In solid carburizing parts loaded into crates along with a carburizer, a substance containing carbon. The carburizer is a mixture of charcoal with carbonate salts (activators), enter in the amount of 20...40%. The addition to coal, carbonate salts (ВаСО3, Na2CO3, K2CO3) activates carburizer due to the formation of carbon dioxide during the decomposition of salts and reactions of coal (ВаСО3=VAO+CO2; CO2+C=2СО). The optimal size of the solid particle carburizing is 3...5 mm. Carburizing box is isolated from the external environment, accurately the joints and cracks special refractory coatings, in placed in an oven heated to 900...950° C.
For gas carburising, use a variety of gases containing carbon: carbon monoxide, saturated hydrocarbons (CH2n+2)— methane, ethane, propane, butane, natural gas, etc. Gas is prepared separately. The gas carburizing temperature of 920...950°C.
The cementation process is achieved by a better distribution of carbon in depth of the surface layer of the part. Therefore, to obtain high hardness and wear resistance of the surface layer in the viscous core of the part after carburizing is subjected to annealing (850...900°C) and low tempering (180...200°C). Cemented layer items after this treatment has a hardness...НRCэ58 62 and the core was of the order of НRСэ 25...35.
Liquid carburizing is used for hardening of steels at low depth of 0.2 mm. It is carried out in molten salts 75 ...85% Nа2СО3 and 10...15% NaCl with the addition of 6% to 10% silicon carbide (SiC); the latter, interacting with the baking soda decomposes and carbon is allocated atomically. The process is conducted at a temperature of 815...850°C depending on the composition of the steel.
In recent times there are recommendations on the application of a vacuum carburization carried out at a temperature of 1040°C (heat. 45 min, exposure time 32 min, the layer depth of 1.25 mm), followed by quenching. This process has several advantages: high speed of cementation, good surface finish, no internal oxidation, a small consumption of carburizing, there is no need to getprivatefield installations. The treatment process is fully automated.
Cemented parts of alloy steels after quenching are recommended to be subjected to treatment with cold (-40...-70°C) with subsequent low vacation. Treatment with a cold dwell time is not less than 2 h. cold Processing is carried out to complete the process of transformation of residual austenite in the structure of the cemented layer to martensite, resulting in increased hardness (HRC>61) and wear resistance of the cemented layer and stabiliziruemost the dimensions of the parts.
Nitriding is the saturation of the surface layer of steel parts in nitrogen in the environment of ammonia or mixtures of ammonia and nitrogen at a temperature of 500...700°C.
The duration of nitriding for 20—80час. The depth of the nitrided layer is 0.1 to 0.5 mm, hardness НV650...1100. The content of nitrogen in the surface layer up to 10...12%.
Distinguish between corrosion and prochnostnye nitriding.
Nitriding is used to improve hardness, wear resistance, fatigue resistance and corrosion resistance of parts operating in the atmosphere, water, steam, etc.
Prochnostnye nitriding is the most effective method of a sharp increase in the surface hardness, strength, durability. High surface hardness of nitrided parts is not changed when re-heating to 600-650°C. the Nitrided machine parts, working in conditions of friction and alternating loads, bending under rotation. The most common nitride is steel, which after nitriding, the hardness reaches НV960...1150.
At temperatures above 400°C, the ammonia dissociates (reaction НN3=3N+N) with the formation of atomic nitrogen. Atomic nitrogen is absorbed and diffuses into the interior of the surface layer, forming a nitrogen phase.
With many alloying elements nitrogen also forms nitrides.
The temperature range for nitriding steel parts is determined by the requirements to the depth and hardness of the layer. When high hardness and a small layer depth is recommended to use low temperature; at great depths, and lower hardness is used a higher temperature; for deep and high hardness is applied two-stage mode.
Nitrocarburizing (cyanidation) is a process of simultaneous saturation of the surface layer of steel parts with nitrogen and carbon. The nitrocarburizing distinguish gas and liquid. When nitrocarburizing surface layer is saturated with carbon and nitrogen from a mixture of ammonia (2...6%) and carburizing gases (coal, propane, etc.) or liquids (prebensen, Tintin, triethanolamine, etc.). When cyanidation is the saturation of the salt melt containing cyanide salts NaCN, CA(CN)2, are suppliers of active atoms of carbon and nitrogen. The nitrocarburizing is divided into low temperature (500...600°C) and high temperature (800...950°C). The thickness of the hardened layer is 0.15 to 0.5 mm.
The dwell time in the gas nitrocarburizing is selected depending on the desired depth of layer and temperature of the process. At the high-temperature nitrocarburizing of the average rate of formation of the hardened layer is 0.08...0.1 mm/h.
Cieniowane thickness of the layer depends on the exposure time. 1 hour of exposure at the working temperature of 820 ...860°C it is possible to obtain a layer thickness of 0,25...0,35 mm.
After cyanidation (nitrocarburizing) the parts are subjected to hardening in oil or water to low and leave at a temperature of 150...170°C for 5h. Gas nitrocarburizing of structural steel ensures a hardness of HRC 58.
Borisovna — the process of saturation of the surface steel (iron) parts of the forest. Used to improve surface hardness (up to НV1300—2500) low alloy cast irons, carbon and high-alloyed steels and special alloys. High surface hardness of the parts increases their wear resistance, especially in conditions of abrasive wear.
The borated hardness of the surface is preserved during the heating to a temperature of ~750°C. the Borated layer has a high heat resistance and acid resistance.
For parts with low specific loads borated layer depth can be selected in the range of 0.25—0.4 mm, at high specific loads, the depth of the layer should not exceed 0,2 mm.
Diffusion boride coatings are used primarily to improve wear resistance. They also have a higher resistance to heat, corrosion and erosion resistance, fire resistance compared to the base metal. Boronovanii steels and refractory metals is carried out by two methods: liquid phase (electrolytic and betalactaminam) and gas-phase.
Borisovna electrolysis is carried out in a melt of borax. Sometimes it is added additives to reduce melting temperature, increase fluidity and facilitate the intensification of the process of saturation, for example, boric anhydride, sodium chloride, yellow blood salt. In this method, the cathode is barely metal anode or graphite electrodes, either the metal crucible, which is barrowsii melt.
Boronovanii in liquid media without electrolysis is usually performed in molten borax. To it add small amounts of substances can serve as reducing agents and form due to the difference in electrode potentials from the saturated metal of the galvanic element: the metal—electrolyte—reductant.
Gas Borisovna carried out in two main ways:
- way of powders, which moreruela the parts are placed in an airtight container, covered with boron powder and heated in a standard furnace with air, neutral or protected environment or under vacuum;
- method of chemical vapor deposition of boron on the surface of the heated parts of the reduction reaction are the halides of boron (ВС13, ВІ3 IN Вг3) hydrogen with the subsequent diffusion of boron into the metal parts, this method is also used of a mixture of borodatov with hydrogen.
Powder boronovanii is carried out at a temperature of 930...1100°C. For boronovanii used powder amorphous boron, boron carbide, ferroboron, ferroboron to which is sometimes added inert fillers (sand, alumina, chamotte).
Regardless of the method of borating the rate of growth of the diffusion layer, its structure and phase composition are determined by three main factors: temperature, process duration and activity buriraya environment. Typically, the boriding temperature is 850...1100°C and soaking time of 1...10h.
Litrovaya — the process of saturation of the surface steel or cast iron aluminum at a temperature of 660...1100°C to increase the heat resistance. Litrovaya subjected to mainly low-carbon steel.
Chrome plating — the process of saturation of the surface steel and cast iron with chromium at a temperature of 950...1050°C to increase the corrosion resistance, kislotoupornye and surface hardness. Chrome plating is subjected to medium and high carbon steel. Chrome steel has okalinolomatel to 800...850°C, high has acid resistance and a high corrosion resistance in a sodium chloride solution.
Siliconizing — process of saturation of the surface steel and cast iron in silicon at high temperatures. Used to increase the resistance of products in nitric, sulfuric, hydrochloric acids.
The sulfidation process of saturation of the surface parts of sulfur. Used to improve wear resistance, improve running and extreme pressure properties of parts. The sulfidation is subjected to the crank shafts, valve tappets, guides, slides and screws lathe machine, molds for injection molding. The depth of the layer is 0.1 mm and above.