Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/60—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using solids, e.g. powders, pastes
  • C23C8/62—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using solids, e.g. powders, pastes only one element being applied
  • C23C8/64—Carburising
  • C23C8/66—Carburising of ferrous surfaces
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/06—Surface hardening
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/02—Pretreatment of the material to be coated
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/04—Treatment of selected surface areas, e.g. using masks
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/20—Carburising
  • C23C8/22—Carburising of ferrous surfaces
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/80—After-treatment
• • E—FIXED CONSTRUCTIONS
  • E21—EARTH OR ROCK DRILLING; MINING
  • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
  • E21B10/00—Drill bits
  • E21B10/46—Drill bits characterised by wear resisting parts, e.g. diamond inserts
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2241/00—Treatments in a special environment

Definitions

• the invention relates to iron and steel, and more particularly to the grades of case hardening steels with a high degree of resilience.
• Such a drill bit is a forged tool consisting of three rotating steel cones "entangled" in one another and making it possible to crush the geological formations during oil or gas exploration operations. These three cones are rotated, via one or more bearings, on three welded steel arms.
• the machining tracks machined on the arms and inside the cones are generally, in conventional production processes, treated superficially by atmospheric carburization to reach a conventional depth, where the Vickers hardness is 550 HV, included on average between 1 and 1.5 mm.
• the present invention relates to a new steel grade that can be used for making cones and arms.
• trephines There are several kinds of such trephines.
• One of them is the bit with "inserted tooth", that is to say in which spikes, most often made of tungsten carbide obtained by powder metallurgy, are crimped into each of the cones.
• the steel subject of the present invention is not limited, in its use, to this type of drill bit, and could also be used for the production of "machined tooth” bits.
• the reference grades used for the manufacture of the cones and arms constituting the drill bits are steels strongly alloyed with nickel at levels of up to 3.5% by weight (especially the grade of 15NiCrMo13).
• This alloying element is usually considered necessary to give the product the level of ductility essential to withstand the severity of the mechanical stresses in service.
• This ductility must be associated with tensile characteristics and high quenchability.
• the properties typically sought for these shades are, in fact:
• this characteristic can be evaluated by the coefficient ⁇ corresponding to the difference in hardness between Jominy points J 5 and J 3 .
• the Jominy curve should satisfy ⁇ ⁇ 3.5 HRC and Ji> 40 HRC; it is recalled that the Jominy curve is a curve which reflects the hardenability of a steel; it is obtained at by means of a standardized test (in particular by the NF EN ISO 642 standard) by measuring the hardness along a generatrix of a cylindrical specimen soaked by a jet of water watering one of its ends; the hardness measured at a depth of x mm from said end is designated J x ;
• An austenitic grain size such that the grain index is greater than ASTM 5;
• US-A-6 146 472 provides an example.
• the increase in resilience is obtained by controlling the magnification resistance of the austenitic grain via the use of an Nb-Al-N microalloy, combined with an optimized heat treatment.
• the resilience values indicated in this document are at best close to 60J, and the examples presented are castings which do not make it possible to check the quenchability criterion ⁇ ⁇ 3.5 HRC.
• US-A-2005/0081962 discloses a carburizing steel not using Ni, but whose resilience does not exceed 51 J, which is not sufficient.
• the object of the invention is to provide a carburizing steel used in particular for the manufacture of drill bits, not requiring addition of Ni and nevertheless meeting all the criteria of ductility, hardenability, Re, Rm and Kv cited above.
• the subject of the invention is a steel for the manufacture of cemented parts, characterized in that its composition, in percentages by weight, is: - 0.1% ⁇ C ⁇ 0.15%;
• traces ⁇ 0 ⁇ 30 ppm Preferably, traces ⁇ 0 ⁇ 30 ppm.
• the subject of the invention is also a case-hardened steel piece, characterized in that it is made of a steel having the preceding composition and in that it has undergone a cementation.
• the invention also relates to a method for manufacturing a cemented part, characterized in that:
• a blank of said workpiece is made of a steel having the preceding composition.
• This shaping can be performed by any method (forging, machining, rolling ...);
• said pressure may be from 5 to 20 mbar, and the succession of steps of the cementation may be the following:
• the invention is based on a careful adjustment of the composition of the steel, to meet all the criteria mentioned above.
• the steel that is the subject of the present invention is also different from that described in US Pat. No. 6,146,472 in that the accessible resilience is significantly greater, and in that the improvement of the resilience is not generated at least mainly, by a control of the grain size.
• This has the advantage of not modifying the ability of the grade to thermomechanical treatment and limit the risk of abnormal magnification of the austenitic grain during cementation.
• the segregating effect of niobium which may lead to a heterogeneous austenitic grain size is, in particular, avoided.
• the level of resilience accessible by the present invention is also significantly higher.
• the type of carburizing used with the steel described in the present invention is not limited to the atmospheric carburization process which could be replaced by other surface hardening processes, for example low pressure carburizing.
• the present invention is based on a steel whose composition is defined below. All grades are given in percentages by weight. By using a composition defined as described below, it is possible to develop, without the voluntary addition of nickel and without using large quantities of other expensive elements, a steel having a hardenability, mechanical characteristics after quenching followed by and a carburizing ability (carbon uptake, resilience to the core, carburizing depth, residual austenite content, etc.) similar to those of the reference grades of 3.5% Ni usually used for the manufacture of carbon fiber drill bits. drilling. The content of C is between 0.10% and 0.15%, a carbon content limited to a relatively narrow range, and which is low, compared with those generally encountered in case-hardening steels.
• the Mn content is between 0.8% and 2%.
• Manganese is used with chromium and molybdenum to compensate for the loss of hardenability associated with decreased carbon content. For its effect to be sufficient, a content greater than or equal to 0.8% is required. Since this alloying element can pose problems of segregation, it is preferable that its concentration does not exceed 2%.
• the Cr content is between 1% and 2.5%.
• chromium is used to ensure a sufficient degree of quenchability to the grade.
• the minimum content of 1% is chosen so that the effect of this alloying element on quenchability is sufficient.
• the maximum content of 2.5% is defined in order to avoid a detrimental effect on the use properties, in particular by formation of coarse chromium carbides.
• Mo content is between 0.2% and 0.6%.
• Molybdenum is a third element used to adjust the quenchability of the grade. It is also an alloying element that can be used judiciously to increase resilience, especially at low temperatures. Molybdenum also exacerbates the effect of boron on hardenability, and can therefore be used for this purpose in the case of boron alloyed grades. For a content of less than 0.2% the increase in hardenability is too low and this value is therefore chosen as the minimum level. For high concentrations, molybdenum tends to decrease the ability of forging steels. In addition, because it is an expensive alloying element, its use at an excessive content would lead to a loss of economic benefit brought about by the non-use of nickel. For these reasons, a maximum content of 0.6% is preferred.
• the Si content is less than 0.35%.
• silicon can be used as a deoxidizing element.
• the residual content of this element does not in any case generally exceed 0.35%. It should also not exceed a content of 0.35% in the steels of the invention, because silicon is an alloying element that can limit, by barrier effect, the carbon uptake during cementation.
• the Ni content is less than or equal to 0.7%, preferably 0.3%.
• one of the objects of the present invention is to make it possible to do without an addition voluntary of this element. However, it is still present in the residual state in the raw materials used to make steel, especially in scrap metal.
• the 0.3% content corresponds to the maximum level most commonly encountered when no voluntary addition of nickel is made during the elaboration.
• the B content is less than 0.005%.
• Boron is an optional element. It can be used to optimally adjust the quenchability of the grade if the levels of Mn, Cr and Mo are not quite sufficient for this purpose. But for this alloy element to actually act on the quenchability, it must be maintained in solid solution. For that, the precipitation of nitrides or oxides of boron must be avoided. This result can be obtained by adding a higher affinity alloy element with nitrogen, for example titanium, and controlling the production process to limit the dissolution of nitrogen and oxygen in the steel.
• the Ti content is less than 0.1% and preferably less than 0.04%. Titanium is optionally added to allow the boron to be maintained in solid solution by precipitation of titanium nitrides which reduce the amount of nitrogen that may be combined with boron. Its content should optimally be chosen according to the amount of nitrogen in the grade. To be completely effective, a stoichiometric amount of titanium must be added to ensure precipitation of all of the nitrogen contained in the steel as TiN, and thus maintain the boron in solid solution. This is verified if the Ti / N ratio is greater than 3.4. In the case of substoichiometric titanium addition, the effect of boron on quenchability can still be expressed but is less pronounced. Beyond the prescribed limit, there is a risk of formation of TiN too coarse during solidification, and moreover the addition of Ti becomes excessively expensive.
• the N content is less than 0.02%, preferably less than 0.01%.
• boron addition from 5 to 50 ppm
• a nitrogen content of less than 0.01% is therefore recommended. If the boron is not used (B ⁇ 5ppm), it is not absolutely essential to strictly control the nitrogen content, which can then go up to 0.02% without detrimental effect on the properties of the elaborated steel. .
• the Al content must be at most 0.1%: Aluminum is an optional element. It can be used as a deoxidizer for steel to replace silicon, and to optimize the behavior of the austenitic grain during cementation.
• the V content is at most 0.3%.
• Vanadium is an optional element. It can be used as a micro-alloy element for better control of grain size during carburizing, providing further improvement in resilience.
• the P content is at most 0.025%. This limit is recommended to avoid risk of weakening the steel. At an excessively high content, this element tends to segregate at the austenitic grain boundaries, which can lead to an increase in the ductile-brittle transition temperature and a decrease in the resilience at room temperature.
• the Cu content is at most 1%, preferably at most 0.6%.
• a maximum content of 1% is recommended because it is an expensive element that provides no benefit of quenchability or resilience.
• the preferred maximum value of 0.6% is a content usually recognized as being below which copper has no significant effect on the mechanical properties of steel. Nevertheless, use at a higher level is possible without modifying the ability of the grade to be used for the manufacture of drill bits.
• the S content is not strictly imposed in the definition of steel according to the most general invention, but it must be controlled according to the intended application.
• a low content will be sought if it is desired to improve the inclusion cleanliness by not forming sulphides (typically ⁇ 0.01%) and a higher content may be chosen (typically from 0.03% to 0.1%) if a gain in machinability is sought and provided that the inclusion cleanliness remains consistent with the requirements of the intended application for steel.
• the O content is most often at most 0.003% (30 ppm), so as to optimize the inclusion cleanliness. This limit may possibly be exceeded if the future application of the steel does not require a very good inclusion cleanliness, and in any case a specific O content does not constitute an intrinsic property of the steel according to the invention.
• the control of the oxygen content is ensured by inerting systems during casting and by a control of the content of deoxidizing elements such as Si and Al.
• deoxidizing elements such as Si and Al.
• the composition of which is controlled so that this chemical equilibrium leads to the establishment of a low dissolved oxygen content in the liquid metal, and then avoiding the reoxidation of the liquid metal until casting by an effective protection against atmospheric oxygen, for example the inerting of the surface of the metal with argon, and the confinement of the pouring jets in refractory tubes themselves filled with argon;
• the dissolved oxygen content is limited by the carbon present in the liquid steel, which causes the departure of the excess dissolved oxygen under form of CO; then, as in the previous case, the low dissolved oxygen content must be preserved until casting by an effective protection of the liquid steel against atmospheric reoxidation.
• enrichment stage at a temperature of 900 to 980 ° C and a carbon potential of between 0.8 and 1.2 for a period of 3 to 20 hours; these conditions may vary depending on the exact composition of the steel being treated, and especially on the carburizing depth involved; typically for a temperature of 960 ° C a treatment of 3 to 6h makes it possible to temper the part to a depth of 1 to 1, 5 mm;
• the criteria for choosing the diffusion temperature are mainly, and conventionally for the person skilled in the art, related to the situation of the phase transformation points of the treated grade, and to the fact that this temperature must not be too high to minimize the deformations of the part during the quenching which follows;
• This type of carburizing is just one example, and other methods can be used.
• the succession of steps may be as follows, to aim at a surface C content typically of 0.5 to 0. , 8%:
• enrichment stage at a temperature of 900 to 980 ° C. and at a carbon potential of between 0.8 and 1.2 for a period of 3 to 20 hours; these conditions may vary depending on the exact composition of the steel being treated, and especially on the carburizing depth involved; typically for a temperature of 960 ' ⁇ a treatment of 3 to 6h allows to temper the part to a depth of 1 to 1, 5 mm avoiding the surface oxidation problems that may be encountered during an air cementation.
• the criteria for choosing the diffusion temperature are mainly, and conventionally for the person skilled in the art, related to the situation of the phase transformation points of the treated grade, and to the fact that this temperature must not be too high to minimize the deformations of the part during the quenching which follows;
• quenching for example with oil or gas (quenching pressure of between 3 and 10 bar), at a temperature of less than or equal to 1 ° C.;
• the mechanical properties obtained on the final product depend not only on the composition of the steel, but also the thermal and thermomechanical treatments it undergoes until the product is obtained. It may however be noted that in the case where the final product must be cemented, the conditions of its hot shaping by forging, rolling or otherwise, are of little importance. Indeed, the Cementation is accompanied by a quenching and tempering operation which gives the product a new structure and erases the consequences of hot shaping. It is then this treatment which confers on the product the essential of its mechanical properties, if it is not itself followed by any other treatment which could modify them.
• the steel Before forging, the steel is in the form of ingots of square section 100x100 mm and 200 mm high. After forging, the 40x40 mm section bars are cooled in still air and then normalized for 2 hours at a temperature of 875, 900 or 925 ⁇ , chosen according to the transformation point Ac3 of the grade. This standardization is intended to homogenize the carbon content and initial microstructure throughout the product.
• Castings Nos. 1 to 4 are those whose composition is in accordance with the present invention.
• Castings Nos. 5 to 10 are those in which at least one of the alloying elements is outside the claimed ranges. All concentrations are given in% by weight, except nitrogen, oxygen and boron, which are given in ppm by weight.
• the table also shows the temperature of the transformation point Ac3 (in ° C) of each of the grades.
• the quenchability of the different samples was evaluated by means of Jominy tests.
• the austenitization temperature was chosen, according to the transformation point of the steel in question, among the temperatures 875, 900 and 925 ° C.
• This heat treatment cycle makes it possible to estimate the resilience expected at the core of the parts treated by cementation.
• a cylinder 1 1NiCrMo13 has therefore been placed in the carburization charge to serve as a reference and determine the reference characteristics that must achieve, for the sample format considered, shades developed in accordance with the present invention.
• the composition of the casting used as a reference is given in Table 3.
• the carbon potential in the diffusion phase was adapted to the treated grade so as to control the residual austenite surface content.

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