Title of Invention

A PROCESS FOR MANUFACTURING A MECHANICAL STEEL PART AND A STEEL FOR ITS MANUFACTURE

Abstract Process for manufacturing a mechanical component made of steel, in which a blank of the steel component is manufactured and a high-temperature carburizing or carbonitriding treatment of at least part of the surface of the blank of the component is carried out, the chemical composition of the steel of which the component is composed comprises, by weight: 0.15% &#8804; C &#8804; 0.35%; 0% &#8804; Si &#8804; 0.6%; 0% &#8804; Mn + Cr + Ni + Mo &#8804; 5%; 0% &#8804; Al :S 0.1%; 0% &#8804; Cu &#8804; 0.5%; 0% &#8804; Ti &#8804; 0.05%; 0.004% < N &#8804; 0.02%; 0% &#8804; S &#8804; 0.15%; P &#8804;, 0.03%; optionally, up to 0.02% tellurium, up to 0.04% selenium, up to 0.07% lead, up to 0.005% calcium, the balance being iron and impurities resulting from the smelting; the chemical composition being adjusted so that the Jominy curve of the steel is such that: 45 HRC &#8804; J<sub>3</sub> &#8804; 50 HRC; 39 HRC &#8804; J11 < 47 HRC; 31 HRC &#8804; J<sub>25</sub> &#8804; 40 HRC; the average values J<sub>3m</sub>, J<sub>11m</sub> and J<sub>25m</sub> of five Jominy tests are such that: J<sub>11m</sub> - J<sub>3m</sub>, X 14/22 - J<sub>25m</sub> X 8/22| &#8804; 2.5 HRC and J<sub>3m</sub> - J<sub>15m</sub> &#8804; 9 HRC. Carburizing or carbonitriding steel for manufacturing this component. Figure for the abstract: none.
Full Text

The present invention relates to the manufacture of steel components, at least part of the surface of which is hardened by carburizing or carbonitriding, followed by quenching in oil or gas.
Many mechanical components made of steel, such as, for example, gears, are surface-hardened by carburizing or carbonitriding. To do this, the components are maintained in a atmosphere rich in carbon or in carbon and nitrogen, at a temperature greater than 900°C for several hours, so as to enrich the steel with carbon or with carbon and nitrogen over a certain depth beneath the surface by diffusion of these elements from the surface, and then the components are quenched in cold, warm or hot oil, or in gas, so as to harden the surface. The carburizing or carbonitriding operation may also be carried out at a temperature greater than 1000°C - it is then said to be a high-temperature carburizing or carbonitriding operation.
In order to manufacture such components, steels containing from 0.15% to 0.35% carbon, alloyed with chromium or alloyed with chromium and molybdenum, or else alloyed with chromium and manganese, are used. This technique, which makes it possible to obtain a high hardness at the surface and near the latter, and which also makes it possible to obtain good mechanical properties in the core of the components, has the disadvantage, however, of generating distortions which may lead to the components being scrapped or requiring to undergo expensive additional machining operations.
The object of the present invention is to remedy this drawback by providing a means for manufacturing steel components at least part of the

surface of which is hardened by carburizing or by carbonitriding, especially at high temperature.
For this purpose, the subject of the invention is a process for manufacturing a mechanical component made of steel, in which a blank of the steel component is manufactured and a carburizing or carbonitriding treatment, especially at high temperature, of at least part of the surface of the blank of the component is carried out. According to this process, the chemical composition of the steel of which the component is

optionally, up to 0.02% tellurium, up to 0.04% selenium, up to 0.07% lead, up to 0.005% calcium, the balance being iron and impurities resulting from the smelting. In addition, the chemical composition of the steel is adjusted so that the Jominy curve of the steel

and:
10 X (J7m - Jl5m)/(8 X (J15m - J25m) ) 2.
Preferably, the chemical composition of the steel is such that:


Preferably, the nitrogen content of the steel is between 0.004% and 0.02%, and the steel may contain from 0% to 0.05% titanium.
The invention also relates to a carburizing steel, the chemical composition of which is that which has just been mentioned; this carburizing steel having a Jominy curve such that:

the average values J3m, J11m, J15m and J25sm of five Jominy tests being such that:
J11m - J3m X 14/22 - J25m X 8/221 J3m - J15m 9 HRC, and preferably
Preferably, the average values J7m/ J11m/ J15m and J25m of five Jominy tests are such that at least one of the following conditions is satisfied:
10 X (J7B - Juni)/(4 X (Ji5m " JaSm) ) 2.15
and:
10 X (J7„ - Jl5m)/(8 X (Jisn. - JaSn.) ) 2.
The invention will now be described in greater detail, but in a non-limiting manner, and illustrated by the examples.
The inventors have recently and unexpectedly discovered that the distortions generated by the quenching which is carried out at the end of the carburizing treatment or at the end of the carbonitriding treatment could be considerably reduced, or even eliminated, as long as, in order to manufacture the component, a steel is used whose Jominy curve has practically no inflection points, unlike the Jominy curves of the steels normally used for this purpose. More specifically, they have discovered that it was desirable to use a steel containing;
- from 0.15% to 0.35% carbon in order for the steel to be easily machinable and in order to obtain a sufficient toughness in the non-carburized or non-carbonitrided parts of the component;
- up to 0.6% silicon in order to ensure sufficient deoxidation of the steel;
- alloying elements, such as manganese, chromium, molybdenum and nickel, in amounts such as their sum remains less than 5% so as to provide sufficient hardenability, 'to adjust the shape of the Jominy curve and to adjust the mechanical properties of the component both in the core and in the carburized or carbonitrided regions;
- up to 0.1% aluminium in order to complete the deoxidation and to control the grain size;
- less than 0.5% copper, which is regarded as an impurity tending to decrease the ductility and toughness of the non-carburized or non-carbonitrided regions;

- optionally, from 0% to 0.05% titanium in order to form hardening nitrides;
- preferably, the nitrogen content, which element is always present and which reacts with aluminium or titanium in order to form nitrides, must be between 0.004% and 0.02%;
- up to 0.15% sulphur in order to improve the machinability; and
- less than 0.03% phosphorus, which is an
impurity having an unfavourable effect on the ductility
and toughness.
The steel may furthermore contain up to 0.02% tellurium, up to 0.04% selenium, up to 0.07% lead and up to 0.005% calcium in order to improve the machinability. The balance of the composition is iron and impurities resulting from the smelting.
The chemical composition is adjusted so that the Jominy curve of the steel is such that:
45 HRC Ja 50 HRC
39 HRC <. jn hrc> 31 HRC J25 40 HRC and so that the average values Jso, Jvm, Jum? Jism and Jasm of five Jominy tests are such that:
Uiim - Jam X 14/22 - Jzsni X 8/22 1 2.5 HRC and,
Jsm - Ji5m 9 HRC, and preferably 8 HRC. The Jominy curve is a curve which characterizes hardenability of the steel. It is obtained by measuring the hardness along a generatrix of a cylindrical test piece quenched by a water jet sprayed onto one of its ends. The hardness measured at a distance of x mm from the sprayed end is called Jx. This test is well known to those skilled in the art. However, there is naturally quite a scatter in such a curve. This is why the shape of the Jominy curve is characterized here, on the one hand, by ranges of values for the points J3, Jn and J25 and by the relationship : J11n - Jan x 14/22 J25m X 8/22| 2.5 HRC, which involves the averages of five different tests carried out on the same steel.

More specifically, 5 identical tests are carried out in succession; for each test, at least the values of J3/ J?/ J11/ J15 and J25/ are measured, five values for Jar J7, J11, J15, and J25, are thus obtained and the average Jjon of the five values is calculated for each point Jx. In this relationship, the vertical bars are the symbol known per se for the absolute value. The relationship itself, combined with the relationship JSB - Jism 9 HRC or 8 HRC, expresses the fact that the Jominy curve does not have pronounced inflection points.
The desired shape of the Jominy curve may be refined by requiring that it satisfy at least one of the following relationships:
10 X (J7m - JlUr.)/(4 X (Jis» - JaSm) ) 2 . 15
and:
10 X (J7„ - JlSm)/{8 X (Ji5m " J25m) ) 2.
Such Jominy curves may be obtained, in particular, with a steel in accordance with what has just been defined, but the chemical composition of which more specifically comprises, by weight:
- from 0.2% to 0.26% and preferably from 0.21% to 0,25% carbon, in order not to have an excessively high hardness in the components before carburizing or carbonitriding and in order to achieve a good carburizability or carbonitridability;
- from 1% to 1.6% and preferably from 1.1% to 1.5% manganese in order to obtain good internal soundness, in order to fix the sulphur and, in combination with chromium and molybdenum, to adjust the hardenability so as to obtain a satisfactory Jominy curve;
- from 0.05% to 0.5% and preferably from 0.1% to 0.45% silicon;
- from 0.4% to 1.5% chromium, and preferably from 0.9% to 1.4%, in order to harden the carburized or carbonitrided layer and, in combination with manganese and molybdenum, to adjust the hardenability so as to obtain a satisfactory Jominy curve;

- from 0.08% to 0.27% and preferably from 0.09%
to 0.26% molybdenum, in order to harden the carburized
or carbonitrided layer, to contribute to increasing the
resistance to oxidation and, in combination with
manganese and chromium, to adjust the hardenability so
as to obtain a satisfactory Jominy curve; the lower
limit corresponds to a minimum value of the molybdenum
content in order for this element to have a significant
effect; and
- from 0% to 0.6% nickel in order to improve
the impact strength of the component.
Preferably, the copper content remains less than 0.3%, the sulphur content is between 0.02% and 0.1% and, better still, remains less than 0.09%, and the aluminium content is between 0.003% and 0.06% and preferably lies between 0.005% and 0.05%.
As indicated above, the steel may furthermore contain one or more elements taken from tellurium, selenium, lead and calcium.
In order to manufacture a component according to the invention, a component blank is manufactured from a steel according to the invention, which blank is carburized or carbonitrided at high temperature and quenched in oil or gas, it being possible for the oil to be cold, warm or hot. The component blank may be manufactured, for example, by forging and by machining.
By way of example, components were produced from six steels according to the invention, the chemical compositions of which were:



After carburizing at 995°C for 10 hours and quenching in warm oil at 98°C, the components exhibited no distortions requiring additional machining. Moreover, the carburizing operation was characterized by a carbon content of 0.94% at 0.1 mm beneath the surface.
By way of comparison, identical components are produced, under the same conditions, from steel of the 27MC5, 29MC5, 27MC5u, 27MC5r, 27CD4u, 30M5 and 20CD4 type according to the prior art. The compositions of these steels were:



After carburizing, the components required machining rework. Moreover, the carbon content in the carburized layer, at 0.1 mm below the surface, was only 0.8%. The latter result shows that, in addition to the lowest sensitivity to distortions, the steel according to the invention carburizes better than the steel according to the prior art.


WE CLAIM:
1. A process for manufacturing a mechanical steel part such as gears and the like wherein a steel blank is produced and a treatment of carburization or carbonitridation, of at least part of the surface of the blank is carried out in a known manner, wherein the chemical composition of the steel from which the part is made comprises, by weight.

and if desired upto 0.02% tellurium, upto 0.04% of selenium, upto 0.07% of lead, upto 0.005%) of calcium, the remainder being iron and impurities, this chemical composition being adjustable so that the JOMINY curve of the steel is:

and so that the mean values J3m, J11m, J15m and J25m of five JOMINY tests are such that:


The process as claimed in claim 1, wherein the chemical composition of the steel is adjusted such that it exhibits the following values:

The process as claimed in claim 1 or 2, wherein the composition of said steel is

The process as claimed in claim 1, 2 or 3, wherein the composition of said

The process as claimed in claims 1 to 4, wherein the chemical compositions of the steel is



The process as claimed in any one of claims 1 to 5, wherein the nitrogen content of the steel is between 0.004% and 0.02%.
Carburized or carbonitrided steel, having the following chemical composition, by weight:

and if desired upto 0.02%) tellurium, upto 0.04%) of selenium, upto 0.07% of lead, upto 0.005% of calcium, the remainder being iron and impurities resulting from the production process.
The carburized or carbonitrided steel as claimed in claim 7, wherein its chemical composition is such that:



The carburized or carbonitrided steel as claimed in claim 7 or 8, wherein its JOMINY curve is such that:


The carburized or carbonitrided steel as claimed in claim 9, which has the following value


The carburized or carbonitrided steel as claimed in claim 9 or 10, wherein the

The carburized or carbonitrided steel as claimed in claim 9, 10 or 11, wherein

The steel as claimed in any one of claims 7 to 12, wherein its nitrogen content is between 0.004% and 0.02%.


Documents:

1524-mas-1998 abstract.pdf

1524-mas-1998 claims-duplicate.pdf

1524-mas-1998 claims.pdf

1524-mas-1998 correspondence-others.pdf

1524-mas-1998 correspondence-po.pdf

1524-mas-1998 description (complete)-duplicate.pdf

1524-mas-1998 description (complete).pdf

1524-mas-1998 form-19.pdf

1524-mas-1998 form-2.pdf

1524-mas-1998 form-26.pdf

1524-mas-1998 form-4.pdf

1524-mas-1998 form-6.pdf

1524-mas-1998 petition.pdf


Patent Number 202088
Indian Patent Application Number 1524/MAS/1998
PG Journal Number 05/2007
Publication Date 02-Feb-2007
Grant Date 07-Sep-2006
Date of Filing 08-Jul-1998
Name of Patentee ASCOMETAL
Applicant Address IMMEUBLE "LA PACIFIC" -LA DEFENSE 7 11/13 COURS VALMY 92800-PUTEAUX.
Inventors:
# Inventor's Name Inventor's Address
1 CLAUDE PICHARD, 31 RUE DE BON PUITS 57860 MALANCOURT LA MONTAGNE.
PCT International Classification Number C22C38/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 97 08770 1997-07-10 France