320 Steels: Metallurgy and Applications(a) Aged 400~ (b) Aged 450~
500'450 - - -~ ' -400 1. = \\'~" 4.07
I \\~ a.os
350 [" \\2.44
I ~'1.97
300 I 1"641 J l I I I I I I
0 1 2 3 4 5 60 1 2 3 4 5 6
Time (h)Figure 4.13 Effect of copper on the secondary hardening response in a 17% Cr 4% Ni
steel (initial conditions 950 ~ C A C, - 78 ~ C, I h) (After Irvine et al. I)
- Sufficient austenite-forming elements to produce a mainly austenitic structure
at 1050~ - Alloying elements to promote tempering resistance/secondary hardening reac-
tions. - An overall alloy content that produces an Ms-Mf transformation range just
below room temperature.
These steels were developed primarily for the aerospace industry but were of
limited commercial success due to the fact that:
- The very tight composition ranges were difficult to achieve in commercial
production. - The meta-stable nature of the steels results in a very high rate of work hard-
ening which limited some aspects of cold formability.
However, FV 520(S) is still in use in the UK and is used primarily in defence
applications such as gun carriages and aircraft.
Steel prices
At the time of reporting (March 1997), stainless steel producers in the UK were
experiencing difficult trading conditions due to the strength of sterling and the
volatile prices for ferro-alloys. Over the past 12 months, they had experienced
price swings for stainless steels of greater than 30% and were therefore very
reluctant to quote the current prices for these grades. Because of this situation,
the only information that can be presented at this time are the prices of some
of the popular grades of strip relative to a value of 1.0 for Type 304 (18% Cr
9% Ni). These data are shown in Figure 4.14 which also includes the relative
price for 2205, the duplex grade, which has gained increased usage over the past
few years. Very clearly, the ratios reflect the alloy content of the steels, Type 409
(12% Cr) being the cheapest and Type 310 (25% Cr 20% Ni) the most expensive
in this particular selection of grades.