**6. Boronizing**

During boronizing, called also boriding, the surface layer of material is saturated with boron. The process is performed in solid, liquid or gaseous medium and is applicable to any ferrous material as well as to alloys of Ni, Co or Ti. In case of steel it is carried out at temperatures between 840 and 1050 oC for up to 10 h creating borides FeB and Fe2B, which have a needle-like structure and hardness reaching 2000 HV. In addition to improving wear resistance, boronizing enhances also the corrosion resistance and oxidation resistance at temperatures of up to 850 oC. The main disadvantage of boronizing is the brittleness of the compound layer, especially the FeB phase.

Thermochemical Treatment of Metals 99

be conducted, exploring exclusively thermodiffusion and the duplex salt bath immersion. During such a treatment, chromizing at 1050 oC is followed by boronizing at 950-1050 oC [93]. For DIN 1.2714 steel the treatment leads to a variety of phases such as CrB, Cr2B, FeB and Fe2B with the boron diffusion in the pre-chromized layer being the rate controlling step. The single-stage *boroaluminizing* is practiced in the gas phase at temperatures of 850-900 oC

*Borocarburizing* is another two-step process where carburizing is followed by boronizing to generate boronitrides. It was proven that carburizing preceding boronizig reduces brittleness of boronized layers since the hardness gradient between iron borides and the carburized substrate becomes shallower. For 17CrNi6-6 steel, heat treated with laser after borocarburizing, three zones are distinguished, iron borides FeB+Fe2B of the modified morphology the hardened carburized zone (heat affected zone) and the carburized layer without heat effect [95]. The laser heat-treated borocarburized layer is characterized by higher hardness than the carburized layer, which is attributed to the presence of FeB and Fe2B phases. For low carbon steels containing Cr and Ni, the borocarburized layer of FeB and Fe2B with a microstructure shown in Fig. 21b, reached a hardness of 1500-1800 HV with a sub-layer zone being in the range of 700-950 HV [96]. An advantage of the borocarburized layer is in the higher frictional resistance as compared with the single treatment of either boronizing or carburizing. As an extension of borocarburizing, carbonitrided surfaces may be subjected to boronizing hence creating complex (B+C+N) diffusion layers [97]. Although *borocarbonitriding* shows a tendency to reduce the depth of iron borides zone and the microhardness gradient across the surface the resultant wear resistance is higher than that after individual processes. Another benefit of borocarbonitriding is borocarbonitriding is the

lower lower temperature and shorter time in comparison with borocarburizing.

**Figure 21.** Cross-sectional microstructure after boronizing: (a) pure chromium, solid medium, 940 oC, 8 h [92]; steel 0.15%C, 1.69%Cr and 1.53%Ni, 930 oC, 20 h [96] (with permission from Elsevier Science)

The purpose of diffusion chromizing is to enrich surface layers of an alloy with chromium. As other diffusion processes it may be carried out by powder pack, salt bath or fluidized bed. The compound surface layer is formed by a reaction between the carbide former, such

with controlled ratios of BF3 and AlF3 [94].

**7. Chromizing** 
