Effects of boron, molybdenum, and titanium on brittleness induced by tempering of structural steels

Conclusions

• 1. The effect or boron on the susceptibility of carbon steels and alloyed steels to reversible brittleness resulting from tempering depends on the composition of the steel. Boron added to pure iron or steel not susceptible to embrittlement by tempering has very little effect on the sensitivity of the material to the cooling rate after tempering. However, the addition of boron to steel susceptible to embrittlement by tempering increases this susceptibility, and the higher the residual concentration of boron, and also the concentration of phosphorus and manganese, the higher the susceptibility.

• 2. The addition of 0.1% Ti to low-carbon steel increases its embrittlement and increases its susceptibility to reversible brittleness resulting from tempering. The lower the concentration of carbon in the steel (i.e., the higher the amount of titanium outside the carbide phase) the more dangerous the effect of titanium and the lower the acceptable concentration of titanium.

• 3. Small amounts of titanium decrease the grain size and decrease the tendency of austenite grains to grow as the result of heating, and also prevents stone-like fracture. The concentration of titanium in structural steels should be 0.02–0.06%.

• 4. The addition of a certain amount of molybdenum has a favorable effect; an increase of the concentration above the optimum value decreases this favorable effect and causes the embrittlement of the steel even after rapid cooling from high tempering temperatures. The optimum concentration of molybdenum in structural steel depends on the carbon concentration of the steel.