RECRYSTALLIZATION OF ELECTROLYTIC IRON

V. G. Dazhin, V. A. Kurdyumov, a n d Y u . D. K o n T k o v

UDC 669-17-174:669.177

We investigated the reerys ta l l iza t ion p rocess in electrolyt ic iron of differing initial hardness (after e lect rolysis) in a wide tempera ture range. Copper samples were plated with iron from an electrolyte of 500 g / l i t e r FeC12.4H20 , pH = 1.5-1.6. At 82-85~ and a current density of 5, 10, 20, 30, and 50 A /d in 2 we obtained deposits with a hardness HV of 150, 250, 330, 410, and 490. Recrysta l l iza t ion annealing was con- ducted in a neutral a tmosphere at 600, 700, 800, and 900~ for 0.5 h. As the result of subsequent meta l lo- graphic analysis we obtained a recrys ta l l iza t ion diagram (Fig. 1). The initial recrys ta l l iza t ion tempera ture was determined f rom the appearance of the f irst nuclei of r ec rys ta l l i zed grains during annealing at t emper - atures of 500-900~ (20 ~ intervals) .

The resul ts obtained (Fig. 1) confirm the data [1] on the fact that the initial reerys ta l l iza t ion t e m p e r - ature of electrolyt ic iron is above that of pyrometal lurgical iron. The main reason for the increase of the initial recrys ta l l iza t ion tempera ture is the saturat ion of the metal with products of the pyrolys is of hydrox- ides, the decomposit ion of which occurs at t empera tures up to 500~ and is accompanied by the formation of a solid solution of oxygen in iron and oxygen-containing inclusions. Hydrogen pins dislocations only in heat - ing to 300~ [2], and therefore its effect on the recrys ta l l iza t ion of electrolyt ic iron is negligible.

Harder coatings, deposited under nonequilibirum conditions, have a s t r e s sed s t ruc ture with s trong dislocation fields. Recrys ta l l iza t ion should occur with a low activation energy, although it can be seen from Fig. 1 that with an increase in initial hardness of electrolyt ic iron the initial recrys ta l l iza t ion t empera tu re increases . This is due to the increase in its impuri ty concentration. The concentrat ion of oxygen entering into the composit ion of these impuri t ies was determined by the vacuum fusion method (Fig. 2).

An increase in the concentrat ion of impuri ty atoms in the form of Cottrell a tmospheres has an effect on the initial recrys ta l l i za t ion tempera ture only at small total impurity concentrat ions. In e lectrolyt ic iron the effect of oxygen is apparent only at substantial concentrat ions (Fig. 2). It can be assumed that oxygen

,o ~ 0,25 / mm 0,20 10- 120

80 / 0.10

.= o, os . j , c.-.

/00 200 300 ~00 I/Y

Fig. 1 Fig. 2

Fig. 1. Diagram of reerys ta l l iza t ion of e lectrolyt ic iron. - - - - ) Initial reerys ta l l iza t ion tempera ture .

Fig. 2. Effect of oxygen content of electrolyt ic iron on the o r ig - inal hardness .

Khabarovsk Polyteehnieal Institute. Transla ted from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 79-80, January, 1970.

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Fig. 3. The beginning of r ec rys ta l l i za t ion of coatings with an initial ha rdnes s of HV 150 (a) and HV 490 (b). (•

a toms favor the fo rmat ion of Suzuki a tmosphe re s , which is par t i a l ly conf i rmed by the fa i r ly slow the rma l act ivat ion of the r ec rys t a l l i za t ion p r o c e s s [3]. Rais ing the annealing t e m p e r a t u r e i nc reases the grain s ize , probably due to the fact that the ra te of grain growth exceeds the fo rmat ion ra te of r ec rys t a ! l i za t ion nuclei at higher t e m p e r a t u r e s .

It can be seen f rom Fig. 1 that soft coatings fo rmed in r ec rys t a l l i za t ion have re la t ive ly fine gra ins . Evidently, with re la t ive ly smal l impur i ty concentra t ions and low dislocat ion dens i t ies the the rmodynamic conditions in different sect ions of the coating become fa i r ly uni form. This induces the format ion of a l a rge number of r ec rys ta l l i za t ion nuclei (Fig. 3). In coatings of high initial ha rdness the conditions for f o r m a - tion of r ec rys t a l l i za t ion nuclei occur f i r s t in p laces with re la t ive ly high excess energ ies , i .e . , at the bound- a r i e s of c racks and l a rge inclusions (Fig. 3). The impor tance of the la t te r in the r ec rys t a l l i za t ion of e l ec - t rolyt ic i ron can be compared with the role of twins in pyrometa l lu rg ica l meta ls [4]. The fo rmat ion of r e - c rys ta l l iza t ion nuclei in ha rde r coatings occurs at e levated t e m p e r a t u r e s , where the number of adsorbed impur i t i es in the gra in boundar ies d e c r e a s e s [5, 6], and t he re fo re the growth ra te of r e c ry s t a l l i z ed gra ins will be higher than in soft coatings and the gra ins will be l a rge r .

It is in te res t ing to note that secondary annealing of r e c ry s t a l l i z ed samples at h igher t e m p e r a t u r e s in- c r e a s e s the gra in s ize , which indicates the collecting r ec rys t a l l i za t ion p r o c e s s in e lec t ro ly t ic iron.

C O N C L U S I O N S

1. An inc rea se of the initial r ec rys t a l l i za t ion t e m p e r a t u r e of e lec t ro ly t ic iron as compared with p y r o - meta l lu rg ica l i ron is due to the la rge amount of oxygen and oxygen-containing impur i t i e s in the meta l , which pin dis locat ions and inhibit the fo rmat ion of r ee rys ta l l i za t ion nuclei.

2. With increas ing initial ha rdness of the coating the initial r ee rys t a l l i za t ion t e m p e r a t u r e and the gra in s ize inc rease .

1.

2. 3. 4.

5.

6.

L I T E R A T U R E C I T E D

Yu. N. Pe t rov et al . , Fiz . Metal. i Metalloved. , 22, No. 5 (1966). V. A. Yagunova and K. V. Popov, Fiz. Metal. i Metal loved. , 12, No. 2 (1961). S. S. Gorel ik, Recrys ta l l i za t ion of Metals and Alloys [in Russian] , Meta l lurgiya , Moscow (1967). L. N. Lar ikov, in: The Phys ica l Bas is of the Strength and Ductil i ty of Metals [in Russian] , Me ta l lu r - gizdat, Moscow (1963}. V. I. Arkharov , Fiz. Metal . i Metal loved. , 12, No. 1 (1961). L. N. Aleksandrov and A. N. Kogan, Fiz. Metal. i Metal loved. , 23, No. 4 (1967).

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