Laboratories 7-8Tempering of Lath MartensiteTemperatureCTransformationRemarks25-100Carbon segregation to dislocations; pre-precipitation clusteringClustering predominant in high-carbon steels100-250Epsilon-carbide precipitation (first stage of tempering)May be absent in low-carbon, low-alloy steels200-300Retained austenite transforms to bainite (second stage)Occurs only in medium-carbon and high-carbon steels250-350Lath-like Fe3C precipitation (third stage)400-600Recovery of dislocation substructure. Lath-like Fe3C agglomerates to form spheroidal Fe3CLath structure maintained500-600Formation of alloy carbides. (secondary hardening or fourth stage)Occurs only in steels containing Ti, Cr, Mo, V, Nb, or W; Fe3C may dissolve600-700Recrystallization and grain growth; coarsening of spheroidal Fe3CRecrystallization inhibited in medium-carbon and high—carbon steels; equiaxed ferrite formedMatSE 471Laboratories 7-8Tempering of Lath Martensite1. ObjectiveThe objective of this laboratory is to monitor the changes that occur during the tempering of a steel using Rockwell hardness testing, microhardness and metallography.2. BackgroundWhen a low to medium carbon steel is quenched sufficiently rapidly, it transforms by shear to lathmartensite (a slightly distorted A2 structure (ex: -Fe) phase). Martensite is very hard and extremely brittle. Because of this brittleness, it is normally tempered prior to use to imbue the material with toughness, while maintaining the hardness, usually, at a variety of high levels. The various changes that occur during the tempering of ferrous martensites are summarized in Table 1. The temperature intervals are very approximate and considerable overlap occurs between each range. Similar information is also presented in Figure 1, which plots hardness as a function of tempering temperature,and also outlines the temperature regions within which each reaction occurs.Table 1Transformations Occurring During Tempering of Ferrous Martensites(After G.R. Speich in Metals. Handbook. 8th edn.. vol. 8. American Society for Metals, Metals Park. Ohio, 1973TemperatureC Transformation Remarks25-100 Carbon segregation to dislocations; pre-precipitation clusteringClustering predominant in high-carbon steels100-250 Epsilon-carbide precipitation (first stage of tempering)May be absent in low-carbon, low-alloy steels200-300 Retained austenite transforms tobainite (second stage)Occurs only in medium-carbon and high-carbon steels250-350 Lath-like Fe3C precipitation (thirdstage)400-600 Recovery of dislocation substructure. Lath-like Fe3C agglomerates to form spheroidalFe3CLath structure maintained500-600 Formation of alloy carbides. (secondary hardening or fourth stage)Occurs only in steels containing Ti, Cr, Mo, V, Nb, or W; Fe3C maydissolve600-700 Recrystallization and grain growth; coarsening of spheroidalFe3CRecrystallization inhibited in medium-carbon and high—carbon steels; equiaxed ferrite formedNow, time and temperature have related effects on tempering. This is shown in figure 2. From figure 2, it can be seen that the data can be related by a simple rate equation:1/t = A exp (-Q/RT) where t is the time to attain a given hardness, A is a constant, Q is an empirical activation energy, and R and T have their usual meanings.Figure 1. The hardness of low and medium iron-carbon martensites tempered for 1 hour between 100°C and 700°C.Figure 2. The effect of time and temperature on the hardness of a tempered 0.82C-0.75Mn steel.3. ProcedureThis is a two-week laboratory and the following tasks will be performed in weeks 1 and 2.3.1 Week 1You will be provided with two steel samples.Austenitize them at 1000°C for 10 minutes and quench into ice brine. Measure the hardness of the samples. Temper the samples for one hour at 650°C* and again ice-brine quench. Measure the hardness of the quenched and tempered specimens. In addition to the above, you will be provided with a series of samples which have already been heat treated. The heat treatment schedule is provided in Table 2. Measure the hardness of all the samples to produce a plot of hardness versus tempering time. Table 2. Quenched and Tempered SamplesAdditionally:(i) Compare the hardness results of your quenched and quenched and tempered samples with those that are provided for you. Discuss the reasons for the differences.(ii) For one particular sample, produce a sufficient number of indents so that the errors in the hardness numbers can be evaluated. 3.2 Week 2 (Lab 8)You will be provided with a quenched and tempered specimen. Polish, etch, and obtain an image of the microstructure. Obtain microstructure images for all the samples listed in Table 2. Do microhardness testing on the 100 hr specimen to determine whether or not there are significant variations in hardness from one region to another. Interpret your results in your report.*For both austenitizing and tempering, monitor the temperature using a thermocouple. 1035 1000°C 45 min water quench1035 1000°C 45 min water quench 650°C 1 hr Air Cool1035 1000°C 45 min water quench 650°C 2 hr Air Cool1035 1000°C 45 min water quench 650°C 5 hr Air Cool1035 1000°C 45 min water quench 650°C 10 hr Air Cool1035 1000°C 45 min water quench 650°C 20 hr Air Cool1035 1000°C 45 min water quench 650°C 40 hr Air Cool1035 1000°C 45 min water quench 650°C 100 hr Air Cool1035 960°C 30 min water