Steels_ Metallurgy and Applications, Third Edition

Stainless steels 319

However, if delta ferrite is present in the microstructure, the ferrite-austenite
interfaces provide further sites for carbide precipitation and transformation to
martensite is produced more readily. Thus where hardening is to be achieved
by primary tempering, rather than refrigeration, the steels should contain 5-10%
delta ferrite. However, the primary tempering route to transformation results in
the formation of lower carbon martensites and lower strength levels compared
with those achieved after transformation by refrigeration. This is illustrated in
Table 4.6, which shows the tensile properties obtained in a particular grade of
steel after the two forms of hardening. This table also shows the low proof
stress value and high tensile strength obtained in the solution-treated 1050"C AC
condition, which gives an indication of the high rate of work hardening in these
meta-stable grades.
As indicated in Table 4.6, these steels are tempered after the hardening treat-
ments and therefore it is desirable to incorporate alloying elements that will
improve the tempering resistance or provide a secondary hardening response.
Thus many of the commercial grades that were introduced in the 1950s contained
about 2% Mo and elements such as copper, cobalt and aluminium were also added
in order to promote secondary hardening reactions in the low-carbon marten-
site. The effect of copper additions on the secondary hardening behaviour of a
17% Cr 4% Ni steel is shown in Figure 4.13 and examples of the compositions
of commercial grades of controlled transformation stainless steels are shown in
Table 4.7. Although the range of composition shown in this table is quite wide,
the steels embody the following features:

1. A chromium content of 15-17% in order to provide good corrosion resistance.

Table 4.6 Properties of a 0.1% C 17% Cr 4% Ni 2% Mo 2% Cu steel

Heat treatment TS 0.2% PS El % 4~/A

(N/mm 2) (N/mm 2)

1050oc 1 h AC

1050oc 1 h AC, 700~ 2 h

10500C 1 h AC, 700oc 2 h, 4500C 6 h

1000~ 1 h AC, -780C 2 h

1000~ 1 h AC, -780C 2 h, 4500C 6 h

1294 213 27.1

1035 689 15.6

1161 961 19.6

1347 874 15.6

1340 1154 22.5

Table 4.7 Compositions of commercial grades of controlled-transformation
stainless steels
........ _ .................. , ..........
Grade C% Mn% Si% Cr% Ni% Mo% Cu% Co% Al% Ti%

Armco 17-4PH 0.04 0.5 0.3 16.5 3.5
AM350 0.1 0.5 0.3 17 4.2 2.75
Stainless W 0.07 0.5 0.3 17 7
Armco 17-7PH 0.07 0.5 0.3 17 7
Armco 17-5PH 0.07 0.5 0.3 15 7 2.5
FV 520(S) 0.07 1.5 0.5 16 5.5 2
SF 80T 0.08 0.2 0.2 17 4 1

3.5

1.5

1.2 2

0.2

1.1

1.2

0.7

0.1