Relevant standards for the metallographic microstructure rating after heat treatment

I. Overview

Metal heat treatment is a process in which metal workpieces are heated to an appropriate temperature in a specified medium, held at that temperature for a certain period, and then cooled at varying rates.

Metal heat treatment is one of the essential processes in mechanical manufacturing. Unlike other machining processes, heat treatment typically does not alter the workpiece’s shape or its overall chemical composition; instead, it enhances or modifies the workpiece’s service performance by altering its internal microstructure or by changing the chemical composition of its surface. A defining characteristic of heat treatment is its ability to improve the intrinsic quality of the workpiece—qualities that are generally invisible to the naked eye.

To impart the desired mechanical, physical, and chemical properties to metal workpieces, heat treatment is often indispensable in addition to the rational selection of materials and various forming processes. Steel is the most widely used material in the machinery industry; its microstructure is complex and can be effectively controlled through heat treatment, making steel heat treatment the principal focus of metal heat treatment. Moreover, aluminum, copper, magnesium, titanium, and their alloys can also have their mechanical, physical, and chemical properties modified by heat treatment to achieve different service performance characteristics.

As humanity progressed from the Stone Age to the Bronze Age and then to the Iron Age, the role of heat treatment gradually came to be recognized. As early as 770–222 BCE, Chinese practitioners in production already observed that the properties of copper and iron could be altered by temperature and by deformation under pressure. The annealing of white cast iron, for instance, was an important process in the manufacture of agricultural tools.

In the sixth century BC, iron and steel weapons were gradually adopted, and to enhance the hardness of steel, quenching technology rapidly advanced. Two swords and a halberd excavated from Yanxiadu in Yixian County, Hebei Province, China, both exhibit martensite in their microstructures, indicating that they were quenched.

With the advancement of quenching technology, it gradually became apparent that the quenching medium significantly influences quenching quality. During the Three Kingdoms period, a Shu craftsman named Pu Yuan is said to have forged 3,000 swords for Zhuge Liang at Xiegou in what is now Shaanxi Province, reportedly by dispatching men to Chengdu to fetch water for quenching. This indicates that even in ancient China, people recognized the varying cooling capacities of different water qualities and also paid attention to the cooling effectiveness of oil and urine. For instance, a precious sword unearthed from the tomb of King Jing of Zhongshan of the Western Han Dynasty (206 BC–AD 24) exhibits a carbon content of 0.15–0.4% in the core but exceeds 0.6% at the surface, demonstrating the early application of carburizing techniques. However, since these techniques were regarded as proprietary “craft secrets” and were carefully guarded against external transmission, their development proceeded rather slowly.

In 1863, British metallurgists and geologists presented six distinct microstructural features of steel under the microscope, demonstrating that steel undergoes internal structural transformations during heating and cooling, with high-temperature phases in steel converting into a harder phase upon rapid quenching. The theory of iron allotropy established by the French scientist Osmond, together with the iron–carbon phase diagram first developed by the British scientist Austin, laid the preliminary theoretical foundation for modern heat-treatment processes. At the same time, researchers also investigated methods for protecting metals during the heating stage of thermal treatment to prevent oxidation and decarburization.

Between 1850 and 1880, a series of patents were granted for protective heating using various gases, such as hydrogen, coal gas, and carbon monoxide. In 1889–1890, the British inventor Lake obtained patents for bright heat treatment of multiple metals.

Since the twentieth century, advances in metallurgical physics and the introduction of other new technologies have led to further development of metal heat-treatment processes. A notable milestone was the adoption, between 1901 and 1925, of rotary furnaces for gas carburizing in industrial production; in the 1930s, the advent of dew-point potentiometers enabled precise control of the carbon potential in the furnace atmosphere, followed by the development of additional methods for more accurate control, such as carbon dioxide infrared analyzers and oxygen probes; in the 1960s, the application of plasma fields revolutionized heat-treatment technology, leading to the development of ion nitriding and ion carburizing processes; and the subsequent application of laser and electron-beam technologies has provided new surface heat-treatment and chemical heat-treatment methods for metals.

II. Processes of Metal Heat Treatment

Heat treatment processes generally comprise three stages: heating, holding, and cooling; in some cases, only heating and cooling are involved. These stages are sequentially linked and must be carried out without interruption.

Heating is one of the essential steps in heat treatment. There are numerous methods for heating during metal heat treatment; historically, wood charcoal and coal were used as heat sources, followed by liquid and gaseous fuels. The advent of electric heating has made temperature control more precise and eliminated environmental pollution. These heat sources can be employed either directly for heating or indirectly, through molten salts or metals, or even through suspended particles.

When metals are heated, the workpieces are exposed to air, which often leads to oxidation and decarburization (i.e., a reduction in carbon content at the surface of steel parts). This has a highly detrimental effect on the surface properties of the parts after heat treatment. Therefore, metals are typically heated in a controlled atmosphere, a protective atmosphere, molten salt, or a vacuum; alternatively, protective coatings or packaging methods can be used to shield the workpieces during heating.

Heating temperature is one of the critical process parameters in heat treatment, and its proper selection and control are paramount to ensuring the quality of the heat-treated product. The appropriate heating temperature varies depending on the metal material being treated and the specific objectives of the heat treatment; however, it is generally set above the phase-transformation temperature to achieve the desired microstructure. Moreover, phase transformations require a certain amount of time; therefore, once the surface of the metal workpiece has reached the specified heating temperature, it must be held at that temperature for a prescribed duration to ensure uniform temperature throughout the cross-section and complete transformation of the microstructure. This holding period is referred to as the soaking time. In high-energy-density heating and surface heat treatments, the heating rate is extremely rapid, so there is typically no soaking time or only a very short one; by contrast, chemical heat treatments often involve relatively long soaking times.

Cooling is also an indispensable step in the heat-treatment process, and the cooling method varies depending on the specific process, with the primary objective being to control the cooling rate. Generally, annealing involves the slowest cooling rate, normalizing a relatively faster rate, and quenching an even faster rate. However, different steel grades may have varying requirements; for example, air-hardening steels can be quenched at a cooling rate comparable to that used in normalizing.

Metal heat-treatment processes can be broadly classified into overall heat treatment, surface heat treatment, local heat treatment, and chemical heat treatment. Depending on the heating medium, heating temperature, and cooling method, each major category can be further subdivided into several distinct heat-treatment processes. For a given metal, employing different heat-treatment processes can produce different microstructures, thereby resulting in varying material properties. Steel is the most widely used metal in industry, and its microstructure is also the most complex; consequently, there are numerous types of steel heat-treatment processes.

Overall heat treatment is a metal heat-treatment process in which the entire workpiece is heated and then cooled at an appropriate rate to modify its overall mechanical properties. For steels, the four basic types of overall heat treatment are annealing, normalizing, quenching, and tempering.

Annealing involves heating a workpiece to an appropriate temperature, holding it at that temperature for a duration that varies depending on the material and the size of the workpiece, and then cooling it slowly. The purpose is to bring the internal microstructure of the metal to an equilibrium state or close to it, thereby achieving desirable processing and service properties, or to prepare the microstructure for subsequent quenching. Normalizing, on the other hand, consists of heating the workpiece to a suitable temperature and then cooling it in still air. The effect of normalizing is similar to that of annealing, except that the resulting microstructure is finer. Normalizing is often used to improve the machinability of materials and, in some cases, serves as the final heat treatment for parts with relatively low performance requirements.

Quenching involves heating and holding a workpiece at a specified temperature, followed by rapid cooling in a quenching medium such as water, oil, or an aqueous solution of inorganic salts or organic compounds. After quenching, the steel becomes harder but also more brittle. To reduce this brittleness, the quenched steel is held at a suitable temperature above room temperature but below 710°C for an extended period, and then cooled; this process is known as tempering. Annealing, normalizing, quenching, and tempering constitute the “four heat treatments” in overall heat treatment, with quenching and tempering being closely related and often used in conjunction—neither can be omitted.

Depending on the heating temperature and cooling method, the “four heat treatments” give rise to various heat-treatment processes. To achieve a specified combination of strength and toughness, the process that combines quenching with high-temperature tempering is known as normalizing. In some alloys, after quenching to form an oversaturated solid solution, the material is held at room temperature or at a slightly higher, appropriate temperature for an extended period to enhance its hardness, strength, electrical properties, magnetic properties, and other characteristics; this heat-treatment process is called aging treatment. When plastic deformation during forging or rolling is effectively and closely integrated with heat treatment, resulting in workpieces that exhibit an excellent balance of strength and toughness, the method is referred to as thermomechanical treatment. Heat treatment conducted in a vacuum or under reduced-pressure atmospheres is called vacuum heat treatment; such processes not only prevent oxidation and decarburization, maintaining a smooth surface finish and enhancing the performance of the workpiece, but also allow the introduction of carburizing agents for chemical heat treatment.

Surface heat treatment is a metal heat-treatment process that involves heating only the surface layer of a workpiece to alter its surface mechanical properties. To ensure that only the surface is heated without excessive heat penetrating into the interior, the heat source used must have a high energy density—i.e., it must deliver a substantial amount of thermal energy per unit area of the workpiece—so that the surface or a localized region can rapidly or instantaneously reach a high temperature. The main methods of surface heat treatment include laser heat treatment, flame quenching, and induction heating. Commonly used heat sources include oxyacetylene or oxypropane flames, induced currents, lasers, and electron beams.

Chemical heat treatment is a metal heat-treatment process that alters the chemical composition, microstructure, and properties of a workpiece’s surface layer. Unlike surface heat treatment, which modifies only the surface chemistry, chemical heat treatment involves heating the workpiece in a medium—gas, liquid, or solid—that contains carbon, nitrogen, or other alloying elements, followed by prolonged holding at elevated temperature to allow these elements to diffuse into the surface layer. After diffusion, additional heat-treatment operations such as quenching and tempering may be performed. The principal methods of chemical heat treatment include carburizing, nitriding, metal impregnation, and composite diffusion.

Heat treatment is one of the critical processes in the manufacturing of mechanical parts and dies and molds. Generally speaking, it can ensure and enhance various performance characteristics of workpieces, such as wear resistance and corrosion resistance. It can also improve the microstructure and stress state of the blank, thereby facilitating subsequent cold and hot working operations.

For example, white cast iron can be transformed into malleable cast iron through prolonged annealing, thereby enhancing its ductility; when gears are subjected to appropriate heat-treatment processes, their service life can be increased by a factor of several or even dozens compared with gears that have not undergone heat treatment; furthermore, inexpensive carbon steels can acquire the properties of certain high-cost alloy steels by alloying with specific elements, enabling them to replace some heat-resistant and stainless steels; and virtually all tools and dies must undergo heat treatment before they can be put into service.

III. Classification of Steel

Steel is an alloy whose primary constituents are iron and carbon, with a carbon content generally below 2.11%. It is an extremely important metallic material in economic development. Based on chemical composition, steel is classified into two major categories: carbon steel (often simply referred to as “carbon steel”) and alloy steel. Carbon steel is an alloy produced by smelting pig iron; in addition to iron and carbon as its main components, it also contains small amounts of impurities such as manganese, silicon, sulfur, and phosphorus. Carbon steel exhibits satisfactory mechanical properties, good processability, and low cost, which has led to its widespread application. However, with the rapid advancement of modern industry and science and technology, the performance of carbon steel can no longer fully meet current requirements, prompting the development of various alloy steels. Alloy steel is a multi-component alloy obtained by deliberately adding certain elements—known as alloying elements—to carbon steel. Compared with carbon steel, alloy steel demonstrates significantly improved performance, resulting in increasingly broad applications.

Due to the wide variety of steel grades, it is essential to classify steels to facilitate production, storage, selection, and research. Based on differences in application, chemical composition, and quality, steels can be categorized into numerous classes:

(1). Classification by Purpose

According to their applications, steels can be classified into three major categories: structural steel, tool steel, and special-performance steel.

1. Structural steel:

(1) Steels used for various machine parts. These include carburizing steels, quenched-and-tempered steels, spring steels, and rolling-bearing steels.

(2) Steel used for structural applications. This includes Class A, Class B, and Special-grade steels among carbon steels, as well as ordinary low-alloy steels.

2. Tool steel: Steel used to manufacture various tools. Depending on the tool’s application, it can be classified into cutting-tool steel, die steel, and measuring-tool steel.

3. Special-performance steels: These are steels that possess exceptional physical and chemical properties. They can be classified into stainless steels, heat-resistant steels, wear-resistant steels, magnetic steels, and others.

(2). Classification by Chemical Composition

Based on chemical composition, steels can be classified into two major categories: carbon steels and alloy steels.

Carbon steel is classified according to carbon content as low-carbon steel (carbon content ≤0.25%); medium-carbon steel (0.25% < carbon content < 0.6%); and high-carbon steel (carbon content ≥0.6%).

Alloy steels are further classified according to their alloying element content as follows: low-alloy steels (total alloying element content ≤5%); medium-alloy steels (total alloying element content = 5%–10%); and high-alloy steels (total alloying element content >10%). In addition, based on the predominant alloying elements present, alloy steels can be categorized into manganese steels, chromium steels, chromium-nickel steels, chromium-manganese-titanium steels, and others.

(3). Classification by Quality

Based on the content of the harmful impurities phosphorus and sulfur in steel, steels can be classified as ordinary steel (phosphorus content ≤0.045% and sulfur content ≤0.055%; or both phosphorus and sulfur contents ≤0.050%) and high-quality steel (phosphorus and sulfur contents ≤0.030%).

In addition, steels are classified according to the type of smelting furnace into open-hearth steels (acid open-hearth and basic open-hearth), basic oxygen steels (acidic converter, basic converter, and top-blown oxygen converter steels), and electric-furnace steels. According to the degree of deoxidation during smelting, steels are further categorized into boiling steels (insufficiently deoxidized), killed steels (relatively thoroughly deoxidized), and semi-killed steels.

When naming steel products, steel mills often combine three classification methods: intended use, chemical composition, and quality. For example, steels may be designated as ordinary carbon structural steel, high-quality carbon structural steel, carbon tool steel, high-grade high-quality carbon tool steel, alloy structural steel, alloy tool steel, and so on. All of these have a phosphorus content of ≤0.040%; high-grade high-quality steel (with a phosphorus content of ≤0.035%,

IV. Mechanical Properties of Metallic Materials

The properties of metallic materials are generally classified into two categories: service performance and processing performance. Processing performance refers to the behavior of metallic materials under specified cold- and hot-working conditions during the manufacturing and machining of mechanical components. The quality of a material’s processing performance determines its ability to be readily formed and machined during production. Because processing conditions vary, the required processing performance also varies, encompassing such characteristics as castability, weldability, forgeability, heat-treatability, and machinability. Service performance, on the other hand, refers to the behavior of metallic materials under actual service conditions and includes mechanical, physical, and chemical properties. The quality of a material’s service performance dictates its range of applications and service life.

In the mechanical manufacturing industry, most machine parts are used under ambient temperature and pressure conditions in non-aggressive corrosive environments, and during service they are subjected to various types of loading. The ability of a metallic material to resist failure under load is referred to as its mechanical properties (or mechanical performance). These properties serve as the primary basis for component design and material selection. Depending on the nature of the applied external load—such as tension, compression, torsion, impact, or cyclic loading—the required mechanical properties will also vary. Commonly used mechanical properties include strength, ductility, hardness, toughness, resistance to repeated impact, and fatigue limit. The following sections will discuss each of these properties in turn.

1. Strength

Strength refers to the ability of a metallic material to resist failure—whether excessive plastic deformation or fracture—under static loading. Since loading can be applied in various modes, such as tension, compression, bending, and shear, strength is accordingly classified into tensile strength, compressive strength, flexural strength, and shear strength. These different types of strength are often interrelated, and in practical applications tensile strength is generally regarded as the most fundamental strength criterion.

2. Plasticity

Plasticity refers to the ability of a metallic material to undergo plastic deformation (permanent deformation) under load without fracturing.

3. Hardness

Hardness is an index used to measure the degree of softness or hardness of metallic materials. In current manufacturing practice, the most commonly employed method for determining hardness is the indentation hardness test, in which an indenter of a specified geometric shape is pressed into the surface of the metal under a defined load, and the hardness value is determined based on the extent of indentation.

Common methods include Brinell hardness (HB), Rockwell hardness (HRA, HRB, HRC), and Vickers hardness (HV).

4. Fatigue

The strength, ductility, and hardness discussed earlier are all mechanical property indicators of metals under static loading. In reality, many machine components operate under cyclic loading, a condition that can lead to fatigue failure.

5. Impact Toughness

A load applied to a machine component at a high rate of loading is referred to as an impact load, and the ability of a metal to resist failure under such loading is known as impact toughness.

5. Annealing–Quenching–Tempering

(1). Types of Annealing

1. Complete Annealing and Isothermal Annealing

Complete annealing, also known as recrystallization annealing and commonly referred to simply as annealing, is primarily used for castings, forgings, and hot-rolled sections made of hypoeutectoid carbon steels and alloy steels; it is sometimes also applied to welded structures. It is typically employed as the final heat treatment for non-critical components or as a preliminary heat treatment for certain workpieces.

2. Spheroidizing Annealing

Spheroidizing annealing is primarily used for hypereutectoid carbon steels and alloy tool steels (such as those employed in the manufacture of cutting tools, measuring instruments, and dies). Its main objectives are to reduce hardness, improve machinability, and prepare the material for subsequent quenching.

3. Stress-relief annealing

Stress-relief annealing, also known as low-temperature annealing (or high-temperature tempering), is primarily used to eliminate residual stresses in castings, forgings, welded components, hot-rolled parts, and cold-drawn parts. If these stresses are not relieved, they can lead to deformation or cracking in the steel components after a certain period of time or during subsequent machining operations.

(2). Quenching

To enhance hardness, the primary methods involve heating, holding at a specific temperature, and rapid cooling. The most commonly used quenching media are brine, water, and oil. Workpieces quenched in brine readily achieve high hardness and a smooth surface, with a low likelihood of soft spots caused by incomplete hardening; however, brine quenching often leads to severe distortion and even cracking. By contrast, oil as a quenching medium is suitable only for certain alloy steels with relatively high supercooled austenite stability or for small-sized carbon steel workpieces.

(3). Tempering

1. Reduce brittleness and eliminate or alleviate internal stresses. After quenching, steel parts exhibit significant internal stresses and brittleness; if they are not tempered promptly, they may deform or even crack.

2. Achieving the required mechanical properties of the workpiece: After quenching, the workpiece exhibits high hardness but significant brittleness. To meet the diverse performance requirements of different workpieces, appropriate tempering can be employed to adjust the hardness, reduce brittleness, and obtain the desired toughness and ductility.

3. Stabilize workpiece dimensions

4. For certain alloy steels that are difficult to soften by annealing, high-temperature tempering is often performed after quenching (or normalizing) to promote the appropriate coalescence of carbides in the steel, thereby reducing its hardness and facilitating machining.

VI. Selection of Common Furnace Types

The furnace type should be determined based on the specific process requirements and the type of workpiece.

1. For parts that cannot be produced in batch quantities, have varying sizes, and come in a wide variety of types, the manufacturing process is required to be versatile.

For versatile applications, a box furnace can be selected.

2. For heating long-axis workpieces and long lead screws, pipes, and other similar components, a deep-well electric furnace may be selected.

3. For small-batch carburized parts, a pit-type gas carburizing furnace may be selected.

4. For the mass production of automotive and tractor gears and other similar parts, a continuous carburizing line or a box-type multi-purpose furnace may be selected.

5. For heating sheet blanks used in stamping parts in mass production, roller furnaces and hearth-type roller furnaces are the preferred choices.

6. For batch production of standardized parts, pusher-type or conveyor-type resistance furnaces (pusher furnaces or belt furnaces) may be used in manufacturing.

7. Small mechanical parts, such as screws and nuts, can be processed using vibration-bottom furnaces or mesh-belt furnaces.

8. For the heat treatment of steel balls and roller bearings, a rotary tube furnace with internal helical coils may be used.

9. For mass production of nonferrous metal ingots and billets, pusher-type furnaces are suitable; for small nonferrous metal parts and materials, air-circulation heating furnaces are preferred.