**5. Carbonitriding**

96 Heat Treatment – Conventional and Novel Applications

corrosion resistance [76].

200

400

600

800

1000

Hardness, HV

1200

1400

1600

1800

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

**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

Duplex- D

0 20 40 60 80 100 120

Carburizing of stainless steel

Martensitic 420- A

Case depth, µm

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.

950 oC for 8 h [75] (with permission from Elsevier Science)

Martensitic 420- B

Austenitic 316- C

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 SAE 1010 steel.

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 TiC0.52N0.48, respectively.