Hastelloy X --- 22Cr 1.5Co 9Mo .6W 18.5Fe balance Ni
Nichrome V --- 20 Cr balance Ni
Waspaloy --- 19.5Cr 13.5Co 4Mo 3Ti 1.4Al .006B balance Ni
HASTELLOY S --- 16Cr 15Mo .3Al .06La balance Ni
C263 --- 20Cr 20Co 6Mo 2.2Ti .45Al balance Ni
As you can see all the nickel alloys listed above contain more than 50% nickel.
Some of them contain fairly large amounts of iron but some of them don’t contain any iron all
Properties of nickel alloys:
Low conductivity – this is why nickel alloyed with chromium is often used for heating elements. Because of a low conductivity heating elements made from nickel alloys get red hot quickly when electric current is applied.
Resistance to oxidation and scaling – nickel alloys can be heated to red hot temperatures repeatedly without scaling.
Corrosion resistance – nickel alloys don’t rust and are very corrosion resistant to certain acids and alkaline solutions.
Strength and hardness - alloy additions of aluminum and titanium allow some heat treatable nickel alloys to be strengthened substantially by heat treatment.
Nickel alloys will not harden from heating and quenching. Only certain nickel alloys can be hardened by heat treatment and it involves holding the metal at an elevated and controlled temperature for a long time. This heat treatment is known as aging or precipitation hardening.
The two main properties of nickel alloys that most determine what welding techniques to use are:
1. Low thermal conductivity
2. Sluggish puddle
A low thermal conductivity means that heat does not dissipate easily. Another way of saying that is that heat builds up very easily.
Stainless steels have low thermal conductivity also and we know one of the tricks to prevent heat build up is to “get in , and get out” you know, don’t fart around ….hit it, get moving, and get out.
Chill blocks to pull the heat out are good for stainless, and good for nickel alloys too.
The sluggish puddle is another property of nickel alloys , especially when tig welding. That is why aerospace welders who tig weld nickel alloys often use an oversized tig cup. The extra argon shielding you get with a big cup helps keep the hot tip of the filler rod shielded and that keeps the puddle more fluid. A more fluid puddle needs less amperage to move the puddle. Less amperage means less heat input, less distortion, less likelihood for cracking, and less heat affected zone….all good things.
3 of the most widely used nickel alloys in the aerospace industry are :
1. Inconel 713c – turbine blades
2. Inconel 718 – engine cases, compressor blades
3. Hastelloy X – hot section parts like burner cans that operate at red hot temperatures and withstand thousands of heating and cooling cycles.
In other industries like power generation, nickel alloys have another application.
Tons upon tons of inconel 625 welding wire are used each year to weld cladding on steel boiler tubes to extend their life.
Inconel 625 is one of the most widely used nickel alloy filler metals because…
Inconel 625 welding wire is used routinely to weld layers of heat and corrosion resistance cladding on less corrosion resistant steel .
It stands to reason that if inconel 625 can be used to clad steel or chromium steel boiler tubes, it is also a good choice as a welding rod for welding steel to other metals.
Another property of nickel is that it remains soft and ductile after the weld is done.
Nickel welding rods will not harden by heating and quenching.
No matter what the base alloy is, a nickel welding deposit will not be hard and brittle.
That’s some useful information.
So lets recap.
We know now that nickel alloys have a low thermal conductivity, resist scaling and oxidation at elevated temperatures, will not harden--even when used to weld carbon steel or cast iron, and we know that nickel alloys can retain these properties even when subjected to multiple heating and cooling cycles.
With these properties, it makes perfect sense that inconel 625 would be used for cladding boiler tubes in power plants.