Comparison ;with Existing Equivalent Data

Since all the previous data for nickel and chromium

equivalents has been presented in the form of a single value, it is difficult to compare directly the commonly used values with this work. Table 1 summarises the results that have been published and is presented as the relative tendency of the elements to form delta ferrite.

Where comparisons can be made for the austenite forming elements the values are given as austenite formers for this work and are compared against delta ferrite formers which for

manganese will be a negative value. However a direct comparison of the data for the austenite forming elements shows generally good agreement with the published values. Manganese has been quoted

as -2 and -0.5 by Thielemann and Newell which certainly

encompasses the range of values from 1.3 to 0.75 determined in the

present work.

(28)

The value of Binder et al for nitrogen compares very well

with the nickel equivalent for nitrogen given in this work at:' both 1050°C and 800°C.

All the chromium equivalent values can be compared directly with the published data and it is interesting to note that

(pG)

Thielemann gives the most potent delta ferrite former as

titanium, which is in agreement with the present work although

the value quoted of 7 .2 is somewhat higher than suggested in this

(22>

work. Also the order of potency suggested by Thielemann

is generally the same as that presented here, but again giving

values that are generally higher. The value of chromium

(22)

equivalent for silicon given by Pryce et al of 3 is in

excellent agreement with an average value obtained by this work

(pR)

at 1050°C. The value for molybdenum, given by Binder of

(PD) (P1) (PP)

1.4 and that of Schaeffler , De Long and Pryce et al

who all give a value of 1, are well encompassed by the range of

values present for molybdenum on Fig 67 at 1050°C. The value of 0.5„for the chromium equivalent for niobium given by Schaeffler^0^

(

)

and De long is lower than the range of values obtained in this

v/ork. The only comparison of the chromium equivalent of alloying additions as sigma formers is the result given for titanium by

(

)

Hattersley et al who worked on very similar alloy compositions

to those used in the present work, essentially studying the effects of titanium on the austenite/(austenite + sigma phase) boundary in Fe-Cr-Ni base ternary alloys. The range of values they obtained was

from 1.2 to 1.9 which compare very well with those presented in Fig 67.

although Hattersley’s values are for a range of temperature. It would appear that the results obtained for the sigma forming capabilities of elements, is the first time that the

distinction has been made between their effects on sigma phase and on delta ferrite. Hence no comparison can be made for the lower temperatures used in the present work. However the potency of the elements as sigma formers Is generally less than their potency as delta ferrite formers, the exception being molybdenum which is a known sigma former.

For the data to be used reliably for the prediction of the constitution of Fe-Cr-Ni alloys'it must be noted that the values refer to alloys that are essentially carbon and nitrogen free, and adjustments must be made to account for amounts of.interstitial elements present in commercial alloys. Also the effects of the alloy additions have been determined for specimens that have been equilibrated for long periods, and may not therefore be represent­ ative of cast or unequilibrated structures. Finally it would seem reasonable to suggest that the values of nickel'and chromium equiv­ alents can be applied only to alloys that are within the range of compositions investigated in this work.

The adjustment of the bulk concentrations of the fourth element addition for the purpose of the determination of nickel and chromium equivalents has been made somewhat arbitarily, by deciding that all the carbon and nitrogen, where appropriate,is available to form carbides, nitrides or a combined carbonitride. In most cases the

adjustments were minimal, the most significant being that for moly­ bdenum because of the MgC type carbide that is formed whereas titanium and niobium for a K C type carbide.

The practical significance of the chromium and nickel equival­ ent values which have been presented, will enable the prediction of phase stability with greater accuracy than previous data has allowed. It has been established in this work that the effect of the various austenite and ferrite forming elements, excepting nitrogen, alter as a function of concentration. The present work has established the equivalents of the austenite/ferrite forming elements, over a range of compositions which cover the common levels of additions to the austenitic stainless steels and iron-nickel based superalloys that

are commonly used as wrappers and canning material in fast reactors.

Further, at lower temperatures the distinction between the effect of the alloying additions as ferrite and sigma formers has been presented,which will again increase the accuracy of the nickel and chromium equivalent approach to predicting phase stability.