The purpose of surface quenching is to obtain a martensitic structure on the surface of the casting, improving its wear resistance while retaining good plasticity and toughness in the core.
Before surface quenching, the volume fraction of pearlite in the casting should not be less than 50%, and preferably greater than 70%. To achieve this, the casting can be pre-treated with normalizing and tempering to increase the pearlite content.
If the pearlite content is too low, increasing the quenching temperature can still harden the casting, but this will result in more retained austenite and secondary cementite.
Induction Heating Quenching
The induction power supply power, power density, and heating time are determined based on the casting dimensions and the required hardened layer depth. For example, for wear-resistant castings suitable for medium loads, the hardened layer depth is generally 0.25~1.5mm, thus a frequency of 10kHz~2MHz and a power density of 10.9~18.6W/mm² can be used. The quenching medium can be water, oil, or air cooling.
When the volume fraction of pearlite in the as-cast microstructure is greater than 70%, a quenching temperature of 900–925°C is used; when the volume fraction of pearlite in the as-cast microstructure is greater than or equal to 50%, a quenching temperature of 955–980°C is used.
This applies to ductile iron castings pre-normalized and tempered at 900°C. Increasing the number of graphite nodules prolongs the heating cycle, and increasing the amount of pearlite in the original microstructure increases the depth of the hardened layer.
For medium-load wear-resistant castings, the hardened layer is relatively shallow; for castings subjected to heavy loads or impact loads, such as gears, crankshafts, and camshafts, the hardened layer depth is in the range of 1.5–6.4 mm. Induction heating is suitable for quenching the inner and outer cylindrical surfaces, as well as flat and toothed surfaces of castings. The gap between the induction coil and the casting should be as uniform as possible. For irregularly shaped castings, the quenching hardness is often uneven, and overheating is prone to occur at sharp corners.
Flame Hardening
Pearlitic ductile iron, such as ISO 700-2 and ISO 600-3, is suitable for flame hardening. The required carbon content is between 0.35% and 0.8% by mass, and the pearlite volume fraction is 70% to 100%. If the carbon content is below 0.35%, the hardness after quenching will be low; if the carbon content is above 0.80%, the casting is prone to cracking.
The heating rate of ductile iron is slower than that of steel. ISO 400-15 ferritic ductile iron, after flame heating at 845-870°C and water quenching, has a hardness of 35-45 HRC. Ferritic + pearlitic ductile iron, such as ISO 500-7, has a hardness of 40-45 HRC after flame hardening. Pearlitic + ferritic ductile iron has a hardness of 50-55 HRC after quenching. Pearlitic ductile iron achieves a hardness of 55-60 HRC after flame heating and water quenching, and 56-59 HRC after oil quenching. Flame quenching depth can reach 0.8-6.4 mm, using an oxy-acetylene flame or an oxy-propane flame with weaker combustion intensity. Neutral or slightly carburizing reducing flames are preferred; oxidizing flames may cause decarburization and overheating. The flame movement speed must be slower than that of steel, and the distance between the high-temperature flame zone and the casting must be controlled to prevent overheating and localized melting.
Flame quenching is suitable for castings requiring localized quenching, such as camshafts, rolls, and gears with complex structures. Flame quenching requires inexpensive equipment but demands high operational skills.
Laser Heating Quenching
The advantages of laser heating quenching include precise control of input power, high power density, and minimal casting deformation. Through an optical system, the laser can reach the internal cavities of castings that are difficult to reach using conventional methods, thus enabling localized quenching of complex-shaped or large castings.
A 15kW CO2 laser was used to generate a 10mm diameter focused spot via an optical system. The scanning range was 25mm along the processing direction at a frequency of 700Hz, and the scanning frequency perpendicular to the processing direction was 125Hz, forming a 22mm × 25mm rectangular spot to heat the cam surface. To form a uniform hardened layer, the camshaft was rotated. To increase laser energy absorption, a coating of manganese phosphate was applied to the cam surface. The input power was 9kW, the power density was 1600W/cm², the processing linear velocity for the cylindrical portion was 7600mm/min, and for the planar portion it was 180mm/min. The hardened layer (≥5OHRC) depth was 0.55mm.
A 400~450W laser was used to scan 10mm × 10mm × 55mm ferritic ductile iron impact specimens with the following parameters: spot diameter 2mm, scanning speed 2~10mm/s, hardened layer depth 0.2~0.5mm, and surface hardness 600~800HV. The laser-hardened layer consists of four layers from the surface inward: a layer of fine fibrous ledeburite, a layer of martensite + spheroidal graphite, a layer of martensite + ferrite + spheroidal graphite (surrounded by martensite), and a layer of martensite + ferrite + pearlite + spheroidal graphite.
Post time: Dec-01-2025