DD279508A1 - METHOD FOR THE GAS CARBONITATION HARDENING OF COMPONENTS OF IRON MATERIALS - Google Patents

Abstract

The invention relates to a method for gas carbonitriding of components made of ferrous materials in nitriding at a temperature range of 700-1 000C. According to the invention, the object is achieved by providing gas mixtures assigned to the surface layer compositions of the components and readjusting the gas compositions in the reaction space with layer-relevant gas parameters and appropriately arranged gas sensors and with the aid of the dimensioning of the ammonia decomposition and the addition of oxygen carriers and carbon carriers to the nominal values of the layer-relevant gas characteristic values become. Fig. 3

Description

For this 3 pages drawings

Field of application of the invention

The invention relates to a method for Gaskarbonitrierhärten of components made of ferrous materials in Nitriding at a temperature range of 700-1000 ⁰ C.

Characteristic of the known state of the art

The gas carbonitriding consists of the enrichment of nitrogen and carbon in the surface layer of the components at temperatures between 700 and 1000 ⁰ C and the subsequent quenching. With increased carbon and nitrogen contents of the austenitic edge structure, the properties of the martensite formed during quenching can be influenced and thus adapted to different requirements. It is possible in principle to produce a bonding layer on the outside of the enriched aistenite layer. It has also been found that when the temperature of the Fe-NC system is exceeded, a thin austenitic surface layer can be formed on the unchanged base material.

(CHATTERJEE, FISCHER and SCHAABER "Hartungtechnische Mitteilungen" 24 11969] 2, pp. 121-124, J. SLYCKE., Csr-bitriding, Division of Engineering Materials Departm. Of Mech. Eng., Linköping University 1979, and LACHTIN and ARSAMASOV "Thermomechanical Treatment Metals ", Metalluergieya Publishing House, Moscow 1 £ S5) deal with the reactions between solid and gas phase as well as with the emerging concentration profiles.

The gas feeds are fixed both in the operation of conventional and in fluidized bed plants on the basis of experience with plant- and batch-related nitration results.

It turns out that a reproducibility of the edge layer properties is not guaranteed.

Object of the invention

The aim of the invention is to improve the reproducibility of the edge layer properties in terms of hardness, corrosion resistance and wear resistance.

Explanation of the essence of the invention

The invention has for its object to provide a method that reliably and easily controls the gas composition in the reaction chamber regardless of the furnace design and the charging.

The object is achieved by a heat treatment in a gas mixture of carbon monoxide, carbon dioxide, ammonia, hydrogen, nitrogen and water vapor.

According to the invention, for the required compositions of the layer, the assigned values of the nitriding index r and the coal index s are determined, taking into account the influence of the alloying elements. Thereafter, the composition of a gas mixture is determined with the specified values of the nitriding and Kohlungsennzahl. Now, a process-specific minimum amount of this gas mixture can be admitted into the reaction space. The layer-relevant gas parameters of the gas mixture which are established in the reaction space are determined with the aid of suitable measuring arrangements, compared with the desired value of the gas mixture and readjusted accordingly.

The values of the nitriding characteristic around the coefficient of carbon number s corresponding to the given time-constant or variable composition of the surface layer are taken from the characteristic concentration diagrams according to FIGS. 1-3 for carbon steels and FIG. 4 and FIG. 5 for alloyed steels.

The diagrams show in each case the existential range of the pure y ^ phase and the regions in which a-y ^ isene or ^ -phase and ε-nitride or y-phase and 9-carbide (cementite) are present side by side.

The gas compositions attributable to the r and s values are determined by predetermining the proportion of ammonia and a carbon carrier in a carrier gas, e.g. As nitrogen and by adjusting the added amount of carbon dioxide and the extent of ammonia decomposition, taking into account the plant-specific ammonia decomposition determined.

The key figures used represent the quotients of the pressures p, (in atm) and (at 1 atm total pressure) from the volume fractions φ, respectively:

₌ P (CO) -P (H ₂ ) ₌ <pR (CO) - <pR (H;) p (HH ₃ ) ₌ (pR (NH ₃ )

^S P (H ₂ O) <pR (H ₂ O) ' ^r p (H ₂ ) 3/2 tpR (H ₂ ) 3/2

Where ρ is the partial pressures and φ is the volume fractions of the gases in the reaction mixture.

For alloyed steels, depending on the qualitative and quantitative alloying additions, the shift of the phase boundary and a possible nitride formation are considered. The change in the austenite dissolved nitrogen and carbon contents by alloying additions at constant nitriding and carbon numbers are taken into account with characteristic concentration diagrams according to FIG. 4 and FIG. 5. In Figure 4, the alloy content is summarized as chromium equivalent. The calculation of the chromium equivalents of the leased steels follows

xinÄq = xct + 0.30x + 0.61 _Mn x _M "+ 0.95x _w (with respect to the Kohlungskennwertes)

XcrAq = X t + 0,28xm "+ 1,04x 0,83xmo * _w (with respect to the Nitrierkennwertes)

The mole fractions X ₁ are calculated from the concentration data of the material analysis.

The mole fractions Xi become dependent on the concrete alloy composition with the conversion formula

(M _Fe , 'Mi /% /

certainly.

(i = Cr, Mn ... C, N, Si ...)

(M _Fe , M, = molecular weights)

In a poor manner, the carbon equivalent is used in characteristic concentration diagrams of Fig. 5 to detect the concentration-reducing influence of alloying elements. The calculation of the carbon equivalent x ^ Aq · takes place after

* c ^s ! Aq = * c + 0.76x _N + 1.94xsi + 0.73x _Ni (in terms of coal coefficient)

Xc '! Aq = Xc + 1.63xn + 4.80x _s , + 1.64x _Ni (in terms of nitriding index)

The agreement of calculations and experimental results of the gas analysis has shown that the accuracy of the method of atmospheric control is good. The composition of the reaction gas mixture is preferably measured with solid electrol / t gas sensors and adjusted to the desired value by the addition of carbon dioxide or other carbon and oxygen carriers and the Ammoniakzorsetzung when given addition of a carbon. Ägers (eg carbon monoxide, endogas or in Spaltretorten processed gas) be varied. The gas state can also be conventionally monitored via the exhaust gas if the conditions in the reaction space remain sufficiently constant. Depending on the temperature, the gas atmospheres are also adjusted to the startup and shutdown phases of the process, but they can also be changed stepwise, intermittently or cyclically during the process in order to intensify the mass transfer and to achieve differentiated results with greater accuracy. A temporally (during the process) variable Schichtz'jsammensetzung is then achieved by varying the gas mixture according to a predetermined regime. The change of the gas mixture is also carried out when the nitriding result can be achieved faster, in particular by the time-limited setting excessive; Equilibrium remote nitriding characteristics of the final state of the

Boundary layer. The comparison of the gas mixture in the reaction chamber with the desired values, as well as the calculation and correction of the fresh gas composition, is preferably carried out with a computer. In order to avoid pore formation, nitrogen concentrations below 1 mass% are set at carbon concentrations <1 mass%. The method enables the computer-aided automated generation of surface layers with differentiated combinations of nitrogen and carbon contents.

With high catalytic activity of the internal surfaces of the plant, marginal nitrogen contents of a few tenths of a percent occur, which can only be slightly influenced by manipulation of the ammonia decomposition. The manipulation of the edge carbon dioxide can be most easily effected by changing the addition of carbon dioxide or other oxygen carriers. Significant influence on the gas parameters have small additions of oxygen or water vapor. On the one hand, air bubbles and gas contamination can be detected immediately, on the other hand, appropriate requirements must be placed on the constancy c he oxidizing impurities.

The process improves the reproducibility of the edge layer properties in terms of hardness, corrosion resistance and wear resistance.

embodiment

The invention will be explained in more detail below using an exemplary embodiment. Show it:

Fig. 1: a characteristic concentration diagram for carbon steels at T = 750 ° C Fig. 2: a characteristic concentration diagram for carbon steels at T = 800 ° C Figure 3: a characteristic concentration diagram for carbon steel at T = 850 ^° C

Gas carbonitriding with CO and H ₂ O addition:

1. Specifications: - Nitrogen content to be set in the surface layer 0.4Ma .-%

- Adjustment of carbon content in the surface layer 0.7% by mass

- nitriding temperature: 850 ⁰ C.

Fresh gas composition:

(Po (NH ₃₎ = 3,0Vol .-% _φ ο (Ν ₂₎ = 50,0Vol .-% (P ₀ (H ₂₎ = 27,0Vol .-% <Po (CO) = 19,0Vol. -% (P ₀ (H ₂ O) = 1.0% by volume

- Materials: Steel C15

2. The target of 0.4 wt .-% N and 0.7 wt .-% of carbon in the surface layer corresponds to FIG. 3 at 850 ⁰ C a Nitrierkennwert r of 0.003, and a carbon characteristic value of s

3. At the o.g. Fresh gas composition, a reaction gas atmosphere was measured with the following chemical composition:

(Pr (NH ₃ ) = 0.1 vol.%

(p _R (N ₂ ) = 50.0% by volume)

(Pr (H ₂ ) = 30.9 vol.%

(Ph (CO) = 18.1 vol.%

(Ph (CO ₂ ) = 0.3% by volume

(Pn (H ₂ O) = 0.6% by volume

4. From the chemical composition of the reaction gas atmosphere, the following process parameters were obtained: nitriding index r = 0.004

Carbon parameter s = 8.85

5. Result:

Experimental investigations have led to a steel C15 forming a boundary layer of about 50 μm with a nitrogen content of 0.42% by mass and a carbon content of 0.65% by mass.

Claims (4)

1. A method for gas carbonitriding of FJauteilen of iron materials in gas mixtures of ammonia, nitrogen, hydrogen, carbon monoxide and other carbon and Sauerstoffträ, orn, orn, characterized in that for predetermined layer compositions and treatment temperatures, the required nitriding characteristics round coal characteristics saus characteristic value-concentration diagrams are determined , determined therefrom, the chemical composition of a gas mixture with these required r and s values, a minimum amount of this gas mixture, however, supplied with an excess of decomposed ammonia to the reaction space, then determined the composition of the adjusting gas mixture in the reaction space and deviations from the desired value using the Variation of the proportions of oxygen and carbon carriers and the variable proportion of decomposed ammonia is regulated. 2. The method according to claim 1, characterized in that for the given, time-constant or variable composition rJer on a non-alloy and an alloyed material layer to be generated, the associated values of r and s are taken from characteristic concentration diagrams and the required R and s values corresponding composition of the gas mixture by specifying the proportions of ammonia, nitrogen and a carbon support and by adjusting the amount of carbon dioxide to be added and the extent of Ammoniakzrjrsetzunrj 'means and as plant-specific Mmdestgasmenge, but with an increased proportion of decomposed ammonia is supplied to the reaction space. 3. The method according to claim 1 and 2, dadurcr characterized in that the oxygen content in the fresh gas and the oxygen potential in the effective reaction gas are measured and the characteristic values in case of deviations from the target word by adjusting ng the amount of added carbon support. *. And the amount of ammonia decomposition regulated be, with process variable management with time variable gas parameters. 4. The method according to claim 1 to 3, characterized in that a time-varying layer composition, including a Voroxydation or a ^{x /} orentkohlung carried by a different composition of the gas atmosphere.