Nickel alloys work harden quickly and the high
pressures developed during machining result into hardening effect that
describes further machining and may also cause warping in small components. By
using cold drawn stress relieved material is recommended for machining. Hot
rolled is less needed and annealed is nominally recommended for several
applications. Specific machining practices are needed. Use sharp tools with
positive rake angles. Adequate feed rate and cut depth are essential and tools
must not be permitted to rub the work. In fact in these supreme conditions,
stress is created that may result into damage of the work. For extreme
dimensional consistency, it is recommended to rough component to size, stress,
relieve it and finally finish it to the size. Stress relieving has nominal
influence on dimensions, however may affect the mechanical characteristics.
Categorization of Alloys:
For machining
of nickel alloys, these are categorized in 4 groups and 2 subgroups:
Group A : Includes alloys that contain 95% or higher nickel
content. These materials have mechanical strength and good toughness. These are
hardenable through cold processing. These are very gummy in the annealed and
hot processed condition and cold drawn material is preferred for excellent
machinability and smooth finish. The alloys belonging to group A are nickel
200, nickel 201, 212
Group
B: This group contains nickel-copper alloys, that
have greater strength and nominally smaller hardness as compare to alloys in
group A. They are only hardenable through cold processing. Cold drawn or Cold
drawn plus stress relieved material offers the excellent machinability and
smooth finish. The alloys in this group are Monel 400, Invar 36, Invar 48 etc.
Group C: Widely comprised of solid solution
nickel-chromium-iron alloys that are identical to austenitic stainless steels.
These are only hardened by cold processing and readily machined in cold drawn
or cold drawn plus stress relieved condition. Group C consists of Inconel 600,
601, nimonic 75, Incoloy 800, Incoloy 800HT, 825 etc.
Group D: It mainly comprises of age hardenable alloys. This
group is further categorized in to –
Group D1- It includes alloys that are not aged and
comprises of Incoloy 925, Nickel Span C902.
Group D2- It consists of alloys of group D1 in aged
condition and various other alloys in aged and unaged forms including Inconel
617, 625, 718, X750, Monel K500, Ni span C902, Hastelloy C276 etc.
Group E – In this group, monel R405 is present that together
offers hardness, strength and corrosion
resistance properties similar to alloy 400 in addition of supreme
machinability. Although, its surface finish quality is not lower than alloy
400.
Cutting fluids
Any kind of cutting fluid can be utilized in the
machining of nickel alloys. In several applications, nickel alloys offer better
performance in dealing with general sulfurized oil, sulphur offers enhanced
lubrication and antiweld characteristics. If the oil temperature and sample
becomes sufficient during machining to result into brown sulphur staining of
the sample, the stain can be easily eliminated by using a cleaning solution of
the sodium cyanide-sulfuric acid kind. It must be done prior to any heat
processing including welding as during additional exposure to high temperature
the staining may result into intergranular surface attack. To prevent
intergranular corrosion, the components must be submerged in a cleaning
solution for a time sufficient to eradicate the stain. Large pace machining
operations that result into high temperatures may preclude the use of a
sulfuried oil due to sulphur embritlement of the carbide equipments. Various
sintered carbides keep a nickel or cobalt matrix that is susceptible to sulphur
corrosion at high temperature. Although, flooding the cutting place with the
cutting liquid normally quenches the tool sufficiently to prevent damage of the
carbide bond.
Water base fluids are normally recommended in the
high speed turning, milling and grinding due to their vast cooling effect. These
can be soluble oils or chemical solutions. Besides of grindings that is
completely based on quenching and flushing, chemical activity is always needed
and is normally offered by chlorine, amines and other chemicals. For slower
processes including drilling, boring, tapping and broaching, extensive lubrications
and rich mixtures of chemical solutions are required. Oils are used for
drilling of Nickel 200 and Inconel alloy X750. In drilling and tapping of small
diameter holes and in various processes in which lubricant travels and chip
flushing are limited, solvents will enhance the functionality. Such slightly
viscous fluids can be utilized solely for dilute mineral and lard oils. The cutting
fluid of the spray mist is sufficient for normal turning processes on various
alloys.
Single point turning tools utilized for cutting
nickel alloys should have positive rake angles so that the metal is cut rather
of push as if negative rake angles were utilized. The secondary function of a
rake angle is to guide the chip far from the finished surface. The side cutting
edge angle is second in significance after the rake angle. It should be
sufficiently big to offer clearance as well as small to offer required support
to the cutting edge. The nose radius that connects the end and side cutting
edges, strengthens the tool nose and assists to radiate the heat produced in
cutting. Nose radii are specified with various other preferred tool angles.
Chip Control:
Nickel alloys show a nominal of chip disposal issues
when cut with tools that are adequately designed chip curlers or breakers. High
speed steel equipments need chip curlers usually referred as lipped tools. It
must include adequate rake angles for the alloy and must be large and adequately
deep to result into curling the chip and breaking it without forcing it. Carbide
tools must have chip breakers. With these equipments, tool rake angles are
plane surfaces that end the chip breaker wall.
Tools
Carbide
tools allow the maximum cutting rates and are preferred for turning operations
including uninterrupted cuts. Cast alloy tools are preferred for turning of
alloys in group A at the adequate cutting rates. High speed steel tools are used
for interrupted cuts like as occur in the hardening of an uneven surface. They
are also used in finishing to close tolerances, finishing to the smooth
surfaces and cutting with nominal work hardening.
Planning & Shaping
The
tools utilized for planning and shaping are identical to lathe tools. For hard
plaining, the top rake angles is of great significance, it must be positive and
of adequate magnitude to receive sufficient cutting action. Adequate chip,
provided from adequate pairing of side cutting edge angle and rake angle, is a
nominal curl that curves before the tool and ruptures as it hits the component.
The
gooseneck kind of planer tool must be used for finishing. Its spring action
develops smooth dust, the cutting edge of the gooseneck tool must be behind the
center line of the clapper box pin so that the tool will spring far from the
cut and doesn’t dig in. Cutting fluids are not important for hardening, however
sulfurized oil must be applied to the sample for smooth finishing cuts. Speeds
are normally 80 – 85% of those utilized for turning. Wide sections can be
parted in a planer with the addition of a gooseneck finishing apparatus.
Broaching
However group
D alloys are broached more evidently in the age hardened condition, large
pressures are needed for such materials in aged or unaged condition. Sulfurized
mineral oil is preferred as cutting fluid.
Drilling
In drilling
of nickel alloys, stable feed rates must be used. If the drill dwells, wide
work hardening of the metal at the end of the hole will make it hard to resume
cutting and may result into damage of the drill when it does take hold. The system
must be firm as feasible. Stub drills are preferred. Drill jigs must be utilized
when feasible. Standard high speed steel drills are satisfactory for the
general purpose drilling of alloys in group A and B. Heavy-duty high speed
steel drills with a heavy web are preferred for drilling of alloys in group C
and D. Cobalt bearing high speed steel drills offer prolong tool performance.
Cutting pressures are decreased and a positive effective rake is sustained. Crankshaft
drills are significant for developing deep holes. These drills keep a heavy web
and a helix angle nominally more than normal, the web is thinned at the chisel
level. The spacing of the grooves must be staggered among two cutting edges. The
influence of this serration will be to develop narrow chips with nominal tendency
to foul in the helical flutes.
Reaming:
Fluted
reamers for nickel alloys are developed as standard items and are featured by
high speed steel component, right and cut, right hand helix and positive radial
rake. The service speed for reaming must be around 2/3rd the speed
for drilling the same material, however not very high to result into chatter. Other
factors performing chatter do not have sufficient firmness in the system in the
setup, misalignment and tedious tools. Reamer feed into the work should be
around 0.0015 inch to 0.004 inch flute per move. Wide feed rate decreases the accuracy
of hole dimensions and finish quality. For reaming of nickel alloys, adequate
stock must be eradicated so that the nonwork hardened or non glazed material is
cut.
As flat and built- up reamers comprise of a specialized
area of finishing in the diameters, cuttings speeds and feeds should be prepared
for each process.
Tapping and Threading
The crucial factor in tapping is the choice of the
suitable drill size. The normal tap drill selection tables used in several
years are based on 75% of complete thread dept and were developed by experience
with small strength materials like brass. Advanced high strength, although
offer sufficient holding strength with small thread depth. This is specifically
true for holes stored to a length of 1.5 times the bolt diameter. Vast cutting
fluid is required for both hand and machine tapping with liquid chlorinated was
recommended.
Lathe Threading – Thread cutting
lathe tools are based to the factors mentioned for turning tools, however the
angles on the threading tools are smaller than those on turning apparatus,
however the angles on threading tools are smaller than the turning tools to
support the small nose of the threading tool.
Die threading: The threading dies should be kept and sharp
and flooded form with the cutting fluid, sulfurized or full mixture of soluble
oil or chemical solution before use.
Thread grinding- Outer
threads can be developed in alloys of group D-2 through form grinding.
Milling
The
basic needs of milling are correctness and smooth finish, hence it is
significant to keep sharp tools and firm machines and fixtures Large speed
steel cutters are fit as milling often includes an interrupted cutting process.
The suitable feed and speed are crucial as light a feed, suitable rubbing will
result into widely work hardened layer. Since rubbing in the initialization of
the cut is prevented by climb milling, this method is recommended to
traditional milling. Besides, the downward motion of the cut supports firmness
and eliminates chatter.
Sawing & Cutting
Nickel
alloys are cut off by traditional methods. The cutting operations includes the
alloys in groups A, B, C, and D-1. Power hackshaws are operable at 90 strokes
per minute for alloys of group A and B and about 60 strokes per minute for
alloys in group C and D-1. The work must be kept immersed in water soluble or
sulfurized cutting oil.
Grinding:
Procedures
for grinding of nickel alloys are alike to the procedures used for steel. When
a nominal magnitude of metal is needed to be eradicated, the finishing process
can be conducted through a grinding equipment by using a hard and a fine grind.
If a fully accurate ground finish is needed, specifically on material of hard
tempera, the work must be permitted to lower the temperature to room level
subsequent to final hardening cut or grind. It permits redistribution of the
internal stresses and cause damage, if improved in the final grinding process.
To achieve the excellent results, the alloys must be wet from the base.
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