LOW-TEMPERATURE OXIDATION OF

M. M. Babich, A. F. Lisovskii, and N. B. Lisovskaya

H A R D - A L L O Y MIXTURES

A number of authors [1-5] have established that the carbon content has a marked effect on the phys i - comechanical proper t ies of alloys.

Under exist ing sintering conditions, the carbon content of an alloy var ies [5]. One of the methods of decarburizing hard alloys is to reac t metal oxides with the carbon of the mixture. In line with presen t -day trends [6], hard-a l loy mixtures contain up to 0.5-1.0% oxygen, depending on the brand. However, this in- formation does not enable us to make even an approximate a s ses smen t of the decarburizat ion of ar tefacts during sintering, because the oxygen content of the mixtures changes markedly during the various stages of the p rocess . Distillation of the p las t ic izer and pres inter ing of the ar tefacts in hydrogen give favorable conditions for removing oxygen f rom the mixture. After presintering, the cooled ar tefacts are discharged in air, where oxygen may again react with the mixture.

During sintering in vacuum or a neutral medium, the oxygen which has reacted with the cobalt of the mixture after presintering may cause decarburization of the alloy. Since little work has been done on low- temperature oxidation of hard-alloy mixtures, we made a closer study of this field.

Investigation Procedure. The experiments were performed on pressed specimen of size 44.0 x 6.5 x 6.5 and 51.0 x 15.0 x I0.0 mm, prepared from VK6, VK6M, and VK6V mixtures, of which the composi-

tions are given in Table i.

Some of the specimens were roasted in a graphite charge in hydrogen at different temperatures. This enabled us to determine the effect of the presintering temperature on the subsequent cooling of the mixture in air at room temperature. Table 2 gives the characteristics of the specimens. In some cases the speci- mens consisted of several moldings, which enabled us to increase the measurement accuracy. For ex- ample, specimen 3 consisted of two moldings, specimens 5, 6, and 7 of four.

Experiments on the low-temperature oxidation of the mixtures were performed in the following se- quence: reduction of the prepared specimens by hydrogen at 500~C for 3.5 h, cooling in hydrogen to room temperature, and discharge on to the pan of an analytical balance (of the ADV-200M type). Owing to reac- tion with atmospheric oxygen, the weight of the freshly reduced specimens increased. Weighing the speci- mens at specific intervals enables us to establish how this increase depended on the period of contact with air. The change in weight is also affected by adsorption of water vapor on the surfaces of the grains of the

specimen. To eliminate the effect of adsorption, we used standards - similar specimens pre-oxidized in air to constant weight. The standards were kept for not less than a week in a desiccator with concentra-

ted H2SO 4.

To ensure equal adsorption of water vapor on the specimen and the standard during weighing, during reduction of the specimen the standard was kept in a cur ren t of hydrogen at the cold end of the same tube

TABLE 1. Chemical Composition of Mixtures, %

Brand co Cto t C fleee 0 z Fe Ni

VK6 VK6M VK6V

5,58 6,12 6,00

5,97 5,69 5,71

0,04 0,05 0,20

0,24 0,45 0,30

0,063 0,100 0,080

0,12

Institute of Hard-Alloy Materials . Translated from Poroshkovaya Metallurgiya, No. 3(75), pp. 96-100, March, 1969. Original art icle submitted May 24, 1967.

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TABLE 2.

Brand

VK6

VK6M

VK6V

Charac te r i s t i cs of the Specimens

Spec.No.

1 2 3 4 5 6 7

8 9

10

Roasting temp.,~

900 I000 1090 1180 1260

I~rlgth, mlTl

44,0 43,9 43,5 50,8 42,1 39,9 35,5

44,0 50,3

44,0

Weight,

12,9044 13,2170 27,9516 59,6502 52,7813 52,7976 52,9186

i3,3232 51,1107

14,2320

Residence g Porosity, % time at

roasting temp., h

52,3 50,6 20- 48,2 16 48,4 6 46,2 6 34,2 5 13,3 4,5

52,6 47,8 6

52,2

as the specimen, but at less than 40~ During discharge, the standard was placed on the other pan of the balance; this automatical ly excluded the effect of adsorption on the balance reading.

Oxidation of the specimens was per formed in air at 22~1~ and a relat ive humidity of 50-80%.

Experimental Results and Discussion. As a l ready stated, the oxidation kinetics were a s ses sed f rom the change in weight of the specimens, which could increase owing to oxidation of ei ther tungsten carbide or cobalt.

We pe r fo rmed experiments which revealed that tungsten carbide does not reac t with atmospheric oxygen at room tempera ture ; therefore in WC - Co mixtures it is cobalt which oxidizes - this is confirmed by x - r a y s t ruc tura l analysis . The experimental data on cobalt oxidation in our specimens (Fig. 1) lie on a s traight line when W is plotted versus log (aT + 1), where W is the amount of oxygen in the mixture in p e r - cent, �9 is the t ime in minutes, and a is the constant equal to unity when the time is expressed in minutes (dimensionality rain-i).

This means that the kinetics of low-temperature oxidation of cobalt in hard-a l loy mixtures are r ep - resented by a logari thmic equation:

W = klog(m: + 1).

F r o m the oxidation cha rac t e r of the specimens, and calculated values given below for specimen 1, we can infer that a very thin oxide film is formed on the cobalt grains:

T, mill 1 10 100 10 000 ~. s 7 10 21 37

In the calculations of the oxide film thickness we assumed that only CoO was formed on the grain sur faces . Determinat ion of the relat ive surface of cobalt in WC -Co specimens is difficult. F r o m data of Tret, yakov [1], we assumed that the grain size of cobalt is 0.1 pro. We assessed the relat ive surface S of cobalt grains f rom the equation:

6.0 S -- -7~ d ,

where 7k is the density in g/cm 3, and d is the par t ic le size in cent imeters [7].

F r o m compar ison of the calculated oxide film thickness with the lattice constant of cobaltous oxide o

(4.25 A), we can assume that oxidation of cobalt is accompanied by formation of nuclei of a new phase, which become la rger during the p rocess , on the most active grain centers . The s t r e s se s in the lattice of the cobalt par t ic les due to grinding of the mixture, the high relat ive surface, and defects of the lattice on the surfaces of its par t ic les apparently promote oxidation of cobalt in hard-a l loy mixtures .

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0 I 2 31og(oT§ 0 .0 a

I 2 3log(or*/) b

Fig. 1. Oxidation kinetics of cobalt in p r e s s e d WC - Co spec imens . Curve Nos. 1, 2, 3, 4, 5, 6, 8, 9, and 10 co r r e sponds to the spec imen Nos. in Table 2.

The lower degree of oxidation of spec imens 3, 5, and 6 (Fig. la) , roas ted at a h igher t empera tu re , is due to a number of changes in the cobalt g ra ins during pres in te r ing . At high t e m p e r a t u r e s the re la t ive surface of the cobal t g ra ins d e c r e a s e s owing to surface diffusion of the a toms, and the cobalt lattice b e - comes imper fec t . F u r t h e r m o r e , so l id-phase s inter ing occurs between the cobalt gra ins , the contacts b e - tween WC and Co inc rease , and WC diffuses into the cobalt [1]. These phenomena as a whole evidently in- c r ea se the r e s i s t ance of cobalt to a tmospher i c oxidation. In fact, in the case of contact with a i r for a day the oxygen content of spec imen 1 was 2.80%, that of spec imen 3 1.55%, and that of 6 only 0.27%. Oxidation of spec imen 7, heated at 1260~ was not observed.

Study of spec imens 1, 8, and 10, p r e p a r e d f r o m mix tu res of different gra in size, r evea led (Fig. la) that f ine-gra ined mix tu re s a re oxidized more rapidly than m e d i u m - or c o a r s e - g r a i n e d mix tu res . The grain size of tungsten carbide, taken as the c r i t e r ion for c lass i fy ing the mix tures , apparent ly has no effect on the oxidation kinet ics of cobalt . In the given case g r ea t e r impor tance a t taches to the d i spe r s i ty of the cobalt and to imper fec t ions of the c r y s t a l latt ice. We did not de te rmine the re la t ive surface of the cobalt gra ins in the mix tu res . However, f r o m the p rocedure used for manufactur ing hard-a l loy mix tu res (the grinding t ime for obtaining f ine-gra ined mix tu res is g r ea t e r than for m e d i u m - and c o a r s e - g r a i n e d m i x - tures) we a s sume that the re la t ive surface of cobal t gra ins in f ine-gra ined mix tu res is g r e a t e r than that of the other mix tu re s . This reason ing a lso holds t rue for a s s e s s i n g the latt ice defects of the cobalt gra ins of the mix tu res and thus affords an explanation of the higher r a t e s of oxidation of spec imen 8 in compar i son with 1 and 10.

A heat ing t e m p e r a t u r e of 1000~ does not e l iminate the d i f fe rences in the r a t e s of oxidation of f ine- and med ium-gra ined mix tu re s (specimens 4 and 9). The role of latt ice defects during oxidation evidently r e m a i n s predominant even for spec imens heated at 1000~C.

C ONC L U S I O N S

1. At room t e m p e r a t u r e the oxidation of ha rd -a l loy mix tu res follows a logar i thmic law. F ine-gra ined mix tu res a re oxidized more rapidly than m e d i u m - and f ine-gra ined mix tu re s .

2. The r e s i s t a n c e s of ha rd -a l loy mix tu res to a tmospher ic oxidation at r oom t e m p e r a t u r e i n c r e a s e s with the p re s in t e r ing t e m p e r a t u r e .

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

1. V . I . T re t ' yakov , C e r m e t Hard Alloys [in Russian], Metal lurgizdat , Moscow (1962). 2. W . D . Jones, P r inc ip les of Powder Metal lurgy [Russian t ranslat ion] , I zd-vo Mir, Moscow (1965). 3. R. Kiefer and L. Sehwarzkopf, Hard Alloys [Russian t ranslat ion] , Metal lurgizdat (1957). 4. R . H . F o r s t e r , J . Aus t ra l . Inst . Metals, 8, No. 4, 405 (1963). 5. M. Pe t rd l ik and B. Dufek, Hutnick~ listy, 10, No. 9, 528 (1955).

248

6o

7.

8.

Technical Stipulations for Commerc ia l Mixtures for Hard-Alloy Manufacture [in Russian], TU 488-61. I. M. Fedorchenko and R. A. Andrievskii , Pr inc ip les of Powder Metallurgy [in Russian], Izd-vo AN USSR (1961), p. 114. I. M. Fedorchenko, in: P rob lem of Powder Metallurgy [in Russian], Izd-vo AN USSR, Kiev (1955), p. 53.

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