Post on 31-Jan-2023
Surface plastic deformation of plasma-arc hardened layersof steel class X12 (D2, D3 AISI)
Ivan IvanovTechnical University – Varna, Bulgaria
Abstract: The paper is presented results from microstructure,
hardness, electron microscopy and roughness analysis of plasma-arc
hardened and surface plastic deformed sidelong specimen of tool
steels class X12.
Key words: plasma-arc hardening, surface plastic deformation,
residual austenite
Introduction:The working capability of cutting and deforming tools depends
on the quality of their heat and mechanic treatment. Poor
implementation of one of the technologies worsens the working
capability and durability of the tool, and, respectively, the
surfaces it is used to treat.
In recent years surface heat treatment with concentrated
energy flows and finishing surface plastic deformation become
widely applied. The usage of CEF as a heat source allows local
hardening at places with highest tool wearing. In addition, we
reach hardness surpassing the hardness after the usual heat
treatment, and for the unhardened parts the initial properties
remain [1,2,4-8].
The application of surface plastic deformation secures the
decrease of the surface roughness, increment of surface hardness,
generation of residual pressure stresses [9,11], and structural
changes in the surface layer. This predisposes the durability of
the details [3,9].
Combining surface quenching and CEF with surface plastic
deformation of the hardened layer leads to additional increase in
the hardness while relatively keeping the phase structure
[1,2,4,5]. There are two major schemes for combined treatment
implementation – plastic deformation before CEF treatment [5,10]
and after CEF treatment [1,2,4,5].
The aim of current paper is to examine the combined plasma-
arc and subsequent surface deformation treatment of steel class
Х12.
Methodology:
We undertake a plasma-arc treatment of specimens of steels Х12,
Х12М, Х12МФ with sizes 20х20х60 mm. The advance heat treatment is
presented in Table 1.
The surface plasma-arc treatment of the specimens of steel
class Х12 was implemented this appliance РМ6601П, which guarantees
linear movement of the plasmotrone. Plasma-generating and
protecting gas is argon.
The plasma-arc quenching regime is presented in Table 2.
The surface elastic-plastic deformation is implemented with a
universal milling machine at rotational movement of the
instrument/tool and linear step-by-step shifting of the sample.
The tool is eccentrically adhered so that we have a width of the
deformed layer of 15 mm. The deforming treatment is implemented
with a sphere with a diameter of d=10,5 mm, and the treatment
power is F=650N. The deforming treatment on the surface of the
specimens is done with a rotation speed of 250 min-1, circle
diameter D=15 mm and linear speed of 12,5 mm/min. The number of
transitions N of the deforming instrument is determined by the
following law (1):
N=2i, i=1÷5
(1)
Scheme of linear surface plastic deformation is presented on
Figure 1.
The hardness analysis is conducted with 0,2,4,8,16 and 32
deformation transitions of the instrument. In order to achieve
better results in the specimens research in the micro hardness
changes we created oblique metallographic specimens. The specimes
are grinded in angle 0,014о in depth 0,3 mm, so that we achieve
large enough surface for layer-by-layer deformation analysis.
The structure is developed with 3%-solution of HNO3 in C2H5OH.
The microhardnesses are measured using the Vikers method with
microhardness measurer ПМТ3 and pressure 100g and microhardness
measurer Heckert and pressure 5kg. The microstructures are
photographed with an optical microscope NEOPHOT 32, and the
electron microscopy ones on an electronic microscope JOEL–JXA-50A
with zoom of 4000 times.
The roughness is measured crosswise the grinding direction. We
have used Taylor-Hobson Surtronic 3 instrument/tool, with standard
width of 4,5 мм and base length of 4 мм. The length of the piece
is 0,8 мм, and the translation speed is 0,25мм/s.
Results and analysis:
Regardless of the advance heat treatment after plasma-arc
treatment a white melted zone appears in the surface layer, with a
characteristic dendrite morphology consisting of more than 90%
residual austenite [1,4]. During the crystallization in the
melting zone a disperse quasi-eutectic is formed around the axes
of the dendrites. The hardness in this zone reaches 500 – 600 HV,
surpassing that of the ordinary austenite. We suppose this is a
consequence of the full solution of the carbides in this zone and
the saturation of the austenite with carbon and all additives.
From the standpoint of classic volume heat treatment the achieved
hardness in the melting zone is not enough for this class of
steels, working at high pressures and relatively high
temperatures.
A large quantity of the residual austenite and its eventual
thermo-deformational transformation during the usage of the
instrument leads to size instability of the instrument as well. On
the other side, the destruction of the carbide texture and its
complete solution in the metal matrix leads to homogenization of
surface layer properties. However, the achieved relatively low
hardness in the melted zone does not contribute to the increase of
the exploitation characteristics of instruments. It is different
with the quenching zone from solid state. The hardness in this
zone reaches 750 – 820 HV and is comparable and even higher than
that achieved in volume quenching. The forming of instruments’
plasma-arc treated surfaces of zones with different phase and
structural content and properties, is not always beneficial for
their exploitation characteristics. The presence of large
quantities of residual austenite in the melted zone, deteriorating
the durometric properties, can be removed in two main ways –
thermal destabilization and plastic deformation. From the view
point of its saturation with carbon and additives, the thermal
destabilization is difficult to achieve in temperatures below
400оС, the residual austenite keeps its hardness, and the further
increase of temperature is not desirable, because of the decrease
of the hardness in the volume quenched part. The deformational
martensite produced as a result of the transformation of the
austenite during exploitation of the instrument is inadmissible.
Te application of surface plastic deformation on plasma-
hardened layers is one of the options to increase the surface
hardness. In current research we observe an increase in every
zones, achieved in plasma-arc quenching regardless of the
structural condition. In greatest extent increase in hardness is
observed in the melted zone – up to 30%, which is logical
considering the quantity of austenite inclined to hammer
hardening. In lower extent, hardening is observed in the quenched
from hard state layers – 10-15% (fig.2). The expectations for γ→α
transformation in zone with 90% Аres. under multiple surface
deformation are not confirmed by the X-ray structural analysis of
specimens of steel Х12 [2]. Nevertheless, the increased hardness
after surface plastic deformation, especially in the melted zone,
and the residual pressure stresses suppose an increase in the
exploitation durability of the instrument, and from an a priori
information is well known that in surface plastic deformation we
achieve favorable pressure stresses. In addition, we observe
decrease in roughness. Largest decrease of Ra is observed after
the second transition of the deforming element. Subsequent
deformation does not contribute to significant changes in
roughness (fig.3).
Micro structurally, we note in the plasma hardened layer a
deformed texture, which is best expressed in the melted layer,
because the austenite possesses more gliding surfaces in its
crystal net than the martensite, and therefore, greater depth of
the deformed layer (fig.4). In the periphery of the zone quenched
from solid state there are formed parallel deformation lines with
distance between them of 1 – 2 μm, and ordered location in the
direction of the lines of dispersed carbides with sizes from 0,5
to 2 μm (fig.4e). We observe crashes of big primary carbides as well
(fig.4 e,f).
Conclusion:
Combining surface plasma-arc and deformation treatment of
steels class Х12 leads to an increase in the hardness in the
melted zone with around 30% and in the zone quenched from solid
state with 10÷15% and hardening depth of 0,2 [mm]. Dispersed
carbides are formed after plasma-arc quenching, and their ordering
in the direction of the deformational lines after the deformation
treatment. The surface plastic deformation decreases the roughness
to Ra 0,63.
References:
1. Kirov S., Ivanov I., Metallographic and electron
microscopy researches of steel H12MF (D2 AISI) after combined
plasma arc and surface deformational treatment, Artcast 2008,
4th International Conference “Casting, from rigor of technique
to art”, 9-10 May 2008, Galati, Romania, Europlus publishing
house, pp.187-191
2. Киров С., Иванов И., Шамонин Ю., Георгиев С.,
Структура и свойства на стомана Х12 след комбинирано плазмено –
дъгово въздействие и повърхностна пластична деформация, V
Международен конгрес „Машиностроителни технологии’ 06”, 20 – 23
септември 2006 г., Варна, България, кн. 2, стр. 36 – 39
3. Метев Х., Кузманов Т., Обработване на закалени
бързорежещи стомани чрез повърхностни механични въздействия, IV
Международен конгрес „Машиностроителни технологии’ 04”, 2004 г.,
Варна, България
4. Иванов И., Георгиев С. Методика и изследване на
повърхностно уякчени слоеве от стомана Х12МФ, Годишник на
Технически Университет –Варна, 2008г. стр.13 – 18
5. Бровер Г. И., Варавка В. Н., Блиновский В. А., О
возможности повышения эффективности лазерной закалки
дополнительным пластическим деформированием, ЭОМ, 1989г., № 3,
стр. 16 – 18
6. Miralles M., Laser hardening of cutting tools,
Master’s Thesis, Lulea University of technology, Spring 2003,
p.87
7. Song R. G., Zhang K., Chen G. N., Electron beam
surface treatment. Part I: surface hardening of AISI D3 tool
steel, Vacuum 69 (2003) 513–516
8. Song R. G., Zhang K., Chen G. N., Electron beam
surface re-melting of AISI D2 cold-worked die steel, Surface and
Coatings Technology 157 (2002) 1–4
9. Сучков, А. и др., Довършващо обработване чрез
повърхностно пластично деформиране, “Техника”, София, 1984г.,
255 с.
10. Г. Д. Гуреев, Д. М. Гуреев, Совмещение лазерного и
ультразвукового воздействий для термообработки поверхности
стали, Вестн. Сам. гос. техн. ун-та. Сер. Физ.-мат. науки, 2007,
1(14), 90–95
11. Вишняков Я. Д., Пискарев В. Д., Управление
остаточными напрежениями в металлах и сплавах, Москва,
Металлургия, 1989г., 254 стр.
Heattreatment Т q.оС Т оС Т an.оС τ, min HRC
Cooling
mediumQuenching 1050 200 - 120 60-62 Oil
Annealing - - 1050 720 25 withoven
Table 1 Advance heat treatment regimes
Speed oftranslation, V
mm/s
Power density,Ns W/cm2
Expense ofplasma-
generating gas,Q, l/min
Distance betweenthe plasmotroneand the surface,
H, mm4÷10 1,5÷1,7*104 4÷6 2
Table 2. Plasma-arc treatment regime parametres
F=650N
n=250 min-1
12,5mm/min
Ø 10,5
Ø15
Figure 1. Scheme of linear surface plastic deformation
0
100
200
300
400
500
600
700
800
900
0 2 4 8 16 32
Number of transitions
HV5
melted zone tempering zone hardened zone
Figure 2. Influence of stage of plastic deformation on microhardness hardened and
annealing specimens
0 0.5 1
1.5 2
2.5
0 2 4 16 Number of translations of the deforming instrument
Ra
Figure 3. Changes in Ra depending on the number of
translations