**6.1. Application range**

For high-carbon-bearing steel AISI 5100, boronizing in solid medium of B4C, SiC and KBF4 at temperatures 800-950 oC for up to 8 h creates the single phase layer of Fe2B with a saw tooth morphology and hardness reaching 1800 HV [83]. The growth rate of boride layer is controlled by boron diffusion in the Fe2B layer with the boronizing activation energy of 106 kJ/mol. For M2 high speed cutting steel, boronizing at 850-950 oC for up to 8 h, produces the smooth and compact layer with a thickness of up to 130 µm and hardness of 1600-1900 HV [84]. For tool steels the high hardness associated with a presence of borides causes a substantial reduction in toughness [85]. When applied to AISI 304L stainless steel by laser technology, boronizing develops the FeB, Fe2B, Cr2B, Cr23C6, Fe3C and B4C phases with surface hardness reaching 1490-1900 HV [86].

Boronizing is applicable to titanium alloys and a pack process at 950oC creates a compact, uniform layer composed of TiB2 and TiB compounds [87]. Also, boronizing of pure nickel in powder-pack at 850-950 oC for up to 8 h creates the 237 µm thick surface layer composed of Ni2B, Ni5Si2 and N2Si phases with a hardness exceeding 980 HV [88]. The laser boronizing of nodular iron increases hardness five times and produces the fine-crystalline, homogeneous structure of iron borides [89]. The commercial boronizing Titancote™B generates a diffusion layer of complex borides with a thickness of 10-200 µm and hardness of 1600-1800 HV with applications in tooling, oil, gas or general components [90]. In addition to titanium, also other refractory metals such as tantalum, niobium, tungsten and also cobalt-chromium alloys benefit from boronizing. One of many advantages is increasing the surface strength without negatively affecting a biocompatibility.

## **6.2. Treatments with a boronizing step**

The two stage treatment called *borochromizing* consists of chromium plating followed by diffusion boronizing and heat treatment. After powder boronizing of the 20 µm thick chromium coating on C45 carbon steel at 950 oC for 4h, the microstructure, thickness and microhardness are similar to the boride layer [91]. An additional treatment with laser, creates a solid solution or boride eutectics with martensite, reducing maximum hardness to 850 HV. An example of the boride layer grown on pure chromium after boriding in a solid medium at 940 oC for 8h, is shown in Fig. 21a [92]. The process of borochromizing can also 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 with controlled ratios of BF3 and AlF3 [94].

*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)
