Foundries are often called upon to produce special grades of Stainless steel.
One of the factors which can be controlled is the volume percent of ferrite. A
separate information sheet discusses the question “Why are Stainless steel
castings Magnetic?” and describes some of the advantages and disadvantages of
having the structure of the metal partially ferritic. But which factors
determine the amount of ferrite and how are they controlled? And how is the
amount of ferrite determined?
But first, what is ferrite? ASTM A800 defines ferrite as “the ferromagnetic,
body-centered cubic, microstructural constituent of variable chemical
composition in iron-chromium-nickel Alloys. This may be formed upon
solidification from the molten metal (delta ferrite) or by transformation from
austenite or sigma phase on cooling in the solid state (alpha ferrite).” In
comparison, austenite is the non-ferromagnetic, face-centered cubic phase which
is found in iron-chromium-nickel Alloys. It is important to note that both
phases are Alloyed, that is, they contain high levels of chromium, nickel,
molybdenum, etc. Ferrite should not be confused with “free iron” or “unAlloyed
iron” which may be on the surface of either wrought or cast material.
Note: Many common materials also exist in different phases. Water may be either
a liquid, a solid (ice), or a gas (steam). Carbon may be either graphite or
diamond. Silicon dioxide (beach sand) may be quartz, cristobalite, or tridymite.
The principal factors which determine the amount of ferrite are chemical
composition, casting section thickness, and heat treatment cycle.
1. Composition
ASTM Specification A800 explains how the average amount of ferrite in the
castings poured from a heat of molten Stainless steel may be estimated from the
chemical composition. Using the formulae given in A800, the amounts of the
“ferrite promoting elements” and the “austenite promoting elements” are combined
into a Chromium equivalent and a Nickel equivalent, respectively. Then the ratio
of the two (Creq/Nieq) is used to determine the expected ferrite from a graph or
from a table.
The ferrite promoting elements are Cr, Mo, Si, and Cb. The austenite promoting
elements are Ni, C, Mn, and N. Ferrite is reduced by lowering the ferrite
promoting elements (the Creq) and/or raising the austenite promoting elements
(the Nieq). Of course, each of the elements must also be held within the limits
established for the specific grade being produced.
2. Section thickness
The ferrite content will vary from one location to another in a given casting.
In general, a thicker section will have a higher ferrite than a thinner section
in the same casting. This effect is due primarily to the lower rate of
solidification in the thicker section. The correspondingly lower cooling rate
during quenching after heat treatment may also have an effect.
By way of further explanation, the material used to develop the relationship
given in ASTM A800 was taken from sand cast test bars, one to six inches thick ,
and solution heat treated at temperatures from 2000 to 2150 F for times of 1 to
4 hours.
It should be pointed out that the Foundry has very little control over the
section thicknesses of the casting; these are determined by the part designer
and the function of the casting. Some sections are cast thicker than originally
designed to provide machining stock or to provide improved casting integrity.
3. Heat treatment
Heat treatment temperature and the hold time at that temperature are known to
affect the final ferrite content of the castings. However, the research has not
yet been done to permit the prediction of the amount of change or even the
direction of the change given a starting chemistry and section thickness.
Fortunately, the effect of heat treatment is usually small relative to those of
composition and casting section thickness.
Ferrite is normally considered to be beneficial. It provides increased strength
and cracking resistance. In fact, the nuclear power industry specifies a minimum
of 5 to 7 % ferrite to assure the required level of weldability (avoidance of
“microfissures” during welding). This same minimum ferrite level provides much
superior castability (resistance to tearing during solidification) as compared
with fully austenitic grades.
Now, some comments about determining the amount of ferrite present in a given
casting. There are three principal techniques: metallographic, magnetic, and
calculation from the composition.
1. Metallographic.
This method is normally considered the “referee” method and is used to calibrate
the other methods. A sample is removed from the casting (thus it is destructive)
or from a representative test coupon (which may or may not actually be
representative as may be concluded from the above discussion). The sample is
polished and then chemically etched to provide the necessary optical contrast
between the ferrite and austenite phases. The polished surface is then examined
using a metallurgical microscope. The volume percent ferrite is determined using
any of a number of well established “manual point counting” techniques.
If the solidification rate is rapid (such as in thin walled castings or in weld
deposits), the ferrite phase is very fine and even the point counting method is
subject to considerable error. This is the primary reason that the welding
industry has opted to assign a “ferrite number” based on magnetic response
rather than to use the volume percent ferrite based on metallographic
measurements.
2. Magnetic
This method depends on the fact that ferrite is magnetic while austenite is
non-magnetic. Two variants of this technique are commonly used: magnetic
attraction and magnetic permeability.
1. Magnetic attraction
In these instruments, a magnet is placed in contact with the metal surface and
the force required to pull it away is measured. This force is an indication of
the amount of ferrite present. Typical instruments in this category are the
Magna-Gage and the Elcometer. A third instrument, the Severn Gage, determines
whether the sample being tested has a greater or lesser attraction to a magnet
than does a reference standard; it thus “brackets” the ferrite content between
two available standards.
2. Magnetic permeability
These instruments apply a low frequency current to a coil which is placed in
contact with the metal surface. The electronic circuitry measures the change in
magnetic properties of the coil and converts these changes into ferrite content.
A typical instrument in this category is the FeritScope.
Note that the magnetic instruments must all be calibrated against standards
having known ferrite contents. Note, too, that the magnetic methods all are
dependent on the magnetic characteristics of the ferrite which, in turn, are
determined by the precise composition, the morphology, and the thermal history
of the ferrite.
3. Calculation
The formula for calculating the ferrite content from the composition is given in
ASTM A800 which was discussed above. This method relies on the accuracy of the
chemical analysis; the uncertainty of the chemical analysis is the reason for
the scatter band in A800.
If you have any further questions regarding the characteristics of cast
Stainless steels, please contact the metallurgical staff at Stainless Foundry &
Engineering, Inc. Top |
