Isothermal Hardening

Heat treating uses strictly controlled temperature modulation to enhance certain desirable characteristics in metals, such as performance and durability. There are five unique heat treatments:

Hardening
Case hardening (or Carburizing)
Normalizing
Tempering
Annealing

Learn more about the full range of heat treating products available with Ajax Electric. Each of these five processes alters the fundamental crystalline structure of the metal to improve particular properties such as toughness or ductility.

What is Isothermal Hardening?

Isothermal hardening is a heat-treating process intended for applications with medium and high carbon-ferrous metals. The primary purpose of isothermal hardening is in reducing distortion while improving the metal’s strength and toughness. The way this is done is heating the work until the metal becomes austenitized and then being quenched to exact temperatures and left there for specific spans of time.

Two well-known tempering methods use Isothermal Hardening; Austempering and Martempering. The two processes share many similarities with their main differences being the temperatures they are being quenched to, the times they remain at those temps, and the end result.

Overview of Austempering and Martempering

To understand Austempering and Martempering fully, it is necessary to reference the S-curve. In heat treatment, the microstructure of the workpiece is transformed to obtain desired properties. S-curves are important to a metallurgist to understand when different transformations occur in metal and at what temperatures.

If we heat steel, or cast iron until we cause its constituents to go into solution, then quench to 1200°F and hold at that temperature long enough, we will cause that microstructure to transform into the softest microstructure, pearlite. If we had instead cooled that Austenite to 600°F before holding, we would avoid the pearlite transformation and instead create a much harder microstructure, Bainite. Finally, if we cool that workpiece rapidly and sufficiently formation of both pearlite and bainite are bypassed becoming the hardest structure, martensite.

Martempering quenches austenite quickly and efficiently as to avoid the formation of either pearlite or bainite, resulting in the hardest microstructure, martenite.
Austempering also cools quickly, but not as low as Martempering, as you only are avoiding the formation of pearlite, and holding a temperature in the quench until the workpiece is completely transformed into bainite.

S-Curves in Austempering and Martempering

Martempering

Cooling curve in relation to the S-curve. Austenitic steel is cooled at a sufficient rate to ovoid the nose of the S-curve, preventing start of pearli-tic or bainitic transformation. It is then held just above the Ms point (start of transformation) to obtain tempera-ture uniformity. When temperature is uniform through-out the steel, it is cooled in air to room temperature. During cooling, martensite forms. Stresses are low.

Austempering

Cooling curve in relation to S-curve. Austenitic steel is cooled at a sufficient rate to avoid tho nose of the S-curve and held just above the Ms point for complete transformation to a bainitic structure while the temperature remains constant, thereby alleviating ther-mal stresses that could cause cracking or distortion. Time-temperature diagrams like this one give heat treat-ers a three-dimensional view of heat treating problems.

Martempering Setup

Complete martempering setup at plant of well-known aircraft manufacturer consists of ( I ) oil quench tank, (2) austenitizing furnace, (3) quenching furnace, and (4) hot water rinse tank. Despite extreme depth of austen-itizing furnace, fifteen and a half feet, temper-ature variation throughout the working area of the bath is less than 10°F. Used for the martempering of long steel aircraft weldments, furnaces consistently treat parts without cracking or decarburization.

Austempering Applications and Materials

Austempered metal is stronger, more ductile, shock resistant, and less prone to distortion. The process is typically used to create durable thin components from carbon steel or ductile iron. Common austempered components include:

Agricultural equipment components
Automobile transmission gears
Construction equipment components
Cutting blades
Gun parts
Clamps
Clips; Seatbelts in cars, Industrial Safety

Martempering Applications and Materials

Martempered metal achieves the toughest Rockwell rating of any Hardening process. The process is used to achieve minimum distortion in very large parts for the aircraft industry, and for extremely tough parts. Some typical martempered parts include:

Crankshafts
Gears
Industrial machinery parts
Helicopter Props

Common Isothermal hardened materials include:

Carbon Steels, Alloy Steels, and Iron are the most hardened materials using these methods due to the high carbon content that helps facilitate the transformations at different temperatures.

The Isothermal Hardening Process

This process hardens carbon steel and other iron alloys with medium-to-high carbon contents by heating the metal to a temperature that disrupts its crystalline structure until it is austenite.

After the work becomes austenite, the processes of Martempering or Austempering diverge:

Martempering

After becoming austenite, the next step is to quench this piece consistently and quickly to prevent the formation of bainite or pearlite.
Once the workpiece has attained a uniform temperature that is right above the start temperature of when Martenite forms, you move onto the final step.
Air cooling to room temperature.

Austempering

After becoming austenite, the next step is to quench this piece to a temperature right above where bainite starts to form.
Once the temperature is achieved, the part must be held until transformation of the austenite is completed to bainite.
Finally the piece is either air-cooled or water-quenched.

Advantages of Isothermal Hardening

Austempering and Martempering offer a wide range of benefits for a variety of industries. The primary advantages include:

Enhanced strength. The hardened bainite or martenite crystalline structure created by this treatment makes metals significantly stronger than untreated metals.
Shock resistance. Austempered metal exhibits higher levels of shock resistance due to its exceptionally strong crystalline structure and ductility. Where other metals might crack or tear, austempered components can withstand impacts while retaining their shape.
Reduced distortion. Austempered steel exhibits reduced distortion due to the quenching process. This makes it particularly useful for thin components that must retain specific dimensions during and after heat treatment.

Considerations for Austempering or Martempering Metal

With many similarities in the process, and end-result in these two processes, some good questions to ask yourself when trying to determine whether to Martemper vs Austemper are the following:

Maximum Section:

Martemping – Carbon Steels under ½”, Alloy Steels up to 6”
Austempering – Carbon Steels under 3/8”, Alloy Steels up to 2”

Attainable Hardness:

Martempering – Up to Rockwell c65
Austempering – Up to Rockwell c35-54

Physical Properties:

Martempering – High Strength, Good Toughness
Austempering – High Strength, Highest Toughness

Distortion Control:

Martempering – Very Good
Austempering – Excellent

For example, a carburized gear with a required hardness of Rockwell c61-62, which is beyond the range or austempering, would have to be martempered. A lawnmower blade that requires a hardness of Rockwell c46-48 would be austempered to give it greater toughness to withstand dents and dings in use than would be attainable with martempering.