The manufacture of cement creates a demanding environment for all equipment, in particular the kiln, due to process-related wear, corrosion, and stress.

Kiln component failures that lead to stoppages are expensive due to the high volumes and efficiencies of production in modern plants. The use of welding and surface coating techniques have been established for many years as a means of combating wear, breakage, and corrosion of kiln components. The correct choice of welding and coating material and method is critical to the success of any repair.

Substantial cost savings can be made by weld reclamation and surface coatings, which impact inventory, production downtime, and service life. What follows will provide an overview of the scope of kiln component repair that have been successful using specialty alloys and modern processes.

Background In an increasingly competitive market, manufacturers need higher productivity at the lowest possible operating costs. This means that plants have to run for longer periods, at higher production rates with reduced maintenance so the operating procedures and materials used in the cement plant have to be chosen with great care. Advances in engineering technologies have assisted engineers in overcoming these new challenges.

The decision to repair rather than replace a worn or damaged component may be justified for a number of reasons: age of plant; availability of spares; cost savings; and improved service performance.

In most cases, welding and thermal spray are becoming the favored methods because advances in techniques and associated consumables mean that components can be frequently restored to their original condition or better by the use of improved materials. With success at the plant, many of these improvements then become standard features in original equipment manufacturer (OEM) parts, such as coated hammers and wearplate lined fans. Repair and maintenance work can be broadly classified by the following three types of repair:

1. Repair of critical, cracked castings (tires, trunnions) Cement kilns are exposed to a number of dynamic forces such as alternating bending, tensile, compressive, torsional, and thermal cycling. Due to the high-fatigue accumulation rate, these stresses can eventually result in cracks that form initially at the sub-surface. The fracture mechanism is one of fatigue and will typically propagate from inclusion weaknesses in the original material. Due to the depth of work-hardening of the tire or trunnion surface and consequential strength increase, cracks will tend to propagate below the surface until the cross-sectional area is large enough to weaken the tire sufficiently for a serious fracture to occur.

Works engineers have different views as to whether kiln tires and trunnions can be successfully repaired by welding. Kiln tire and trunnion fracture represent major welding repairs (Photos 1 and 2). Practical limitations include the need to maintain correct pre-and-post-weld heat (PWHT) on such a large part (in order to avoid the formation of brittle heat-affected zones) and the reliability of the repair not to recrack.

These components are generally made from carbon or low-alloy steels. There are two common routes available for weld procedures: the use of a filler alloy, such as ferritic-base, or a dissimilar alloy or non-ferritic. The ferritic-base route (low hydrogen - e.g. E7018) would be the preferred route compositionally, since the consumable more nearly matches that of the parent material. However, if the service conditions remain the same, the component will fail or crack again in exactly the same way at some future time. It therefore makes sense to consider a repair material that has a greater capacity to absorb and resist stresses produced during normal operation. In addition, due to the air hardenability of the weld metal and the relatively hard heat-affected zone, the preheat requirement is quite high, causing procedural and practical problems, including the need for PWHT. The problems with PWHT on such a section are the concentration of heat for the mass of the section together with heat loss through conduction.

Of the non-ferritic alloys, one is generally accepted as being the most reliable, having the approval of major OEMs. A procedure has been developed in close collaboration with the specialty consumable supplier Eutectic+Castolin, The Welding Institute (TWI), FLS Parts and Service Division, and certain welding contractors. The advantages of this procedure are that preheat levels are more easily attained with no need for PWHT. That is providing that certain techniques are rigidly adhered to during the welding operation, the structures are not excessively large, and the structures do not contain high residual stresses following years of service.

The performance of these products in relation to tensile strength, particularly elongation and impact values (even without PWHT), are superior to the parent material. These properties coupled with good metallurgical integrity at the weld-parent metal interface and ease of welding give tough, ductile welds resistant to cracks and a high level of repair reliability, according to Eutectic + Castolin.

The problem of surface hardness degradation also is avoided. Key physical and metallurgical factors for a successful long-term repair are Coefficient of Thermal Expansion (CET), dilution effects, and brittle-phase (Sigma) precipitation. All are superior for Eutectic NucleoTec 2222 over the ferritic alloys. The lower initial weld hardness is rapidly increased during service by work hardening.

The extent of cracking varies from through-cracks across the full component width to multiple cracks, which can join to spall off whole sections and stop production. Detailed procedures of the repair are described in other literature, but essentially are the following:

* Cutting out of the crack (mechanically or with arc gouging);

* Restraining bars and brackets are installed to limit tire movement during cutting and welding;

* The joint preparation is fully ground and shot-blasted and surface NDT is carried out;

* Attachment of copper backing bar under the tire together with run-on and run-off plates ensures that all weld starts and stops are outside of the finished body of the repair;

* The electrical preheat equipment is then installed; and

* The part is welded using a method which enables two welders to work simultaneously, giving a critical reduction in repair time. Reclamations are typically completed in seven to 12 days, including all post-welding grinding.

To fit a new tire including shell would cost approximately $200,000 with a 24-week delivery, which typically can represent $6.6 million in lost production. Obviously a repair option was financially attractive but technically demanding. Detailed heat-treatment procedures and the use of a welding product with superior mechanical properties to the kiln tire or to a low-hydrogen electrode (which in practice could have been used) produced a successful repair for about $230,000. The repaired tire is still running after one year of service. Putting it back in production quickly gave more time to plan a replacement tire.

2.Rebuilding of critical, worn castings (girth wheel gears) Frequently, the amount of wear experienced can be high resulting in significant loss of section and alteration in the component dimensions. These dimensions can be restored using austenitic-based welding materials, which make an excellent underlay prior to applying a more wear-resistant coating.

Gear tooth-One such case was the excessive wear (metal plastic flow and section chipping) of the gear teeth on a kiln drive gear. Metal-to-metal wear and excess stress/work hardening leading to micro-cracking and chipping were analyzed. Lack of lubrication, kiln distortions, or long-term operation can cause this. The result is not only an inefficient gear operation, but also can lead to transfer of principal stresses to other areas of the tooth, eventually causing catastrophic cracking. Here, an alloy is needed that has a similar hardness or better (to resist initial wear), but is less sensitive to stress-induced cracking.

A two-metal system is used for weld repair and to extend the performance of the original casting. The first layer is tough and non-work-hardening for rebuilding the tooth profile. The final and harder overlay reduces metal-to-metal wear. This combination is not possible in a casting, the closest would be differential case hardening. Major cost savings are possible as the gear does not need to be removed from the kiln and a new part does not need to be kept in inventory. Photo 3 shows the use of a template to restore original profile.

3.Coating protection of critical, worn or corroded parts (fuel-feed nozzles, kiln shell, lamella seals) It is generally considered that the best wear- resistant materials that see heavy-duty wear are iron-based castings with a high chrome and carbon content. These alloys can be further hardened by heat-treatment and are represented by white cast irons and Ni-Hard materials. These, and similar castings, have dominated the industry for grinding and wear applications. Their wear properties, however, are ultimately limited by the hardness of the primary chrome carbide hard-phase, which is only 980-1300 HV (Vickers scale). Harder phases exist such as Chromium Boride (1400-1800 HV) and Tungsten Carbide (2200-2800 HV).

Such hardness levels and related microstructures can translate to wear resistance gains of three to seven times over standard chromium carbide materials. Casting alloys with such alternative hard-phase containing alloys also would be problematic and manufacturing costs for a similar- sized component would be prohibitive (due to the increased cost of these elements). The casting would probably also be too brittle for engineering applications. The solution is to use these alloys as selective coatings in the high-wear areas of the cast component. Additional wear life is added to the original casting with no change to the casting process or brittleness, and worn casting can be reused after coating.

New alloy and process technique developments have enabled this approach to be successfully applied in the cement industry with considerable cost savings. Processes are all low-heat input and include Plasma Transferred Arc (PTA) [3], Twin-arc Wire [6], and High-Velocity Oxy Fuel (HVOF) [5] Thermal Spray. Alloys are based on Tungsten Carbide (WC) and Chromium Borides (CrB), and applications include hammers, roller presses, and transport screws where the substrate is a casting. The key to success is matching the alloy, process, and procedure to the substrate and application. Examples of this approach in the kiln area are:

Lamella seals-The metal on an overlapping leaf-spring lamella seal on a rotary kiln is chosen principally for its spring qualities and thermal resistance. The severe wear observed between the seal leaves and rubbing strip is due to metal-to-metal fatigue combined with three-body abrasive wear from kiln-charge dust. The seal limits air intake to the kiln and dust to the plant. Enhancing the seal-metal wear properties would degrade the metal's desired spring properties and also demand a complete rebuild [6].

As the wear is only a surface condition, a thin coating was applied over the existing coat through hardened rubbing strips, in-situ using the Twin Arc Wire Thermal Spray Process (Photo 4). The coating alloy was a high-temperature, self-lubricating metal with abrasion resistance containing chromium borides. A thin coating was sufficient, and it allowed seal tolerances to be maintained. The previous service life for a through-hardened rubbing plate was 18 months. The coated part has already been in service 26 months without any major signs of wear.

Kiln shell corrosion from alternative fuels-Coatings also can be used against corrosion as well as wear. Again, a highly alloyed metal tailored to resist the specific corrosive environment can be deposited selectively on an existing component, where corrosion is taking place. Corrosion-induced thinning of the kiln shell behind the refractory bricks was recently witnessed and associated with the use of alternative fuels (containing chlorine). Such thinning is potentially dangerous and any necessary premature kiln shell replacement expensive and disruptive. Here a 0.03-in., nickel-based, high-chromium coating was deposited in-situ by thermal spraying the affected metal [4]. A drastic reduction in thinning rate was monitored after one year of operation.

Burner nozzles and coal classifiers-The transport of the fine, pulverized coal from the mills to the inside of the kiln during firing can cause excessive erosive wear in key components along the route. This is exacerbated if lower grade fuel is used with its associated high quartz/pyrites content. Wear leads to inefficient combustion and therefore higher production cost. Two components are the burner nozzle and the coalclassifier. Both have had their life considerably extended by application of a t ungsten carbide-containing thermal spray coating. The classifier rotor is a dynamic component and must maintain its low weight and geometry. Consequently a 0.005-in., ultra-hard HVOF WC-Co was used. The nozzle is a static component allowing the use of thicker coating (0.1-in.) and a tough NiCrBSi matrix deposited by PTA.

Conclusions The use of modern welding and thermal-spray technologies, coupled with new alloy developments, have resulted in significant savings in total kiln operation costs by increasing kiln productivity and drastically reducing costs through reduced kiln downtime associated with component failure, wear or corrosion.