23. - 25. 5. 2012, Brno, Czech Republic, EU

DIFFUSION BORONIZING OF Cr-V LEDEBURITIC STEELS

Mária HUDÁKOVÁa, Peter JURČIa, Viktória SEDLICKÁa

aSTU – FACULTY OF MATERIALS AND TECHNOLOGY, Paulínska 16, 917 24 Trnava, Slovak Republic

maria_hudakova@stuba.sk, peter.jurci@stuba.sk, viktoria.sedlicka@stuba.sk

Abstract

Four Cr-V ledeburitic cold work tool steels with different chromium and vanadium contents have been

powder boronized and subsequently re-austenitized, nitrogen gas quenched and tempered to standard core

hardness prescribed for a given material. The microstructure, phase constitution and microhardness of

boronized layers were investigated. The boronized regions are, except those developed on the steel with

ultra-high vanadium content, composed of both the FeB- and the Fe2B-phases. The thickness of boronized

layers increases with increasing boronizing time. The effect of the steel composition on the layer thickness is

the following: The maximal thickness has been found for the steel with 12%Cr and 0.95%V. At this level of

Cr-content, the effect of the vanadium on the thickness is negative – the higher the vanadium content the

thinner is the layer. Higher vanadium content but lower chromium content generally led to even much thinner

compound layers. The microhardness of the layers developed on the steels with low and medium-vanadium

content was very high and it exceeded 2000 HV 0.1 in selected cases. On the other hand, the steel with

ultra-high vanadium content and very low Cr-content had the lowest microhardness.

Keywords: boronizing, boronized layer, boronizing powder mixture, ledeburitic steels

1. INTRODUCTION

Boronizing is thermo-chemical treatment, which results in a saturation of metallic surfaces with boron. As a

product of the treatment, thin, very hard wear resistant and corrosion resistant compound layers are formed

[1 - 3]. Below the compound layers, transition areas are formed as a result of certain, but very limited solid

solubility of boron in the -(or )-phase. These areas, however, have significantly lower hardness than the

boron compounds. Depending on the nature of the substrate material and processing conditions, single

phase (Fe2B) or double phase (FeB+Fe2B) layers can be formed. The initial material state and chemistry, as

well as the parameters of the boronizing itself can have a substantial impact on the results of the treatment.

The thickness of compound layers can reach up to 60-100 m for tool steels [4, 5]. Due to high alloying of

tool steels, also other elements can easily form the borides in the layers, mostly Cr if the alloy contains

chromium in sufficiently high amount [6]. Phase constitution of boronized layers changes from the free

substrate to the layer/base material interface as the boron content decreases in the same direction. The free

surface side of boronized layer is often formed by the FeB-phase and its content decreases in favour of the

increase of Fe2B amount [7]. Close the base material also complex borides like (Fe,Cr)2B or (Fe,Cr)B for the

chromium ledeburitic tool steels can be formed [6, 7]. Hardness of boronized layers can achieve over 2000

HV 0.1 for Cr- ledeburitic steels as well as for high speed steels [7, 8]. The aim of the conference paper is to

determine the effect of chromium and vanadium content on the formation and properties of boronized layers

on Cr-V ledeburitic tool steels. The steels grades K110 (D2), K190, CH3F12 and VANADIS 6 were chosen

as experimental materials.

2. EXPERIMENTAL

Four Cr-V ledeburitic steels, Table 1, were used as experimental materials. The samples were ground to

a final roughness of Ra = 0.1-0.2 m, cleaned, degreased and boronized using the Durborid® powder

mixture in hermetically sealed containers. The boronizing temperature was 1030 oC for all the materials and

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the processing time was chosen from the range 30 – 150 min. After the boronizing, the containers were air-

cooled to a room temperature and the samples were removed from them. Afterwards, the samples were re-

austenitized in a vacuum furnace up to 1025 oC (Vanadis 6 steel up to 1000

oC) for 30 min, nitrogen gas (6

bar) quenched and twice tempered at 530 oC for 2 h.

The light and scanning electron microscopy (JEOL 7600F operating at acceleration voltage of 15 kV) after a

deep etching were used for the microstructural evaluation. For the EDS mapping and point chemical

analysis, the EDS-detector was used whereas the acceleration voltage of the SEM was lowered to 1 kV.

Microhardness of boronized layer, transient region and core material was measured with a Buehler

Indentament 1100 tester, at a load of 100 g (HV 0.1) and loading time of 10 s. At least ten measurements

have been made to obtain the mean value and other statistical data, according to method elaborated in [9].

Table 1 Chemical composition of used steels

Steel grade Chemical composition [wt. %]

C Si Mn Cr Mo V Co

K110 1.55 0.25 0.35 11.80 0.80 0.95 -

K190 2.30 0.4 0.4 12.50 1.10 4 -

CH3F12 3.04 - - 3 - 12 0.8

Vanadis 6 2.1 1.0 0.4 6.8 1.5 5.4 -

3. RESULTS AND DISCUSSION

Figure 1 shows the microstructure of core materials after heat treatment. The materials consist of the matrix

(tempered martensite) and carbides. The carbides differs as from the point of view of their nature so in the

size and shape. The K110 steel contains mostly chromium and the carbides are M7C3. In the case of

Vanadis 6 and K190, two carbides can be found in the structure, namely M7C3 and MC [10]. The last

material, CH3F12, contains mostly vanadium and the carbides are of MC-type. Moreover it should be noted

that the K110-steel was manufactured via classical ingot metallurgy while all the other materials were made

via P/M. This difference is reflected in significantly coarser eutectic carbides in K110-steel, compare Figs. 1a

and 1 b-d.

30 m a

30 m b

30 m d

30 m c

Fig. 1 Light micrographs

showing the microstructure

of experimental steels, a –

K110, b – K 190, c –

CH3F12, d – Vanadis 6

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Figure 2 shows boronized layers developed on the K110 steel. The thickness of the regions increases with

processing time. The layers have two-phased constitution, e.g. both the FeB- and Fe2B-phase appear in the

microstructure. Below the compound regions, there is a transient area with enhanced portion of insoluble

carbides. As clearly shown, the interface between compound region and transition area is so-called

„irregular“ in samples processed for shorter time while it changes to typical “sawtooth” morphology for

samples processed for longer times.

Figure 3 shows representative micrographs of boronized layers developed on the Vanadis 6 steel. The

thickness of the layers increases with processing time. Nevertheless, the layers are thinner compared to

those formed on the K110 steel for the same processing time. As clearly shown, all the layers are two-

phased. On the free surface, there is the FeB region (dark) and in between, the Fe2B- phase appears

(bright). Below the compound regions, there is a transient region, typical through enhanced amount of

insoluble carbides. The interface between compound layer and transition region exhibits symptoms of typical

“sawtooth” morphology – in contrast to that of K110 steel, in the short-time processed samples, also.

In the case of boronizing of high chromium steels, the FeB-layer tends to form more easily and it can make

up to 50% of the total compound layer thickness [11 - 13]. In current work, a similar effect of vanadium has

been found – the FeB-phase makes only 10% of the total layer thickness of practically no vanadium

containing K110 steel while up to 50 % of the layer formed on the Vanadis 6 steel (less chromium but much

more vanadium content).

Elevated carbide content in the transient regions can be attributed to the almost complete insolubility of the

carbon in borides. Therefore, it is transported from the surface towards the substrate and forms a “carbide

excess” in the transition areas, in high carbon steels in particular.

Figure 4 shows the thickness of boronized regions for all the investigated steels, as a function of processing

time. Generally, the thickness of layers increased with prolonged processing time. However, the thickness of

layers has been determined to be different for the materials with various both the chromium and the

vanadium content. The thickest regions were found for K110 steel, containing dominant part of chromium

and only very limited content of other elements. The layers on the K190 steel are thinner – it should be noted

that the steel contains 4 %V at similar chromium content. The CH3F12 material had thinner layer in the case

a b

c d

Fig. 2 Microstructure of

boronized layers

developed on K110 steel,

a – processing time of 30

min, b – 45 min, c – 75

min, d – 150 min.

30 m 30 m

30 m 30 m

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of longer processing time and thicker on short-time processed samples. Here, it should be noted that the

material contains mainly vanadium (in carbides mostly) and low amount of chromium. Finally, common effect

of high chromium and high vanadium content is demonstrated upon example of Vanadis 6 steel. It is clearly

evident that high content of both the Cr and the V induced the thinnest boronized regions. It is known that

chromium inhibits the layer growth rate [14]. But, obtained results indicate that vanadium inhibits the growth

rate in much more distinctive manner than chromium.

Fig. 4 Thickness of boronized regions for all the examined steels, as a function of processing time

Figure 5 presents representative SEM micrographs of boronized layers on the K110 steel and corresponding

EDS-maps. It is shown that there are some original carbides conserved in the compound layer, Figs. 5b,c.

These carbides contain mainly chromium, Fig. 5c and less iron, Fig. 5d. The carbides in the transient region

(newly formed) are chromium rich, also, but the Cr-content is much lower than that in original carbides.

Figure 6 presents SEM micrographs of boronized layers on the CH3F12 steel and corresponding EDS-maps.

The original carbides in the material are the MC-particles. In the transient region, Fig. 6b, these particles

c 30 m

30 m a b 30 m

Fig. 3 Boronized layers on the Vanadis

6 steel formed at 1030 oC for: a - 45

min., b - 75 min., c - 150 min.

0

10

20

30

40

50

60

70

80

90

100

K110 K190 CH3F12 Vanadis 6

Steel grade

Th

ick

ne

ss

of

lay

er

[ m

]

30 min 45 min 75 min 150 min

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(dark contrast due to lower atomic weight of V) are surrounded by newly developed carbides (bright),

containing much more chromium and negligible amount of vanadium, Figs. 6 c, d.

Fig. 5 Microstructure of boronized layer developed on K110 steel, a – overview, b – detail, c – EDS-map of

Cr, d – EDS map of Fe.

Fig. 6 Microstructure of boronized layer developed on CH3F12 steel, a – overview, b – detail, c – EDS-map

of Cr, d – EDS map of V.

Figure 7 shows SEM micrograph and EDS-mapping of boronized layer on the Vanadis 6 steel. Here, original

both the Cr-based and V-based carbides are clearly visible in the compound layer, Fig. 7 a,c,d. Further, it is

shown that the boronized layer is composed of two distinctively different regions. Close to the surface, there

15 m b 50 m a

c d

50 m a b 5 m

c d

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is the FeB-phase with enhanced boron content, Fig. 7b, and lowered Cr-content, Fig. 7 b, c. The Fe2B-phase

with lower boron content is located in between. There is also evidence of newly formed carbides in the

transient region – these particles are chromium rich as indicated in Fig. 7c.

Fig. 7 Microstructure of boronized layer developed on Vanadis 6 steel, a – SEM micrograph, b – EDS-map of

boron, c – EDS-map of Cr, d – EDS map of V.

Fig. 8 Hardness of boronized layers experimental materials

The hardness of the compound boronized layers commonly increased with prolonged processing time, Fig.

8. The effect of the material chemistry can be summarized as follows: The hardness was the highest for Cr-

ledeburitic steel K110-grade. The K190-steel with 4%V had lower hardness and, in addition, measurement of

FeB-phase was impossible due to the fact that it was too brittle. Therefore, the results are comparable for the

Fe2B only. Common effect of high chromium and high vanadium content can be commented as negative on

the hardness as demonstrated upon example of Vanadis 6 steel.

0

500

1000

1500

2000

2500

30 45 75 150 30 45 75 150 30 45 75 150 45 75 150

Processing time [min]

Mic

roh

ard

ne

ss

HV

0.1

FeB Fe2B diffusion inter-layer

K110 Vanadis 6 CH3F12 K190

7 m a b

d c

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4. CONCLUSIONS

All the materials consist of the matrix (tempered martensite) and carbides, whereas their nature depends on

the material chemistry and manufacturing route used. Boronized layers developed on all the materials are

two-phased. Their thickness increases with processing time. It seems that besides known inhibiting effect of

chromium, vanadium inhibits the growth rate of the layers, also.

Transient areas contain enhanced portion of carbides. This can be referred to almost complete insolubility of

carbon in borides whereas carbon atoms, released due to decomposition of part of carbides, are transported

away the surface. Here, they form chromium rich particles in all the investigated materials.

The effect of Cr and V, respectively, on the hardness follows the impact of these elements on the thickness.

The highest hardness was found for the layers on K110-steel whole the lowest was one was recorded for the

layers on Vanadis 6 steel.

AKNOWLEDGEMENTS

This paper is the result of the project implementation: CE for the development and application of

diagnostic methods in the processing of metallic and non-metallic materials, ITMS:26220120048

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