Method and steel component

Abstract

Method for heat treating a steel component (28, 36) comprising the steps of: a) carbonitriding the steel component (28, 36) at a temperature of 930-970 C., b) cooling the steel component (28, 36), d) re-heating the steel component (28, 36) to a temperature of 780-820 C. and d) quenching the steel component (28, 36). The method comprises the step of either e) performing a bainite transformation at a temperature just above the martensite formation temperature, transforming 25-99% of the austenite into bainite at the temperature and then increasing the temperature to speed up the transformation of the remaining austenite into bainite, or f) holding the steel component (28, 36) at an initial temperature (T.sub.1) above the initial martensite formation temperature (Ms), and lowering the initial temperature (T.sub.1) to a temperature (T.sub.2) that is below the initial martensite formation temperature (Ms) but above the actual martensite formation temperature during the bainite transformation.

Claims

1. A method for heat treating a steel component comprising the steps of: a) carbonitriding the steel component at a temperature of 930-970 C., b) cooling the steel component, c) re-heating the steel component to a temperature of 780-820 C., d) quenching the steel component, and e) holding the steel component at an initial temperature (T.sub.1) above the initial martensite formation temperature (Ms), and lowering the initial temperature (T.sub.1) to a temperature (T.sub.2) that is below the initial martensite formation temperature (Ms) but above the actual martensite formation temperature during the bainite transformation.

2. The method according to claim 1, wherein step e) further comprises subsequently raising the temperature from (T.sub.2) to a temperature (T.sub.3) that is above the initial martensite formation temperature (Ms) during the bainite transformation.

3. The method according to claim 2, wherein step e) further comprises the step of transforming at least 15-40% of the austenite into bainite at a temperature (T.sub.1) above the initial martensite formation temperature (Ms) before lowering the temperature (T.sub.1) to a temperature (T.sub.2) below the initial martensite formation temperature (Ms) but above the actual martensite formation temperature.

4. The method according to claim 2, wherein step e) further comprises the step of maintaining the temperature (T.sub.3) above the initial martensite formation temperature (Ms) until complete bainite transformation is achieved.

5. The method according to claim 4 wherein step a) further comprises carbonitriding the steel component at a temperature of 930-970 C. for 5-10 hours.

6. The method according to claim 5, wherein the steel component further comprises steel with a carbon content of 0.6 to 1.20 weight %.

7. The method according to claim 5, wherein the steel component further comprises steel with a carbon content of 0.2 to 0.6 weight %.

8. The method according to claim 7, wherein the steel comprises, by weight, max 20 ppm S and max 15 ppm O and includes sulphide inclusions and less than 5% of the sulphide inclusions contain encapsulated or embedded oxide inclusions.

9. The method according to claim 8, wherein the maximum length of the sulphide inclusions is 125 mm at a Reduced Variate equal to 3.

10. The method according to claim 9, wherein the steel comprises max 10 ppm O or max 8 ppm O.

11. The method according to claim 9, wherein all of the sulphide inclusions have an aspect ratio less than 3:1.

12. The method according to claim 11, wherein the steel comprises an element selected from the group: Ca, Mg, Te, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu.

13. The method according to claim 12, wherein the steel comprises, by weight, 10-30 ppm of said element.

14. The method according to claim 13, wherein the steel 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, and 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.

15. The method according to claim 14, wherein the steel component is provided with a carbonitrided layer having a thickness (d) of 0.3-1.2 mm whereby all of the carbides in the carbonitrided layer have a maximum longitudinal dimension of 0.2-0.3 mm.

16. The method according to claim 15, wherein the steel component is provided with a carbonitrided layer having a ratio (d:D) of depth (d) of the carbonitrided layer measured from the surface of the steel component to maximum transverse dimension (D) of the steel component of 1:4000 to 1:17,000 or more.

17. The method according to claim 16, wherein the steel component has a substantially bainitic structure and a hardness of at least 62 HRC.

18. A method for heat treating a steel component comprising the steps of: a) carbonitriding the steel component at a temperature of 930-970 C., b) after step a), cooling the steel component, c) after step b), re-heating the steel component to a temperature of 780-820 C., d) after step c), quenching the steel component, e) after step d) holding the steel component at an initial temperature (T.sub.1) above the initial martensite formation temperature (Ms), and lowering the initial temperature (T.sub.1) to a temperature (T.sub.2) that is below the initial martensite formation temperature (Ms) but above the actual martensite formation temperature during the bainite transformation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended figures where;

(2) FIG. 1 shows steps a)-d) of a method according to an embodiment of the present invention,

(3) FIG. 2 shows a bainite transformation method according to the prior art,

(4) FIG. 3 shows a bainite transformation according to step e) of a method according to an embodiment of the present invention,

(5) FIG. 4 shows a bainite transformation according to step e) of a method according to an embodiment of the present invention, p FIG. 5 shows a bainite transformation according to step f) of a method according to an embodiment of the present invention,

(6) FIGS. 6 & 7 show steel components according to embodiments of the invention, and

(7) FIG. 8 shows a flow chart of a method according to the present invention.

(8) 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.

DETAILED DESCRIPTION OF EMBODIMENTS

(9) FIG. 1 shows steps a)-d) of a method according to the present invention. The illustrated method comprises the steps of a) carbonitriding a steel component at a temperature of 930-970 C. for 5-10 hours. The process environment may be provided by the introduction of methane/propane/natural gas (for carbon) and ammonia (for nitrogen) into a furnace in the presence of a controlled carrier gas. By maintaining the proper ratios of the working gases, the component is provided with a thin carbonitrided layer of carbon- and nitrogen-rich steel.

(10) According to an embodiment of the invention the method includes supplying a higher concentration of ammonia at the beginning of the carbonitriding step a) to boost the carbonitriding process. For example, 9.5% ammonia may be used initially; this may be lowered to 6.5% ammonia and then 0%. 9.5% ammonia may be used for about 70% of the carbonitriding step a).

(11) The component is then cooled to a temperature below the A.sub.1 transformation temperature (step b)) and then re-heated to a temperature of 780-820 C. (step c)), i.e. a temperature higher than the A.sub.1 transformation temperature and lower than the carbonitriding temperature, and is subsequently quenched (step d)) to achieve the full case hardness. Quenching may be carried out in an oil or salt bath with bath temperatures selected to achieve the optimum properties with acceptable levels of dimensional change. Hot oil/salt bath quenching can be used to minimize distortion of intricate parts.

(12) FIG. 2 shows a plot of temperature versus log time for three conventional bainite transformation heat treatments i, ii, iii. Ms denotes the temperature at which martensite starts to form. Bs denotes the start of bainite transformation and Bf denotes the end of bainite transformation. Steel is firstly austenitized and then quenched. The steel is then isothermally tempered by heating at a temperature just above the martensite formation temperature (Ms).

(13) Conventionally, in order to obtain maximum hardness, a tempering temperature close to the initial martensite formation temperature (Ms) has been used (plot iii in FIG. 2). However, this results in a very long transformation time which is not economical. The transformation time may be reduced by increasing the temperature at which the steel is tempered (plot i in FIG. 2). However, this will reduce the hardness of the bainite formed.

(14) FIG. 3 shows a bainite transformation according to step e) of a method according to an embodiment of the present invention, in which a bainite transformation is performed on a steel component that has been subjected to steps a)-d) of a method according to an embodiment of the present invention, at a temperature just above the martensite formation temperature (Ms). 60-80% of the austenite is transformed into bainite at that temperature and then the temperature is increased to speed up the transformation of the remaining austenite into bainite. The dotted line in FIG. 3 shows a conventional isothermal bainite transformation which gives the same hardness but requires a significantly longer transformation time.

(15) FIG. 4 shows a bainite transformation according to step e) of a method according to an embodiment of the present invention, whereby the dotted line shows a conventional isothermal bainite transformation with the same transformation time but which gives a significantly lower hardness.

(16) FIG. 5 shows a bainite transformation according to step f) of a method according to an embodiment of the present invention in which a steel component, that has been subjected to steps a)-d) of a method according to an embodiment of the present invention, is held at an initial temperature (T.sub.1) above the initial martensite formation temperature (Ms). The initial temperature (T.sub.1) is then lowered to a temperature (T.sub.2) that is below the initial martensite formation temperature (Ms) but above the actual martensite formation temperature during the bainite transformation. According to an embodiment of the invention 50-90% of the austenite is transformed to bainite before increasing the temperature to T.sub.3 to speed up the transformation of the remaining austenite to bainite.

(17) After a steel component has been subjected to method steps a), b), c), d), and e), or method steps a), b), c), d), and f) the component is cooled to room temperature and may then be used in any application in which it is likely to be subjected to stress, strain, impact and/or wear under a normal operational cycle.

(18) FIG. 6 shows an example of a steel component according to an embodiment of the invention, namely a rolling element bearing 28 that may range in size from 10 mm diameter to a few meters diameter and have a load-carrying capacity from a few tens of grams to many thousands of tonnes. The bearing 28 according to the present invention may namely be of any size and have any load-carrying capacity. The bearing 28 has an inner ring 30 and an outer ring 32 and a set of rolling elements 34. The inner ring 30, the outer ring 32 and/or the rolling elements 34 of the rolling element bearing 28, and preferably at least part of the surface of all of the rolling contact parts of the rolling element bearing 28 may be subjected to a method according to the present invention.

(19) FIG. 7 shows a component 36, namely a shaft shown in cross section, according to an embodiment of the invention. The component 36 has been provided with a carbonitrided layer 38 on its outer surface using a method according to an embodiment of the invention. The depth of the carbonitrided layer 38 measured from the surface of the component 36 is d and the maximum transverse dimension of the component 36 (the diameter of the shaft in this case) is D. The ratio (d:D) of the thickness d of the carbonitrided layer 38 to the maximum transverse dimension D of the component 36 is 1:4000-17,000 or more.

(20) A steel component according to the present invention may be manufactured from steel having the following composition:

(21) 0.70-0.95 weight-% carbon

(22) 0.05-1.5 weight-% silicon

(23) 0.15-0.50 weight-% manganese

(24) 0.5-2.5 weight-% chromium

(25) 0.10-1.5 weight-% molybdenum

(26) max. 0.25 weight-% vanadium

(27) the remainder being Fe, and normally occurring impurities comprising 10-30 ppm Ca, max 20 ppm S and max 15 ppm O, preferably max 10 ppm O or most preferably max 8 ppm O.

(28) About 1% of the sulphide inclusions of such steel contains encapsulated or embedded oxide inclusions. On the contrary, in standard steel, about 80% of the steel's sulphide inclusions contain encapsulated or embedded oxide inclusions. It has been found that the fatigue strength (measured in rotating beam tests at 950 MPa) of a steel component according to the present invention is substantially higher than the fatigue strength of standard steel.

(29) FIG. 8 is a flow diagram which outlines the steps a)-f) of a method according to an embodiment of the present invention.

(30) Further modifications of the invention within the scope of the claims would be apparent to a skilled person.