**7. Chromizing**

98 Heat Treatment – Conventional and Novel Applications

compound layer, especially the FeB phase.

surface hardness reaching 1490-1900 HV [86].

without negatively affecting a biocompatibility.

**6.2. Treatments with a boronizing step** 

**6.1. Application range** 

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

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

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

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

**6. Boronizing** 

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 as Cr deposited on the surface and carbon in the substrate. The outcome shows similar properties to coatings produced by CVD and PVD. In some sources, the process is divided into soft chromizing, when carbon content in a substrate is below 0.1% and hard chromizing for the carbon content in a substrate exceeding 0.3%. As negative features of chromizing, the shallow penetration depth and the distinct interface with the substrate are often quoted. Both features are caused by the diffusion kinetics of chromium in steel.

Thermochemical Treatment of Metals 101

**8. Thermo-reactive diffusion** 

kinetics for a bath containing more than 10% of ferro-niobium.

**9. Hybrid thermochemical treatments** 

The *thermal diffusion* (TD), *thermo-reactive deposition/diffusion* (TRD) or *TD*-*Toyota diffusion process* is a high temperature treatment which generates a surface layer of carbides on steel as well as other carbon-containing materials such as nickel or cobalt alloys. In the treatment, carbon in the steel substrate diffuses into the deposited layer with a carbide-forming element such as vanadium, niobium, tantalum, chromium, molybdenum or tungsten. Then, the diffused carbon reacts, forming a compact, metallurgically-bonded coating with a thickness of up to 20 µm. The process is carried out at temperatures from 800 to 1250 oC for up to several hours. Due to the high temperature, steel requires bulk hardening either directly from the TD temperature or after the separate re-heating cycle. The typical hardness of vanadium carbide coatings, obtained using the salt bath TD process, reaches 3200-3800 V [104]. Also, niobium carbide NbC coatings exhibit the high hardness, wear resistance and low friction coefficient along with the high melting point. Coatings are produced by the steel immersion in the molten bath consisting of borax (Na2B4O7), boric acid (B2O3) and ferroniobium at 900-1100 oC for up to 10 h [105]. The diffusion of elements from niobium carbide coating to the steel and from the substrate to the coating was found to control the process

There are a number of surface modification technologies where thermochemical process is a single step in the multi-step treatment. An example of such hybrid is a concept of creating functionally graded materials, exploring a combination of coating and thermochemical treatment (Fig. 22). The single step process of deposition of thick coating with the high hardness is often difficult since they develop microcracks due to a generation of high internal stress [106]. Functionally graded materials offer new strategies for the implementation of high-performance structures in engineering components. They are comprised of continuous or discontinuous varying composition and/or microstructure over definable geometric orientations or distances. As a result they exhibit some unique properties which are beneficial for specific engineering applications. For example, the use of functionally graded systems in high-temperature components can enhance the adhesion and thermo-mechanical response of ceramic coatings deposited on metallic substrates [107].

The difference in phase transformation temperatures between the steel substrate and the Fe-10%Ni electrolytic deposit is an important factor of the thermal treatment proposed [108] [109]. At temperatures below 727 oC, the steel containing 0.9% C is composed essentially of pearlite, i.e. α + Fe3C. At the same time, the temperature of the α - γ transformation of the coating is approximately 680 oC. By selecting the temperature between 680 and 727 oC, the thermal diffusion treatment can be conducted at the coexistence of α (substrate) - γ (coating) diffusion couple. By contrast, during annealing at a temperature above 727 oC, both the steel substrate and the coating are composed exclusively of austenite (γ). The co-existence of the α - γ or the γ - γ diffusion couples leads to the essentially different redistribution of carbon

across the coating thickness and the surface region of the steel substrate.

## **7.1. Process and applications**

The typical chemistry of chromizing powder consists of 30% of ferrochromium (71%Cr, 0.03%C and Fe as a balance), 2.5% ammonium chloride activator and 67.5% of alumina powder filler [98]. The diffusion depth depends on the temperature and substrate chemistry. It obeys the parabolic rate law and increases with chromizing time and carbon content in the matrix. For temperature of 950 oC and reaction time of 9 h, the diffusion layer thickness reaches 13.2, 22.5 and 27.0 for AISI 1020, 1045 and 1095 steels, respectively. The growth mechanism of chromium diffusion coatings on ferrous alloys was intensively studied in 1980s [99] along with the role of pack geometry, substrate composition, type of halide activator, inner filler, time temperature and chromium source.

Chromizing kinetics can be improved by a combination of conventional thermochemical process with recently developed surface mechanical attrition treatment. The latter aims at refining grains of surface layers into a nanometer range by the repeated plastic deformation such as high velocity ball impacting or mechanical grinding [100]. When a 20 µm thick surface layer with grain size of 10 nm was formed on AISI H13 tool steel, it provided a substantial enhancement of chromium diffusion. The two-step thermochemical treatment of chromizing, with the first step conducted within the stability limit of nano-structures at 600 oC for 2 h, followed by the second-step treatment at 1050 oC for 4 h, created the 30 µm thick layer with a gradient of chromium concentration. The layer contained (Cr,Fe)23C6 and (Cr,Fe)2N1-x particles with a size below 200 nm.

## **7.2. Treatments with a chromizing step**

Chromizing is often combined into a two-step treatment with nitriding, nitrocarburizing or boronizing. For AISI 1010 steel, nitrocarburized at 572 oC for 2 h, and subsequently chromized by the pack method in a powder of ferrochromium, ammonium chloride and alumina at 1000oC for up to 4h, the layer thickness reaches up to 13 µm with a hardness of 1800 HV [101]. The layer consists of Cr2N and (Cr,Fe)2N(1-x) phases. In another example, AISI 1045 steel was first nitrided with 2 µm thick compound layer and hardness of 740 HV and then chromized in powder mixtures consisting of ferrochromium, ammonium chloride and alumina at 1000oC for 2 h [102]. Chromizing of nitrided layer resulted in formation of Cr2N chromium nitride and Fe3N iron nitrides. Although an increase in hardness was observed, it did not lead to an improvement in wear resistance. When combining chromizing with boronizing, pack chromium treatment of previously boronized bearing steel provides high wear resistance, particularly in sliding applications [103].

## **8. Thermo-reactive diffusion**

100 Heat Treatment – Conventional and Novel Applications

**7.1. Process and applications** 

as Cr deposited on the surface and carbon in the substrate. The outcome shows similar properties to coatings produced by CVD and PVD. In some sources, the process is divided into soft chromizing, when carbon content in a substrate is below 0.1% and hard chromizing for the carbon content in a substrate exceeding 0.3%. As negative features of chromizing, the shallow penetration depth and the distinct interface with the substrate are often quoted.

The typical chemistry of chromizing powder consists of 30% of ferrochromium (71%Cr, 0.03%C and Fe as a balance), 2.5% ammonium chloride activator and 67.5% of alumina powder filler [98]. The diffusion depth depends on the temperature and substrate chemistry. It obeys the parabolic rate law and increases with chromizing time and carbon content in the matrix. For temperature of 950 oC and reaction time of 9 h, the diffusion layer thickness reaches 13.2, 22.5 and 27.0 for AISI 1020, 1045 and 1095 steels, respectively. The growth mechanism of chromium diffusion coatings on ferrous alloys was intensively studied in 1980s [99] along with the role of pack geometry, substrate composition, type of halide

Chromizing kinetics can be improved by a combination of conventional thermochemical process with recently developed surface mechanical attrition treatment. The latter aims at refining grains of surface layers into a nanometer range by the repeated plastic deformation such as high velocity ball impacting or mechanical grinding [100]. When a 20 µm thick surface layer with grain size of 10 nm was formed on AISI H13 tool steel, it provided a substantial enhancement of chromium diffusion. The two-step thermochemical treatment of chromizing, with the first step conducted within the stability limit of nano-structures at 600 oC for 2 h, followed by the second-step treatment at 1050 oC for 4 h, created the 30 µm thick layer with a gradient of chromium concentration. The layer contained (Cr,Fe)23C6 and

Chromizing is often combined into a two-step treatment with nitriding, nitrocarburizing or boronizing. For AISI 1010 steel, nitrocarburized at 572 oC for 2 h, and subsequently chromized by the pack method in a powder of ferrochromium, ammonium chloride and alumina at 1000oC for up to 4h, the layer thickness reaches up to 13 µm with a hardness of 1800 HV [101]. The layer consists of Cr2N and (Cr,Fe)2N(1-x) phases. In another example, AISI 1045 steel was first nitrided with 2 µm thick compound layer and hardness of 740 HV and then chromized in powder mixtures consisting of ferrochromium, ammonium chloride and alumina at 1000oC for 2 h [102]. Chromizing of nitrided layer resulted in formation of Cr2N chromium nitride and Fe3N iron nitrides. Although an increase in hardness was observed, it did not lead to an improvement in wear resistance. When combining chromizing with boronizing, pack chromium treatment of previously boronized bearing steel provides high

Both features are caused by the diffusion kinetics of chromium in steel.

activator, inner filler, time temperature and chromium source.

(Cr,Fe)2N1-x particles with a size below 200 nm.

**7.2. Treatments with a chromizing step** 

wear resistance, particularly in sliding applications [103].

The *thermal diffusion* (TD), *thermo-reactive deposition/diffusion* (TRD) or *TD*-*Toyota diffusion process* is a high temperature treatment which generates a surface layer of carbides on steel as well as other carbon-containing materials such as nickel or cobalt alloys. In the treatment, carbon in the steel substrate diffuses into the deposited layer with a carbide-forming element such as vanadium, niobium, tantalum, chromium, molybdenum or tungsten. Then, the diffused carbon reacts, forming a compact, metallurgically-bonded coating with a thickness of up to 20 µm. The process is carried out at temperatures from 800 to 1250 oC for up to several hours. Due to the high temperature, steel requires bulk hardening either directly from the TD temperature or after the separate re-heating cycle. The typical hardness of vanadium carbide coatings, obtained using the salt bath TD process, reaches 3200-3800 V [104]. Also, niobium carbide NbC coatings exhibit the high hardness, wear resistance and low friction coefficient along with the high melting point. Coatings are produced by the steel immersion in the molten bath consisting of borax (Na2B4O7), boric acid (B2O3) and ferroniobium at 900-1100 oC for up to 10 h [105]. The diffusion of elements from niobium carbide coating to the steel and from the substrate to the coating was found to control the process kinetics for a bath containing more than 10% of ferro-niobium.

## **9. Hybrid thermochemical treatments**

There are a number of surface modification technologies where thermochemical process is a single step in the multi-step treatment. An example of such hybrid is a concept of creating functionally graded materials, exploring a combination of coating and thermochemical treatment (Fig. 22). The single step process of deposition of thick coating with the high hardness is often difficult since they develop microcracks due to a generation of high internal stress [106]. Functionally graded materials offer new strategies for the implementation of high-performance structures in engineering components. They are comprised of continuous or discontinuous varying composition and/or microstructure over definable geometric orientations or distances. As a result they exhibit some unique properties which are beneficial for specific engineering applications. For example, the use of functionally graded systems in high-temperature components can enhance the adhesion and thermo-mechanical response of ceramic coatings deposited on metallic substrates [107].

The difference in phase transformation temperatures between the steel substrate and the Fe-10%Ni electrolytic deposit is an important factor of the thermal treatment proposed [108] [109]. At temperatures below 727 oC, the steel containing 0.9% C is composed essentially of pearlite, i.e. α + Fe3C. At the same time, the temperature of the α - γ transformation of the coating is approximately 680 oC. By selecting the temperature between 680 and 727 oC, the thermal diffusion treatment can be conducted at the coexistence of α (substrate) - γ (coating) diffusion couple. By contrast, during annealing at a temperature above 727 oC, both the steel substrate and the coating are composed exclusively of austenite (γ). The co-existence of the α - γ or the γ - γ diffusion couples leads to the essentially different redistribution of carbon across the coating thickness and the surface region of the steel substrate.

environment and from the substrate

Thermochemical Treatment of Metals 103

is equal to 2.5×10-11 m2s-1 which

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

1000 oC the diffusion coefficient of carbon in austenite DC

means that carbon is capable penetrating the entire coating thickness.

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

corresponds to the mean root square displacement of almost 270 µm after 30 min. This

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

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)

γ

**couple** 

400 µm thick.

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