Question:

I know that the effects of boron to the grade 4130 allow a deeper hardness penetration. What are the short- and long-term negatives that come with 4130 with high boron?

Answer:

The normal range for boron as an element to increase hardenability in 4130 (or any low alloy steel) is 0.0005-0.003 weight %. We are not quite sure what you mean by "high boron," but we assume it is somewhere in this range. Your question seems to be directed at what the effect of boron is when it is properly added and properly protected (usually by Ti to tie up nitrogen) such that the desired hardenability effect is realized (and is within the 0.0005-0.003% range). We were not completely sure if you were asking if there is a negative effect if it is present in amounts greater than 0.003%. 

Many boron grades (including 41B30) are used with no negative short- or long-term issues. But various issues can and do occur depending on a variety of different factors. In carburized components, for example, boron will increase core hardenability and can actually decrease case hardenability.

Note that while boron is usually used to increase hardenability, there are applications that use boron in a manner to increase toughness rather than hardenability. Also, while the common way to "protect the boron" is via titanium addition, there is also a method to add enough boron (without any purposeful Ti addition) to tie up all the free nitrogen and have enough free boron remaining to provide the hardenability increase. When used to increase toughness, the increased hardenability is not desired and the steelmaking approach is different.

How the 4130 is to be used (for example, through-hardened, induction hardened or carburized) will also have an effect. Depending on how you plan to process and use the steel will change the answer because the effects of boron additions can be affected in this manner.

There are some good books that discuss boron in steel in detail. An excellent synopsis of the effects of a large number of steelmaking elements including boron can be found in the book Ferroalloys and Alloying Additives Handbook by Deeley, Kundig and Spendelow published by Shieldalloy Corp. and Metallurg Alloy Corp. in 1981. It is available from several used-book dealers online. You might also refer to my Heat Treat Doctor article “Boron in Steel” published in the May 2007 issue of Industrial Heating by clicking here.