By heating the surface of steel partscoolingAnd change the surface layermechanical property Metal heat treatment process.Surface hardeningIt is the main content of surface heat treatment to obtain high hardnessSurface layerAnd advantageousinternal stressDistribution to improveworkpieceWear resistance and fatigue resistance.[1]
By heating and cooling the surface layer of the workpiece and changing the structure of the surface layer, the metal heat treatment process with the required properties can be obtained.The surface heat treatment of steel parts can obtain martensite structure with high hardness on the surface, while retaining the toughness and plasticity of the center, improving the comprehensive mechanical properties of the parts.For example, for some shafts, gears and parts bearing variable load, surface heat treatment can be carried out to make the surface have a high wear resistance and greatly improve the overall fatigue resistance of the workpiece.The main content of surface heat treatment is surface quenching of steel, which can be divided intoFlame surface hardeningandInduction surface hardening。
Classification and principle
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Surface hardening
The workpiece is quickly heated by different heat sources, and when the surface temperature of the part reachescritical pointWhen the temperature above (at this time, the temperature of the workpiece center is below the critical point), cool it quickly, so that the surface of the workpiece is quenchedHard tissueThe heart remains the original tissue.In order to only heat the surface of the workpiece, the heat source used is required to have a highenergy density 。According to different heating methods,Surface hardeningCan be divided intoinduction heating(High frequencyintermediate frequency, power frequency) surface quenching, flame heating surface quenching, electric contact heating surface quenchingelectrolyteHeating surface quenchingLaser heatingSurface quenching, electron beam surface quenching, etc.Induction heating andFlame heatingSurface hardening.
Chemical heat treatment
Place the workpiece in the medium containing active elements for heating and insulation, so that the active atoms in the medium penetrate into the surface of the workpiece or form a compoundoverburdenTo change the structure and chemical composition of the surface layer, so that the surface of the part has special mechanical or physical and chemical properties.Generally, other appropriate heat treatment is required before and after chemical infiltration, so as to maximize the potential of the infiltration layer and achieve the best match between the core of the workpiece and the surface in terms of structure, performance, etc.Depending on the elements infiltrated,Chemical heat treatmentCan be divided intocarburization、nitriding 、Boronizing、Siliconizing、Sulfurizing、Aluminizing、Chromizing、Zincifying、CarbonitridingAluminum chromizing, etc.
Contact resistance heating and quenching
Apply a voltage of less than 5V to the workpiece through the electrode. A large current flows through the contact between the electrode and the workpiece, and a large amount of resistance heat is generated to heat the workpiece surface toquenching temperature The electrode is then removed, and the heat is transferred to the inside of the workpiece and the surface is cooled rapidly to achieve the purpose of quenching.When processing long workpieces, the electrode moves forward continuously, and the parts left behind are hardened continuously.This method has the advantages of simple equipment, convenient operation, easy automation, minimal distortion of the workpiece, no need for tempering, and can significantly improve the wear resistance and scratch resistance of the workpiece, but the hardening layer is thin (0.15~0.35mm).The uniformity of microstructure and hardness is poor.This method is mostly used for cast ironMachine tool guideOfSurface hardeningThe application scope is not wide.
Electrolytic heating quenching
Place the workpiece in the solution of acid, alkali or saltelectrolyteMiddle, the workpiece is connected to the cathode,Electrolytic cellConnect anode.After the DC power is connected, the electrolyte is electrolyzed, oxygen is released on the anode, and hydrogen is released on the workpiece.Hydrogen forms a gas film around the workpiece and becomes a resistance body to generate heat, rapidly heating the workpiece surface toquenching temperature Then power off, the gas film disappears immediately, and the electrolyte becomesQuenching mediumThe surface of the workpiece is cooled and hardened rapidly.The common electrolyte is 5~18%sodium carbonateIn water.The electrolytic heating method is simple, the processing time is short, the heating time is only 5~10 seconds, the productivity is high, and the quenching distortion is small. It is suitable for the mass production of small parts, and has been used for engine exhaustValve rodEndSurface hardening。
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Laser heat treatment
The application research of laser in heat treatment began in the early 1970s, and then entered the production application stage from the laboratory research stage.When the focus is highenergy density When the laser (10W/cm) irradiates the metal surface, the metal surface rises toquenching temperature 。As the temperature of the irradiation point rises very fast, the heat cannot be transferred to the surrounding metal in time, so it is stoppedLaser irradiationThe metal around the irradiation point will riseQuenching mediumIt absorbs a large amount of heat due to the effect of, which makes the irradiation point cool rapidly, obtaining extremely fine structure and high mechanical properties.If the heating temperature is high enough to melt the metal surface, a smooth surface can be obtained after cooling. This operation is called polishing.Laser heatingIt can also be used for local alloying treatment, that is to say, the parts of the workpiece that are easy to wear or need heat resistance shall be coated with a layer of wear-resistant or heat-resistant metal, or a layer of coating containing wear-resistant or heat-resistant metal shall be coated, and then laser irradiation shall be used to melt it rapidly to form wear-resistant or heat-resistant metalheat-resisting alloyLayer.A layer of chromium shall be plated on the parts that need heat resistance, and then laser shall be used to melt them rapidly to form hard resistancetemperingThe service life and heat resistance of the workpiece can be greatly improved by the chromium containing heat resistant surface layer of the workpiece.
Electron beam heat treatment
Research and application began in the 1970s.It was used for continuous annealing of thin steel strip and steel wire in the early stage, with the energy density up to 10W/cm.Electron beam surface quenching shall be carried out in vacuum, and other characteristics are the same as that of laser.Whenelectron beamWhen bombarding the metal surface, the bombarding point is heated rapidly.The penetration depth of the electron beam through the material depends onAccelerating voltageandMaterial density。For example, the theory of 150 kW electron beam on iron surfacepenetration depthApproximately 0.076 mm;It can reach 0.16 mm on aluminum surface.The electron beam bombards the surface in a very short time, and the surface temperature rises rapidly, while the substrate remains cold.When the electron beam stops bombarding, the heat is quickly transferred to the cold base metal, so that the heated surface is quenched by itself.In order to effectively carry out "self cooling quenching", the volume of the whole workpiece and the volume of the quenched surface layer must be kept at least 5:1 ratio.The surface temperature and quenching depth are also related to the bombardment time.Electron beam heat treatmentHigh heating speed,AustenitizingThe time is only a few tenths of a second or even shorter, so the workpiece surfacegrainIt is very fine, has higher hardness than the general heat treatment, and has good mechanical properties.
equipment
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According to the basic process of heat treatment, heat treatment equipment is divided into main equipment and auxiliary equipment.Main equipment includesHeat treatment furnaceHeat treatment heating equipment, cooling equipment, etc.Auxiliary equipment includes inspection equipment, calibration equipment, cleaning equipment, quenching medium circulation equipment, fire prevention and dust removal equipment, etc.
1. Heat treatment furnace
Heat treatment furnace is a kind of heating equipment for heat treatment.The temperature is generally lower than that of the heating furnace, so it is required to strictly control the furnace temperature and furnace atmosphere.It is composed of furnace body, heating device, mechanical device and electrical equipment.According to different energy sources, it can be divided into fuel furnace and electric furnace;According to the working temperature, it can be divided into high temperature furnace (higher than 1000 ℃), medium temperature furnace (600-1000 ℃) and low temperature furnace (lower than 600 ℃);It can be divided into air furnace, controlled atmosphere furnaceFlowing particle furnaceVacuum furnace, glow ion furnace, etc;It can be divided into periodic furnace, semi continuous furnace and continuous furnace according to operation procedures and mechanization degree;According to different process uses, it can be divided into normalizing furnace, annealing furnace, tempering furnace, quenching furnace, carburizing furnace, nitriding furnace, etc;According to different shapes and furnace shapes, it can be divided into box type furnace, well type furnace, trolley type furnace, pusher type furnace, rotary hearth furnace, vibrating hearth furnace, conveyor belt furnace, etc;According to the degree of specialization, it can be divided into general furnace and special furnace.[2]
2. Cooling equipment
Common cooling equipment includes water tank, oil tank, bath furnace, slow cooling pit, etc.The media include: tap water, brine, engine oil, nitrate solution, etc.
3. Inspection equipment
Commonly used inspection equipment includes Rockwell hardness juice, Brinell durometermetallurgical microscopePhysical property tester, measuring tool, NDT equipment, etc.
