Heat treating helps metal fabricators make strong, durable materials. The hardening procedure accurately heats and treats metal to improve mechanical qualities. Manufacturers control this heat treatment to assure high-quality products.
To help better explain the process, we’ve written an ultimate guide on the steel hardening process. We’ll go over internal structures, best techniques for testing and hardening, and even cover the varieties of hardened steel. Let’s get started!
Steel Internal Structures and Hardness
Steel isn't universal; it's versatile because different applications demand different characteristics. The strength, hardness, and flexibility of steel depend on its internal structure, a microscopic arrangement of atoms. As engineers needed tougher materials , they developed processes to harden steel. Raw steel may not be strong enough for a skyscraper or vehicle chassis, but altering its internal structure can dramatically boost its strength and hardness. Steel hardening involves regulated transformations that favor a better internal arrangement. Let's explore steel's interesting microscopic building blocks to comprehend this.
Martensite
In the world of steel microstructures, martensite is the hardest. It forms when steel is rapidly cooled (quenched), trapping carbon atoms within the iron's structure to create a very strong solid solution. The resulting microstructure consists of needle-like crystals, making the steel very hard but also somewhat brittle. Martensite is also significant in iron-carbon and iron-nickel-carbon alloys.
Pearlite
Pearlite represents the softer side of steel. It forms when steel cools slowly, allowing iron atoms to settle into a layered pattern of ferrite (iron) and cementite (iron carbide). This combination results in a material that is strong yet bendable, suitable for applications requiring flexibility.
Austenite
Austenitic steel features a tough internal structure, characterized by a face-centered cubic lattice that allows for a high degree of ductility and carbon solubility. This structure is stable at high temperatures, typically between 1333.4°F and 2732°F,making austenite robust and workable under hot conditions.
Techniques for Steel Hardening
Steel hardening can be achieved through several techniques, including thermal, mechanical, chemical, or a combination of these methods. Among these, thermal hardening is the most prevalent. This process typically consists of three main stages: heating, holding, and cooling. In the initial stage, the metal is heated to a high temperature to induce internal structural changes, making it more malleable for reshaping. Before you get started, make sure you get a proper heat treat oven. The various steel hardening methods include:
Hardening Through Solid Solution Alloying
This method involves adding an alloying element to the base metal to form a solid solution. When the metal solidifies, the alloy atoms within the crystal lattice increase its hardness. The effectiveness of this process depends on the size difference between the solute and solvent atoms. Larger solute atoms create compressive strain fields, while smaller solute atoms cause tensile strain fields. This distortion of the lattice increases the hardness and strength of the metal.
Cold Working
This technique involves deforming the metal at temperatures below its recrystallization point, significantly altering its properties. Cold working enhances yield strength, tensile strength, and hardness while reducing ductility. Strain hardening, a crucial strengthening mechanism, occurs due to the buildup and entanglement of dislocations during plastic deformation. Although approximately 90% of the energy from cold working is released as heat, the rest is retained within the crystal lattice, increasing its internal energy.
Quenching and Tempering
In quenching, steel is heated above its critical temperature into the austenite range, maintained at this temperature, and then rapidly cooled, typically in water, oil, or molten salt. For hypoeutectoid steels, the heating temperature is set 30-50ºC above the austenite solubility limit. For hypereutectoid steels, the heating temperature is above the eutectoid point. Quenching induces a martensitic transformation, significantly hardening the steel. However, this makes the steel very brittle. To reduce brittleness and alleviate internal stresses, tempering is essential. This involves reheating the quenched steel to a lower temperature to achieve the desired balance of hardness and toughness. You can use an industrial kiln for precise temperature control.
Case Hardening
Case hardening, as its name suggests, is designed to create a durable surface that can withstand wear and friction in applications like crankshafts and bearings. This steel hardening technique typically employs one of three main methods:
Flame Hardening and Induction
This process entails subjecting steel to a carbon-rich environment at elevated temperatures. This atmosphere is typically created using high-quality coal or dissociated natural gas. Carbon atoms penetrate the metal's subsurface, forming a high-carbon layer. Upon rapid cooling (quenching), this layer transforms into a tough, wear-resistant martensitic surface.
Carburizing
Carburizing is a steel hardening process where low-carbon steel is heated to high temperatures (typically 1560°F to 1740°F) in a carbon-rich environment. Carbon atoms diffuse into the steel's surface, creating a high-carbon layer. The steel is then rapidly cooled, resulting in a hard, wear-resistant surface while maintaining a softer, tougher core. This method is commonly used for parts like gears and camshafts that require both strength and durability.
Nitriding
This process modifies the surface composition through the introduction of nitrogen. Initially, the steel is heat-treated to achieve a tempered martensitic structure. It is then exposed to an ammonia atmosphere at approximately 500-550°C for 12-36 hours. This treatment incorporates small alloying elements such as aluminum or chromium, enhancing the formation of finely dispersed nitrides. These nitrides significantly elevate surface hardness and resistance to wear, surpassing the hardness of martensite alone.
Related: The Types of Heat Treatment
Steel Hardness Testing
Hardness is quantified through various index numbers rather than a single unit of measurement. The specific index used to describe a material's hardness depends on the type of hardness test employed. Common methods for measuring hardness include:
Vickers
The Vickers hardness test utilizes a diamond pyramid with a square base as the indenter. This pyramid is pressed against the material's surface under a specified load for approximately 10-30 seconds. The size of the resulting indentation's diagonals is measured, and the material's hardness is calculated using these measurements.
Brinell
In this test, a predetermined load is applied to the material's surface using a steel or carbide ball of known diameter. The Brinell Hardness Number (BHN) is derived from the diameter of the resulting impression. By measuring both the impression diameter and the ball diameter, the BHN is calculated using a specific formula.
Rockwell
The Rockwell test measures hardness by using an indenter (either a hard steel ball or a diamond cone) to penetrate the material twice. The first penetration, using a minor load, sets a reference depth. The second penetration, using a major load, measures the additional depth of indentation. The difference in depth determines the Rockwell hardness value, which is read directly from a dial or digital display.
Knoop Micro
This testing method is ideal for evaluating the hardness of very thin materials or those that are prone to fracturing easily. It uses a diamond-tipped indenter shaped like an elongated pyramid to make a small indentation on the surface. By meticulously examining the indentation under a microscope and measuring the length of the long diagonal, the material's hardness is calculated.
Varieties of Hardenable Steel
Classification of various type of steel:
1. Tool Steel
Tool steels are ideal for making sturdy drills and cutting instruments. Tungsten, cobalt, vanadium, and molybdenum distinguish these steels from others. Cold working, which applies extreme pressure, and steel heat treatment operations like quenching, which rapidly cools the material, give tool steels their outstanding hardness. Tool steels become highly wear-resistant after hardening, making them perfect for demanding industrial applications.
2. Carbon Steel
Steel gets its strength from a special ingredient: carbon. Carbon steel is an iron alloy that mixes in up to 2% carbon to create different strengths. Tiny amounts of other elements can also be added for a specific job. Depending on how much carbon is used, there are three main types of carbon steel: low, medium, and high. Each offers a different balance of strength, flexibility, and ease of shaping.
3. Stainless Steel
Stainless steels are a category of steel distinguished by their exceptional resistance to corrosion and wear. This enhanced durability stems from the inclusion of chromium, a key alloying element, at a concentration between 10% and 20%. Furthermore, stainless steels can be further classified based on their internal structure and the presence of additional elements.
Related: What is Heat Treatment?
4. Alloy steels
Hardenable alloy steels are a speciality. Copper, nickel, manganese, boron, and vanadium are added to alloy steels, unlike carbon steels. These additional ingredients are vital to hardening. They prevent austenite, a high-temperature steel structure, from decomposing. During quenching, martensite, a hard microstructure, forms due to this delay. Second, solid solution hardening strengthens alloy steels. This technique adds fine particles to the steel's matrix, making it harder and less deformable. These mechanisms make alloy steels harden well, making them ideal for demanding applications.
Conclusion
Steel by itself is a strong material, but steel hardening takes it to the next level. This process significantly boosts the metal's strength and durability, making it ideal for demanding applications. Imagine transforming ordinary steel into a dependable material capable of withstanding heavy loads and harsh wear. That's the power of steel hardening, and we have explored the various techniques used to achieve this remarkable transformation.