Heat & Corrosion Resistant Materials/Composites : When "Good Enough" Just Isn't Anymore
The dual pressures of rising costs and competitive pricing have put extraordinary demands on managing successful heat-treating operations. Rigid quality standards are a given and cannot be compromised. A shrinking skill base complicates the situation even more. Heat treaters have, over the years, learned to get along with equipment that used to be considered "good enough," but not anymore.
Today's extraordinarily competitive environment has caused managers to re-examine every facet of cost. Many operations now feature sophisticated computer-based data collection systems to capture their true costs. Even companies without those tools have re-focused on each and every element of cost. That attention, in turn, has enabled managers to make intelligent trade-off decisions in their cost reduction efforts.
Defining Which Costs Can Be LoweredFor example, where managers once simplified their budgets by replacing high-temperature alloy components in a furnace, there is now an active evaluation of alternate materials. This isn't a new phenomenon, but now it is more critical than ever. Alloy deterioration and its related downtime and replacement costs are known to have a direct negative effect on the bottom line.
Alloy components such as radiant tubes, trays, baskets, burner components, and recuperators all require periodic replacement. The limitations imposed by creep deformation of metals are all too well understood by heat treaters. Nearly every heat-treating facility, whether captive or commercial, has a pile of severely distorted alloy behind the shop. Alloy manufacturers continuously strive to improve their materials with the objective of longer life and improved performance. Despite those efforts, the cost of alloy replacement has been, and continues to be, a major concern. Recent steep increases in the price of nickel have made the impact of alloy replacement all the more serious. Figure 1 shows the recent history of nickel prices, which may be too painful for some readers to view. They already know the impact on their bottom line.
Similarly, efforts to improve the productivity of furnace equipment have received renewed emphasis. This is not limited to just downtime reduction, but also seeking ways to increase throughput. Many managers learned long ago that they could push more work through their furnace if, for example, they increased the burner inputs. Metallurgists and operations managers have long known that in most cases, they don't compromise quality if they bring the load up to transformation temperature faster. The portion of the cycle that represents ramping up to temperature often comprises a significant portion of the total heat-treating cycle. Figure 2 represents a typical batch furnace temperature profile. By simply firing harder, the recovery portion of the cycle can be shortened, often by 40 to 50 percent. Many have done just that, but the downside of that strategy is even shorter alloy life.
Much has been made lately of trying to maximize the ratio of workload to total load (sometimes referred to as net-to-gross). All furnaces, whether vacuum or atmosphere, have to heat the entire load, not just the parts being processed. Baskets, trays, and part fixtures all have to be heated. Often, the ratio of actual workload to total load can be 0.5, or even less if a lot of special fixtures are used. The simple solution is to reduce the weight of the tray, basket, and fixture. Clever engineering and design work has, in fact, accomplished just that. Unfortunately, the downside to those efforts has sometimes included reduced life for those alloy components. Thinner cross sections weakened the parts, and creep distortion increased. The trade-off between increased loads and poorer alloy life requires very careful data collection and analysis.
The price of natural gas is another huge cost factor that has recently sustained heart-stopping increases. Furnace builders, as well as operators, have at least a subtle understanding that recuperation can offer significant fuel savings. Advanced burner systems can also offer major benefits with efficiencies previously unattainable. Radiant tube inserts to capture heat are also effective. Recuperators, single-ended recuperator burner systems, and tube inserts offer great cost-saving potential. All these, if fabricated from nickel/chrome alloys, have the same inherent limitations imposed by poor life and temperature capability of the alloy.
What Can You Do to Lower Costs?Most of the foregoing discussion is well understood by anyone working in the trenches day after day. Most of you already know that "good enough" isn't going to cut it in today's business environment. So what can you do to keep your costs competitive?
Advanced non-metallic materials can provide some of the answers. Like a lot of inventions and innovations that have originated in the United States, the real push to adopt new technology often occurs elsewhere. That certainly has been the case with substituting ceramics for nickel-chrome alloys.
Much of the early development of silicon carbide, for example, originated at U.S. companies. The Gas Research Institute (now part of the Gas Technology Institute) did much in the ‘80s and ‘90s to facilitate material improvements and their early adoption. The Europeans, however, led the way in the widespread, now almost universal, use of silicon carbide for burner components, radiant tubes, recuperators, and furnace components.
The Europeans were faced with very high energy costs long before U.S. industries began to feel the same effects. They realized that downtime caused by alloy failure was unacceptable. "Good enough" was replaced by a quest for the very best. The use of advanced burner systems incorporating silicon carbide recuperators became commonplace. More recently, carbon/carbon composites used in vacuum furnace fixtures have begun the same early adoption. An examination of the material properties reveals why these advanced materials are gaining acceptance all over the world.
The first and most obvious property is creep resistance. Creep, the mode of failure for alloys in the vast majority of heat-treating applications, is defined as elongation (deformation) with time under load. The rate of creep is directly proportional to time, load, and temperature. The higher the load and temperature, the more severe the deformation and the shorter the time to failure. Figure 3 shows the distribution of radiant tube life in carburizing furnaces. As can be seen, some fail quite quickly, while others last four or five years. Half fail within two years.
The difference in alloy tube life is somewhat dependent upon the particular alloy but is primarily a function of the use temperature and atmosphere. Companies that aggressively fire the radiant tube pay the price in shorter life. Some companies, for reasons of throughput or the kind of work they do (i.e. solution treating of stainless steel), simply accept poor tube life as a matter of business as usual. Until advanced materials, which have excellent creep resistance, came along, there really was no choice.
Almost all domestic and foreign burner companies now offer systems employing silicon carbide or other advanced materials. Burner components, recuperator subsystems, and radiant tubes now offer creep resistant components. These materials still creep (all materials do), but the rate is so slow that there is no perceptible deformation over the lifetime of the furnace or, if you prefer human terms, the tenure of a typical heat-treating facility maintenance manager. Figure 4 shows the results of an accelerated creep test on a silicon carbide based material (left) versus a premium alloy (right).
All metal alloy tubes fail. As mentioned, their life expectancy is a function of the alloy, the use environment, and temperature. Advanced ceramic materials also have a downside. Their fracture toughness is generally poor, and whenever there is an application in which mechanical impact is likely, the use of these materials is not recommended. Proper system design and installation can usually prevent these problems.
High-Efficiency Burner SystemsHigh-efficiency burner systems employing advanced materials are almost the de facto standard in Europe. They are rapidly catching on in the U.S. and elsewhere. They rely upon pre-heated air to improve efficiency. Some of these systems use conventional materials, but because of their compact design and higher heat fluxes, the metal components have unacceptably short service life. The extensive use of ceramic components in advanced regenerative burner systems has achieved unprecedented efficiencies of nearly 80 percent. Single-ended recuperator systems with efficiencies of more than 70 percent are not uncommon. Other burner systems employing advanced materials in recuperators and staged combustion have similar high efficiencies.
By comparison, the efficiency of unrecuperated natural convection burners can be as poor as 25 percent. Heat treaters don't need a super computer to calculate the potential savings. The bottom line is that paybacks of less than one year are readily attainable. These new systems are available from a variety of companies. Figure 5 shows the wide range of fuel efficiencies from commonly available combustion systems.
Radiant Tube InsertsRadiant tube inserts are certainly not a new idea. Almost everyone knows they work well at reducing energy costs, and almost no one uses them. Those who have tried them know there are a couple of reasons why they aren't universally used. First, the metal inserts usually distort so badly that they wind up plugging the tube. Ceramic inserts don't distort, but many early experiments with them simply used the wrong material. Mullite and alumina inserts failed (often miserably) because those materials had poor thermal shock resistance. Newer versions employing silicon carbide have excellent thermal shock resistance, and their unique spiraled design has proven highly effective in extracting the energy in the exhaust stream (see Figure 6).
Furnace ThroughputImproving furnace throughput can be a confusing and sometimes difficult subject. Bottom line to furnace throughput is that those who take time to carefully analyze how to shorten the recovery time are achieving huge savings. This is true of both continuous and batch furnaces. In order to bring the load up to temperature more quickly, two things are necessary. One is that you must apply more energy (fire the burners harder). Two, you must have components that can take the higher firing rate without adverse affects. That almost always requires advanced materials. Fortunately, materials such as silicon carbide have demonstrated their ability to meet these twin requirements.
Nonmetallic Furnace FixturesNon-metallic fixtures and separators are catching on in many vacuum furnaces and even in some atmosphere furnaces. Carbon/carbon composites, once thought to be too expensive for heat treating, are proving to be effective ways to improve the net-to-gross ratio. These composites actually get stronger at higher temperatures and are extremely light in weight. Novel ways to prevent carbon pickup have been developed, which make these advanced materials even more useful. They combine the advantages of long life, fuel savings, and in the case of gas quenching, they can also shorten quench time. Figure 7 shows a separator grid made of one continuous fiber with inserts.
ConclusionEver increasing costs and competitive pricing are forcing the conclusion that "good enough" cannot be tolerated. New technologies, including the adoption of advanced materials, can have a direct positive effect on the bottom line. Cycle time reductions, improved net-to-gross ratios, fuel savings, downtime reductions, and component replacement cost reductions are all now routinely achieved by those willing to employ new materials.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: radiant tubes, advanced ceramics, silicon carbide, heat treating