Plasma hardening of medium carbon steels

Abstract

The structure, size and hardness of the zone of hardening of pre-annealed steels (30XHMA, 34XH1M, 35XH2Φ, 38XΓH, 40XHMA, 40XH2MA, 45) was studied after treatment with a plasma arc of direct polarity. It was shown that plasma hardening forms a thin layer of martensitic-austenitic structure with variable composition and hardness on the treated steel surface. The zone subjected to hardening was represented by marten site of different dispersion and ferrite veins. Between the quenching zone and the base metal there was a transition zone with a gradient structure of plate perlite and elements of perlite-ferrite base, the share of which was gradually increasing. The structural-phase structure of this zone provided a smooth transition of mechanical properties from the hardened layer to the base metal. Dependences for management of a structural condition, hardness and depth of a zone of hardening were determined.

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Plasma hardening of medium carbon steels
To cite this article: E N Safonov and M V Mironova 2019 J. Phys.: Conf. Ser. 1353 012065
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HIRM-2019
Journal of Physics: Conference Series 1353 (2019) 012065
IOP Publishing
doi:10.1088/1742-6596/1353/1/012065
1
Plasma hardening of medium carbon steels
E N Safonov and M V Mironova
Ural Federal University, Nizhny Tagil technological Institute (branch), 59,
Krasnogvardeyskaya ave., Nizhniy Tagil, 622031, Russia
E-mail: E.N.Safonov@urfu.ru
Abstract. The structure, size and hardness of the zone of hardening of pre-annealed steels
(30ХНМА, 34ХН1М, 35ХН2Ф, 38ХГН, 40ХНМА, 40ХН2МА, 45) was studied after
treatment with a plasma arc of direct polarity. It was shown that plasma hardening forms a thin
layer of martensitic-austenitic structure with variable composition and hardness on the treated
steel surface. The zone subjected to hardening was represented by marten site of different
dispersion and ferrite veins. Between the quenching zone and the base metal there was a
transition zone with a gradient structure of plate perlite and elements of perlite-ferrite base, the
share of which was gradually increasing. The structural-phase structure of this zone provided a
smooth transition of mechanical properties from the hardened layer to the base metal.
Dependences for management of a structural condition, hardness and depth of a zone of
hardening were determined.
1. Introduction
Medium-carbon low-alloy steels are typically used in a thermally hardened state to provide the required
strength. This does not allow realizing the maximum hardness of the working surface of the parts, which
is accompanied by a corresponding decrease in their life, especially in friction conditions. A rational
solution to this problem is the use of hardening of working surfaces, for example, plasma hardening [1-
8]. At the same time, the physical and chemical state and structure of steel in the inner layers of the
product do not change. Surface hardening allows increasing the wear resistance and service life of parts
due to a favorable combination of high hardness of the surface working layer with a sufficiently strong
and plastic core, the structure and properties of which are formed at the previous stages of production.
The purpose of the work is to determine the rational parameters of the surface plasma hardening
mode of medium-carbon structural steels based on the study of the structure, depth and hardness of the
local hardening zone.
2. Materials and methods
The structure, size and hardness of the zone of hardening of pre-annealed steels (30ХНМА, 34ХН1М,
35ХН2Ф, 38ХГН, 40ХНМА, 40ХН2МА, 45) after treatment of their surface by a plasma arc of direct
polarity were studied. Argon was used as a plasma-forming and protective gas. The range of variation
of parameters of the process mode: arc current 120...300 A, the speed of movement of the plasma torch
1...5 cm/s. The linear energy of the arc was changed in the range 1320...3400 J/cm preventing macro-
melting of the treated surface.
The structure and dimensions of the heat-treated zone were investigated on transverse micro-grinding
with AXIOVERT 40 microscope at magnifications of ×50 and ×1000. Assessment of dispersion of

HIRM-2019
Journal of Physics: Conference Series 1353 (2019) 012065
IOP Publishing
doi:10.1088/1742-6596/1353/1/012065
2
structural components was carried out according to State Standard 8233-56 "Steel. The standards of the
microstructure". Hardness was measured by the device ERGOTEST COMP 25. Tests were carried out
with a Vickers pyramid under load 98.07 H. Phase composition of the surface layer of steels 34ХН1М,
35KHN2F, 38KHSN, 45 studied using x-ray diffractometer DRON-1 in the iron K radiation. The
volume fraction of phases was determined with respect to the integral intensities of austenite lines (311)
and martensite lines (112-211) – (121). To estimate the carbon content in the phase components, the
parameters of their lattices were determined.
3. Results and discussion
As a result of the studies it was found that with the increase in the linear energy of the plasma arc, the
depth and hardness of the local hardening zone increase. The increase in the arc speed at a fixed current
value is accompanied by a decrease in these indicators. This is due to the decrease in the linear energy
of the heat input process. Figure 1 shows the dependence of the hardness change in the depth of the
thermal influence zone for a number of steels under different values of linear energy.
Figure 1. Change of hardness (HV 10) in depth (L) of the local zone of plasma hardening of steels at
different values of linear energy (J/cm)
Comparing the data obtained, it can be concluded that the depth and hardness of the plasma-
quenching zone increase with the increase in the arc energy. Steels with higher carbon content have a
greater depth and hardness of the hardened zone. However, it should be noted that the surface hardness
of the heat-treated zone for steel 40ХНМА and 40ХН2МА slightly below the maximum values observed
in its depth.
X-ray analysis shows the presence of a thin (~ 0.01...0.03 mm) surface layer of samples treated with
plasma arc, residual austenite. Its volume fraction varies depending on the composition of the steel and
the linear energy of the arc (Tables 1, 2).
Evaluating the results of steel 45 processing, it can be noted that the value of the linear arc energy of
1320 J/cm is insufficient for its full hardening (Figure 1).
The initial structure of steels is represented by ferrite and perlite (30...50 %). The typical structural
state of the plasma quenching zone observed at an increase of ×50 for the studied steels is shown in
figure 2. It is seen that the structure naturally changes in depth. In the process of plasma heating in the
areas of perlite, carbon austenite is formed, the composition of which is close to eutectoid. In the surface
layer, where the heating temperature and its duration are maximum, the diffusion equalization of the
carbon concentration occurs. As a result of the development of this process, a thin layer of martensitic-

HIRM-2019
Journal of Physics: Conference Series 1353 (2019) 012065
IOP Publishing
doi:10.1088/1742-6596/1353/1/012065
3
austenitic structure is fixed on the treated surface after cooling, with a variable content of residual
austenite depending on the treatment mode and steel composition.
Table 1. Dependence of the local depth of the hardening zone, the volume fraction residual austenite
(γr, %) in the surface layer and its hardness from linear arc energy at plasma hardening of steel 45
Linear arc energy, J/cm
Depth of zone, mm
γr, %
HV 10
1320
0.40
40
360
1710
0.75
29
545
1800
0.95
28
665
2100
1.10
28
730
2400
1.30
17
757
2700
1.50
< 5
780
Table 2. Dependence of the lattice parameter of martensite (а (211)) and the volume fraction of
residual austenite (γr, %) in the surface layer, as well as its hardness from linear arc energy at plasma
hardening of steels
Linear arc
energy,
J/cm
34ХН1М
35ХН2Ф
38ХГН
а(211),
о
А
γr, %
HV 10
а(211),
о
А
γr, %
HV 10
а(211),
о
А
γr, %
HV 10
1320
2.8737
37
470
2.8742
45
560
2.8740
39
545
1500
2.8729
< 5
675
2.8740
25
674
2.8695
< 5
600
1650
—
< 5
634
2.8730
< 5
695
2.8720
< 5
653
1800
—
—
—
2.8736
< 5
660
2.8716
< 5
633
It can be noted that the main structure of the quenching zone is needle martensite. The length of its
needles gradually decreases from 16 µm (8 score) at the sample surface to 2 µm (2 score) at a depth of
up to 1.5 mm from the surface. In the transition zone from the hardened metal to the main one, which
has not experienced phase transitions during processing, martensite is gradually replaced by perlite with
an interplate distance of 0.2...0.3 mm (sorbit). This increases the proportion of structurally free ferrite,
which together with perlite is the initial structure of the investigated steels.
According to the experimental data, the regimes with minimal heat input allow obtaining martensite
with a significant (30...40 %) fraction of residual austenite in the surface layer of the studied steels. The
surface hardness is 360...670 HV 10, the depth of the hardened zone – 0.4...0.8 mm. Processing modes
with high values of linear arc energy are characterized by increased heat input. In the studied range, they
form on the surface mainly a martensitic structure with a maximum hardness (690...780 HV 10) and a
depth of the quenching zone (1.3...1.5 mm).
Several hypotheses of the austenitization mechanism are known, but an austenite is formed that is
inhomogeneous in carbon for all mechanisms [9]. The reason for the stabilization of austenite in the
surface layer of ferrite-pearlite steels is the hereditary preservation of sites with a high concentration of
carbon in austenite. Austenite is formed from perlite by high-speed short-term heating of the surface by
a moving heat source. At the same time, the process of diffusion equalization of the carbon concentration
in the volume of the formed austenitic grain does not have time to develop. The increased carbon content
provides a corresponding decrease in the Mн point near the interface and the subsequent fixation of
austenite during cooling.
In the process of plasma heating there are various stages of formation of austenite in the depth of the
zone of thermal influence. This is due to the heat flow spreading from the metal surface. In the upper
layers, is heated to a high temperature, is the transformation of excess ferrite in the austenite and ferrite

HIRM-2019
Journal of Physics: Conference Series 1353 (2019) 012065
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doi:10.1088/1742-6596/1353/1/012065
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saturation of the former sites of carbon. However for all investigated steels there are signs of
incompleteness of this process, especially after processing with minimal heat investment (Tables 1, 2).
Figure 2. Structural state of the plasma hardening zone of steel 30ХНМА
The X-ray analysis records the presence of austenite and martensite with different carbon content in
the surface layer. It can be explained by the fact that in the process of high-speed heating, the pearlite
colony passes into an austenitic state with a carbon concentration close to eutectoid. Further, carbon
diffuses within the grain of structurally free ferrite. This is a condition for its complete transformation
into austenite. The degree of completion of the carbon redistribution process is determined by the
parameters of the thermal heating cycle, the composition and the initial structure of the steel, in
particular, the grain size. As a rule, in the conditions of continuous heating-cooling, characteristic of the
considered technology, due to the short-term stay at a high temperature, the alignment of the austenite
composition with carbon is not achieved. After cooling, in areas with a high concentration of carbon,
residual austenite and high-carbon martensite are formed, between the cementite plates, where the solid
solution is less saturated with carbon, low-carbon martensite occurs.
It was previously shown [8] that in hypereutectoid steels containing excess cementite, austenite is
most saturated with carbon, formed after plasma treatment in the range of increased values of linear
energy. The obtained experimental data allow concluding that the influence of the regime parameters
on the concentration of residual austenite in the surface layer of pre-eutectoid and hypereutectoid steels
differs. With increasing arc energy in the surface layer of the zone of thermal influence of hypereutectoid
steels, the degree of dissolution of excess cementite increases, austenite is saturated with carbon, which
leads to the formation of a significant proportion of residual austenite together with carbon tetragonal
martensite of increased hardness during cooling.
In steels with pre-eutectoid ferrite, the increase in heat deposition is accompanied by a decrease in
the carbon concentration in the resulting austenite. This is due to its diffusion into the ferrite with a
corresponding decrease in the stability of austenite. Martensite is formed in the process of cooling carbon
inhomogeneous austenite. It inherits this microchemical heterogeneity. The proportion of low-carbon

HIRM-2019
Journal of Physics: Conference Series 1353 (2019) 012065
IOP Publishing
doi:10.1088/1742-6596/1353/1/012065
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martensite increases, and the residual austenite decreases. Consequently, in pre-eutectoid steels, the
largest proportion of residual austenite in the surface layer is fixed after treatment with the minimum for
each grade values of the linear energy of the plasma arc. This ensures the formation of austenite during
heating and martensite during cooling. Moreover, for each steel grade, depending on the concentration
of carbon and alloying elements affecting the temperature of MN, these values differ and should be
determined experimentally. Often, these values of the arc energy are insufficient to obtain a hardened
zone of the required depth and hardness (Figure 1, Steel 45). Therefore, for plasma hardening of
medium-carbon (pre-eutectoid) steels it is possible to recommend regimes with a maximum value of the
arc running energy, taking into account the degree of melting of the surface.
4. Conclusion
1. Plasma hardening of the investigated steels with linear arc energy within 1320...3260 J/cm forms on
the treated surface a thin (~ 0.01...0.03 mm) layer of martensitic-austenitic structure with a hardness of
360...730 HV 10 with a volume fraction of the phases varying depending on the treatment mode and
composition of the steel.
2. The hardened zone is represented by a martensite whose needle length gradually decreases from
16 µm (8 score) at the sample surface to 2 µm (2 score) at a depth of up to 1.5 mm from the surface, and
by ferrite veins.
3. Between the quenching zone and the base metal there is a transition zone with a gradient structure
of plate pearlite with a hardness of 520...260 HV 10 and elements of the pearlitic-ferritic basis, the
proportion of which gradually increases. The structural-phase state of this zone provides a smooth
transition of mechanical properties from the hardened layer to the base metal.
4. With an increase in the running energy of the arc, as well as the carbon concentration in the steels
studied, the depth and hardness of the plasma quenching zone increase.
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