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Possibilities of spray hardening tool displacement of rocks C. Florea, V. Florea A. Drescher,G.Praporgescu

Universitatea din Petrosani,20,str. Universitatii ,332006, Petrosani ,Romania

E-mail : carmenflorea2004@yahoo.com

Abstract

Following the processes of friction, impact or abrasion, some small particles of solid materials can separate and become hot because of energy used in processes. Question. If these particles are oxidizable materials, such as iron or steel, they can undergo oxidation, reaching even higher temperatures. In this context during rock cutting process can be generated sparks which very often are sources of ignition. These particles (sparks) may ignite combustible mixtures and certain mixtures of dust / air (especially mixtures of metal powder / air). If deposited dust, or methane, sparks can cause a burning or smoldering (and this can be a source of ignition for an explosive atmosphere) or an explosion. Choice for hardening tungsten carbide spray mining tool is the result of positive results of underground experiments in potentially explosive environment, with these materials have proved to ignite gaseous mixtures. Charging is to design spray filler metal melted and sprayed in very fine particles on the surface of the tool cutting edge dislocation of rocks. If tungsten carbide load, heat source (electric arc, oxyacetylene flame, plasma jet, etc..) Melted filler material and a surface layer of base material, forming a molten metal bath are distributed tungsten carbides. In this case the deposit is made of tungsten carbide grains with original properties unaltered, cemented into a mass of binder with high mechanical characteristics (hardness to 65 HRC) steel produced by primary crystallization of the liquid from the basic metal and metal addition, allied with elements from dissolving carbides.

1.Introduction

The main physical - mechanical you should possess the necessary materials perforaj execution mining tools that can operate in hazardous environments, caused by combustible gases are:

a - mechanical strength (hardness and toughness) high; b - high temperature resistance; c - mechanical and thermal shock resistance; d - resistance to abrasive wear.

The characteristics listed are usually in inverse ratio. This complex of special properties antiscânteie complete this feature, which is defined as non-appearance of spark to the

shock produced by the impact of parts (deployment tools) to rock.

If wear caused by rocks on metallic materials and their influence in the process of aging, but requires laboratory tests and in situ.

Resistance to mechanical efforts of rock is considered as "opposition" to their component particles under the action of external loads which tend to defeat the forces of cohesion between particles and cement the connection.

Occurrence of abrasion wear prerequisite is the existence of difference of hardness of surfaces in contact. In this context, it is clear that the rocks are discontinuous bodies, heterogeneous and anisotropic, with different hardness even for the same sample at the same time, it is necessary to note that theories of elasticity and plasticity, can not be used where the rocks, than the limited research areas [1], and mechanical testing laboratory for the study of wear produced by rock on metallic materials can be performed on samples of irregular shapes.

Material of the base of the rock perforaj mining tools can be carbon steel alloyed with Si and Mn, with a pearlitic structure feritico-average resistance (strengthening material takes place under the influence of alloying elements in solution). Other materials that can use different tools and parts that work in hazardous environments caused by combustible gases are:

a - tool steels; b - high speed steel with high toughness; c - manganese steels with high capacity for hardening; d - white cast iron alloyed with Mo or Ni Cr; e - hard cast alloys.

Spray charging mining tools to design perforated filler metal melted and sprayed very fine particles on the surface of their cutting edges.

If spraying hardening alloys and tungsten carbide cobalt can use the following methods, [2]:

a - loading oxyfuel flame spraying (conventional method); b - loading oxyfuel flame spraying high speed (High Velocity Oxygen-Fuel HVOF); c - load arc spray; d - loading plasma arc spraying.

Of these there are loading process oxyfuel flame spraying at high speed as it provides:

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a - obtaining a high density (low porosity) due to high energy impact of tungsten carbide powders; b - high corrosion resistance due to low porosity (less than 1%); c - high wear resistance due to tungsten carbide deposited; d - very good connection between the deposited layer and the base material due to improved adhesion of the particles deposited at high speed; e - deposited layer thickness is small (min. 0.05 mm), achieving a significant saving, and residual stresses developed are very low, limiting deformation parts.

2. Perforated plates with hardened edges oxyfuel flame spraying at high speed.

Cutting edge of detachable heads used in perforajul rotating plates are reinforced with tungsten carbide applied by brazing in order to obtain a propritatilor mentioned and reduced wear.

Figure 1 Perforated plates with hardened edges spray.

Experiments that were conducted with edged tools hardened by spraying (Figure 1) to drive out hard tungsten carbide plates in order to avoid their detachment, frequently observed during operation in spcecial because, poor quality soldering.

In general, hard alloys based on tungsten carbide Physical and mechanical properties meet that gives them wide uses in various fields, properties can be modified depending on destination, structural control and controlled alloying WC-Co system , it can make the following considerations:

a - fine-grained structures lead to higher hardness at the expense of resistance to bending and compression; b - increasing cobalt content increases the resistance to bending and hence the toughness, but toughness and decreasing compressive strength; c - the presence of TiC and hard TAC layer composition leads to an increase in hardness, but also to lower resistance to bending and compression.

In this context, prototype tools for the perforaj revolution (Figure 1) were made by high-speed flame spraying, the tungsten carbide grains on their cutting edges.

If high-speed flame spraying (HVOF process - Figure 2), speed of impact of particles on the workpiece surface is much higher than with conventional flame spraying process, as used gun is equipped with a nozzle expansion providing a high speed jet of gas flow [3], [4] ..

Figure 2. Scheme flame spraying process at high speed Filler material is driven at high speed in the jet, which causes a strong acceleration of powder particles. Fuel gas is propane, propylene, acetylene, hydrogen, natural gas or kerosene. In Table 1 Typical operating parameters and productivity HVOF spray process load.

Figure 3 shows schematically HVOF spray charging equipment, realizing torch cooling water. Charging parts spray flame at high speed (HVOF) technology involves the following steps: surface preparation charge subject, preheat parts, loading parts and spray the final heat treatment.

Processing (cleaning) substrate surface and integrity of critical importance to achieve a high adhesion between the substrate and the layer deposited by spray.

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TABLE 1 Process Gas flow,

m3/h Temp. Flame

/plasma 0C

Adhesion strength between the substrate and

deposited layer, MPa

Rezist. coeziunii

dintre particule

Impact velocity of the particle

m/s

Oxide content,

%

Maximum porosity,

%

Maximum speed of spraying,

kg/h

The power,

kW

The energy required melting kW/kg

High-speed flame spraying (HVOF)

28-57 3100 > 70 Very high 610-1060 0,2 2 14 100-270 22-200

Figure 3. Schematic representation of loading equipment spray HVOF. 

The size of substrate surface roughness is very important for high-speed flame spraying to achieve a high adhesion between the substrate and deposited layer by spraying. Substrate surface roughness affects the morphology of filler droplets deposited by spray deposited layer roughness influence. Morphological appearance of the droplets of filler material initially deposited (first pass) on the substrate surface affects the integrity of the interface formed between the deposited layer and substrate adhesion leading to modification of the deposited layer and substrate. This is due

to mechanisms (mechanical, metalurgico-chemical and physical) through which the deposit layer. Initially, powder particles deform at impact with the workpiece surface to form the mechanical connection to the substrate surface roughness (Figure 4). The relationship between deposited layer and substrate is strengthened following links metalurgico-chemical nature (achieved by diffusion or alloying with intermetallic compound formation) and physical connections (made by particle adhesion to the substrate through van der Waals forces).

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Figure 4. Powder particle surface adhesion to the substrate

processing

Figure 5 presents the cross-sections of pieces of steel, their surfaces were machined to different conditions, substrate surface roughness can be modified (eg in 6 to 15 µm) by changing the pressure of compressed air used to blasting. Also, the surface of the piece is influenced by the average diameter of the particle used for blasting.

Figure 5. Cross sections of machined steel parts in

different conditions: a - polished surface (using suspension containing

fine abrasive grains (1 μm ); b - the surface blasted with grains having an average

particle size of 125 μm; c - the surface blasted with grains having an average

particle size of 300 μm; d - the surface blasted with grains having an average

particle size of 1500 μm: In general, when there is a temperature difference between substrate and deposited layer, thermal stresses may occur. These tensions can be reduced or even eliminated by pre substratului.Se recommended working temperature of the steel parts surface to be located between 90-1500C. For this project, working temperature is an object of research.

Surface pre-perforated edge tools was achieved with HVOF gun

Materials used for charging high-speed flame spraying are: WC-12% Co powder, fuel gas (propane), oxygen and carrier gas (nitrogen).

Size and shape of the particles of powder, used to load spray flame at high speed, quality of the deposited layers influence.

Figure 6. Morphological appearance of isolated droplets

deposited on the workpiece surface Figure 6 shows a dense distribution of carbide particles retained in the matrix of cobalt metal. The drop looks flattened submitted indicating that the metal matrix powder was completely melted. You notice a lot of tungsten carbide particles of small size (similar to the original size), embedded in the matrix metal. Average particle size of WC in the deposit is 1.34 ± 0.32 μm.

Generally these hard materials are used to provide a layer made of abrasion wear resistance. High resistance to wear, provided by these materials is due to hard carbide particles are distributed in the matrix metal.

3. Conclusions Assessment of abrasion wear behavior can be based on weight loss tool used in the drilling of rocks. Weight loss due to abrasive wear layer deposited WC-12% Co powder was 6 mg. Generally the wear resistance of these materials depends on the content of carbide particles deposited layer and the link between these particles and metal matrix are distributed. Wear resistance of the deposited layers increases with carbide content and carbide particle size reduction in layer

References

[1]. Ciocârdia C., Drăgulănescu E., Drăgulănescu I.: “Sintered carbide alloys", Technical Publishing, Bucharest, 1985.

[2]. ***: STAS 11684/6-85 „Thermal spray coatings with metal powders. Weld metal melting coating. Execution requirements. "

[3]. ***: SR EN 657:2005 „Thermal pulverization”. [4]. M. Ivosevic, V. Gupta, R. A. Cairncross, T. E. Twardowski, R.

Knight, J. A. Baldoni: “Effect of Substrate Roughness on Splatting Behavior of HVOF. Sprayed Polymer Particles: Modeling and Experiments International“, Thermal Spray Conference – ITSC-2006 Seattle, Washington, U.S.A., May 2006