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.
