Steels_ Metallurgy and Applications, Third Edition

144 Steels:MetallurgyandApplications

500

200

.~_ t- 10o

1t)

c ~ 50

t-

O

.-~- 20 -

10 --

0.0002

Experimental

! Predicted

0.0005 0.001 0.002 0.005 0.01 0.02

Precipitate fraction, f

Figure 2.4 The dependence of precipitation strengthening on precipitate size (~) and
fraction according to the Ashby-Orowan Model, compared with experimental observations
for given micro-alloying additions (After Gladman et al. 6)

agent at a temperature of 920"C. However, in A1-V steels, aluminium is the more

powerful nitride former, and in the presence of 0.04% AI, significant levels of

vanadium will go into solution at 920~ and be available for the precipitation of

V4C3 on transformation to ferrite. Vanadium steels therefore provide significant

precipitation-strengthening effects, i.e. up to 150 N/mm 2 per 0.10% V.

The strengthening effect of precipitated particles is dependent on both the

volume fraction and particle size of the precipitates. This is illustrated in

Figure 2.4, which was derived by Gladman et al. using the Ashby-Orowan

model for precipitation strengthening. Whereas the volume fraction of precipitate

is controlled by aspects such as solute concentration and solution treatment

temperature, the particle size will be influenced primarily by the temperature of

transformation, which is controlled by the alloy content and cooling rate effects.

Transformation strengthening

As stated earlier, both alloying elements and faster cooling rates depress the

temperature of transformation of austenite to ferrite and, ultimately, the effect will

be sufficient to cause transformation to bainite or martensite. The consequence

of this progression is illustrated in Figure 2.5, which relates to steels containing

0.05-0.20% C. Thus the strength is increased progressively with the introduction

of lower temperature transformation products but, of course, with some sacrifice

to toughness and ductility. However, in the context of structural steels, there is

a demand for quenched and tempered low-alloy grades with yield strengths up

to 700 N/mm 2. Such steels are normally alloyed with molybdenum and boron to