The effect of titanium on creep strength in 2.25 Pct Cr-1 Pct Mo steels
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I.
with particular regard to the effects o f carbon content and austenitizing temperature.
INTRODUCTION
THEmechanical properties of low carbon structural steels, particularly after hot working, can be markedly improved by small additions of niobium, vanadium, and titanium. The additions are normally made at concentrations in the range 0.05 to 0.15 wt pct. Niobium has been shown to improve strength through its action as a grain refining agent. The largely insoluble Nb (C,N) particles retard grain growth during austenitizing and prevent recrystallization during hot working.~ Vanadium carbide, on the other hand, is much more soluble than niobium carbide in austenite and therefore passes into solution during austenitizingfl Strengthening in vanadium alloy steels is derived from the fine scale precipitation of V4C3 in ferrite either during cooling or upon tempering. 3 Titanium is believed to act in a manner intermediate to that of niobium and vanadium with the ratio of strengthening by grain refining to strengthening by precipitation being dependent upon the austenitizing temperaturer It has been reported that the presence of titanium in 2.25 pct Cr-1 pct Mo steels can lead to an increase in the creep rupture strength, but at the expense of creep ductility. 5 Furthermore, the presence of titanium has been observed to modify both the carbide coarsening kinetics and precipitation processes. 6'7 In the present study, the role of a low level of titanium (nominally 0.04 wt pct) on creep deformation in 2.25 pct Cr-I pct Mo steels has been examined Table I.
II.
EXPERIMENTAL
The steels used in the present study were prepared by the vacuum induction melting and casting of high purity 2.25 pct Cr-1 pct Mo steels, three of the six casts being doped with an iron-30 wt pct titanium alloy prior to casting. The alloys contained 0.02, 0.06, or 0.09 pct carbon, with or without 0.04 pct titanium; the analyzed compositions are given in Table I. After hot working the ingots to 14 mm square bars, samples 12 mm in diameter and 150 mm in length were prepared. These were homogenized in vacuum at 1250 ~ for thirty minutes and either air cooled (series A) or furnace cooled (series B). The specimens were then reaustenitized at either 950 ~ (series A) or 1100 ~ (series B) for thirty minutes and oil quenched. Austenitizing at 950 ~ resulted in a prior austenite grain size of between 40 and 50/zm for the titanium-free steels, while the titaniumdoped steels exhibited a bimodal grain size distribution of 30/120/zm. 8 Austenitizing at 1100 ~ produced grain sizes of 80 to 95 m in the titanium-free steels and 40 to 45 jzm in the titanium-doped materials. All the samples were then tempered for three hours at 700 ~ and water quenched. Creep specimens with gauge diameters of 6.5 mm were machined from the tempered samples and tested in air at
The Compositions of the Steels (Wt Pct)
Alloy
C
Cr
Mo
Mn
P
Si
Ti
S
N
BD1 BD3 CE1 CE3 AFI AF3
0.018 0.022 0.06 0.06 0.09 0.09
2.34 2.27 2.25 2.21 2.35 2.25
0.99 0.99 1.00 0.99 0.99 0.98
1.02 1.01 1.
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