Type Analysis | Description | Physical Constants | Corrosion Resistance
Mechanical Properties | Workability

Type Analysis

Description

Inconel nickel-iron-chromium alloy 706 is a precipitation-hardenable alloy 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 substantial nickel and chromiun contents provide good resistance to oxidation and corrosion. 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 temperatures. 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.

Physical Constants

Density                                                                                     
   Annealed, lb/cu in .............................................. 0.291              
                  Mg/cu m ................................................ 8.05                      
   Precipitation-Hardened, lb/cu in ........................ 0.292                   
                                 Mg/cu m ................................. 8.08                    
Melting Range, °F ......................................... 2434-2499             
                     °C ............................................. 1334-1371                     
Specific Heat, 70°F, Btu/lb-°F................................ 0.106                                             
                    21°C, J/kg-°C ........................................ 444         
Permeability at 200 oersted (15.9 kA/m)
   Annealed
          74°F(23°C) .................................................... 1.011
       -109°F(-78°C) .................................................. 1.020
       -320°F(-196°C) ........................................... Magnetic  
   Precipitation-Hardened
          74°F(23°C) .................................................... 1.010   
       -109°F(-78°C) .................................................. 1.040
       -320°F(-196°C) ........................................... Magnetic
Curie Temperature, °F ............................................. <-109
                            °C ................................................... <-78

Modulus of Elasticity *

*Determined by dynamic method on precipitation-hardened matererial.
**Calculated from moduli of elasticity.

Thermal Properties

*Calculated from electrical resistivity.
**Average coefficient between 78°F(26°C) and temperature shown.
***Calculated from chemical composition.

Corrosion Resistance

The composition of Inconel alloy 706 enables it to resist corrosion in various environments. The alloy's chromium content provides resistance to oxidizing media, and its nickel content provides resistance to reducing environments. The table below lists corrosion rates for the alloy in several acid solutions at boiling temperatures. Other tests have shown the alloy to have poor resistance to boiling 50% sulfuric acid and to boiling 38% and concentrated hydrochloric acid.

*Mils penetration per year.

Mechanical Properties

The high-strength of precipitation-hardened Inconel alloy 706 is maintained at temperatures up to 1300°F(705°C). Optimum mechanical properties for the intended service temperature are achieved by the use of one of two heat treatments.

Heat Treatments

The heat treatments used for alloy 706 are designed to produce either high tensile properties for room and moderate-temperature applications or high stress-rupture properties for applications up to 1300°F(705°C).
For optimum creep and rupture properties, the alloy receives a three-part heat treatment:

Heat Treatment A
Solution Treatment-1700-1850°F(925-1010°C) for a time commensurate with section size, air cool.
Stabilizing Treatment-1550°F(845°C)/ 3 hr, air cool.
Precipitation Treatment-1325°F(720°C)/ 8 hr, furnace cool at 100°F (55°C)/ hr to 1150°F (620°C)/ 8 hr, air cool.
For tensile-limited applications, a two-part treatment is used:

Heat Treatment B
Solution Treatment-1700-1850°F(925-1010°C) for a time commensurate with section size, air cool.
Precipitation Treatment-1350°F(730°C)/ 8 hr, furnace cool at 100°F (55°C)/ hr to 1150°F (620°C)/ 8 hr, air cool.
The heat treatments described above are based on metal temperatures and are derived from batch rather than continuous furnace operations. The solution-treatment procedures may not be applicable to continuous operations since continuous heat treatment is normally performed by short-time exposure in the hot zone of a furnace set at higher temperatures.

Tensile Properties

Room-temperature tensile properties of cold-rolled sheet in the solution-treated and precipitation-hardened conditions are given in the table below. All specimens were form the transverse direction.

Shear and Bearing Strength

The shear strength of Inconel alloy 706 in various forms and conditions is shown in table below. The values are from guillotine shear tests in which the specimens were loaded in double shear. Tensile properties of the specimens are included for comparision.
Alloy 706 sheet has high bearing properties. Table below gives bearing yield strength and ultimate strength of specimens from precipitation-hardened cold-rolled sheet. The data are from pin-type bearing tests*. The specimen was subjected to tensile stress while supported by a 0.250" diameter pin inserted with a tight fit through a hole whose center was 0.375" from the edge of the specimen (edge distance ratio of 1.5).

*Pin diameter of 0.250". Edge distance ratio of 1.5.

Working Instruction

Inconel alloy 706 has good working characteristics and is readily fabricated by conventional procedures for high-strength alloys. Of particular significance are the machinability and weldability of the alloy.

Joining
Inconel alloy 706 has excellent weldability. The alloy's columbium/titanium precipitation-hardening system provides a delayed response to precipitation-hardening temperatures. The sluggish aging response imparts a high degree of resistance to postweld strain-age cracking. The alloy resists underbead cracking (microfissuring). Most joints that are not highly restrained can be repair-welded and directly re-aged with no cracking. Pierce-Miller patch tests performed on four thickness of annealed sheet showed no cracking after specimens were welded, age-hardened, repair-welded, and re-aged. Repair welds in highly restrained joints, however, may require solution treatment before being re-aged.
Welding procedures for alloy 706 are the same as for Inconel alloy 718 is the recommended process. The alloy has also been joined by the flash-butt process; sound welds were produced with less upset pressure than that needed for alloy 718.
Precipitation-hardened welds in the alloy 706 exhibit high strength levels. The table below gives tensile properties of transverse specimens from autogenous gas-tungsten-arc welds in 0062" sheet. Tensile properties of joints welded with the gas-tungsten-arc process and Inconel Filler Metal 718 are listed below. The values are for transverse specimens from butt joints in 0.500" thick flats.

Transverse Tensile Properties of Autogenous Gas-Tungsten-Arc Weld*

*Specimens were from joints in 0.062" sheet. All specimens fractured in weld metal.

Transverse Tensile Properties of Gas-Tungsten-Arc Weld Made with Inconel Filler Metal 718*

*Specimens were from butt joints in 0.500" thick flat. All specimens fractured in weld metals.

Inconel 706 - Current Inventory Stock