Sminlesssteels 351at temperatures below about 50"C, resulting in low proof stress values. However,
this behaviour also leads to a very high rate of work hardening and produces high
levels of tensile strength. Conversely, steels such as Types 309 and 310 are stable
enough to resist the formation of strain-induced martensite and their proof stress
values increase significantly below 100C, following the characteristic of fcc
alloys in showing a marked increase in flow stress with decreasing temperature.
However, the suppression of strain-induced martensite leads to low rates of work
hardening and therefore to relatively low levels of tensile strength.
As indicated in Figure 4.32, the elongation values of these materials are also
strongly correlated with alloy content at temperatures below about 100C. Thus
stable grades such as Types 309 and 310 exhibit a modest peak in ductility
at temperatures of the order of -50"C whereas the less stable grades such as
Types 304 and 316 show sharply defined maximum values at temperatures of
about +20 and -20"C respectively. The reason for this behaviour is complex
but can be explained in terms of the temperature dependence of the flow stress
and work hardening rate. 3~
Impact properties
The impact properties of the standard austenitic grades over a similar range
of temperature are shown in Figure 4.33 (a) and (b). Again with the exception
of Types 309 and 310, the materials fall within reasonably well defined scatter
bands but with a very clear differentiation between Charpy and Izod values. In
the Charpy test, the energy values decrease at temperatures below about 100*C,
whereas in the Izod test the values decrease progressively as the temperature
is increased. However, these variations are largely a function of the operating
conditions in the testing machines used for this work, and the main point to
emerge is the excellent toughness provided by these steels at extremely low
temperatures.
Steels for boilers and pressure vessels
Carbon, alloy and stainless steels are used in boilers operating at elevated
temperatures and also in pressure and containment vessels operating at room
and sub-zero temperatures. Whereas the metallurgy and applications of these
various types of steel have been discussed in discrete chapters of this text, it
was considered more appropriate to deal with this general topic under a single
heading at this stage. Thus, although this chapter is concerned primarily with
stainless steels, this particular section will also deal with the use of carbon and
alloy steels in boilers and pressure vessels.
Steel specifications
Pressure vessels are manufactured from plates, sections, tubes and forgings and
steel specifications for these and other product forms have been formulated
specifically for use in pressure vessels. A summary of the relevant BS and