**9.3. Carbonitriding and diffusion annealing at a coexistence of the α-**γ **diffusion couple**

This hybrid treatment explores simultaneous carbonitriding at the coating gas interface and carburizing at the coating-substrate interface. At carbonitriding temperatures the substrate also acts as a source of carbon and, in fact, during these processes the flux of the element causing hardening (C,N) is moving from two interfaces the substrate/coating and gas/coating [109]. The resultant microstructure is shown in Fig. 25.

**Figure 25.** Microstructure of Fe-10%Ni coating after nitrocarburizing at 670 oC for 1.5 h in solid medium [109] (with permission from Springer Verlag)

The microhardness profile across the coating exhibits the maximum located in the subsurface region (Fig. 26). A comparison with the corresponding microstructure indicates that the hardness peak is caused by a layer of carbonitrides, typically situated in the near-surface region. It should be emphasized that during carbonitriding, the microstructural changes in the coating are accompanied by the changes in the substrate. The extent of those changes is essentially the same as that described previously for diffusion annealing.

**Figure 26.** Hardness depth profile within Fe-10%Ni coating on steel substrate after nitrocarburizing [109] (with permission from Springer Verlag)

## **10. Summary**

104 Heat Treatment – Conventional and Novel Applications

(with permission from Springer Verlag)

0

100

200

300

400

Microhardness, HV

500

600

700

**Carburizing**

substrate-coating interface

**couple** 

**Figure 24.** Hardness depth profile within Fe-10%Ni coating on steel substrate after carburizing [109]

0 50 100 150 200 250 300 350 400

920oC-1.5h-air

920oC-1.5h-water

710oC-1.5h-air

gas-coating interface

Coating thickness, µm

**9.3. Carbonitriding and diffusion annealing at a coexistence of the α-**γ **diffusion** 

This hybrid treatment explores simultaneous carbonitriding at the coating gas interface and carburizing at the coating-substrate interface. At carbonitriding temperatures the substrate also acts as a source of carbon and, in fact, during these processes the flux of the element causing hardening (C,N) is moving from two interfaces the substrate/coating and

**Figure 25.** Microstructure of Fe-10%Ni coating after nitrocarburizing at 670 oC for 1.5 h in solid

medium [109] (with permission from Springer Verlag)

gas/coating [109]. The resultant microstructure is shown in Fig. 25.

This chapter shows a variety of surface modification technologies, exploring the phenomenon of thermochemical diffusion. Although an idea of the thermochemical treatment originated at the beginning of the 20th century, it is still a subject of scientific research. At the commercial level, there is a continuous improvement of existing technologies, expansion to novel treatments and a search for unique applications. Of particular interests are hybrids which explore a combination of conventional thermochemical processes with new techniques of surface engineering, including surface deformations, cladding, coatings or laser modifications. In practice, a selection of the optimum technique depends on the component size, geometry, material chemistry, service requirements and the process economy. In recent years, also an environmental aspect is getting a growing attention. The key to benefit from opportunities created by thermochemical treatments is knowledge of capabilities of each technology for a particular substrate material under specific service conditions and its implementation at the stage of a component design.
