Engineering steels 249transportational stresses. Whereas the storage containers could experience up to
12 refills per annum, the transportable containers might be refilled twice daily.
Harris et al. carried out fatigue crack growth tests on Cr-Mo and C-Mn steels
in hydrogen gas at 150 bar and their results are shown in Figure 3.31. Whereas
the bulk of the test data fell within a relatively narrow scatter band, the major
exception was the Cr-Mo steel with the relatively high hardness of 324 HB. On
the other hand, the fatigue crack growth of all the steels in hydrogen was very
much greater than that induced in a nitrogen atmosphere. The acceleration factor
by which the crack growth rate in hydrogen exceeds that in nitrogen at a stress
intensity value of 20 MNm -3/2 is shown as a function of hardness in Figure 3.32.
This figure demonstrates very clearly the influence of the hardness (or strength) of
the steel on the fatigue behaviour, the acceleration factor increasing from below
20 to above 100 as the hardness is increased from 274 to 324 HB.
Subsequent to the above work, amendments were incorporated into the standard
for transportable gas containers and appear as Appendix E to BS 5045: Part 1
(Containers for use in hydrogen trailer service). This restricts the hardness range
to 230-290 HB. In addition, the yield stress is not allowed to exceed 680 N/mm 2
and the tensile strength must be within the range 800-930 N/ram 2.
Higher strength steels
As indicated earlier, the carrying capacity of gas containers has been increased
progressively over the years and this has been achieved primarily through the use
of steels with progressively higher yield strength. This trend is continuing and
Naylor 19 describes development work on gas container steels with minimum yield
strength values of 950 N/ram 2. These higher strength values are achieved with
micro-alloying additions of vanadium and also with higher levels of molybdenum
or silicon. Naylor reports that cylinder-manufacturing trials were being carried
out on these experimental compositions.
Bearing steels
Bearings constitute vital components in most items of machinery, permitting
accurate movement under low frictional conditions. In addition, they are also
required to transmit high loads and provide long service lives under arduous
fatigue conditions. Stemming mainly from the requirement of the aeroengine
industry, major effort has been devoted to improving the level and consistency
of bearing fatigue performance and, with the adoption of cleaner steelmaking
techniques, it is claimed that bearing life has increased by a factor of 100 since
the early 1940s. zz
The grade of steel adopted internationally for through-hardened bearings is
SAE 52100, the 1.0% C 1.5% Cr composition. This material is generally solu-
tion treated at a temperature of about 850~ followed by oil quenching and
tempering in the range 180-250~ This results in a microstructure of lightly
tempered martensite, primary (undissolved) carbides and up to about 5% retained
austenite. For larger beatings, carburizing grades such as SAE 4720 (2% Ni-Mo)