HEAT TREATMENT OF MARAGING STEEL 00N18K9M5T
S .S . Ryzhak , O .K . Revyak ina , V .V . Sachov , and Ya .M.
Potak
UDC 669.15-194.55:621. 785. 784
Overheating of steel 00N18K9M5T during hot working or heat
treatment leads to severe grain growth which is not corrected by
normal quenching (1 h, 820~
The purpose of this investigation was to find a heat treatment
to refine the grains of steel overheated in the course of hot
working in order to improve the mechanical properties. We also
determined the mechan- ical propert ies after various aging
treatments. The composition of the steels is given in Table 1.
The steel was melted in an open induction furnace and then
remelted in a vacuum-arc furnace.*
To refine the grains the bars of melt 2, overheated at 1100~ for
2 h, werestngle-andtr ip le-quenched from 900, 920, 950, and 975~
and then quenched from 820~ (1 h) and aged 3 h at 480~
The best results with regard to microstructure, fracture, and
reduction in section were obtained after triple recrystal l izat
ion at 950~ Triple heating at 900~ also improved the reduction in
section eonsiderably (Fig. 1).
The effect of overheating on susceptibil ity to brittle fracture
was determined on samples (melt 1) with a section of 2.5 x 10
mmwhich had a surface crack 2.5 mm long. The samples were tested in
tension by the method described in [1]. Longitudinal samples were
taken from bars 130 ram in diameter (grain size of grade 1-3) and
from hot-rol led plate 2.5 mm thick (grain size of grade 8). For
comparison, samples of
TABLE 1. Chemical Composition of Steel 00N18K9M5T
d Composition, %
Z c I t ' I co A,t IB N, ,~,o, TI Zr
0016 117,93 508 907 0 88 0 18 0003810,028 0'010 18,19 8 13 9,21
0'7a Io 16] 0'0045 0,020
3 0:on [18,70 %14 9,18 0:72 ~0:10: o:ooas 0,02a Ngte: Mn and Si
< 0.1%; P and S
o b, kg/mm ~
22O 2fO 2O0 ~:lt i
kg-m/cm 2 2i
3O
I -/~--'''~'- J[3 t~rg"es" I l l .h h
tO f 30 rain 0
gO0 925 950 Prehminary quenching temp.
TABLE 3. Strength of "Light" Melts of Steel 00N18K9M5T in
Relation to Aging C onditions (Quenched from 820~ 1 h)
_..~-.
Melt No t Ti, %
4 5 6 7 8 9
10
0,64 0,64 0,69 0,65 0,56 0,57 0,fi6
%, ~g/'mm2
450~ 3 h I480~ 3h
189--1891191--206 191--192 188----199 193--t98119i--195
181--1841191 172--1801167--173 184---188 172--181 186--200
182--190
450~ 16 h
224 211--215 208--210 201 196--198 201--204 207--209
Fig. 1. Effect of prel iminary quenching on the mechanical prop-
erties of overheated xnelt 2 after quenching from 820~ (1 h) and
aging 3 h at 480~ Initially over- heated to ll00~
~ b, 00. 2, ~ kg/mm2 ,
ZOO
150 / , a n, kg-m/cm 2
,ii ,o _,,, %7 20]0 ~na 300 ~00 500 ~ Ob ' Aging temperature
kg/mm 2 ~eo~
'~176 ;~ '*~ - ' - ~ '%,o
,so I ~176 Ja tO t5 h
Aging 'time
Fig. 2. Effect of aging temperature (3 h)and aging time on the
mechan- ical properties of Samples from melt 3 quenched from 820~
(1 h).
the elevated susceptibility to cracking. The data in Table 2
show that even with severe overheating 00N18- K9M5T steel is less
susceptible to cracking then 1~I643.
The effect of aging temperature and time was de- termined with
rods 12 and 14 mm in diameter quenched from 820~ (melt 3). Figure 2
shows the variation of the mechanical properties with aging
conditions for steel 00N18KgM5T. 'To increase the impact toughness
the steel can be used without aging or with overaging. For the same
strength the ductility is higher in unaged samples.
The investigation shows that the accepted aging procedure for
steel 00N18K9M5T (3 h, 480~ [2, 3] is not always the optimal
treatment. The strength can be increased with longer aging at 480
or 450~ or even at 425~ (8-16 h).
Reducing the aging temperature along with in- creasing the aging
time does not impair the plasticity or ductility (at the same
strength).
Obviously, in production it is better to use low- temperature
aging with a longer aging time for more precise control of the
strength of machine parts.
In addition, the duration of aging can be used to increase the
strength of "light" melts with relatively low concentrations of
elements.
,Light" melts do not reach the required strength (o- b -> 195
kg /mm 2) after the standard aging (3 h at 450 or 480~ Increasing
the aging time to 16 h at 450~ brings the strength up to
specifications without lowering the plasticity (Table 3).
CONCLUSIONS
1. Grain refining of overheated steel 00N18K9M- 5T requires
triple water-quench or air-quench from 900-950~ (holding 1 h)
before quenching from 820~
2. For the same strength, the ductility of the unaged steel is
somewhat higher than that of the overaged steel.
3. The strength of ~light ~ melts can be increased substantially
by increasing the aging time to 16 h at 450~ without lowering the
plasticity.
432
