Hybrid coatings on centrifugal fans

Cen­trifu­gal fans in steel indus­try are often exposed to rough and chal­leng­ing con­di­tions like vibra­tions, high tem­per­a­tures and abra­sive par­ti­cles. While vibra­tions can most­ly be avoid­ed by bal­anc­ing mea­sures and tem­per­a­ture resis­tance achieved by the right choice of mate­r­i­al and dimen­sion­ing of cool­ing discs, a resis­tance against abra­sive par­ti­cles still is a chal­lenge for many cen­trifu­gal fans. So the dust load in the air as well as the size, the speed or the hard­ness of the par­ti­cles deter­mine the run­ning time of a cen­trifu­gal fan in a steel plant. That was the main incen­tive for TLT Tur­bo to invest great efforts to con­sid­er­ably increase the run­ning time of cen­trifu­gal fans in dust loaded conditions.

Already since approx. 1970, TLT-Tur­bo has per­formed par­ti­cle jet wear exper­i­ments on var­i­ous fan mate­ri­als and coat­ings. For that, TLT-Tur­bo used a par­ti­cle jet test stand, where a stan­dard­ized test sand (with stan­dard­ized grain size dis­tri­b­u­tion) was blast­ed under a defined pres­sure / speed as well as spe­cial defined angles to sur­faces of dif­fer­ent fan mate­ri­als and coat­ings. By exact­ly mea­sur­ing the mate­r­i­al loss after a defined amount of sand / time of blast­ing, TLT-Tur­bo deter­mined ero­sion rates that cor­re­spond with the wear resis­tance of the inves­ti­gat­ed mate­ri­als and coat­ing. Fig­ure 2 shows the old test stand.

Research to increase running time

In 2014, a new mate­ri­als lab­o­ra­to­ry was planned and has been suc­ces­sive­ly extend­ed since then. Next to high pre­ci­sion devices for deter­min­ing an exact weight loss, coat­ing thick­ness, micro­scop­ic exam­i­na­tion of sur­faces etc., the par­ti­cle jet test stand was also upgrad­ed. TLT-Tur­bo is now able to per­form par­ti­cle jet wear exper­i­ments with orig­i­nal cus­tomer dust and dif­fer­ent speeds of the blast­ing par­ti­cles. Addi­tion­al to the find­ings of over 650 par­ti­cle jet wear exper­i­ments, TLT-Tur­bo has been able to gain an even deep­er under­stand­ing of the influ­ence fac­tors and mech­a­nism of wear on cen­trifu­gal fans. Var­i­ous approach­es to mas­ter a com­bi­na­tion of par­ti­cle-induced wear and cor­ro­sion or wear dur­ing sin­ter­ing can also be inves­ti­gat­ed on a sci­en­tif­ic base in the new mate­ri­als laboratory.

Next to the pop­u­lar hard­fac­ing solu­tions to pre­vent wear induced plant shut­downs, TLT-Tur­bo now is able to offer new and sophis­ti­cat­ed thin lay­er coat­ings like H‑101 and H‑106. The choice of these thin lay­er coat­ings is based on par­ti­cle jet wear exper­i­ments. To cre­ate the opti­mal solu­tion for the cus­tomer in order to achieve the high­est run­ning time and a com­par­a­tive­ly cheap price, TLT-Tur­bo inves­ti­gat­ed pos­si­bil­i­ties to com­bine hard­fac­ing with thin lay­er coat­ings to hybrid coat­ings. To under­stand the virtue of a hybrid coat­ing it is advis­able to take a clos­er look at hard­fac­ing and thin lay­er coat­ings first.

Hardfacing as popular wear protection

In dif­fer­ent appli­ca­tions in steel indus­try, weld­ed coat­ings on cen­trifu­gal fans have already been used since the 1980s. So in between, hard­fac­ing is a well-estab­lished and stan­dard­ized process (robot based auto­mat­ic coat­ing of impeller blades) and the costs per square meter are com­par­a­tive­ly cheap. Anoth­er advan­tage is if wear induced local dam­ages in the coat­ing occur, the cus­tomer can repair it direct­ly at the affect­ed area in the impeller. By increas­ing the num­ber of weld­ed lay­ers on the most exposed areas, the run­ning time can be fur­ther increased. The only obsta­cle is the acces­si­bil­i­ty of the area in the impeller when it is inte­grat­ed in the cus­tomer plant.

On the oth­er side, hard­fac­ing also has some dis­ad­van­tages. If applied on fine grained steel, the incom­ing heat into the base mate­r­i­al will impair the mechan­i­cal prop­er­ties of the mate­r­i­al. On a rotat­ing part like an impeller, exposed to high cen­trifu­gal forces and vibra­tions, TLT-Tur­bo is not will­ing to risk insuf­fi­cient mechan­i­cal prop­er­ties. Instead, TLT-Tur­bo welds in hard­faced plates of uncrit­i­cal base mate­r­i­al that get attached to the impeller of fine grain steel by plug weld­ing or screw­ing (if replace­able wear pro­tec­tion is required). The neces­si­ty of the uncrit­i­cal base mate­r­i­al con­tributes to the impeller weight. That dis­ad­van­tage is fur­ther increased if the weight of the hard­fac­ing itself is tak­en into account. The hard­fac­ing has a thick­ness of 3–4mm that adds to the thick­ness of blades, par­tial­ly on the cen­ter disc or even on the cov­er discs. Since cen­trifu­gal fans oper­ate below their first res­o­nance fre­quen­cy and the res­o­nance fre­quen­cy gets deter­mined by the stiff­ness of the shaft and the impeller weight, addi­tion­al weight of the impellers requires a high­er stiff­ness of the shaft. This gets imple­ment­ed by a high­er shaft diam­e­ter and, by that, high­er weight. Also big­ger bear­ings or heav­ier sub­struc­tures might get nec­es­sary. So, addi­tion­al weight on the impeller also caus­es con­sid­er­able addi­tion­al weight for the rest of the rotor, and con­se­quent­ly, addi­tion­al costs.

Anoth­er dis­ad­van­tage of cen­trifu­gal fans with hard­fac­ing is the rough sur­face. This coarse­ness caus­es aero­dy­nam­ic dis­tur­bances and has a neg­a­tive impact on the effi­cien­cy of cen­trifu­gal fans in steel indus­try. Fur­ther­more, the risk of cak­ing (adhe­sion of dust par­ti­cles on the sur­face) on blades or hard­faced areas ris­es. Fig­ure 3 shows an exam­ple of a typ­i­cal sur­face of a weld­ed coat­ing. Com­pared to sophis­ti­cat­ed thin lay­er coat­ings a weld­ed coat­ing fur­ther­more has a high­er ero­sion rate. Fig­ure 4 com­pares a typ­i­cal ero­sion rate of a weld­ed coat­ing (green) with a typ­i­cal thin lay­er coat­ing (red) and TLT-Turbo’s H‑101.

The infe­ri­or ero­sion rate of a weld­ed coat­ing (espe­cial­ly at steep angles like 50–90°) only part­ly can be com­pen­sat­ed by high­er coat­ing thick­ness. A typ­i­cal weld­ed coat­ing thick­ness lies at 3–4mm, an HVOF coat­ing often at 0.5mm.

Thin layer coatings as new alternative

For a cou­ple of years now, thin lay­er coat­ings have been in spe­cial focus of TLT-Turbo’s Anti-Wear research. Exper­i­ments on TLT-Turbo’s par­ti­cle jet test stand showed that espe­cial­ly High Veloc­i­ty Oxy­gen Fuel (=HVOF) coat­ings proved to have supe­ri­or ero­sion rates. This is a more cost­ly but also high qual­i­ty way of flame spray­ing. Espe­cial­ly in steel indus­try TLT-Turbo’s HVOF coat­ings like H‑101 are increas­ing­ly estab­lished. Table 1 shows an exam­ple of the exten­sion of oper­at­ing time by usage of dif­fer­ent HVOF coat­ings for a cen­trifu­gal fan in a Euro­pean steel plant.

Fig­ure 4 and Table 1 have already shown H‑101’s supe­ri­or ero­sion rate com­pared to typ­i­cal weld­ed coat­ings. How­ev­er, fur­ther par­ti­cle flow exper­i­ments with new chem­i­cal com­po­si­tions revealed even bet­ter coat­ings. The most promis­ing coat­ing now is H‑106. Fig­ure 5 shows a com­par­i­son between H‑106 and H‑101. In addi­tion to the improved ero­sion rate of H‑106, also a high­er thick­ness of up to 1mm is pos­si­ble. But due to price pres­sure for cen­trifu­gal fans and a slight­ly reduced coat­ing qual­i­ty at high thick­ness­es, 1mm is only rec­om­mend­ed on areas with extreme wear. A fur­ther advan­tage of the HVOF coat­ings is their smooth sur­face. In con­trast to hard­fac­ing, HVOF coat­ings have a hydraulic smooth sur­face for supe­ri­or aero­dy­nam­ic effi­cien­cy. Addi­tion­al­ly, the lack of cracks and the low poros­i­ty of TLT-Turbo´s new HVOF coat­ings pre­vent local wear and chip­ping and pro­tect the under­ly­ing base mate­r­i­al from cor­ro­sive under­min­ing. Fig­ure 6 shows a close-up view of a typ­i­cal HVOF surface.

How­ev­er, HVOF coat­ings also have a few dis­ad­van­tages. The main dis­ad­van­tage is the price per square meter which is high­er than a typ­i­cal price of a com­mon hard­fac­ing. In CAPEX ori­ent­ed mar­kets that often pre­vents the choice of a HVOF coat­ing. Anoth­er dis­ad­van­tage of HVOF coat­ings is that their robust­ness (e.g. against major mechan­i­cal impacts like a blow of a ham­mer) can´t com­pete with hard­fac­ing yet. That means it is advis­able to han­dle a HVOF coat­ing with more care than a weld­ed coat­ing. Else you risk a crack in the coat­ing which is a start­ing point for fur­ther chip­ping and expose uncoat­ed sur­faces to abra­sive dust. Fig­ure 7 shows the result of a crack in the coat­ing between a weld­ed-in coun­ter­sunk head screw and a wear blade.

Rea­sons for the crack might be a mechan­i­cal impact on the back­side of the screw, a too strong tight­en­ing of the coun­ter­sunk head screw or pre­loads caused dur­ing the weld­ing-in of the screw into the plate. Also, a repair of a HVOF coat­ing in the steel plant is expen­sive and dif­fi­cult due to the instal­la­tion situation.

Final­ly, a direct coat­ing of an impeller is often not pos­si­ble due to the acces­si­bil­i­ty of the areas to be coat­ed. The indi­vid­ual geom­e­try of the impeller and the posi­tion or the def­i­n­i­tion of the coat­ing areas deter­mines that pos­si­bil­i­ty. To coat plates like radi­al blades or exposed areas on the cen­ter disc before weld­ing is also pos­si­ble. How­ev­er, the weld­ing of the sep­a­rate parts togeth­er after coat­ing impairs the coat­ing qual­i­ty in the inter­sec­tion areas between the blades and the cen­ter disc.

Hybrid coating

Since both approach­es – Hard­fac­ing and HVOF – have their jus­ti­fi­ca­tion and are estab­lished in steel indus­try, TLT-Tur­bo searched for ways to com­bine their advan­tages. In order to add HVOF coat­ings like H‑106 on a typ­i­cal weld­ed coat­ing some research was done. Final­ly, the efforts to merge these coat­ings togeth­er were suc­cess­ful. So TLT-Tur­bo is able to pro­vide new wear pro­tec­tion pos­si­bil­i­ties for cen­trifu­gal fans in steel indus­try. Fig­ure 8 shows a suc­cess­ful merge of both coat­ings. The most rea­son­able approach for TLT-Tur­bo now is to offer a cost effi­cient and robust hard­fac­ing on the entire pres­sure side of blades and on the areas on cen­ter disc and cov­er disc that suf­fer most from wear. An eval­u­a­tion of typ­i­cal wear zones of cen­trifu­gal fans revealed that the wear is typ­i­cal­ly dis­trib­uted uneven­ly on the blades of an impeller. Often, the lead­ing edge and the out­er area of a radi­al blade are the most exposed parts and have the high­est mate­r­i­al loss. On these com­par­a­tive­ly small areas, there­fore, an addi­tion­al coat­ing like H‑106 is applied. The thick­ness of that coat­ing can also be adapt­ed, depend­ing on the wear pat­tern with­in the blade.  With that mea­sure, a typ­i­cal run­ning time with­out wear induced stops in the steel plant can be more than dou­bled com­pared to sole­ly hard­faced impellers.

A fur­ther advan­tage of the solu­tion is the com­par­a­tive­ly cheap price for the blade with only slight­ly high­er costs than a pure weld­ed coat­ing. This of course pre­sumes that not the entire blade gets the hybrid coat­ing, only those areas that are most prone for wear. Depend­ing on the increase of oper­at­ing time required by the steel plant, typ­i­cal costs for a wear pro­tect­ed impeller are shown in Fig­ure 9.

Also the cus­tomer will ben­e­fit from the robust­ness of the wear pro­tec­tion in the impeller. Mechan­i­cal impacts on the sur­face of the hybrid coat­ing only would have a small effect on the HVOF sur­face. If local chip­ping occurs, it will be lim­it­ed until the next crack of the under­ly­ing weld­ed coat­ing. If small local dam­ages emerge, they won’t leave the base mate­r­i­al exposed. In the pre­vi­ous exam­ple with the sole­ly HVOF coat­ed blades (Fig­ure 7), an under­ly­ing hard­fac­ing would have absorbed the impact and no crack would have occurred.

In con­trast to a pure HVOV coat­ing, local dam­age to a hybrid coat­ing can also eas­i­ly be repaired in a steel plant. Mea­sures to repair a hybrid coat­ing would only be tak­en if the hard­fac­ing had already been dam­aged or van­ished. All that needs to be done when local dam­ages occur is to weld a new lay­er of hard­fac­ing over the affect­ed area. This can be done on site and even by the cus­tomer himself.

Fur­ther­more, at least at the hybrid coat­ing areas, the sur­face is smooth and aero­dy­nam­i­cal­ly ben­e­fi­cial. Small cracks get filled by the over­lay­ing HVOF coat­ing and only big cracks might stay open. Fig­ure 10 showed 2 com­par­a­tive­ly deep cracks part­ly filled with H‑101.

Also, the addi­tion­al weight caused by HVOF is quite lim­it­ed due to the local hybrid areas and their thick­ness of most­ly only 500µm. So the total weight rough­ly equals that of com­mon wear pro­tect­ed impellers. If how­ev­er a pure HVOF coat­ing will be replaced by a hybrid coat­ing, e.g. at exchange­able wear blades, the thick­ness of the base mate­r­i­al of the wear blade need to be reduced to com­pen­sate the addi­tion­al weight by the welding.

Conclusion

A cen­trifu­gal fan in a steel plant is often exposed to abra­sive dust which results in mate­r­i­al loss and a short­age of run­ning time for the fan. With TLT-Turbo´s new approach of hybrid coat­ings, even most chal­leng­ing cus­tomer require­ments can be met. Espe­cial­ly with the inclu­sion of H‑106, a dras­ti­cal­ly increased run­ning time is now pos­si­ble for the most wear exposed sur­faces at the impeller.

A supe­ri­or robust­ness, which is often demand­ed by a steel plant, is ensured as well as the customer’s option to repair dam­ages inde­pen­dent­ly and self-direct­ed­ly. The costs for the new hybrid coat­ing are also only slight­ly high­er than those of a com­mon hard­fac­ing. To use the advan­tages of the new hybrid coat­ing in your steel plant, TLT experts can be con­tact­ed at any time.