**9.2. Carburizing and diffusion annealing at a coexistence of the** γ **-** γ **diffusion couple**

102 Heat Treatment – Conventional and Novel Applications

environment and from the substrate

**couple** 

directions.

**Figure 22.** Concept of thermochemical treatment of a coating, exploring simultaneous diffusion from an

**9.1. Carburizing and diffusion annealing at a coexistence of the** α **-** γ **diffusion** 

During annealing at temperatures below α-γ transformation of the steel substrate, the coating already contains a significant amount of the γ phase, which is the solid solution of Ni in γFe (fcc). At the same time, the substrate during annealing remained fully pearlitic. Thus, at the annealing temperature, cementite coagulated, dissolved and acted as a source of carbon, diffusing towards coating. Due to a relatively low temperature, the transport of large amounts of carbon for long distances within coating was difficult. After 30 min of annealing at 710 oC, the mean square root displacement of the carbon in austenite is as low as 32 µm. However, the carbon concentration gradient within the coating caused substantial modifications of the microstructure formed after cooling from annealing temperatures.

An example of cross-sectional image of the microstructure formed after cooling from the two-phase (α-γ) range of coating, is shown in Fig. 23a. The coating cooled from the onephase region γ comprises continuously graded microstructures caused primarily by differences in carbon content at the coating-gas and coating substrate interface. Since the substrate was not transformed, the decarburization is seen as a thin ferritic layer, adjacent to fully pearlitic microstructure. The hardness depth profile for the treatment performed at 710 oC, is represented by the lower curve in Fig. 24. An increase in hardness is seen in the region adjacent to the substrate and to the outer surface, due to diffusion of carbon from these two Significantly different changes in coating microstructure are observed after annealing at temperatures higher than the α-γ transformation of the steel substrate [109]. For example, at 1000 oC the diffusion coefficient of carbon in austenite DC γ is equal to 2.5×10-11 m2s-1 which corresponds to the mean root square displacement of almost 270 µm after 30 min. This means that carbon is capable penetrating the entire coating thickness.

At temperatures above 727 oC, diffusion of carbon within the substrate, towards the substrate-coating interface, takes place in the austenite. As a result, the distribution of carbon in the substrate after cooling has a significantly different character than that described for α-γ diffusion couple. In general, the substrate does not show a ferritic layer but a continuously graded microstructure composed of ferrite and pearlite with an increasing contribution of pearlite, while moving inward from the substrate-coating interface. After 30 min annealing at 1000 oC, the ferritic and pearlitic region is approximately 400 µm thick.

Carburizing at 920 oC allows a higher enrichment of the coating in carbon and the higher hardness after cooling as showed by two upper curves in Fig. 24. The lower hardness in the regions close to the substrate and the outer surface can be explained on the basis of microstructural observations (Fig. 23b). While the coating carburized at 710 oC has a microstructure of acicular ferrite and bainite, the coating carburized at 920 oC is composed of martensite and retained austenite [109]. The high volume fraction of retained austenite in the regions close to the substrate and the outer surface caused the lower hardness.

**Figure 23.** Microstructure of Fe-10%Ni coating on steel substrate after carburizing at temperatures of 670 oC (a) and 920 oC (b) [109] (with permission from Springer Verlag)

Thermochemical Treatment of Metals 105

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

**Figure 26.** Hardness depth profile within Fe-10%Ni coating on steel substrate after nitrocarburizing

0 50 100 150 200 250 300 350 400

gas-coating interface

Coating thickness, um

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

[109] (with permission from Springer Verlag)

0

100

substrate-coating

interface

200

300

400

Microhardness, HV

500

600

700

**10. Summary** 

component design.

essentially the same as that described previously for diffusion annealing.

Nitrocarburizing KCN-45% KCNO-45% K2CO3-10%

670oC -1.5h- air cooling

**Figure 24.** Hardness depth profile within Fe-10%Ni coating on steel substrate after carburizing [109] (with permission from Springer Verlag)
