In order to gain toughness in hardened steel, the process usually adopted is:

Question

Prepp Team · Accepted Answer

When steel is subjected to a hardening process, it is typically heated to a high temperature and then rapidly cooled, usually by quenching. This rapid cooling transforms the steel's internal structure into a very hard and strong phase called martensite. While martensite provides excellent hardness, it also makes the steel extremely brittle, meaning it can easily crack or break under impact or shock loads. To overcome this brittleness and improve its ability to absorb energy before fracturing, a subsequent heat treatment is essential. This process aims to achieve a balance between hardness and ductility, making the steel suitable for various demanding applications, such as tools, springs, and gears.

Tempering for Steel Toughness

The heat treatment process specifically adopted to gain significant toughness in hardened steel is tempering. After the initial hardening (quenching) process, steel contains a high amount of untempered martensite, which is very hard but inherently brittle. Tempering involves reheating this hardened steel to a specific temperature below its lower critical temperature (typically below \(727^{\circ}\text{C}\)), holding it at that temperature for a predetermined time, and then allowing it to cool, often in still air.

Primary Objective: The main purpose of tempering is to reduce the brittleness of hardened steel, relieve internal stresses induced during quenching, and thereby increase its ductility and toughness, all while retaining a substantial portion of its desired hardness.
Microstructural Transformation: During tempering, the unstable, strained martensite microstructure undergoes a transformation. Carbon atoms within the martensite begin to precipitate out, forming fine carbide particles (like cementite) within a ferrite matrix. This new microstructure, often referred to as tempered martensite, is much more stable and less brittle than untempered martensite.
Temperature Influence: The specific tempering temperature plays a crucial role in determining the final properties. Lower tempering temperatures yield steel with higher hardness but moderate toughness, whereas higher tempering temperatures result in steel that is softer but significantly tougher and more ductile.

Other Heat Treatment Processes and Their Purposes

It is important to differentiate tempering from other common heat treatment processes, as each serves distinct metallurgical objectives and is not primarily focused on imparting toughness to *already hardened* steel.

Annealing

Annealing is a heat treatment process that typically involves heating steel above its critical temperature, holding it at that temperature for a sufficient period, and then cooling it very slowly, usually inside the furnace. The main goals of annealing are:

To soften the steel for improved machinability or cold working.
To relieve internal stresses that might have developed during previous manufacturing processes like casting, forging, or cold working.
To refine the grain structure and achieve a more uniform microstructure.
To enhance ductility and toughness, but in a different context than tempering of hardened steel.

While annealing does improve ductility, its primary application is not specifically to make *hardened* steel tough; it's generally applied to unhardened steel to prepare it for subsequent processing or to achieve a soft state.

Normalizing

Normalizing involves heating steel above its critical temperature, holding it at that temperature, and then cooling it in still air. This process is distinct from annealing in its cooling rate. Normalizing is primarily used to:

Refine the grain size and achieve a more uniform and homogeneous microstructure compared to annealed or as-cast/forged conditions.
Eliminate internal stresses and improve mechanical properties such as strength and ductility.

Normalizing provides a good balance of strength and ductility but is not directly performed on *hardened* martensitic steel to specifically increase its toughness in the way tempering does.

Spherodising

Spherodising is a specialized type of annealing process primarily applied to high carbon steels, including tool steels. It involves prolonged heating at a temperature just below the lower critical temperature or by temperature cycling, followed by very slow cooling. The main objective of spherodising is to:

Cause the lamellar (plate-like) cementite in the pearlite microstructure to coalesce into spherical (globular) particles embedded in a soft ferrite matrix.
Significantly improve the machinability and ductility of high carbon steels, making them easier to cut and form.

Spherodising aims to make the steel softer and more workable, rather than specifically increasing the toughness of steel that has already been hardened.

Comparative Summary of Heat Treatment Processes

Process	Primary Goal	Effect on Hardness and Toughness	Typical Application Scenario
Tempering	Reduce brittleness and increase toughness and ductility of hardened steel.	Decreases hardness slightly, significantly increases toughness.	Applied after quenching for tools, springs, dies, gears to make them usable.
Annealing	Soften steel, relieve internal stresses, refine grain structure, improve machinability.	Softens steel significantly, increases ductility and toughness (overall properties).	For softening steel before machining or cold working, or stress relief in castings/forgings.
Normalizing	Refine grain size, improve uniformity, relieve stresses.	Provides a good balance of strength and ductility, generally tougher than annealed but harder than fully annealed.	For improving mechanical properties and homogeneity of as-cast or as-forged components.
Spherodising	Improve machinability and ductility of high carbon steels by forming globular carbides.	Significantly softens high carbon steels, improves formability.	Applied to high carbon steels to facilitate machining.

Therefore, when the objective is specifically to gain toughness in steel that has undergone a hardening treatment, tempering is the universally adopted process. It effectively transforms the brittle martensite into a structure that can absorb more energy, thereby enhancing the steel's resistance to impact and fracture.