diffusion boronizing of cr-v ledeburitic...
TRANSCRIPT
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
[email protected], [email protected], [email protected]
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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