Advanced Concepts in Thermal Analysis for foundry metals
Shringae and Gas Arrests
Shringae and Gas Arrests
Shringae and Gas Arrests
The following introduction to shrinkage is from a paper I did for the Ductile Iron Society back in 2001 revised in 2007. The paper is available for downloading.
Introduction to Shrinkage and Expansion
Ductile Iron consists of primarily two materials: a steel matrix surrounding graphitic nodules. The steel matrix can be ferritic,
pearlitic or martensitic, or a combination of any two. The majority of ductile castings are generally ferritic with less than 10%
pearlite. A small amount of retained austenite is generally present and, in combination with micro carbides, retains about 20% of
the carbon(1). This carbon can then be transformed into graphite during heat-treating.
The steel matrix will shrink considerably when cooling from 2000 degrees to room temperature. Offsetting this is the transformation
of dissolved carbon into nodules of graphite, which occupies 9 to 11% more volume as graphite than as carbon.
One insidious form of shrinkage is a suck-in. It is caused by the same factors as shrinkage, but shows no internal porosity as the
volume loss is transferred to the surface of the casting. Suck-ins are caused by the combination of a high shrinkage iron, and a
thin or weak casting wall that cannot resist the internal pull. This could be due to a combination of a casting designed hot spot
and/or hotter than normal iron. Eutectic and hypereutectic irons are more susceptible to suck-in than hypoeutectic iron because
they are slow to form thick walls. All though these castings might not show internal shrinkage, they should be counted as having
shrinkage nonetheless.
Two other forms of voids appear in iron: micro-shrinkage, and gas or blows. Gas is caused by Nitrogen and Hydrogen being present
in the iron(9), but is not a true shrinkage, though some people mistake it for shrinkage.
The micro-shrinkage appears in the grain boundaries(5)(10)(11) as the final solidification takes place, and can be caused by
micro-segregation where the grain boundaries become enriched in low melting elements and phases(8). Another possible cause of
grain boundary shrinkage can be just the internal stress in the casting caused by not having macro-shrinkage. Macro-shrinkage
relieves the shrinkage-induced stress of the casting and reduces the endothermic signature of the end of freezing arrest. So
when macro-shrinkage is avoided, the remaining unfulfilled shrinkage is transferred to the grain boundaries.
Grain-boundary shrinkage may be preferred over macro-shrinkage, but these voids could also contribute to reducing fatigue life.
Fortunately, heat-treating allows dissolved carbon (occupying no volume in the casting) to move to the graphite nodules where
it adds to the volume of graphite. This graphite growth increases expansion forces, and helps collapse the grain boundary voids,
and improves fatigue life.
When we melt a material we add heat and that loosens the connections between atoms so that they become futher apart and can flow easier.
In a general statement of materials science, a solid hss 6 nearest neighbors and a liquid has 5. This increased seperation is referred
to in physics as work
and requires energy (heat). When metal cools, it gives up that heat as the atoms move back closer together.
So we say metal freezing is exothermic (giving off heat) metal melting is endothermic (adsorbing heat).
When we form a void in metal during cooling we are pulling atoms apart and forming an interior surface. That action takes work, is endothermic
and adsorbs heat. So, on the cooling curve, shrinkage and gas holes have a unique signature, the curve moves in the opposite direction of
metal crystal formation. For the green Cooling rate curve, this direction is up instead of down.
The next question is whether it is shrinkage or gas. If you examine the walls of the hole, you will see either a very smooth wall, or a rough
wall which, under a microscope, shows dendrite crystals. The smooth wall indicates gas, and the rough wall indicates shrinkage. Another indicator
is during what portion of the cooling curve the endothermic bump occured. There may be an exception, but in my experience, gas holes occur earlier
than shrink holes do. So most of the bumps at or near zero cooling rate are gas, and most of those toward the end of the eutectic are shrinkage.
In the example picture above, the green curve bends upward in a shrinkage arrest. The black curve (4th derivative) does a sharp dive though zero
and returns to deliminate the shrinkage arrest. Please note that the black curve before and after is stable. That is a measure of the background
noise of the system. the dive though zero shows the 4th derivative is easily 3 to 4 times background noise during this event. Statisticly this event
has a probability of being real in excess of 99.9%. That is about as good as it gets. The top picture shows 3 larger voids. The round one was gas,
the other two were shrink.
It is possible with gassy iron the voids will block temperature signatures from reaching the thermal couple and interfere
with the readings taken from the curve after the gas defects. Slag also will turn solid towards the end of the eutectic and produce some unusual
arrests and running the readings of the solidus characteristics.
For a full copy of this paper click the download button below
David A. Sparkman, 2006
Download Now
References
- T. Skaland and O. Grong: Nodule Distribution in Ductile Cast Iron,AFS Transactions 91-56, p 153-157 (1991).
- Torbjorn Skaland: A Model for the Graphite Formation in Ductile Cast Iron, University of Thronheim, Sweden. (1992)
- R.W. Heine: Nodule Count: The Benchmark of Ductile Iron Solidification, AFS Transactions 93-84, p 879 (1993)
- R.W. Heine: Carbon, Silicon, Carbon Equivalent, Solidification, and Thermal Analysis Relationships in Gray and Ductile Cast Irons, AFS Transactions 72-82, p 462 (1972)
- D.M Stefanescu, H.Q. Qiu and C.H. Chen: Effects of selected metal and mold variables on the dispersed shrinkage in spheroidal graphitic cast iron, AFS Transactions 95-057, p 189 (1995)
- T.N. Blackman: Graphite Flotation in Ductile Iron Castings, AFS Special Report (1988)
- A.G. Fuller, T.N. Blackman: Effects of Composition and Foundry Process Variables on Graphite Flotation in Hypereutectic Ductile Irons, AFS Special Report (1988)
- R. Boeri, F. Weinberg: Microsegregation in Ductile Iron, AFS Transactions 89-106, p 179 (1989)
- Richard Fruehan: Gases in Metals, ASM Handbook volume 15 Castings, p 82 (1992)
- D.A. Sparkman, C.A. Bhaskaran: Chill Measurement by Thermal Analysis, AFS Transactions 96-127, p 969 (1996)
- David Sparkman: Using Thermal Analysis Practically in Iron Casting, Modern Castings November 1992, p 35