Structure and properties of hot work steel alloyed with WC.pdf

of Achievements in Materials

and Manufacturing Engineering

VOLUME 24

ISSUE 2

October

2007

Structure and properties of hot-work

tool steel alloyed by WC carbides

by a use of high power diode laser

M. Bonek a, *, L.A. Dobrzański a , A. Klimpel b

a Division of Materials Processing Technology, Management and Computer Techniques

in Materials Science, Institute of Engineering Materials and Biomaterials,

Silesian University of Technology, ul. Konarskiego 18a, 44-100 Gliwice, Poland

b Welding Department, Silesian University of Technology,

ul. Konarskiego 18a, 44-100 Gliwice, Poland

* Corresponding author: E-mail address: miroslaw.bonek@polsl.pl

Received 20.03.2007; published in revised form 01.10.2007

Manufacturing and processing

AbstrAct

Purpose: The paper presents the effect of alloying with tungsten carbide on properties of the X40CrMoV5-1

steel surface layer, using the high power diode laser (HPDL).

Design/methodology/approach: The structural mechanism of surface layer development was determined and the

effect of alloying parameters, gas protection method, and thickness of paste layer applied onto the steel surface on

structure refinement and influence of these factors on the mechanical properties of surface layer was studied.

Findings: Selection of laser operating conditions is discussed, as well as thickness of the alloying layer, and

their influence on structure and chemical composition of the steel. Analysis of the process conditions influence

on thicknesses of the alloyed layer and heat-affected zone is presented.

Practical implications: Laser surface modification has the important cognitive significance and gives grounds to the

practical employment of these technologies for forming the surfaces of new tools and regeneration of the used ones.

Originality/value: The outcome of the research is an investigation showing the structural mechanisms

accompanying laser alloying.

Keywords: Heat treatment; Laser; Tool materials; Wear resistance

1.Introduction

form of heat in the surface layer. Moreover, methods which are

not based on partial melting of surface alone, but on partial

melting with the simultaneous introduction of the alloying

elements with high hardness, like carbides, are being employed

more and more widely for modification of the surface layer. High

cooling rates are obtained in this process and its end result is the

fine-grained material with the super-cooled phases. Material

transport in the liquid metal, caused by the surface tension forces,

features the main factor deciding development of the alloyed

layers. The non-uniform material heating resulting from the laser

beam impact causes development of a big surface tension gradient

Improvement of surface properties of materials used to date

features one of the goals set to the research institutions active in

the materials engineering area. Possibilities of increasing the

functional properties of the hot-work alloy tool steels by

modification of their chemical composition in a conventional way

are very limited already. Laser technique features the especially

promising tool for solving the contemporary surface engineering

problems thanks to the physical properties of the laser beam,

making it possible to focus precisely the delivered energy in the

Short paper 175

1. Introduction

Journal of Achievements in Materials and Manufacturing Engineering

Volume 24 Issue 2 October 2007

on the surface of the liquid. The force is directed outside from the

beam centre where is the highest temperature value to its edge and

causes movement of the molten material [1-5, 12-15].

Density of power delivered to the surface layer of the

processed materials is lower for the more and more often used

HPDL type lasers compared to its single mode distribution

characteristic for other laser types; however, the energy is

distributed more uniformly across the rectangular area of the laser

beam focus [6]. Therefore, the HPDL laser is ideally suited for

modification of the surface layer of materials. Moreover,

employment of the appropriate shielding gases and correctly

selected nozzle shape, and its position in respect to the processed

material, have the objective to ensure the high quality of weld

face and repeatability of the obtained results. The advantages of

laser treatment compared to other surface layer modification

methods are: high processing rate, possibility to carry out

treatment without protective guards, modification of small,

arbitrarily selected fragments of the processed surfaces

responsible for tools and machine elements life, as well as its

material economy [7-11].

The goals of this work were: determining the technological

conditions for alloying with the tungsten carbide the hot-work

alloy steel surface layer using the high power diode laser (HPDL)

and determining the relationship between the laser treatment

parameters and structure and chemical composition of the steel.

2.Investigationprocedure

The material under investigation were specimens from the

X40CrMoV5-1 hot-work alloy tool steel, obtained from the

vacuum cast, delivered in the form of bars with O.D. 75 mm. The

chemical composition of the steel is given below in table 1.

Specimens of O.D. 70 mm and 6 mm thick were turned from the

material delivered in a softened state, further they were

austenitized in a salt bath furnace and tempered in the chamber

furnace with argon protective atmosphere.

The specimens were heated gradually to the austenitizing

temperature with holding at 650°C for 15 min and were

austenitized for 30 min at the temperature of 1060°C. Cooling

was carried out in the hot oil. The specimens were tempered

twice after quenching, each time for 2 hours, at the temperature

of 510°C.

HAZ

Table 1

Chemical composition of X40CrMoV5-1 steel

Weight, (%)

Steel

type

Mn Si

W Mo

Fig. 1. Experimental setup with high power diode laser HPDL

ROFIN DL 020, NM – native material, HAZ - heat effected zone,

RZ – remelted zone, AM – alloying material

X40Cr

MoV5-1

0,41 0,44 1,09 5,40 0,01 1,41 0,95 0,015 0,010

Table 2

Properties of the WC powder

Powder

Average

grain

diameter,

µm

Melting

temp., °C

Density,

g/cm 3

Hardness,

HV 30

20 - 30

2730 - 2870

15,6

1550

Table 3

Technical date for the HPDL ROFIN DL 020

Wavelength of the laser radiation, nm

808 r 5

Maximum output power of the laser beam

(continuous wave), W

2500

Power range, W

100-2500

Focal length of the laser beam, mm

82 / 32

Fig. 2. Boundary of the remelted steel surface layer after alloying

with the parameters: scanning rate – 0.5 m/min, beam power – 1.1

kW, WC layer thickness – 0.06 mm

Laser spot size, mm

1.8 u 6.8 /

1.8 u 3.8

Power density range in the laser beam focal

plane, kW/cm 2

0.8-36.5

176

Short paper

M. Bonek, L.A. Dobrzański, A. Klimpel

2. Investigation procedure

Manufacturing and processing

Fig. 4. a) Microstructure of the X40CrMoV5-1 steel (SEM) after

alloying with the following parameters: beam power – 0.9 kW,

layer thickness – 0.11 mm b) the plot of intensity versus energy

dispersive X-ray radiation, representing the pointwise WC carbide

chemical composition analysis

Fusion was carried out in the direction perpendicular to the

longer side of the focused beam with the multi-mode energy

distribution, which ensures obtaining the wide weld face in the

statement (fig. 1).

Metallographic examinations of the material structures after

laser alloying its surface layer were made on LEICA MEF4A

light microscope with magnifications from 100 to 1000x.

Structure examinations and thickness measurement of the

relevant zones in the surface layer were made also on the

transverse micro-sections on the OPTON DSM 940 scanning

electron microscope with magnifications from 1000 do 5000x.

Examinations of the chemical composition in micro-zones and

analyses of the linear distribution of alloying elements in

specimens from the investigated steel were made on the OPTON

DSM-940 scanning electron microscope with the Oxford EDS

LINK ISIS energy dispersive X-ray spectrometer with the 20 kV

accelerating voltage.

Fig. 3. Surface layer of the X40CrMoV5-1 steel after alloying

with the following parameters: beam power – 1.3 kW, layer

thickness – 0.11 mm; a) SEM structure, b) linear analysis of the

chemical composition changes

The specimens’ surfaces were sand blasted and ground on a

surface grinder with the magnetic holder. The paste containing the

WC tungsten carbide powder with the inorganic binder was applied

on the degreased specimens as 0.06 mm and 0.11 mm thick layers,

which properties can be seen in table 2. The specimens from the

X40CrMoV5-1 steel, mounted on a turntable, were penetrated with

the Rofin DL 020 high power diode laser (HPDL) beam. The

technical parameters of the laser can be seen in table 3. Two fusion

tracks were made on each specimen face with radii of 12 mm and

22 mm. Dimensions of the laser beam focused on the material

surface are 1.8 x 6.8 mm. The working focal distance (measured

from the head protective glass) is 92 mm.

3.Discussionofinvestigation

results

Basing on metallographic examinations, occurrences of two

zones in the surface layer of the investigated steel were found out:

the remelted zone and the heat-affected zone, whose thicknesses are

dependant on the employed laser treatment parameters (laser

treatment type, laser beam power, thickness of the alloying layer).

Structure and properties of hot-work tool steel alloyed by WC carbides by a use of high power diode laser

177

3. Discussion of the investigation results

Journal of Achievements in Materials and Manufacturing Engineering

Volume 24 Issue 2 October 2007

In case of alloying with the tungsten carbide powder, whose

melting point is much higher than the melting point of steel, with the

initially specified laser power, inundation occurs of the un-molten

WC powder grains into the molten steel substrate. Strong circulation

of the liquid metal was discovered with rapid solidification after the

laser beam has passed, leading to freezing of the structure (fig. 2). It

was found out, basing on metallographic examinations, that the

structure of the material solidifying after laser fusion is characterized

by occurrences of areas with the diversified morphology (fig. 3a)

connected with crystallization of the steel. Depending on the

processing parameters used various material mixing mechanisms

were observed. At low laser material penetration energy values the

capillary lines are not connected and the fusion structure is relatively

hogeneous. Along with the laser power growth the occurrences of

capillary lines' swirls were observed. Big dendrites occur in the

boundary zone between the solid and liquid phases, whose main axes

are oriented according to the heat abstraction directions. Repeated

crystal growth direction changes are observed, characteristic for these

zones. Structure of fine equiaxial crystals with the carbide lattice

develops in the central zone of the fused area where heat abstraction

takes place in all directions. Linear analysis of the chemical

composition revealed tungsten presence in the fused zone only (fig.

3b). No tungsten presence was found in the heat affected zone.

Undissolved tungsten carbides were revealed surrounded by the

dense lattices of the superfine eutectic mixture (fig. 4). It was found

out that laser fusion results in refinement of structure in the entire

laser power range. Different grain size was revealed during

examinations in the particular zones of the surface layer after laser

alloying. Matrix grains shape and size does not change during

fusion. Only at the crystallization front, between the fused and heat

affected zones, the elongated and smaller grains were found, which

are subjected to partial melting and re-crystallization during laser

modification in the statement.

references

[1] J. Mateos, J.M. Cuetos, E. Fernandez, R. Vijande, Tribological

behaviour of plasma sprayed WC coatings with and without

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engineering, Publisher „Akapit”, Kraków, 2000, (in Polish).

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Polok, Properties and structures of gradient layer surface of

32CrMoV12-28 hot work steel alloyed using high power

diode laser, International Conference on Manufacturing and

Materials Processing 2006 (ICMM 2006), Kuala Lumpur,

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4.conclusions

Alloying the investigated steel with tungsten carbide causes

development of the surface layer, in which one can point out the

fused zone, heat affected zone, and interface zones. Their

thickness is closely connected with the fusion parameters, and

thickness of the fused layer changes significantly along with the

laser beam power increase. A fine-grained, dendritic structure was

obtained in the fused zone, with the crystallographic orientation

connected with the dynamical heat abstraction from the laser

beam impact zone. Occurrences of the un-fused tungsten carbide

grains were observed in the structure and the increased tungsten

content compared to the native material, whose variable

concentration is connected with the molten metal fluctuation in

the pool during alloying.

Acknowledgements

This work was partially financed within the frameworks of the

Polish State Committee for Scientific Research PBZ-100/4/2004

headed by Prof. L.A. DobrzaĔski.

178

Short paper

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References

4. Conclusion

Acknowledgements

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