A New High-Temperature Alloy for Extreme Temperatures
Kanthal APMTTM Essentials
Kanthal APMT is an advanced powder-metallurgical, dispersion-strengthened FeCrAlMo alloy, which is optimized for oxidation resistance up to 1300°C (2372°F) and continuous operation as a construction material at temperatures up to 1250°C (~2300°F). The alloy is now introduced in a wide range of product forms, which opens up possibilities for components designed for enhanced processing temperatures and improved energy efficiency in a variety of high-temperature processes where oxidation or high-temperature corrosion limits component life.
The unique combination of properties makes APMT a problem solver and a facilitator. APMT features the spontaneous formation of an Al2O3 surface layer during operation, which provides a dense, adherent and self-healing protection against destructive corrosion and oxidation at very high temperatures. The alloy also shows excellent form stability, exceeding that of Ni-based alloys at higher temperatures due to a unique powder-metallurgical production process.
This article was originally published on December 5, 2014.
Sandvik’s Kanthal-branded materials and systems have long been the standard solution for electrical heating elements when it comes to FeCrAl (Kanthal®) and NiCrFe (Nikrothal®) metallic alloys as well as Super™ (MoSi2) and Globar® (SiC) ceramic heating elements.
For thermal processing and process heating, Kanthal APM™ and Kanthal APMT are more recent additions. Development is taking another step by introducing APMT for components like retorts for PM sintering, furnace rollers and other furnace components.
Traditionally, wrought or cast Ni-based alloys are commonly used for high-temperature constructions, but their limited oxidation resistance above 1000-1100°C (1832-2012°F) and sensitivity to carburizing is a problem in many high-temperature industrial processes.
Conventional wrought high-temperature alloys are far superior in this respect, with oxidation resistance up to almost 1400°C (2552°F) due to their ability to form a very slow-growing and protective alumina scale during service. The drawback of these alloys is relatively low mechanical strength at high temperatures, which strongly limits their application in mechanically stressed components.
By introducing a unique rapid-solidification powder-metallurgical process route (RSP), the mechanical strength of FeCrAl alloys was greatly improved, and the resulting APM was first presented to the wider market with a paper published in this magazine in 1989. Since then, the alloy has become the preferred choice for high-performance electrical heating elements and radiant tubes worldwide (Fig. 1).
The further enhanced mechanical strength of the recently developed APMT now bridges the gap between high-temperature metallic alloys and ceramics in terms of application temperature (Fig. 3).
Lifetime Improvement in Many Industrial Atmospheres
Lifetime is typically limited by creep deformation and/or oxidation. At lower temperature and high loads, failure is often controlled by the mechanical properties, whereas the oxidation process is critical at high temperature and below a certain load level. At high temperatures, the adherent, thin alumina layer that forms on the APMT during service provides great advantages compared to chromia-forming Ni-based alloys in terms of slow oxidation rate; very high scale adherence; chemical stability toward water, carbon and sulphur; and negligible amounts of gas phase and particulate emissions. The result is higher maximum operating temperature and longer service life.
APMT forms protective alumina in most common, controlled atmospheres such as endo- and exo-gas and hydrogen. The carburization resistance of APMT is excellent in comparison to all chromia-forming materials, and it is highly resistant to combustion atmospheres up to 1300°C (2372°F) from fuels such as natural gas and coal. It is also almost entirely insensitive to metal dusting and sulphur compared to Ni-based alloys since no low-melting compounds will form. A word of caution is in order for service in dry N2 or N2/H2 mixtures where Ni-based alloys may be a better choice due to risk of unstable alumina formation (Fig. 3).
In general, the creep-rupture strength for APMT is comparative to Ni-based alloys above 900°C (1652°F), and the advantage in favor of APMT becomes greater at higher temperature. Rupture and creep-rate data is well defined up to 1300°C (2000-2400°F), which is a range where Ni-based alloys suffer from severe oxidation and loss of structural integrity (Fig. 4).
Welding and Fabrication
APMT is normally welded with TIG/GTAW. Preheating to 250°C (482°F) and post-weld annealing to 850°C (1562°F) is necessary, however, and some creep strength is lost in the weld. As a rule of thumb, the magnitude of strength loss corresponds to a 100°C (180°F) temperature rise of the base alloy, and mechanically loaded welds should therefore be located where temperature or stress are below their peak levels. In addition to TIG welding, several joining techniques – such as laser or MIG welding and brazing – can be used.
APMT is ductile at room temperature with elongation to rupture between 10 and 25%, depending on product form, but it is nevertheless recommended that plastic deformations are performed using a preheat to ≥250°C (482°F). Bending over a radius gives better stress distribution than a V press and is preferred when possible. Elaborate forming operations are readily done at red-hot temperature at which the ductility is very high.
Machinability of APMT is comparable to that of forged or rolled ferritic steels. The cutting speed, however, normally needs to be reduced as compared to standard grades. Cutting may be performed with standard methods, but water cutting has the advantage of not negatively affecting the stress state and surface conditions.
Industrial Heating Applications
Due to the combination of strength and oxidation resistance at temperatures greater than 1250°C (~2300°F), the alloy is a facilitator and problem solver in components such as furnace rollers operating at 1200°C, hot-wall retorts for sintering at >1250°C, reformer tubing, thermocouple protection (resistance to metal dusting) and heat shields. A wide range of product forms is now available, including wide hot-rolled plate and extruded tube to support the application of the material.
Batch processes using vacuum or low pressure at high temperature conventionally use cold-wall designs with outer water-cooled vacuum chambers and a graphite- or Mo-heated process chamber inside it. Due to its combination of adequate oxidation resistance and sufficient mechanical strength, APMT greatly expands the upper temperature limit for hot-wall furnace design in which the chamber wall combines the functions of keeping a low pressure (vacuum) and transferring heat to the parts. Here, the alloy is a facilitator, since APMT hot-wall retorts (Fig. 5) can be applied up to 1250°C (~2300°F), which is approximately 200°C (360°F) more than what is possible with the best Ni-based alloys. The heat is then provided by conventional external heating. The advantages are great in terms of increased productivity and lower cost due to:
• Lower investment
• Faster cycle time
• Effective and easy-access loading
• One heating system can serve two or more process chambers
• Reduced energy consumption due to reduced or eliminated need for water cooling
Hot-wall APMT systems are applied at ~1250°C (~2300°F) for PM sintering of magnetic and medical-implant materials such as CoSm and CoCr alloys.
Muffles for Heat Treatment and Furnace Furniture
Muffles with custom cross sections can be made from wide plate, but seamless extruded tubes have also been used for high-temperature anneal of stainless steel wire, bar and tube when size permits. The alumina scale that forms (even at very low oxygen levels) gives very long service life, and it also stops carburization and buildup of coke deposits and prevents metal-metal sticking to the treated product. A wide range of bar, wire and plate dimensions is available for components such as trays, baskets and load supports for batch and continuous pusher and belt heat-treatment furnaces (Fig. 6).
Thermocouple protection tubes and thermowells see all environments and temperatures possible. At temperatures above 1100°C (2012°F) in oxidizing environments, APMT replaces brittle ceramic protection tubes, and in petrochemical processes, for example, its outstanding resistance to carburizing and sulphidation solve severe corrosion problems experienced with Ni-based alloys.
Furnace rollers made from conventional NiCrFe wrought or cast alloys suffer from severe oxidation and often carburization when applied in oxidizing conditions at the high temperatures needed for treatment of certain stainless steel grades. This means frequent and costly refurbishments and also gives product-quality issues, since flakes and corrosion dents on the roller surface affect the surfaces of the treated product. Time between reconditioning may be as short as 6-12 months, which means frequent maintenance stoppages as well as costly storage of large numbers of spare rollers. Water cooling of the rollers is often used in high-temperature continuous furnaces, which reduces the corrosion problems with conventional materials but at a huge cost in terms of energy losses.
Kanthal APMT rollers (Fig. 7) offer all the expected advantages – namely greatly reduced oxidation rate, much better roller surfaces (and thus also on the treated product) after long service time and very large improvement in service life. A specialty stainless steel producer operating a furnace at ~1200°C (2192°F) has experienced a life increase from 6-12 months to at least four years. Furthermore, by eliminating the need for cooling, APMT helps operations reduce energy consumption in many cases. IH
1. Bo Jönsson, Qin Lu, Dilip Chandrasekaran, Roger Berglund, Fernando Rave, “Oxidation and Creep Limited Lifetime of Kanthal APMT®, a Dispersion Strengthened FeCrAlMo Alloy Designed for Strength and Oxidation Resistance at High Temperatures,” Oxid. of Metals, 2012
1. B. Jönsson and C. Svedberg, “Limiting Factors for Fe-Cr-Al and NiCr in Controlled Industrial Atmospheres,” Mater. Sci. Forum, 251–4 (1997) 551–8.
1. Kanthal APMT datasheet available at http://www.kanthal.com/en/products/material-datasheets/wire/na/kanthal-apmt/