Technical Article


An Examination of the Impact of Grain Structure on Tensile Test Results


by Kevin Day, PhD., Process Engineer, Cannon-Muskegon Corporation Abstract


M


any of the common metal specifications include heat qualification testing require-


ments that must be met by casting and testing tensile bars. Controlling the grain structure of the cast tensile bars is a key component in obtaining consistent tensile results. Ideally, the grain struc- ture in the gage section of a tensile bar will be a fine equiaxed structure. This structure will give consistent results that are representative of the capability of the metal. If the structure instead consists of large oriented grains, the tensile test results will be impacted by that orienta- tion. In addition, if the grain orientation is not constant between tensile bars, the tensile test results may show significant variability. A study was undertaken to examine how different tensile bar styles will give different gage section grain structures. The resulting grain structures were then related back to the tensile properties of the test bars.


Background Impact of Grain Size and Structure on Mechanical Testing In addition to meeting a given chemistry range, most metal specifications include a requirement that the material must meet certain mechanical properties when tested. Typically, the mechanical properties are verified by casting and testing tensile bars. Some common mechanical properties that are tested are tensile strength, yield strength, percent elongation, and stress rupture properties. The size and structure of the grains


in a polycrystalline metal strongly influence the mechanical properties of the material. This is because grain boundaries will act as a barrier to dislocation motion during plastic deformation. Generally speaking, a fine grained


20 ❘ May 2019 ®


material will be harder and stronger than the same material exhibiting a coarse grain structure. This is due to the fine grained material having a greater total grain boundary area to inhibit dislocation motion. This relationship between the yield strength and the grain size can be generalized by the Hall-


σy = σO + kyd -1/2


Petch equation: where d is the average grain diameter .


and σy and ky are material constants[1]


In addition to grain size, the orientation of the grains may also influence the mechanical properties of the material. An extreme example is shown in Figure 1, where the mechanical testing results would be different depending on if the material was tested in the x or y-direction. The impact of grain orientation can be particularly important when there are coarse grains in the gage section of a test bar.


in Figure 2 can often lead to failing or non-repeatable test results.


Impact of Casting Conditions on Grain Size and Structure There are multiple casting conditions that will impact the final grain size of a cast part. These conditions include, the chemistry of the alloy, the pour temperature,


the temperature of the


mold, the shape of the part being cast and the associated gating. The chemistry of a given alloy


will play a role in grain structure of the solidified part. In particular, the difference between the solidus and liquidus temperatures (the freezing range) will be important in the formation of porosity in the casting. Alloys that exhibit a wide freezing range will be more difficult to cast fully sound than an alloy with a narrow freezing range. This is because a wide freezing range will allow more shrinkage to occur, which could result in increased porosity in the casting if the shrinkage isn’t adequately fed. A large temperature gradient is a key


Figure 1: Schematic depiction of a material with grain orientation.


Lastly, a metallurgically sound


casting is important for obtaining reliable, repeatable mechanical testing results. Figure 2 depicts a cast tensile bar with significant porosity in the gage section. The amount of porosity present


factor in making a sound casting that has a small grain size because it promotes rapid solidification and minimizes the opportunity for grain growth. The pour temperature and the temperature of the mold are obvious factors that will influence the temperature gradient. A pour temperature should be chosen such that it utilizes the minimum amount of superheat required to cast successfully. A low amount of superheat will require less heat to be extracted from the metal and will promote grain nucleation and help minimize grain growth. Similarly, a low mold temperature will promote a larger temperature gradient and help minimize grain growth. The desire the minimize the pour temperature and mold temperature will need to be balanced


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