Cutting and Forming
Mild steel and heat resisting alloys do handle differently, and it is well to know the product you are working with to get the most out of it. There are some rules to know, but for the most part designing and fabricating alloys is using common sense based on the properties of the alloys and what you expect them to accomplish.
In the first place, the yield strengths of heat resisting alloys in the annealed condition are a little higher, and their tensile strengths a lot higher, than mild steel. Shear capacity, for example, has to be about 50% greater. On our shear rated 3/8” (9.5 mm) mild steel, we regularly shear 1/4” (6.35 mm) heat resisting alloy. The hydraulic shear, rated 3/4” (19 mm) mild steel, will handle 1/2” (12.7 mm) alloy plate. Good shearing practice cuts about 20% of the metal and fractures the remaining 80%. Heavier thickness plate is best cut by abrasive wheels, which produce a smooth, close tolerance surface.
Bending and Forming
Austenitic heat resisting alloys should almost always be bent cold. Heating without adequate temperature control is dangerous because of the narrow working range and the possibility of over or under heating. In the 1200 to 1600°F (650-870°C), or red heat, range, both 18-8 stainless and the nickel heat resisting grades will tear or rupture in forming. It will take more power to form these alloys than it takes to form mild steel, but because of good ductility, the alloys will take a lot of deformation without rupture. Extremely severe forming may require annealing between operations.
The ferritic grade RA446 does not form well at room temperature. Plates 1/4” (6.35 mm) and thicker should be preheated 250-400°F (120-205°C) for any bending or forming. Failure to preheat may result in some plates cracking apart, while others may be formed successfully.
These alloys always harden on deformation and cannot be worked beyond a limit without rupture. Our stock materials have all been scientifically annealed. A given size will have limits on hardness, elongation, and reduction of area. A typical plate might have a Rockwell B hardness of 84, and elongation of 35% and a reduction of area of 60%. Every lot of RA material is checked for these properties, and the mill certifications of the material delivered to you are kept on file for ten years at Rolled Alloys. Records on your order are kept by Rolled Alloys for six years.
After work hardening, but before rupturing, the material can be restored to its original mechanical properties by annealing. The process varies with the alloy, the mass, and the hardness. A piece of 3/16” (4.8 mm) plate, for example, that had been formed into a 4”(100 mm) tube might have had its hardness raised from the original Rockwell B 84 up to RB 96. It could be returned to RB 84 by heating to 1950°F (1065°C) and holding at this temperature for five minutes, then cooling quickly with an air blast. There probably would be little reason for annealing this shape, unless it was to be formed again, and it were required to be soft to permit further cold work.
These solid solution strengthened materials, therefore, can be hardened only by cold working, and softened by annealing. Occasionally tooling for aerospace requires to be stress relieved after rough machining, which in the case of RA330, may be accomplished by heating for one hour per inch (25 mm) of thickness to 1800°F (982°C), and furnace cooling until black, then air cooling.
The austenitic alloys will take a bend of 180° with a minimum inside radius equal to twice the thickness of the material. They will sometimes accept a bend flat on themselves, but they are not guaranteed to do so. The fabricator must perform bends with small radii and his own risk and be prepared to weld cracks that may develop. For extreme bends or the harder alloys it is better to bend across the grain, rather than having the grain parallel to the bending axis. The work hardened surface of a sheared or punched edge limits the amount of forming possible before cracking. As a minimum precaution, the shear burr or drag should be ground off. If severe forming is anticipated, the work hardened metal must be removed from the edge to be formed.
The photo on page 80 shows the effect of edge condition on formability of 1/2” (12.7 mm) RA333 plate. With an as-sheared edge, the plate cracked after only a 40° bend. Removing the shear burr permitted a 90° bend with some cracks. With a ground or bandsawn edge, material from the same plate was bent 180°, nearly flat on itself, with no cracking.
The first sign of over stretching is an orange peel appearance. This in itself is seldom detrimental, but the fracture soon to follow with further forming is incurable except by welding. It is far better to avoid a design that makes use of minimum radii. A generous radius is better for keeping the metal solid in service as well as during forming, because it gives the structure freedom to expand and contract, minimizing the thermal stresses created by heating and cooling.
A thermal expansion of some 3/16” per lineal foot (16 mm per meter) is going to occur between room temperature and the average service temperature. The resulting stresses are great, and the metal is going to move; so it should be pointed in the right direction.
RA 602 CA, RA333, and RA 253 MA are the strongest metals of the group in most temperature ranges; so they are slightly tougher to work. RA309 and RA310 are a little weaker. RA600 is somewhat softer and weaker. Its high nickel makes it “gummier” than alloys with more iron. The ferritic RA446 is less ductile and requires preheating before bending.
RA333 1/2 inch (12.7 mm) plate, formed with different edge preparations. Left - Sheared edge ground. Bent 180° flat on itself, no cracks. Middle - Shear burr removed. Bend 90° before cracking. Right- As-sheared, burr up. Cracked at 40° bend angle.
Spinning and Deep Drawing
Spinning and deep drawing can be accomplished by taking into consideration the physical properties, work hardening, and annealing. RA330 spins rather well, roughly comparable to 304 stainless. None of the heat resistant alloys will deep draw as well as 304 stainless. Dies for drawing the heat resistant alloys ought not be proofed with 304, as results will be different.
Illustrated below is an 11 gage RA85H spun half for a radiant tube return bend.
Hot forging should be used only if cold pressing cannot do the job. We know the materials are forgeable, because they all came from large cast ingots; but the working ranges are narrow, and close control of temperature, time, heating atmosphere and reduction are all important.
Heat resistant alloys must be heated throughout the section thickness. Typically, forging should begin when the metal is around 2100-2200F (1150-1200C), and finish before the metal cools below 1700F (930C). The exact temperature ranges vary from alloy to alloy. Forging either too hot or, more likely, too cold may cause cracking.
Never, attempt to bend or form any austenitic alloy in the 1100-1600F (590-870C) temperature range. Whether 304 stainless or nickel alloy 600, all will tear when formed at these temperatures. Unlike carbon steel, heating locally with a torch to make bending easier just doesn’t work. It is too difficult to heat nickel alloys uniformly hot enough throughout the section.
1 1/2” (38 mm) diameter RA330 scale: 1 3/8 X
Torch heated to bend. Although the operator thought it was hot enough, the brown temper color in the crack is typical of about 1200°F (650°C)