Abstract - Six matrix formulas are introduced in this paper, and the moulds are designed to manufacture eighteen matrix samples through the method of hot pressing sintering， and then Rockwell hardness and bending strength of the samples are tested. Based on the analysis of test results, the optimal matrix formula of impregnated diamond bit is recommended. And tungsten carbide (WC) as a kind of framework ingredients which has high melting temperature can improve Rockwell hardness and wear resistance of matrix. According to the result of experiment, the average value of hardness and bending strength are bigger and bigger as WC volume fraction is higher and higher.
Keywords - impregnated diamond bit, matrix formula, hot pressing sintering method, and performance test
I. INTRODUCTION
In recent years, as the national economy and the urbanization are developing quickly, the government has made good efforts to prospect the resources of minerals and oil. As a kind of tools to prospect the resources, the diamond bits have excellent advantages of impact resistance and wear resistance to drill rocks and have been widely used in drilling field [1-4]. During the process of drilling, in order to improve the efficiency, the matrix performances including the Rockwell hardness and bending strength must fit with drillability of stratum [3, 5]. And the performances are greatly related to the matrix formula and sintering method. So the selections of matrix formula and sintering method are very important to manufacture good quality of diamond bits [6][7].
II. MATERIALS AND METHODS
A Experimental design
(1) Design of matrix formulas
The performances of matrix of diamond bit mainly depend on the formula of matrix, which includes ingredients and volume fraction. The ingredients are mainly divided into two kinds: one is framework ingredients, which is used as hard spots; and another is binder metal of low melting point, which is used to bond framework ingredients and diamond. The volume fraction of binder metal must be moderate, and if the volume fraction is too high, the thick metal layer will be generated among the framework ingredients and the diamond, Rockwell hardness and wear resistance of matrix will be reduced, and if too low, the framework ingredients will not be bonded solidly, which can easily cause diamond bits broken [8] [9].
According to the above analysis and practical experiences, six formulas of matrix are designed, and then they are tested after sintered, at last the optimal formula is recommended when different stratum are drilled. And both ingredients and volume fraction of each formula are shown as table Ⅰ.
TABLE I
Matrix formulas of bit
Formula / Ingredient and volume fraction/%WC / YG6 / Ni / Mn / Co / ZQSn663
Ⅰ / 45 / 0 / 7 / 5 / 3 / 40
Ⅱ / 40 / 20 / 5 / 5 / 0 / 30
Ⅲ / 40 / 15 / 7 / 5 / 3 / 30
Ⅳ / 40 / 15 / 5 / 5 / 0 / 35
Ⅴ / 45 / 15 / 5 / 5 / 0 / 30
Ⅵ / 50 / 15 / 5 / 5 / 0 / 25
(2) Mould design
Graphite is selected as the material of moulds, as it has high temperature resistance and enough strength. In addition, it also does not contaminate the matrix. In order to reduce error, three samples are manufactured for every formula. And it is well known that the densification of matrix has an important effect on performances of matrix. So these samples need to be compressed to improve the densification. And the compression is stopped when the samples are compressed to a certain position in the process of sintering. The mould is made up of three parts: cover mould, top column and bottom column. Firstly the bottom columns are put into the cover mould, and then ingredients which have already mixed well are put into the cover mould, and at last the top columns are put into the cover mould. Then the top column are compressed into the cover mould, the process of compression is stopped [10-15]. The mould is showed as Fig.1.
The dimension of mould is as follows: the thickness of cover mould is 25mm; the bottom columns are 5mm; the top columns are 15mm, and cross-section of three parts is same, as showed in Fig. 2.
(3) Design of sintering
The important function of matrix is to connect diamond and steel body, which has an important effect on the lifespan of bits. So all ingredients must be mixed well in order to improve impact resistance and wear resistance of matrix. The recommended time of mixing is 24 hours at least [16].
After the mixed ingredients are put into the mould, the samples are ready to be sintered. Compare to the normal method of manufacturing impregnated diamond bit, the hot pressing sintering method is convenient and selected to sinter samples. Temperature is a main factor to affect the alloying extent of matrix, and the pressure is mainly to control the matrix densification. At the same time, the rate of heat increasing shouldn’t too high. Because the heating rate is too quick, gas generating in the mould can’t be discharged completely. This can disturb the ingredients, even to bring the ingredients out of the mould. The recommended heating rate is 250~350°C/ min. Heat preservation is necessary to reduce temperature difference of matrix, and it also gives enough time to complete alloying process. This is helpful to improve the strength of cementing diamond.
Sintering parameters are selected as follows: the temperature of sintering is 960℃; the pressure is 150kgf/cm2; the time of heat preservation is 5 minutes. Sintering furnace is showed in Fig. 3.
B Performance test
(1) Rockwell hardness test
Based on the same parameters, eighteen samples of six formulas are sintered. And then these samples are taken out from the mould. And after cleaned, they are polished to expose a fresh surface by grinding wheel. In the end, the samples are tested on Rockwell hardness tester. Four random points are tested on the fresh surface, and the average values are calculated.
(2) Bending strength test
The bending strength of matrix is a very important performance for diamond bit. If the value is too low , the bit is broken easily. After Rockwell hardness is tested, bending strength of the samples is tested. The test results are shown as table 3.
The standard bending strength tester is used to test bending strength of these samples. In addition, in order to reduce error, a pressure sensor is used to show the broken stress. The bending strength is got through following calculation equation (1):
(1)
Where is, P—broken stress, N; L—distance between two fulcrums, L＝24mm; b—width of the sample, mm；h—height of the sample, mm.
According to the dimension of mould, the b and h can be calculated, that is, b=5mm and h=8mm, the bending strength can be calculated after the broken stress is tested.
TABLE II
Test data of Rockwell hardness
Formula / NO. / Rockwell hardness value / average value1 / 2 / 3 / 4 / average
Ⅰ / 1-1 / 28.2 / 28.1 / 31.2 / 29.7 / 29.3 / 29.4
1-2 / 28.0 / 28.5 / 28.1 / 27.7 / 28.1
1-3 / 32.2 / 31.1 / 29.5 / 30.4 / 30.8
Ⅱ / 2-1 / 31.8 / 32.1 / 30.9 / 30.7 / 31.4 / 30.6
2-2 / 28.3 / 28.8 / 30.8 / 30.1 / 29.5
2-3 / 31.9 / 30.4 / 29.9 / 32.2 / 30.9
Ⅲ / 3-1 / 38.3 / 39.1 / 39.5 / 37.8 / 38.7 / 38.4
3-2 / 39.5 / 36.1 / 37.0 / 35.4 / 36.9
3-3 / 39.1 / 39.7 / 38.9 / 40.7 / 39.6
Ⅳ / 4-1 / 34.4 / 35.7 / 33.9 / 35.1 / 34.8 / 36.2
4-2 / 38.0 / 36.1 / 36.5 / 37.8 / 37.1
4-3 / 37.3 / 38.1 / 36.5 / 34.9 / 36.7
Ⅴ / 5-1 / 42.7 / 46.0 / 44.4 / 48.1 / 45.3 / 44.8
5-2 / 44.7 / 46.2 / 42.6 / 48.4 / 45.5
5-3 / 42.2 / 43.1 / 45.2 / 43.9 / 43.6
Ⅵ / 6-1 / 44.9 / 46.3 / 48.4 / 47.5 / 46.8 / 48.2
6-2 / 49.4 / 48.5 / 48.0 / 48.9 / 48.7
6-3 / 48.2 / 48.7 / 50.1 / 49.3 / 49.1
III. RESULTS
The test results of Rockwell hardness are shown as tableⅡ. For further analysis, the fitting curve is drawn as fig.5. The test results of bending strength are shown as table Ⅲ.
TABLE III
Test data of bending strength
Formula / Bending strength value (MPa) / average valueNO.1 sample / NO.2 sample / NO.3 sample
Ⅰ / 436 / 388 / 397 / 407
Ⅱ / 538 / 439 / 439 / 472
Ⅲ / 583 / 724 / 610 / 639
Ⅳ / 516 / 604 / 584 / 568
Ⅴ / 938 / 781 / 816 / 845
Ⅵ / 831 / 789 / 906 / 842
In order to contrast difference values of bending strength, the fitting curve is drawn as Fig.6.
IV. DISCUSSION
The six formulas are based on practical experiences, and the test results are different, because the volume fraction of framework ingredients and binder metal are adjusted. Based on the test data, the conclusions are listed as follows:
(1) According to the fitting curves, the average value of hardness and bending strength are bigger and bigger as WC volume fraction is higher and higher. So WC as a kind of framework ingredients which has high melting temperature can improve Rockwell hardness and wear resistance of matrix.
(2) Compare formula Ⅰ with formula Ⅱ, although WC fraction of formula Ⅰ is higher than formula Ⅱ, but formula Ⅱ does not include YG6, and YG6 is made up of 94%WC and 6%Co, so in fact total WC fraction of formula Ⅱ is higher than formula Ⅰ. The Rockwell hardness value of formula Ⅱ increases by 4% than formulaⅠ, and bending strength value by 16%. Formula Ⅱ should be selected when the stratum is hard and low-abrasive, because if the Rockwell hardness is low, diamond is exposed easily, and the drilling efficiency would be improved.
(3) Compare formula Ⅲ with formula Ⅳ, WC fractions of both is same, and binder metal fractions are adjusted only. The Rockwell hardness value of formula Ⅲ increases by 6.1% than formula Ⅳ, and bending strength value increases by 12.5%. So the formula Ⅲ including Co can improve alloying extend of matrix. Formula Ⅲ is recommended when the hardness and abrasiveness of stratum are moderate.
(4) Compare formula Ⅴ with formula Ⅵ, WC fraction of formula Ⅵ is higher than that of formula Ⅴ, and bender metal fraction of formula Ⅵ is lower than that of formula Ⅴ correspondingly. According to the test data, Rockwell hardness value of formula Ⅵimproves by 7.6% than that of formulaⅤ, and bending strength values of both formulas are almost same. So formula Ⅵ should be selected when the stratum is soft and high-abrasive in order to improve the life of bit.
(5) The sintering process and sintering parameters are important to the sample manufacturing. Especially the sintering temperature and time of heat preservation greatly affect the performances of matrix. In this experiment, the matrix is heated to 960℃, and the time of heat preservation is 5minutes, and these samples are natural cooled in the sand. And this gives enough time to complete alloying process. In addition, it is helpful to form the excellent metallographic structure.
V. CONCLUSION
(1) From the experiment, WC as the framework material can obviously increase the hardness, bending strength, and structure strength and wear resistance of the matrix .And with the increasing of Co content, alloying degree and toughness of the matrix can be greatly improved.
(2) As the experiment results are shown, number Ⅱ formula is suitable to application in the rock of hardness and weak-abrasiveness. Number Ⅲ formula is suitable to application in the rock of medium hardness and medium—abrasiveness. Number Ⅳ formula is suitable to application in the rock of softness and strong—abrasiveness.
(3) The experiment is performed as followed: in the sintering process the ultimate temperature need to be heated to 960 ℃, and keep this temperature for 5 minutes, then put the heated matrix into perlite powder for natural cooling that would be benefit to form good metallographic organization of the matrix.
(4) The research of matrix formula is a complex project, and is not summarized by three formulas. Combining with the production practice and the practical application, this thesis is only to test the several representative formulas, and still need further research.
ACKNOWLEDGMENT
The work was supported by the special program of the ministry of land and resources public welfare industry research（SinoProbe-09-05）; National 863 plan project (Grant No. 2009 AA06Z201); National Natural Science Youth Foundation of China (Grant No. 51004052)
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