The following points highlights the four main methods used for nitriding of steels. The methods are:- 1. Gas-Nitriding 2. Single-Stage and Double-Stage Nitriding 3. Ionitriding (Plasma), or Glow-Discharge Nitriding 4. Liquid Nitriding.
Method # 1. Gas-Nitriding:
Gas nitriding is the most commonly used nitriding method. In the original method, components are heated at about 510°C (500-575°C) in an atmosphere of dissociated ammonia gas. At the nitriding temperatures, ammonia dissociates at the steel surface to give nitrogen in the atomic-form and which is absorbed by the steel-
NH3 <=> 3/2 H2 + N(Fe) …(8.49)
where, N(Fe) is the nitrogen absorbed at the surface of the steel. The equilibrium constant of the reaction 8.49 is-
where, K and k are constants at a given temperature. Equation 8.51 gives the % nitrogen at the surface of the steel, or it is given by the maximum solubility of nitrogen in ferrite at that temperature, whichever is lower.
However, ammonia gas itself, at the nitriding temperatures, dissociates as:
NH3 à 1/2 N2 + 3/2 H2 …(8.52)
but, this reaction is very slow. To control the nitrogen potential in the nitriding furnace, a mixture of gases (NH3, N2, H2) is fed to the furnace. In actual practice, ammonia gas is dissociated in a separate reaction chamber, which is then mixed with appropriate amount of NH3, and supplied to the nitriding furnace.
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Water vapours of ammonia gas are removed by passing it through a filter of unslaked lime. A continuous check of the nitrogen potential of the atmosphere is kept by determining the composition of the exit gases by means of a dissociation pipette.
The molecular nitrogen also decomposes at the nitriding temperatures at the steel surface to be absorbed and then diffused inside as:
where, k’ is a constant. However, at a given temperature, the constant k’ is orders of magnitude smaller than the constant k of the equation 8.51. For example, a mixture of NH3 and H2 having 18% NH3 and 82% H2 (at one atmospheric pressure) gives the same percentage of nitrogen dissolved in iron as 5 x 103 atmospheres of N2 at 500°C. Hence, it is not practically easy to use nitrogen gas for nitriding.
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In practice, nitriding is done in dissociated anhydrous ammonia at a potential when iron-nitride is just formed at the surface, i.e., nitriding is done under the saturated conditions. Depending upon the alloy content and the temperature, it takes 48 to 96 hours to get a case of 1 mm thickness. Fig. 8.40 illustrates effect of time on case depth at a nitriding temperature of 525°C in Al-Cr-Ni-Mo steel.
Fig. 8.41 illustrates that case hardness decreases as the nitriding temperature increases, though case-depth is higher at higher temperature of nitriding (Fig. 8.37). Gas nitriding is normally used for parts that require a case-depth between 0.2 and 0.7 mm. Nitriding may be done in electric-heating furnaces. Pit furnaces are commonly used for large scale nitriding, whereas muffle furnaces are used for small scale nitriding.
The components to be nitrided should be cleaned well and degreased. There should not be any trace of rust or mill scale. Selective nitriding, or the areas of the part to be allowed to remain soft, need either an electrolytic coating of copper, tin, or nickel, which prevents nitrogen from diffusing into the steel, or by painting with the “stopping-off” agents containing tin to the surfaces to be protected against nitriding.
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As tin becomes molten at the nitriding temperature, it is essential that this coating should be so thin (< 0.01 mm) that it does not spread to surfaces that are to be nitrided. When the nitriding has been completed, the parts are allowed to cool to at least 200°C in an atmosphere of ammonia, and only then the gas supply is cut off. Nitrided part after gas-nitriding normally exhibits a characteristics matte-grey colour due to some oxygen present in the atmosphere.
Method # 2. Single-Stage and Double-Stage Nitriding:
In the single-stage nitriding process, the temperature is in the range of 495°C to 525°C and the dissociated ammonia in the mixture is 15-30%. Time of nitriding depends on the case-depth desired as illustrated in Fig. 8.40. Single-stage nitriding having low dissociated NH, maintains high nitrogen potential in the atmosphere, and thus, also produces thick and brittle white-layer at the surface of nitrided case, which is undesirable.
The double-stage nitriding, also called ‘Floe process’ has the advantage of reducing the thickness of the white layer, or completely eliminating it. Normally, the first-stage is carried out in about the same conditions as in single stage method, i.e., in range of 495°-525°C for 15-20 hours under normal gas atmosphere of high nitrogen activity, i.e., with dissociated ammonia of ≈ 20% to achieve faster growth of nitrided case. The second-stage can be done in the same temperature range, but in an atmosphere of low nitrogen activity, i.e., with 75 to 80% of dissociated ammonia.
Normally, the temperature range is between 550°-565°C. In the second-stage, the activity of nitrogen is lower than that required for the formation of iron nitrides. Thus, iron nitrides, if formed in first stage, dissociate during this period, and the thickness of the white-layer gets reduced from its normal thickness of – 0.05 mm to less than 0.01 mm.
If the second stage is continued further, the white layer may be completely eliminated, but some nitrogen from the nitrided case may also escape leading to a decrease in the surface hardness. Elimination of white-layer also means the case-depth is increased by that thickness. Thus, the two-stage nitriding apparently gives more case depth.
The higher nitriding temperatures in the second- stage results in- lower case hardness; increased case depth; may lower the core hardness depending on the prior tempering temperature.
Other Gas Mixtures:
The formation of white layer can be prevented by using a mixture of NH3 and H2. Nitriding can begin with higher activity of nitrogen than required to form Fe4N, but then before Fe4N nucleates, the NH3/H2 ratio is reduced (this helps to reduce the time of nitriding) so that the activity of nitrogen is lesser than required to nucleate Fe4N.
The process requires closer control over the process, but it gives any desired case depth, high surface hardness and no white layer on the surfaces. In another practice, a mixture of 20% NH3 and 80% N2 has been used to have low activity of nitrogen with a tougher case.
Pressure nitriding is nitriding with ammonia under pressure by using a sealed retort which had been charged with parts, evacuated and filled with ammonia to a predetermined pressure, normally 50-100 g of NH, per square meter of surface to be nitrided, and then heated. This method can handle shapes that are difficult to be handled by other methods. White layer can be controlled, but if kept below 0.00025-0.0005 mm, the case depth cannot exceed 0.5 mm to 0.63 mm.
Bright nitriding method uses NH3 and H2 gas mixture. In this method, furnace gas in continuously withdrawn to be passed through NaOH-water solution to remove traces of HCN produced. This improves the rate of nitriding. As this method produces little, or no white layer, that is why the name-bright nitriding. If present, then white layer consists of more ductile Fe4N phase.
Method # 3. Ionitriding (Plasma), or Glow-Discharge Nitriding:
It is a case-hardening process of enriching the surface of the steel with nascent nitrogen by glow discharge method. Parts to be nitrided are cleaned, degreased and are then charged in the vessel, which is then evacuated to 0.05-0.10 torr. The part is made the cathode. The parts are heated by electrical heaters to heat them to 375° to 650°C. Then the gas mixture of N2 and H2 are backfilled with reduced pressure of 1-10 torr.
High voltage is impressed between the cathode (part) and anode to form plasma, through which nitrogen ions are accelerated to impinge on the parts. This ion-bombardment cleans the surface, heats the parts and provides the nascent nitrogen. Fig. 8.44 illustrates the plasma region.
Voltage (D.C.) and current are kept in the ion nitriding region (Fig. 8.44). Once a glow is established, it completely envelops the part (see Fig. 8.45) and then the nitriding starts. The anode is kept cool by circulating water around it. The temperature of the parts can be controlled by controlling the current density and or the pressure of N2 and H2 gases.
Under the high voltage of 500-1000V, nitrogen gas is dissociated, Glow-discharge ionised and accelerated towards the parts. The positively-charged-nitrogen-ion then picks up an electron from the part (cathode) and emits a photon. This photon emission causes visible glow discharge. The nitrogen atom, thus formed, strikes the part to heat it. A desirable glow discharge-thickness is about 6 mm. unless the parts have holes. The nitrided case depth depends on the nitriding current, temperature and process lime.
Fig. 8.47 illustrates effect of time on case depth for three steels. Fig. 8.48 shows effect of time and temperature on case depth. Fig. 8.49 illustrates hardness gradient in some steels in the nitrided case. It is possible to control the chemistry of white layer as illustrated in Fig. 8.46. After completing ion-nitriding, the parts are cooled in inert atmosphere of N2 gas.
Plasma nitriding has better control over uniformity of case depth, the chemistry of case and even results in low distortion of parts. It is possible to select either e or Y layer, or even complete elimination of white layer.
Some of other advantages of nitriding are:
1. White layer and its chemistry can he controlled in ion-nitriding or may be eliminated completely. ε layer has good wear and fatigue resistance (used for fuel-injection system), γ’ is softer and tougher. White layer is kept as it has lubricity.
2. Growth of ion-nitrided parts are normally within design allowances and thus may be used directly.
3. Fatigue life is better than that obtained in gas nitriding by about 80% in 30CrMoV1 and 130% in plain carbon steels.
4. Low gas and power consumption makes this process a cheap process.
5. Speed of nitriding is almost five times of gas-nitriding.
6. Mechanical masking even by mild steel which has indefinite life makes it much cheaper method.
7. Since ion-nitriding uses low temperatures, even cold-worked steels can be ion-nitrided without effecting the basic properties.
8. Case-depth can be controlled better.
9. No environmental hazards. The process can be automated.
Disadvantages:
1. The equipment is complex and needs closer control.
2. The parameters of the process have to be strictly controlled.
3. High initial equipment cost.
4. Different shapes and size parts cannot be ion-nitrided together.
5. Highly skilled personnel are required.
Method # 4. Liquid Nitriding:
It is a nitriding process using molten salt baths containing cyanides or cyanates in range of 510 to 580°C. It provides similar advantages as gas nitriding. Liquid nitriding is used where low case-depth is required and is then cheaper. Tools and small components are often liquid-nitrided. Table 8.10 gives composition of some nitriding baths. Normally the cyanide salt is melted and aged at the nitriding temperature for 12-24 hours, before the parts are immersed into the bath.
During ageing period, cyanate content increases to desired level of 45%, and cyanide content decreases. Nitriding action of the bath depends on cyanate content of bath rather, the ratio cyanide to cyanate is critical. Cyanide-free salt baths have been introduced.
During ageing, the following reactions occur, say in NaCN bath:
Nitrogen, as in reaction (8.18) is absorbed by steel which then diffuses inside. Baths are dredged periodically to remove the sodium carbonate, and fresh sodium cyanide is added. KCl acts as diluting agent to promote fluidity.
A working bath may have the typical composition of:
Figure 8.50 illustrates case depths for two steels after liquid nitriding. The parts to be nitrided should be thoroughly cleaned and preheated (350-480°C) before immersing in bath. Carbonate content be kept below 25%. Aerated bath nitriding is a process in which calculated amount of air is pumped into bath to increase chemical activity (equation 8.16) and agitation. It gives a case of 0.3 mm in plain carbon steels in 90 minutes. It is a process actually specified for nitriding of plain carbon steels.
Tufftride Process:
It is a liquid nitriding process in which salts are melted in a titanium crucible. Iron crucible was used to decompose the salts by reaction and change the cyanate content of the bath. Tufftriding is done at a temperature of 570°C in a bath of molten salts (85% shall be consisting of 40% NaCNO, 60% NaCN and 15% Na2CO3) through which dry air is pumped in. Preheated (350-480°C) parts (to be nitrided) are immersed in it.
As temperatures used are low, mainly nitrogen (some carbon also and thus, sometimes called nitro-carburising) diffuses. A thin layer of ε-mainly Fe3 (N, C) (7-15 µm) forms on the surface of the steel. Below this is a diffusion zone consisting of nitrogen dissolved in ferrite and some γ’ phase. Total depth is 0.15-0.5 mm. The hardness of the layer is similar to gas nitriding i.e., 300-350 VPN for plain carbon steels and 600-1100 VPN for alloy steels.
The ε layer is tough, wear and galling resistant. The components after tuff riding (normally for 10-180 minutes), are cooled in an oxidising bath, (hydroxide salt bath) maintained at 350-400°C for 10-20 minutes. This improves corrosion resistance too. Fatigue properties are also improved.