ZA200607903B - Steel for mechanical parts, method for producing mechanical parts from said steel and the thus obtainable mechanical parts - Google Patents

Description

<n, - " @ CT : Steel for mechanical parts, method for producing mechanical parts from said s teel and the thus ob tainable mechanicaml parts

The invention relates to the field of st eel metallurgy and, more particularly, s teels for mechanical components, such as pinions.

Steels for gear manufacturing must have = high level of resistance to contact fatigue. Most of thie time, component s produced from these steels are subjected to a carburising -or carbonitriding treatment which is intended to provide them with sufficient surface hardness and mechanical strength whilst maintaining a high level of core strength owing, in particular, to a carbon content in the or-der of from only 0.10 to 0.30%. The ca rbon content of the carburised layer may be up to approximatel y 1%.

Various documents describe gear manufacturing steels which are intended to be caxburised. These include US-aA-5 518 685 , in which the contents of Si and Mn are maintained within relatively low limits (from 0.45 to 1% and from 0.40 to 0.70%, ’ respectively) in order to prevent intergramnular oxidation during the carburising operation. JP-A-4-21757 describes steels for gear manufa cturing which are in tended to be carburised using plasma or at reduced pres sure then shot blasted, and which can have contents of Si and Mn higher than t he previous steels. They have a high leve_l of resistance tom t he surface pressure acting on the pinion wshose life span is thus increased.

WO-A-03 012 156 proposess a steel for mecharnical components, such as pinions, the composition of which is: 0.12% £ Cc < )

® 0.30%; 0.8% < Si £ 1.5%; 1.0% < Mn < 1.6%; 0.4% < Cr < 1.6%;

Mo < 0.30%; Ni < 0. 6%; Al < 0.06%; Cu < 0. 30%; S £ 0.10%; P < 0.03%; Nb £ 0.050%. This steel has the adv-antage of minimising the operational plastic deforma tions of the component as a whole owing, in particular, to a judicious balance of the contents of silicon and man ganese. The carburising or the carbonitriding must pre ferably take place under non-oxidising conditions, for examples, at reduced pressure, so that the relatively high contesnts of silicon and manganese do not lead to problems of intergranular oxidation.

Generally, the carburising or carbonitridirig takes place at a temperature in the order of from 850 to 930°C. However, the current tendency is to attempt to carry outs this operation at higher temperatures (high-temperature carbutarising or carbonitriding) in the order of from 950 to 1050°C. This increase in the processing temperature either allows the length of the proces sing time to be reducecd with the same carburised depth or allows the carburised depth to be increased with the s ame length of processimg time. The manufacturer thus has the choice of being a ble to increase the productivity of the installation or inc rease the effectiveness of the products obtained.

However, the application of a high~temperat ure carburising or cabronitriding operation to the known steel s which have been described presents a number of problems. Fi _rstly, the high temperature may lead to an increase in poor ly controlled grains, which is detrimental to the mechanical properties of the component. Secondly, the carburising or carbonitriding is followed by quenching during which the component is subjected to deformations. These may require the component to be i remachined or, in the most extreme cases, czause it to be ! i

= x3 ® 3 oo NT rej ected. These problems are accentuated wehen the guenchirmg is carried out on a compoment which has ju st been subjected to .a carburising or carbomitriding operati on at high temperature and not at a more normal tempe rature.

The object of the inventien is to provide, for metallurgissts carrying out high-tempera-ture carburising or carbonitridimg of rmechanical components, in particular pi nions, a steel whieh overcomes the problesms mentioned abo ve whilst mairmtaining the required rmechanical proper ties, and which is also compatible with the carburising and c-arbonitriding opemations carried out at more normal tempe-eratures.

To t=his end, the subject-rmatter of the inveention is a stee 1 for mechanical components, characterised im that the compoosition thereof is, im percentages by wweight: - 0.19% £ C £ 0.25% ~ - 1.1% £ Mn £ 1.5%; - 0.8% S$ 81 ££ 1.2%; - 0.01% £5 <£ 0.09%; - trace levels £ P < 0.025%; - trace levels £ Ni £ 0.25%; - 1% £ Cr £ 1.4%; - 0.10% £ Mo £ 0.25%; - trace levels £ Cu £ 0.30%; - 0.010% < Al £ 0.04 5%; - 0.010% £ Nb £ 0.04 5%; - 0.0130% £ N £ 0.03 00%; - optionally trace levels < Bi < (O.10% and/or tr=ce levels £ Pb £ 0.12% and/ or trace levels £ Te < 0.015% and/or tr-ace levels < Se < 0.03 0% and/or trace levels <£ Ca £ 0.0050%;

i © — 4 - : the bamlance being iron and impurities resulting ®rom the

Production operation, the chemical composition being adjusted

SO tha t the mean values Ji,, Ji1n, Jism and Jzsm for five Jominy tests are such that: a = | Jum = Jn x 14/22 — Jo5n x 8/22 | £ 2.5 HRC; and

BH = J3n ~ Jism £ 9 HRC.

Preferably, the composition thereof is adju.sted so that

B = Jsn — Jism £ 8 HRC. referably, the compositi on thereof is: ~ 0.19% £ C £ 0.25%; - 1.2% £ Mn £ 1.5%; - 0.85% < 585i £ 1.2%; - 0.01% £ 5S £ 0.09%; — trace levels £ P £ O.025%; —- 0.08% £ Ni £ 0.25%; - 1.1% < Cr £ 1.4%; - 0.10% £ Mo £ 0.25%; ~ 0.06% < Cu £ 0.30%; - 0.010% Ss Al £ 0.045%; - 0.015% < Nb £ 0.045%; - 0.0130% £ N £ 0.0300%; optionally trace levels < Bi < 0.07% and/or trace levels =< Pb < 0.12% and/or trace levels < Te < 0.0 10% and/or trace l-evels < Se £ 0.020% and/o r trace levels < Ca <£ 0.045%, the balance being iron and impur ities resulting fr om the productDon operation.

Optimally, the composition thereof is: - 0.20% £ C £ 0.25%; ~- 1.21% £ Mn £ 1.45%; - 0.85% £ 81 £ 1.10%; - 0.01% £5 £ 0.08%; —- trace levels £ P £ 0_020%; - 0.08% < Ni £ 0.20%; i [

- 5 - ToL oT _ - 1.10% £ Cr <£ 1. 40%; - 0.11% £ Mo £ 0. 25%; - 0.08% < Cu £ 0. 30%; - 0.010% < Al £ 0.035%; ~ 0.025% £ Nb £ 0.040%; - 0.0130% £ N £ 0 .0220%:; optionally trace levels < Bi < 0.07% and/ or trace levels < Pb < 0.12% and/or trace levels < Te < 0.010% and/or tra ce levels < Se < 0.020% and/or trace levels < Ca < 0.045%, the balance being iron and dmpurities resulting from the production operation.

The subject-matter of the invention is also a m ethod for producing a mechanical component from carburiseed or carloonitrided steel, characterised in that a stesel of the abowe-mentioned type is used for this purpose, on which a machining operation, carburi sing operation or carbonitriding operation is carried out, thien a quenching operation.

The carburising or carbonitr iding is preferably carried out at aa temperature of from 950 to 1050°cC.

The subject-matter of the in-vention is also a st_eel mech anical component, such as a gear component, characterised in t hat it is produced using the above method.

As w ill be appreciated, the -invention is based on a precise adjustment of the content rariges of the main all oy elements, as well as the simultaneous Presence, in well-de _fined conteants, of aluminium, niobi um and nitrogen.

The desired effects are substantially of two types.

4 1 . - @ : Firstly, the selection of the contents of the main allomy elements is intended to achieve a Jomin y curve with no significantly marked inflection point. This condition a llows minimal deformations to be achieved dur ing the quenchin g operation. In this respect, the Carburi sing or the carbonitriding which is carried out at high temperature is, as has been mentioned, particularly demanding.

It should be noted th at the Jominy curve of a steel which is

Produced using a conv entional standardissed test characterises the quenchability of -the steel. It is pr-oduced by measur-ing the hardness of a cyl_indrical test piece= which has been quenched using a jet of water which spra ys one of the en_ds thereof, along the length of one of the generating lines thereof. The hardness is measured at sev eral distances x (in mm) from the sprayed end, and the corres ponding value is designated Jy. Jum refers to the mean val-ue obtained durimg five tests for measuring the hardness at distance x.

As disclosed in docume nt EP-A-0 890 653, to which the reader m ay refer for further details, the applicant demonstrated that a composition of the steel which produces a Jominy curve with no inflection poimt was advantageouss for producing gxreatly reduced deformations during the Ggruenching operati_on following a carburising or carbonitriding operation. This

Jominy curve with no inflection point is produced when th e values Jim, Jism, Josn aned Jism satisfy the Following rezlationships: = a= | Jim - Tym x 14/22 = J2sm Xx 8-22 | £ 2.5 HRC; = B= Jam - Jism £ 9 HRC, or preferably £ 8 HRC. i i

® oC

The compositiosn of the steel accordin.g to the present invention is t herefore adjusted so th.at this relationship is also produced in this instance.

The compositioen is also adjusted, in particular owing to the combined prese nce of aluminium, niobi um and nitrogen in defined conten ts, so that the size of the grains remainss controlled, ev en when the carburising or the carbonitricdding is carried out at high temperature.

Finally, the c omposition of the steel must of course provide the desired me chanical properties for the use of the component. The criteria to be monitor ed more particularly include the ca rburised depth (convent ionally defined as the depth at which the measured hardness is 550HV), the devi_ation of hardness be tween the surface and t he core of the carburised com ponent, which must be a s low as possible i_.n order to minim ise the deformations du ring quenching, anc3 the core hardness which must be high so t hat the component responds effec tively to stresses duri ng operation, and therefore has a high level of strength in terms of endur—ance and fatigue.

The invention will be better understo«wd from a reading of the following desc ription, given with reference to the appemmded drawing which shows the Jominy curves of four reference steels and three steels according to the invention.

The steel accozxding to the invention Dds intended primari._ly for producing rmechanical components which are subjected to high levels of stress, such as gear elements, and which are intended to be carburised or carbonitzxided (preferably a t low pressure or in a non-oxidising atmosphere to prevent

Le CA ® _g - ' oxidation of the most oxid isable elements), beoth at normal temperatures of approximat ely from 850 to 930°C and at high tempearatures in the order of from 950 to 1050*C. These components must have a high level of fatigue endurance, high stren:gth, and must be only slightly deformed during thermal treat ments, such as the quenching cperation following the carbu rising or carbonitrid-ing operation. It has the following compo sition (all the percentages are percentacges by weight).

The c arbon content thereof is between 0.19 ancl 0.25%. These contemts are normal for gear manufacturing steels.

Furtheermore, this range allows the contents of the other elememts to be adjusted, which allows the desi red shape to be producced for the Jominy curve. The minimum con_tent of 0.19% is fuzxxther justified by the core hardness which can be achiewred thereby after queraching. At more than 0.25%, there is a misk that the hardness will be too high t o preserve the desirable machinability for the steel. The pre ferred range is from O.20 to 0.25%.

The manganese content there of is between 1.1 amd 1.5%. The minimum value is justified by the production of the desired

Jominy= curve in conjunction with the contents of the other elemermts. At more than 1.5%, there is the risk of the appear-ance of segregations and also banding during the anneal ing operations. Furthermore, such a high content would bring about excessive corrosion of the heat-resistant coating of the steel ladle during the production operat-ion. It would not be desirable to further restrict this range of contents since producing the precise grade desired in the steelworks could Je excessively difficult. The preferred range is from 1.2 to 1.5%, preferably from 1.21 to 1.45%.

Claims (8)

1.. Steel for mechanical components, charact erised in that t he compositicn thereof is, in percentages by w eight:

- 0.19% £ C £ 0. 25%;

- 1.1% €£ Mn S 1. 5%;

- 0.8% < 51 £1. 2%;

- 0.01% £ S§ < 0. 09%; - trace levels < P < 0.025%; - trace levels £ Ni £ 0.25%; - 1% S Cr £ 1.4%;

- 0.10% £ Mo £ 0.25%; - trace levels £ Cu £ 0.30%;

- 0.010% £ Al £ 0.045%;

- 0.010% £ Nb £ 0.045%;

- 0.0130% £ N £ Q.0300%; - optionally trace levels £ Bi £ 0 .10% and/or trace levels £ Pb £ 0.12% amd/or trace levels S Te £ 0.015% and/or tra=ace levels £ Se =

0 .030% and/or trace lewels < Ca S$ 0.0050%; tke balance being iron and impurities resulting from the production operation, the chemical compositi on being adjusted so that the mean values Jin, Jiim, Jisa and Jzs n for five Jominwy tests are such that: o = | Jim — Jam X 14/22 —- Jas, Xx 8/22 | ££ 2.5 HRC; and B = Jn — Jism £ 9 HRC.

2.. Steel for mechanical components accorcding to claim 1, clmaracterised in that +the composition thereof is adjusted so that B = Jan — Jism £ 8 HRC.

o - 25 -

3. =Steel for mechanical components according to cl aim 1 or 2, characterised in that the composition thereof is:

- 0.19% £ C £ 0.25%;

- 1.2% <£ Mn £ 1.5%;

- 0.85% £ 585i £ 1.2%;

- 0.01% £ Ss £ 0.09%; - trace levels <= P < 0.025%;

- 0.08% < Ni £ 0.25%;

- 1.1% £ Cr £ 1.4%; ~- 0.10% £ Mo £ 0.25%;

- 0.06% £ Cu £ 0.30%;

- 0.010% < Al < 0.045%;

- 0.015% £ Nb £ 0.045%;

~ 0.0130% £ N £ 0.0300%; - optionally trace levels £ Bi < 0.07% and/or trace leve-1ls < Pb £ 0.12% and/or trace levels £ Te < 0.01 0% and/or trac e levels < Se < 0.020% and/or trace levels < Ca < 0.045%, the balance being iron and impurities resulting from the prod uction operation.

4. S teel for mechanical components according to cla im 3, char.acterised in that the composition thereof is:

~ 0.20% £ C £ 0.25%;

- 1.21% £ Mn £ 1.45%;

- 0.85% £ Si £ 1.10%;

- 0.01% £ S £ 0.08%: - trace levels < P £ 0.020%;

- 0.08% <£ Ni € 0.20%;

- 1.10% £ Cr £ 1.40%;

- 0.11% S$ Mo S$ 0.25%;

- 0.08% £ Cu £ 0.30%;

- 0.010% S$ Al £ 0.035%;

- 0.025% < Nb < 0.040%;

® TE

- 0.0130% SN £0.0220%; —- option ally trace levels < Bi < 0.07% and/or t race levels < Pb < 0.12% and/or trace levels < Te £ 0.010% an d/or trace levels < Se < 0.020% and/or trace levels < Ca < 0.0 45%, the balance being iron and impurities resulting from the production operati on.

5. Method for producing a mechanical component from carburised or carb onitrided steel, characterised in that a steel according to any one of claims 1 t« 4 is used for this purpose, on which a machining operation, carburising operation or carbomitriding operation is carried out, therm a quenching operation.

6. Method according to claim 5, characterised in that the carburising or carkoonitriding is carried out at a temperature of from 950 to 1050°C.

7. Steel mechanical component, character-ised in that it iss produced using the method according to claim 5 or 6.

8. Mechanical component according to claim 7, characterisead in that it is a gear component.