All alloying elements with the possible exception of Co, lower temperature of the start of the martensite formation, as well as the finish of the martensite formation, i.e. at 100% martensite.
All alloying elements except Co delay the formation of ferrite and cementite. It is very difficult to formulate any general rules regarding the influence exerted by the various alloying elements. However, it has definitely been found that some elements affect the bainite transformation more than the pearlite transformation, while other elements act in the opposite manner
Effect on the temperature of martensite formation
All alloying elements with the possible exception of Co, lower Ms the
temperature of the start of the martensite formation, as well as Mf, the
finish of the martensite formation, i.e. at 100% martensite. For the
majority of steels containing more than 0,50% C, Mf lies below
room temperature.
This implies that after hardening these steels practically always contain
some residual austenite. Ms may be calculated from the equation given
below, by inserting the percentage concentration of each alloying element
in the appropriate term. The equation is valid only if all the alloying
elements are completely dissolved in the austenite.
Ms = 561 - 474C - 33Mn - 17Ni - 17Cr - 21Mo
For high-alloy and medium-alloy steels Stuhlmann has suggested the
following equation:
Ms(°C) = 550 - 350C - 40Mn - 20Cr - 10Mo - 17Ni - 8W - 35V - 10Cu + 15Co + 30Al
It can be noted that carbon has the strongest influence on the Ms
temperature. Figures 1 and 2 show diagrams with an example of experimental
results of the effect of Mn and Ni on the Ms temperature of various types
of steel.
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Figure 1.
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Effect of Mn on the Ms - temperature (after Russel and McGuire,
Payson and Savage, Zyuzin, Grange and Stewart)
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Figure 2.
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Effect of Ni on the Ms - temperature (after Russel and McGuire,
Payson and Savage, Zyuzin, Grange and Stewart)
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Effect on the formation of pearlite and bainite during the isothermal
transformation
All alloying elements except Co delay the formation of ferrite and
cementite. It is very difficult to formulate any general rules regarding
the influence exerted by the various alloying elements. However, it has
definitely been found that some elements affect the bainite transformation
more than the pearlite transformation, while other elements act in the
opposite manner.
Certain elements will, paradoxically, accelerate the transformations if
their concentration increases beyond a certain limiting value, this limit
been affected by other alloying elements present. For case-hardening and
tool steels the time taken to initiate the pearlite-bainite transformation
is reduced as the carbon content exceeds about 1%. For tool steels and
constructional steels Si-concentrations of 1,5% and above have been found
to promote pearlite formation.
As a general principle it may be stated that by increasing the
concentration of one alloying element by some few percent and the basic
carbon content being kept about 0,50%, only a relatively small retardation
of the transformation rates is noticed. For plain carbon steels a
successive increase in C from 0,30% to 1% produces but a negligible effect.
It is only in conjunction with several alloying elements that a more
noticeable effect is produced.
The diagram in Figure 3, applicable to steel W 1 (l% C) will serve as a
basis for this discussion. The shortest transformation time for this steel
is less than 1/8th second. Note that the time scale is logarithmic; hence
there is no zero time. As has been mentioned previously, both
pearlite and bainite form simultaneously in this steel at about 550°C.
Since the curves overlap it is customary to draw only one curve. With
increasing contents of certain alloying elements, however, the noses of
the pearlite and bainite curves will separate.
The structures shown in Figure 3 are obtained by austenitizing samples of
steel W 1 at 780°C for 10 min and quenching in a salt bath at various
temperatures. After holding them for predetermined times at various
temperatures they are finally quenched in water. Before the salt-bath
quenching the steel contains undissolved carbides but in view of the
composition of the austenite the steel may be regarded as an eutectoid
one. The diagram should be studied with the aid of the explanatory text
below.
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Figure 3.
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TTT diagram for isothermal transformation of steel W 1 (1% C)
A = austenite, B = bainite,
Ms = start of martensite transformation,
M50 = 50% M, P = pearlite
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- Quenching in a liquid bath at 700°C; holding time 4 min. During this
interval the C has separated out, partly as pearlite lamellae and partly
as spheroidized cementite. Hardness 225 HV.
- Quenching to 575°C, holding time 4 s. A very fine, closely spaced
pearlite as well as some bainite has formed. Note that the amount of
spheroidized cementite is much less than in the preceding case. Hardness
380 HV.
- Quenching to 450°C, holding time 60 s. The structure consists mainly
of bainite. Hardness 410 HV.
- Quenching to 20°C (room temperature). The matrix consists of, roughly,
93% martensite and 7% retained austenite. There is some 5% cementite as
well which has not been included in the matrix figure. Hardness 850 HV.