Heat-Resistant Materials: Alloy Systems
Adapting to change is a basic human trait, but in the world of heat treating, we often need a gentle push to help move us forward. In the immortal words of Bob Dylan, “The Times They Are a Changin’,” and when it comes to heat-resistant alloys, we must adapt to compete. Let’s learn more.
The family of heat-resistant alloys represented by cast HT and its close cousins (e.g., HK, HU) as well as their wrought equivalents (e.g., 330) are to the heat treater what motherhood and apple pie is to our society as a whole – sacred cows! We’ve used these alloys for so long that we often don’t look to see if something better has arrived.
Yet newer alloys and alloy systems are available that offer unique advantages with surprisingly competitive paybacks. While time (and space) prohibits us from addressing this subject in great depth, one example of a more advanced alloy system will suffice to make our point.
602 CA® Alloy
602 CA® is a wrought material developed by VDM and distributed in the U.S. under the designation RA 602 CA®. Inspired by aerospace coatings applied to turbine blades, this product is a nickel-based alloy that employs a high chromium content along with aluminum and yttrium additions that produce a tightly adherent oxide, allowing the alloy to operate at temperatures in excess of 1230°C (2250°F). The alloy is extremely resistant to grain growth at high temperatures and is resistant to carburization. The aluminum in the alloy allows for the formation of a continuous, homogenous and self-repairing alumina subscale, while the yttrium improves the adhesion and spalling resistance of the chromium- and aluminum-oxide scales.
Creep and Rupture Properties
This alloy also has excellent creep-rupture properties. Creep and rupture strength (Tables 1-2) are important benchmarks in determining the life expectancy of high-temperature alloys.
The tenacious oxide layer present on this alloy is also responsible for its excellent carburization resistance (Table 3) in a heat-treat atmosphere at a 0.80% carbon potential. The data shows that this alloy is significantly more resistant to carburization than typical austenitic alloys and Inconel. Carburization leads to embrittlement/cracking, and alloys that are more resistant will retain their ductility longer.
A common concern involving components exposed to extremely high temperatures for long periods of time is (brittle) fracture. At temperatures exceeding the annealing temperature of a heat-resistant alloy, grain growth can be expected (Table 4) and leads to loss of ductility. RA 602 CA shows no appreciable grain growth.
Metal Dusting (aka Catastrophic Carburization)
Metal dusting is a form of carburization at relatively low temperatures that leads to rapid, catastrophic corrosion of heat-resistant alloys. For years, RA333® has been an excellent choice to negate these effects, but availability in some forms in recent years has limited its use. RA 602 CA has also proven superior in resisting the effects of metal dusting due to its chemistry additions.
RA 602 CA is weldable by GTAW, GMAW, SMAW and PAW. Proper selection of shielding gases is critical. Shielding gases (or elec-trode coatings in the case of SMAW) are dependent on the welding process. Welding guidelines are available from the supplier, including welding instructions for dissimilar metals.
This alloy may be hot worked in the 900-1200°C (1650-2190°F) range and immediately quenched in water, but it should not be formed between 595-815°C (1100-1500°F). Heating must be done in a tightly temperature-controlled furnace and a neutral to slightly oxidizing atmosphere. Fluctuating between an oxidizing and reducing atmosphere must be avoided. Natural gas should not contain more than 0.5% sulfur or 0.1% by weight of fuel oil. Never use a torch to heat the material because this will often lead to cracking.
The high carbon content (0.15-0.25%) in the material causes rapid work hardening. Components made from this alloy may be bent 120 degrees around a radius equal to three times the material thickness (3T) for material up to 0.4-inch thick. As with all nickel alloys, the shear drag (burr) must be removed to prevent crack initiation.
Typical Heat-Treat Applications
RA 602 CA is used for many applications, including radiant tubes, furnace rolls, muffles, retorts, atmosphere and vacuum furnace fixtures, and grids to name a few.
One steel mill has reported using this alloy for slab reheat furnace rolls at temperatures of 1250°C (2280°F) for over two years. The key to success is the high creep strength imparted by the carbon content and the constant rolling motion that prevents stresses from the slab weight from being concentrated on any particular point.
Radiant tubes (Fig. 1) fabricated from this alloy are an alternative to cast tubes with, in general, a significant weight reduction. For example, an 8 mm (5/16 inches) thick cast tube can be made from 3-mm (11-gauge) sheet. When firing continuously in the working zones of continuous or high-production batch furnaces, significant energy savings can be achieved from the lower mass, which in turn allows more of the heat generated to be used for heating material instead of keeping equilibrium in the tube. It also allows for lower firing temperatures because there is a smaller gradient across the tube due to the section thickness. Exact savings will be reliant on furnace condition, insulation integrity and operating conditions but should be 10% or more depending on the thickness reduction.
Lightweight rod-mesh baskets and liners are being fabricated from this alloy to take advantage of its higher creep strength, re-sistance to grain growth and retention of ductility. The result is improved basket life and fewer issues when straightening of baskets is required. It has been reported that high-pressure gas quenching of high-speed tool steels hardened at temperatures in excess of 1065°C (1950°F) allows for a weight reduction of up to 10%.
Low-pressure vacuum carburizing is another example of where alloy 602 CA is making inroads. The aluminum content allows for retention of the alloy’s oxide layer, while more traditional alloys (such as Inconel 600, 601 and RA330) lose their protective chromium and/or silicon oxides.
Alloy 602 CA is one example of a family of new high-temperature alloys available from a multitude of suppliers that are increasingly being used in the heat-treat industry. There are many other applications for these alloys throughout the thermal-processing industry as both cost-effective alternatives to more traditional wrought alloys or as substitutes for cast alloys in high-temperature applications. As a heat treater, it is worth the time and effort to investigate and use this new generation of materials. IH
1. 602 CA® is a registered trademark of Outokumpu ThyssenKrupp.
2. Rolled Alloys Bulletin No. 1602USe 02-12.
3. Herring, Daniel H., “A Survey of High Temperature Alloy Selection in Heat Treating,” Industrial Heating, September 2006.
4. VDM Data Sheet, 2012.
5. Kelly, James, Heat Resistant Alloys, Bulletin 401, Rolled Alloys, 2006.
6. Mr. Marc Glasser, Manager of Metallurgy, Rolled Alloys Corporation, technical and editorial review.