August 2022 Volume 4

MATERIALS

Question: Can you explain the differences between normalizing and annealing? This is a very important question and it is critical to understand the metallurgical distinctions between these terms, as they often get intermingled in error. When applied correctly, either of these treatments can be critical to the successful manufacture and performance of forged products. As we will discuss, these operations can be used to obtain final part properties, or as a preparatory treatment to a starting condition for subsequent manufacturing steps. In order to understand the true differences, it is necessary to form a groundwork for temperature terminologies that come up often in heat treating, as they are critical to these operations. First, we will be discussing what is referred to as the “critical temperatures,” also denoted as Ac1 and Ac3. These are the temperatures that we need to heat above in order to form austenite. Ac1 (the lower critical temperature) is the temperature at which the austenitic phase begins to form and Ac3 (the upper critical temperature) is the point at which complete conversion to austenite is complete. Heating above Ac3 is the necessary starting point for both annealing and normalizing operations – as it is for quench and temper treatments, which have been covered in this column previously. Next, we have terms denoted Ar3 and Ar1 . These are also critical temperatures; however, they are specific to the points at which austenite decomposes to the room temperature stable structures of ferrite and pearlite. For cooling, the numbers are now reversed, so Ar3 is the temperature at which the decomposition reaction starts and Ar1 is that point at which it is complete. Notably, values for all critical temperatures of common alloys can be found in multiple sources such as steel supplier handbooks. Essentially, normalizing and annealing are structured around different methods of heating above Ac3, then manipulating cooling rates through the Ar3-Ar1 ranges in order to achieve different hardness/strength levels for different applications. It is critical that Ac temperatures are not confused or used in place of Ar temperatures because they are often significantly different for the same material. With a cursory framework of critical temperatures in place, we will start with the annealing process. In the simplest terms, the purpose of annealing is to soften the material. As applied to forgings, this process is usually assigned to improve machinability, as the forging process can result in unacceptable hardness gradients caused by rapid/non-uniform cooling from the forging temperature. Additionally, the part may just be too hard in general in the as forged state. Practically, annealing consists of re-heating the forgings to slightly above the critical temperature range to form a uniform austenitic Heat Treat Corner By Chuck Hartwig

microstructure in the material followed by slow cooling. Note that we do not heat significantly above the upper critical temperature for annealing as this risks creating too much of a thermal gradient when we start cooling through the critical temperature range. If no specific instructions are given, it is commonly accepted that the cooling method for annealing is cooling inside the furnace chamber with the heating source turned off. This cooling rate must be held to a temperature range that is safely below Ar1. An important consideration here is that if the process control is running off of the furnace thermocouple, the actual workload will be at a much higher temperature. Consequently, the ending temperature for the anneal cycle based on the furnace control thermocouple needs to be a few hundred degrees below the theoretical Ar1 value of the alloy being annealed. The most critical aspect of an annealing cycle is the cooling rate of the load as it cools through the Ar3-Ar1 temperature range. This is usually in the range of 1450°F to 1100°F, but each alloy is particular. To produce the softest material with the highest ductility, the load must be cooled very slowly through this range. In doing so, we create a microstructure of coarse ferrite along with coarsely spaced pearlite because the very slow cooling rate during austenite decomposition allows more time for carbon to diffuse into intergranular spaces and coarsely spaced carbide lamellar structures in pearlite. In terms of heat treating specifications, annealing cycles are usually specified with a maximum hardness. Sometimes, specifications will use a minimum and a maximum. This can present more of a challenge for process design because it then becomes possible to cool too slowly. It is possible to design an annealing cycle to maintain hardness within a specified min/max range. It just becomes a matter of tailoring the cooling rate carefully and also controlling the furnace type and load size to certain parameters. In these cases, it also becomes critical to employ furnaces that use PLC recipe and ramp rate controls to ensure that the desired temperature profile is being followed for each cycle. Next, wewill discuss the process of normalization, themost common thermal treatment applied to forged products. Normalizing is often a necessity after forging due to the high temperatures used in forging that result in grain growth and overall grain size heterogeneity within the material. Grain size is typically defined by an “ASTM Grain Size” number. These numbers range from 1-14, with 1 being the largest grain size. Generally, an ASTM grain size of 5 or finer is suitable for most forging applications. Many forging processes will result in enlarged grains below 5. Enlarged grains (as well as heterogenous mixed size grains) result in lowered strength, lowered toughness, and sub optimal response to subsequent heat treating – whether it is quench and temper or

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