Inconel 706 Inconel Alloy 706 N09706 Alloy Seamless Tubes

Inconel alloy 706 UNS N09706 is precipitation-hardening alloy of nickel-chromium-iron, readily fabricated and machinable, that provides high mechanical strength in combination with good fabricability. The characteristics of the alloy are similar to those of Inconel alloy 718 except that alloy 706 is more readily fabricated, particularly by machining.

The limiting chemical composition of Inconel alloy 706 is shown in the above table. The substantia nickel and chromiun contents provide good oxidation resistance and corrosion resistance. The primary precipitation-hardening constituents of the alloy are columbium and titanium. The aluminum content also contributes to the hardening response. The precipitation-hardening system in Inconel alloy 706 provides the desirable characteristic of delayed hardening response during exposure to precipitation temperature. That characteristic gives the alloy excellent resistance to postweld strain-age cracking.

Inconel alloy 706 is used for a variety of applications that require high strength combined with ease of fabrication. In the aerospace field, the alloy is used for turbine discs, shafts, and cases; diffuser cases; compressor discs and shafts; engine mounts; and fasteners. In addition to aerospace applications, the alloy is used for turbine discs in large industrial gas turbines.

Inconel 706 Forming

This alloy has good ductility and may be readily formed by all conventional methods. Because the alloy is stronger than regular steel it requires more powerful equipment to accomplish forming. Heavy-duty lubricants should be used during cold forming. It is essential to thoroughly clean the part of all traces of lubricant after forming as embrittlement of the alloy may occur at high temperature if lubricant is left on.

Inconel 706 Machinability

Conventional machining techniques used for iron based alloys may be used. This alloy does work-harden during machining and has higher strength and “gumminess” not typical of steels. Heavy duty machining equipment and tooling should be used to minimize chatter or work-hardening of the alloy ahead of the cutting. Most any commercial coolant may be used in the machining operations. Water-base coolants are preferred for high speed operations such as turning, grinding, or milling. Heavy lubricants work best for drilling, tapping, broaching or boring. Turning: Carbide tools are recommended for turning with a continuous cut. High-speed tool steel tooling should be used for interrupted cuts and for smooth finishing to close tolerance. Tools should have a positive rake angle.

Cutting speeds and feeds are in the following ranges:

For High-Speed Steel Tools For Carbide Tooling Depth Surface Feed Depth Surface Feed of cut speed in inches of cut speed in inches inches feet/min. per rev. inches feet/min. per rev. 0.250″ 12-18 0.010 0.250″ 30-40 0.010 0.050″ 15-20 0.008 0.050″ 40-50 0.008 Drilling: Steady feed rates must be used to avoid work hardening due to dwelling of the drill on the metal. Rigid set-ups are essential with as short a stub drill as feasible. Heavy-duty, high-speed steel drills with a heavy web are recommended. Feeds vary from 0.0007 inch per rev. for holes of less than 1/16″ diameter, 0.003 inch per rev. for 1/4″ dia., to 0.010 inch per rev. for holes of 7/8″diameter. Slow surface speed, as 8-10 feet/minute, are best for drilling. Milling: To obtain good accuracy and a smooth finish it is essential to have rigid machines and fixtures and sharp cutting tools. High-speed steel cutters such as M-2 or M-10 work best with cutting speeds of 5 to 15 feet per minute and feed of 0.001″-0.004″ per cutting tooth. Grinding: The alloy should be wet ground and aluminum oxide wheels or belts are preferred.

Inconel 706 Welding

The commonly used welding methods work well with this alloy. Matching alloy filler metal should be used. If matching alloy is not available then the nearest alloy richer in the essential chemistry (Ni, Co, Cr, Mo) should be used. All weld beads should be slightly convex. It is not necessary to use preheating. Surfaces to be welded must be clean and free from oil, paint or crayon marking. The cleaned area should extend at least 2″ beyond either side of a welded joint. Gas-Tungsten Arc Welding: DC straight polarity (electrode negative) is recommended. Keep as short an arc length as possible and use care to keep the hot end of filler metal always within the protective atmosphere. Shielded Metal-Arc Welding: Electrodes should be kept in dry storage and if moisture has been picked up the electrodes should be baked at 600 F for one hour to insure dryness. Current settings vary from 60 amps for thin material (0.062″ thick) up to 140 amps for material of 1/2″ and thicker. It is best to weave the electrode slightly as this alloy weld metal does not tend to spread. 

Cleaning of slag is done with a wire brush (hand or powered). Complete removal of all slag is very important before successive weld passes and also after final welding. Gas Metal-Arc Welding: Reverse-polarity DC should be used and best results are obtained with the welding gun at 90 degrees to the joint. For Short-Circuiting-Transfer GMAW a typical voltage is 20- 23 with a current of 110-130 amps and a wire feed of 250-275 inches per minute. For Spray-Transfer GMAW voltage of 26 to 33 and current in the range of 175-300 amps with wire feed rate of 200-350 inches per minute are typical. Submerged-Arc Welding: Matching filler metal, the same as for GMAW, should be used. DC current with either reverse or straight polarity may be used. Convex weld beads are preferred.

Microstructure of Modified Inconel 706 Superalloys

Inconel 706 does not fully meet the stringent requirements of the application in new steam turbines. The thermal stability of Inconel 706 is insufficient for a long term service above 700 degC, which leads to a dramatic loss of creep and tensile strength. Two methods of compositional modification were followed to optimize the microstructural stability of Inconel 706. One is by adding rhenium to the standard composition of the superalloy and second is by refining the chemistry of Inconel 706 resulting in a new alloy composition named DT 706 alloy. The main aim of this study was to investigate the complex microstructure in Inconel 706 alloy with high resolution techniques like electron microscopy (HREM) and three dimensional atom-probe (3DAP). The microchemistry around precipitates and the local structural variations involved in phase formation and transformation sequences of the fine precipitates and the co-precipitates (as small as 10 nm) were studied.

The analysis was performed to understand not only the transformation sequences but also the stability of each precipitate type. Microstructures in different heat treated conditions and after long time ageing at 750degC for 750 h and 5000 h were therefore studied in Inconel 706 alloy and compared with the modified alloys. The addition of Re to Inconel 706 composition did not show the desired effect which therefore suggests that alloying with Re is not the right choice in order to stabilize the structure of Ni-Fe wrought superalloys such as Inconel 706. On the other hand, it was observed that the thermal stability of DT 706 alloy is significantly improved. Therefore, DT 706 alloy has an advantage over the Inconel 706 alloy.