The key difference between tempering and austempering is that tempering is useful in removing the excessive hardness of steel, whereas austempering is important in reducing the distortion of iron alloys.
Tempering and austempering are closely related processes which involve heat treatment of metal alloys, specifically iron alloys such as steel. However, the steps of each process and the final outcome are different from each other.
CONTENTS
1. Overview and Key Difference
2. What is Tempering
3. What is Austempering
4. Side by Side Comparison – Tempering vs Austempering in Tabular Form
5. Summary
What is Tempering?
Tempering is a process that involves heat treating to increase the toughness of iron-based alloys. Also, this process is very important in removing some of the excessive hardness of steel. In this process, first we need to heat the metal to a temperature below the critical point for some time, and then we need to allow the object to cool in still air. The temperature determines the amount of hardness we can remove from the steel. However, this temperature to which we are going to heat the metal depends on the composition of metal or alloy and its properties. For example, low temperatures are favorable for very hard tools, but soft tools such as springs require high temperatures.
Figure 01: Tempering Colors of Steel
Usually, in industries, we perform the tempering step after the quenching. Therefore, the workpiece of the tempering process is the quenched object, and we need to heat the object with control to a certain temperature that is below the lower critical point of the object. During this heating, the grain structures of the object (ferrite and cementite) tend to convert into an austenite grain structure. This is a single-phase solid solution.
What is Austempering?
Austempering is a process in which the metal alloy develops a metallurgical microstructure. The application of this process is mainly on ferrous alloys having medium to high carbon content. Here, steel and ductile iron are most notable among the alloys. In steel, this process forms a microstructure called “bainite” while in ductile iron it produces “ausferrite” microstructure.
Figure 02: Time-Temperature Transformation Diagram Showing the Cooling Curve for Austempering in Red Color
Primarily, we use this process to reduce distortion of the alloy, thereby improving the mechanical properties of the material, i.e. we can increase strength, toughness, etc. In this process, we need to heat the material to the hardening temperature, then cooling it rapidly to the martensite temperature. Then, we need to hold the temperature at this level for a sufficient time to get the bainite structure.
What is the Difference Between Tempering and Austempering?
Tempering and austempering are important in enhancing the mechanical properties of a metal alloy, specifically the iron alloys. The key difference between tempering and austempering is that tempering is useful in removing the excessive hardness of steel, whereas austempering is important in reducing the distortion of iron alloys.
When considering the theory of these two processes, during the heat treatment of the tempering process, the temperature determines the amount of hardness we can remove from the steel. However, in the austempering, formation of either “bainite” structure or “ausferrite” structure strengthen the alloy.
Summary – Tempering vs Austempering
Tempering and austempering are important in enhancing the mechanical properties of a metal alloy, specifically the iron alloys. The key difference between tempering and austempering is that tempering is useful in removing the excessive hardness of steel, whereas austempering is important in reducing the distortion of iron alloys.
Reference:
1. “Austempering – Hardening and Tempering” Bodycote, Available here.
Image Courtesy:
1. “Tempering standards used in blacksmithing” By Zaereth – Own work (CC0) via Commons Wikimedia
2. “Austempering” By Zapatilla – Own work (Public Domain) via Commons Wikimedia