Spraytime

There is a certain sinking feeling that settles in when an essential piece of equipment goes offline unexpectedly. A machine that is fundamental, the beating heart of a manufacturing operation, suddenly ceases to beat, and a clock begins ticking. Each minute the unit is down, product is not being made, people are idle, and costs continue to add up without revenue to maintain the delicate balance of efficiency. I hate to even talk about it for fear that it might jinx us, but, until machines can be made of indestructible stuff, wear, outage, and downtime will continue to be a part of our lives.

This truth was fundamental to the early development of thermal spraying. A process that could quickly replace worn metal with new metal, without distortion or compromise, became an instant solution for problems that had plagued machines since the dawn of the Industrial Age. Before materials scientists began looking to spraying as a way of enhancing surfaces, technicians and entrepreneurs were out on the streets lugging wire spray guns and cylinders of oxygen and acetylene into factories, mills, and utilities to get failed machinery back online quickly — Figs. 1 and 2. 

Why Choose Thermal Spray for Restoration?

Thermal spraying is chosen for restoration for a number of reasons, but some of the most common scenarios are the following:

• Parts for the failed machine are no longer manufactured or available.
• The cost of complete replacement or spares might be entirely prohibitive for the business.
• The lead time for fabrication of a replacement item might be longer than the business can afford to be down.
• An existing essential wear protection coating has begun to deteriorate, putting the life of the component at risk.
• A machining error was made, too much stock was removed, and now the piece is nonconformant.

In each of these cases, thermal spraying can typically restore the item to like-new condition or better, and can do so in a matter of days or weeks, rather than months, and often for a fraction of the cost of replacement.

Such is the case for a few items on our shop floor at Hayden Corp., West Springfield, Mass. In three case studies (see sidebar on pages 14 and 15), we’ll look at components in harsh service environments that have been given new life through thermal spraying. One item is the badly pitted hydraulic ram from an earth mover. In the second, leaking pump sleeves are restored to better-than-new condition through the use of more durable materials. And, in the third, we’ll see a rotating element that is given virtually unlimited life through a regular maintenance practice involving thermal sprayed ceramics.

Giving New Life to Old Parts 

When thermal spray is considered for a repair or restoration, it is typically best to assess the nature of the original failure, as this will likely steer the repair approach. In most cases, the metal that has been lost can be replaced with something similar, but thermal spraying, because it is not a welding process, has the advantage of being able to apply something dissimilar and even longer wearing. If the damage was caused by galling (metal-on-metal wear usually characterized by adhesive transfer of metal from one surface to another), it may be a good idea to restore the worn surface(s) with dissimilar materials to reduce the likelihood of metal transfer. Abrasive wear and erosion, caused by cutting or grinding of one material by another, are typically improved by replacing the lost material with something considerably harder, such as a chromium or tungsten carbide, or a ceramic, such as chromium oxide or alumina.

Analyzing the Cost of Replacement vs. Repair

The economic case for a thermal spray repair is often straightforward. When damage occurs, the cost for complete replacement of the damaged part is added to the opportunity cost of the downtime during the manufacturing lead time for the new component (or the cost for an on-hand spare), and the cost for disassembly and removal as well as installation and reassembly when the replacement part arrives. By contrast, the cost of a thermal spray repair is typically the cost of preparatory machining to remove the damage, the cost to coat the repair zone, and the cost of finish machining after coating. Disassembly and reassembly costs still apply, but some thermal spray repairs can be done in situ, eliminating the expense and time of assembly and shipping — Fig. 3. The total work scope is considerably less than full replacement because most of the original component is still functional and correct, and only the damaged zone must be addressed. As a result, a thermal spray repair is typically far less expensive and the lead time far lower, and that sinking feeling we talked about at the beginning of this article will dissipate more quickly.

After determining that thermal spraying is a viable option for a project, the next challenge is to determine exactly how the repair is to be executed. What is the dimensional requirement, and what is acceptable? If original drawings of the component are available, the answer is clear. However, in many cases, drawings are not available. Another excellent but uncommon resource is an undamaged spare part. The next best guide for determining the target buildup is the form, fit, and function of the part with those that mate to it. If a bearing rides on the worn surface (which is often the case), the bearing manufacturer will have size and tolerance data for the fit. As a last resort, there are often undamaged surfaces immediately adjacent to the wear zone that can provide a usable dimension, if the undamaged area appears to be of the same surface as the material lost. In some cases, such as the worm gear case study that follows, 3D scanning can provide worthwhile reference information.

Set the Strategy

Once the repair strategy is set, the work of executing the repair can begin. Most used machine parts will arrive covered in oil, grease, dirt, or worse. All of this is hazardous to the performance of a sprayed coating and must be removed thoroughly, by solvent or aqueous parts washer, for instance. With the part cleaned and the damage area clearly visible, final plans for the repair design can be set. It is best to ensure that the final coating thickness is uniform, greater than 0.003 in. to ensure integrity, and less than the maximum recommended thickness for the material being applied. Some sprayed materials can be applied to virtually unlimited thickness, but most hard materials will be prone to cracking if the thickness is too great. If the thickness of the repair is greater than the maximum thickness recommended for the coating, a bond coat of nickel chromium or nickel aluminide beneath the hard surface can usually make up the difference. Usually, the damage zone should be machined to clean up any unevenness. Such an undercut should also extend slightly beyond the damage zone so that final machining can ensure a clean, flush match with the remaining adjacent surfaces, and the edges of the undercut should be chamfered. A 45 deg is typical.

Material Matters

The material selected will guide the particular thermal spray process to be used. Most metals can be sprayed by arc wire, plasma, or high velocity oxygen fuel (HVOF). Carbides are best handled by plasma and HVOF. Ceramics are typically applied by plasma. Other process selection factors include the surface finish required (HVOF coatings have lower porosity and can generally be ground to a finer finish) and cost (arc wire coatings are least expensive while HVOF coatings are generally the most expensive). If the repaired surface must be machined extensively after coating, such as by threading or milling, a laser clad overlay is likely to be more durable and effective than a thermal spray repair.

Coating operations tend to be simple because the nature of the repair will usually be determined by people familiar with thermal spray’s capabilities and limitations, and the repair plan will likely have been developed with best practices in mind. Some useful general guidance includes erring on the side of extra thickness, when possible, to avoid additional rework; ensuring the coated area has a generous overlap beyond the repair undercut pocket so that final machining creates a seamless blend at the edges; and taking extra care during demasking and deburring to avoid accidental chipping into the repair zone.

In nearly all cases for repair, the coating will need to be machined or ground to final dimension and finish. Tooling and grinding thermal spray coatings present challenges all their own, due to the brittleness of hard coatings and the comparatively weak bond between the coating and substrate (compared to wrought material). The scope of machining techniques for finishing thermal spray coatings is beyond this article (the September 2011 issue of Welding Journal includes a good, comprehensive guide). In general, low surface speeds, sharp inserts, and light cuts are recommended for tooling, and, whenever possible, grinding is preferable, with removal rates of 0.0005 to 0.0015 in. per pass. When the coated zone must mate with a concentric part, it is often best to err on the high side of the dimensional tolerance and polish the surface in to fit. From time to time, a repair made on-site must be machined in place, and specialized equipment can be brought in to turn or mill features with the repaired component still in place or partially disassembled — Fig. 4. Softer metallic coatings can often be brought to size and finish with emery tape or other hand polishing methods.

Executing the Repair

In an ideal case, a repair would begin with an engineering assessment of the damage and its associated effects. One or more repair approach alternatives would be considered, including the costs and durability of each option. From there, one option would be down-selected, and planning can begin for an outage. The repair plan would include a schedule of operations, including a timeline, with adequate accommodation for contingencies, and actual project milestones would be benchmarked against the plan to ensure that the outage was proceeding as planned. Following the job completion, a postmortem meeting would assess planning effectiveness and identify logistical or technical problems for consideration in the next repair.

In reality, many repairs follow a much more condensed timeline. A component failure causes a machine outage, and the clock is ticking to come up with a solution that will get operations back online quickly. This is where thermal spray’s advantages truly shine. The low thermal impact of applying a thermal spray coating (when compared to welding, for instance) means that lost material can be replaced quickly, with very little risk of distortion, and finished in place. It is often as easy to apply a superior material as it would be to apply material identical to that which was lost, and so a higher-integrity repair is often the result. And, most thermal spraying for repair can be done using equipment that is field portable, making it easy to set up in a facility’s maintenance or machine shop or do the work directly on the installed component.

Conclusion

Thermal spraying is often an alternative repair method with substantial advantages. Leveraging low cost, simplicity, and portability, it can be the fastest way to return mission-critical equipment to service. In making the selection for a repair strategy (thermal spray or otherwise), it is important to weigh all of the costs (direct and opportunity) into each alternative. In this regard, thermal spraying can often be the fastest and least costly repair method available. Contingencies (utility issues, potential machining issues, etc.) are a critical part of the plan when the repair timeline is constrained. It is important, too, to recognize there are potential risks that may arise. A coating may fail to bond adequately, unexpected heat or pressure may create a delamination in service, or machining limitations may make it impossible to achieve the desired surface finish. Nonetheless, once all factors are considered, thermal spraying may be the ideal choice to get essential equipment back in service, on time and under budget, and eliminate that sinking feeling.

Disclaimer: The historical photos used in this article were taken in 1965 and are not in accordance with the safety practices used today.

DANIEL C. HAYDEN (daniel.hayden@haydencorp.com) is presdent of Hayden Corp., West Springfield, Mass.

Thermal Spray Repair Case Studies

CASE STUDY 1: Hydraulic Rams

Hydraulic rams often see tremendous abuse because they are placed in service in challenging and dirty environments. Those that see limited actuation or whose stroke doesn’t permit full retraction often suffer from pitting and corrosion on the portion of the cylinder that is always left exposed. In image 1, the ram had heavy pitting near the knuckle and abrasion and scoring along the actuation length. A lathe operation turned the ram to remove the heavy pitting and take a skim cut along the full actuation length — image 2. The heavily undercut areas were built up with nickel chromium bond coat, and then the whole length was finished with an HVOF hard coat of chromium carbide — image 3. After finish grinding, the ram was as good as new — image 4.

CASE STUDY 2: Pump Sleeves

HVOF-applied chromium carbide was selected for the following application because it is hard, wear resistant, dense, and easily ground to a fine finish that will not deteriorate the cylinder’s seals. It was also chosen because the chromium carbide/nickel chromium composite is exceptionally corrosion resistant. The result was a hard surface considerably thicker than the original chromium plating that may even tolerate a few regrinds over the course of its service life. 

Pump sleeves are the rotating elements that pass through a gland from the wet interior of a pump to the dry exterior. Due to the need to control leakage at this zone, packing materials are compressed against the sleeve to form a seal as it rotates. This combination of active loading and constant rotation leads quickly to circumferential wear and leakage. As seen in image 5, worn pump sleeves (left) can be fully restored with stainless steel (second from left) or can be enhanced by repairing with HVOF tungsten carbide (second from right) or chromium oxide ceramic (right), both of which are far harder than stainless and will last considerably longer. The ceramic option has the added benefit of being chemically nonreactive.

CASE STUDY 3: Splined Drive Shafts

The splined drive shafts in image 6 are among the more expensive parts to manufacture of the machine in which they are used. In service, however, the only surfaces that wear are the sealing areas at each end. The end user maintains a rotating stock of spares that are regularly sent for strip grinding, coating, and finishing of the seal zones using plasma sprayed chromium oxide ceramic. The repair cost is minimal since only the seal areas must be machined, coated, and ground. And, as there are spares in rotation, their machine downtime is reduced to the hours needed for replacement.

The repair method was to plunge grind the worn areas, leaving the undercut just shy of the planned depth. Any remaining coating was removed by grit blasting. In image 6, the wear zones were then plasma sprayed with chromium oxide, selected because of its hardness and density. Grinding brought the coated zone and adjacent shoulders to size and finish — images 7 and 8. Note that the repair process will typically leave a little residual coating on the shoulders adjacent to the coated zone; in this application, provided the outside diameter was correct, the overspray was not an issue. 

All photos courtesy of Hayden Corp.