Steels_ Metallurgy and Applications, Third Edition

312 Steels: Metallurgy and Applications

1. In the solution-treated condition, the hardness was 455 HV and there was some
   evidence for the precipitation of small particles of Fe3C within the martensitic
   structure, i.e. slight autotempering.


2. After tempering at 350~ the amount of Fe3C precipitation had increased,
   but at 450 HV the loss of hardness was still negligible.


3. Tempering at 450~ caused slight secondary hardening and this was associated
   with the precipitation of Cr7C3 and a small amount of MEX (based on Cr2C).


4. A major loss in hardness occurred at 500~ and this was associated with
   the precipitation of relatively large particles of chromium-rich M23C6 at the
   martensite plate and prior austenite grain boundaries.


5. Further softening occurred as the tempering temperature was increased and
   this was associated with the solution of Cr7C3 and the growth of M23C6
   particles.

On tempering at 600oc, the steel had lost virtually all of the hardness associated
with a martensitic structure and there was a particular need to improve the
tempering resistance of 12% Cr turbine blading material at temperatures up to at
least 650~ This was concerned with a requirement for good impact properties
but brazing operations for the attachment of wear-resistant shields also raised the
temperature of the blades locally to a temperature of about 650~
As indicated in Chapter 3, additions of molybdenum and vanadium are very
effective in improving the tempering resistance of low-alloy martensitic steels and
the same is true for their 12% Cr counterparts. This is illustrated in Figure 4.9,
which shows the effects of molybdenum and vanadium in a base steel containing
12% Cr, 2% Ni and 0.1% C. In either case, the addition of nickel is made to the
steels to counteract the ferrite-forming potential of these elements. In the case of
molybdenum (Figure 4.9(b)), a progressive increase in hardness is obtained on
tempering at temperatures up to 500~ This is due to an intensification of the
secondary hardening reaction and a fine dispersion of MEX precipitates persists

500

400

~300

200

-~ 12% Cr Base

_ (a)

"~'~712. ~~ oB a sex ~

_.-.- 2.0% Mo (b)

~. 3.1% Mo

,12% Cr Base

• ..__,=,,--o 9O/o v

_ (c)

I I I I I I I , ~ , I I I I I I I I

13 14 15 16 17 18 19 13 14 15 16 17 18 19 13 14 15 16 17 18 19

T (20 + Log t) x 10 -3

4oo soo 000 7004oo soo 6oo 700400 500 000 700

1 hour at temperature (~

Figure 4.9 Effect of alloying elements on the tempering characteristics of a 0.1% C
12% Cr steel (After Irvine and Pickering 6)