FACTORS DETERMINING THE STABILITY

OF AUSTENITIC AND NICKEL STEELS*

I. L. Mirkin and R. P. Zaletaeva

A N D S T R E N G T H

UDC 669o15'24-194:620.18

For the major i ty of s t ruc tura l mater ia ls operating at low tempera tures under static loads the ha rd - ening crea ted in the manufactur ing process is re ta ined for the entire working life of the piece. On the con t r a ry , in alloys working at elevated tempera tures i r revers ib le p rocesses of softening and failure occur m~der the influence of the t empera tu re , and it is necessa ry to c rea te sufficiently high hardening which is re ta ined for the entire working life.

We consider it essent ial to emphasize the fact that this requi rement holds true also for local r e - peated overheating of mic rovo lumes , par t icular ly f rom impact or dynamic cycl ic s t r e s se s .

Alloying is of fundamental impor tance in c rea t ingh igh-s t r eng tha l loys . Microalloying or the addition of smal l amounts of var ious e lements (from thousandths to tenths of one pe r cent) is usually for the purpose of 1) Combining cer ta in impuri t ies in the alloy ( low-melt ing-point metals , sulfur, nitrogen, etc.) into more stable o r r e f r a c t o r y chemical compounds; 2) affecting the condition of the grain boundaries by means of su r face - active e lements .

Fo r crea t ing on the grain boundaries a continuous film with a thickness of 10 atoms consist ing only of the added elements it is n e c e s s a r y to add in all only 0.04 at.% for a gra in size of grade 7 and 0.003 at.% for grade 1.

Analytical calculat ions by the two- laye r atomic model of the c rys ta l recent ly showed [1] that in the center of dislocations there are large elast ic displacements of atoms which rapidly decrease with distance, but still constitute substantial values (tenths of one per cent) at fourth and fifth neares t neighbors .

In the lattice with a foreign atom the displacements will be smal l e r . In proport ion to the increase in the concentrat ion of alloying elements all atoms of the lattice of the alloy will be near neighbors of the foreign atom and will be e las t ical ly displaced f rom their previous equil ibrium positions in the uncon- taminated solvent.

The grea tes t e last ic displacement is undergone by those atoms of the basic meta l -so lvent which are neighbors of the atoms of the alloying element .

For austenitic and nickel alloys with a face -cen te red cubic latt ice the coordination number is 12. With 8.5 at.% of the alloying component any atom of the solvent will have a foreign atom as a near neighbor,

with 16 at .% an average of two foreign atoms as near neighbors , and

//8 260

220 "J " / ~F-ez W

180 ' ~./ "~ N~3AL

i40

Q 65O 750 850 950 ~

Effect of temper ing t e m p e r - ature on hardness of alloys hardened by excess phases. Tempered 4 h.

SO on.

The majority of high-strength and heat-resistant austenitie steels

and nickel alloys consist of a complex saturated solid solution, constituting 80-95% of the volume of the alloy, and dispersed strengthening excess phases, constituting 5-20% of the volume. These phases are generally car-

bides, borides, and intermetallic compounds [2-6].

The kinetics of the precipitation and growth of the crystats of these excess phases, their degree of dispersion, and their amounts determine to a substantial extent the toughening of the alloy and the heat resistance or weakness of the alloy. As the theory and experiments show, the basic factors determining the kinetics of the process of weakening are the de- gree of supersaturation of the solid solution, the crystal and chemical

* Presen ted at the Conference on P rog re s s ive Methods of Heat Treatment , Leningrad, June, 1966.

TsNIITMASh. Transla ted f rom Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 63-65,

NTovember, 1966.

948

Characteristics of Phases

Chemical Lattice a ~ c ~ __e Hv Phase formula a

y-solution Varying composition fcc 3,55--3,6 - - - - 8 0 - - 2 2

AB (Fe, NI) AI bcc of 2,85--2,90 - - - - - - CaC1 type

Fe~ (W, Mo, Nb) Hex 900 AB~ Fe_~, Mo, Fe~, Nb, 4 , 7 4 - - 4 , 7 5 7 , 7 1 - - 7 , 7 3 1,63 765--915,

F%W <3OO

Ni a, AI fcc 420--500 ABa Ni a (AI, Ti) 3,56 - - - -

A2B~ Fe~W Rhomb 4,73 55,76 5,44 - -

I '10,51--10,73 - - - - >800 ]~23C6

Cr2~C6 (Cr, Mo, W, Fe)o~C o

ee~ (W, M?), C ]

Mixed cubic

Mn'Mm"C 10--11 - - .-- .~- 1000

M~B~ (Mo, Cr, W, Ni)~B~ . . . . .

M~B5 (Mo, Cr, W, N i ' ~ B 5 . . . . .

types of the precipitating phases, the degree of difference of the concentrations of elements in them and in the original mother liquor, and their interphase surface tension, which determines the potential and work of formation of the nuclei of the new phase. Also important is the thermal mobility of the components, on which depends the change of the structure and properties under conditions of aging or operation under the influence of temperature and stress over a prolonged period of time (often up to 5000 h, and up to 80,000h

in individual cases).

We examined a large number of solid solutions varying widely in composition with different con- centrations of nickel (20, 40, 60%), molybdenum, tungsten (up to 5%), cobalt (up to 20%), niobium, aluminum (up to 2%), and titanium. The investigation confirmed the fact that the change of the composition of the solid solution in a single-phase alloy leads to comparatively small changes in the characteristics of density, hardness, and yield point (not exceeding 30%).

However, in passing from iron alloys to nickel alloys the same properties change substantially. But where even a small change of alloying affects the formation of phases, i.e., changes the type of excess phase, the kinetics of its precipitation and coagulation, it substantially changes the properties of the alloy and their endurance as a function of temperature and working life. The comparison of different groups of alloys made it possible to reveal and determine the relative importance of the basic factors character- izing the kinetics of the precipitation of excess phases, the potential and work of formation of nuclei, and the rate of growth of the new phase. These factors depend on: i) the degree of supersaturation of the so- lution; 2) specific rate of diffusion of the elements in the solid solution necessary for building crystals of the strengthening phase in the mother liquor with estimation of the components subject to removal from the sections of the matrix in the area of the future crystal; 3) variation of the concentrations of these ele- ments in the matrix and the excess phase (degree of enrichment or impoverishment of the sections of their formation); 4) degree of difference in the type of crystal lattice (type of chemical bond) of the new phase and the matrix.

We found a great difference in the kinetics of aging resulting from the precipitation of various strengthening phases and in the thermal stability of these alloys. Thus, tungsten compounds precipitate and coagulate with more difficulty and more slowly (and later these alloys soften during overaging) than other compounds. Depending on the type of the excess phase, the maximum effect of dispersion harden- ing is reached at different temperatures (see figure) -at 700~ for carbides, at 750~ for the y '-phase, at 850-900~ for tungsten compounds.

949

The diffusion coefficient of tungsten in austenite is several orders lower than that of aluminum and the difference in the type of lattice is substantial (see table). Consequently, at 750~ the primary mass of particles of Ni3AI precipitates during a time of less than i00 h, while Fe2W requires more than 3000 h. Softening in the first case begins after heating for i0 h at 800~ and in the second case only at 900~

Alloys hardened with borides have a high structural stability.

However, it should not be supposed that at working temperatures of 570-600~ and aRer stabiliziing heat treatment there are no phase transformations in austenitic steels (not to mention pearlitie steels). Thus, in KhI4NI4V2M steel at 585~ the size of the austenite grains increases several times, but this re- quires upwards of 50,000 h (5 years) of continuous heating at this temperature [7].

The coagulation of carbides proceeds extremely slowly at these temperatures, the carbide par- ticles growing by only one atomic layer per day of heating.

LITERATURE CITED

i. I.L. Mirkin and A. A. Yudin, Transactions of TsNIITMASh [in Russian], Moscow, Mashinostroenie (1966), Book 59.

2. I.L. Mirkin, R. P. Zaletaeva, and L. G. Golen'shina, Transactions of TsNIITMASh [in Russian], Moscow, Mashinostroenie (1964), Book 45.

3. I.L. Mirkin and M. I. Fantaeva, Transactions of TsNIITMASh [in Russian], Moscow, Mashinostroenie

(1959), Book 93. 4o R.P. Zaletaeva and L. G. Golen'shina, Transactions of TsNIITMASh [in Russian], Moscow, Mashino-

stroenie (1966), Book 59. 5. A.S. Tereshkovich, Transactions of TsNIITMASh [in Russian], Moscow, Mashinostorenie (1964),

Book 45. 6. I.L. Mirkin, V. Z. Tseitlin, and G. G. Morozova, Transactions of the Scientific-Technical Session on

Heat-Resistant Alloys [in Russian], Izd. AN SSSR (1958), Vol. 9. 7. M.I. Solonouts, Teplo~nergetika, No. 12 (1964).

All abbreviations of periodicals in the above bibliography are letter-by-letter translitera-

tions of the abbreviations as given in the original Russian journal. Some or all of this peri- odical literature may wel l be available in Engl ish translation. A comple t e l i s t of the cover - to -

cover E n g l i s h t r a n s l a t i o n s a ppe a r s at the back of the f i r s t i s s u e of t h i s year,

950