strength.
The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended schematic figures where;
It should be noted that the drawings have not been drawn to scale and that the dimensions of certain features have been exaggerated for the sake of clarity.
The surface enrichment step a) may include case carburizing the at least one part of the surface of the metal component followed by martensitic or bainitic quenching or cooling. Alternatively, the surface enrichment step a) may include carbonitriding the at least one part of the surface of the metal component followed by martensitic or bainitic quenching. Changing the microstructure of the surface of the metal component using such surface enrichment may improve it wear resistance, corrosion resistance, load bearing capacity, surface hardness, core hardness, compound layer thickness, abrasive wear, adhesive wear, and/or fatigue resistance and enhances its ability to relax stress concentration at the edges of any indentations in its surface.
The induction hardening step b) may also be followed by martensitic or bainitic quenching.
Optionally, the method provides the step of tempering the at least one part of the surface of the metal component in between the surface enrichment step a) and the induction hardening step b). Such intermediate tempering may be carried out in a furnace or by means of induction tempering. Intermediate tempering may be carried out for 4 hours at a temperature of 390° C. for example or for any other suitable time and at any other suitable temperature.
Optionally, the method provides the step of deep cooling the at least one part of the surface of the metal component to below −20° C. after both of the steps a) and b) have been carried out.
Optionally, the method provides the step of tempering the at least one part of the surface of the metal component after both of the steps a) and b) have been carried out. Such final tempering may be carried out in a furnace or by means of induction tempering. Final tempering may be carried out for 1 hour at a temperature of 160° C. for example or for any other suitable time and at any other suitable temperature.
A method according to an embodiment of the present invention may be used to provide a metal component that has a case depth up to 1+Dw/30 mm, where Dw is the maximum transverse dimension of the metal component in millimeters, for example the diameter of a rolling element, a surface carbon content of 0.5-2.5 weight-% or 0.5-1.5 weigh-%, and/or a surface nitrogen content of 0-1 weight-% or 0-0.4 weight-%, and an induction hardening depth of up to 1.3*(1+Dw/30) mm after being subjected to the method.
The metal component may provides steel containing 0.5-5.0 weight-% Cr, 0.1-5.0 weight-% Mo and 0.1-1.1 weight-% C, the remainder being Fe and optionally any one or more of the following Si, Mn, Ni, and/or V, and normally occurring impurities.
According to an embodiment of the invention the metal component provides steel containing 0.5-2.0 weight-% Cr, 0.1-0.5 weight-% Mo and 0.1-1.1 weight-% C the remainder being Fe and optionally any one or more of the following Si, Mn, Ni, and/or V, and normally occurring impurities.
According to another embodiment of the invention the metal component provides steel containing 0.5-0.7 weight-% C and less than 1 weight-% Mn, the remainder being Fe and optionally any one or more of the following Cr, Mo, Si, Ni, and/or V, and normally occurring impurities.
According to a further embodiment of the invention the metal component provides steel containing less than 0.2 weight-% C, 4.0-4.5 weigh-% Cr, 4.0-4.5 weight-% Mo, 3.0-4.0 weight-% Ni and 1.0-1.5 weight-% V, the remainder being Fe and optionally any one or more of the following Si, and/or Mn, and normally occurring impurities.
The metal component may provide one of the following steels: C56E2, 42CrMo4, 50CrMo4, 20NiCrMo7, 16MnCr5, 18NiCrMo14-6, 18NiCrMo7-6 a high carbon bearing steel grade, such as 100Cr6.
The surface of a metal component subjected to a method according to the present invention may be provided with a surface hardness of 700-1000 HV, and a core hardness of 200-550 HV depending on the grade of steel used.
According to an embodiment of the present invention the metal component has residual stresses lower than −300 MPa, lower than −400 MPa or lower than −500 MPa at a depth of 0-0.5 mm below its surface after being subjected to the method. The magnitude of residual stresses is strongly dependent on the induction hardening depth. If a smaller induction hardening depth is chosen, low residual stresses, i.e. lower than −300 MPa may be achieved.
Further modifications of the invention within the scope of the claims would be apparent to a skilled person.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1450792-5 | Jun 2014 | SE | national |
This is a United States National Stage Application claiming the benefit of International Application Number PCT/SE2015/050656 filed on Jun. 5, 2015, which claims the benefit of Swedish Patent Application 1450792-5 filed on Jun. 27, 2014, both of which are incorporated herein by reference in their entireties.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SE2015/050656 | 6/5/2015 | WO | 00 |
