292 Steels: Metallurgy and Applicationsbased on a two-stage process, the first employing a conventional electric arc
furnace for the rapid melting of scrap and ferro-alloys but using cheap, high-
carbon ferro-chrome as the main source of chromium units. The high-carbon
melt is then refined in a second stage, using either an argon-oxygen decarburizer
(AOD) or by blowing with oxygen under vacuum (VOD). The AOD process is
now employed for over 80% of the world's production of stainless steel and
produces 100 tonnes of material in less than one hour. However, in addition to
achieving faster production rates, the intimate mixing with special slags results
in very efficient desulphurization. Other benefits also accrue from the facility to
produce carbon contents of less than 0.01% and hydrogen levels of 2-3 ppm.
Substantial cost savings were also achieved with the adoption of continuous
casting in place of ingot casting and these overall gains in production have led to
a significant cheapening of stainless steel relative to two of its main competitors,
namely plastics and aluminium. For the future, there is the prospect of the direct
introduction of cheap chromium ores and their reduction by coal in a converter
which would lead to further cost reduction.
In terms of product innovation, perhaps the greatest benefits have been obtained
from relatively simple changes, such as the introduction of stainless grades with
low carbon contents, i.e. below 0.03% C. This modification has virtually elim-
inated the risk of intergranular corrosion in unstabilized austenitic grades and
has also improved the corrosion performance and ductility of ferritic grades.
However, steel users have been reluctant to take advantage of higher strength
austenitic steels, such as those based on 0.2% N, which can lead to significant
cost reductions through the use of reduced thicknesses in pipework and pressure
vessels. This contrasts sharply with the situation outlined earlier in Chapter 3
where the micro-alloy grades are now used extensively in place of plain carbon
steel. When these high-nitrogen stainless grades were introduced in the UK in the
mid-1960s, their high proof strength values could not be used to full advantage
because of limitations imposed by the design codes of the day. Welding prob-
lems were also encountered due to the fact that the high nitrogen content led to
the formation of a fully austenitic weld metal and susceptibility to solidification
cracking. However, these problems have now been resolved and therefore there
is the prospect of greater utilization of these materials in the future.
The 1970s saw the introduction of the low interstitial ferritic grades, with
combined carbon and nitrogen contents of less than 200 ppm. These steels are
based on compositions such as 18% Cr 2% Mo and 26% Cr 1% Mo and offered
the prospect of being a cheaper alternative to an austenitic grade such as Type 316
(18% Cr, 12% Ni, 2.5% Mo). However, whereas the low interstitial grades
exhibit good corrosion resistance, particularly with regard to chloride-induced
stress corrosion, they tend to retain the problem of conventional ferritic steels
in relation to grain coarsening and loss of toughness after welding. On the other
hand, high-alloy steels involving duplex austenite plus ferrite microstructures are
now gaining acceptance, because of their higher strength and better resistance to
stress corrosion than conventional austenitic grades.
Whereas the consumption of bulk steel products is likely to remain fairly static,
stainless steel is still very much in a growth market. This relates to the fact that