Boriding is also referred to as Boronising, is the diffusion of Boron into the steel surface to form Metal Borides (Fe2B, CrB, MoB2). The surface layer is smooth and continuous, of exceptional hardness and highly wear resistant.

Boriding can also be applied to stainless steels and nickel alloys with very high hardness’s achieved.

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 (even on mild steel)
  • significant increase in abrasive and adhesive wear resistance
  • smooth and continuous layer prevents under-film corrosion
  • no flaking due to diffusion mechanism
  • no grinding or finishing required after process
  • minimal to negligible size change
  • increased in corrosion resistance on steels
  • 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.)

Layer Hardness

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 hardness of up to 2800 Hv can be achieved. For popular austenitic steels such as 304, 316 and 904 stainless steels, hardness’ are from 1600 – 2200 Hv.

Boronising Graph1

Thickness of Boride Layer

Boride cross sectionSince the process is diffusion based, both time, temperature and alloy content have an affect on layer thickness. Typically the range is 20 – 50 µm for high alloy steels, and up to 0.2 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. See here for further info on Boriding of valves and pump sleeves.

Austenitic Stainless Steels can also be borided very successfully with typical layer thickness of 20-30μm and very little drop in corrosion resistance.  On polished austenitic stainless surfaces, exceptionally good surface finishes can be achieved with superior scuff and scratch resistance.

Borided 304 Ball valve

To read more about boriding of pump sleeves please click here.

Acid Resistance

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.

Boriding Acid resistance

Contact Us for Assistance

Boriding is a treatment not without its drawbacks and limitations. Limitations can be regarding size, geometry, area requiring processing etc. Contact us regarding the application and to determine the part’s suitability for Boriding. A drawing or sketch of the part will be necessary to determine pricing and suitability.