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Engine Crankshaft Induction Hardening Process
In the domestic main engine factories, crankshaft parts factories, induction hardening has become the main means of strengthening the crankshaft, and continue to expand the application. Crankshaft using induction hardening for surface strengthening treatment, technical and economic indicators are ideal.

1. Crankshaft induction hardening advantages
(1) Steel crankshaft by induction quenching + low temperature tempering (or autothermal tempering), compared with the modified state can improve the crankshaft fatigue strength of 100% or more, while greatly improving the wear resistance of the journal surface (quenching + back Fire hardness can be precisely controlled at 50 ~ 55HRC), and other means of strengthening is difficult to achieve the above two indicators at the same time.
(2) Induction hardening production efficiency is high, and induction hardening and can be produced by the beat, can be directly arranged in the crankshaft production line, saving logistics costs and time.
(3) Only the crankshaft on the journal and other hardened surface hardening surface heating, and high efficiency, time is short, lower than other heat treatment energy consumption by 80% or more.

(4) Due to the small heating range, short time, so that the deformation of the crankshaft deformation, less oxidation decarbonization, can reduce the finishing allowance, reduce the workload of machining and so on.


2. Material of induction hardened crankshaft
(1) Crankshaft material car crankshaft commonly used materials are quenched and tempered steel 42CrMo, 35CrMo, 40Cr, 40MnB, non-quenched and tempered steel 48MnV, and ductile iron and so on. Materials such as Cr, Mo and other alloy components can significantly improve the hardenability of materials, thereby enhancing the strength of the crankshaft, but its price is higher, and quenching cracking tendency, the need to use the appropriate quenching and cooling medium to avoid quenching cracks, Should be considered to meet the performance requirements of the premise of preferred ductile iron and non-quenched and tempered steel to reduce production costs. As long as the tooling is appropriate, the process is reasonable, accurate operation, whether non-quenched or quenched steel or ductile iron crankshaft, induction hardening can achieve technical requirements.
(2) The preparation of rough preparation of quenched and tempered steel quenched and tempered treatment can be meticulous uniform organization, parts of the internal stress is very good, the induction hardening is very good: quenching uniform, quenching deformation and cracking tendency is small. Non-quenched and tempered steel using V, Ti and other elements to join the refinement of grain, in a substantial reduction in production costs while improving the mechanical properties, but its preparation is different from the quenched and tempered steel, so in the development of induction heat treatment process According to its organizational characteristics to be analyzed.

The pretreatment of ductile iron crankshaft is mainly composed of both positive and arcing. Normalizing treatment can improve the pearlite content, eliminate casting stress, the organization is also more uniform, and then by induction hardening easy to get tissue, hardness uniform hardened layer. Casting ductile iron rough, as-cast pearlite content is generally 75% to 85%, more ferrite, to get the ideal quenching organization, the induction quenching process requires a higher. Therefore, the development of the process to consider the different pre-heat treatment on the impact of induction hardening.


3. Specification for heat treatment of induction hardening
(1) the preparation of induction hardening process to achieve the desired results, and heating power, the workpiece material, control gap, heating time, cooling time, the workpiece heat, quenching and cooling medium concentration and pressure are related. Different materials should be used for different materials, such as alloy steel toughness is good, suitable for high-power heating and rapid cooling, and ductile iron is brittle material, not suitable for high power, long heating and quenching, heating time and cooling time should According to the actual situation of the field equipment, the workpiece hardened layer requirements. Alloy steel and ductile iron crankshaft IF induction quenching process parameters.
(2) The technical requirements of induction hardening are: surface hardness, hardening zone, hardened layer depth, metallographic structure, quenching deformation and quenching crack. Quenching hardening layer Metallographic structure: steel crankshaft needle or fine needle-like tempering martensite, should not appear free ferrite; ductile iron crankshaft to allow in the vicinity of spherical graphite with a small amount of undissolved ferrite, but not ring. Hardened layer depth and surface hardness is an important indicator of high fatigue strength, have a best range, too high and too low to reduce the fatigue strength. Hardening layer depth is too shallow and the surface hardness is too low to reduce the strength of parts, wear resistance; the other side when the hardening layer depth is too deep, the compressive stress peak from the surface inward, the surface compressive stress decreases, so that the strength decreases. Hardness is too high to bring the harm is obvious, it makes the parts of the brittleness increased in the crankshaft by the bending and torsional fatigue load and the impact of the case of a serious reduction in strength.

(3) After quenching after tempering induction hardening, the surface of the organization from pearlite into martensite, resulting in the existence of residual thermal stress workpiece, the organization of stress, the workpiece must be tempered in time, the general provisions of the 4 ~ 8h , Otherwise it will cause the crankshaft surface cracking.


4. Induction hardening operation essentials
(1) The surface to be treated should be no cracks, defects, burrs, oil and decarburization, etc., or quenching after quenching crack, burn the workpiece, hardness is not enough defects.
(2) Design and manufacture or use of sensors, the structure and size should be able to meet the technical requirements.
(3) The inductor and the workpiece in the process, should maintain a suitable relative position, control the gap in the 1.5 ~ 2.5mm.
(4) The correct choice of electrical parameters, so that the device is in the best working condition. Steel crankshaft selection power at (100 ± 5) kW, ductile iron crankshaft selection (75 ± 5) kW. Steel crankshaft can be treated with higher power, because of its good ductility, suitable for quenching; ductile iron is brittle material, suitable for lower power quenching, or particularly easy to produce quenching cracks.
(5) The workpiece surface temperature measurement is very important, should use the photoelectric pyrometer or infrared radiation thermometer, continuous tracking measurement control workpiece temperature, keep the heat from 150 ~ 220 ℃, timely adjustment of equipment operating parameters, the use of waste heat to achieve the workpiece Self-tempering, which is also a key measure to prevent quenching cracks.
(6) Determine the cooling parameters according to the material, the shape of the workpiece, the size and the heating method and the required hardening layer depth, including the cooling method, the quenching and cooling medium and the cooling time.

(7) Ductile iron crankshaft rounded corner is not suitable for hardening, hardened layer should leave the fillet 3 ~ 6mm, so hardened and non-hardened area at the junction of the residual tensile stress away from rounded corners to improve fatigue strength, to prevent rounded corners Excessive stress caused by early non-fatigue fracture.


5. Application
Ductile iron crankshaft induction hardening plus rounded roll, more and more crankshaft manufacturers used by the composite reinforcement process will improve the use of reasonable fatigue strength, and greatly improve the surface roughness and wear resistance, Operable.
The following is a model of QT800-3 four-cylinder crankshaft IF induction hardening example, the effect is better than other surface heat treatment.
(1) Processing process iron mold casting → normalizing treatment → high temperature to tempering → roughing → a flaw detection → induction quenching → medium temperature tempering → secondary flaw detection (quenching crack detection) → finishing.
(2) Induction hardening process specifications according to the crankshaft induction quenching technical requirements
(3) Equipment and tooling equipment is dedicated to the crankshaft semi-automatic quenching equipment, SCR intermediate frequency power supply, saddle-shaped sensor, the concentration of 5% to 8% PAG quenching fluid.

(4) After the test results after crankshaft quenching the hardened layer of the journal for the 3.0 ~ 4.0mm, quenching + tempering surface hardness of 45 ~ 52HRC, hardened area and the side of the spacing in the 5 ~ 6.5mm, quenching Hard area microstructure martensite 6 to 7, no quenching crack; quenching + tempering radial round runout ≤ 0.5mm (measuring the middle main journal), to meet the technical requirements.


6. Conclusion

Crankshaft is one of the most ideal applications of induction heat treatment technology. After crankshaft induction and quenching, the performance of the crankshaft is greatly improved, which is beneficial to improve the engine performance and greatly reduce the production cost. Induction hardening technology is increasingly used in the production of high-end crankshaft products. At the same time affect the quality of quenching a lot of key factors, such as equipment, tooling, materials, process parameters, equipment tooling maintenance, process optimization, process monitoring, can effectively control the quality of quenching with high power engine demand Increase, crankshaft induction hardening technology will be a wider range of applications, crankshaft manufacturers will also get a good income.


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