Silicon carbide (carborundum) is a binary inorganic chemical compound of silicon and carbon. The chemical formula is SiC. It is found in nature in the form of the mineral muassanite. Silicon carbide powder was first obtained in the laboratory in 1893. It is used as an abrasive, semiconductor, in microelectronics (in the powertrains of electric cars), and for imitating diamond inlays in jewelry.

Silicon carbide of natural, mainly cosmic origin is a very rare element, so all silicon carbide available today is synthetically produced. Basically by sintering silica with carbon at high temperatures. Pure silicon carbide is colorless. Its shades of brown to black color are due to iron impurities. Finely ground silicon carbide turns into powder and it is in this form that it is used for dressing stones.

Silicon carbide powder is an abrasive material that is used to create an aqueous suspension when smoothing the surface of a sharpening stone. Due to its properties, it gives a uniform suspension of milky color, which is well retained between rubbing surfaces. It is used for work together with lappers.

What is the purpose of leveling sharpening stones? Without leveling the plane of the stone, we cannot properly control the angle between the feed and the surface of the stone, this will result in a disturbed angle, blockage of the edge. In order to check the degree of wear of the stone surface, you can take a ruler and place it with its edge on the stone and place a light source behind it. If, having looked between the ruler and the stone we will see a gap, it means that the stone needs to be straightened on the working surface.

The most affordable way to level stones is by dressing on glass. For artificial stone, the size of the glass is preferably one and a half times the size of the stone. For natural stones, the size should be several times larger, as natural stones are harder and the glass will be seriously worn. It is very important that the glass is thick and will not sag.

The main question that arises when preparing to level stones is what type of powder to use for a particular stone? A rule of thumb for this is that the powder grain should be 2-3 times the size of the stone grain.

For example, for Arkansas stones, this system might apply:

For Washita stone leveling, F120 silicon carbide powder is used.
For leveling Arkansas Translucent stone, F800-1200 powder is used.
For leveling Arkansas Black stone, F1200 powder is used.

However, these are only theoretical, each sharpening enthusiast must develop his own system in this matter and follow it.

The alignment technique used by most sharpeners is as follows: a pencil grid is applied to the stone, which must be completely rubbed out on a lapping stone with the powder applied. A simple pencil can be used. The grid should be applied to the stone wiped dry. For the leveling process, a small amount of powder is poured on the glass and water is added so that the glass is completely covered with a thin film of water. That is, when working, the water should not leave the edges of the glass. Then the stone is made movements, they are best to make a figure eight with two hands, and moving the stone from the left edge of the glass to the right and back. It is very important in such work do not forget to constantly add water, because if the suspension becomes too thick, it will lead to a piling of the surface of the stone. The work should be carried out until the mesh is completely gone. After that you can move to a finer grinder, or finish leveling. It is very important in the process of work to chamfer the bars, since their edges become very sharp and can damage the knife when sharpening. When using coarse powder, it should be remembered – you do not need to put a lot of pressure on the sharpening stone, otherwise, again, the surface may be bogged down. Leveling stones on silicon carbide is a meticulous, time-consuming job that requires patience and concentration.

This alignment on silicon carbide, is also used in what is called “stripping” or “shaking” the stones. The same movements as for leveling are used to remove the baked layer on the stone with which it comes from the factory. In the process, the grains of the synthetic stone are “opened”. This procedure is mandatory for stones, without it the stone’s efficiency will not be high.

It is also important not to forget that silicon carbide powder is a volatile substance, which can get into the respiratory tract of a person in dry form. It is therefore necessary to work with it carefully, use construction respirators and keep the workplace away from foodstuffs.

 

Powder steels have been used for knife manufacturing for more than 30 years. During these years the price for such steels has significantly decreased, they have become more affordable and are used in a wide variety of knives, including not only in the premium segment. What is the difference between powder steel and “ordinary” steel and how is it created?

Powdered steel is steel ground to powder, which undergoes a process of atomization, crystallization and baking. As a result of this processing cycle, the so-called “powder conversion” takes place – the steel receives a large amount of carbides and can also be alloyed with additional elements in greater quantities than standard rolled counterparts.

The structure of any hardened steel consists of two essential elements: carbides and martensite.

Martensite is the main structural component of hardened steel (matrix). It is an ordered supersaturated solid solution of carbon in α-iron of the same concentration as the original steel material (austenite). The martensite structure is non-equilibrium and has high internal stresses, which largely determines the high hardness and strength of steels with martensitic structure.

Carbides are compounds of metals and nonmetals with carbon. The peculiarity of carbides is the greater electronegativity of carbon, compared to the other element. Carbides are refractory solids. They are non-volatile and insoluble in any of the known solvents. Carbides are used in the production of cast irons and steels, ceramics, various alloys, as abrasive and grinding materials, as reducing agents, deoxidizers, catalysts, etc. Carbides are used in the production of silicon carbides. Silicon carbide SiC (carborundum) is used to make grinding wheels and other abrasives; iron carbide Fe3C (cementite) is used in cast irons and steels; tungsten carbide and chromium carbide are used to produce powders for gas-thermal spraying.

Most steels used to make blades have a structure after heat treatment: martensite + carbides (+ residual austenite + non-metallic inclusions, etc). Carbides, harder and more brittle than the martensite matrix, increase the wear resistance of the steel, but deteriorate its mechanical characteristics, negatively affecting strength and toughness. The degree of reduction in strength properties depends on the amount of carbide phase, its type, the size of carbides and their clusters, and the uniformity of carbide distribution in the structure.

In addition, pronounced carbide heterogeneity creates problems in grinding and increases the tendency to leashes and cracks. Steels with a large number of large and irregularly distributed carbides are less amenable to hot deformation. Such steels develop a heterogeneous structure when heat treated, and the results of heat treatment are less predictable.

Consequently, to increase the wear resistance of steel and long term sharpness retention, it is necessary to increase the amount of carbide phase, and to maintain acceptable mechanical performance to reduce and improve their distribution. Several methods can be used to achieve this goal. Among them:

1. Optimizing the steel composition. For example, it is possible to saturate the steel with carbides of other types, most often large amounts of vanadium.

2. Microalloying. </The saturation of steel with elements that improve the distribution of carbides and slightly reduce their size.

3. High-intensity plastic deformation. As the degree of deformation increases, carbides are partially crushed and their distribution is improved (especially when special deformation techniques are used).

4. Increase in the rate of crystallization. This is the principle behind powder metallurgy technology. In order to increase the cooling rate, the ingot size must be reduced. At ingot size of about 150 microns, the cooling rate reaches 104105 k/s, at such speeds and sizes eutectic (liquid solution crystallizing at the lowest temperature for alloys of this system) is very thin, and the size of carbides does not exceed 23 microns. In order to achieve this it is necessary to apply the powder method or powder conversion method.

Powder method (powder conversion).

Remaking – one of the stages of metal production or processing in ferrous and non-ferrous metallurgy. To the processing include: melting and casting of metal, crimping, rolling, pipe and hardware production. The essence of the technology of powder metallurgy method consists in obtaining powders of pure metals and multi-component alloys with their subsequent step-by-step waste-free transformation into ready-to-use materials, products and coatings of the required functional parameters.

 

Properties of Powders

Metal powders vary in their physical, chemical and processing properties. The category of physical properties includes the particle size and particle size distribution, their specific surface area characteristics, as well as their density and deformability, which is called microhardness.

The set of chemical properties is determined by the chemical composition of the raw materials and the method/method of manufacture. The permissible concentration of undesirable impurities in finished powder products should not exceed the value of 1.5-2%. One of the most important chemical properties is the degree of gas saturation of the powder, which is especially important for powders produced by reduction, from the composition of which it is difficult to remove a certain part of gaseous reducing agents and reaction products.

The main methods of making powders from raw materials are:

1. Physical and mechanical method

In this method, the raw material is converted into powder without disturbing the chemical composition, by mechanical grinding, both in the solid aggregate state and as a liquid melt. Physical and mechanical grinding is carried out by crushing and milling; atomization and granulation. When crushing and milling solid raw materials, the original particle size parameters are reduced to specified values.

2. Chemical-metallurgical method

This method of obtaining metal powders can also be realized in a variety of ways, among which are the most popular:

• Chemical recovery of metal from raw materials (reduction method). It uses various chemical substances-reductive agents, which affect the salts and metal oxides to separate the non-metallic fraction (salt residue, gases).

• Electrolysis – the method of manufacturing powders consists in the deposition of particles of pure metal on the cathode under the influence of direct current on the corresponding electrolyte in the form of solution or melt.

• Thermocarbonyl dissociation (carbonyl method). Carbonyl powders are made by decomposition in a given temperature regime carbonyl metal compounds into the initial components: particles of pure metal and gaseous carbon monoxide CO, which is removed.

• The process of manufacturing powder steel includes a number of stages: preliminary preparation of the powder mixture (charge); molding; sintering.

• Preliminary preparation of powder mixture

• The transformation of already manufactured metal powder into final products begins with the preliminary preparation of the initial mixture (charge), which will be subsequently subjected to molding and sintering. The process of preparation of the initial charge is three-stage and is carried out sequentially in the form of: annealing, then sorting into fractions (classification) and directly mixing.

Recrystallization annealing of powders is necessary to improve their ductility and pressability. By annealing, residual oxides can be reduced and internal stress, the naklep, can be removed. For annealing, the powders are heated in a reducing and protective gas or vacuum environment.

Classification of powders is carried out by their separation into fractions (depending on certain size parameters of particles) using special vibrating sieves with cells of appropriate diameters. Air separators are also used for separation into fractions, and centrifugal dispersed sedimentation is used to classify liquid mixtures.

The powder material is directed by a turbine-driven air stream into the separation area, where centrifugal force separates and settles the heavy coarse particles, which are removed in the downward direction through a discharge valve. The fine light particles are drawn upwards by the cyclone air flow and are directed for additional separation.

Mixing is the most important of the preparatory operations, it is carried out by preparing a homogeneous substance – charge – from metal powders of different chemical and granulometric composition (alloying additives of powders of non-metallic elements are possible). The homogeneity of the charge depends on how thoroughly mixing takes place, which is extremely important for the final functional properties of the finished metal-ceramic products. Most often mixing of powder components is carried out mechanically with the use of special mixers. Mixing, not accompanied by grinding, is performed in continuous mixers of drum, screw, paddle, centrifugal and other types. At the end of the process, the charge is thoroughly dried and sieved.

Forming

Forming (shaping) in powder metallurgy is a technological stage, the purpose of which is the compaction of a given amount of ready bulk charge entering the mold and its compression to give the form dimensions of the product ready for subsequent sintering. Deformation of particles during molding by its genesis can be simultaneously elastic, brittle and plastic. In most cases, the charge is molded by placing it in sturdy steel molds and then pressing it under pressure from 30 to 1200 MPa using mechanical, pneumatic or hydraulic presses.

Baking

The final stage of the powder metallurgy process method is the heat treatment of the molded billets. It is carried out by sintering. Sintering is one of the most critical process procedures within the PM process, whereby low-strength billets are transformed into exceptionally strong sintered bodies. In the course of sintering, gases adsorbed in the billets are removed, undesirable impurities are burned off, residual stresses in the particles and contact points between them are removed, oxide films are eliminated, diffusion transformation of the surface layer takes place, and the shape of pores is qualitatively transformed. Sintering is carried out by two methods: solid-phase (no liquid melt of one of the components is formed as the blanks are heated), and liquid-phase. Sintering results in a metal bar or plate that becomes the basis for the knife.

Benefits of powder steels

Due to the small size and uniform distribution of carbides in powder steels, the degree of alloying and the volume of the carbide phase can be significantly increased, thereby increasing the resistance properties of the steel. Better mechanical properties are achieved, in particular powder steels are much better at grinding and forging. When steel is quenched, a more saturated solid solution, finer and more uniform grains are obtained, which contributes to a certain increase in hardness, heat resistance, mechanical properties and corrosion resistance. Powder technology makes it quite easy to produce high-nitrogen steels by solid-phase nitriding methods. In general, powder processing has almost no disadvantages, improving all qualities of steel.