The document discusses nickel-based super alloys, including their properties, applications, common alloying elements, and weldability issues. Super alloys exhibit excellent mechanical strength and creep resistance at high temperatures due to their face-centered cubic crystal structure and alloying with nickel, cobalt, chromium, and other elements. Weldability problems with nickel alloys include hot cracking caused by sulfur and porosity caused by nitrogen, which require careful control of welding parameters.

1. GOVERNMENT COLLEGE OF ENGINEERING,SALEM
DEPARTMENT OF METALLURGICAL ENGINEERING
Weldability studies of Ni based super alloys
Guided by Submitted by
Mr.R.Vinothbabu,M.Tech P.Archunan (1763002)
Assistant Professor M.E-Welding Technology

2. SUPER ALLOYS
 Super alloys or high performance alloys, are alloys that
exhibit excellent mechanical strength and creep resistance
at high temperatures, good surface stability, and corrosion
and oxidation resistance.
 They typically have an austenitic face- centered cubic
crystal structure with a base alloying element of nickel,
cobalt, or nickel-iron.
 The development of super alloys has primarily been driven
by the aerospace and power industries.

3. SUPER ALLOYS
 Many of the industrial nickel-based super alloys contain
alloying elements, including chromium (Cr), aluminium
(Al), titanium(Ti), molybdenum(Mo), tungsten (W),
niobium (Nb), tantalum (Ta) and cobalt (Co)
 PROPERTIES
 Corrosion resistance
 Excellent mechanical strength and creep resistance at high
temperatures
 Good surface stability
 Resistance to mechanical and thermal shock

4. SUPER ALLOYS
 APPLICATIONS
 Turbine blades and jet/rocket engines
 Nuclear reactors
 Heat exchanger tubing
 Industrial gas turbines
 Very low temperature cryogenic applications
TYPES OF SUPER ALLOYS
 Iron base super alloys
 Nickel base super alloys
 Cobalt base super alloys

5. Ni based super alloys
 The essential solutes in nickel based super alloys are
aluminium and/or titanium, with a total concentration
which is typically less than 10 atomic percent.
 This generates a two-phase equilibrium microstructure,
consisting of gamma (γ) and gamma-prime (γ'). It is the
γ' which is largely responsible for the elevated-temperature
strength of the material and its incredible resistance to
creep deformation
 Two families of alloys - solid solution strengthened
alloys and precipitation hardened alloys

6. Weldability problems
 The most serious cracking problem with nickel alloys is hot
cracking in either the weld metal or close to the fusion
line in the HAZ with the latter being the more frequent.
The main source of this problem is sulphur but
phosphorus, lead, bismuth and boron also contribute
 Stainless steel wire brushing followed by thorough
degreasing with a suitable solvent is necessary prior to
welding
 Porosity can be a problem with the nickel alloys, the main
culprit being nitrogen. As little as 0.025% nitrogen will
form pores in the solidifying weld metal

7. Weldability problems
 With the gas shielded processes, gas purity and the
efficiency of the gas shield must be as good as possible.
 The weld pool, in addition to this surface film, is also
sluggish and does not flow freely as with a carbon or
stainless steel.
 Although stringer beads may be used, a slight weave to
assist the weld metal to wet the side walls of the
preparation is beneficial. In addition, weld preparations
must be sufficiently wide to enable the welder to control
and direct the weld pool; an included angle of 70 to 80° is
recommended for V butt welds.
 U preparation included angle of 30 to 40° is acceptable.

8. Weldability problems
 A further characteristic of nickel alloys is that
the amount of penetration is less than with a
carbon or stainless steel. Increasing the
welding current will not increase penetration.
 It is recommended that the thickness of the
root face should not be greater than 1.5mm in
a TIG butt weld.
 Precipitation hardening alloys require closer
control of the welding process variables
because of the possibility of ageing and
formation of refractory oxides during welding

9. Phase structure of super alloys
 The constitution of superalloy microstructure depends on
the alloying elements, which influence the strength of the
matrix, feature of the precipitate, precipitation of carbide
particles, resistance to oxidation or hydrogen environment,
etc
 The structure is composed of austenitic matrix γ with face
centered cubic lattice (FCC) and other secondary phases:
 γ' – FCC structure – Ni3(Al,Ti)
 γ''− tetragonal body centered – Ni3Nb
 η – hexagonal ordered – Ni3Ti
 δ – orthorhombic structure – Ni3Nb
 Carbides (MC, M23C6, M6C , M7C3)

10. Phase structure of super alloys

11. Effect of alloy elements

12. Effect of alloy elements
 Solution hardening effective elements are W,Mo,Cr,
 Solution hardening weaker elements are Fe,Cu,Co
 Gamma prime formers are Ni,Ti,Al
 Gamma prime is affected by Co,Fe,W,Mo,Ta
 Carbide formers are Ti,Zr,Nb,Ta
 Carbide affected by Co,Mo,Fe,W
 From this Co, Fe,Mo,W are used for same purpose like
solid solution strengthening,affects gamma prime phase and
affects carbides

13. Effect of alloy elements
 Copper improved the resistant to non-oxidizing acids.
 Chromium increases resistance to oxidizing environments.
 Iron primarily decreases cost and price of the alloy but it
does not improve and anti-corrosive properties of nickel.
Iron increases solubility of carbon in nickel.
 Cobalt increases solubility of carbon in nickel, similar to
Fe, which increases resistance to high temperature
carburizing.
 Molybdenum increases resistance against non-oxidizing
acids. Mo increases resistance against pitting and crevice
corrosion. Mo is an important strengthening element for
alloys with increased firmness at high-temperature
applications.

14. Effect of alloy elements
 Tungsten increases resistance to non-oxidizing acids and
local corrosion, similar to Mo. It is also a significant
strengthening element
 Silicon is contained in nickel only in small amounts either
as a trace element from the deoxidation process or as an
additive to improve the resistance to high-temperature
oxidation