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What is the purpose of carburizing low-carbon steel castings?
Improve surface hardness and wear resistance
Low-carbon steel castings have low carbon content, insufficient surface hardness and wear resistance. Carburizing can significantly increase the carbon content on the surface of steel parts. After quenching and tempering, tempered martensite and carbides form on the surface. The hardness reaches HRC58-64, greatly improving wear resistance. This makes it suitable for gears, shafts, and other easily worn parts.
Enhance fatigue strength
Carburizing creates a high-carbon layer on steel part surfaces. It produces residual compressive stress, counteracts tensile stress from alternating loads, slows fatigue crack growth, and helps parts endure greater alternating loads. Improve fatigue strength, such as engine crankshafts that can withstand more complex stress changes and extend service life after carburizing.
Improve anti-seizure performance
For moving parts like sliding bearings and piston pins, carburizing improves surface hardness and finish. It lowers the surface friction coefficient, reduces metal contact and biting tendency, allows normal operation under high speed and heavy load, and prevents damage or equipment failure caused by biting.
Ensure the toughness of the core
During carburizing treatment, the core of low-carbon cast steel still maintains a low carbon content. After proper heat treatment, the core has good toughness and plasticity, so that the parts have high hardness and high strength on the surface. At the same time, the core can withstand impact loads and bending loads. It is suitable for mechanical parts under complex loads. This ensures the parts are less prone to breakage during use. It effectively resists surface wear and fatigue damage.
The operation of low-carbon cast steel carburizing process is as follows:
Preparation work
Material selection: Select low-carbon cast steel with a carbon content of 0.1%-0.25%, such as 20 steel, 20Cr, etc., to ensure the toughness of the core and the carburizing effect.
Equipment inspection: Check the heating, temperature control, ventilation and other systems of the carburizing furnace to ensure normal operation and furnace temperature uniformity within ±5℃.
Workpiece processing: Process the workpiece to a size close to the final size, leaving a 0.5-1.5mm carburizing processing allowance, use sandpaper or shot blasting to clean the surface oil, rust, etc., and dry after cleaning.
Furnace loading
Reasonable placement: The workpieces are evenly placed on the carburizing furnace basket or fixture, with a spacing of 10-20mm to ensure furnace gas circulation and uniform carburizing.
Furnace loading control: Load the furnace according to the rated furnace loading capacity of the equipment to avoid excessive impact on the uniformity of the carburizing atmosphere and heat transfer.
Carburizing process
Heating stage: Heat up to 850-950℃ at a rate of 10-20℃/min. Pay attention to the temperature uniformity in the furnace as the furnace heats up.
Insulation stage
Strong infiltration period: After reaching carburizing temperature, introduce enriched gases like propane or butane. Control carbon potential at 1.0%–1.2%. The strong carburizing time depends on the required case depth, usually lasting 2 to 6 hours.
Diffusion period: After strong carburizing, introduce diluent gases like nitrogen or carbon dioxide. Lower furnace carbon potential to 0.8%–1.0%. Maintain diffusion for 1–3 hours to equalize surface carbon concentration.
Temperature and carbon potential control: Use thermocouples and other precise temperature control, and the deviation is controlled within ±10℃. Use oxygen probes and other methods to monitor and control the carbon potential in real time to ensure the quality of carburizing.
Quenching and cooling
Quenching: After carburizing, the workpiece is cooled to 820-860℃ with the furnace and kept warm for 30-60 minutes, and then quenched with oil cooling or water-soluble quenching agent. Small pieces can be quenched with salt water.
Tempering: Temper in time after quenching, tempering temperature 180-220℃, keep warm for 2-4 hours, eliminate internal stress, improve toughness and stability.
Quality inspection
Depth of carburized layer inspection: Use metallographic method, hardness method, etc. to measure the depth of carburized layer, and the deviation is controlled within ±0.1mm.
Surface hardness inspection: Use Rockwell hardness tester to measure surface hardness, generally the surface hardness after carburizing and quenching is HRC58-64.
Metallurgical structure inspection: Observe the metallographic structure, the carburized layer should be fine needle-shaped martensite and evenly distributed carbides, and the core is low-carbon martensite or ferrite + pearlite.
What is the appropriate thickness of the carburized layer of low-carbon cast steel?
The carburizing thickness of low-carbon cast steel needs to be determined according to the specific service conditions and performance requirements of the parts. The following is a reference for the appropriate carburizing thickness in different application scenarios:
Gear parts
General mechanical transmission gears: For small and medium modulus gears in mechanical transmission, carburizing thickness is usually 0.8-1.2mm. This provides excellent wear resistance and contact fatigue strength on the surface. The core retains sufficient toughness to withstand normal transmission loads.
Heavy-loaded high-speed gears: For heavy-loaded high-speed gears in aircraft engines and large reducers, carburizing thickness is 1.2-2.0mm. This enhances the gear’s wear resistance, anti-bonding, and anti-fatigue capabilities. It helps the gear cope with complex conditions of high speed and torque.
Shaft parts
Shafts that bear medium loads: such as machine tool transmission shafts, the carburizing thickness is 0.5-1.0mm, which can improve the wear resistance and contact fatigue strength of the journal surface, and ensure the accuracy and reliability of the shaft in long-term operation.
Shafts that bear heavy loads: such as rolling mill roller shafts, carburizing thickness of 1.0-1.5mm, can make the shaft surface have high hardness and strength, the core maintains good toughness, and resists wear, deformation and fatigue damage under heavy load.
Mold parts
Small cold stamping die: carburizing thickness of 0.5-0.8mm can improve the surface hardness and wear resistance of the mold, so that the mold is not easy to wear and scratch during the stamping process, ensuring the quality of stamping parts and the life of the mold.
Large hot forging die: carburizing thickness of 1.0-1.8mm, enhances the high temperature strength, wear resistance and thermal fatigue resistance of the mold surface, so that it can withstand high temperature, high pressure and repeated impact loads, and extend the service life of the mold.
Other parts
Piston pin: carburizing thickness of 0.8-1.2mm, can make the piston pin surface hard and wear-resistant, the core tough and impact-resistant, meet its working requirements in the engine.
Camshaft: The carburizing thickness of an automobile engine camshaft is typically 0.6-1.0mm. This improves wear and fatigue resistance on the cam surface. It ensures accurate valve opening and closing control. The carburizing process extends the camshaft’s service life.