THERMAL FATIGUE OF LASER TREATED HOT-WORK TOOL STEELS ТЕРМИЧЕСКАЯ УСТАЛОСТЬ ТЕПЛОСТОЙКИХ ИНСТРУМЕНТАЛЬНЫХ СТАЛЕЙ, ОБРАБОТАННЫХ ЛАЗЕРОМ

Abstract

The present paper deals with the thermal fatigue micro-cracks in the laser-hardened layers of steels AISI L6, Н21 and Н11. The thermal cycling tests were carried out by heating in a molten aluminum alloy– Tmax=680 o C and cooling in water – Tmin=20 o C. The parameters of the maximum – l max and average – l av length of the micro-cracks in the cross section of the sample were used for the quantitative estimation of their propagation. Oxidation and Al alloy soldering were registered on the samples surface by SEM even in the first 100 thermal cycles. EDX analysis shows oxidation of the macro-crack surfaces and presence of the oxides of the Fe 3 O 4 type inside them. It was established that the mechanisms of the micro-cracks appearance and propagation are identical in the steels treated by laser and in the steels subjected to the volume heat treatment. Micro-cracks appearance on the laser-hardened layers occurs mainly on the sample surface in the first dozens of thermal cycles due to the accumulation of the local plastic deformation, whereas their propagation on the initial stage is due to the oxidation of their surfaces.

Full text

THERMAL FATIGUE OF LASER TREATED
HOT-WORK TOOL STEELS
ТЕРМИЧЕСКАЯ УСТАЛОСТЬ ТЕПЛОСТОЙКИХ ИНСТРУМЕНТАЛЬНЫХ
СТАЛЕЙ, ОБРАБОТАННЫХ ЛАЗЕРОМ
Dr. Eng. Dikova Ts., Prof. DSc. Stavrev D.,
Technical University of Varna,
E-mail: tsanka_dikova@abv.bg, d_stavrev@abv.bg
Abstract: The present paper deals with the thermal fatigue micro-cracks in the laser-hardened layers of steels AISI L6, Н21 and Н11. The
thermal cycling tests were carried out by heating in a molten aluminum alloy– Tmax=680 oC and cooling in water – Tmin=20 o C. The
parameters of the maximum – lmax and average – lav length of the micro-cracks in the cross section of the sample were used for the
quantitative estimation of their propagation. Oxidation and Al alloy soldering were registered on the samples surface by SEM even in the
first 100 thermal cycles. EDX analysis shows oxidation of the macro-crack surfaces and presence of the oxides of the Fe3O4 type inside them.
It was established that the mechanisms of the micro-cracks appearance and propagation are identical in the steels treated by laser and in the
steels subjected to the volume heat treatment. Micro-cracks appearance on the laser-hardened layers occurs mainly on the sample surface in
the first dozens of thermal cycles due to the accumulation of the local plastic deformation, whereas their propagation on the initial stage is
due to the oxidation of their surfaces.
KEYWORDS: THERMAL FATIGUE MICRO-CRACKS, LASER SURFACE TREATMENT, HOT-WORK TOOL STEELS
1. Introduction
Die casting is a highly productive process in which cheap
castings with complex configuration, high accuracy and smooth
surfaces are obtained mainly from non-ferrous metals. It is realized
by means of the repeated processes of heating and cooling of the
surface layer of the die at high temperatures, dynamic loadings and
considerable impact of the molten metal. As a result, die casting
dies are destroyed during the operation on their surfaces due to the
thermal fatigue. Repeated heating and cooling cycles initiate
thermal stresses in the details which bring to the origination of the
local micro-cracks after exceeding the certain value, depending on
the material properties and the working conditions. They develop
into macro-cracks leading to destroying in a comparatively short
period (102–104 cycles) i.e. in the range of the low-cycle fatigue [1,
2]. The processes of oxidation and creep contribute considerably to
the initiation and propagation of the cracks [3-5].
There exist two main ways for improving the thermal fatigue
resistance. 1) Development and usage of new materials with high
levels of hot strength, creep strength and tempering resistance,
along with the low thermal expansion and high thermal conductivity
[6-8]. 2) Usage of different kinds of surface treatment – chemical
heat treatment, PVD, CVD, duplex-processes, treatment by
concentrated energy fluxes (laser, plasma and electron beam) by
means of which only the surface layer microstructure and properties
are changed. Coatings resulted from the first group of the processes
lead to the improvement of the resistance to the molten metal
soldering, corrosion and erosion resistance, whereas they worsen
the thermal fatigue resistance [9-11]. The laser treatment by
different schemes of operating could form conditions for increasing
the thermal fatigue resistance. It was found that the alternation of
zones with different microstructure and properties is an obstacle to
the propagation of the thermal fatigue cracks [12, 13]. On the other
hand the thermal fatigue micro-cracks appear in homogeneous
microstructures on a later stage, while their growth is prevented by
dispersive microstructures [14, 15].
Since the laser technology develops intensively in the last 20
years there is no sufficient information concerning to the influence
of the additional types of treatment on the obtained surface layers,
appearance of defects and their destroying. The aim of this research
is to investigate the mechanisms of initiation and propagation of the
thermal fatigue micro-cracks in laser-hardened layers of hot work
tool steels.
2. Experimental details
Hot-work tool steels AISI L6, Н21 and Н11 were used for the
experimental investigations (Table 1). Prismatic samples with
dimensions of 20х20х50mm3 for L6 and H21 steels and
15х15х60mm3 for H11 steel were manufactured and subjected to
preliminary quenching and tempering. The surface heat treatment
was done by a continuous wave CO2 laser at different regimes –
with melting and without melting of the surface. The thermal
cycling tests were carried out by heating in a molten aluminum
alloy– Tmax=680 oC and cooling in water – Tmin=20 oC. Thermal
fatigue micro-cracks were investigated by using optical and SEM
metallography, EPMA and EDX analysis. Their propagation was
studied in each 100, 200, 300 and 400 cycles for L6 and H21 steels.
Since the micro-cracks were found out in the first 100 cycles, in the
steel H11 they were investigated on the initial stage of the
experiment – from 10 up to 50 cycles. The parameters maximum–
lmax and average – lav length of the micro-cracks in the cross section
of the sample were used for the quantitative estimation of their
propagation.
3. Results
3.1. Kinetics of the thermal fatigue micro-cracks
During thermal cycling the micro-cracks appear mainly on the
surface of the laser-hardened layer as well as on the base material.
Investigation on the base material of L6 and H21 steels shows their
appearance in the first 100 cycles (Fig.1a). The initial stage study of
the H11 steel confirms their presence even in 10 cycles. Micro-
cracks in L6 steel have a larger length, but their sizes are
comparatively similar. In the sample of H21 steel the presence of
different size carbides located in the type of stripes brings to the
appearance and propagation of the particular micro-cracks with a
large length (Fig.2a).
In the zone of the laser treatment without melting, conditions
for earlier appearance of the micro-cracks as well as for their
accelerated growth along the grain boundaries exist due to the
tempering processes and increasing the carbide phase quantity.
Because of the presence of non-metallic inclusions micro-cracks
appearance under the surface becomes possible. In their
enlargement they can fuse with the micro-crack originated on the
surface and form a new larger size micro-crack (Fig.2b).
The microstructure in the melted zone of L6 steel is of a block-
type which does not change during thermal cycling. Originated
Table 1.
Chemical composition of the investigated steels.
Chemical composition, %
Steel C Si Mn Cr Ni W V Mo
L6 0,59 0,22 0,65 0,70 1,55 - - 0,25
H21 0,37 0,35 0,20 2,35 - 7,80 0,40 -
H11 0,38 1,00 0.30 5,00 - - 0,50 1,50

micro-cracks grow along the blocks boundaries and reach the
comparatively big maximum length lmax even in 200 cycles (Fig.1c,
Fig.3a). Whereas in H21 and H11 steels fine-grained microstructure
in the melted zone results in the initial stage of the thermal cycling
– in 10 cycles for H11 steel [16]. This is the reason for preventing
the growth and further propagation in depth of the initially
originated micro-cracks (Fig.1c, Fig.3b). On increasing the cycle’s
number up to 400, the increasing of maximum lmax and average lav
length of the micro-cracks rather slight. (Fig.1c).
3.2. SEM investigation
Investigation by EPMA analysis shows the presence of a thin
layer of Al and Fe compounds and oxides on the samples surface
even on the initial stage due to the methodology of the thermal
cycling tests (Fig.4). Inside the micro-cracks up to 400 cycles only
oxides were fixed.
The detailed investigation by EDX analysis detects a clearly
expressed oxide layer and slight increasing of the Al concentration
in the surface layer – up to 2,7% in L6 steel and up to 5,6% in H21
steel (Fig.5, Table2). Incompact laminar texture Fe oxides were
observed inside the micro-crack in the melted zone of the L6 steel.
They were of the Fe3O4 type mainly and their formation was fixed
at considerable magnification of the micro-crack tip (Fig.5).
Oxidation process of the side surfaces of the micro-crack and the
started oxide destroying were noticed very well. Similar results
were obtained in the investigation of the micro-crack on the base
material surface of the H21 steel up to 400 thermal cycles.
3.3. Hardness changes
As a result of the accelerated running of the tempering
processes during the thermal cycling of the L6, H11 and H21 steels
the hardness of the laser-hardened layers decreases (Fig.6).
Hardness decreasing is more intensive on the initial stages and it
stabilizes gradually with the increasing of the cycles numbers. The
hardness of L6 and H11 steels practically equalizes with that of the
base material, whereas in H21 steel the hardness of the melted zone
(in regimes with melting) remains comparatively higher.
4. Disccusion
During the thermal cycling, due to the repeatedly changing
temperatures, deformation processes, originating and increasing of
the defects in the structure of the material occurs, known as thermal
fatigue. Repeated heating and cooling cycles initiate thermal
stresses in the sample which lead to the origination of the local
micro-cracks after exceeding the certain value, depending on the
material properties and the working conditions. The parameters of
the thermal cycle – maximum temperature, duration and the
temperature amplitude ∆Т=Тmax-Тmin exert main influence on the
thermal fatigue processes.
This investigation indicates that the mechanisms of micro-
cracks origination and development during thermal cycling are
identical in steels treated by laser and in steels subjected to the
volume heat treatment.
4.1. Micro-cracks origination
The initial micro-cracks originate mainly on the surface of the
Fig.1. Changes of maximum – l max and average - l av. length
of the surface micro-cracks in L6 and H21 steels in thermal
cycling.
a) base metal;
b) zone of the surface treatment at a regime with no melting
(Es=2810 J/cm2);
c) melted zone of the laser re-melted layer(Es=4762 J/cm2).
0
20
40
60
80
100
120
140
160
0 100 200 300 400
N
, cycles
l, mm
l max – H21
l av – H21
l max – L6
l av – L6
a)
0
20
40
60
80
100
120
140
0 100 200 300 400
N
, cycles
l, mm
l max – H21
l av – H21
l max – L6
l av – L6
b)
0
20
40
60
80
100
120
140
160
0 100 200 300 400
N
, cycles
l, mm
l max – H21
l av – H21
l max – L6
l av – L6
c)
Fig.2. Thermal fatigue micro-
cracks on the surface of:
a) base metal of H21 steel
after 400 thermal cycles and
b) laser treated layer of L6
steel after 300 cycles (regime
without melting - Es=2810
J/cm2).
8
µ
m
a)
25
µ
m
b)
15
µ
m
a)
12
µ
m
b)
Fig.3. Thermal fatigue micro-cracks
on the surface of the melted zone of
L6 steel - a) and H21 steel - b) after
400 thermal cycles (regime of laser
surface melting - Es=4762 J/cm2).

material. During the thermal cycling the mechanical properties
worsen and the creep processes accelerate along with the increase of
the cycle temperature [4,17]. The hardness of the laser-hardened
layers decreases even on the initial stage and it equalizes with that
of the base material even after 10 cycles for H11 steel. The tensile
strength and the creep strength of this steel at the maximum cycle
temperature of 400оС are respectively 1160 MP and 100 MP and
they decrease further at the temperatures above 600 оС [8], which
results in increasing the plastic deformation. The data for L6 and
H21 steels are analogical [18].
In the first seconds of heating and cooling, temperature
gradients within the range of 23*103 - 26*103 К/m [16] were
obtained near the sample surface, provoking high thermal stresses.
Cyclic change of the stresses, hardness and creep strength decrease
lead to the accumulation of the plastic deformation on the sample
surface after a certain number of cycles [6,7,9]. As a result, zones
with definite relief consisting of grooves and bulges were obtained
and they acted as stress concentrators.
Experiments create conditions for corrosion and oxidation
which can accelerate the process of the micro-cracks initiation. A
thin layer of inter-metallic compounds (according to the Al-Fe-Si
diagram) and oxides was formed on the samples surface. The both
types of the compounds are hard and brittle and they lead to the
decreasing of the heat conductivity [19, 20]. Their breaking is
possible during the thermal cycling and the nuclei of the first micro-
cracks - micro-pores were originated. When the thermal stresses
exceed the material strength the most probable places for micro-
cracks initiation are the grooves on the surface layer
In melting zone of steels treated by laser influence, metal
refinement, decreasing of the defects of the structure and increasing
of the initial surface roughness in the melted zone were obtained.
Owing to the alloying of the solid solution the heat conductivity of
the melted zone decreases and leads to the additional increase of the
stresses. The probable compression stresses on the surface due to
the martensite transformation in the melted zone do not exert
particular influence because they relax even after the first cycles at
the temperature range of the investigation. For that reason, the later
stage of the micro-cracks appearance on the melted zone surface
could hardly be expected to occur in comparison to those on the
base metal.
The same tendency was observed at the regime of laser
treatment without melting. On using the operating mode with the
pre-set parameters, tempering processes run in the surface layer
leading to the increasing of the carbide phase quantity. Owing to
different thermal properties of the particular phases, temperature
gradients and stress gradients occur on their boundaries. In this way
the micro-cracks appear on the surface along the boundary between
the carbide and the solid solution.
4.2. Micro-cracks propagation
Micro-cracks propagation in the surface layer of the base
metal as well as of the laser-hardened layer was done mostly by
inter-crystalline mechanism due to the separation of the carbide
phase along the grains boundaries at the high temperature of the
thermal cycle.
The oxidation plays an important role in the micro-cracks
growth on the initial stage of the thermal cycling. Oxides mainly of
the Fe3O4 type were formed inside the initially originated micro-
cracks due to the gas corrosion (Fig.4, Fig.5). They prevent the
closing of the micro-cracks during cycling in tension-compression
[21]. On the other hand – the relative volume of the oxides is bigger
than that of the steel [3] leading to the increase of the compression
stresses in the heating phase. Owing to these, tensile stresses in the
cooling phase increase and provoke the micro-crack growth [5]. The
brittle oxides can be easilly broken (Fig.5) due to the tensile stresses
creating conditions for further oxidation and for penetrating the Al
alloy in a sufficient micro-crack width.
4
3
2 1
5
7
8
6
Table 2.
Chemical composition, at.%
№ Fe O Al
1 39,97 55,75 2,73
2 48,14 50,89 0,53
3 44,55 54,34 0,31
4 99,83 0,00 0,17
5 44,04 55,44 0,16
6 44,31 55,24 0,08
7 44,97 53,55 0,19
8 99,14 0,00 0,00
Fig.5. Chemical composition changes in different regions of
the thermal fatigue micro-crack (melted zone of steel L6 after
400 thermal cycles).
a)
Fe O Al
Fe O Al
b
)
Fig.4. Thermal fatigue micro-cracks in the melted zone of surface layer of L6 steel – a) and in the base metal of H21 steel – b) after
400 thermal cycles: microstructure, element’s distribution map and linear analysis of the chemical composition changes.

In laser treated L6 steel sharp increase of the maximum length
lmax of the micro-cracks was observed after 200 cycles whereas the
character of the average length lav change is similar to that of the
base metal (Fig.1b, Fig.1c). Probably this is due to the increased
quantity of the carbide phase of the M3C type [16] which was
precipitated at comparatively low temperatures and it has a high
tendency to grow. At the used regime without melting, it begins to
precipitate even during the laser treatment. Further thermal cycling
accelerates this process creating conditions for facilitated
propagation of the initiated micro-cracks. The block character of the
microstructure at regime with melting additionally helps that
process. Some of the micro-cracks grow to a large depth, in this
way the stresses in the surrounding material relax, retarding the
propagation of the adjacent micro-cracks [6,9].
The character of the changes of the maximum lmax and average
lav micro-cracks length in H21 steel treated at a regime without
melting is analogous to that of the base material due to the similar
microstructure. At a regime with melting, a rather slight increase of
the two parameters was indicated after 200 cycles. Melting of the
surface layer results in strong decrease of the carbide heterogeneity
and obtaining of the comparatively homogeneous and dispersive
microstructure. Fine-grained microstructure and the comparatively
higher hardness were retained in the subsequent thermal cycling due
to the poor tendency to growth of the carbide M6C precipitated [16].
All these factors interfere the growth and propagation of the micro-
cracks.
Effective detention of micro-cracks propagation occurs when
the depth of the dispersive microstructure is enough. If the depth of
the melted zone is commensurable with the length of the initial
micro-cracks, they exceed it and continue to propagate rapidly in
the base material. From this point of view the most suitable regime
of laser treatment is the regime ensuring the depth of the melted
zone not less than 0,2 mm.
5. Conclusions
- During thermal cycling the micro-cracks appear mainly on
the surface of the laser-hardened layer as well as of the base
material.
- Processes of decreasing the hardness and the creep strength,
accumulating plastic deformation on the surface, oxidation and
corrosion significantly influence the appearance of micro-cracks.
- Oxidation plays an important role in the micro-cracks
growth on the initial stage of thermal cycling.
- Mechanisms of thermal fatigue micro-cracks initiation and
propagation are identical in the steels treated by laser and in the
steels subjected to the volume heat treatment.
- Kinetics of the micro-cracks development is different at the
different kinds of treatment due to the microstructure of the surface
layer.
- Effective detention of micro-cracks development occurs
when the depth of the dispersive microstructure is enough. The
most suitable regime of laser treatment is the regime ensuring the
depth of the melted zone not less than 0, 2 mm.
6. Acknowledgements
The authors thank Prof. Shigeru Yamaguchi, Prof. Sachio Seto
and MSc Eng. Ryusuke Horiuchi from Tokai University, Japan, for
their help in SEM investigations.
7. References
1. Veronskij A., Thermal fatigue of metals, M.Metalurgia, 1986;
2. Voronova O.I., Casting production, 1996, №7, p.15;
3. Kovrigin V.A., MITOM, 1980, №9, p. 51-53;
4. Danzer R. et al, 12th International Die Casting Congress &
Exposition, Minneapolis, Paper No G0T83-012, Oct 31-Nov 3,
1983 (4 p.);
5. Persson A., Hogmark S., Bergstrom J., J.Mater.Process
Technol. 148 (2004) 108-118;
6. Persson A., Hogmark S., Bergstrom J., International Journal of
Fatigue 26 (2004) 1095-1107;
7. Sjostrom J, Bergstrom J., J.Mater.Process Technol. 153-154
(2004) 1089-1096;
8. Dobrzanski L., Mazurkiewicz J., Hajduczek E., Madejski J.,
J.Mater.Process Technol. 113 (2001) 527-538;
9. Persson A., Hogmark S., Bergstrom J., Surf. Coat. Technoll.
191 (2005) 216-227;
10. Mitterer C., Surf.Coat.Technol., 125 (2000) р.233-239;
11. Pellizari M., Molinari A., Staffelini G., Surf. Coat. Technol.,
142-144 (2001) 1109-1115;
12. Zhou H., Zhang Z.H., Ren L.Q., Song Q.F., Chen L., Surf.
Coat. Technol., article in press (2005);
13. Соколов Б.К., 6 Международна Конференция АМТЕХ
2001 03-05 октомври 2001, Созопол;
14. Jiang W., Molian P., Surf. Coat. Technol., 135 (2001) 139;
15. Staverv D., Dikova Ts., II Euro-Asian Scientific-Practical
Conference “Strength of Heterogeneous Structures PROST 2004”,
Abstracts of Papers, MISA, Moscow, Russia, April 2004, p. 71;
16. Dikova Ts.,Investigation the Behavior of Steels 5ChNM
(L6), 3Ch2W8F (21) and 4Ch5MFS (H11) in Laser Treatment and
Thermal Cycling, PhD thesis, Varna, 2005, 44 p.,;
17. Maillot V., Fissolo A., Degallaix G., Degallaix S., Int. J.
Solids and Structures, 42 (2005) 759-769;
18. Poznjak L.A.,Tool steels, Kiev,Naukova dumka,1996,488p.;
19. Pirzada D., Baburaj E., Govindaraju R., Froes F., Surface
Engineering 2000, Vol.16, No. 2, 164-168;
20. Srivastava A., Joshi V., Shivpuri R., Wear 256(2004) 38-43;
21. Samrout H., Adbi R., Int. J. Fatigue, 1998, Vol. 20, No. 8,
555-563;
Fig.6. Changes of average surface hardness of steels
H11 – a), L6 – b) and H21 – c) after thermal cycling.
H11
0
200
400
600
800
1000
0102050
cycles
HV5 base metal
laser melted layer Es=4762 J/cm2
laser melted layer Es=6196 J/cm2
laser melted layer Es=6857 J/cm2
laser melted layer Es=7074 J/cm2
a
)
L6
0
200
400
600
800
1000
0 100 200 300 400
cycles
HV5 base metal
laser treated layer Es=2810 J/cm2
laser melted lay er Es=3867 J/cm2
laser melted lay er Es=4444 J/cm2
laser melted lay er Es=4762 J/cm2
b
)
H21
0
200
400
600
800
1000
0 100 200 300 400
cycles
HV5 base metal
laser treated layer Es=2810 J/cm2
laser melted layer Es=3867 J/cm2
laser melted layer Es=4444 J/cm2
laser melted layer Es=4762 J/cm2
c
)