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Definition
Boron steel is a generic term used to define a family of low hardened
and tempered steels.
Studies on this kind of steel began
during the Second war, and they gained importance in the fifties due
to the crisis in availability of precious elements caused by the war,
when it became necessary to find alternative elements to those swhich
cost much more.
Following a long period of experiments,
boron steels became established in the seventies and eighties with
the introduction of advanced iron metallurgy technology and today
they are very important in the hardened and tempered steels sector.
Field of application
The fields of application are differents
and tied to the characteristic of boron which, when added in low percentages
(these steels are also called "micro-alloyed boron steels"),
greatly increase the hardenability of low alloy carbon steels, with
a limited increase in production costs.
In general, boron steels are used
in the same sectors where hardened and tempered steels are used, for
springs (in particular silicon steel) and to a lesser extent with
casehardened steels.
Besides the nuts and bolts sector,
it is widely used on tractors, both on parts subject to wear (bottoms
of tracks and teeth on excavators) as well as mechanical parts.
This steel is widely used in industries
connected with agriculture where it has substituted hoes and other
parts made out of silicon steel because of the ecological advantage
of hardening using water instead of oil.
Boron steel is now widely used
to make the forks on lift trucks where it has proved to be versatile
and reliable.
In conclusion, the definition of
boron steel as an alternative steel is today an understatement: this
class of steel has its specific features with a consolidated manufacturing
technology, and is very versatile and completely reliable where it
is used.
Operating conditions
As heat treated steels are the most used, the same overall conditions
apply to boron steels.
Therefore
the best operating conditions are those requiring "heat treatments"
and in this specific case in the hardened and tempered state..
Boron effect
The addition of low percentages of boron (usually 10 - 50 ppm) improves
hardenabilty compared to steel with the same chemical composition
but without boron.
The efficiency of
boron in regard to hardenability can be tested according to the GROSMAN
method given in the ASTM A 255-89 standard.
By following the Grossman
method and using I.D to indicate the ideal diameter of a steel bar
with a structure having 50% of martensite at the core following ideal
hardening, the B.E. (boron effect) is defined:
| E.B. = | D.I. with boron (measured on steel with boron) |
| ------------------------------------------------------------------------------------------------------- |
| D.I. wihout boron (measured on steel without
boron) |
The greater the efficiency of boron the poorer the base steel
is. That is B.E. is inversely proportional to the prcetage of Carbon
and other alloy elements.
For 30MCB5 type steel, I.D. = 1.7 MIN.
The efficiency of the Boron element is achieved through that part
of the Boron (Soluble boron) which does not combine with the dissolved
gasses in the steel (principally nitrogen and oxygen).
The manufacturing technogy solves this problem using specific N2 and
O2 "blockers", that is Titanium and Aluminium.
Therefore this family of steel will contain minimum percentages of
these elements, usually 0.030% by weight.
Mechanical workability characteristics
In the "natural milling" state the hardness characteristcs
do not differ much from those in the corresponding basic steel.
30MCB5 steel, in the natural milling state, has a resistance of about:
R=650-700N/mm2, whereas when hardened and tempered (hardening in water
at 860°C + tempering at 550°C) it is about R =~ 900 N/mm2
(ref. ø 40 mm). For tempering at lower temperatures (about 220°C)
a resistance of R =~ 1500-1700 N/mm2 (ø 40 ) is reached.
Toughness characteristics
Boron heat treted steels have excellent resistance to brittleness.
30MCB5 heat treated steel has an impact strength (KV) much higher
than 27 J even at -20°C (on hardened and tempered material with
hardening at about 600°C). In the case of hardening at lower temperature,
at +20°C the same resistance strength is guaranteed.
Welding characteristics
The welding characteristics are very much the same as that found in
corresponding basic steel.
Forging and heat treatment
Forging temperature must be between 900 and1200°C and the stay period
after reaching the required temperaure must be as short as possible
so as to avoid the boron present losing its properties. Hardening
in most cases from the heat during forging and the hardening medium
(normally water) must have a constant temperature.
The choice of temperature to be used for tempering depends on the
mechanical characteristics required: between 180 - 220°C the highest
resistance values are reached. With temperatures between 450 - 550°C
the toughness characteristics are much improved but resistance values
are lower.
Forgeability annealing
Boron steel has natural cold forging characteristics and therefore
rarely requires annealing. In some cases where particular and difficult
cold forming is needed, annealing for about 6/8 hours can be carried
out to give the steel a hardness of about 150 HB.
Conclusion
Boron steel is the best way of achieving a high quality product at
a relatively low cost.
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