50CrV4 QT
50CrV4 QT
50CrV4 QT
M. Kuffová1 *, P. Čelko2
1
Academy of the Armed Forces of Gen. M. R. Štefánik, Department of Mechanical Engineering,
Demänová 393, 031 01 Liptovský Mikuláš, Slovak Republic
2
Alexander Dubček University of Trenčín, Faculty of Special Technology,
Pri parku 19, 911 06 Trenčín – Záblatie, Slovak Republic
Received 11 November 2014, received in revised form 14 May 2015, accepted 29 June 2015
Abstract
The presented work is focused on the fatigue endurance improvement by using plasma
nitriding. In this research the microstructure and fatigue properties of low-alloyed steel
50CrV4+QT were evaluated. The low-alloyed (manganese-chromium-vanadium) steel is com-
monly used after heat treatment – quenching and tempering – QT(.7). This steel is widely
used for highly loaded machines and parts of road vehicles: crankshafts of diesel engines, shafts
of driving gears, connecting shafts, pins, springs, axle shafts and piston rods. Steel is suitable
for quenching and tempering and contains chemical elements which predetermine it to the
chemical-thermal treatment: plasma nitriding. The fatigue limit was determined at the rotat-
ing beam fatigue. The fatigue limit improvement at 1 × 107 cycles was 44 %. Explanation of
the improvement is based on stabilized gradient of properties in treated layer.
K e y w o r d s : plasma nitriding, low-alloy steel 50CrV4, rotating beam fatigue tests, fatigue
endurance
Nitride layer structure is formed in accordance Plasma nitriding principle is based on the direct
with binary phase diagram Fe-N, partially depicted ionization of nitrogen. The treated material is stored
in Fig. 1. Nitride layer is created (Fig. 2) [1]: in a vacuum container (recipient) where the container
– α-phase – Feα (N) solid solution of nitrogen con- is connected as an anode while nitride components as
taining max. 0.1 % N at an eutectoid temperature a cathode. The largest voltage drop occurs in few mil-
with body centred cubic structure [1]. limetres from the surface of the component, this fact
– ε labelled Fe2−3 N which is stable at temperat- appears as light emitting anomalous discharge glow
ures lower than 670 ◦C, provided higher nitrogen con- (Fig. 4). Due to a strong electric field (DC – voltage
tent. Iron atoms in it occupy the nodes of a hexagonal 400–1000 V) between the recipient body and the com-
close packed structure; nitrogen then takes a variable ponent surface ionization of a gas or mixture of gases
part of the interstitial positions. Nitride ε appears in occurred, e.g. nitrogen and hydrogen. Positive ions are
M. Kuffová, P. Čelko / Kovove Mater. 53 2015 443–450 445
accelerated towards the component surface, impacting ε-phase. The layer containing a major percentage of
on it with high kinetic energy. Part of the incident ions nitrogen (8–11.2 % N) causes its subsequent absorp-
is converted into heat. Ions of iron and alloying ele- tion into the material.
ments are knocked out from the surface of the parts. Under the electric field effect, there is huge migra-
Afterwards, they react further with the atomic nitro- tion of the working gas molecules. As a consequence
gen in plasma. Created nitrides in turn condense on of their impact, gas is splitting and ionizing. Posit-
the component surface. After the nitrogen ions im- ive ions are accelerated towards the cathode, it means
pact on the component surface, the adsorbed layer of towards the surface of nitrided samples [6].
nitrides with high nitrogen content is created, called Velocity growth of ions towards the cathode as well
446 M. Kuffová, P. Čelko / Kovove Mater. 53 2015 443–450
C Mn Si P S Cr Ni V Cu
Standard 0.46–0.54 0.5–0.8 0.15–0.4 max. 0.03 max. 0.03 0.8–1.1 max. 0.4 0.1–0.2 max. 0.25
Analysis 0.49 0.54 0.19 0.023 0.017 1.02 0.06 0.11 0.13
as their kinetic energy growth are not linear. Abrupt T a b l e 2. Mechanical properties of low-alloyed steel
growth is observed in the close proximity of the ni- 50CrV4+QT
trided surface within the cathode voltage loss. Con-
sequently, the highest intensity of processes proceed- Yield strength Rp0.2 (MPa) 900
Tensile strength Rm (MPa) 1100–1300
ing in anomalous discharge glow is concentrated to a
Elongation A5 (%) 9
narrow band around the components without respect Contraction Z (%) 40
to their shape and distance from the recipient body Notch impact strength KCV (J cm−2 ) 35
– anode (Fig. 5). Molecules splitting and atom ioniz- Fatigue limit σ c for a rotating beam 410–450
ation are carried out within this band preferentially. fatigue (MPa)
Anomalous glow discharge is planar and its lumines-
cent corona copies the component surface. This is very
important feature which is essential for using of dis-
charge glow for the diffusion heat treatment.
Component surfaces have to be pure, dry and T a b l e 3. Heat treatment of steel 50CrV4
without passivative layers, e.g. oxides layer. Plasma
nitriding allows affiliating the step called cleaning in Process Temperature ( ◦C) Cooling
plasma into the nitriding process. It is the process
Quenching 820–860 oil
within which ineligible surface oxides are de-dusted Tempering 550 air
by the instrumentality of accelerated atoms [6].
Plasma nitriding process can be divided into fol-
lowing steps:
1. Induction heating by recipient body at the tem- holes, it is necessary to use additional electrodes.
perature for cleaning in plasma (lower than nitriding
temperature).
2. Cleaning in plasma (de-dusting) – with or 4. Experimental material
without it.
3. Additional heating at the nitriding temperature. As an experimental material, steel 50CrV4 was
4. Abidance in nitriding temperature during nitrid- used in accordance with Standard EN 89-71, equal
ing process. to Slovak steel 15 260, Slovak Standard STN 41
5. Cooling of the components after nitriding pro- 5260 [8]. According to the other Standards: 1.8159
cess. (W.Nr.), 6150 (AS 1444-86), Gr.6150 (ASTM A322-
Advantages of plasma nitriding [7]: 82), 50ChFA (GOST 14959-79).
– High variability of process parameters which en- Chemical composition of experimental material as
ables either the balanced and porousless white layers well as mechanical properties are shown in Tables 1
or elimination of their formation. and 2.
– Growth rate of plasma nitride layers is 30–40 % The low-alloyed (manganese-chromium-vanadium)
higher in comparison with the common methods. steel is commonly used after heat treatment – quench-
– Low nitriding temperatures together with ad- ing and tempering – QT(.7). Optimal diameter for
equate nitriding depth. this heat treatment is 80 mm. Welding of steel is dif-
– Economic efficiency in comparison to the nitrid- ficult. Hot shaping is good. Machining after soft an-
ing in gas (20 times less costs for gas consumption). nealing is good. This steel is widely used for highly
– Lower expenses for device maintenance. loaded machines and parts of road vehicles: crank-
Disadvantages of plasma nitriding [7]: shafts of diesel engines, shafts of driving gears, con-
– High acquisition costs and low usage of inside necting shafts, pins, springs, axle shafts and piston
space. rods. Steel is suitable for quenching and tempering
– Necessity of careful material setting and treat- and contains chemical elements which predetermine it
ment of material with the same chemical composition. to the chemical – thermal treatment, plasma nitriding.
– Nitriding in holes is difficult, it is impossible to Parameters of heat treatment are presented in
form the nitriding layer in small holes. In case of bigger Table 3.
M. Kuffová, P. Čelko / Kovove Mater. 53 2015 443–450 447
Temperature ( ◦C) Gases H2 : N2 (l/h) Time (h) Pressure (Pa) Voltage (V) Pulse time (µs)
Plasma nitriding – 5 h
390
Hardness of core 413 399 450 0.193
394
Fig. 11. R. R. Moore model: High speed rotating beam fatigue testing machine, detail of rotating part.
σ c at 1 × 107 (MPa),
State n = 9250 rpm => f = 154 Hz
Fig. 15. Wöhler graph of experimental material after 5 h
untreated 450 plasma nitriding.
nitrided 650
7. Discussion of results