Why Bearing Steel Needs Forging and Preparatory Heat Treatment
High-carbon chromium bearing steel such as GCr15, covered by GB/T 18254 in China and by the ball and roller bearing steel section of EN ISO 683-17 internationally, is highly alloyed and hardens to a very high hardness. That combination also makes the cast structure coarse and chemically segregated, with carbides concentrated in bands and at grain boundaries.
Forging breaks up the cast structure and closes internal porosity, and the preparatory heat treatment that follows brings the steel to a soft, machinable and structurally uniform condition before the rings are turned and finally hardened. Skipping or shortening these steps shows up later as distortion, cracking or short bearing life.
From Ingot to Billet: Cropping and Diffusion Annealing
The route begins with ingot casting. The top of the ingot, which contains the shrinkage pipe and the heaviest segregation, is cropped off so that the defective material does not enter the product. This cropping step is decisive for bearing quality, because inclusions and porosity concentrated in the discarded cap cannot be removed by any later process.
The cropped ingot is then given a diffusion anneal, a long high-temperature soak that lets alloying elements and carbon redistribute through the section. Diffusion annealing reduces the macrosegregation inherited from casting and softens the ingot enough for the first forging operation. It is a slow, energy-intensive treatment, but it produces a far more uniform starting structure than the same steel supplied as continuously cast bloom.
Forging Practice and Carbide Segregation
Forging is carried out in stages. The ingot is first upset and worked into a polyhedron so that deformation acts in several directions, which breaks up the dendritic network and disperses the banded carbides. The blank is then forged to the required bar or ring section with a controlled forging ratio so that the deformation reaches the centre of the section.
Temperature control is critical. The initial forging temperature must not be too high, because overheating coarsens the austenite grain and can cause burning at the grain boundaries; it must also not be so low that the steel is worked in a brittle range. After the final forging pass the blank is cooled in a controlled way, using dispersed air cooling or spray cooling, so that carbide networks do not precipitate along the prior austenite grain boundaries.
Bearing parts are commonly produced by the forging route rather than by simple continuous casting because the forged blank has a sounder original structure and a more uniform carbide distribution, which is essential for rolling contact fatigue resistance.
Spheroidize Annealing and Network Carbide Control
After forging and blanking, the material is spheroidize annealed. The treatment holds the steel just below the lower critical temperature for a long period and then cools it slowly, converting the lamellar carbide into small globules dispersed in a ferritic matrix. The result is a soft, machinable structure with an even carbide distribution and a hardness low enough for turning, and it also prepares the structure so that the final hardening treatment gives a fine, uniform martensite.
Spheroidize annealing is applied in different forms to bars, plates or blocks, wire and pipe, and to rolled rings, because the section and the intended forming operation differ. Where forging or cooling has left a network of carbides along the grain boundaries, a normalising treatment is used to break the network before spheroidize annealing, since an unbroken network is difficult to remove afterwards and acts as a crack path.
Inspection and Quality Points
Check the cropping position and discard weight, so that the shrinkage pipe is fully removed.
Record the diffusion anneal cycle, because under-soaking leaves segregation that the forge cannot remove.
Control the forging start and finish temperatures, with no reheating that coarsens the grain.
Control the post-forging cooling rate to avoid carbide networks at the grain boundaries.
Verify spheroidize annealing by hardness and by metallographic assessment of the carbide shape and distribution.
Assess carbide banding, decarburisation depth and internal soundness before the blank is released to machining.
FAQ
Q: Why is the top of the ingot cropped?
It contains the shrinkage pipe and the worst segregation, which cannot be repaired later in the process.
Q: What does diffusion annealing achieve?
It reduces macrosegregation and softens the ingot, giving a more uniform structure before forging.
Q: Why is the ingot upset to a polyhedron first?
Working the steel in several directions breaks up the dendritic network and disperses banded carbides.
Q: What is the purpose of spheroidize annealing?
It produces globular carbides in a ferritic matrix, lowering hardness for machining and preparing the structure for hardening.
Q: How are network carbides removed?
A normalising treatment is applied to break the network before spheroidize annealing.
Q: Does the forging route cost more?
Yes, the ingot, diffusion anneal and forging sequence costs more than continuous casting, but it delivers a better original structure and higher reliability.
