**3.1. Process technology**

The salt bath nitrocarburizing by Tufftride® is performed in a mixture of alkali cyanate and alkali carbonate in the temperature range of 480-630 oC [56]. The gas nitrocarburizing has been developed as a cleaner alternative to the salt bath technology. Besides ammonia, required to supply the nascent nitrogen, the nitrocarburizing atmospheres contain carbonbearing additives like exothermic and endothermic gases CO2, CO and H2 as products of the dissociation of methanol. There is also a hybrid treatment, integrating the low temperature plasma nitriding and additions of carburizing species to the plasma media to cause the simultaneous incorporation of nitrogen and carbon. However, using the glow discharge of plasma containing nitrogen and carbon species it is difficult to produce a single ε-Fe2-3(N,C) phase compound layer on engineering steels [57]. Instead, the plasma nitrocarburizing generates a compound layer with mixed phases of ε-Fe2-3(N,C) and γ'-Fe4(N,C), known to be detrimental in tribological applications, especially under impact loads. According to [58], excluding for low-carbon steel or to some extent for medium carbon unalloyed steel, gas nitrocarburizing does not produce compound or diffusion layers faster than gas nitriding. Moreover, the properties of nitrocarburized parts are not always superior to those obtained by nitriding and the nitrocarburizing process is more difficult to control.

There is a substantial difference between gas and plasma nitrocarburizing when considering an environmental aspect. As shown in Table 3, in addition to drastic reduction of CO, CO2 and NOx emission, reaching 500-5000 times by plasma technology, the total gas consumption is at least 10 times lower than in the gas process [57].


**Table 3.** Emission data for plasma and gas nitrocarburizing [57]

Among different nitrocarburizing techniques, a special attention is paid to low pressure processes, performed in a mixture of NH3 and CO2. Since CO2 containing gas has a high oxygen potential, especially under low pressure conditions, oxygen atoms accelerate the formation of Fe3(N,C) and contributes to the growth of adherent nitrocarburized layers [59]. The process named Nitreg-ONC® is based on Nitreg® technology but generates a modified complex compound layer which contains an increased concentration of carbon, oxygen and sulphur [60]. As a result, surface retains its high wear resistance, anti-scuffing and anti-seizing properties. Another advantage is the substantially increased corrosion resistance which for carbon steels reaches a level comparable with stainless grades. The increased corrosion resistance is associated with the superficial oxide structure which is not penetrable by corrosive fluids. In general, the treatment is considered a superior alternative to chrome plating.

## **3.2. Surface layer structure**

92 Heat Treatment – Conventional and Novel Applications

**3. Nitrocarburizing (ferritic nitrocarburizing)** 

carburizing process at temperatures of the ferritic state.

**3.1. Process technology** 

performed as a pre-treatment before the plasma nitriding. In addition to Al2O3 and Al-

During nitrocarburizing, nitrogen and carbon are supplied to the surface of steel in ferritic state at temperatures usually between 500 and 580 oC. The general classification of thermochemical treatments involving nitrogen and/or carbon is shown in Fig.17. According to some terminology, the high temperature equivalent of ferritic nitrocarburizing is called as austenitic nitrocarburizing. There is also a term of ferritic carburizing, describing the

50wt%Mg powders, nitrogen gas is introduced and the content is heated to 630 oC.

**Figure 17.** Classification of basic thermochemical treatments involving nitrogen and carbon

by nitriding and the nitrocarburizing process is more difficult to control.

The salt bath nitrocarburizing by Tufftride® is performed in a mixture of alkali cyanate and alkali carbonate in the temperature range of 480-630 oC [56]. The gas nitrocarburizing has been developed as a cleaner alternative to the salt bath technology. Besides ammonia, required to supply the nascent nitrogen, the nitrocarburizing atmospheres contain carbonbearing additives like exothermic and endothermic gases CO2, CO and H2 as products of the dissociation of methanol. There is also a hybrid treatment, integrating the low temperature plasma nitriding and additions of carburizing species to the plasma media to cause the simultaneous incorporation of nitrogen and carbon. However, using the glow discharge of plasma containing nitrogen and carbon species it is difficult to produce a single ε-Fe2-3(N,C) phase compound layer on engineering steels [57]. Instead, the plasma nitrocarburizing generates a compound layer with mixed phases of ε-Fe2-3(N,C) and γ'-Fe4(N,C), known to be detrimental in tribological applications, especially under impact loads. According to [58], excluding for low-carbon steel or to some extent for medium carbon unalloyed steel, gas nitrocarburizing does not produce compound or diffusion layers faster than gas nitriding. Moreover, the properties of nitrocarburized parts are not always superior to those obtained

During nitrocarburizing of iron, the microstructural evolution of the compound layer starts with the formation of carbon-rich cementite and develops into the direction of nitrogenricher and carbon-poorer phases of ε and γ' [61]. Both steps are a consequence of higher solubility of nitrogen in α-Fe than carbon and lower rate of nitrogen transfer from the gas into the solid phase. The compound layer is typically composed of carbonitrides of iron ε-Fe3(N,C)1+x and γ'-Fe4(N,C)1-z along with θ-Fe3C cementite (Table 4) [62] [63]. As during nitriding, beneath the compound layer the diffusion zone forms with carbon and nitrogen being dissolved in the ferritic matrix. It is well documented that the best properties are achieved when the compound layer contains predominantly the single ε phase (Fig. 18a). The compound layer, typically in the range of 20 µm, leads to significant improvements in hardness, wear and corrosion resistance. A presence of ammonia in gas nitrocarburizing atmosphere affects the compound layer structure and a presence of cementite Fe3C. During ferritic carburizing of iron at a temperature of 550 oC in gas atmospheres containing a certain

#### 94 Heat Treatment – Conventional and Novel Applications

amount of NH3, massive layers of cementite Fe3C can be grown [64]. In order to generate thicker layers, the nitrocarburizing process is conducted at temperatures exceeding the Fe-N eutectoid point of 590 oC. After austenitic plasma nitrocarburizing at 700 oC for 3 h of 0.45% C steel the layer contains mainly the ε-Fe2-3(N,C) phase but unlike after ferritic nitrocarburizing process, the austenite layer forms between the ε phase and diffusion zone (Fig. 18b) [57].

Thermochemical Treatment of Metals 95

In addition to application of nitrocarburizing to carbon and nitriding steels to increase their surface hardness and tribological performance it is also used to stainless steels and special alloys. After low-temperature plasma nitrocarburizing at 450 oC of austenitic stainless steel AISI 304, the dual layer structure grows with a nitrogen-enriched layer on top of a carbon enriched layer. Both layers are free of nitride and carbide precipitates [70]. In addition to increased surface hardness up to 1500 HV and improved wear resistance, the corrosion resistance is also positively altered. There is a difference in corrosion resistance between processes conducted at various temperatures below 450 oC. When treatments conducted at 380oC and 415oC lead to similar properties, increasing temperature to 430 oC causes slightly higher corrosion resistance. The latter is attributed to the formation of a small amount of chromium nitride in the nitrogen-enriched surface layer. The overall improvement in corrosion resistance after nitrocarburizing of AISI 304 stainless steel is thought to be due to the extremely large supersaturation of an upper part of the nitrogen-enriched layer with both nitrogen and carbon. Also sintered Astaloy CrM® + 0.3% C, nitrocarburized in a salt bath at 580 oC for at least 2 h, experiences an increased corrosion resistance [71]. Its surface

layer after the treatment is dominated by the ε-iron carbonitride Fe2-3(CN).

content is deliberately selected to retain the core toughness.

surface. The key reactions of carburizing involve [72]:

The objective of carburizing is to enrich surface layers of steel or other alloys with carbon. To achieve the sufficient carbon solubility and penetration depth the treatment is carried out at relatively high temperatures of 900-950 oC. As a result, steels, which do not have the sufficient carbon content within their volume, obtain the hard surface. The reduced carbon

The endothermic carburizing atmospheres consist of a mixture of carburizing ingredients such as CO and CH4 and decarburizing ones such as CO2 and H2O. To control the process, the carburizing potential of the furnace atmosphere requires the measurement of all the gas constituents CO, CO2, CH4 and H2O. The driving force for carburizing is determined by the gradient between potentials of carbon in the furnace atmosphere and carbon at the steel

A variety of applications of steel carburizing were explored for decades with typical examples of automotive gears. This includes also stainless steels, in particular the ferritic and austenitic stainless grades. Recently, the carburizing process creates a growing attention in area of martensitic stainless steels. A comparison of hardness depth profiles for all three

(g Fe) 2 2CO C CO → + <sup>−</sup> (3)

CH C 2H 4 (g Fe) 2 → + <sup>−</sup> (4)

CO H C H O 2 (g Fe) 2 <sup>−</sup> +↔ + (5)

**3.3. Applications** 

**4. Carburizing** 

**Figure 18.** Microstructure differences after low temperature and high temperature plasma nitrocarburizing in atmosphere of 87% N2 + 8% H2 + 5% CO2: (a) Armco iron, 570oC for 3h; (b) 0.45% C steel, 700oC for 3 h [57] (with permission from Elsevier Science)

There are benefits to surface corrosion resistance after combining the plasma nitrocarburizing and oxidizing [65]. The carbonitrided SKD61 steel with a 10 µm thick compound layer (predominantly ε-Fe2-3(N,C) and small proportions of γ'- Fe4(N,C)) and 200 µm thick diffusion layer subjected to plasma oxidation at 500 oC for 1 h creates 1-2 µm thick magnetite Fe3O4 layer on top of the compound layer [66]. According to the anodic polarization test, a significant improvement in the steel corrosion resistance is achieved.


**Table 4.** Characteristics of phases in Fe-N-C system at 580-590 oC

#### **3.3. Applications**

94 Heat Treatment – Conventional and Novel Applications

(Fig. 18b) [57].

amount of NH3, massive layers of cementite Fe3C can be grown [64]. In order to generate thicker layers, the nitrocarburizing process is conducted at temperatures exceeding the Fe-N eutectoid point of 590 oC. After austenitic plasma nitrocarburizing at 700 oC for 3 h of 0.45% C steel the layer contains mainly the ε-Fe2-3(N,C) phase but unlike after ferritic nitrocarburizing process, the austenite layer forms between the ε phase and diffusion zone

**Figure 18.** Microstructure differences after low temperature and high temperature plasma

steel, 700oC for 3 h [57] (with permission from Elsevier Science)

Phase N (at. %) C (at. %) Crystallography Atom

α-Fe 0-37 0-0.02 Fe bcc, N, C in

γ'-Fe4N1-z 19.4-20 <0.7 Fe fcc, N ordered in

ε-Fe3(N,C)1+x 15-33 0-8 Fe hcp, N ordered in

θ-Fe3C 0 25 Fe complicated

**Table 4.** Characteristics of phases in Fe-N-C system at 580-590 oC

nitrocarburizing in atmosphere of 87% N2 + 8% H2 + 5% CO2: (a) Armco iron, 570oC for 3h; (b) 0.45% C

There are benefits to surface corrosion resistance after combining the plasma nitrocarburizing and oxidizing [65]. The carbonitrided SKD61 steel with a 10 µm thick compound layer (predominantly ε-Fe2-3(N,C) and small proportions of γ'- Fe4(N,C)) and 200 µm thick diffusion layer subjected to plasma oxidation at 500 oC for 1 h creates 1-2 µm thick magnetite Fe3O4 layer on top of the compound layer [66]. According to the anodic polarization test, a significant improvement in the steel corrosion resistance is achieved.

orthorhombic

arrangement

octahedral sites

C in bicapped trigonal prisms

octahedral sites

octahedral sites

central

Reference

[17]

[64] [67]

[61] [68] [69]

[61] [68] [69]

In addition to application of nitrocarburizing to carbon and nitriding steels to increase their surface hardness and tribological performance it is also used to stainless steels and special alloys. After low-temperature plasma nitrocarburizing at 450 oC of austenitic stainless steel AISI 304, the dual layer structure grows with a nitrogen-enriched layer on top of a carbon enriched layer. Both layers are free of nitride and carbide precipitates [70]. In addition to increased surface hardness up to 1500 HV and improved wear resistance, the corrosion resistance is also positively altered. There is a difference in corrosion resistance between processes conducted at various temperatures below 450 oC. When treatments conducted at 380oC and 415oC lead to similar properties, increasing temperature to 430 oC causes slightly higher corrosion resistance. The latter is attributed to the formation of a small amount of chromium nitride in the nitrogen-enriched surface layer. The overall improvement in corrosion resistance after nitrocarburizing of AISI 304 stainless steel is thought to be due to the extremely large supersaturation of an upper part of the nitrogen-enriched layer with both nitrogen and carbon. Also sintered Astaloy CrM® + 0.3% C, nitrocarburized in a salt bath at 580 oC for at least 2 h, experiences an increased corrosion resistance [71]. Its surface layer after the treatment is dominated by the ε-iron carbonitride Fe2-3(CN).

### **4. Carburizing**

The objective of carburizing is to enrich surface layers of steel or other alloys with carbon. To achieve the sufficient carbon solubility and penetration depth the treatment is carried out at relatively high temperatures of 900-950 oC. As a result, steels, which do not have the sufficient carbon content within their volume, obtain the hard surface. The reduced carbon content is deliberately selected to retain the core toughness.

The endothermic carburizing atmospheres consist of a mixture of carburizing ingredients such as CO and CH4 and decarburizing ones such as CO2 and H2O. To control the process, the carburizing potential of the furnace atmosphere requires the measurement of all the gas constituents CO, CO2, CH4 and H2O. The driving force for carburizing is determined by the gradient between potentials of carbon in the furnace atmosphere and carbon at the steel surface. The key reactions of carburizing involve [72]:

$$\text{2CO} \rightarrow \text{C}\_{\text{(g-Fe)}} + \text{CO}\_2 \tag{3}$$

$$\text{CH}\_4 \rightarrow \text{C}\_{\text{(g-Fe)}} + 2\text{H}\_2 \tag{4}$$

$$\text{CO} + \text{H}\_2 \leftrightarrow \text{C}\_{\text{(g-Fe)}} + \text{H}\_2\text{O} \tag{5}$$

A variety of applications of steel carburizing were explored for decades with typical examples of automotive gears. This includes also stainless steels, in particular the ferritic and austenitic stainless grades. Recently, the carburizing process creates a growing attention in area of martensitic stainless steels. A comparison of hardness depth profiles for all three families of stainless steels is shown in Fig. 19. A relatively novel, low temperature gas carburizing at 470oC increases the surface hardness of AISI 316 austenitic stainless steel from 200 HV to 1000 HV through extreme supersaturation of up to 12 at.% carbon in the solid solution [73] [74]. After treatment, two types of carbides M5C2 and M7C3 form with long needles or laths morphology, exhibiting the special orientation relationship with the austenitic matrix (Fig. 20). It is claimed that the carburizing technique which combines the superplastic deformation and the carbon diffusion generates a thicker layer and substantially higher hardness [75]. For duplex stainless steel JIS US329J1 the surface hardness of 1648 HV is achieved as compared to 1300 HV for conventional carburizing. The plasma carburizing of AISI 410 stainless steel in a gas mixture of 80% H2 + 20% Ar with 0.5- 1% of CH4 by volume, leads to surface hardness of 600-800 HV with no evidence of reduced corrosion resistance [76].

Thermochemical Treatment of Metals 97

During carburizing of silicon with carbon, pre-deposited from a carbon source a 3C-SiC(111) film forms because it is well lattice-matched with Si(110). The buffer layer of 3C-SiC(111)

**Figure 20.** Optical microstructure of AISI 316 austenitic stainless steel after carburizing at 470 oC for 246 h (a) [74] and TEM image with selected area electron diffraction pattern showing carbide morphology

Carbonitriding is a process similar to carburizing whereby a source of nitrogen is added to the carburizing atmosphere which results in simultaneous incorporation of carbon and nitrogen into alloy surface. Sometimes carbonitriding is confused with nitrocarburizing. It is usually a two-step treatment, conducted at temperatures of 800-940 oC in an environment containing both carbon and nitrogen and is followed by quenching. At carbonitriding temperatures, which are substantially higher than that used during nitriding or nitrocarburizing, steel is in the austenitic state, having high solubility of carbon. To improve toughness, quenching is followed by the second step of low-temperature tempering or stress relieving. At the processing stage, nitrogen inhibits diffusion of carbon, resulting in thinner case, improves hardenability and forms nitrides. After treatment, a presence of nitrogen in carburized steel increases hardness, wear resistance and delays tempering. The latter is of importance for elevated temperature applications. Carbonitriding is widely accepted for surface improvement of plain carbon steels, having low hardenability. According to the comparative study of both processes, carbonitriding and nitrocarburizing develop the compressive stress and are associated with the size and shape distortion [80]. However, nitrocarburizing causes lower compressive stress and size/shape distortion, as is the case for

Since carbon and nitrogen form with titanium the hard carbides and nitrides, carbonitriding is applicable to titanium and its alloys. In case of laser gas assisted carbonitriding of Ti-6Al-4V alloy, the 55 µm thick layer composed of TiCxN1-x, TiN and TiC phases grow [81]. In case of pure titanium, carbonitriding at 850 oC for 5 h forms the near-surface layer of carbonitrides and thick layer of α-stabilized solid solution of titanium with nitrogen and oxygen [82]. As the partial nitrogen pressure changes from 105 Pa to 100 Pa and to 10 Pa the surface hardness decreases and composition alters to TiC0.25N0.75 to TiC0.50N0.50 and

(b, c) [73] (with permission from Elsevier Science)

**5. Carbonitriding** 

SAE 1010 steel.

TiC0.52N0.48, respectively.

consists of hexagonal arrays that act as templates for graphene nucleation and growth.

**Figure 19.** Hardness depth profiles for carburized stainless steel of different grades: A – AISI 420 martensitic stainless steel, carburized in low temperature plasma at 450 oC for 4 h in 1% CH4 [76]; B – as A but CH4 concentration of 0.5%; C – AISI 316 austenitic stainless steel, gas carburized at 470 oC for 246 h [74]; D –JIS SUS329J1 duplex stainless steel, superplastically deformed and carburized in powder at 950 oC for 8 h [75] (with permission from Elsevier Science)

In the area of non-ferrous alloys, carburizing is used to increase the wear resistance of some titanium alloys. As a result of double-glow plasma carburizing of the Ti2AlNb orthorhombic alloy, the layer of 40 µm with a hardness of 1051 HV and decreasing carbon content develops [77]. Also plasma carburizing of pure titanium in hydrogen free atmosphere is capable of creating the superficial carburized layer with special characteristics [78]. Of novel applications, carburizing of silicon is portrayed as an inexpensive *in situ* method of forming graphene on silicon wafer [79]. The process is seen as an alternative to the silicon technology. During carburizing of silicon with carbon, pre-deposited from a carbon source a 3C-SiC(111) film forms because it is well lattice-matched with Si(110). The buffer layer of 3C-SiC(111) consists of hexagonal arrays that act as templates for graphene nucleation and growth.

**Figure 20.** Optical microstructure of AISI 316 austenitic stainless steel after carburizing at 470 oC for 246 h (a) [74] and TEM image with selected area electron diffraction pattern showing carbide morphology (b, c) [73] (with permission from Elsevier Science)
