difuznÍ boridovÁnÍ oceli pro prÁci za...

METAL 2009 19. - 21. 5. 2009, Hradec nad Moravicí

DIFUZNÍ BORIDOVÁNÍ OCELI PRO PRÁCI ZA TEPLA DIFFUSION BORONIZING OF HOT WORK TOOL STEEL

aPeter Jurči

bMária Hudáková

ČVUT v Praze, Fakulta strojní, Karlovo nám. 13, 121 35 Praha, ČR

STU, MtF v Trnavě, J.Bottu 52, 917 24 Trnava, SR

Abstract H11 hot work tool steel was boronized at various processing parameters, austenitized,

quenched and tempered to a core hardness of 47 – 48 HRC. Microstructure, phase constitution

and microhardness of boronized layers were investigated. Effect of boronized region on the

bulk properties was determined by the Charpy impact test. Structure of boronized regions is

formed by the compound layers and diffusion inter-layer. They are formed with diffusion

regions and compound region, which consisted of only Fe2B-phase but in the case of longer

processing time also of the FeB – phase. Microhardness of boronized layers exceeded

considerably 2000 HV0.1. However, boronizing led to a substantial lowering of the Charpy

impact toughness of the material.

Abstrakt

Ocel pro práci za tepla typu H11-type byla boridována při různých parametrech

procesu, austenitizována, kalena a popouštěna na tvrdost 47 – 48 HRC. Byla analyzována

mikrostruktura, fázové složení a tvrdost boridovaných vrstev. Nakonec byl zkoumán vliv

boridovaných vrstev na vlastností masivního tělesa pomocí Charpyho zkoušky rázové

houževnatosti. Mikrostruktura boridovaných oblastí je tvořena difuzní vrstvou a vrstvou

sloučeninovou. Tato pozůstává pouze z fáze Fe2B, avšak při delších časech se tvoří rovněž

borid FeB. Mikrotvrdost boridovaných vrstev překračovala značně 2000 HV0.1. Na druhé

straně ovšem boridování vedlo ke značnému snížení houževnatosti materiálu.

1. INTRODUCTION

Boronizing is a thermo-chemical treatment that forms thin, hard and wear resistant

compound layers on the metallic surfaces. These layers have a much higher hardness than

those formed due to the nitriding or carburizing. For tool steels, their hardness exceeds often a

limit of 2000 HV 0.1. Due to the diffusion of boron into the steel substrate, boronized layers

can exhibit a good adhesion on the steel surface.

One of the most important problems of boronizing is the choice of optimal processing

temperature with respect to subsequent quenching and tempering. Boronizing is carried out

either in various powder mixtures in hermetically sealed containers or in salt baths. This

makes it difficult to quench the material directly and, after processing, it must be cooled down

slowly to a room temperature and heated again up to the austenitizing temperature which

increases the risk of undesirable grain growth. In addition, many materials have the

austenitizing temperature higher than that of boronizing, which makes strong limitations in

formation of boronized layers on the surface of them.

Boronizing of tool steels is an object of scientific interest over many years. The

investigations are focused to the optimization of parameters with respect to minimize some

undesirable effects like partial stripping of layers and embrittlement of bulk material.

Thickness of boronized layers developed on tool steels is much lower than that of layers on

plain carbon or low alloyed structural steels since the alloying elements inhibit the born

diffusion into the material. Typically, the thickness of compound layers consisting of FeB-


and/or Fe2B- boride reaches up to 60-100 µm for various Cr and V containing tool steels

[1,2]. 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 [3,4]. On the

other hand, carbon is insoluble in borides and it segregates into the material depth, forming

additional portion of carbides. 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 formed mostly by the FeB-phase and its

content decreases in favour of the increase of Fe2B amount [4]. Hardness of boronized layers

can achieve over 2000 HV 0.1 for Cr- ledeburitic steels as well as for high speed steels [4,5].

Besides positive effects of thermo-chemical treatment upon important mechanical

properties like hardness and wear resistance, also undesirable effects like embrittlement can

take place in some cases. Lowering of the three point bending strength for various nitrided

PM ledeburitic steels was found recently [6,7]. This is connected with a cleavage region in

surface processed material, with minimal plastic deformation, and with a very low fracture

toughness of the core material [7]. Also for hot work steel of a H13-type, the lowering of

absorbed Charpy impact energy was found. However, although also for the boronized layers

such an effect can be expected, neither experiences are known nor exact determination of the

effect of boronized layer upon the fracture toughness was published.

2. EXPERIMENTAL

The hot work steel THYROTHERM 2343 EFS (0.37 %C, 1% Si, 5.3 %Cr, 1.3 %Mo,

0.3 %V, Fe bal) has been used for experiments. Round shaped plate specimens, intended for

the structural investigations and microhardness measurements, of 20 mm in diameter and 5

mm in thickness were fine ground to a surface roughness of Ra = 0.8 µm. Besides the

specimens for microstructural evaluation, also the samples for the Charpy impact testing

according to the NADCA 202-97 standard were made.

Both types of specimens were cleaned, degreased and powder boronized in hermetically

sealed containers at a temperature of 1030 oC for 30, 45, 75 and 150 min. After the

boronizing, the containers were furnace cooled down to a room temperature, and then the

specimens were removed and subjected to a vacuum heat treatment. The heat treatment

consisted of austenitizing at 1020 oC/30 min., nitrogen gas quenching (pressure of 6 bar) and

triple tempering at the temperatures of cycles 570, 610 and 550 oC, each for 2 hours. After

each tempering cycle, the samples were cooled down slowly to a room temperature. Resulting

core hardness of the steel was 47-48 HRC.

Scanning electron microscopy after a deep etching was used for the microstructural

evaluation. Scanning electron microscopy was used also for the fractography. Microhardness

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

indenter placed in a Zeiss Neophot 21 light microscope, at a load of 100 g (HV 0.1). X-ray

patterns of the boride layers were recorded using a Phillips PW 1710 device with Fe-

monochromatic radiation. Data were recorded in the range 27 – 120o of the two-theta angle.

Charpy impact testing was carried out on an instrumented machine with the maximal impact

energy of 300 J.

3. RESULTS, DISCUSSION

Microstructure of the core material, Fig. 1 consists of fine tempered martensite. Detail

SEM micrograph obtained at higher magnification, Fig. 2 shows that the material contains

fine martensitic needles uniformly distributed throughout the specimen, with neither presence

of undissolved carbides nor pro-eutectoidal phases at the grain boundaries. It indicates that the

heat treatment after the boronizing was performed in an appropriate way.


Figure 1 (left) showing the microstructure of the core material.

Figure 2 (right) showing a detailed SEM micrograph of the core material.

On the other hand, closer the boronized layer the density of undissolved carbides

increases, Figs. 3 - 5. The reason is, as abovementioned, the diffusion of carbon from the

surface to the core material due its insolubility in borides.

Figs. 3 – 5 – Microstructure of the material close to boronized layer, 3 – far away the surface,

4 – closer the surface, 5 – below the boronized region

Boronized layers formed at 1030 oC for 30, 45 and 75 min. are formed only of Fe2B –

phase, Fig 6 a,b,c. The layer formed at the same temperature but for a processing time of 150

min. contains also the FeB – phase on the free surface, Fig. 6d. As clearly shown, the

boronized layer is cracked at the two boron phases interface. It is known that if both boron


phases are formed then the FeB phase contains tensile stresses and the Fe2B compressive

stresses. This can be the principal explanation of cracking of FeB – layer in the last mentioned

case. The monophase layers (Figs. 6a,b,c) are homogeneous with no presence of

macrocracking.

Fig. 6 a-d – Boronized layers on the H11-steel surface, formed at 1030 oC for a-30 min., b-45

min., c-75 min., d-150 min.

Fig. 7 a-d – Boronized layer formed at 1030 oC for 30 min.. a – overview, b-free side region,

c-substrate side region, d-detail from c.

A B C D


Figure 7 shows the boronized layer formed at the parameters 1030 oC/30 min. The total

thickness of boronized region is of about 60 µm (7a) and it can be divided into two basic

parts. The first one, close the surface, is the compound layer, containing a lot of pores mostly

in thin near-surface area (7b). In between, there is the diffusion region. As clearly shown, the

compound layer contains only the Fe2B - phase. The boundary between the compound layer

and the diffusion region is typical by so-called “teething”, Fig. 7a,c. This phenomenon is

typical for the growth of boronized layer on low- or medium-alloyed tool steels. Diffusion

region contains also some carbides (7c,d) due to the fact that carbon is not soluble in borides

and during the boronizing, it diffused into the core material as above discussed.

X-ray diffraction fixed the Fe2B-phase in all of the specimens, Figs. 8,9. For the

material boronized at 1030 oC for 150 min. also the FeB-phase was identified. This

observation corresponds well with the microstructural observations, Fig. 6.

Figure 8 showing the X-ray patterns from boronized layer formed at 1030 oC for 75 min.

Figure 9 showing the X-ray patterns from boronized layer formed at 1030 oC for 150 min.

Hardness measurements of boronized layers are summarized in Table 1. If only Fe2B

compound is formed, and the processing time was short, then it had the average

microhardness of 1473-1483 HV 0.1. Diffusion region was considerably softer - its

microhardness was 638 HV 0.1. If the processing time was longer (75 min.) then the hardness

of boronized layer increased to 2221 HV 0.1. Such a high value indicates that at least tracks

of FeB-phase could be formed – however, X-ray diffraction did not confirm this. For the

specimens processed for 150 min. also the FeB compound was formed, and its average value


of microhardness exceeded 2300 HV 0.1. In this case, the microhardness of Fe2B – layer

reached only up to 1700 HV 0.1.

Boronizing Microhardness HV 0.1 Charpy impact

energy absorbed [J] FeB Fe2B diffusion region

1030 oC/30 min. --- 1483 416 -

1030 oC/45 min. --- 1473 406 -

1030 oC/75 min. --- 2221 638 17.8

1030 oC/150 min. 2325 1686 560 12.4

Table 1 – Mechanical properties of boronized layers and bulk material

The material after heat treatment without boronized layer had the Charpy impact

strength of more than 300 J. The presence of boronized layer on the surface lowers the

Charpy impact strength dramatically and the lowering is more evident as the thickness of

boronized layer increases, with a simultaneous occurrence of the FeB-phase in the surface

region. The impact strength of layers formed for short-time processing was not measured yet

– however it can not assume any significant improvement. Various authors [8-10] have found

that the fracture toughness of boronized layers is very poor – it ranged between 2.1 and 4.8

MPa.m1/2

for some ledeburitic steels and other high alloyed steels. Boronized layers have thus

several times worse resistance against propagation of brittle cracks than the steel substrate

(although also the fracture toughness of high alloyed and heat treated tool steels is not good)

and any sub-microscopical defects can easily act as fracture nuclei.

Figure 10 shows the fracture surface of the material without boronized layer on the

surface. The surface exhibits clearly evident ductile morphology with some secondary cracks

and deep dimples. Detail micrograph, Fig. 11, demonstrates that the propagation of the

fracture is connected with an evident plastic deformation. This is a natural explanation of high

absorbed Charpy impact energy and good impact strength of the material.

Figures 10, 11 – Fracture surface of the specimen without boronized layer, left – overview,

right detail.

The situation in the case of boronized material differs clearly from that non – boronized.

Figure 12 shows the results of fractographical analysis of boronized Charpy specimens at

1030 oC for 150 min. The fracture is iniciated at the tensile side of the specimens (A),

probably by cracking of boronized layer (B) and propagated downwards the material through


diffusion inter-layer (C). Cracking of the boronized layer is evidently supported by its

morphology – i.e. columnar character of the microstructure contributes to the brittle character

of failure. The inter-layer exhibits clearly transcrystalline cleavage character of the fracture,

i.e. only minimum energy is absorbed by plastic deformation. Further propagation of the

fracture, also in the core material, is realised mostly via transcrystalline cleavage and this is

the principal explanation of a dramatical embrittlement of the bulk material due to the

boronizing.

Figure 12 showing the results of fractographical analysis of Charpy specimen boronized at

1030 oC for 150 min., a – overview, b – fracture of compound layer, c – diffusion region, d –

core material.

4. CONCLUSIONS

The main goal of experimental effort is to develop the optimal boronized layer with a

good adhesion on the substrate, sufficiently high hardness, good resistance against cracking

and as minimal as possible negative influence on the bulk properties.

1) All the developed layers have a thickness exceeding 50 µm. No cracks or inhomogeneities

on the layer/substrate interface were found, but if the layer consisted of two phases,

longitudinal cracks on the boundary of these phases was detected.

15 µm B 100 µm A

10 µm C 20 µm D


2) The layer formed for 150 min. consisted of two phases, FeB and Fe2B, and had a

microhardness of 2325/1700 HV 0.1 for both sub-layers. The layers formed for 30, 45 and 75

min. consisted only of Fe2B-phase with a microhradness of about 1500 HV 0.1, but that

developed for the processing time of 75 min. was siginificantly harder.

3) The presence of boronized layers on the surface lowered the Charpy impact energy of the

bulk material in order of magnitude.

4) Further, and probably larger experimental investigations are necessary to find the

boronizing parameters to form the layer with the optimal combination of properties, i.e. with a

high hardness combined with as less significant worsening of Charpy impact strength as

possible.

REFERENCES

[1]: Sedlická, V., Hudáková, M., Moravčík, R., Grgač, P.: In: Proc.of Conf.“Coatings and

layers 2005“, Trenčín, Slovak Republic, 168 – 173.

[2]: Grafen, W., Edenhofer, B.: Surf. Coat. Techn. 200 (2005) 1830 – 1836.

[3]: Kusý, M., Sedlická, V., Hudáková, M., Grgač, P.: In: Proc.of 21st Int. Conf. on Heat

Treatment, Jihlava, 28.-30. November 2006, Czech Republic, 289-294.

[4]: Hudáková, M., Kusý, M., Sedlická, V., Grgač, P.: Materiali in Tehnologije 41 (2007) 2,

81-84

[5]: Hudáková, M., Kusý, M., Sedlická, V., Grgač, P.: In: Proc.of Conf.“Coatings and layers

2007“, Rožnov pod Radhoštěm, 29.-30.October 2007, Czech Republic. CD-ROM

[6]: Jurči, P., Hnilica, F., Cejp, J.: Materiali in Tehnologije, 41 (2007), 5, 231 – 236.

[7]: Hnilica, F., Čmakal, J., Jurči, P.: Materiali in Tehnologije, 38 (2004), 5, 263 – 268.

[8]: Campos, I. et al.: Applied Surf. Sci., 254 (2008) pp. 2967 – 2974

[9]: Sen, U., Sen, S.: Mater. Charact. 50 (2003) pp. 261 - 267

[10]: Li, Ch. et al.: Surf. Coat. Techn. 202 (2008) pp. 5882 – 5886.

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