PTA-surfacing of composite materials

size=Article: Author - Som А.I.

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Today the composite alloys on basis of fused tungsten carbide (FTC) [1, 2] are the most effective for surfacing of the parts working in conditions of intensive abrasive wear. Despite of its high prices, they are irreplaceable in some cases, especially in metal mining industry. Composite alloys are applied with success for surfacing of such parts as locks of well tube, drilling bit rolling cutters, excavator bucket teeth etc. [3-5]. Service life of parts, surfaced by these alloys, exceeds durability of parts, surfaced by hypereutectic alloys of type high-chromium cast iron a few times more.

Thereupon search of effective ways of surfacing of the composite alloys providing the best combination of service properties of surfaced metal is rather actual. Among the known technological processes used for surfacing of the composite alloys, especially it is necessary to distinguish the Plasma Transferred Arc surfacing (PTA-surfacing) [6]. It is the most suitable for these purposes as provides high quality of surfaced metal, uses adding materials in the form of powders and could be applied both in mechanized, and in manual variants.

Present paper is devoted to research of a wear resistance of surfaced metal, a dependence on quantity and forms of reinforcing particles of FTC, a way of their input in a welding pool, and also a type of matrix.

In experiments the powders from Ni- and Fe-base alloys (tab. 1) are used as a matrix material. The fraction of powders was 56-200 microns. A choice of these alloys is not casual. Self-fluxing Ni-base alloys are widely used for surfacing of composite materials. They have rather low fusion temperature (1000-1100 °С), moisten grains of FTC well and are enough wear-resistant.

The Fe-base alloy of the given chemical composition is offered for these purposes for the first time. It belongs to a class of high-vanadium cast irons and combines high wear resistance with the plasticity which is necessary for providing of resistance of a working layer to impact load. Besides it is much cheaper than Ni-base alloys.

As reinforcing materials the powders of crushed and spherical FTC manufactured by Paton Welding Institute are used. The sizes of particles of these powders were within the limits of 100-300 microns.

The surfacing was carried out on samples of size 60х100х20 mm from steel 20 by plasma torch РР-6-02 with the Plasma-Master Co., Ltd. equipment (fig. 1). The section of deposited bead was 25x5 mm, productivity of surfacing was 5 kg/h.

Feeding of matrix and reinforcing powders was carried out by two ways - as a mix and separately. In the first case to exclude separation, powders were mixed by confluence of two streams feed from separate feeders, directly before an input into plasma torch. In the second - the matrix powder was feed in an arc through central nozzle of plasma torch, and FTC - in a welding pool directly through the special channel.

The purpose of feeding of FTC particles in a welding pool was as much as possible to reduce thermal influence of an arc thereby to protect them from dissolution. Especially it is important when the Fe-base alloys are used as a matrix [2, 3].

In table 2 there are all tested variants of surfacing. A separate feeding was used mainly for spherical FTC, as more perspective for this way.

The samples of size 16х16х6 mm for tests for wear resistance and metallographic analysis were cut out from surfaced plates. The top piece of a surfaced layer of these samples were ground down to a level at which FTC grains settled down relatively evenly throughout section. Tests were carried out by machine SR "stationary ring " [7]. The scheme of tests is given below in the fig. 2, results of tests are produced in the fig. 3.

Test conditions: abrasive - quartz sand in size of particles 0,2-0,4 mm, length of friction is 400 m, specific pressure of sample is 0,5 МPа, speed of sliding of sample is 0,6 m/sec. Steel 45 was used as the standard.

Discussion of results

1. The surfacing by a mix of powders

The best results are achieved under the FTC content in a mix about 50 volume % (variants 4, 5). It concerns both to spherical, and to crushed FTC. Good formation of beads and uniform distribution of FTC particles throughout section (fig. 4) is provided in this case, that in its turn provides the maximal wear resistance of surfaced metal (fig. 3). If FTC in a mix is more than 50 volume % then to form the bead well it is necessary essentially (in 40-50 %) to increase surfacing current, i.e. to increase heat-embedding into a part. In its turn it results in appreciable dissolution of particles, embrittlement of matrix and, as consequence, in decrease of wear resistance.

In surfaced layer there is no more than 30 % spherical and no more than 25 % crushed FTC on volume (fig. 5).

Under the FTC contents < 50 volume % (variants 1, 2) all it settles down in the bottom part of a layer, leaving the unfilled top part. Naturally, wear resistance of the top part, despite of some additional alloying C and W, remains low (fig. 3)

Researches of distribution of matrix microhardness throughout the height of a layer and micro X-ray spectrum analysis show, that in all cases though to a variable extent there is dissolution of carbides, at that it is more appreciable in the top part i.e. in an effective area of a plasma arc. Even in variants 4 and 5 with 50 volume % of FTC the matrix hardness grows from HV01 366 kgs/mm2 at a fusion line up to 727 kgs/mm2 in the top part due to alloying by carbon and tungsten.

Microhardness of the kept particles of spherical and crushed FTC in an alloy is approximately equal and comes to 1850-1900 kgs/mm2. Microhardness of half-dissolved particles changes within the limits of 1300-1600 kgs/mm².

In all investigated variants there were microcracks in a surfaced layer. Its minimum was in variants 4 and 5, and maximum was in 8 and 9. In variants 1 and 2 where the top part of beads was empty of carbides, cracks settled down along a section border of the layers (fig. 6, а)

When using of spherical FTC because of high internal pressure also there were scabbings of surfaced metal.

2. Surfacing by separate feeding of powders

Carbides dissolution during separate feeding of matrix and reinforcing materials is minimal, even in case of a Fe-base matrix (variant 12, fig. 7). It is confirmed by the micro X-ray spectrum analysis data and microhardness gaugings of matrixes. Microhardness of a Ni-base matrix varies in limits HV01 360-420 kgs/mm2, on Fe-basis - within the limits of 540-640 kgs/mm2. Formation of deposited beads is good, cracks are absent in most cases. They are observed similarly to surfacing by a mix when there is a partial filling of bead volume by carbides (variants 3, 6) (fig. 6, b)

Table 1. Chemical composition of the alloys used as a matrix metal

Type of alloy	Content of elements, mass %									Hardness, HRC
	C	Si	B	Ni	Cr	V	Mo	Fe	Mn
Ni-base	0,5	2,6	2,2	base	13,5	-	-	2,1	-	40
Fe-base	2,2	0,6	-	2,7	18,2	7,8	2,5	base	0,8	44

Table 2. Variants of realized surfacing and estimate of wear-resistance

№	Type of matrix	Theoretical content of FTC, volume %	Way of powder feeding		Form of FTC particles		Presence of cracks	Loss of sample mass during friction, g alloy = 2,5mm
			Mix	Separate	Spherical	Crushed
1	Ni-base	40	Х		Х		+	0,062
2	--//--	40	Х			Х	+	0,045
3	--//--	40		Х	Х		+	0,069
4	--//--	50	Х		Х		-	0,021
5	--//--	50	Х			Х	-	0,081
6	--//--	50		Х	Х		+	0,058
7	--//--	50		Х		Х	+	0,106
8	--//--	60	Х		Х		+	0,079
9	--//--	60	Х			Х	+	0,092
10	--//--	60		Х	Х		-	0,028
11	Fe-base	50	Х		Х		+	0,055
12	--//--	50		Х	Х		+	0,042
13	--//--	60		Х	Х		-	0,031

Wear resistance of a Ni-matrix layer when uniform its filling by carbides (variant 10) is close to value which we had during the surfacing by mix (fig. 2). When using the Fe-base alloy as matrix, it is a little bit higher (variant 13).

As a whole, wear resistance of composite layers with spherical FTC under other equal conditions is higher than with crushed FTC. Spherical FTC is less dissolved in a welding pool and is very convenient for PTA surfacing.

In the fig. 8 there are examples of surfacing by composite alloys of real parts.