Question
What is boronizing, and how does it differ from other case-hardening processes like carburizing or carbonitriding?

Answer
Boron is added to steels for its unique ability to increase hardenability when present in concentrations as low as 0.00005% (the TTT curve is shifted to the right). It has long been used as a replacement for other alloying elements in heat-treatable steels due to both cost and availability. And boron is used to control phase transformation and microstructure in HSLA steel sheet.

When boron is used as an alloying element in plain-carbon and low-alloy steels, it is added to increase the core hardenability and not the case hardenability. In fact, boron can actually decrease the case hardenability in carburized steels. Boron “works” by suppressing the nucleation (but not the growth) of proeutectoid ferrite on austenitic grain boundaries. Boron’s effectiveness increases linearly up to around 0.002%, then levels off. Boron concentrations over 0.003% are seldom used due to hot working and embrittlement concerns.

Boron is much more effective at low carbon levels with its contribution falling to zero as the eutectoid carbon content is approached. As a result, it is best to limit case carbon contents to about 0.70%C. Boron is unique among alloying elements in that its hardenability factor increases with the amount of martensite, having a relatively greater effect on a section that can be fully hardened as opposed to one having quenched to, say, 50% martensite. In addition, boron neither raises nor lowers the Ms temperature and has no effect on retained austenite. It does not change the fineness of pearlite or produce any solid-solution strengthening. There is also a slight (but tolerable) increase in susceptibility toward temper embrittlement. Finally, the Ti:N ratio in the steel will influence the effectiveness of the boron addition.

The ASM Handbook, Volume 4, pages 437-447 does a nice job of presenting comparative data on steels that have been borided versus carburized or carbonitrided, nitrided or nitrocarburized. Table 2 in that article provides some interesting information (see below), but the reader should review the entire article as it covers the uses, advantages, disadvantages and a host of other information that is quite useful.

For example, the hardness of the borided layer depends on the composition of the base steel. From Table 2:

Process/Hardness Range (HV)

Carburized, low-alloy steel/650-950
Nitrided steels/650-1700
Borided mild steel/1600
Borided H13 die steel/1800
Borided A2 tool steel/1900
Hard Cr plate/1000-1200

It should be noted that borided layers "have extremely high hardness values (between 1450 and 5000 HV) with high melting points of the constituent phases." (Note: The hardness of the boride layer depends on the base element. For example, FeB is 1900-2100 HV, Fe2B is 1800-2000 HV, while Ti2B is 3000 HV.)