When you add the right amount of ferroboron alloy, most of the boron stays in solid solution – that's the good kind. But once you go over the limit, the nitrogen and oxygen in the steel are already "used up" and can't tie up any more boron. That's when the extra boron starts forming coarse borides or carbon-borides instead. And that's exactly when the trouble begins.
What Happens When You Overdo Boron?
1. Prone to Cracking at High Temperatures
- This is the most obvious issue. When boron content is too high, a layer of coarse borides precipitates at the grain boundaries, destroying the "bonding force" between grains. During hot rolling or forging, cracks form directly on the surface of the steel billet; in severe cases, the entire piece may break.
Decreased Impact Toughness
- Steel used for gears and drive shafts must be impact-resistant. If boron levels exceed the standard, impact energy absorption can be cut in half. When used in parts, they will fracture immediately upon encountering impact loads.
Reduced Service Life
- Excess boron forms hard inclusions, such as boron nitride. Under repeated stress, cracks begin to propagate from these inclusions. Gears will soon develop tooth damage, and springs will snap during use.
Cracks After Welding
- High-boron steel has very poor weldability. Fine cracks are prone to form near the weld, which may not be visible immediately after welding but will open up after some time or when subjected to stress.
Difficult Cold Working
- Cold heading bolts? They crack. Turning? The cutting tools wear out extremely quickly. Bending sheet metal? It cracks as soon as it's bent. In short, it's a hassle to work with in every way.
How much boron should be added?
Tolerance limit of boron varies across steel grades. Below is the industry-recognized specification range:
| Steel Grade Type | Optimal Boron Content (Mass Fraction) | Approximate Critical Hazard Threshold |
|---|---|---|
| Alloy Structural Steel (e.g. 40CrB, 20MnB) | 0.0005% ~ 0.0035% | > 0.007% |
| Gear Steel | 0.0005% ~ 0.0030% | > 0.006% |
| Spring Steel | Normally no boron addition | - |
| Non-Quenched & Tempered Steel | 0.0005% ~ 0.0025% | > 0.005% |
| Low-Carbon Boron Steel | 0.0005% ~ 0.0040% | > 0.008% |
Core Principle: Boron dosage shall be precisely controlled in ppm unit (1 ppm = 0.0001%), instead of rough calculation by kilogram.
Is there a remedy for ferroboron overdose?
To be honest – no, not really. Once boron forms those coarse borides in the steel, normal heat treatment like normalizing, annealing, or quench-and-tempering won't take them out. The only way is to remelt the whole heat. And that is an expensive lesson.
If you have any questions regarding the ferroboron addition process or boron content control for specific steel grades, please feel free to contact us. With extensive practical experience in ferroboron applications and the addition of special alloys, we can provide you with specific process recommendations.
