In our previous article about Additive Manufacturing, we discussed the cracking phenomenon associated with metals produced using LPBF-type techniques. We pointed out, the process of heating and cooling plays a pivotal role in the cracking process and that it shapes a metal’s microstructure, consequently influencing its thermo-mechanical properties.
In this article we will talk about heat treatment of metals in more detail. This will include a comparison of traditional martensitic steels with so-called “maraging” steels.
Introduction
In the realm of metallurgy, the art of fortifying metals to endure the challenges posed by thermo-mechanical stresses finds its foundation in the indispensable practice of heat treatment. Whether the objective is the construction of a resilient bridge, or the engineering of a steadfast aircraft engine part, the comprehension of the intricate mechanisms underlying heat treatment assumes paramount significance.
Basics
Heat treatment involves heating the metal to specific temperatures and then cooling it down in various ways. This process can make the metal harder, softer, tougher, or more resistant to wear and tear, depending on the desired outcome.
- Heating: The journey begins with heating the metal to a carefully controlled temperature. This stage, known as the austenitizing phase, allows the metal’s atomic structure to transform into a more malleable form.
- Cooling: The journey begins with heating the metal to a carefully controlled temperature. This stage, known as the austenitizing phase, allows the metal’s atomic structure to transform into a more malleable form.
- Tempering: The final step, tempering, involves reheating the metal to a specific temperature. This relieves internal stresses created during quenching and fine-tunes the metal’s hardness and toughness.
Types of Heat Treatment
- Annealing: Slow cooling after heating makes the metal softer and more ductile. It relieves internal stresses and makes it easier to shape.
- Hardening: Rapid cooling (quenching) after heating creates a harder and more brittle metal. It’s ideal for tools and sharp objects like knives.
- Tempering: Reheating after quenching balances hardness and toughness, producing a more durable and resilient metal.
- Normalizing: Similar to annealing, but with a faster cooling rate, normalizing improves the metal’s uniformity and strength.
Note: We excluded Case-Hardening on purpose
Atomic Structure and Heat Treatment
At the heart of heat treatment is a profound understanding of a metal’s atomic structure. Most metals consist of a crystalline structure, where atoms arrange themselves in a repeating pattern. The arrangement of these atoms directly affects the metal’s properties.
- Grain Structure: Metals are composed of grains, which are like tiny crystals within the material. Heat treatment can influence the size and distribution of these grains. Smaller grains often result in improved strength and hardness.
- Phase Changes: When a metal is heated, its atomic arrangement can change. One key transformation is from a ferrite or pearlite structure to austenite (see next section), which is a more disordered, high-temperature phase. This change is crucial for altering the metal’s properties during heat treatment.
The Iron-Carbon Phase Diagram
For many common metals, including steel, the iron-carbon phase diagram is an invaluable tool. This diagram illustrates how the structure of iron changes with varying carbon content and temperature. It’s essential for understanding how different heat treatment processes work. We will concentrate only on a few of the aspects of the phase diagram:
Martensite Phase: Martensite is a metastable phase with a body-centered tetragonal (BCT) crystal structure. It forms when austenite is rapidly cooled (quenched) to lower temperatures. Martensite is extremely hard and is responsible for the hardness of hardened steels. It appears as a non-equilibrium phase in the phase diagram and is not bound by specific temperature ranges.
Austenite Phase (γ-iron): Austenite is a solid solution of carbon in iron that exists at elevated temperatures. It has a face-centered cubic (FCC) crystal structure. Austenite is stable at high temperatures and is non-magnetic. It is represented by the region in the phase diagram above about 727°C (1341°F).
Ferrite Phase (α-iron): Ferrite is another solid solution of carbon in iron but exists at lower temperatures than austenite. It has a body-centered cubic (BCC) crystal structure. Ferrite is stable at lower temperatures and is also non-magnetic. It appears as a field to the left of the phase diagram, below about 727°C (1341°F).
Cementite Phase (Iron Carbide, Fe3C): Cementite is a compound of iron and carbon (Fe3C) and is quite hard and brittle. It forms at higher carbon concentrations and higher temperatures. Cementite is represented in the phase diagram at temperatures above about 727°C (1341°F).
Pearlite Phase: Pearlite is a lamellar mixture of ferrite and cementite. It forms at lower temperatures when austenite or cementite decomposes. Pearlite is present in the phase diagram below about 727°C (1341°F) and consists of alternating layers of ferrite and cementite.

Martensitic and “Maraging” steels:
Martensitic and maraging steels are important metallurgical concepts, each with distinct characteristics and applications in the context of heat treatment and materials engineering. They mainly differ in chemical composition and heat treatment.
Martensite
An example of traditional martensitic steel is the AISI D2 tool steel with the following chemical composition: C 1.40-1.60 wt-%, Cr 11-13 wt-%, Mo 0.70-1.20 wt-%, V 0.70 wt-%, Si 0.60 wt-%, Mn 0.6 wt-%, S <0.03 wt-%, P <0.03 wt-%, balance Fe.
- Austenitizing: The process of forming traditional martensite typically begins by heating the steel to its austenitizing temperature, where its crystal structure transforms into austenite, a high-temperature phase.
- Rapid Quenching: The critical step in forming traditional martensite is rapid quenching. After austenitizing, the steel is quickly cooled by immersing it in a quenching medium such as oil, water, or air. This rapid cooling prevents the crystal lattice from returning to its original state and locks it into the martensitic phase (supersaturated). That means, there isn’t enough time for all the carbon atoms to diffuse out of the iron lattice, leading to a concentration of carbon that exceeds its normal solubility limit in iron. This is why it’s called „supersaturated”.
- Resulting Properties: Traditional martensite is extremely hard and brittle due to its highly strained and distorted crystal lattice. This phase is characterized by its hardness and wear resistance.
- Tempering: To reduce brittleness and improve toughness, traditional martensite is often tempered. Tempering involves reheating the steel to a specific temperature and then cooling it slowly. This process relaxes the crystal structure and balances hardness with improved ductility and toughness.
Maraging Steel
Typically, maraging steels have lower carbon content than martensitic steels such as aforementioned AISI D2. An example of maraging steel is the EN 1.2709 grade steel with the following chemical composition: C <0.03 wt-%, Ni 8.5-9.5 wt-%, Co 7.5-8.5 wt-%, Mo 1.5-1.8 wt-%, Ti 0.4-0.6 wt-%, Al 0.05-0.2 wt-%, Si <0.1 wt-%, Mn <0.1 wt-%, P < 0.01 wt-%, S < 0.005 wt-%, balance Fe. We will delve a bit deeper into the intricacies of heat treatment for maraging steels.
- Solution Annealing (Austenitizing): Heating: The process begins with heating the maraging steel to its austenitizing temperature. This temperature can vary depending on the specific grade of maraging steel. This heating stage transforms the material’s crystal structure into austenite (high-temperature phase). Hold Time: The steel is held at this temperature for a specific period, allowing the austenite to fully form and dissolve any precipitates or phases present in the original material.
- Rapid Quenching: Quenching: After the solution annealing stage, the maraging steel is rapidly quenched in a cooling medium like oil or air. This rapid cooling prevents the reformation of any undesired phases and „freezes“ the austenite in a supersaturated state. Resulting Structure: The rapid quenching results in a metastable microstructure with supersaturated austenite. This is a critical precursor for achieving the desired strength and toughness in maraging steels.
- Aging (Precipitation Hardening): Heating for Aging: The quenched steel is then subjected to aging at a lower temperature, typically between (450°C and 510°C). This aging temperature depends on the specific maraging steel grade and desired properties. Aging Time: The material is held at this temperature for an extended period, often several hours to several days. During this time, intermetallic precipitates, such as nickel, cobalt, and molybdenum, begin to form within the steel matrix (precipitated intermetallic phase). These precipitates strengthen the material by acting as obstacles to dislocation movement. Resulting Properties: The aging process transforms the supersaturated austenite into martensite with fine, uniformly distributed precipitates. This results in the exceptional strength and toughness that maraging steels are known for.
- Optional Tempering: In some cases, maraging steels may undergo a tempering process to adjust their final properties. Tempering involves reheating the material to a specific temperature and then cooling it slowly. This can help balance hardness and toughness, depending on the application requirements.
We conclude: martensite is a phase formed during rapid quenching in heat treatment and is characterized by extreme hardness, while maraging steels are a specific group of materials that utilize controlled aging of martensite to achieve exceptional strength and toughness. They are typically used in applications where high-strength materials are required, such as aerospace and defense applications.
Why are Maraging Steels Not Suited for High-Temperature Applications?
One might ask: Why not use this “super-metal” called maraging steel for high-temperature applications?
Aging Embrittlement: The unique microstructure of maraging steels, which includes the presence of supersaturated martensite and precipitated intermetallic phases, can be altered at high temperatures. The coarsening of precipitates and the reversion of martensite can occur, negatively affecting the material’s properties. In this context “Aging Embrittlement” refers to the softening of maraging steels at temperatures above 400°C. This is because the aging process that gives them their remarkable strength and hardness at lower temperatures is reversed to some extent at higher temperatures. As the temperature increases, the precipitated intermetallic phases in maraging steels, originally formed through precipitation heating (phases consisting of alloy elements), can coarsen, leading to a reduction in strength.
Let’s Summarize:
General Content
In this article we explored the critical role of heat treatment in shaping the microstructure and thermo-mechanical properties of metals, with a focus on the comparison between traditional martensitic steels and maraging steels. The fundamental steps of heating, cooling, and tempering are explained, emphasizing their influence on a metal’s properties. Various types of heat treatment, including annealing, hardening, tempering, and normalizing, are discussed.
Insight was gained on how the exceptional strength and toughness of maraging steels are achieved through a special heat treatment process. Finally, in this article the limitations of maraging steels for high-temperature applications due to aging embrittlement, which can result in a reduction of strength at temperatures above 400°C is addressed.
Conclusions for Additive Manufacturing
In the context of Additive Manufacturing heat treatment plays an as important role as it does for conventional metal parts. Typically, metal powder used for LPBF-type 3D printing methods and produced by melting and atomization of metal alloys are not already heat treated, i.e., they are in their raw state (sometimes annealing is already done). Therefore, in Additive Manufacturing, heat-treatment, such as stress relief or stress relief annealing is part of the AM post-processing steps.
It’s worth noting that maraging steels have enjoyed massive attention in the world of Additive Manufacturing. Their exceptional thermo-mechanical properties and the ease of 3D printing, with little susceptibility to cracking, have made them a hot topic in the AM community. However, it’s essential to clarify that maraging steels are not „hot-working“ steels (as unfortunately advertised by many). Because, as this article emphasizes, at elevated temperatures (e.g. hot-working environments such as aluminum die-casting) this metal loses its thermo-mechanical resistance.
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