UDC 669.018.8 I N V E S T I G A T I O N OF C A S T A L L O Y S OF T H E S Y S T E M C r - C o - B R E S I S T A N T TO A B R A S I V E WEAR AND C O R R O S I O N IN AGGRESSIVE MEDIA A. V. Smirnov and N. A. Syroegina Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 5, No. 1, pp. 54-56, 1969. This article describes the results of tests on abrasion- and corrosion-resistant alloys in the chromium corner of the Cr-Co-B system. The aim of this investigation was to explore the possibilities of developing inexpensive abrasion- and corrosionresistant pen-point alloys suitable for fabrication of steel pens. It is known [1] that the best pen-point materials are found in alloys based on certain metals of the platinum group, especially osmium-iridium alloys. Although the hardness of these aUoys is relatively low (HVa/30 -~ 500), their abrasion resistance is very much higher than that of other alloys of the same or even higher hardness. However, high cost and scarcity of alloys of this kind made it necessary to look for other alloys possessing similar properties. Among many patents relating to abrasion-resistant pen-point alloys the most interesting is the patent [2] covering alloys in the Cr-Co-B system. The fixst paper concerned with this system and citing a section of its ternary equilibrium diagram at 800~ was published in 1966 [3]. The results of our preliminary studies of alloys of the Cr-Co-B system showed that using chromium as the alloy base (90-95%) ensures a high corrosion resistance, that cobalt additions increase the alloy strength (due to the formation of a solid solution), and that boron forms hard chromium borides and improves the welding charaetexistics of the alloys in question. Wear-resistance tests on emery paper showed that in this respect the Cr-Co-B aIloys are superior to other alloys (such as stellite VaK) widely used as abrasion-resistant materials. To determine the optimum composition, about 20 experimental alloys were prepared; 99,98% Cr, 99.96% Co and 99.95% B were used as the starting materials, the boron and cobalt contents varying between 1.0 and 9.0%. The nominal composition and chemical analysis of four of the alloys studied are given in Table 1. The alloys were made by levitation melting (in a high-frequency electromagnetic field) in pure helium; this work was done in the HighFrequency Heating Laboratory of Physicotechnical Institute AS USSR. Table i Nominal composition, % Alloy No.

Cr

94

93

Co

4

B

' C h e m i c a l analysis, % Cr

Co

B

95.3

3.40

95.3 93.70 92.22

2.78 4.26 4.85

t.32 1.90

1,64

2.73

The levitation melting method eliminates the risk of the starting materials being contaminated during melting due to the interaction with the crucible material, but the quantities of altoy that can be made by this method are relatively low. SeveraI experimental alloys of the same nominal composition were therefore made using an ordinary high-frequency induction set of the LPZU-80 type. The melting was done in alumina crucibles in air; cast wire specimens (2-3 mm in diameter, 100-130 mm long) were obtained by drawing molten alloy into quartz capillary tubes. Investigation of experimental alloys included chemical, metallographic, and X-ray diffraction analyses and hardness measurements, the latter using both a pyramid indenter and a scratch hardness tester (to determine H100 and HAs, respectively). In view of the interrelation between scratch hardness HAs and resistance to abrasive wear [1], particular importance was attached to scratch-hardness tests. Most of the case-alloy specimens consisted of two phases: a chromium boride Cr4B dispersed in a matrix of a sotid solution of Cr and Co; alloys with 9 - ! 0 % B contained traces of a third phase Cr2B. Most aIIoys had a fine dendritic structure resembling that of Os-Ir alloys (see figure). 39

Our experimental alloys were based on binary Cr-B alloys of the eutectic composition [4] ; their hardness was H100 = 572 kg/mm 2. According to the constitution diagram of the system Cr-B [3], an alloy with 3% B by weight should form a pure eutectic containing about 70% of fine dendritic eutectic with 66% Cr4B. In alloys with 4% B there appear needles of Cr4B (Hi00 = 2290 k g / m m z) whose quantity increases with increasing boron content. Alloys containing more than 7% B are very hard but brittle. The results of metallographic examination and X-ray diffraction and hardness measurements were used to select four Cr-rich alloys (alloys no. 2, 3,5, 8) containing 2-4% Co and 2-3%B for further tests. The abrasive wear tests were carried out on a Kh4-B machine [5], on cylindrical specimens (2 mm diameter, 8-10 mm long) prepared by a method described in [5]. A 9KhS steel specimen (H100 = 795 kg/mm 2) was used as a standard. A grade KSM28 waterproof carborundumMicrostructure (x500) of a cast Cr-Co-B coated cloth was used as the abrasive surface; the load on the specimen alloy No. 8. was 0.3 kg. The degree of wear was determined as the change in specimen length for a friction path of 15 m at a specific load of 9.55 kg/cm z. The relative wear resistance (Table 2) was calculated from the formula

= ( a U a l M) (aJaM)2, where At s and AZM are the absolute values of linear wear of standard and experimental specimens, respoctively, and d s and dM are the actual diameters of the standard and experimental specimens, respectively. Table 2

Alloy No.

Method of preparation

H100, kg/mm 2

ds

A A A

549 556 797 831 842

2.04 2.04 2.04 2.04 2.04

B, B

dM.

1.98 2.02 2.04 1.97 1.97

al s

0.084 0.083 0.084 0.084 0.080

MM

0.065 0.060 0.050 0,040 0.030

66.2 68.1 81.1 108.7 118.3

Note: A) Alloys made by a levitation melting method using high-frequency induction heating; B) alloys melted in air in an ordinary high-frequency induction set. The wear resistance values were re-calculated for a lead-tin standard alloy BM [5] ; the wear resistance of the steel 9KhS standard relative to this alloy is 48.3. Data in Fig. 2 show that the wear resistance of group A alloys increases with increasing boron content and that the wear resistance of specimens made by drawing liquid alloys (melted in air) into quartz capillaries is considerably higher than that of alloys made by levitation melting in helium. Comparison of our results with the corresponding data for other known wear-resistant alloys showed that for cast alloys of the V3K (stellite) type s = 32; in the case of a hard-facing alloy BKh (50% chromium boride and 50% Fe) we have s = 80.8, i.e. o the relative wear resistance of chromium alloys is always twice that of stellite, and in three cases out of five it is higher than that of alloy BKh. The corrosion resistance of the alloys studied in six grades of ink concentrated with hydrochloric and nitric acids was determined by measuring the specimen weight loss after six months in these media. No evidence of corrosion was observed. Summary 1. The high abrasion resistance of Cr-Co-B alloys containing 2-3% B and 2-4% Co is due to the presence of 60% chromium boride in a near-eutectic composition and due to their fine dendritic structure analogous to that of Os-Ir alloys. 40

2, The relative wear resistance of alloys of this kind is twice that of the wear resistance of grade VSK steIlite. 3. The al!oys are distinguished by a high resistance to corrosion in aggressive media. REFERENCES 1. 2. 3. 4. Russian], 5.

O. Winkler, Z. Electrochem., 49, no. 3, 221-229, 1943. German patent no. 974966. Yu. B. Kuz'ma and M. V. Chepiga, Neorganicheskie materialy, no. 7, 1966. F. I. Shamrai and T. I. Fedorov, collection: Investigations of Metals in the Liquid and Solid State [in Izd. Nauka, 255-264, 1964. M. M. Khrishchov and M. A. Babichev, Studies of the Wear of Metals [in Russian], Izd. AN SSSR, 1960.

13 January 1968

Aeronautical Instrumentation Institute,

Leningrad

41