The Three difficult parts of Shrink

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1 There are three parts to understanding what we see in shrinkage by thermal analysis. The first part is that that shrinkage absorbs heat. The second part is that shrinkage occurs due to stresses set up in the cooling metal, because feeding of more metal has been cut off. And the third part is that the results of stress in the metal can actually be seen as a cooling inflection in the thermal curve. Part One: Shrink produces cooling Shrink is a place where we would expect to find metal, but it isn t there. This can be inside the casting or on the surface of a casting in the form of a suck-in, where the surface is pulled in as a kind of dent. Likewise, gas bubbles act like shrink in absorbing heat and relieving stress. The reason for this is a simple principle of physics. As metal solidifies, it becomes more ordered, has less energy, and must lose or give up heat. Shrinkage occurs when the metal is pulled apart to form a void. It is a form of disorder and requires energy to form an interior surface, so it absorbs energy or heat. This means that when we look closely at a thermal analysis, we should look for both heat-producing and heat-absorbing events. During heat-producing events, there are different phases forming, and during heat-absorbing events, there are voids forming both shrink and gas bubbles. Part Two: Shrink is caused by stress Stress plays a significant role in forming voids in castings. Forming an interior surface a void requires a lot of energy. Think of how we test the physical properties of a metal by pulling a test bar. Pulling the test bar apart requires enormous forces. While a void in the metal is much smaller, the initial formation of such a void still requires significant force. Stress is the force pulling on the metal much like the tensile machine builds up sufficient forces to break the test bar. When a casting starts to solidify, it generally does so from outside and inside surfaces enclosing liquid metal between these walls. Liquid metal is said to be where an atom has five nearest neighbors. Think of a die with six sides. Five sides are held fast by other atoms, but the sixth side is open and the atom can move into that space, and so it acts like a liquid that can flow. When solid, the metal has neighbors on all sides, and so it is more compact and occupies less space. The common metal iron undergoes a difficult transition from liquid to solid that includes an unbalanced contraction and expansion. The matrix metal is ferrite, while a significant amount of the carbon, in most cases, forms graphite. In gray iron, the graphite growth will come close to matching the shrinkage, leaving about a 2 to 3 percent total loss. In white irons, there is very little graphite growth to offset the matrix shrinkage, so casting dimensions are reduced.

2 Ductile iron is unique in the irons because, although there is sufficient graphite growth to offset the matrix contraction, the graphite growth is delayed, and this imbalance leads to stress. This stress then provides the necessary tension to possibly pull open voids. Part Three: Shrink and stress can be seen by thermal analysis Shrink is often seen in metals toward the end of the eutectic solidification. At this point the stresses become great enough to open up a void. Gases, on the other hand, generally cause voids earlier in the eutectic. Of course gas voids will remove some of the stresses because they add needed volume to the metal in a casting, so it is rare to see both gas and shrinkage voids in the same sample. Fig 1. Acceptable Shrinkage Fig 2. Increased Shrinkage One interesting thing we have noticed is that the shrink signals can be multiple or can be consolidated. The multiple shrinkage signals seem to be associated with interdendritic shrink or what is commonly called microshrink. Large single arrests seem associated with macroshrink. Fig 3. Acceptable Shrinkage Fig 4. Increased Shrinkage Some markers are the behaviors of the 2 nd derivative (blue curve) as it approaches the Solidus. Residual stress in the casting manifests itself as a more endothermic (heat adsorbing) Solidus. This can be seen in several different ways: the shape and height of the 2 nd derivative is different as seen in figures 5 and 6. The height of the green cooling rate curve (1 st derivative inverted) is higher, and the area between the baseline curve and the solidus changes.

3 Fig 5. Second derivative less shrink Fig 6. Second derivative increased shrink Above there is a slight but increased difference in the 2 nd derivative just before it peaks and the peak values are different with the lesser shrink showing a steeper slope and a higher maximum. Fig 7. Cooling rate less Shrink Fig 8. Cooling rate increased shrink There is of course one problem in using these two measures to judge shrink and that is the variability in the sample sizes and temperatures. So there is one more thing that can be done to put an actual number on this shrinkage effect. The SSCRS point above is the point where the energies of solidification have finished, and the casting is cooling down in a steady state of normal heat loss. This point is called the Steady State Cooling Rate of the Solid or SSCRS and it anchors the end of the baseline curve. A similar point called the Steady State Cooling Rate of the Liquid or SSCRL anchors the start of the baseline curve. The baseline curve is shown in magenta in figures 9 and 10 for the two example curves. Fig 9. Baseline curve with less shrink Fig 10. Baseline curve with increased shrink

4 By integrating the area between the magenta baseline curve and the green cooling rate curve, we can obtain a number for the heat of fusion. This is the total energy given off during solidification (fusion) of the sample and is shown in light green in Figures 11 and 12. The red area is the endothermic cooling caused by the stress in the grain boundaries. Clearly the sample with more shrinkage has less cooling and therefore less residual stress. Fig 11. Baseline Curve with Less Shrink Fig 12. Baseline Curve with increased shrink MeltLab uses integral calculus to measure both the heat of fusion value and the cooling due to stress value. Since these values can be affected by how much iron is in the cup, the two are ratioed and the cooling value is expressed as a percent of the total heat of fusion. Higher percentages mean a more solid casting. Then there is the other problem that reminds us of the Sherlock Holms story of the dog that did not bark. What can cause the shrink signals to disappear? When we don t find shrinkage signals, we would expect to find the stress being built up in the grain boundaries. These castings have minimum shrinkage, and as the late-growing graphite takes on volume, the grain boundaries compact. When there are a lot of shrinkage or gas signals, we see the heat energy at solidus greatly diminished or eliminated. See figure 13. Fig 13. No cooling due to large shrinkage arrest. Occasionally, though, we see no internal shrinkage signals and no major stress buildup. That mystery is explained by external shrinkage, or what is commonly called suck-in. In this case there is no evidence of a void being formed inside the casting, but there is also no buildup of

5 stress or any cooling reaction at the solidus point. This is because the stress energy has pulled in the skin of the casting and caused an external shrink a suck-in defect. Fig 14 external shrink or Suck-in defect Conclusion During heat-producing events, there are different crystalline phases forming, while during heatabsorbing events, there are voids forming both shrink and gas bubbles. With MeltLab, we can see both the heat producing and the heat absorbing events. Properly evaluated, they will tell us when we have a shrink prone metal and when we don t. It is a tool. It doesn t fix gating or temperature problems so MeltLab can t always tell you all the reasons why. But it can tell you that your iron is indeed shrink-prone and by how much. It can also tell you about how well inoculated you are, how the graphite is growing, your C.E., proeutectic carbides and late carbides, nodularity, and much more. As you change your melting practices, MeltLab can help guide you by showing what works and what doesn t. Not all casting designs or all customers need the most shrink-resistant iron. But when you are running a critical job, it is good to know when everything will be OK before you pour those castings.