This is the diffusion of Boron into the steel surface to form Metal Borides (Fe2B, CrB, MoB2). Although this process is not new, it is underutilised in the South African market and is largely unknown to most engineers and heat treaters. Boriding is also referred to as Boronising.
Boriding contributes unique properties that are very suited to severe high wear environments and can also be applied to stainless steels and nickel alloys with very high hardness’s achieved. See here for the advantages of using Boriding for pump sleeves.
The boriding process normally takes place at temperatures between 800 °C and 950 °C. Boriding is not a coating process but rather a surface conversion. As such, properties and structure of the resulting layer are strongly influenced by the base material properties and alloying elements.
The advantages of boriding are typically:
- radical increase of surface hardness
- radical increase in abrasive wear resistance
- greater hardness than carburized or nitride layers
- greater case depth than nitriding and nitrocarburising
- excellent adhesion due to diffusion mechanism
- no grinding or finishing required after process
- minimal to negligible size change
- certain steels can be through hardened directly from the process
- increase in corrosion resistance due to ceramic layer
- no increase in surface roughness in most cases
- can be selectively processed to target areas
- suitable for very small components
- can be done on blind holes and recesses
- stainless steels can be treated for high surface hardness of ~ 80 HRc (eqv)
For ferrous materials such as cast iron and steels, the hardness of borided layers reach values of 1600 to 2000 Vickers. Nickel based alloys can also benefit from boronising and hardnesses of up to 2800 Hv can be achieved. For popular austenitic steels such as 304, 316 and 904 stainless steels, hardnesses are from 1600 – 2200 Hv approx.
Thickness of Boride Layer
Since the process is diffusion based, both time, temperature and alloy content have an affect on layer thickness. Typically the range is ± 20μm for high alloy steels, and up to 0.3 mm for low alloy steels.
The greater the alloy content, the shallower the layer thickness and the less tooth like the structure. The image below shows the typical cross-section of an unalloyed steel or cast iron.
Boriding also significantly improves the corrosion resistance of mild steels and low alloy steels in acid environments. This is beneficial for many applications where corrosion may accelerate wearing conditions such as in pumps and valves.
Austenitic Stainless Steels can also be borided very successfully with typical layer thickness of 20-30μm and very little drop in corrosion resistance. Since there is no microstructural phase change when austenitic steels are processed, there is very little size change. On polished austenitic stainless surfaces, exceptionally good surface finishes can be achieved with superior scuff and scratch resistance.
To read more about boriding of pump sleeves please click here.
Boriding can considerably increase the resistance of low alloy steels to acids. Borided austenitic stainless steel also shows excellent corrosion resistance in hydrochloric and other acids: as shown in the figures below with 1045 steel on the left and 321 Stainless steel on the right in a warm acid at 56°C. Click on the image for an enlarged view.
Contact Us for Assistance
Boriding is a treatment not without its drawbacks and limitations. Limitations can be regarding size, geometry, area requiring processing etc.
It is best for interested parties to contact us specifically regarding their application to determine case by case suitability. A drawing or sketch of the part will be necessary to determine pricing and suitability.