It should be becoming clear that the properties of materials are dependent on the prior processing. In particular, various heat treatments for a given steel alloy allow the development of a number of alternative microstructures, which in turn, affect the mechanical properties of the steel. In this lab, we explore several (of the infinite set of possibilities for) tempering temperatures and times, to determine their effect on the mechanical properties of a medium carbon steel. The mechanical properties are investigated using both hardness testing and fracture toughness testing. The relationship between these properties will also be examined.

A basic understanding of the heat treating processes for steel should be available from the information in class notes and referenced readings [1-3]. There are a number of different heat treatments available for steel, depending on the microstructure and the properties that the steel must obtain. Samples are austenitized in the austenite region of the iron- carbon phase diagram, and then cooled (or quenched) at various rates. Different cooling rates give rise to different microstructures. Slow cooling from the austenite region promotes the formation of pearlite, the equilibrium transformation product. Very fast cooling rates suppress the time dependent transformation to pearlite and lead to development of martensitic structures, which are usually hard and brittle. These structures are then further processed by "tempering" the steel. Tempering consists of reheating the samples to moderate temperatures for a specific time interval to achieve the desired mechanical property. The furnace temperatures and tempering times must be determined experimentally to optimize mechanical properties of interest.

The term "hardness" represents the resistance of a material to permanent damage due to an external mechanical stimulus. There are a number of alternative methods for measuring hardness, but one of the most convenient procedures is the Rockwell Hardness Test. In this procedure, the hardness of various materials is measured relative to an arbitrary scale. There are several scales that are available, and of those, the Rockwell C scale will be used for the steel specimens in this lab. Hardness can be related to the tensile strength of materials (with some involved calculation), and because hardness is much simpler to measure, this test is often used in place of a more complicated tensile testing procedure.

After calibrating and choosing the appropriate scale for measurement, the specimen is placed on an anvil platen on the hardness tester, beneath a diamond indentor, and a load is automatically applied, forcing the indentor into the specimen. The hardness of the specimen then appears on the digital read out; again this measurement is relative to the Rockwell C scale. Hard materials will show higher Rockwell hardness than softer materials.

Two types of standardized notched-bar impact tests are commonly used to determine the tendency for a material to behave in a brittle manner. Charpy impact testing is favored over the Izod testing, where the differences lie in specimen and test geometries. Impact tests detect differences in the ductile - brittle behavior of materials in a way which is not readily observable by standard tensile testing. The results from notched-bar testing are not easily expressed in terms of design requirements, however, and there is no general agreement on the interpretation or significance of results obtained from this type of testing. Nevertheless, the Charpy impact test remains an important method for characterizing the mechanical behavior of materials.

Specimen dimensions are shown in Figure 8.16 in your text. The notch is located in the center of the test bar. The specimen is supported horizontally at two points, and is then impacted from a pendulum of specific weight on the side opposite the notch. The specimen fails (breaks) in flexure under this impact. The principal measurement from the impact test is the energy absorbed in fracturing the specimen. This energy is measured by differences in potential energy, again using a relative scale in ft-lbs. Materials that show 0 ft-lbs energy absorbed indicate that it is very easy to fracture the material: it shows brittle failure. Alternatively, materials which absorb energy (energy > 0 ft-lbs) during fracture will exhibit ductile behavior.

Heat Treatment Samples: 1095 steel Charpy Test specimens labeled accordingly

A - As received

B - Austenitized (870°C, 15 min.) + Water Quench

C - Austenitized (870°C, 15 min.) + Air Cooled

D - Austenitized (870°C, 15 min.) + Water Quench + Tempering @ ________ for _______

E - Austenitized (870°C, 15 min) + Water Quench + Tempering @ ________ for _______

Measure the average hardness of 5 points on each of 2 adjacent sides for each specimen.

Determine the fracture energy in ft-lbs required to break each sample.

Using the information from all specimens heat treated and mechanically tested, make a series of graphs which show the trends and relationships between tempering times, tempering temperatures, hardness and toughness. Using the standard laboratory report format, discuss the justifications for these results, based on the theory of phase transformations. Martensite and cast iron are both hard and brittle materials. Explain the difference in your discussion. In your Introduction, give a short description of the classification system for plain carbon steels.

Steel Type: 1095