ACIERS AU BORE ET PROCEDE DE RECUIT CONNEXE

BORON-CONTAINING STEELS AND PROCESS FOR THEIR ANNEALING

Abrégé anglais

ABSTRACT
The invention relates to boron-containing steels and a process
for the annealing of steel products having boron contents of up
to 200 ppm in the temperature range of between 850 and 1050°C.
The characterizing feature of the invention is that the products
are annealed in the state of equilibrium in a non-oxidizing
gaseous atmosphere with a boron potential supplied by a
pulverulent boron oxide source having a value such that the
products retain or adsorb their boron content. The boron source
is quantitatively about 100 g of B2O3 per m3 of annealing space.
In an annealing gas atmosphere mainly consisting of hydrogen the
steam partial pressure can be adjusted at atmospheric pressure
the presence of B2O3 to the required boron activity within the
range of between 10-3 to 10-5.
The products can also be simultaneously case hardened during
annealing. The case hardening agent can be mixed with
pulverulent boron oxide as the boron source.

Revendications

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CLAIMS
1. Boron-containing steels, characterized in that in the
annealed condition they have a predefined (dissolved) boron
content in the range of Up to 200 ppm.
2, A process for the annealing of steel products having boron
contents of up to 200 ppm in the temperature range between 850
and 1050°C, characterized in that the products are annealed in
the state of equilibrium in a non-oxidizing gaseous atmosphere
with a boron potential supplied by a pulverulent boron oxide
(B203) source having a value such that the products retain or
absorb their boron content.
3. A process according to claim 2, characterized in that 100 g
of B203 are used per ?3 of annealing space.
4. A process according to claims 2 or 3, characterized in that
in an annealing gas atmosphere mainly consisting of hydrogen the
steam partial pressure can be adjusted at atmospheric pressure in
the presence of B203 to the required boron activity within the
range of between 10-3 to 10-5.
5. A process according to one of claims 2, 3, or 4, characterized
in that the products are case hardened during annealing.
6. A process according to claim 5, characterized in that the
case hardening agent is used mixed with pulverulent boron oxide.

Description

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BORON-~O~TAINING s~EELs AND P~OOESS POR I~E~R ANN~ALIN~
The invention relates to boron-containing steels and a process
~or ~he annealing of steel products haYing boron contents of up
to 200 ppm in the temperature range of between ~50 and 1050C.
In the anhealing of steels containing boron in the dissolved ~orm
- i.e., up to 100 ppm - in th~ range of elevated temperature
around 1000C it ha~ ~een repQatedly observed tha~ the products
~re undesirably impoveri5hed ~n boron, at least in the su~face
area~ Thus, ~or example, ~. ~noue ~nd Y. Ochid~, ~ d~scribed in
~ectur~ No. S. 1351, pr~sent~d at the 102nd.ISIJ M~eting 1981,
fo~nd ~fter an annealing of low-alloy ~teel contain~ng O.2% C and
30 ppm B ~t 900 C fo~ 2 hour~ of ~lowing ~gon a ~oron
impov~rishment up to a depth o~ 0.4 mm from the ~ur~ace. ~ho
same ~esult was o~ained w~n annealing was p~r~orm~d in air
1nstoad o~ argon~ ThQy th~r~for~ recommended that boron-
conta~nlng ~t~ should bQ ann-al-~ in a ~acuum of 13.3 mPa in
tho presence of a ~irCOniUm gottQr~ P.E. Busby, N.~. Warga and
C. Wells al~o ob~erved, as described in: Jo~rnal o MQtals,
Nov~mb~r 1953, pp. 1463-8, in annealing~ in the au~tenite rango -
i.e., above ~Po~t 900-~ - a simultan~ous e~aporat~on o~ carbon
and boron from a low-al7Oy ~teol hav~ng ~.43% ~ and 38 ppm B in
wet hydrogen. ~n Journal o~ Netal~, February 1954, p~. 185-~9O
.E. Nicholson d~tected a fairly heavy enrichment o~ ~oron in the
core of the sample~, as~ri~ing ~bis ~o ~he formation o~ ~o~on
compounds. It was recomm~nded that when add;ng ~oron, more
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particularly the oxygen oontent should be ta~en in~o ac~ount,
since a portion thereof would com~ine with koron to form ~oron
oxide, ~hus reduoing the soluble proportion of bor~n having a
hardena~ility-enhancing e~fect.
Freguently boron-containing steels exhibit very different
hardenability. Since no syste~atic investigation ~as been .
carried out in this res~ect, the causal connection has hitherto
rema~ned unc~arified.
It is ~n ~je~t of the ihvcntion, in the high te~perature
anne~ling o~ steQl products ~ontaining ~oron in thR dis801~ed
~nrm. to or~lrent bo~on ~ o~ o ~n~ th~ b~ is
even supplied to the steel in a woll-defined predeterminable
guantlt~ ~n the dissolved rOrm~ w~hout a h~rd ~oriae l~yer
for~ing on thQ surface.
To ~~ d ~ in~Lion provi~os ~ st~l a~ set ~orth in
Claim 1 and ~ process whereln the products are ~nnealed in the
~tat~ of equ~lib~ium in ~ non-oxidi~ing gaseous at~osphere ~ith a
boron pot~ntial sup~lied by a pulverulent boron oxide (B203)
~ource having ~ val~e ~uch ~ha~ the product~ retain or ab80r~
th~ir boron c4ntent~
The process according to the inv~ntion cannot ~e comp~red to~ the~
kn~wn ~v~ L~ , wh~r~n the ~oron 4~tiv~ty ~ th~
~nnealing atmosphere is of ~ quitR dif~erent order of magni~ude
around or above 1, ~hi~e in th~ process according to t~e
inv~ntion it is lower ~y several powers o~ ten. Accordingly, in
~ .

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~oriding the steel absorbs in the borided zone with the formation
o~ iron boride (~eB and Fe2B~ boron contents in t~ % ~ange -
i.e., cont~nts far above t~e solubility limit (in the case of
g~mma iron about 75 ppm B at 1000C). Boriding is described in
the specialist book "Boriding" by ~ a~ von M~usC~ka (1~77),
published by ~arl ~nser. Boriding processes are ~lso known from
German OS 2 126 37~ and British Patent Specification 1 435 O~S.
In the process according to the inYention annealing is per~o~med
in the state of equilibrium - i.e~, with constant te~erat~re ,
pressu~e and concentration conditi~ns, including constant ~oron
acti~ity and con~tant boron partial pressure in ~he ~nnealing
atmosphere. ptg. 1 shows clearly th~ depcnaonc~ of ~he boron
content on boron activity. It can be seen that the boron con~ent
increases in a te~perature-~ependent gradient wlth increa~ing
boron acti~ity and amounts to about 70 ppm B in th~ ~as~ of gamma
iron at 1000C with a bo~on activity aB = 10-3, eorr~sponding to
a molar frac~ion of ~oron of Xg 3.5 . 10-4. In th range o~
soluble boron ~ont~nts in iron ~ ., up to about 200 ~pm B -
the ~oron activity as s~own ~n Fig. ~ ther~fore lies in the range
o~ 10-3 to 10-4, while in boriding, a~ al~eady mentioned, it $~
subst~n~ial~y higher. The ~ro~ess according to th~ invention i~
t~erefore a~med at the layer-~res ad~u~men~ o~ a cont~nt of
dissol~e~ boron in th~ matrix, without the nec ~i~y o~ re~oving
a hard boriding layer produce~ as a result.
As experiments have shown, the use of 100 g of B203 per m3 of
annealing space as the boron source is adequate in condit~ons o~
eguilibrium fo~ ~e annealing, which takes several hour~. ~he

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annealing conditions tpreSsure, temperature, compo$ition of the
annealing gas mix~ure~ are adjusted, and t~en annealing is
per~o~med in the state of equilibrium, so that reproducible
conditions are o~tained.
If the annealing gas atmosphere mainly consists ~f hydrogen, in
th~ pre~ence of B~03 the bore potentia~ can be considerably
reduced, since at the most t~aces o~ oxygen are still contained
in the annealing gas a~mosphere.
Th~ n~utr~l ~tmoephere al~o prot~ct~ ~he st~ol against ~al;ng
and boron impoveri~h~ent~ In a ~re~erred em~odiment of the
process according ~o t~Q invention it is rec~mmended that t~e
~team par~al ~ressu~e in ~he ~2 annealing gas atmosphere i8
~d~usted to the reguired boron activi~y and carefully measured,
and i~ t~e steam ~artial pressure changes, it is regulated to thQ
required ~alue in the ~ange ~f 10 3 ~o lo-~, sin~e via ~he
partial pressure relation derived from the formation equation of
boron oxide
I
~B =~ R P~20-3
the boron activity a~ in ~ pure hydrogen atmosphere ~Qp~nd6
solel~ on th~ ~tea~ partial preSsurQ P~20 ~R ~ co~s~ant).
~is provi~es the possibility of influen~ing the boron activity
in the annealihg gas atmosphere by altering the steam partial
~ressure.

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Preferably annealing is performed in the ~oron-containing g~seous
atmosphere in the annealing box witho~t circula~ion of the gaeous
atmosphere, since in ~hat case the annealing a~mosphere can ~e
prevehted f~om ~eing i~poverished in boron by boron oxide
condensing out. Instead, the boron potential Qn rem~in
pres~rved even for ~ours in the annealing g~s atmosp~ere.
~,on~rary to the prev~ opinio~ ~iGllol~on loc. Cit. and
Gra~keJPaju in: Steel ReSearch 8/88, ~a~e 336), by the p~ocess
~cc;or~lln~ invent Ton b~ron-~;ont~ pro~
case hardened, and even p~oducts fro~ a steel con~ainln~ no ~oro~
~n bo aa~o h~rdened ~nd ~t th~ 9am~ ~im~ all~ wi~h ~ hl~
~oron in a quan~i~y o~ up to about 200 ppm B.
Nicholson had s~cceeded in alloying up iron from ~n iron boride
coating layer with ~oron only when the sa~le ~s co~pletely
enclosed with ~oron powder d~ring annealLng in a hyd~ogen
atmosphere. In con~rast, in vacuo or using h~lium as a
protective ga$, boron could not b- successfully di~fu~ed into an
lron sa~ple. Grabke/Paju (loc. cit.) only recently considered
that the simultaneous car~urization of steel and alloying up with
~oron was impossible.
The bo~on source, consisting of cheap ~ulverulent boron oxide,
can be mixed with the cas8 hardening agent in 4nnealing with
simultaneous caxb~riza~ion.

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Wording on ~ig . 1: Ordinate ~ boron ac~ivi~y ; abscissa ~ molar
~raction of boron.
( C~aption ); Boron activity isothe~ms in alp~a and gamma iron
(after ~. Brodowsky and ~.J. Wernicke 70))