STUDY OF THE STRUCTURE AND PROPERTIES OF CERTAIN ALLOYS FOR HARDFACING DRILL BITS

Yu. A. EVTYUSHKIN

(Presented by the scientific seminar of the Department of Equipment and Technology of Welding Production)

This article describes the results of work on hardfacing bits for percussion-cable drilling, carried out at the Department of Welding Production of TPI and in the Tel’ma drilling detachment of the Irkutsk Geological Administration. The macro- and microstructure and the physicomechanical properties of the hardfacing metal were studied in the metallography laboratory of SFTI [1].

The basic tool in percussion-cable drilling is the drilling assembly, consisting of a bit and a percussion rod weighing up to 3000 kg [2]. The bit is subjected to intense abrasive wear under impact loads (especially when working in boulders). In this process, the edges 1—2 of the bit (Fig. 1) wear down and its diameter decreases. Drilling productivity drops sharply. Usually a bit is considered dulled when its diameter has decreased by 4—6 mm.

Under normal conditions, a bit weighing 160 kg is dressed by the forge method 70—80 times before it is completely worn out. For one dressing (taking wear into account), 1.0—1.4 kg of metal is consumed. A bit is rejected at a length of 600—700 mm and a weight of 64—80 kg. Thus, the total consumption of U7A bit steel, taking rejection into account, is almost doubled.

Fig. 1

The requirements imposed on the hardfacing metal and the choice of its composition depend on a number of factors: operating conditions, the method of dressing and heat treatment, the chemical composition and weldability of the bit material. On this basis, two types of alloys were selected for hardfacing—U35Kh25R alloy (T-590 grade electrodes) and Kh8 steel (experimental electrodes).

T-590 electrodes provide the chemical composition of the hardfacing metal in the following ranges for the main components: carbon—2.8–3.5%; chromium—23–26%. The deposited layer has low deformability and ductility and high hardness. It is not amenable to either heat or mechanical treatment and is capable of hardening even when cooled in air.

The structure of the hardfacing metal consists of chromium carbides, ledeburitic eutectic, and retained austenite (Fig. 2). Although the fusion boundary is structurally distinct and sharply delineated (Fig. 3), in terms of mechanical properties it has a smooth transition (Fig. 4). In tests of the deposited layer in chipping, failure occurred through the hardfacing metal.

The higher strength of the fusion zone in comparison with the hardfacing metal is evidently explained, first, by a reduction in the amount of carbon due to the base metal and, second, by equalization of the chemical composition by diffusion, since the hardfacing was carried out with preliminary and subsequent heating to 550–600°.

During tests of hardfaced bits it was found that bits in which the thickness of the deposited layer was less than 3 mm showed durability when drilling rocks of category IV equal to that of factory-made bits. When drilling rocks of category III and below, the durability of the hardfaced bits exceeded that of factory-made bits by three to four times.

With an increase in the thickness of the deposited layer, its brittleness increased and chipping was observed. The increased durability of a thin deposited layer is explained by its depletion in carbon due to the base metal.

Often the wear of bits reaches 7 mm per side. Restoring such a size by hardfacing with T-590 electrodes does not provide normal durability. In this case it is advisable to deposit the first layer with electrodes of type E-42, thereby ensuring a reduction in the carbon concentration in the second layer, deposited with T-590 electrodes. This method provides in the transition layer a structure consisting of a mixture of troostite and martensite, which has a comparatively higher impact toughness. The durability of such bits when working in rocks of category IV–V is somewhat higher than the durability of bits without hardfacing.

However, when drilling rocks above category VI, hardfacing with a hard but brittle alloy of the U35Kh25R type does not give the desired result.

It is known [3] that chromium steels possess good wear resistance with satisfactory toughness. Experimentally, an alloy of the following composition was selected for hardfacing bits: carbon—0.6–1.0%; chromium—8–10%. Under conditions in which the bit was cooled in air after hardfacing, this chemical composition of the alloy provided a structure of the hardfacing metal consisting of a mixture of troostite and alloyed martensite (Fig. 5).

When bits hardfaced with these electrodes were tested, they showed twice the durability when drilling rocks of category VI.

In conclusion it should be noted that, since the U35Kh25R alloy is not subjected to heat treatment and forging, restored bits should be used until the deposited layer is completely worn out, which is not always possible because of nonuniform wear of the cutting edge of the bit. Otherwise, the remaining deposited metal makes repeated forge dressing difficult.

The best results in this case are given by alloy Kh8. In heat-treatment technology it is close to bit steels and is capable of...

Fig. 2. X500

Fig. 3. X500

Fig. 4

Fig. 5. ×500

be subjected to forging. The technology for restoration by electrode hardfacing fits well into the technological process of dressing drill bits. Almost analogous properties can be obtained when hardfacing drill bits with EN60M electrodes.

REFERENCES

1. V. N. Kashcheev, K. V. Savitskii, Yu. A. Evtyushkin. Development of materials and modes for hardfacing drilling tools in order to improve their operating properties. (Research report of the SFTI at TSU). Tomsk, 1966.

2. I. P. Zorin, A. M. Storozhenko. Percussion-cable drilling. Gosgortekhizdat, 1960.

3. New Problems in Welding Technology. Collection of articles edited by Academician B. E. Paton. Tekhnika Publishing House, 1964.