FR2461019A1 - Gas phase soft nitriding metal workpieces - using gas mixt. contg. relatively low amt. of ammonia - Google Patents

Abstract

Method uses a carrier gas of mainly nitrogen in a heat treating furnace heated to 550-620 degrees C. The atmosphere of the heat treating furnace is a mixt. by vol. of ammonia (NH3) 10-30% and either CO 2.5-4.5% and the balance N2 or CO 1-3% and the balance endothermic treated furnace gas. The soft nitriding method provides fatigue strength and wear resistance to a ferrous member to form a crystal structure with the Epsilon-phase of the Fe-N-C system in the surface layer. In the conventional method using a large amount of NH3, amount of CO influences the mechanical properties of the ferrous member. This invention provides a atmosphere which economises in the use of ammonia.

Description

 The present invention relates to an improved process for the treatment of soft niltrurati'on by means of an iron parts gas using a carrier gas containing mainly nitrogen gas.

 In general, the iron members to be used as parts to build machines must have excellent resistance to fatigue and wear. To satisfy these conditions, several processes are known in which iron parts are nitrided in order to form a hard layer consisting of a nitrided compound at the surface part of the iron parts, this giving the parts the required characteristics.

 Among these processes, there is known a nitriding process using a gas and a nitriding process in a salt bath. These methods which are typical have the following drawbacks. The gas nitriding process undesirably requires an extended treatment time and causes a problem in terms of fatigue resistance, since the parts treated by this process become relatively brittle while having a high hardness due to forming a hard, deep layer. Consequently, the process cannot be suitable if the parts are to have excellent resistance to fatigue with a certain degree of hardness.

 The nitriding process in a salt bath is advantageous in that this treatment is carried out at low temperature for a short period and the parts obtained have a relatively high hardness and an excellent resistance to fatigue and they are then suitable for use. in the construction of parts for iron machines which must have excellent resistance to fatigue.

However, contrary to the advantages indicated above, the nitriding process in a salt bath must use cyanides and cyanates which are very toxic and constitute problems in terms of air pollution because of cyane, that is to say dinitrile, hydrocyanic acid, and these products are also difficult to transport and handle.

 In recent years, gentle nitriding processes using a gas have been studied, proposed and practiced in which the iron parts are gently nitrated under conditions of relative safety using a gaseous medium containing nitrogen gas to improve fatigue resistance and wear resistance of parts.

These methods of gentle nitriding by means of a gas make it possible to obtain treatment results similar to that of nitriding by means of a salt bath, and they have the advantage of not polluting the environment with cyane. .

 This process of gentle nitriding by means of a gas consists in forming at the surface part of iron parts a layer of crystalline structure called phase e consisting of iron, nitrogen and carbon (Fe-N-C). The known carrier gases used in the treatment of gentle nitriding by means of a gas have the following compositions 1. Combination of ammoniacal gas and endothermic gas (RX gas).

NH3 gas 50%
gas RX 50%
(NH3 50%, CO 12.5%, H2 15.6%, CO2 0.13%,
N2 complement, the percentages being indicated in
volume here as well as below).

2. Combination of ammonia gas and cracked methanol gas.

NH3 gas 50%
methanol gas
cracked 50%
(NH3 50%, CO 16.7%, H23.3%, CO2 0.2%, N2 complement) 3. Combination of ammonia gas and exothermic gas
(NX gas)
NH3 gas 20%
80% NX gas
(NH3 20%, CO 1.4%, H2 0.7%, CO2 0.04%, N2
complement)
The processes of gentle nitriding by means of a known gas in which the vehicle gases indicated above are used have the following drawbacks. In the processes in which vehicle gases 1 and 2 are used, ammonia gas which is relatively expensive is used in large quantities (40 to 60% by volume), which increases the cost price of the treatment and the price of the parts treated. .In addition, gas
CO is in an excessive amount, so that the carbon content of the layer of the compound obtained becomes high and thus the hardness of the layer becomes too high due to the high carbon content. As a result, the layer tends to be hard and brittle. Especially in the case of long pieces which have to be corrected after tensioning, there is a high probability of producing cracks. In addition, the treated parts are covered with carbon on their surface and they must be cleaned in a complementary phase after treatment. In the case where the vehicle gas 3 is used, the CO content is so small that the formation of phase c is not ensured, which results in a low resistance to fatigue.

 The reason why these problems are encountered in the methods of the prior art is explained below.

I1 est important de remarquer que le traitement de nitruration doit être effectué en utilisant une composition gazeuse qui peut produire des valeurs optimales de (N) et (C).The fundamental principle on which the process of soft nitriding with gas is the formation of a layer of
Fe-NC phase in the surface part of the iron parts by diffusion of nitrogen (N) and carbon (C) from the surface
to the interior of the rooms in accordance with the following reactions

It is important to note that the nitriding treatment must be carried out using a gaseous composition which can produce optimal values of (N) and (C).

Based on the reaction formula (a), the potential of (N) can be expressed as follows:

The potential (C) based on the reaction formula (b) can be expressed as follows

The activities of nitrogen (N) and carbon (C) are determined from equations (al) and (b1). In the above reactions KAN and KC represent the equilibrium constants of the reaction formulas (a) and (b> respectively).

 In the case of the gentle nitriding process using a gas in which 40 to 60% of ammonia gas is used, and 60 to 40% of an endothermic gas (RX gas), the nitriding treatment is not possible unless ammonia gas has to be used in large quantities to keep the activity (N) and the value a (N) constant because of the high H2 content of the endothermic gas. The use of expensive ammonia gas results in an increase in the cost price of the treatment, which is contrary to the requirement to reduce the cost price of the parts treated.

 Whereas in the case of the gentle nitriding process by means of a gas in which 10 to 30% of ammonia gas is used and 90 to 70% of an exothermic gas (NX gas), the CO content of the mixed gas is very small, so that it is almost impossible to keep the activity of (C) in the above equation (bl) at a satisfactory level.

Thus, this process is not interesting if we compare it to the process in which ammonia gas and endothermic gas are used in the sense that the treated parts are more susceptible to fatigue when they are subjected to stress concentrations when they are applied on machines.

The object of the present invention is to solve the problems inherent in the process of gentle nitriding by means of a gas of known art such as a problem of cost price dt with a large consumption of expensive ammonia gas in large quantity, d on the one hand, and problems of low resistance to fatigue of the treated parts and of carbon deposition on the treated surface of the parts on the other hand
As described in methods 1 to 3 of the prior art, in which large quantities of ammonia gas are used, it has been found that the content of CO gas has a great influence on the hardness and on the fatigue and fatigue strengths. the wear of the parts treated by the gentle nitriding process using a gas.

 The present invention provides a method of gentle nitriding by means of a gas which comprises a step of heating the iron parts in a heat treatment oven carried out between 550 and 6200C in a controlled gas which comprises, by volume, 10 to 30 % of ammonia gas, 2.5 to 4.5% of carbon monoxide (CO) and an additional quantity of a gas containing mainly nitrogen N2. The total carbon monoxide can be free carbon monoxide added in an amount of 2.5 to 4.5% by volume of the controlled gas. Alternatively, an endothermic gas containing carbon monoxide can be added such that the content of the carbon monoxide component in the endothermic gas is within the defined range above. Alternatively still, carbon monoxide total can be made up of 1 to 3% by volume of carbon monoxide and of a complement of carbon monoxide compound coming from an exothermic gas containing especially nitrogen N2. In the latter case, the free carbon monoxide can be replaced by an endothermic gas so that the carbon monoxide component from the endothermic gas is contained in an amount of 1 to 3% by volume of the controlled gas.

 Consequently, the object of the invention is to provide a process for gentle nitriding by means of an improved gas in which the ammonia gas which is used in large quantities in the processes of the prior art is present in relatively small quantities, with a reduction in the operating costs of the mild nitriding treatment using a gas and an improvement in the cost price of the soft nitriding treatment using a gas, which leads to a reduction in the cost price of the parts treated compared to those obtained by the methods of the prior art.

 The subject of the invention is also - a method of gentle nitriding by means of a gas which can give treated parts resistance to fatigue and to wear which are similar or superior to those obtained by the methods of prior art and which is therefore very useful and interesting from an economic point of view given the reduction of ammonia gas consumed.

- a process of gentle nitriding by means of a gas which is free of carbon deposition on the surfaces of the treated parts so as to provide a clean treated surface.

- A process of gentle nitriding by means of a gas in which a carrier gas is used which is much more reduced in quantity of ammonia gas if it is compared with what occurs in the products of the prior art and which contains a proper amount of carbon monoxide.

Other advantages and characteristics of the invention will emerge from the following detailed description of several preferred embodiments given with reference to the appended drawings in which:
- Fig. 1 is a graph representing the relationship which exists between the carbon monoxide content expressed as a percentage and by volume which appears on the bee and the rate of X-ray intensity of phase E which appears on the ordinates; and
- Fig. 2 is a graph representing the relationship between the form coefficient represented on the abscissa s the fatigue of the parts treated in accordance with a process of the known technique given for comparison and a process according to the invention.

In the implementation of the process according to the invention, the gentle nitriding treatment by means of a gas is carried out by introducing into a heat treatment oven a controlled quantity of gas which is composed essentially by volume of 10 to 30% ammonia gas (NH3), and 2.5 to 4.5 total carbon monoxide (CO) and the balance in nitrogen gas N2 or 10 to 30% ammonia gas (NH3),
r 1 to 3% of free carbon monoxide and the complement of exothermic gas. It should be noted that the total carbon monoxide content must be within the range from 2.5 to 4.5, whether or not free carbon monoxide or a combination of free carbon monoxide and a carbon monoxide compound contained in a mixed gas such as an exothermic gas, an endothermic gas or a cracked methanol gas.

The carrier gas (controlled gas of soft nitriding) useful for the treatment of soft nitriding by means of a gas in accordance with the invention corresponds to the following formula
1. Ammonia gas (NH3) 10 to 30 | in volume
Carbon monoxide (CO) 2.5 to 4.5%
N2 gas supplement
2. Ammonia gas (NE3) 10 to 30% by volume
Carbon monoxide (CO) 1 to 3%
Exothermic gas (gas
NX) complement
The controlled gas corresponding to the above formulation is introduced into a heat treatment oven where the gentle nitriding treatment by means of a gas is carried out at a temperature between 5500 and 6200C.

 As described with reference to methods 1 to 3 described above of the known technique, the amount of ammonia gas depends very much on the amount of CO. In the controlled gases which can be used in accordance with the present invention, the quantity of ammonia gas is reduced by 10 to 30% but the quantity of CO is increased to a level such that satisfactory fatigue resistance is obtained, but deposition is avoided. of free carbon on the surfaces of the treated parts.

 N2 or an exothermic gas (NX gas) is used as the carrier gas.

 By decreasing the amount of hydrogen (H2) in a gas mixed with ammonia gas (NH3), the concentration of ammonia gas decreases. In this case, the amount of CO is small enough to stop the formation of phase c, carbon monoxide CO is introduced into the vehicle gas, the ammonia gas content of which is reduced.

 The composition of exothermic gas (NX gas is indicated with the composition of an endothermic gas (RX gas) commonly used.

Exothermic gas (NX gas)
CO 1.8%
H2 0.9%
C02 0.05%
N2 complement
Endothermic gas (RX gas)
CO 24.5%
H2 31.2%
C 2 0.26%
0.4% H2O
N2 complement
By introducing carbon monoxide CO, it is possible to use free carbon monoxide or an endothermic gas or a cracked methanol gas rich in CO. In short, it is important that the ammonia gas, CO, and N2 or an exothermic gas are mixed in suitable ratios so that the potentials of N and C necessary for the formation of phase e are controlled so as to be maintained at constant levels.

 The present invention is illustrated in particular by the following examples.

EXAMPLE 1.

 A cold rolled steel was heated (designated SPCC according to Japanese standards having a composition of less than 0.12% in C, less than 0.5% in Mn, less than 0.04% in P, less than 0.045% in S and an additional Fe) in an atmosphere containing 20% NH3, 2.5% CO and the additional N2 gas at 6000C for one hour.

 As a result of the X-ray diffraction on the surface of this treated steel, it has been discovered that the intensity rate of X-rays of phase e reaches 64% which is a favorable result.

EXAMPLE 2.

Example 1 was reproduced without using gas
CO, the treated steel then had a rate of x-ray intensity of phase e of 21%, the remainder consisting of phase Y and a, so it was practically not usable.

 Furthermore, the above process has been reproduced using different amounts of CO and NH3, the results are shown in FIG. 1.

 As is clear from the figure, the intensity level of practically usable x-rays (above about 60%) cannot be obtained unless CO is used in an amount greater than 2.5% and NH3 is used in an amount greater than 10%.

 It has been discovered that when CO and NH3 exceed 4.5% and 30% respectively, the surface hardness becomes greater than the required hardness, which tends to produce cracks when the treated part has to be corrected in tension. In addition, no carbon deposition is observed when the CO content is less than 4.5%. The use of CO and NH3 in excessively increased quantities is not favorable since this leads to an increase in the cost price.

In Fig. 1 the quantity (% by volume) of carbon monoxide (CO) is represented on the abscissa while the intensity rate in (%) of the x-rays of phase e is indicated on the ordinate.

EXAMPLE 3.

 The cold rolled steel parts subjected to the test (S48C indicates in Japanese standards as having a composition of 0.45 to 0.51% in C, 0.15 to 0.35% in Si, 0.6 to 0.9% in Mn, less than 0.03 in P, less than 0.035% in S and the balance in Fe) were heated at 6000C for 2 hours in an atmosphere containing 15% NH3, 11% of a gas endothermic (RX gas) and additional N2 gas.

 The endothermic gas used was composed of 24.5% CO, 31.2 t of H2, 0.26% of CO2, 0.4% of H2O and the complement of N2 and the amount of CO located in the range indicated above.

 The parts thus treated were subjected to the Ono fatigue, curvature and rotation test, the results are shown in FIG. 2. The fatigue limits were found to be 50 kg / mm2, 35.5 kg / mm2 and 25.1 kg / mm2 when the shape coefficients were 1.0, 2.1 and 3.4, respectively. These results were superior to those obtained by a method of the known technique (in which 50% RX gas and 50% NH3 gas are used).

 The Ono curvature and rotation fatigue test is used to determine the curvature and rotation fatigue limit of a metallic substance in the form of a standard test piece, the bending and rotation operations being repeated more than 104 times at room temperature in air. By curvature and rotation means the rotation with respect to a bending moment, of a test piece having its axis in a plane of the bending moment, during which repeated stresses are exerted on the piece to be tested to determine - the fatigue limit.

 The tester includes a rotary shaft for rotating the standard workpiece, a drive motor for the shaft, and a load mechanism having a metal installation suspended from a mounted ball bearing in the inner end part of a cross member of the rotary shaft, and a coil spring.

 When reproducing Example 3 using different amounts of% CO, it was found that the CO content was in the range of 2.5% to 4.5% in order to reach the same level of fatigue limit as in the case of the processes of the known technique.

 In Fig. 2, the form coefficient is indicated on 2 abcisses and the fatigue limit in kg / mm2 is indicated on the ordinate.

 In the examples above, we have illustrated the controlled gases consisting of ammonia gas, carbon monoxide (CO) or RX gas rich in CO and mixed N2 gas, but similar results are obtained when using a controlled gas which contains ammonia gas, carbon monoxide (CO) or an RX gas rich in CO component, and, instead of N2 gas, an NX gas (exothermic gas) containing mainly nitrogen In this case, the CO gas is added in an amount between 1 and 3% since the NX gas contains a certain amount of CO and the total amount of CO gas is sufficient to be in the range between 2.5 and 4.5%.

 As can be understood from the above, in accordance with the invention, the amount of ammonia gas in the controlled gas is reduced between 10 and 30% instead of 40 to 60% as used in the manufacturing processes. the prior art. Carbon monoxide is introduced into a carrier gas, free from which the treated parts have fatigue and wear resistance which are similar or superior to those obtained by the processes of the known technique. The present invention therefore has several advantages such as in particular a considerable reduction in the expensive consumption of ammonia, the reduction in the costs of treatment of gentle nitriding by means of a gas, and a treatment of gentle nitriding by means of a gas. economical with results similar or superior to those obtained by known methods of the technique as well as a reduced cost price of the treated parts.

Claims (6)

 1. Method of gentle nitriding by means of a gas, characterized in that it comprises a step of heating iron parts in a heating treatment oven between approximately 550 to 620oC in a controlled gas which is made up of 10 to 30 t of ammonia gas, 2.5 to 4.5 * of total carbon monoxide (CO) and a gas supplement containing mainly nitrogen N2.  2. Method according to claim 1, characterized in that- the total carbon monoxide consists of 2.5 to 4.5% free carbon dioxide.  3. Method according to claim 1, characterized in that the total carbon monoxide is constituted by an endothermic gas containing carbon monoxide, in an amount such as the carbon monoxide contained in the endothermic gas which is found in said amount of total carbon monoxide.  4. Method according to claim 1, characterized in that the controlled gas is a mixture of ammonia gas, free carbon monoxide and N2 gas respectively in said quantities.  5. Method according to claim 1, characterized in that the total carbon monoxide consists of 1 to 3% of free carbon monoxide and of a complement of a carbon monoxide compound originating from a gas exothermic mainly containing nitrogen N2.  6. Method according to claim 5, characterized in that the free carbon monoxide is replaced by an endothermic gas so that the carbon monoxide compound in the endothermic gas is found in the same amount as the carbon monoxide free.