Butterfly valves are often made from CF8M stainless steel, and although the corrosion resistance is high, the wear resistance is relatively low. Traditional hard coatings of these parts such as thermal spray have two distinct issues:
- they show an adhesion issue once the coating is chipped, and
- the surface roughness is high and to smooth this out is costly, if not impossible.
Boriding diffuses into the surface therefore does not suffer from adhesion problems. It leaves a smooth surface (basically as smooth as it was prepared to before coating) and the surface hardness is .1600 Hv (~80 HRc) which is significantly harder than tungsten carbide. The corrosion resistance of the CF8M is also still high. Boriding is a win on many fronts.
Pump sleeves have proven to be an effective application of the boriding technology by dramatically increasing the wear resistance of the sleeve exterior. Typically, pumps see very high wear on pump sleeves and this necessitates using expensive processes such as hard chrome, thermal spray etc to combat the wear action.
The preparation required for these processes are expensive, and after coating, the sleeve has to be ground again – which again adds cost.
Coatings can also suffer from adhesion failure where delamination or chipping can occur. Sub-layer corrosion from porosity or cracks in the coating can also cause delamination leading to a sharp and deep scar that quickly slices the packing seal and requires immediate replacement.
In contrast, Borided pump sleeves can be made from conventional hollow bar material such as ST52, machined to the final size and given a basic and inexpensive preparation process before treatment. Very light finishing off is only required after the Boriding treatment. Boriding converts the surface to ceramic iron boride which diffuses into the surface of the sleeve with a hardness of over 75 HRc. Due to the saw-tooth like diffusion pattern, chipping and delamination does not occur and wear is progressive.
Boriding is a simple, high performance, cost effective process. An overall better economy is achieved with Boriding with regards to wear performance and reduced cost.
The advantages of Boriding for pump sleeves are the following:
- no complicated preparation or finishing off of parts
- smooth, even wear with no delamination or release of hard particles
- boriding is a smooth, pore free coating and hence no corrosion initiation points.
- no under-coating corrosion at the coating/steel interface
- boriding is highly resistant to thermal shock
- Stainless steels, nickel based alloys and cobalt based alloys can also be treated with little or no corrosion resistance but with dramatic improvement in wear resistance.
For more information concerning your particular application, please contact us.