Boriding (also referred to as Boronizing), is the diffusion of Boron into the surface of a steel. The resulting surface layer is smooth and continuous and of exceptional hardness. The surface hardness of the Borided mild steel exceeds 1600 Hv (~75 HRc) which is a carbide-like hardness, and which makes it highly wear resistant.
Boriding is one of the few processes can surface harden stainless steels such as 304 and 316, achieving a surface hardness of ± 1800 Hv (~80 HRc). This provides the stainless with exceptional anti-galling, sliding wear resistance and very good abrasion resistance.
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:
- significant increase in abrasive and adhesive wear resistance
- no flaking due to diffusion mechanism
- no grinding or finishing required after process
- minimal size change
- increased in corrosion resistance on steels
- smooth and continuous layer prevents under-film corrosion
- 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
- Nickel-based alloys can also be treated
Formation and Thickness of the Boride Layer
Boriding forms with a very familiar saw-tooth pattern in the a steel while on stainless a smooth, thin layer forms due to the high alloy content. 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 as shown below. The image below shows the typical cross-section of an unalloyed steel or cast iron.
|Boride Layer on Mild Steel||Boride Layer on Stainless Steel|
Corrosion Resistance Acid Resistance
Boriding significantly improves the corrosion resistance of mild and low alloy steels, particularly in acid environments. This is beneficial for many applications where corrosion may accelerate wearing conditions such as in pumps and valves.
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 (En8) 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 is applied to components in the following areas:
- pumps and valves
- pipes and tubing
- brick making
- wear resistant machine components
To read more about boriding in pump and valve applications, please click here.
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.