lab report ( paraphrase only)

The rate of cooling that the steel is subjected considerably affects the level of hardness and performance. In this sense, the quenching is one of the most commonly used heat treatments in industry, in which the steel is subjected to a quick cooling after the austenitization. This treatment increases the samples hardness, making it possible to use steels with less content of alloy elements in applications where mechanical resistance is required. Although the hardness is strongly dependent on the carbon content, the hardenability also depends on the presence of alloy elements and cooling rates.

Determining a relationship between the heating, cooling process will help the heat treating industry by reducing the time spent in the research and development stage. As a corollary this means less money spent. The Jominy End Quench Test which is the complete version of conducted experiment in this lab determines the capacity of steel to harden under defined heating and cooling parameters. The addition of the grain size relationship coupled with the results of the test would produce better defined heating and cooling rates. This allows the engineer to correctly identify the correct steel alloy and heat treatment depending on its intended application, such as rock breaker pistons, or aircraft undercarriages. Extensive research has already been done in hardenability of steel, but has not yet made any significant correlations between hardness and grain size. It is imperative to find a relationship because grain size influences the outcome of many mechanical properties of steel such as hardness, ductility and yield strength. A desired austenitic grain size can be obtained experimentally by using a specific temperature and time.

1.1 Heating

Through the experimentation of different temperatures and times it is possible to determine the optimal temperature and time for a specific grain size. These two factors play a major role in the austenitic grain growth of 1045 steel. The first place to look to determine which temperatures to use can be found by using the phase diagram. The phase diagram shows at what temperature the material reaches its austenizing temperature. This is the temperature at which the carbon redistributes and the structure becomes crystalline. On the following diagram this section is represented by the γ on the left hand side of the diagram. To find the correct temperature the weight percentage of carbon needs to be known. In this case it is 0.45% which is determined by the last two digits of the steel. A vertical line is then drawn up to the lower austenite boundary and then a perpendicular line drawn to the temperature on the side. In the case of 1045 steel it is 843°C. The following figure in next page is the phase diagram for steel.

Figure 1. Phase Diagram for Steel

The closer you are to the austenizing temperature the slower the grain growth of the structure. Given this fact, more time spent at the austenizing temperature the larger grain size. When heating a jominy bar in the furnace it is required to add a half an hour to the desired time spent in the furnace. The addition of the half hour is called normalizing. This process provides the sample time to reach the desired temperature and become uniform on a structural basis.

1.2 Quenching

Quenching is a process where a heat treated part is cooled down rapidly. There are many ways to quench a part as well as multiple quenching mediums such as air, oil, water, brine or spray. This process is just one step in the manufacturing process and depending on the desired result, one of the many methods of quenching can be used. As a result of the cooling process, the structure of the metal is altered. Through the use of a Time-Temperature-Transformation diagram the desired result can be used to determine the proper cooling process. From the following diagram you can see the cooling time is the determining factor in the resulting microstructure. Given a desired microstructure, the cooling rate can then be found. Based on that information the correct quenching process can then be determined.

Figure 2. Schematic illustration of TTT diagram of 1045 Steel and correlation with hardness.

In this experiment, quenching is used to cool the part and just as important to stop the grain growth process. The bigger a grain grows the higher the hardness value. In this experiment the focus is on the austenitic grain growth. When the sample is quenched, it immediately changes in the microstructure.

2. Experimental Procedure

This experiment was consisting hardness measurement and 3 process of Austenizing, quenching and Tempering which will be listed in order of sequence below:

Seven specimens of Steel 1045 were given to conduct the experiment. The hardness of the samples was measured using Rockwell A scale.

1. Austenizing: all 7 samples were heated up to (870 +/- 15 C) for 1/2 hour to achieve austenitic microstructure.

2. Quenching: 1st sample was quenched in water immediately after heating to achieve martensitic microstructure. Measurement of hardness on Rockwell A scale was done after that.

3. 2nd sample was cooled in the air after heating. Measurement of hardness on Rockwell A scale was done after that.

4. 3rd sample was cooled inside the furnace after heating. Measurement of hardness on Rockwell A scale was done 24 hours after the experiment.

5. Tempering: remaining 4 samples also were quenched after the first heat treatment process, then they were heated up in 4 different temperatures for 1/2 hour for tempering. After tempering, measurement of hardness on Rockwell A scale was done. The tempering temperatures were; 250, 300, 400, and 500 C.

Note: before taking the hardness measurement sample’s surfaces were grinded using disc grinder to remove the oxide layer and achieve precise measurement.

3. Results & Discussion

Below table summarizes the obtained data from hardness measurement in different conditions.

Table 1. Hardness results for 7 samples in various heat treatment conditions.

Its clearly obvious that after austenization, and quenching in water hardness of the sample increased significantly from the average of 79 up to 93. In this phase transformation, Austenitic FCC steel microstructure, transform to the hard and brittle martensitic BCT microstructure. In simple words, the microstructure of steel in 870C is freezed immediately to room temperature.

As shown in the table samples that were air cooled and furnace cooled have a much lower hardness. This drop in the hardness is the result of diffusion of the carbon atoms inside the microstructure as a result of time and temperature. By giving more time and temperature to the sample atoms have the time to diffuse and move inside the microstructure and produce a perlitic microstructure which is much softer than martensitic.

For the four samples that were tempered after the initial heat treatment, hardness is relatively the same. To analyze the difference between these microstructures further investigation is needed which was out of scope and time of this experiment such as; Metallography and Mechanical Testing.

1. Purpose of Quenching and Tempering: Quenching and tempering are processes that strengthen and harden materials like steel and other iron-based alloys. The process of quenching or quench hardening involves heating the material and then rapidly cooling in water, oil, forced air or inert gases such as nitrogen. The process is tightly controlled, with the heating temperature, cooling method, cooling substance and cooling speed all dependent upon the type of material being quenched and the desired hardness. After the material has been quenched to its hardest state, the process of tempering is used to achieve greater toughness and ductility by decreasing hardness. Tempering is achieved by heating the quenched material to below the critical point for a set period of time, then allowing it to cool in still air. Both the temperature and heating time depend on the composition of the material and will determine the amount of hardness removed.

By tempering quenched steel, it becomes less brittle and more ductile without sacrificing too much hardness. It is the combination of these two processes that produces a harder, tougher steel that’s more weldable and ductile than ordinary carbon steel. The toughness-factor means greater resistance to wear and abrasion. That is why quenched and tempered steel is particularly useful in machinery and structures where greater abrasion resistance and higher yield strength are necessary, such as mining, quarrying, earthmoving and construction.

2. Source of error and Scatter in hardness data: the main source of error in the hardness data after any heat treatment process is the presence of oxide and scale layers. As we know oxide has a very high hardness and if any layer of oxide is on the surface will damage the integrity of the data.

Hardness readings are most accurate when the result is near the center of the scale. If a reading comes up near the upper or lower bound of a scale, it may be best to retest using another scale to ensure accuracy. Any readings that fall completely outside the recommended range are suspect, and another hardness scale must be used.

When taking a hardness measurement, it is important that the sample be properly fixtured. Parts should not be free to move on the anvil. For microindentation hardness testing, the samples should be metallographically mounted in resin. The mount should then be ground down to the section of interest and polished for best results.

Other write up questions were thoroughly discussed in the introduction part.

1