butt welding of hard-alloy cermet parts

4

Click here to load reader

Upload: m-m-babich

Post on 09-Aug-2016

216 views

Category:

Documents


1 download

TRANSCRIPT

Page 1: Butt welding of hard-alloy cermet parts

B U T T W E L D I N G O F H A R D - A L L O Y C E R M E T P A R T S

Mo M. B a b i c h

For the manufacture of cer ta in types of tools, it is neces sa ry to have large hard-a l loy par ts that can- not be produced by means of the ordinary methods: plate billets with a length of 400-1800 mm for cutters of jointing machines, shel lers , and center less gr inders , billets for hollow rol l ing-mil l ro l le rs , billets for long cyl inders , etc.

In the operation of composite cut ters made of short plates and brazed to each other with copper or b ras s or mechanical ly fastened to each other, the alloy fails soon at the butt joint or the location of brazing. The well-known methods of joining hard-a l loy parts are based on the diffusion welding of individual parts to a steel holder in high v a c u u m (10-2-10 -4 mm Hg) with heating to a tempera ture below the eutectic Tree value [1]o

As a result of our investigations, we have developed a technologically efficient method for the indus- t r ial manufacture of hard-a l loy par ts of grea t length. A method for manufacturing bard-a l loy billets of any length by butt-welding short plates, r ings, cyl inders , or billets with different shapes, s intered in the usual manner , was developed in 1960-1963 and utilized in the industry.* (Author's cert if icate No. 140581 of 1961) $ [2].

The butt welding method is based on the ability of hard-a l loy par ts to adhere to each other and become welded at the place of contact as the liquid phase appears in the process of sintering~ As the liquid phase develops, the free surface of all tungsten carbide grains at the pa r t ' s surface is wetted and covered with a l iquid-pbase layer , as a resul t of which the sur faces in contact adhere to each other and form a compact joint whose strength is not infer ior to that of the alloy.

The butt welding method consists of the following.

1. The surfaces of the plates to be welded a re prepared by grinding until a c l a s s -7 to c lass-12 finish without the "piling-up" of the edges is achieved.

2. The surfaces to be welded are e lectroplated with a cobalt layer with a thickness of 1-2 # m for a l - loys with a cobalt percentage of less than 6%. Alternat ively, "solder" plates of a tungsten c a r b i d e - c o b a l t alloy, where the cobalt percentage exceeds by 5-10% the cobalt content in the part to be welded, a re used for the welding of the above alloys. The "solder" plates, which have a thickness of less than 1 ram, are prepared by press ing or roll ing a suitable mixture of tungsten carbide and cobalt powders with subsequent s inter ing under earbur iz ing conditions at 1260-1280~

3. The sur faces a re welded under a p r e s su re of 0.1-1.0 k g / c m 2 in a protective gaseous medium or vacuum by heating them rapidly in a ver t ica l e lec t r ic furnace or a special adapter (Fig. 1) to the melt ing tempera ture of the cobalt phase on the surfaces in contact or in the "solder" plates.

The butt welding of hard-a l loy par ts is per formed according to two different methods :

a) The end-faces of the plates, p ressed tightly together, between which a thin "solder" plate consist ing of a coba l t - r ich alloy is inserted, are heated to a tempera ture exceeding the tempera ture at which the liquid phase appears . The heating is per formed by means of high-frequency current in a special adapter (Fig. 1). Af ter they have been prepared for welding, plates 3 are fastened by means of bolts in the two heads of the

* In collaborat ion withA. P. Kozlov, P. Eng., a staff member of the OKB Design Bureau of the Institute of Superhard Materials , the author and V. Bondarenko, P. Eng., developed the adapters and introduced the butt-welding method in 1961-1963 at the experimental plant of the Institute.

Patent No. 3279049 for a s imi lar method of manufacturing par ts f rom previously s intered billets -was is- sued in 1966 in the USA.

Institute of Superhard Materials . Translated f rom Poroshkovaya Metallurgiya, No. 2 (74), pp. 100- 104, February , 1969. Original ar t ic le submitted January 25, 1967.

161

Page 2: Butt welding of hard-alloy cermet parts

Fig. 1 Fig. 2

Fig. 1. Schematic diagram of the adapter for the butt welding of hard-a l loy plates. The i tems are explained in the text.

Fig. 2. Hard-al loy plates for cutters, p repared by using the butt welding method.

Fig. 3 Fig. 4

Fig. 3. Photomicrograph of an unetched section of the transit ion zone in the weld seam of the VK15 alloy with VK30-alloy "solder" ; xl00. Bright accumulations of cobalt that did not fuse due to insufficient heating of the core section of the par t a re visible at the location of the "solder" plate. There a re no cobalt accumulations at the surface; the liquid phase formed in the fusion of cobalt has migrated to the ad- jacent sections of the alloy (the right-hand par t of the photograph).

Fig. 4. Photomicrograph of an unetched section of the welding zone of the VK15 al- loy without "so lder" ; •

the graphite or copper grips 4. The plate ends to be welded a re heated by high-frequency current , using an inductor 5, and a universal graphite holding piece 1 and 2. The welding is perfDrmed under a contact p r e s - sure of 0.1-1.0 k g / c m 2.

b) The par ts to be welded are heated in a ver t ical furnace in a hydrogen a tmosphere or vacuum under an external p re s su re of 0.1-1.0 k g / c m 2 or under their own weight beyond the tempera ture at which the liquid phase develops.

Table 1 provides the dimensions and charac te r i s t i c s of cer ta in hard-a l loy billets obtained by means of the butt welding method. Figure 2 shows photographs of plates for cutters. There are no t races of weld- ing or alloy inhomogeneities on the f rac ture surfaces and sections of normal ly welded parts .

It was found that, in heating to the temperature at which the liquid phase appears on the butt faces, the clean surfaces of carbide grains a re rapidly covered with a liquid film and f i rmly welded at the places of contact. The f i ssure- l ike c learances between the parts are filled with the liquid phase, which is forwarded by capi l lary action f rom the adjacent volumes in the alloy, and a re simultaneously reduced as the surfaces approach each other as a resul t of the deformation of roughnesses under the action of the external p ressu re and the surface tension forces . After the f i ssure- l ike cavities a re filled, the liquid phase migra tes to the internal volumes, which is accompanied by the displacement of carbide grains and the equalization of the thicknesses of veins between the grains.

162

Page 3: Butt welding of hard-alloy cermet parts

TABLE 1

Dimensions of corn- Billets Billet dimensions, mm

ponent parts , mm

Plates for cutters Billets for hollow ro l le rs

Billets for boring bars

Billets for pump cylinders

Length, up to 1200 Outside diam., 50; inside

diam., 30; length, 240 Outside diam., 50; inside

diatom., 30; length, 320 Outside diam., 82; inside

diam., 50; length, 220- 32O

Length, 150-310 Outside diam., 50; inside

diam., 30; length, 80 Outside diam., 50; inside

diam., 30; length, 80 Outside diam., 82; inside

diam., 50; length, 110

In welding par ts made of different types of al loy or of the same low-cobalt alloy with the application of "solder" plates whose cobalt content exceeds by 5-10% the cobalt content in the parts to be welded, the liquid phase, af ter it appears in the "solder" and in the par t to be welded, migra tes under the action of ca- pi l lary p res su re f rom sections with a higher cobalt content to sections with a lower cobalt content. As a r e - sult of this, there occurs a certain equalization of the a l loy ' s composition or, more accura te ly , of the thick- nesses of the liquid-phase veins. The rupture surfaces and microsec t ions do not display differences or sharp discontinuities in the s t ructure of the seam and the welded parts~ Figure 3 shows the photomicro- graph of an unetched sect ion of the transit ion zone of a weld seam. In the seam sections where cobalt had melted, the alloy composition was equalized as a resul t of migrat ion of the liquid phase, and the butt line cannot be detected even with large magnifications. In the middle of the welded surfaces , where cobalt did not fuse due to the tempera ture drop over the c ross section of the part , the s t ruc tura l differences remained, which is c lear ly visible on the section.

The investigation resul ts and the experience in work have shown that, for reliable welding, it is neces - s a r y that the "so lder" plates be saturated with carbon and contain a larger percentage of it than the parts to be welded~ Thus, conditions a re created for wetting the entire butt surface with the liquid phase, filling in the roughnesses , and eliminating the gas in ter layer before the liquid phase appears in the par ts to be welded, i.e., before the liquid phase s tar ts to migra te f rom the "solder" to the parts .

If the liquid phase appears at a higher temperature in the "solder" , i.e., if it appears there later than in the parts to be welded, welding may not take place, since the migrat ion of the liquid phase would then oc- cur fas ter and before the entire surface is bound.

As a resul t of heating beyond the tempera ture at which the liquid phase appears in the "solder" plate and below the tempera ture at which it appears in the par ts , f i rm welding takes place, and the differences be- tween the cobalt percentages in the "solder" and in the parts are preserved. In order to eliminate differen- ces in the s t ructure and composit ion of the seam and the par ts , it is neces sa ry to per form normal iz ing an- nealing at a tempera ture of 1380-1390 ~ during which the liquid phase migra tes and the alloy composition is equalized. However, if the par ts to be welded are very long and the welding heating and the subsequent an- nealing are of shor t duration, the percentage of cobalt along the length of the parts is nonuniform, varying gradually f rom the minimum to the maximum value in correspondence with its percentage in the ini t ia lbi l le ts . .

In the case of shor t -dura t ion welding (with heating over a period of 10-15 min) of par ts made of VK15 without "solder" , Co accumulat ions, which have formed as a resul t of the filling of c learances with the liquid phase and have not dissipated in welding, remain at the location of the f issure c learances (Fig. 4). Cobalt accumulat ions and other s t ruc tura l nonuniformities were not detected af ter annealing over a period of 30 min at a t empera ture of 1390~

A weak mechanical bond is obtained in welding p a r t s in a hydrogen medium containing sur face-ac t ive substances (sulfur, phosphorus, arsenic , or their volatile compounds). This is due to the non-wettabili ty of some of the tungsten carbide grains by the liquid phase as a resul t of the adsorption of molecules of the above impurit ies on the grain surfaces . Therefore , in o rder to secure a s t rong bond over the entire surface, the welding must be per formed in vacuum or in a protect ive medium that is free f rom such impurit ies.

CONCLUSIONS

1. Our method of butt welding makes it possible to produce hard-a l loy parts of any length f rom plates, r ings, cyl inders , or billets with other shapes, obtained by means of the usual s intering process .

163

Page 4: Butt welding of hard-alloy cermet parts

2. Butt welding of ha rd -a l loy pa r t s with a cobalt percen tage in excess of 6% can be p e r f o r m e d suc- cessfu l ly without using the " so lde r . "

3. In o rder to secure a s t rong bond between the su r faces in welding low-cobal t a l loys , it is n e c e s s a r y to place between them thin " so lde r" plates consis t ing of a mixture of tungsten carbide and cobalt where the percentage of the la t ter exceeds by 5-10% the cobalt percentage in the pa r t s to be welded, or a cobalt l ayer with a thickness of 1-3 #rn, act ing as " so lder , " is deposited by e l ec t ro lys i s on the su r faces to be welded.

4. With a h igher cobalt content in welding and the subsequent annealing, the in te r l aye r s vanish as a r e su l t of migra t ion of the liquid phase.

1o

2,

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

N. F. Kazakov, Vacuum Welding of Metals [in Russian], NTO (Scientific and Technical Division) Mash- p rom, Kiev (1958). M. M. Babich, Au thor ' s cer t i f ica te No. 140581, Ofi ts ia l 'nyi byull. " Izobre teniya , Promyshlennye Obraz t sy ," Tovarnye Znaki, No. 16 (1961).

164