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Metal Science and Heat Treatment Voi. 38, Nos. 9 - I0, 1996
TECHNOLOGY OF HEAT TREATMENT
UDC 621.785.796
VOLUME-SURFACE HARDENING OF BY A HIGH-SPEED WATER STREAM
RAILROAD TRANSPORT PARTS
V. M. Fedin I
Translated from Metallovedenie i Termicheskaya Obrabotka
Metailov, No. 9, pp. 2 - 6, September, 1996.
Large production volumes of rolling stock and track structure
require the introduction of effective strengthen- ing methods at a
minimum expenditure. This stimulates a search for ways of
increasing the service life of parts of railroad transport.
Volume-surface hardening is an efficient method of thermal
strengthening. The method consists in through or deep furnace or
induction heating of parts before hardening and subsequent intense
cooling. The hardenability of the steel used is consistent with the
thickness of the strengthened layer, which creates a hardness
gradient over the thickness of the parts, i.e., a high surface
hardness and a ductile core. In turn, this creates a favorable
distribution of internal stresses and provides a high cyclic
endurance of the parts in operation. The possibility of using
volume-surface hardening to strength railroad transport parts is
consid- ered with allowance for the special features of their
production and operation.
The method of volume-surface hardening developed in the 1960s on
the initiative and under the guidance of Shepe- lyakovskii is
widely used in industry, especially for strength- ening gears,
half-axles, spiders of Cardan joints, compound springs, bearing
races for railroad cars, and the like [ 1, 2]. Re- search conducted
at the All-Russia Research Institute of Rail- road Transport has
shown that the application of this method can be extended to
strengthening a wide range of parts of roll- ing stock and track
structure [3, 4].
The production of such parts has the following special
features:
1. The production is large-scale, and hence large vol- umes of
metal are consumed. This makes the producers use predominantly
carbon and low-carbon steels not containing expensive alloying
elements. These steels have a low or re- stricted hardenability. Is
some cases it is possible to use com- mercial steels with a
guaranteed level of hardenability.
2. Most parts are fabricated from rolled stock or castings with
minimum mechanical processing and retention of the black
surface.
3. Large production volumes require environmentally safe methods
of heat treatment not involving gaseous atmos- pheres, quenching
oils, or polymer quenching media.
4. In order to provide repair by welding, a considerable part of
the products are fabricated from low-carbon or low-al- loyed steels
with high critical cooling rates. The efficiency of
1 All-Russia Research Institute of Railroad Transporl,
Russia.
hardening such steels by conventional heat treatment meth- otis
and, in particular, by volume water quenching is not high. It is
expedient to use intense cooling by a shower or a stream of water.
However, such cooling is hardly possible for large- size parts with
a complex geometry, because this requires cooling equipment with a
complex design and complicated maintenance and a large consumption
of water.
In some cases parts of low-carbon steels with a well- manifested
gradient of the properties in the cross section can be strengthened
effectively with the use of quite simple and reliable equipment,
cooling in which results in the formation of a martensitic
structure in the surface layers. The term "vol- ume-surface
hardening" is applied to steels of this kind.
The presence of noumartensitic structures in the surface layers
worsens the properties of the parts [5], which should be taken into
account when choosing this method of harden- ing for a specific
part.
The most important condition for appropriate strengthen- ing of
railroad transport parts consists in the creation of a
high-strength wear-resistant surface layer in the loaded zones or
over the entire surface. These properties are exhibited by a
surface layer with a martensitic structure obtained in steels with
an inherited fine-grained structure containing fine austenite
grains. As a rule, such a structure is obtained by hardening after
induction heating. However, induction heat- ing can hardly be used
for large parts of a complex shape.
The requirements on the quality of the heating and the structure
should be differentiated depending on the carbon
365 0026-0673/96/0910-0365515.00 0 1997 Plenum Publishing
Corporation
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366 V.M. Fedin
ou ; o0.2, N / rnm 2
Cl u
800 x - l l - - "~ i ' ' x "
600 ~' ~ 00"2
/, J
200 300 400 HV
Fig. 1. Relationship between the hardness and the strength
parameters of steel 20GL.
~Sh, n ln l
211
0 2 4 6 8 10 h, mm
Fig. 2. Variation of the wear resistance (lsh is the crater
length) in Spindel tests for wear resistance over the cross section
of the functional zone of a clutch lock of steel 20GL (h is the
distance from the surface): 1 ) atk'r vol- ume-saa-faee hardening;
2 ) after high-frequency surfacing by the technology of the
Bezlaitsk Steel Plant.
content in the steel. Specifically, in low-carbon steels with a
high fracture toughness the grain size is not a substantial fac-
tor for the mechanical properties, in contrast to steels with a
medium or high carbon content. For this reason, even fur- nace
heating in hardening low-carbon steels with grain size No. 7 - 8
provides a high level of mechanical properties. High-carbon steels
55S, 65S, and 58 (55PP) (0.55 - 0.65% C) possess a high set of
strength and toughness properties only after induction heating [ 1
]. In order to strengthen a low-carb- on steel to the maximum
level, it is important to eliminate in- terim cooling, which causes
segregation of excess (structur- ally free) ferrite, and conduct
the hardening at a high cooling rate, leading to formation of
predominantly martensitic struc- tures.
The high toughness of the low-carbon martensite, which is
partially decomposed in the cooling process, makes it pos- sible to
eliminate subsequent tempering of the parts, which is often used in
practice [6].
The relationship between the strength and the hardness of
low-carbon steels after hardening and tempering is illustrated by
Fig. 1. The possibility of a linear correlation between the
strength and the hardness of the steel o, = x. HB has been shown in
[7, 8]. It can be seen from Fig. 1 that the curves de- scribing the
function c~, =f(HB) consist of several rectilin- ear regions due to
the fundamentally different structural states of steel 20GL at
different levels of hardness. At a hardness below 250 HV, which
corresponds to a predominantly ferrite- pearlite structure, the
proportionality factor x is equal to 0.33, which corresponds to the
data of [7, 8]; at a hardness exceed- ing 360 HV (a predominantly
martensitic structure) o, grows much more intensely. In the
intermediate region of 250 - 360 HVo, depends weakly on HV. The
dependence of a0. 2 on HV behaves in a similar way.
The elongation and the hardness are uncorrelated. The highest
value ~i = 12- 18% corresponds to a ferrite-pearlite structure. In
steel 20GL with a predominantly martensitic structure a quite high
ductility is retained at a high strength for ~5 = 10%.
An electron-microscopic thin-foil study of the structure of
low-carbon steel St3ps after volume-surface hardening has shown
that the strengthened surface layer possesses a bainite- martensite
structure (75% martensite) with regions of troos- tite and ferrite.
With increasing distance from the surface the amount of products of
the martensite decomposition that oc- curs in the process of
hardening cooling increases. At a dis- tahoe of 2.5 - 3.0 mm from
the surface the volume fraction of bainite is below 50% and sorbite
appears.
These data show that the use of a high-speed stream of water in
industrial cooling equipment for low-carbon steels (0.15 - 0.30% C)
does not always ensure a purely martensitic structure. However, the
surface layer strengthened by this method has a rather high set of
mechanical properties [9].
The deep layers outside the zone of effective strengthen- ing
contain all the mentioned structural components with a high
proportion of degenerate pearlite, and the structure is coarsened
due to the growth of segregated carbides.
For many railroad transport parts, and especially cast parts of
car clutches, the wear resistance should be very high. In this
respect hardening of steels with 0.17- 0.25% C by a high-speed
water stream can compete with surfacing.
We tested specimens for wear resistance under the effect of an
abrasive medium by the method of Spindel. The results of these
tests agree well with operational data. The data pre- sented in
Fig. 2 show that the wear resistance of a clutch lock of steel 20GL
subjected to high-frequency surfacing exceeds that after
volume-surface hardening only within a facing layer 1.5 mm thick;
after abrasion of this layer the wear in- creases markedly. For an
admissible abrasion depth of 4 - 5 mm, which is greater than the
strengthened zone with a hard- ness of at least 35 HRC in parts
hardened by a high-speed water stream, the endurance of parts
strengthened by both variants is virtually the same.
In order to provide a high wear resistance and cyclic en-
durance of the parts, the thickness of the high-hardness sur- face
layer and the strength level of the core should be suffi- cient for
preventing failure under the action of the operational
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Volume-Surface Hardening of Railroad Transport Parts by a
High-Speed Water Stream 367
stresses. In addition, the thickness of the strengthened surface
layer should be greater than the depth of propagation of sur- face
defects typical for articles with a retained black surface
(castings, rolled stock). These defects include surface con-
tamination, cracks, seams, scale, scratches caused by me- chanical
damage, a decarburized layer, and the like [10]. Castings
(especially large ones) can contain blisters and other
discontinuities in the surface layers. In accordance with the
standards in force the depth of the layer with defects in rolled
stock should not exceed the negative allowance for the thick- ness
of the profile, i.e., 0.2 -0.5 mm in most cases. The kind and size
of admissible defects in castings are regulated by specifications
for various products, but in the absence of flaw detection the size
of the defects in an article can be quite large.
When choosing the depth of strengthening, the thickness of the
surface layer containing defects and a heightened con- cenwation of
microcraeks should be taken into account. It has been shown in [11]
that this layer forms in operation, espe- cially due to fatigue and
wear of the parts. The thickness of the layer with defects grows
with the thickness of the article, and can attain 0.7 nun in, for
example, rolls 110 mm in di- amete~
By imposing a deeply strengthened layer with high resid- ual
compressive seesses over the defective layer, the influ- ence of
the defects as sources of fracture in cyclic loading is
compensated. For example, castings of steel 20GL with a square
cross section 130 x 130 mm in size and a wall thick- ness of 20 mm
(boxes of clutches of railroad cars) after a vol- ume surface
hardening that provided a high-hardness layer with a thickness h =
6 - 8 mm broke in cyclic tests in places other than the stress
concentrator (a 3-ram-deep transverse notch imposed on them). It
has been shown in [12] that this can be explained by the positive
effect of residual compres- sive stresses in the notch zone that
balance the tensile stresses due to the external load.
The black surface of a part with scale retained on it also plays
the role of a heat insulator that decelerates the cooling of the
part even if it is hardened by a high-speed water stream. In some
cases we could not detect signs of hardening under a layer of dense
and thick scale.
This indicates that the state of the surface of a part should be
taken into account when designing strengthening regimes and
preliminary tests of them on actual specimens of rolled stock and
castings. Scale should be removed from the surface to be
hardened.
Another important condition for effective use of volume- surface
hardening of parts is the sufficient strength of the core in the
presence of a gradient of properties over the cross sec- tion,
which should guarantee the creation of high residual compressive
stresses in the loaded surface layers that increase the cyclic
endurance and service life of the parts.
The admissible depth of surface hardening and the strength of
the deep layers and the core of a part can be evalu- ated by the
method of "effective hardness" described in [1]. Figure 3 presents
in relative units the distribution of the hard-
HV / HV,=
0.6 ~2
0.4 ~
02 ~
0 0.2 0.4 0,6 0.8 I h/R
Fig. 3. Variation of the relative hardness over cross secfious
of parts of diffe~ ent steels after volume-surface hardening (/-/V~
is the hardness o f the surface, HV is the hardness of the middle,
h is the distance from the surface, R is the radius or the
half-thickness of the part): 1 ) clutch box of steel 20GL with a
wall thickness of 20 ram; 2 ) rail chair of steel SUII~ with a
thickness of 16 ram; 3, 4 ) helical springs of a car suspension of
steel 58 (55PP) with a rod diameter of 21 nun and of steel 55S with
a rod diameter of 30 rnrn; r~o fively.
ness HI," over the cross section of railroad transport parts of
various steels for which the technology of volume-surface hardening
has been developed. The straight line in the same figure
characterizes the distribution of working stresses ex- pressed in
terms of hardness over the cross section of a part subjected to
torsion and bending. It can be seen that for all the parts the
distribution of the properties over the cross section is favorable,
because in any zone of the cross section the working stresses do
not exceed the strength of the maten'al. In this case the principle
of equal strength is realized to a certain degree with maximum use
of surface strengthening.
The maximum permissible depth of surface strengthening is
limited by the impossibility of stable realization of the prin-
ciple of volume-surface hardening for a surface layer exceed- ing
0.25 of the thickness of the part [1] and the need to obtain a
hardness gradient over the cross section that would be suffi- cient
for creating high residual compressive stresses in the surface
layers. Data on hardness gradients and levels of resid- ual
internal compressive stresses for different railroad trans- port
parts are presented in Table 1.
For volume-surface hardening to be used efficiently the
hardenability of the steel should be consistent with the size
of
TABLE 1
Article Steel s (D), nun HVIHVsur ocomp , N/ram2
Clutch box 20GL 20 0.67 200 Rail chair 3ps 16 0.58 200 Helical
spring 55S ~ 30 0.45 850
58 (55PP) O 21 0.48 860
Notatlee: s (D)) thickness or diameter of the articles; HV) core
hardness; HV~ ) surface hardness; acomp ) residual compressive
stress.
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368 V.M. Fedln
v=, deg/sec
21111
150
100
50
10 20 30 40 50 60 s(D),mm
Fig. 4. Dependence of the critical cooling rate (Vcr) necessary
for realization of the effect of volume-sm'fa~ hardening of plate
(I) and cylinder (2) parts on the thickness (diameter) of the
parts.
the cross section of the article or its strengthened parts.
Figure 4 presents calculated critical cooling rates for
volume-surface hardening of steel articles with a cylindrical or
plate shape that produces a strengthened layer with a thickness of
13 - 25% and 10 - 25% of the thickness of the plate and the cylin-
der, respectively.
Experience in using the method has shown that the har-
denability of carbon and low-alloyed steels, which can be subjected
to volume-surface hardening, can be evaluated by the "method of
multipliers" with an accuracy sufficient for practical purposes
[13]. The formula for calculating the har- denability is
D~ = D I "f-C-C (1 + 4.1Mn) (i + 0.64Si)
x (1 + 0.52Ni) (1 + 0.27Cu) (1 + 2.33Cr),
where D is the ideal critical diameter, mm; D l is a coeffi-
cient dependent on the size of the austenite grain (in accord- ance
with the experimental data of B. K. Ushakov D I = 6.2 for grain
size No. 11 - 12, D I = 6.75 for grain size No. 10); C, Mn, Si, Ni,
Cu, Cr are the mass fractions of the corre- sponding elements,
%.
The critical diameter calculated by this formula for steel 58
(55PP) in the entire range of compositions of this grade is D~=9.6
- 18.1 and 10.5- 19.7 mm for grain sizes Nos. 11 and 10,
respectively. In accordance with the experimental data presented in
[1] the actual level of the hardenability of this steel corresponds
to D~ = 10- 14 mm, which shows that the calculation is quite
reliable if we take into account the very low probability that the
molten metal will have maxi- mum contents of all the listed
elements.
The critical cooling rate for producing a strengthened layer
with the requisite thickness can be determined using the curves
presented in Fig. 5. These curves have been plotted by solving the
Fourier differential equation of heat conduction with boundary
conditions of the third l
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Volume-Surface Hardening of Railroad Transport Parts by a
High-Speed Water Stream 369
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