CAST GEARS OF BORON-CONTAINING STEEL
d.t.s. Isagulov A.Z., Sultangazievà À.B. Askarova A.A.
Karaganda state
technical university, Kazakhstan
Cast gears are parts designed for the torque transfer along its axis and
for supporting the machine rotary parts. Gears are used if it is necessary to
convert the reciprocating motion in the machine into the rotary motion and vice
versa. A gear is used as a part of a
reducer in the hoisting mechanism of the crane, transmissions, automobile back
axles.
The basic criterion of workability is bearing strength, twisting
strength; gears are subjected to
bending due to efforts occurring in the parts of transmissions, under these parts weight and under their own mass,
they transfer the torque.
A
rational selection of the material for cast gears assists not only their
reliability and durability increase but it improves the conditions of the
material workability and the shafts quality by the value of deformation under
thermal processing. The shaft manufacture quality, accuracy and cost depend in
a significant extent on the tendency to deformation when thermally processed as a decisive factor in the selection of the
method of final fine finishing and the value of allowance for polishing.
Steels for manufacturing the shafts are to
possess high strength and sufficient viscosity. The shafts subjected to
attrition are to have hard and attrition-resistive surface.
For gears manufacturing there are used several grades of steel, which
use expediency is explained either by technological or operational conditions.
The gear work is accompanied by the phenomena effecting their reliability and
durability in operation. Wear, alternating and impact loads, bending,
temperature effect and a number of other phenomena observed in the shaft work
are to be taken into account when selecting a material and their manufacturing
technology.
To
assure high operational characteristics of the machine parts a steel grade is
of great importance. Besides, the steel grade effects significantly the whole
technological process of the parts manufacturing including forging,
cutting, machining and thermal
processing of the parts. Therefor the steel grade is to assist in a maximum
degree the achieving of high and stable mechanical and operational properties
of the parts, to possess good manufacturability at all the stages of the
technological process and is not to be expensive and deficit.
The basic materials used for obtaining needed mechanical and operational
properties of a gear are structural carbon and alloyed steels. There are
recommended to use small- and middle-carbon content up to 0.40% and 0.004% of
boron steels subjected to cementation: 20GR 40G2R, 20R, 30GR, 35GR. The
structural steel for manufacturing is to be fine-grained.
Alloyed structural steels possess the best
mechanical properties after thermal processing. This is explained by the fact
that alloying elements delay diffusion processes and have a greater effect on
the phase transformations taking place in steels when tempered; they delay
martensite decomposition and carbide particles coarsening. Alloying elements
especially strongly increase the yield point σ0.2, relative shrinkage ψ and impact
viscosity KCU.
The
will to increase the quality of high-strength complicated-profile parts of an
automobile without additional costs for their production puts on the first
place the problem of widening turnout and use of economically alloyed including
micro-alloyed with boron steels. The characteristic feature of boron
–containing steels is their technological plasticity, favorable ratio between
the strength and plastic properties in the annealed and thermally hardened
state, high level of hardenability characteristics [1].
In the
home practice there have been developed and widely used boron-containing steels
of the following alloying systems: Ñ-Mn-B, Fe-Si-B,
Fe-Si-Mn-B. However, in spite of the obvious technological advantages of
boron-containing steels, their implementation in mass production is restricted
by a number of technological difficulties, to which there should be referred
the necessity ro preventing boron binding in nitrides in melting steel, as the
hardenability characteristics are effected by only “effective” (not bound in
nitrides) boron.
The
analysis of the world tendency shows growing interest of industry to melting
and using boron-containing steels and alloys. This is conditioned by
exclusively high positive effect of boron micro-concentrations (10-4-10-3
%) on operational characteristics of metals. To achieve similar results, the
demand for boron is 100-300 less that for molybdenum, chrome, vanadium and
other alloying elements. In other words, boron is an economically alloying
element and so attracts attention.
Thermal-dynamic
analysis shows that for the efficient protection of boron (providing its
content in a solid solution at the level of 0.0010 %) and increasing
the coefficient of boron assimilation up to 50 % in traditionally used
in automobile production steels, it is necessary to increase (at the existing
level) titanium and aluminum content up to the level no less than 0.025…0.030
and 0.050...0.060 %, respectively; to reduce nitrogen content up to 0.005…0.008
% [2].
An important reserve
of the hardenability level of
micro-alloyed with boron steels is grinding the austenite grain that is
practically achieved by micro-alloying
with strong carbon-nitride-forming elements (Al, Ti, Zr, Nb,
V, etc.). Their introducing into steel alongside with boron micro-additions
ensures nitrogen and carbon binding in stable fine-dispersed carbon-nitrides of
Ìå(CÕN1-Õ ) type
that, on one hand, assist the boundaries migration braking and so preserving
fine-dispersed grain structure up to sufficiently high temperatures, and, on
the other hand, having high chemical similarity to nitrogen and oxygen, bind
them into nitrides and oxides, providing boron protection that permits to
increase the concentration of “effective” boron and so to increase the steel
hardenability.
A
serious consumer of boron-containing steels are machine-building works and
mechanical-repair shops of metallurgical complexes and MCCs for manufacturing
high-strength gears and fastening for heavy-duty reducers of rolling mills,
mining cutters-loaders, large-capacity open cast vehicles and other equipment
made of boron-containing steel.
1. Lyakishev N.P., Pliner Yu.L., Lappo S.I. Boron-containing steels and
alloys. Ì.: Metallurgy, 1986. 192 p.
2. Òikhonov À.Ê. Steels for automobile production / Technology of
metals. 2008. No 12. P. 47-51.