Experiment E: Martensitic Transformations

Transcription

1 Experiment E: Martensitic Transformations Introduction: The purpose of this experiment is to introduce students to a family of phase transformations which occur by shear rather than diffusion. In metals, these transformations are referred to as martensitic transformations. Each student will study the characteristics of the martensite transformations in an eutectoid steel (the same steel which is used in the nucleation and growth experiment), aluminum bronze alloys and a series of Fe-Ni alloys of varying compositions. Background: Certain metal and ceramic systems experience transformations in which shape changes occur without diffusion. These shape changes involve large shear forces and dilatation of the crystal structure. In metals, these phase transformations are referred to as martensitic. They are important in both ferrous and non-ferrous alloys (e.g. aluminum bronzes and shape memory alloys) and are utilized in order to improve properties such as strength and hardness. In metals, the magnitude of these changes can be large enough to allow for observation through the light microscope. In certain ceramics (e.g. partially stabilized zirconia, barium titanate), these phase transformations are referred to as displacive and are exploited in order to improve properties such as strength and fracture Figure 1. Plate martensite in high carbon steel. toughness. With these types of ceramics, displacive transformations may significantly decrease the energy available for crack propagation thus reducing or halting the growth of cracks. The changes that occur within the ceramic structure are small in nature and are best examined by various diffraction methods, for example x-ray diffraction. It is highly recommended that the student review the section of the textbook on martensite transformations prior to the experiment. Safety: It is the responsibility of each TA and each student to be aware of the many hazards in this laboratory and make use the appropriate safety equipment when performing this experiment. The main potential hazards in this experiment are extreme heat, infrared radiation, high voltage, hot gases, cryogenic materials and hazardous chemicals. The following MSDS are available Pitch Coke (Calcined Petroleum Coke), Barium Chloride (Heat Treating Salts), Sodium Chloride (Heat Treating Salts), Potassium Chloride (Heat Treating Salts), Methanol, 95% Ethanol, Nitric E1

2 Acid and Sodium Metabisulphite. Procedure: Part One: In the first part of this experiment, the formation of martensite in a eutectoid steel is investigated. The steel must first be austenitized at 900 C for 1 hour using either a furnace or a salt bath in JHE 244. The furnace temperature or salt temperature will be preset by the TA. The samples with small holes drilled through them will be provided to each student along with wire for holding the specimens. Thread and twist the lengths of wire through the holes for easy handling. Figure 2. TTT diagram with continuous cooling curves. If a furnace is used for the austenizing heat treatment, pack the samples with pitch coke in the Inconel box provided. Place the box in the pre-heated furnace. In the salt bath, the samples can be hung by their wires. When the austenizing treatment is done, use tongs to remove the samples from the furnace or salt bath and transfer them as quickly as possible to a bucket of cold water. Rapid transfer is critical to this procedure. Mount, polish and etch each sample and observe it in the microscope. Part Two Five samples consisting of Fe-Ni alloys of varying compositions are provided. The nickel compositions in weight percent are as follows: Alloy 1: % wt. Ni Alloy 2: % wt. Ni Alloy 3: % wt. Ni Alloy 4: % wt. Ni Alloy 5: 33.88% wt. Ni In addition, these samples are low in carbon and other residual elements. A table of M s, and M f for Fe-Ni alloys is shown on the next page (Trans. AIME, 206 pp.1393). Also included are the A s and the A f temperatures at which austenite starts and finishes forming upon heating the martensitic microstructure, respectively. E2

3 Based on the results provided in this table, the lab group has to determine which sample corresponds to which alloy and to carefully observe and sketch the type of martensite, which forms. The following information should help with the analysis. The initial quenching is done in water at or slightly below room temperature, a thermometer is available to measure the temperature. The following mediums are available for subsequent quenching: water, methanol, ice, dry ice and liquid nitrogen (do not put thermometers in the liquid nitrogen). Use appropriate thermometers for all solutions (except liquid nitrogen). The freezing point of dry ice is -77 C. Quenching mediums below 0 O C can be made by dissolving the dry ice in methanol (use a Nalgene beaker - NO GLASS beakers). Liquid nitrogen has a temperature of -196 C (use a Nalgene beaker NO GLASS beakers). Do not put the methanol/dry ice mixture or liquid nitrogen in a sealed container, it will explode. The martensite can be revealed by etching with 2% or 5% nital after polishing. Aqueous 10% sodium metabisulphite can be also used to darken the martensite, if necessary, but the sample will have to be lightly repolished before further quenching. It is important to follow good E3

4 metallographic procedures in order to see the martensite clearly. Transformations of austenite to martensite occur almost instantaneously when the austenite is quenched to a temperature below the M s temperature. When martensite forms at a low temperatures, it is clearly visible without etching because of the considerable surface relief which it produces. Before coming to the lab, students should consider how they will go about differentiating the samples. Before doing anything, a plan should be discussed with the TA Note that the TA can only offer suggestions. However, the TA will help with observations of the martensite structures in the light microscope to make sure the salient features of the microstructures are noted for the samples produced. Part Three Three specimens of copper with between 11 and 12 wt.% aluminum labeled X21, X22 and X23 and two specimens of a hypoeutectoid plain carbon steel labeled X24 and X25 are available in the microscope room desiccators for observation. The copper aluminum alloy is commonly known as aluminum bronze. Both alloy systems contain an eutectoid and undergo martensitic transformations when rapidly cooled from high temperatures. In the copper - aluminum alloy system, this eutectoid occurs at 11.8 wt. % Al and is created by the breakdown of a compound, beta (β), to a solid solution of aluminum in copper called alpha (α) and another compound called gamma (γ). Each specimen has already been mounted, polished and etched and is designated by the number on the bottom of the mount. If the specimens require repolishing or etching, please contact the technical staff. Each specimen should be observed visually and at high and low magnifications with a bench microscope. Systematically scan the whole section. Select regions that are representative of the majority of the specimen. Sketch the observed microstructures on unruled white paper, which will be provided. The sketches indicate whether or not a clear understanding of the basic structures observed have been achieved. Each sketch should show the principal characteristics of each specimen. Refer to the questions in the Lab Report section as a guide to what you should be thinking about when observing the various specimens. The 3T04 atlas contains additional images that may be helpful. Please return the specimens to the desiccators after observations are completed. Specimen X21, Copper with 11.8% aluminum, eutectoid alloy (chill cast, reheated for 1 hour at 900 C and slowly cooled) The surface of this specimen has an iridescent appearance. The material has completely transformed to the eutectoid, which consists mainly of a fine lamellar arrangement of the alpha solid solution (light) and the gamma compound (dark). However, in some regions, the alpha has a coarser more irregular pattern. E4

5 Specimen X23, Copper with 11.3% aluminium, hypo-eutectoid alloy (sand cast, annealed at 900 C, slowly cooled) Rather jagged needles of a are set in a background of eutectoid. The eutectoid is mainly irregular and relatively coarse in pattern; the gamma particles are large enough to be clearly seen as blue in colour. Sometimes, small brown regions are found in the eutectoid areas; these represent regions where there is a fine lamellar structure that is unresolvable. Specimen X22 (as X21, but water quenched after slowly cooling to 530 C) This specimen has been partially transformed. Nodules of eutectoid are found at the grain boundaries of the original beta phase. The structure of the eutectoid is lamellar except near the original beta boundaries, where it is coarser and more irregular. The remainder of the structure is light with a faint acicular pattern. This is known as beta prime and is a martensite transformation product of beta. Note that the beta phase is body-centered cubic, while the beta prime and (Cu) solid solution phases are distorted face-centered cubic and face-centered cubic, respectively. Specimen X % carbon steel (normalized from 870 C) This structure of X24 consists of fine pearlite and ferrite almost approaching a Widmanstatten pattern. Alloys undergoing eutectoid reactions also lend themselves to heat treatment. The most important example is the quench-hardening and tempering of steel. In general, air cooling (normalizing) from the austenite region gives finer structure than slow cooling (annealing) resulting in higher hardness and strength in the normalized condition. However, considerably more marked effects are obtained by quenching the steel and then modifying or tempering the structure to give the final desired result. The above specimen illustrates some difference in structure as a result of air-cooling instead of slow cooling. But, more pronounced differences occur when the steel is water quenched from the austenite condition. This treatment prevents the formation of ferrite and pearlite. However, on reaching a relatively low temperature, the austenite structure is so unstable that a rapid martensitic change takes place, involving adjustment of atomic positions. Diffusion does not control this type of solid state reaction. The microstructure then consists of many interlacing martensitic needles, with a very small amount of residual austenite. There are no sharp boundaries between the needles and the matrix and the structure has a diffuse acicular appearance. The carbon does not escape from solution, the overall atomic pattern is rendered more complex, it is distorted and in a state of stress. For these reasons, the material becomes hard and brittle (unless the carbon content is low). Reheating, that is tempering, is then used to release the carbon as fine carbide particles, to a degree determined by the final properties required. Specimen X % carbon steel, water-quenched from 870 C (1 hour treated) A typical martensitic structure. Away from the edge of the specimen, small precipitates of ferrite may be found. These have either an angular shape, or sometimes, they have saw-teeth E5

6 contours. They have formed where the rate of cooling has not been so high as at the surface. Lab Report The lab report should address the following points: Why does the steel not transform to pearlite on quenching to room temperature? Refer to the TTT diagram(s) in the response. What is the difference between lath and plate martensite? Why are martensite particles plate- or lath-shaped? Relate the observed microstructures in the X21-X23 series to the Cu-Al phase diagram (next page). Describe the transformations which occur on quenching and on slow cooling. Specimens X22 and X25 1) What are the metallographic characteristics of martensite? 2) In X22, there are nodules of eutectoid. Explain why. 3) How could equilibrium structures be obtained in these alloys? Make sure the sketches clearly show examples of the different accommodation mechanisms observed. E6

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