Metallurgists have long known that small amounts of chromium, molybdenum, nickel and tungsten make steel stronger and more heat-resistant. The nickel-based superalloy known as Hastelloy contains all four, with a bit of cobalt and vanadium to boot. It’s tough stuff.
It’s also been around for a long time. Inventor and entrepreneur Elwood Haynes obtained the first patent on Hastelloy A in 1921. The superchargers that gave World War II military aircraft more power at higher altitudes contained Hastelloy B turbine blades, while nozzle skirts and heat exchangers made of Hastelloy C helped bring the Apollo 11 astronauts to the moon.
Those alloys are still in use today. So are Hastelloy X and numerous letter-and number-designated variants in between, all registered to Haynes International. The workhorse of the group is probably Hastelloy C-276, a heat-resistant superalloy (HRSA) known for its exceptional resistance to cracking and pitting in the presence of acids, chlorides and oxidizing salts.
These attributes make Hastelloy C-276 the top dog for a wide range of chemical, oil and gas, pharmaceutical and waste-treatment applications, even at temperatures up to 1900°F (1038°C).
Hastelloy variants remain popular in the aerospace industry, too, where they are widely used in gas turbine components.
‘Have to Get Everything Just Right’
The strength that makes Hastelloy appealing simultaneously makes machining it more challenging, as Mike Restall is well aware.
A senior CAM programmer and application specialist at the Sandvik Coromant Center in Mebane, North Carolina, Restall spends his days developing effective machining strategies for such alloys.
Read More: 5 Metal-Cutting Tips For Nickel-Based High-Temp Alloys
“For aerospace use, the most common is Hastelloy X, which has a machinability rating just 16 percent that of 1112 steel,” he says. “It’s easier to cut than Inconel 718, but not by much.”
Many of the parts Restall is tasked with turning are forged. Here, he typically begins with a 3/4-inch round ceramic insert such as Sandvik Coromant’s 670 grade (a whisker-reinforced ceramic) to “peel away the nasty, scaly outer surface,” followed by a smaller 6160-grade ceramic insert to rough the balance of the workpiece.
Restall might begin the finishing process—which he defines as removal of the final 1 millimeter of material—with a CNMG or DNMG-style insert, depending on the part’s geometry. He says an S205 CVD-coated carbide works well, although notching at the depth-of-cut line can be expected (as with all superalloys).
One productive alternative to carbide is a 7014 cubic boron nitride (CBN) finishing insert, which Restall suggests can run at speeds up to 900 surface feet per minute (SFM), more than twice that of carbide.
A similar approach applies with milling. Radial depths of cut during roughing are light—between 5 percent and 15 percent of the cutter diameter, using high axial engagement.
With ceramic tooling, machining is always performed dry. Otherwise, high-pressure coolant (HPC) should be used with carbide tools. Trochoidal toolpaths that leverage the chip-thinning effect and maintain constant chip loads are advisable, as is a rigid machine tool with plenty of horsepower.
“With Hastelloy and other heat-resistant superalloys, it’s crucial to develop a repeatable, secure process, particularly if the part needs to be certified for aerospace use,” Restall says. “That’s because once you’re locked in, it’s very difficult to change anything without first going back to the customer for approval. This is why I strongly recommend that shops engage with their cutting tool provider early on. You have to get everything just right with these tough materials.”
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35I own my shop it is very small. I only do manual machining ,Lathe,vertical milling. mostly my customer base is made up of farmers,maintenence work on machinery. So not any exotic materials used.
35Good night sir
I am milling C276 very first time I tried to use APMT inserts of endmill of a P M grade but failed. Please recommended type of tool and inserts grade for pocket cut . And also recommend SFM and Rpm of tool you suggested .
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