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Definition
As provided in the UNI EN
10020-89 standard, the spring steels we supply come under the category
of alloyed steels, defined as special, for mechanical constructions.
Field of application
The application of the product is
for manufactures of springs of various types (leaf and parabolic springs,
torsion bars, buffer volute springs, helical springs, etc.) for the
automotive industry, heavy industry and railroad transport. These
steels are also used in agricultural machinery and in general whenever
the steel must be indeformable, wear scratch resistant.
Operating conditions
Spring steel is similar to heat treated steel but differs from the
latter because of the special application which requires tempering
at a much lower temperature (about 450°C instead of about 600°C),
thus achieving high yield points, both in regards to absolute value
and tensile strenght (high Rs/R ratio).
Therefore, even if they can sometimesbe used in a natural or annealed
state for maximum resistance to wear and to take advantage of the
high "rigidity", the correct operating condition is when
hardened and tempered, given that the best combination of characteristics
is obtained, as is well known, had due to the tempering of the martensite
achieved through hardening and tempering, and not because of the pearlitic
or mixed structures.
Types of steel for springs - characteristics
The ideal material for manufacturing springs has to deflects out of
shape, wihout undergoing any plastic deformation and the same time,
the material must posses a high resistance to fatigue.
Keeping in mind that springs are mechanical members, and subject to
repeated stress cycles, the steel must possess a low elastic modulus
so as to bend well even under light loads, a high elastic limit to
support high stress levels and excellent toughness in the hardened
and tempered state.
Depending on the size
of item to be manufactured and on the severity of the opearating conditions
required, the type of steel may vary from plain silicon steel with
varying levels of carbon, up to the addition of chrome, chrome-vanadium
and SI-CR-NI.
What is really important is not so much the individual values of the
analysis of the steel, but the assurance that together, these values
correspond to steel with the quality levels that are required. The
determining factor lies in the raw materials and the manufacturing
process.
Once this has been established, in order to satisfy most of the normal
requirements all that is needed is to choose the suitable steel so
that the desired stress loads are met.
As the number and percentage of the alloy elements increases, the
penetration of the hardening increases as well.
The aim to reach the best resistance to fatigue characteristics also
governs the choice of the type of steel selected.
It should be noted, that resistance to fatigue indicates the capacity
of the material to oppose the phenomena of reduced mechanical strength
compared to the original value after it has undergone a cycle of varying
stress intensity over a period of time.
Resistance to fragile breaking strength, that is to say toughness,
is not a requirement for these steels, so the impact strength parameter
which indicates this value is not standardized and the same applies
for necking down.
As regards toughness it should be pointed out that with the same number
of alloy elements, by increasing the percentage of carbon the toughness
decreases, whereas it remains almost the same if the percentage of
the other elements is increased and the carbon level is kept constant.
Annealing
Spring steel is usually supplied in the natural rolled state. On request
annealing treatment can be carried out to improve cold machinability.
Conclusions
Spring steels are therefore very suitable in all those uses where
elasticity, impact strength and wear and scratch resistance are required.
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