Steels_ Metallurgy and Applications, Third Edition

300 Steels: Metallurgy and Applications

Table 4.1

Element Change in delta ferrite

per 1.0 wt%

N -200

C -180

Ni -10

Co -6

Cu -3

Mn -1

Austenite

formers

W +8

Si +8

Mo +11

Cr +15

V +19

AI +38

Ferrite

formers

Thus carbon and nitrogen are particularly powerful austenite formers and the

latter is incorporated in certain grades of stainless steel, specifically for this

purpose. Elements such as titanium and niobium are also ferdte formers in their

own fight but have an additional ferrite-promoting effect by virtue of the fact

that they are also strong carbide and nitride formers and can therefore eliminate

the austenite-forming effects of carbon and nitrogen.

Whereas alloying elements oppose each other in terms of austenite or ferrite

formation at elevated temperatures, they act in a similar manner in depressing

the martensite transformation range. Andrews 2 has derived the following formula

for the calculation of Ms:

Ms(*C) = 539- 423C - 30.4Mn- 17.7Ni-- 12.1Cr- 7.5Mo

Therefore, in predicting the room temperature microstructure of stainless steels,
consideration has to be given to two major effects:

1. The balance between austenite and ferrite formers which dictates the
   mierostructure at elevated temperatures.


2. The overall alloy content which controls the Ms-Mf transformation range and
   the degree of transformation to martensite at ambient temperature.

A convenient but very approximate method of relating composition and

mierostrucmre in stainless steels is by means of the Schaeffler diagram which has

been modified by Schneider. 3 This is illustrated in Figure 4.7, which indicates

the structures produced in a wide range of compositions after rapid cooling from

1050"C. In this diagram, the elements that behave like chromium in promoting

the formation of ferrite are expressed in terms of a chromium equivalent:

Cr equivalent = (Cr) + (2Si) + (1.5Mo) -t- (5V) + (5.5A1) + (1.75Nb)

+ (1.STi) + (0.75W)