EP1044287A1

EP1044287A1 - Nitriding steel, method for obtaining same and parts formed with said steel

Info

Publication number: EP1044287A1
Application number: EP99946245A
Authority: EP
Inventors: Philippe Dubois
Original assignee: Aubert and Duval SA
Current assignee: Aubert and Duval SA
Priority date: 1998-09-30
Filing date: 1999-09-28
Publication date: 2000-10-18
Anticipated expiration: 2019-09-28
Also published as: ES2374261T3; FR2783840A1; WO2000018975A1; CA2312034A1; ATE531832T1; CA2312034C; DK1044287T3; EP1044287B1; FR2783840B1

Abstract

The invention concerns a steel composition comprising, expressed in wt.%: 0.2 to 0.4 % of C; 0.8 to 2 % of Cr; 0.6 to 2 % of Mo; 0.05 to 0.4 % of Al; and as the case may be, not more than 0.5 % of Si; not more than 1.5 % of Mn; not more than 1.5 % Ni; not more than 0.5 % of V; the rest consisting of iron and residual impurities; said composition Cr, Mo, V and Al contents expressed in wt.% satisfies the following relationship: 4 </= 3Cr + Mo + V + 2Al </= 8. The invention also concerns a method for making treated and nitrided parts, made of said compositions.

Description

NITRURATION STEEL, PROCESS FOR OBTAINING SAME AND PARTS FORMED THEREFROM

The present invention relates to a composition of nitriding steel, parts formed with this steel, as well as a method for manufacturing parts produced in this steel.

Nitriding is a thermochemical treatment of surface hardening by introducing nitrogen into the steel. This process is used in all fields of mechanics and is used in particular to manufacture gears, splines, transmission shafts, cam shafts, heat engine distribution parts, pump bodies, injector bodies , crankshafts, extrusion screws, hydraulic distributors, guide rails, bearing cages, control gauges and shaping tool parts.

Nitriding generally takes place in a temperature range of about 450 to 600 ° C, temperatures for which the diffusion of nitrogen is relatively slow. Conventional nitriding steels, such as 40CrAIMo6.12, 25NiAICr14.12, 30CrMo12 or 32CrMoV13

(the chemical compositions of which are recalled in Table 1 below), only make it possible to obtain modest thicknesses of nitrided layers, of the order of about 0.7 mm at most, the nitriding steps having durations excessively long, up to around 200 h.

Table 1: Typical chemical compositions in percent by weight

These shortcomings limit the use of nitrided steels for technical reasons, for example when the applied stresses induce high stresses beyond the nitriding depths achievable with current steels, or even when the machining resumes after nitriding are significant. . However, the limitation is also economic due to the excessively long duration of the nitriding cycles.

These limitations could be circumvented by the use of steels containing lower chromium contents, which leads to an increase in the diffusion coefficient of nitrogen in the alloy. This is the case, for example, of 35CrMo4 or 42CrMo4 steels (the chemical compositions of which are given in Table 1 above). However, the maximum surface hardnesses that it is possible to obtain with these steels are of the order of 700 HV which is too low in most cases. In addition, the tensile, resilience and toughness characteristics of the underlay are notably insufficient for many applications.

The main object of the present invention is therefore to provide a nitriding steel composition making it possible to conserve the tensile, resilience, toughness, hardenability, fatigue, and surface hardness properties of the nitrided layers of nitriding steels. type 32CrMoV13, while increasing the nitrogen diffusion kinetics, to allow either greater depths of nitriding, or reduced nitriding times.

A first subject of the invention is thus a composition of nitriding steel, comprising, expressed in% by weight,

0.2 to 0.4% of C, 0.8 to 2% of Cr, 0.6 to 2% of Mo, 0.05 to 0.4% of Al, and, where appropriate, at most 0, 5% of Si, at most 1.5% Mn, at most 1.5% Ni, at most 0.5% V, the remainder consisting of iron and residual impurities, the contents of this composition in Cr, Mo, V and Al, expressed in% by weight, satisfying the following relationship:

4 <3Cr + Mo + V + 2AI <8. The sulfur is preferably limited to 0.015% and the phosphorus to 0.020% by weight. Elements such as calcium, cerium, titanium, zirconium, niobium which are used either to deoxidize the steel or to refine the grain size are preferably limited to 0.1% by weight each.

The carbon range is 0.2-0.4% by weight in order to obtain, after quenching and returning to a temperature compatible with subsequent nitriding, a maximum tensile strength in the range from 900 to 1500 MPa. Tight carbon forks are also interesting:

• the range of 0.2-0.35% by weight makes it possible to maximize the characteristics of ductility, resilience and toughness, * the range of 0.3-0.4% by weight makes it possible to maximize the elastic limit,

• the range of 0.25-0.37% by weight provides a good compromise for all of the above characteristics.

The carbon also contributes to the quenchability of the alloy as well as to its resistance to tempering and to its resistance to softening during the nitriding cycle.

The silicon must be limited because it leads, during nitriding, to the precipitation of carbonitrides which participate little in hardening, but reduce the rate of diffusion of nitrogen. For these reasons, it is limited to 0.5% by weight at most, and preferably 0.35% by weight. Chromium is one of the predominant elements for obtaining the characteristics of the nitrided layer, but it leads to a significant precipitation of nitrogen in the form of carbonitrides, which reduces the rate of diffusion of nitrogen in the nitrided layer. Chromium also has a beneficial influence on the hardenability of the undercoat. These considerations lead to limiting the chromium content to 0.8-2% by weight, preferably to 1.1-1.8% by weight.

Compared with a 32CrMoV13 type steel, the loss of hardenability induced by the lowering of the chromium content is partially compensated by an increase in the manganese and nickel contents.

These two elements must not, however, be added in too high a content, since this leads respectively to segregation of chemical composition and to embrittlement of the undercoat during nitriding. For these reasons the contents are limited to 1.5% by weight at most for manganese and nickel. Tighter ranges are preferred: 0.2-1.1% by weight for manganese and 0.5-1.3% by weight for nickel.

Similarly, compared to a 32CrMoV13 type steel, the lowering of the chromium content induces a reduction in hardening during nitriding, which is compensated by an increase in the molybdenum and vanadium contents, as well as by the addition of aluminum. These three elements contribute to increasing the hardenability of the steel.

Furthermore, the molybdenum and vanadium elements increase the resistance to tempering of the steel and limit its softening during nitriding. Their content must be limited because too large a quantity would lead to embrittlement of the steel underlayment. The contents are therefore limited to 0.6-2% by weight for the molybdenum, to at most 0.5% by weight for the vanadium, and to 0.05% -0.4% by weight for the aluminum. Tighter ranges are preferred: 0.8-1.5% by weight for the molybdenum, 0.1-0.4% by weight for vanadium and 0.1-0.3% by weight for aluminum.

A second object of the invention is a process for manufacturing treated and nitrided parts, comprising the following operations: a - constitution of a filler intended to obtain a composition in accordance with the present invention, as described above, b - preparation of said charge in an arc furnace, c - heating and hot transformation of the ingot, d - heat treatment for homogenizing the structure and refining the grain, e - heat treatment for use, and f - nitriding.

The steel according to the invention can be obtained by conventional production techniques, but to obtain better results in resilience, tenacity and fatigue it is preferable to carry out a reflow by consumable electrode either under slag (ESR) or under reduced pressure

(VAR), following the development in the arc furnace.

To further increase these performances, it is possible to carry out the first production under reduced pressure (VIM) and to continue with a reflow by consumable electrode.

The ingots obtained by any one of the preceding routes undergo a reheating at high temperature to homogenize the structure and then thermomechanical transformations aimed at giving the product produced in this alloy a sufficient degree of wrinkling which will be preferred greater than or equal to 3 Lower working rates may be allowed for large parts. These thermomechanical transformations are based on conventional procedures, such as rolling, forging, stamping or spinning.

Several alternative embodiments are possible with regard to step d of the method according to the invention. Processed products can simply be softened at a temperature below the point critical (ACi), or annealed at a temperature above the critical point (ACi), which then assumes a sufficiently slow start of cooling. When looking for the best characteristics it is however preferable to carry out a normalization from a temperature above the critical point (AC ₃ ), followed by air cooling and a softening income at a temperature below the critical point (Ad).

As an indication, the critical point temperature (ACi) is generally in the range from 700 ° C to 790 ° C, while the critical point temperature (AC ₃ ) is generally in the range from 800 ° C at 890 ° C.

The products are then quenched and returned in the form of a rolled bar, forged or stamped blank, pre-machined parts. The quenching takes place from an austenitization temperature above the critical point (AC ₃ ), in the range from 900 to 1000 ° C., for example. The quenching fluid is adjusted, in a conventional manner, according to the section of the products.

The tempering is then carried out at a temperature adjusted as a function of the mechanical characteristics sought for the core, in the range of approximately 550 ° C. to approximately 750 ° C. The choice must take into account the temperature at which nitriding will take place. A tempering temperature at least 30 ° C higher than the nitriding temperature is preferred. In certain particular cases, nitriding can take the place of income. Nitriding, step f of the process according to the invention, is then carried out on a finished or almost finished workpiece. The time and temperature parameters are to be set according to the compromise sought in surface hardness, depth and microstructure for the layer. It is possible to carry out gaseous nitriding with ammonia, or ionic nitriding with nitrogen or else nitriding in a salt bath capable of releasing nitrogen on the surface of the steel. The process used has little influence on the hardness gradient of the nitride layer, which essentially depends on the chemical composition of the steel.

A third object of the invention consists of the treated and nitrided parts made with the steel according to the invention. These parts can advantageously be manufactured by means of the manufacturing method according to the invention, but also by any other method chosen according to the final application.

The embodiments of the invention which follow show that the combination of the elements carbon, silicon, manganese, nickel, chromium, molybdenum, vanadium and aluminum in the proportions by weight indicated above results in a steel having simultaneously excellent tensile characteristics. , resilience, transition of resilience, tenacity, fatigue, resistance to income of the heart, associated with excellent surface hardness and depth of the nitrided layer.

The classic steel chosen as a comparison element is steel

32CrMoV13 which has been developed in the past to optimize the compromise between core characteristics / surface hardness / kinetics of nitriding and which remains one of the best nitriding steels available today.

As for the nitriding step, it was systematically carried out with the gaseous nitriding process using ammonia.

Examples The symbols used in the following have the following meanings:

R _m = maximum resistance

R po, 2 = conventional elastic limit at 0.2% deformation Asd = elongation in% on the basis 5 d (d = diameter of the test piece) Z = necking

HV ₅₀ = Vickers hardness under a load of 50 kg HRC = Rockwell C hardness

KV = impact energy in impact bending on V-notch test piece KCU = impact energy in impact bending on U-shaped test piece

KQ = tenacity. The examples are supplemented by the figures of the accompanying drawing plates, in which:

FIG. 1 represents the hardness profile of two samples, the preparation of which is described in example 1,

FIG. 2 represents the hardness profile of two samples, the preparation of which is described in Example 2,

FIG. 3 represents the hardness profile of two samples, the preparation of which is described in Example 3, • FIG. 4 represents the hardness profile of two samples, the preparation of which is described in Example 4,

FIG. 5 represents the hardness profile of two samples, the preparation of which is described in Example 5,

FIG. 6 represents the hardness profile of two samples, the preparation of which is described in Example 6,

FIG. 7 represents the hardness profile of two samples, the preparation of which is described in Example 7,

FIG. 8 represents the hardness profile of two samples, the preparation of which is described in Example 8, • FIG. 9 represents the hardness profile of two samples, the preparation of which is described in Example 9. Example 1

A 35 kg ingot was produced in the chemical composition indicated in percentage by weight below, in accordance with the indications of the present invention:

C 0.30% If 0.06% Mn 0.28% Cr 1, 20% Ni 0.05%

Mo 1.00% V 0.25% Al 0.09% the remainder consisting of iron and residual impurities. This ingot was produced by arc fusion, it was then homogenized at high temperature (1100 ° C) to obtain a uniform structure, then it was forged. The forged products were slowly cooled in the oven. They have been standardized in order to dissolve the carbides, to homogenize the austenitic structure and to refine the grain. Bars from this ingot were austenitized at 940 ° C, quenched in oil, then returned to a temperature of 650 ° C.

The mechanical characteristics obtained are indicated in the following table:

Samples were nitrided at 520 ° C for 100 h. The hardness profile obtained is presented in Figure 1, as well as that obtained for the same cycle with a 32CrMoV13 steel.

This profile shows that the steel according to the present invention has, for the same nitriding cycle, a surface hardness equivalent to that of 32CrMoV13 steel and a significantly greater nitrided depth.

Example 2

C 0.35% If 0.05%

Mn 0.28% Cr 1.21% Ni 0.50% Mo 1.01% V 0.25%

Al 0.18% the remainder consisting essentially of iron and residual impurities.

This ingot was produced by arc fusion, it was then homogenized at high temperature (1100 ° C) to obtain a uniform structure, then it was forged. The forged products were slowly cooled in the oven. They have been standardized in order to dissolve the carbides, to homogenize the austenitic structure and to refine the grain.

Bars from this ingot were austenitized at 940 ° C, quenched in oil, then returned to a temperature of 650 ° C. The mechanical characteristics obtained are indicated in the following table:

Samples were nitrided at 520 ° C for 100 h. The hardness profile obtained is presented in Figure 2, as well as that obtained for the same cycle with a 32CrMoV13 steel.

Example 3

C 0.35% If 0.06% Mn 0.69% Cr 1, 20% Ni 0.50%

Mo 1.21% V 0.35% Al 0.30% the remainder consisting essentially of iron and residual impurities. This ingot was produced by arc fusion, it was then homogenized at high temperature (1100 ° C) to obtain a uniform structure, then it was forged. Forged products were cooled slowly in the oven. They have been standardized in order to dissolve the carbides, to homogenize the austenitic structure and to refine the grain.

Bars from this ingot were austenitized at 940 ° C, quenched in oil, then returned to a temperature of 650 ° C.

The mechanical characteristics obtained are indicated in the following table:

Samples were nitrided at 520 ° C for 100 h. The hardness profile obtained is presented in Figure 3, as well as that obtained for the same cycle with a 32CrMoV13 steel.

Example 4

C 0.40%

If 0.06%

Mn 0.70% Cr 1, 19%

Ni 1.00% Mo 1.31% V 0.35% Al 0.32% the remainder consisting essentially of iron and residual impurities.

The mechanical characteristics obtained are indicated in the following table:

Samples were nitrided at 520 ° C for 100 h. The hardness profile obtained is presented in Figure 4, as well as that obtained for the same cycle with a 32CrMoV13 steel.

Example 5

A 35 kg ingot was produced in the chemical composition indicated in percentage by weight below, in accordance with the indications of the present invention: C 0.38%

If 0.06%

Mn 0.31%

Cr 1.06%

Nor 0.99%

MB 1.50%

V 0.25%

Al 0.16% the remainder consisting essentially of iron and residual impurities.

The mechanical characteristics obtained are indicated in the following table:

Samples were nitrided at 520 ° C for 100 h. The hardness profile obtained is presented in Figure 5, as well as that obtained for the same cycle with a 32CrMoV13 steel.

This profile shows that the steel according to the present invention has, for the same nitriding cycle, a surface hardness equivalent to that of 32CrMoV13 steel and a significantly greater nitrided depth. Example 6

C 0.35%

If 0.06%

Mn 0.64%

Cr 1, 38%

Ni 0.51%

Mo 1, 20%

V 0.34%

Al 0.09% the remainder consisting essentially of iron and residual impurities.

The mechanical characteristics obtained are indicated in the following table:

Samples were nitrided at 520 ° C for 100 h. The hardness profile obtained is presented in Figure 6, as well as that obtained for the same cycle with a 32CrMoV13 steel.

Example 7

C 0.32% If 0.05%

Mn 0.90% Cr 1, 33% Ni 0.77% Mo 1, 12% V 0.27%

Al 0.19% the remainder consisting essentially of iron and residual impurities.

Samples were nitrided at 520 ° C for 100 h. The hardness profile obtained is presented in Figure 7, as well as that obtained for the same cycle with a 32CrMoV13 steel.

Example 8

C 0.31% If 0.27% Mn 0.91% Cr 1.34% Ni 1.04%

Mo 1.17% V 0.23% Al 0.30% the remainder consisting essentially of iron and residual impurities. This ingot was produced by arc fusion, it was then homogenized at high temperature (1100 ° C) to obtain a uniform structure, then it was forged. Forged products were cooled slowly in the oven. They have been standardized in order to dissolve the carbides, to homogenize the austenitic structure and to refine the grain.

The mechanical characteristics obtained are indicated in the following table:

Samples were nitrided at 520 ° C for 100 h. The hardness profile obtained is presented in Figure 8, as well as that obtained for the same cycle with a 32CrMoV13 steel.

Example # 9

An ingot of 1000 kg was produced in the chemical composition indicated in percentage by weight below, in accordance with the indications of the present invention:

C 0.32%

If 0.03%

Mn 0.86%

Cr 1, 35%

Ni 0.78%

MB 1.15%

V 0.28% Al 0.19% the remainder consisting essentially of iron and residual impurities.

This ingot was obtained by vacuum fusion and then remelting by consumable electrode, it was then reheated to high temperature (1100 ° C) in order to homogenize the structure, then it was rolled to result in cylindrical bars with a diameter of 100 mm. These bars have undergone a normalization treatment in order to dissolve the carbides, homogenize the austenitic structure and refine the grain size.

Samples taken from these bars were austenitized at 940 ° C, quenched in oil and returned to 650 ° C.

Part of the samples were used to determine the mechanical characteristics in the quenched quenched state. The results obtained are indicated in the following tables:

* value obtained on CT 25 type test pieces according to ASTM E 399-90.

The rest of the samples were nitrided by applying a cycle of

100 h at 520 ° C. Among these samples, some were protected from nitriding in order to determine the evolution of the mechanical characteristics of the heart. The results obtained are presented in the following table:

FIG. 9 also makes it possible to compare the hardness profiles obtained after nitriding with this steel and with a 32CrMoV13 steel.

Finally, the limit of endurance in rotary bending at 2.10 ⁷ cycles was determined according to standard NFA 03-102, using test pieces having a stress concentration factor Kt = 1.035. Two cases were studied, steel in the quenched and annealed state, as well as steel in the quenched, annealed and nitrided state as described above. The results obtained are shown in the following table and compared to the best known values for 32CrMoV13 steel:

Nitriding depth

The depth of the hardness gradient makes it possible to measure the performance of a steel according to the invention in terms of kinetics of nitriding. This depth is conventionally defined in Europe by measuring the depth at which the hardness is equal to that of the core + 100 HV (Vickers hardness). In the United States, the convention is to define the depth at which the hardness is equal to 50 HRC (i.e. 513 HV, value obtained by conversion according to ASTM E140).

The depths obtained for each of the 9 previous examples according to these two conventions are collated in the following table:

These various results clearly show that the steel according to the invention, once quenched and tempered, presents an excellent compromise between mechanical tensile strength, resilience and tenacity of the undercoat. In addition, after nitriding, it has an excellent compromise between surface hardness, depth of nitriding and the duration of the nitriding cycle.

The parts manufactured using the steel according to the invention can be, in particular, bars, sheets, forged or stamped blanks, tubes or wires.

It goes without saying that the embodiments of the invention which have been described above have been given for information only and in no way limiting, and that many modifications can be made without departing from the scope of the invention.

Claims

1. Composition of nitriding steel comprising, expressed in% by weight,

0.2 to 0.4% of C, 0.8 to 2% of Cr, 0.6 to 2% of Mo, 0.05 to 0.4% of Al, and, where appropriate, at most 0, 5% of Si, at most 1.5% of Mn, at most 1.5% of Ni, at most 0.5% of V, the balance consisting of iron and residual impurities, the contents of this composition in Cr , Mo, V and Al, expressed in% by weight, satisfying the following relationship:

4 <3Cr + Mo + V + 2AI <8. 2. A nitriding steel composition according to claim 1, comprising, expressed in% by weight, 0.25 to 0.37% of C, 1.1 to 1.8% of Cr, 0.8 to 1.5 % of Mo, 0.1 to 0.3% of Al,

0,

2 to 1.1% of Mn, 0.5 to 1.3% of Ni, 0.1 to 0.4% of V, and, where appropriate, at most 0.35% of Si, the balance consisting of iron and residual impurities, the contents of this composition in Cr, Mo, V and Al, expressed in% by weight, satisfying the following relationship:

4 <3Cr + Mo + V + 2AI <8.

3. Composition of nitriding steel according to one of claims 1 or

2, further comprising at most 0.015% by weight of S and 0.020% by weight of P.

4. A nitriding steel composition according to any one of claims 1 to 3, further containing at most 0.1% by weight of each element Ca, Ce, Nb, Ti, Zr.

5. A method of manufacturing treated and nitrided parts, comprising the following operations: a - constitution of a filler intended to obtain a chemical composition according to any one of claims 1 to 4, b - preparation of said filler in a arc, c - heating and hot processing of the ingot, d - heat treatment for homogenization of the structure and refinement of the grain, e - heat treatment for use, and f - nitriding.

6. The manufacturing method according to claim 5, wherein the elaboration in an arc furnace (step b) is followed by remelting by a consumable electrode.

7. The manufacturing method according to claim 6, wherein the elaboration in an arc furnace (step b) is carried out under reduced pressure.

8. The manufacturing method according to any one of claims 5 to 7, wherein step d comprises normalization at a temperature higher than that of the critical point AC ₃ , air cooling and a softening income at a temperature lower than that of the critical point ACi.

9. The manufacturing method according to any one of claims 5 to 8 wherein step e comprises quenching from an austenitization temperature in the range of 900-1000 ° C, followed by tempering at a temperature of 550-750 ° C.

10. The manufacturing method according to claim 9 wherein the tempering temperature is at least 30 ° C higher than the nitriding temperature.

11. Parts, bars, sheets, forged or stamped blanks, tubes and steel wires having a composition according to any one of claims 1 to 4.

12. Parts, bars, sheets, forged or stamped blanks, tubes and steel wires according to claim 11, characterized in that they are obtained by a process according to any one of claims 5 to 10.