dapting 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. Lets 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! Weve used these alloys for so long that we often dont look to see if something better has arrived. Yet newer alloys and alloy systems are avail-able that offer unique advantages with surpris-ingly competitive paybacks. While time (and space) prohibits us from addressing this subject in great depth, one example of a more advanced alloy system will suffi ce to make our point.

602 CA Alloy[1]

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 1230C (2250F). 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.[2]

Creep and Rupture PropertiesThis alloy also has excellent creep-rupture properties. Creep and rup-ture strength (Tables 1-2) are important benchmarks in determining the life expectancy of high-temperature alloys.[3]

Carburization ResistanceThe 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 signifi -cantly 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.

Grain GrowthA common concern involving components exposed to extremely high temperatures for long periods of time is (brittle) fracture. At

Heat-Resistant Materials: Alloy Systems

Daniel H. Herring | 630-834-3017 | heattreatdoctor@industrialheating.com

The Heat Treat Doctor

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24 September 2013 - IndustrialHeating.com

Table 1. Total creep strength for indicated strain rate

Temperature, C (F)

Creep strength, MPa (psi)

Creep strength, MPa (psi)

0.0001% 0.00001%

650 (1200) 184.8 (26,800) 120.0 (17,405)

705 (1300) 127.5 (18,500) 80.0 (11,550)

760 (1400) 65.0 (9,427) 37.0 (5,366)

815 (1500) 29.0 (4,210) 13.3 (1,925)

870 (1600) 16.5 (2,390) 8.9 (1,291)

930 (1700) 10.3 (1,490) 6.3 (915)

980 (1800) 6.8 (980) 3.9 (555)

1040 (1900) 4.0 (585) 2.2 (312)

1095 (2000) 2.3 (330) 1.03 (152)

1150 (2100) 1.0 (145) 0.40 (58)

Table 2. Rupture strength to break at indicated time[4]

Temperature, C (F)

Rupture strength, MPa (psi)

Rupture strength, MPa (psi)

1% in 10,000 hours 1% in 100,000 hours

650 (1200) 215.0 (31,180) 165.0 (23,931)

705 (1300) 149.6 (21,700) 94.0 (13,750)

760 (1400) 78.0 (11,310) 40.0 (5,802)

815 (1500) 33.8 (4,900) 17.1 (2,450)

870 (1600) 22.5 (3,260) 12.2 (1,769)

930 (1700) 15.4 (2,230) 8.0 (1,160)

980 (1800) 10.1 (1,460) 5.1 (735)

1040 (1900) 6.8 (980) 3.3 (479)

1095 (2000) 4.5 (655) 2.1 (311)

1150 (2100) 3.0 (435) 1.4 (203)

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.

Table 3. Weight change (mg/m2h) for cyclic carburization data in CH4/H2 atmosphere, Ac = 0.80%.

[4]

Temperature, C (F)

850 (1562)

1000 (1832)

1150 (2102)

310 130 305 ----

800AT 143 339 813

600 50 190 626

601 64 170 508

RA 602 CA 13 70 175

Metal Dusting (aka Catastrophic Carburization)Metal dusting is a form of carburization at relatively low temperatures that leads to rapid, catastrophic corrosion of heat-re-sistant 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 ad-ditions.

Weldability/FormabilityRA 602 CA is weldable by GTAW, GMAW, SMAW and PAW. Proper selec-tion of shielding gases is critical. Shield-ing gases (or electrode coatings in the case of SMAW) are dependent on the welding process. Welding guidelines are available from the supplier, including welding in-structions for dissimilar metals. This alloy may be hot worked in the 900-1200C (1650-2190F) range and im-mediately quenched in water, but it should not be formed between 595-815C (1100-1500F). Heating must be done in a tightly temperature-controlled furnace and a neu-tral to slightly oxidizing atmosphere. Fluc-tuating between an oxidizing and reduc-ing 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 harden-ing. 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 ApplicationsRA 602 CA is used for many applications, including radiant tubes, furnace rolls, muffl es, retorts, atmosphere and vacuum furnace fi xtures, and grids to name a few. One steel mill has reported using this alloy for slab reheat furnace rolls at tem-peratures of 1250C (2280F) for over two years. The key to success is the high creep strength imparted by the carbon content and the constant rolling mo-tion 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 signifi cant weight re-duction. For example, an 8 mm (5/16 inches) thick cast tube can be made from 3-mm (11-gauge) sheet. When fi ring con-tinuously in the working zones of continu-ous or high-production batch furnaces, signifi cant energy savings can be achieved from the lower mass, which in turn al-lows more of the heat generated to be used for heating material instead of keeping equilibrium in the tube. It also allows for lower fi ring temperatures because there is a smaller gradient across the tube due to the section thickness. Exact savings will be reliant on furnace condition, insula-tion integrity and operating conditions but should be 10% or more depending on the thickness reduction. Lightweight rod-mesh baskets and lin-ers are being fabricated from this alloy to

take advantage of its higher creep strength, resistance 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 1065C (1950F) allows for a weight reduction of up to 10%. Low-pressure vacuum carburizing is an-other example of where alloy 602 CA is making inroads. The aluminum content allows for retention of the alloys oxide layer, while more traditional alloys (such as Inconel 600, 601 and RA330) lose their protective chromium and/or silicon ox-ides.[5]

Summing UpAlloy 602 CA is one example of a family of new high-temperature alloys available from a multitude of suppliers that are in-creasingly being used in the heat-treat in-dustry. 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

References available online

26 September 2013 - IndustrialHeating.com

Fig. 1. RA 602 CA radiant tubes after one year of service

Table 4. Effect of time on ASTM grain size for various alloys at 1120C (2050F) for 990 hrs.[2]

Time (hour) 0 2 24 72 184 344 510 670 830 990

ASTM Grain Size

RA 602 CA 7 7 7 7 6.5 6.5 6.5 6.5 6.5 6.5

601 5 5 1.5 1 1 0 0 00 00 00

601 GC 5.5 5.5 5 5 3.5 3.5 3 3 3 2.5

RA330 7 3.5 3.5 3 3 2.5 2 2 2 1.5

RA333 4 4 4 3 2.5 2 2 2 2 1

600 8 4 4 0 0 0 00 00 00 00