fabricability and design considerations of heat resistant alloys

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fabricability and design considerations of heat resistant alloys
FABRICABILITY AND DESIGN CONSIDERATIONS
OF HEAT RESISTANT ALLOYS
James Skarda
Rolled Alloys Inc.
Temperance, Michigan, USA
Abstract
ing contamination, or to contain a generated
atmosphere that is neutral to the work, or to
develop enhanced properties of the work bei ng
processed. Examples of this would be muffles,
retorts and radiant tubes.
The second application is conveyors which
include fixtures, jigs, grids, trays or boxes
that hold the work as it is carried throu p, h the
fu rnace. Each has its own design criteria.
Common to both is to use the least amount of
material that is necessary to resist the opera ting stresses. The difference is the frequency
and rate of heating and cooling.
Muffles, retorts and radiant tubes general ly are not subject to the rapid fluctuations
associated with conveying systems but are
exposed to thermal aradients because of the
temperature differences on the inside and outside surfaces. Other considerations are expan sion rates and awareness of the fuel and thermal
efficiencies.
A rather oovious statement is to use only
the mass (thickness) that can support the
applied stress. This will certainly reduce
material costs. However, even if this were not
a factor, there are other valid considerations.
An aspect is the fuel efficiency as related
to the heat required to raise the temperature to
operating levels. It can be formulated by the
equation: Q = SM (T2 - T 1). This indicates
the amount of energy Q requi red to rai se the
temperature. The temperature of a container
can be calculated by multiplying the specific
heat times the mass (M), or weight, times the
difference (T 2 - T 1). The AT is dictated by
the process requirements and variations possible
are quite limited if not fixed. Since variation
in specific heats is insignificant, the real variant
occurs in metal thickness. The thicker the wall,
the more energy is required to raise the temperatu re to the desi red level.
Another consideration is heat transfer.
The factors affecting environment resistance
thermal fatigue, shock resistance and fabrica bility of heat resisting alloys as used in thermal
processing equipment in heat treating equipment. A review of metal properties as related
to design considerations and fuel efficiencies
in muffles, retorts, fixtures and radiant tubes.
The surface characteristics of oxidation, green
rot and metal dusting as affected by envi ron ments, sigma formation, are reviewed with
general guidelines for fabrication and welding
techniques.
FOR THE PU RPOSE of this paper, we will con centrate on the family of alloys, primarily nickel and chromium combinations, that serve
industry in thermal processing equipment) in
particular. To differentiate these alloys from
the more exotic and special conditions of space
and turbine blade materials, the general para meters are:
Long - time use (10,000 hours)
primari Iy single phase, fabricabi lity, avai labi Iity
in a variety of product forms and reasonably
priced for use in a temperature range of 12000F
to 2300 oF. It should be noted that the tempera ture is related to metal temperatures, not
processing or operating temperatures.
There are several alloys that can perform
satisfactorily in this temperature range and
selection becomes a matter of detailing environments, cycles and heating rates. Weill touch
on two areas; design and fabrication considera tions and alloys.
FACTORS AFFECTING HEAT
RESISTANT ALLOYS
The two main applications as related to
the heat treat industry are containers and con veyors. By an arbitrary definition a container
is a method to contain an atmosphere prevent-
Fou rier l slaw :
279
9-.=
t
KA (T 2 - T 1)
[
indicates
does occur.
Metal dusting, sometimes known as metal
erosion, will also form due to specific atmosphere or environmental conditions. It will
usually occur in stagnant atmospheres that
are reducing or carburizing to the metal in a
temperature range of 800°F to 12000F. The
corrosion product, when still found in place,
is a black dust composed of graphite, metal,
metal carbides and metal oxides. This dust
mixture is usually magnetic. The tendency
increases with the nickel content. Sections
of anchor bolts are subject to these conditions.
The surface appearance of an anchor bolt is
shown in Figure 3.
Sigma will cause the properties of some
heat resistant alloys to change after a few
hundred or thousand hours in service and
become brittle losing their toughness and ductility. This usually happens with high chromium, low nickel grades such as 309 or 310
(Figure 4). The most common problem is the
formation of a very brittle phase usually in the
grain boundaries identified as sigma. Differentiating this from carbide precipitations,
which occur in the same temperature range,
there is minimal chromium depletion. Sigma
forms in the 1100° to 1600 0F temperature
range peaking at 1300 0F. It is a time temperature formation. The amount of sigma
usually found in heat resistant alloys is not
seriously detrimental to the alloy at high
temperature. However, sigma can completely
embrittle an alloy when it reaches room
temperature and can be a source for failure
during frequent thermal cycling. It can be
eliminated by heating above 1600 0 F.
the ability to transfer heat through metal and
can also be related to thermal efficiency. This
states that the rate of heat transfer is inversely
proportional to the thickness and proportional
to the surface and temperature difference. The
Q /t is the heat energy per unit of time, K =
thermal conductivity coefficient, T 2 - T 1 =
temperature differences between the outside
and inside surfaces, similar to applications of
muffles or radiant tubes, A = su rface area,
L = mass (wall thickness). As the thickness
increases, the rate of heat transfer will decrease.
To illustrate the effect of mass on thermal
gradients, generally it's more of a problem in
conveyor systems, an example is round bars
used in a heat treat basket (Figure 1). Making
the bar "stronger" by increasing the cross section does not necessarily make it better because
it can increase the thermal gradients between
the core and outside surfaces. In this particu lar illustration, replacing a 3/8" diameter with
a 1/2" diameter round bar increased the cross
section by 78%. The larger cross section increased the thermal gradients that occurred
whenever the basket was quenched. I twas
unable to absorb the increased and repeated
stresses and developed a maze of intergranular
fatigue cracks. The bar then fails, not by
load, but by thermal gradients. A better way
to make the bar stronger is to change the alloy
content.
Oxidation resistance is another property
important in performance levels in thermal
equipment. Basically RA alloys have oxidation
resistance to 20000F. The relative oxidation
resistance of various alloys can vary up or
down, depending upon the load and the rate
and frequency of heating and coo Ii ng. Whi Ie
we do think, along with several other testers,
a 20 hour cycle test more closely approaches
the conditions normally associated with the
heat treating industry. Figure 2 illustrates
the cyclic scale resistance of various alloys at
20000F for 500 hours.
Two other surface conditions can occur to
alloy at elevated temperatures - green rot and
metal dusting. While not typical sources of
failures, both conditions are possible and could
be of interest. Because of its appearance,
green rot is sometimes taken for a sulphurous
deposit, and when observed becomes a mystery
when neither feed stock or the envi ronment
contai ns sulphur. I t is considered to be the
selective reduction of nickel and i ron under
conditions that do not reduce the more stable
chromium oxide. The selective reduction forms
voids leaving a greenish appearing chromium
oxide. Upon initjal inspection it looks like
rotted wood that is yellow-green in appearance.
It appears that the propensity to form is rela ted to the nickel content; the more the nickel
the more likely to occur. Because of the nar;ow
parameters required by the environment to
produce green rot it will only occasionally be
observed, but should be recognized when it
FABRICATION AND DESIGN
Selection of an alloy for a specific application may involve two considerations : what
composition should be used, and should it be
wrought or cast form. Although the latter
might be considered a design function, it is
included here because, (1) the basic principles of design discussed under that subject
are applicable to structures in general,
whether cast, or w rought, and, (2) the
procedure followed in selecting the composition
would apply to either cast or wrought form.
The Advantages of Cast Are:
280
1.
Initial cost - Since the casting is essentially a finished product as - cast, its cost per
pound may be less than a fabricated item.
2.
Strength - Simi lar compositions are inherently stronger at elevated temperature in cast
form than in wrought; this is attributable
to the very coarse grained as - cast structure and the fact that most equivalent cast
compositions are modified with hic-her
carbon content to improve castabllity.
3.
Shapes - Certain shapes can be produced
as a casting are not made as a mill form, or
that cannot be fabricated economically from
the wrought forms available.
4.
Compositions - Some compositions are
available in castings that lack sufficient
ductility to be worked into wrought forms.
These fissures can also affect the ability
to take forming. Use as large a bending radius
as possible . The A IS I's recommendation for
austenitic steels is forming to an inside radius
twice the thickness of the material, and on more
highly alloyed metal it could be four times the
thickness. In the real world, this is usually
much too generous for most fabrications and
we often see metal bent to much smaller radi i.
If the inside radius has to be sharp, it is good
practice to dress the edges to reduce the
amount of fissures that can be generated
through shearing or punching . The method
for best edge conditioning is saw cutting and
grinding.
Welding heat resisting alloys requires
specific techniques easily acqui red through
experience, but does not requi re sophi sticated
techniques.
The Advantages of Wrought Are:
1.
Section Size - There is practically no limit
to the section sizes available in wrought
iron. Thinner sections often permit a
weight reduction of 50% or more, making
the initial cost of fabrication no more than,
and perhaps less than, a casting; decreased
tare weight results in substantial reduction
in operati ng costs in many instances.
2.
Thermal Fatigue - Thinner sections that
reduce thermal stresses, and the inherently
greater ductility of wrought materials,
usually promote better resistance to thermal
fatigue, especially under cyclic conditions.
3.
4.
5.
6.
Welding Guidelines Are :
Soundness - Wrought materials are normal ly free of internal and external defects
such as shrinks, porosity, cold shuts, etc.
Surface Finish - The smooth surface of
wrought materials is often beneficial in
avoiding focal points of concentrated or
accelerated corrosion or carbon attack.
Availability - Wrought heat resisting alloys
are available from stock in numerous forms,
permitting immediate procurement, and mini mizing the need for excessive inventory
or maintenance supplies.
1.
Correct joint preparation - use beveled
joints. This is particularly important in
heavy plate.
2.
Correct power setti ng - in general, use
as low heat input as possible.
3.
Use stringer beads rather than weavi ng.
4.
Keep interpass temperatures low.
5.
Maintain a reinforced, or convex, bead
contour.
6.
Do not preheat, or postheat.
Implied in the suggested guidelines is an
attempt to get all the weld metal to solidify at
the same instant. However, because of the
low thermal conductivity of alloys, the base
metal does not dissipate the heat as rapidly and
reduces the cooling rate of the weld deposit.
Because of the sharp thermal gradients that
might exist in such a condition, there is a
tendency for bead cracks. If bead cracks do
occu r, it is good practice to reduce the dwell
time, or use multipass welding, laying down
less metal per pass.
Perhaps the most common cause of the
weld failures in service is lack of adequate
penetration. I f penetration is not complete so
that a cavity remains within the joint, the
repeated expansion and contraction of the
metal around the cavity initiates fracture from
within. This is aggravated by the fact that
the absence of metal in the cavity prevents
normal distribution of stresses, creating stress
concentration. These sharp and irregular contours frequently existing within the void fur ther contribute a notch or stress riser effect.
(Figures 5, 6a and 6b). To achieve complete
penetration, chamfering or gapping of the
joint is necessary.
Smoother weld contours with freedom from
craters are roost desirable, especially for service
No pattern cost.
In fabrication, the heat resi sting alloys
work differently than mild steel. Whether
bending, forming shearing, machining, or wel ding, there are some differences that should
be considered based on the realization of the
alloy's properties.
The most important thing to keep in mind
when shearing an alloy for high temperature
application is to avoid cracks and rough edges.
Torn metal is the focal point for stresses which
can lead to failure and a shorter service life.
The yield strengths are a little higher in alloy
in the hot rolled, annealed condition and the
tensile strengths are a lot higher compared to
mild steel. Shearing capacity has to be apprOXimately 50% greater. Good shearing practice is to cut 20% of the metal and the remaining
80% is fractured. A larger percent of the frac ture ratio can result in tearing and develop
small edge fissures which become notches when
used at high temperatures due to expansion of
the metal. Punching is a more critical opera tion and more susceptible to tearing. If economically feasible, drill rather than punch holes
and on thick plates, saw cut rather than shear.
281
resulted in an exothermic reaction raising the
temperature above the melting point. It
should be noted that any deposit on a container wi II prevent heat dissipation and wi II cause
hot spots . The point here is that nickel chromium alloys do not melt at 1800° or 2000oF.
If there is evidence of molten metal, the temperatu re at that point probably was in the
vicinity of 2400oF.
Outstanding performance has often been
observed. Illustrated is a muffle used in a
continuous hardening operation (Figure 7).
The muffle has been removed for straightening and put back into service. The picture
shows the condition of the muffle after 10
years, operating at 1550 to 161 OOF, two shifts
per day, five days per week, idle at 1400oF,
endothermic atmosphere.
Admittedly, this is exceptional life, probably a record, but it does indicate the capability
of a fabricated muffle, with the best combination of alloy and fabricating techniques compatible to the envi ronment.
in carburizing atmospheres. Butt welding is
preferable to lap type of welding and wi II be
discussed later.
Heat resisting alloys are not free machining.
They require sharp and heavy tools. Machina bility rating is roughly 40% that of mild steel
machine stock. Relatively low speeds with cuts
deep enough to get under the skin are recommended. The rake of the tool should be such that
it does not rub on the material which would
work harden it, causing havoc with the tool
life.
Basic guidelines for design is to maintain
minimum cross section, uniform cross section
and allow for thermal expansion. Attempts
should be made to eliminate fixed positions or
rigidity. This is really not a reflection on the
welding characteristics but a recognition of
thermal expansion . An attempt at this is an
articulated design which allows for freedom of
metal movement. Often, applied stresses for a
conveying system do not permit this, but
when permissable it is quite effective. Uniform
cross section is also an important consideration.
Again, we relate to thermal gradients developed
by differences of metal temperature. As indi cated previously, butt welding is the preferred
method of joining because it maintains uniform
cross section. If a joi nt is lap welded, the
cross section will generally be doubled. Due
to the low thermal conductivity of heat resis ting alloys, this can result in a significant
difference of temperature between the cross
section area. The importance of this is also
illustrated by a typical cast alloy grid used in
continuous pusher furnaces. The inner sections of the crossing member are cored. This
results in a hole at each intersection which not
only saves material but accomodates uniform
cross section at each intersection.
The above should not be considered intimi dating. The suggestions are not really due to
the difficulties of fabricating the alloys, but
rather a reflection of the way it is used. The
harsher the service the more critical it is to
develop good fabricating and welding techniques.
A NEW ALLOY
A recent alloy development is the addition
of small amounts of rare earths added during
pouring of a heat to result in micro- alloying
(MA) that greatly enhances the high tempera tures properties of chromium- nickel austenitic
alloys. RA 253MA is the first commercially
available grade to use micro-alloy to extend
the useful range of a lean nickel - chromium
alloy base (21Cr- 11Ni - 1. 7Si - 0.04Ce). It has
had extensive testing, including over 2.6
million hours of creep rupture testing through
20000F and has received ASME approval to
1650 o F. The improvement in the oxidation is
indicated in Figure 8. The yield strength
and the tensile strength of RA 253MA do not
drop off rapidly with increasing temperature.
At room temperature the yield point is over
40% higher than T 309. At 1500 0 F the yield
strength is over 60% higher. It also has at
least 4 times the creep strength of T 309 at
1600 o F.
Figure 9- 10 illustrate comparative creep
and stress rupture data . A practical test
illustrates the higher strength levels.
Figure 11 shows 1 mm (.039") thick sheetmetal rings made of different materials. The
rings were fastened to a steel plate and
annealed in a car type fu rnace at 1830 0 F
(1000 0 C) . After 35 hours the rinQs of T321
and 310 were heavily deformed due to creep.
The RA 253MA was fully intact with no measurably deformation equaling the creep
strength of the N i alloy.
APPLICAT IONS
Fabricated r adiant tubes are used exten sively in thermal processing equipment with
proven cost- effective service life. On occasion,
a tube will fail prematurely, not related to the
engineering properties of the metal.
Illustrated is an example of a tube which
obviously shows melting of the base metal
(Figure 6). Records indicated that the operating temperature never got beyond 1850 oF,
which is probably true, and the past performance had been excellent. In this particular
case , soot formation had accumulated on the
surface of the tube, causing two problems .
It became carburized , which can lower the
melting point by perhaps 100°F. The burning
of the soot d ue to oxi dizing cond itions p robably
282
FIG. 1
Effects of Thermal Gradients
I\)
CD
W
ABOUT 7X
ETCHANT MIXED ACIDS
Similar fatigue cracks in 112" and %1" dia. bars from the same basket, illustrating the
effect of cross section on thermal gradients and resultant stresses. In quenching service even small cross sections often fall ultimately in this manner.
FIG. 2
Cyclic Oxidation Test
20000F
20· Hour Cycles in Air
50 I
N
<XI
~
-
§
40 I
I
II
N
E
(,)
C ')
.sc
30 I
I II I
·co
-
CJ
~
C')
~
20 I
.,1
10 I
100
200
'J
Note: Actual weight loss figures are valid
A:»)::C I
only for the specific conditions of the test.
Neither this nor other laboratory oxidation
data should be used to quantitatively predict
metal wastage in actual service.
300
400
500
Exposure Time, Hours
Failure of Furnace Anchor Bolts Made of
RA 330 at White Motor Company
I\)
(X)
0'1
Photograph of the pitted area of anchor bolt.
FIG. 3
TTY-Diagram for Sigma Phase Precipitation
FIG. 4
Aging Temperature, °C (OF)
1000
(1800)
e=253MA
0 =3105(1)
~___- -
0 -3105
X =310
(1700)
I\)
(J)
m
900
X-310
e-253MA
(1500)
800
(1400)
e
700
100
200
( 1) Si = 1. 83
500
1000
2000
5000
10000
Aging Time, Hours
c
o
..1&
ic
l.
-a
j-
287
288
Weld Penetration
FIG.6A
FIG. 68
CORRECT
INCORRECT
This fully welded joint can resist both thermal and
mechanical fatigue.
The unfused void in this fillet weld acts as a stress riser
and may cause premature failure.
I\)
ex>
<D
290
FIG. 8
Cyclic Oxidation Test
2000°F 20-Hour Cycles in Air
10
, _
8
I\)
~
N
E
u
0)
6
E
c
'i
"~
4
0)
:i
3:
2
100
200
300
Exposure Time, Hours
400
500
FIG. 9
Minimum CreeD Rate
0.0001 Percent Per Hour
20,000
10,000 ~"""'Iiii::li
.........
---. -
., .......... '-~-............ I'....
" ..."'"
-" "" "'-, ' ....... '"
~
~ ,,~,
5,000
~,~
'ec..n
N
<0
N
"
til
!
~,
2,000
U;
Cii
"'
~
1,000
500
'"
"
",-KADI
~" ~
~
.., ~ ~.
"""
~-,
,
'" , RA:M
~.
"
" ' ...
"
...:
"
-vvT
~330'
'"'\
" ' ...
~A446
200
1300
53MA
~
.......
~
I'-
'\.
100
1200
......."".
"-
1400
'"
1500
",
,~
,,
,,
1600
Temperature OF.
1700
1800
Stress to Rupture
FIG. 10
10,000 Hours
20,000
10,000
~
..
-
"""""",
',"",,"
~
....~
~
'"'
"""" X
""1IIIii ~,
'~',
5,000
,~
I\)
CD
W
'in
Q.
2,000
en
U)
-e
en
1,000
'" ~
,
~
25311A '" RA 333
...... -.
'~'
RA
""
'310'" ~,
, ....
'"
'~
....RA 446
""""""
~
"'
~
' ...." ~
1'."""""""-
"'"
-'-.
""""" ""'-
~
500
'
....
~
200
100
1200
1300
1400
1500
1600
Temperature of
'"
" '~
rw;;;::
...
~
~
1700
1800
In.
FIG. 11
Relative Creep Strength
1 mm (.0393") thick rings after
annealing 1832 FO for 35
hours.
F\)
<0
~
1.
2.
3.
4.
RA 235MA
Ni base alloy (60 0/0 Ni)
AISI 321
AISI 310
::JA
.