The crankshaft bears the maximum load and all the power in the engine, bears the strong direction of the constantly changing bending moment and torque, and also withstands the wear and tear for a long time in high-speed operation, therefore, the crankshaft material is required to have high rigidity, fatigue strength and good wear resistance.
Crankshaft quality and performance requirements are high: tensile strength (body) бb ≥ 650 MPa, elongation δ ≥ 3%; hardness (HB) 250 or more, quenched hardness (HRC) 55 or more; crankshaft surface roughness of Ranbsp;6.3 ~ Ranbsp;3.2, crankshaft size accuracy of CT5 ~ CT6 level; crankshaft after processing, does not allow any naked eye visible defects.
Xinyuanzhu Group adopts shell process for mass production of small and medium-sized cast crankshafts with various grades such as QT650-3, QT700-4, QT780-3 and QT900-5, etc. The more common types of defects, causes and countermeasures when crankshafts are produced by shell process.
1.Cratering, shrinkage defects
It is a unique scrap phenomenon of ductile iron, and one of the common defects of crankshaft.
1.1 Analysis of the causes of internal crankshaft shrinkage and shrinkage
Causes and solidification mode, solidification sequence, the carbon equivalent of the iron and riser shrinkage, the temperature of the iron and cooling medium and other factors have a lot to do with.
1.11 Carbon equivalent is low
Low carbon equivalent reduces the precipitation of eutectic graphite during solidification, which reduces the role of graphitization expansion and increases the solidification shrinkage, and also reduces the fluidity of the molten iron, which in turn reduces the self-replenishing shrinkage capacity of the molten iron and makes the crankshaft easy to produce shrinkage holes or shrinkage loosening.
1.12 Low shell stiffness
For a long time after casting, ductile iron is solidified in a plastic thin shell with low strength. When solidification, the graphitization of eutectic expansion is very large, and the consequent eutectic expansion force is also very large, coupled with the role of hydrostatic pressure of ferrofluid, it is easy to make the crankshaft wall case outward, so that the crankshaft shape expansion, can also produce shrinkage, shrinkage.
1.13 Pouring temperature is not qualified
Pouring temperature is too high, will make the liquid metal liquid shrinkage increase; pouring temperature is too low, and will reduce the riser''s ability to fill shrinkage, causing the crankshaft to produce shrinkage holes or shrinkage loose.
1.14 Poor breeding
When ductile iron is treated with ferrosilicon for breeding before pouring, if the breeding is poor, it will lead to the precipitation of a large amount of carburized body when the iron solidifies, thus increasing the solidification shrinkage and producing shrinkage holes or shrinkage loosening.
1.15 Irrational design of pouring system and gate
The incorrect location and size of the internal gate may cause graphitization expansion of the molten iron during solidification, and the molten iron will flow backwards through the internal gate to the straight sprue, thus making the crankshaft produce shrinkage holes and shrinkage loosening. Improper design of the gate size and gate neck size may cause the crankshaft not to achieve sequential solidification, which may lead to crankshaft shrinkage and shrinkage.
1.16 Poor cooling effect
Shell-type process using shell back-fill iron shot process, iron shot temperature is an important parameter. When the iron shot temperature is low, the cooling speed is fast, and the crankshaft has high strength and dense organization; on the contrary, the iron shot temperature is high when it is easy to produce shrinkage and shrinkage defects.
1.2 Measures to prevent internal crankshaft shrinkage, shrinkage and loosening
1.21 Stable control of chemical composition
For ductile iron crankshaft, the factors affecting shrinkage and shrinkage are mainly carbon equivalent and carbon content, so the first problem to be solved is how to control carbon equivalent and carbon content in a stable manner. With the melting process of scrap steel plus carbon additive, the absorption rate of carbon additive directly affects the carbon content of the molten iron, and the particle size and the order of adding carbon additive will affect the absorption rate of carbon additive.
1.22 Strengthen the ability of the riser to compensate for shrinkage
The vertical pouring process has the advantages of dense crankshaft organization and easy floating of dross, but the low temperature of the molten iron at the riser leads to poor shrinkage effect of the riser. The test results often use the spherical bubble, the spherical bubble ferrofluid tension is larger, the shrinkage effect is significantly improved than the cylindrical bubble.
1.23 Reasonable determination of pouring temperature range
If the pouring temperature of molten iron is too high, the tendency of liquid shrinkage will be large and shrinkage holes will be easily produced; if the pouring temperature is low, the liquidity of molten iron will be reduced, and it will also lead to insufficient reaction of heating block and weaken the effect of shrinkage. After years of production experience, it is believed that 1400-1440 ℃ is the most ideal pouring temperature.
1.24 Control casting stiffness
After loading, the degree of iron shot shaking solid is controlled automatically, and fixed equipment parameters are set to avoid human factors affecting the effect of shaking solid, which ensures that the shell does not swell the box under the pressure of the iron liquid and well ensures the rigidity of the shell.
1.25 Strengthening the cooling of iron pellet
The temperature of iron shot is the core parameter of the shell type process, which has a great influence on the internal quality of the crankshaft and its mechanical properties. In summer, the high temperature and insufficient cooling of iron shot often lead to the concentration of shrinkage holes and shrinkage defects. The iron shot cooling equipment is the key.
1.26 Local Excitation Cooling
As the crankshaft journal and connecting rod neck at the ferrofluid thermal effect is stronger, often leading to shrinkage defects. If necessary, you can also locally use the cooling material like zirconium sand to improve the local cooling effect.
2.Slag porosity defect
Slag hole is also one of the main defects of the crankshaft in the author''s factory. Slag pores are small, numerous and diffusely distributed, mainly distributed on the upper surface of the crankshaft balance block and shaft shoulder, etc.
2.1 Analysis of the causes of slag porosity defects
The main reason is the oxidation of the iron liquid after spheroidization treatment, the dross melting point is low, good mobility, easy to enter the casting in the filling process. In addition, the spheroidized treatment of the molten iron in the transport, pouring package, pouring and filling process will have tumbling, splashing phenomenon, in this process will also occur in the molten iron oxidation.
2.2 Prevent slag porosity defects countermeasures
2.21 Reasonable determination of pouring process temperature range
The lowering of temperature is the root cause of iron oxidation, for this reason the pouring temperature is increased from about 1360 ℃ before to 1390 ~ 1450 ℃ at present, and controlled at the upper limit in winter, which effectively prevents the rapid oxidation of iron at low pouring temperature.
2.22 Reasonable choice of filter
In order to reduce slag hole defects, the author''s factory has been using straight-hole ceramic strainers in the production of crankshafts. Small crankshafts generally use a filter mesh with an aperture of 1.8 mm×1.8 mm, and large crankshafts use a filter mesh with an aperture of 2.2 mm×2.2 mm. In order to further improve the filtering effect of the filter screen, the foam filter screen is tested and used to replace the straight hole filter screen.
In addition, the strength as well as the wall thickness of the strainer must be ensured to avoid scrap caused by the scouring of the iron liquid.
Operational aspects need to be noted.
2.231 After the spheroidization process, the slag should be carefully picked up.
2.232 It is better to spread a layer of covering agent before pouring.
2.233 Smooth and fast pouring process to prevent turbulent flow of molten iron
2.234 Clean up the slag adhering to the pouring ladle in time.
Xinyuanzhu Group specializes in the production of Ductile iron castings, Pig iron gray iron casting, Steel casting, Brass Casting, Malleable Iron Casting, and undertakes batch casting processing of castings. Welcome guests to cooperate with pictures or samples.