Steel Impact Test and DBTT Theory PDF

Preview

Summary

Download Steel Impact Test and DBTT Theory PDF

Description

CHARPY IMPACT TESTING and the DBTT Fracture can occur in either a ductile or brittle fashion. Most very pure FCC metals are ductile at all temperatures, whereas many high yield strength materials fail in a brittle manner. Some materials like steels, however, undergo a transition from brittle failure at low temperature to ductile failure at higher temperature. The temperature at which this transformation occurs, known as the ductile to brittle transition temperature (DBTT), is affected by several factors such as the state of stress in the metal, the rate at which the stress is applied, and changes in composition and microstructure. Ductile failure is accompanied by a large degree of plasticity in the metal. This plasticity, which is due to dislocation motion, requires shear stresses (dislocations move under the application of shear stresses). In the typical uniaxial tensile test of a smooth sample, the maximum shear stress occurs on a plane offset from the applied tensile stress by 45 degrees and is equal to half of the applied tensile stress. If the sample has a flaw such as a notch in it, the tensile stress in the area immediately surrounding the notch tip not only has a component in the original tensile direction, but it also has a component in the radial direction thus becoming a "triaxial" stress state. This radial component of the tensile stress reduces the shear stress which, in turn, reduces dislocation motion and suppresses plasticity. Ductile failure is therefore less likely to occur and the sample may fracture in a brittle manner. It doesn't take as low a temperature for brittle fracture to occur. The transition temperature, therefore, is raised due to the presence of a notch in the specimen. It is also known that the rate of loading affects the failure mode. A typical tensile test utilizes a strain rate ε of 10-5 to 10-3 s-1. If the sample is loaded much more quickly, say at a strain rate of 102 s-1, brittle fracture is more likely to occur. This rate of loading can be achieved by impact testing the sample. Impact testing, therefore, also raises the temperature at which brittle failure can occur. Other factors such as compositional and microstructural modifications can also alter the DBTT (for example, the addition of nickel or quenching to martensite decreases the DBTT in steel) The "Charpy impact test" utilizes a notched specimen and high strain rates. This subjects the material to more severe conditions that raise the DBTT for the material. In the test, a sample is machined as shown in Figure 1 and is then loaded into the testing apparatus.

Figure 1. Charpy V-notch test apparatus [1]. When the pendulum hammer in the apparatus is raised to a set height above the specimen and released, it will swing down and impact the specimen on the side opposite the machined notch. Assuming the sample doesn't stop the hammer (a rare occurrence), the hammer will continue to swing after impact. The final height the hammer achieves after striking the specimen is recorded by the testing apparatus. By measuring the preset height of the hammer, and the final height the hammer achieved after striking the specimen, the amount of energy the hammer lost in breaking the specimen can be calculated. (Assuming that the pendulum pivot has relatively low friction, the energy is

just the difference in the two heights times the weight of the hammer.) Generally the apparatus has a dial gauge that directly reads the impact energy. This energy absorbed by the specimen, as it breaks, can qualitatively be related to it's fracture mode (ductile or brittle.) Ductile fracture, which involves substantial plastic deformation, requires much more energy than brittle fracture. Note that the fracture energy measured in a Charpy impact test is a relative energy and can only be used to compare dimensionally identical specimens; it cannot be easily used directly in engineering calculations. By testing the fracture energy of a sample at many different temperatures, a curve such as Figure 2 can be developed. Because the transition from completely brittle (low impact energy) to completely ductile (high impact energy) occurs over a broad range of temperatures, several procedures exist for the determination of the actual "transition temperature." The definition we will use is that the transition temperature is the temperature where the fracture energy is halfway between the pure brittle and pure ductile impact energies. This can be determined using a fitted plot of the impact energies versus temperature similar to that shown in curve A in Figure 2.

Figure 2. Representative Charpy impact test results as a function of temperature [1]. The ductile-brittle transition also leads to a noticeable change in the appearance of the fracture surfaces. Note the ductile shape change of the Charpy samples in Figure 3 for temperatures ≥ -12°C. Only the -59°C sample retains it’s original rectangular cross sectional shape.

Figure 3. Examples of fracture structures from A36 steel samples tested at different temperatures. Notice the transition in appearance between -59 and -12° [1] A scanning electron microscope (SEM) is a tool that can be used to examine the details of fracture surfaces. An electron microscope operates similar to a digital optical microscope except that electrons are used instead of photons and electromagnets are used as lenses. In addition to providing higher resolution, an SEM also offers greater depth of field. This means rough surfaces will be kept in focus better than with an optical microscope where the sample must be very flat. Figure 4 shows scanning electron microscopy images of ductile microvoid coalescence and brittle cleavage fracture that are commonly seen in the impact testing of steels.

Figure 4. Panel (a) at left shows a typical ductile microvoid coalescence fracture surface. Panel (b) at right shows a typical brittle cleavage fracture surface. [1]

References [1] Callister, W.D. Jr., 2010, Materials Science and Engineering: An Introduction, John Wiley and Sons, New York, N.Y., USA, Chap. 8....