1120 Significance of Alloying Element Levels in Realizing the Specified Tensile Properties in 18 wt % Nicke l M araging Steel
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par ticl es Ni3(T i,Mo), l eaving unde tecta ble a moun t ( < 0.1
wt%) of Ti in th e matrix [7]. It is hen ce concluded that a
somewhat higher level of Ti would have facilitated
r eaching the specified str en gth l evel . Th e level of alumi-
num (0.09%) is slightly below th e middle of th e specified
range (0.1%). This is expected to have very small effect
on the strength, considering that strengthening effect of
Al is 5.6 - 6.3 kg/sq·mm (55 - 62 MPa) per 0.1 wt% [8].
If the microstructure a ft er a ging contain s con sider able
amount of austenite, this could lead to a relatively low
level of yield strength, as austenite is a soft phase com-
pared to aged martensite. In the present case, however,
austenite is present in the aged micr ostructur es a t a l evel
of < 2%. Hence th is i s not a caus ative factor for the steel
not respo nd ing sa tis fac torily to the aging treatment.
Aging has been continued till 15 hours and tensile
properties evaluated. There is a steady increase in the
strength with aging time; however, even after aging for
15 hours, not all values of 0.2% proof strength met the
speci fi ca ti on. Furth er it i s n ecessa r y i n in dus tr ial pr a ctice
to have a steel composition which will respond to the
aging treatment and lead to attainment of specified me-
chanical properties in a relatively short time, say 3 to 6
hours, from the product ivity point of vie w.
It th us becomes clear that adhering to AMS 6512 with
respect to chemical composition, by itself, is not suffi-
cient to meet th e specification with r espect t o mechan ical
properties. As mentioned in the results section, heats
made with the standard practice, where the levels of in-
dividual alloying elements are maintain ed in th e middle /
upper half of t he specified range show ed acceptable 0.2%
PS an d UTS val ue s a ft er 3. 5 h ours of a gin g. Th e s ol uti on
to consistently producing the material with total confor-
mity to AMS 6512 hence lies in producing the melts with
levels of Ni, Co, Mo and Ti in the middle or even in the
upper h alf of the range specified in the AMS.
5. Conclusions
1) Maintainin g the levels of alloying elements within the
range specified in AMS 6512 for 18 wt% nickel marag-
ing steel is not sufficient to realize the strength levels
specifi ed in the sam e Standar d.
2) I t is believed that the r elatively low levels of Ni, Mo
and Ti within the specified range, tried out for reducing
the cost of production, lead to a relatively low volume
fraction of Ni3(Ti,Mo); this causes lower than the re-
quired precipitation strengthening effect.
3) The relatively low level of Co tried out appears to
be r esul ting in a less decr ease in solid solubi lity of Mo in
the martensitic matr ix, thereby leading to reduced extent
of precipitation of Mo and consequently reduced amount
of preci pitation str ength ening.
4) The standard practice of melting, with particular
emphasis to pegging the levels of Ni, Co, Mo and Ti in
the middle or upper half of the range specified in AMS
6512, has to be adopted if material meeting this specifi-
cation in all respect s is to be produc ed.
6. Acknowledgement s
M Nageswara Rao is grateful to the Management of VIT
Un iversity for their kind consent to publish this paper. K
Sivasubramanian is indebted to the management of
MIDHANI for encouragement and per mission to publish
the results.
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