Hard alloys over the past two decades have become very widespread in the industry. They are used in the mining industry – for drilling, metalworking industry – for cutting, stamping and drawing, as well as for surfacing wear parts.
The wide spread of hard alloys in the industry is explained by the fact that tools equipped with hard alloys allow many times to increase the productivity of existing equipment and reduce the cost of manufactured products and that parts directed by hard alloys work abrasion significantly (sometimes tens of times) longer than non-melted parts.
The main constituent of all alloys are carbides of metals: tungsten, molybdenum, chromium, titanium, manganese. Carbides impart high hardness and wear resistance to alloys. In addition, the composition of hard alloys include cobalt, nickel, iron.
Solid alloys are divided into cast, powdered and cermet.
Cast and powdered hard alloys.
These alloys are used for surfacing wear parts.
Cast solid alloys – stellite and stellite-like – are distinguished by high corrosion resistance, in particular in sulfuric acid; They retain their resistance at high temperatures (stellites – up to 8000, stellite-like up to – 6000).
Stellites and srirmat are widely used in engineering for surfacing parts and tools that work without impact, and where the part after machining must be smooth and clean (mainly in sliding friction), for example: for bending and drawing matrices, machine centers, measuring Staples, rings for broach. In view of the high heat resistance of these alloys, they are also used for surfacing parts working at high temperatures, for example: for parts of metallurgical equipment, hot-cutting knives, valves of internal combustion engines.
Surfacing of cast hard alloys can be performed on steel (iron) and cast iron parts, regardless of their cross-section and configuration. Covering the working surface of the part with a layer of alloy is produced with the help of a gas burner with an acetylene-oxygen flame.
Powdered solid alloys – VKAR and STEINITE – are used mainly for welding parts that produce rough work, where the maximum number of pores and shells is allowed and treatment of the welded surface is not mandatory (cheeks of crushers, excavator teeth, excavators, etc.).
Vokar contains 86% of tungsten, 9.5 – 10.5% of carbon, up to 0.5% of silicon and up to 2.5% of iron; Stannite – 16-20% chromium, 8-10% carbon, 13-17% manganese to 3% silicon, the rest – iron.
Powder-like hard alloys are welded by the direct-current arc by the method of Benardos (using a carbon electrode). The surface to be welded is installed horizontally, a thin (0.2-0.3 mm) layer of flux (calcined borax) and a layer of powdered hard alloy (batch) with a thickness of 3 to 5 mm are applied to it. The electrode is connected to the negative pole, the part is connected with a positive . The electric arc formed between the electrode and the workpiece melts the burden and adjacent layers of the parent metal, and a small bath of the molten hard alloy and base metal is formed. The electrode is given a progressive zigzag motion, and the arc is continuously transferred over the surface of the hard alloy.
Metal-ceramic hard alloys
These alloys are applied in the form of plates to the cutting tool. Tools with hard alloy plates are now widely used in the factory practice for high-speed cutting of metals.
A characteristic feature of metal-ceramic hard alloys is their high hardness and the ability to maintain cutting properties at temperatures up to 1000 – 11000.
The main cutting component of metal-ceramic hard alloys is tungsten carbides; Some grades of alloys contain, in addition, titanium carbides. Cobalt is used as the binding metal.
For the manufacture of plates of metal-ceramic hard alloys, the powdery components are thoroughly mixed and the mixture is pressed under a pressure of 1000 to 4200 kg / cm2. Semi-finished products obtained in molds are placed in an electric furnace, where their sintering takes place at a temperature of 1400-15000. During sintering, the binding metal (cobalt) melts and, wrapping the carbide grains, binds them. In the production of hard alloys, pressing and sintering operations are often replaced by a single operation – hot pressing.
Plates of hard alloys serve to equip tools, drills, cutters and other tools. The equipment is made by soldering the plates on the holders or by mechanically attaching the plates to the holders.
Light metals and their alloys
Alloys of aluminum with silicon
Called also silumin, in the technique find use of silumin, close to the eutectic composition (from 6 to 13%). These alloys have good casting properties (high fluidity and low shrinkage), high density and increased mechanical properties compared to aluminum. Increased mechanical properties are achieved by modifying, consisting in processing molten silumin with a modifier (metallic sodium or a mixture of fluorine salts of sodium and potassium). A small amount of modifier (about 0.01% by weight) dramatically changes the structure of silumin: the crystals become shallow, and the fracture acquires a velvety appearance. Silumin, not subject to modification, have a coarse-grained structure and inferior mechanical properties.
When introducing a small amount of magnesium and manganese into the composition of silumin, their mechanical properties are further improved.