Basic knowledge of metal heat treatment

Basic knowledge of metal heat treatment (3)
Classification of steel

Steel is an alloy with iron and carbon as the main components, and its carbon content is generally less than 2.11%. Steel is an extremely important metal material in economic construction.

Steel is divided into two categories: carbon steel (referred to as carbon steel) and alloy steel according to its chemical composition. Carbon steel is an alloy obtained by smelting pig iron. In addition to iron and carbon as its main components, it also contains a small amount of impurities such as manganese, silicon, sulfur, and phosphorus. Carbon steel has certain mechanical properties, good process properties, and low price. Therefore, carbon steel has been widely used. However, with the rapid development of modern industry and science and technology, the performance of carbon steel can no longer fully meet the needs, so people have developed various alloy steels. Alloy steel is a multi-element alloy obtained by adding some elements (called alloying elements) purposefully on the basis of carbon steel. Compared with carbon steel, the performance of alloy steel has been significantly improved, so it is increasingly widely used.

Due to the wide variety of steel materials, in order to facilitate production, storage, selection and research, steel materials must be classified. According to different purposes, chemical composition and quality of steel, steel can be divided into many categories:

1. Classification by purpose

According to the purpose of steel, it can be divided into three categories: structural steel, tool steel, and special performance steel.

Structural steel: 1. Steel used for various machine parts. It includes carburized steel, quenched and tempered steel, spring steel and rolling bearing steel.

2. Steel used as engineering structure. It includes A, B, special steel and ordinary low alloy steel springs in carbon steel.

Tool steel: steel used to make various tools. According to different uses of tools, it can be divided into cutting tool steel, die steel and measuring tool steel.

Special performance steel: steel with special physical and chemical properties. Can be divided into stainless steel, heat-resistant steel, wear-resistant steel, magnetic steel, etc.

2. Classified by chemical composition

According to the chemical composition of steel, it can be divided into two categories: carbon steel and alloy steel.

Carbon steel: According to carbon content, it can be divided into low carbon steel (carbon content ≤0.25%); medium carbon steel (0.25%<carbon content<0.6%); high carbon steel (carbon content ≥0.6%) .

Alloy steel: According to the content of alloying elements, it can be divided into low alloy steel (total content of alloying elements≤5%); medium alloy steel (total content of alloying elements=5%-10%); high-alloy steel (total content of alloying elements> 10%). In addition, according to the different types of main alloying elements contained in steel, it can also be divided into manganese steel, chromium steel, chromium nickel steel, chromium manganese titanium steel and so on.

3. Classified by quality

According to the content of harmful impurities phosphorus and sulfur in steel, it can be divided into ordinary steel (phosphorus content ≤0.045%, sulfur content ≤0.055%; or phosphorus and sulfur content ≤0.050%); high-quality steel (phosphorus and sulfur content ≤ 0.040%); high-quality steel (phosphorus content ≤0.035%, sulfur content ≤0.030%).

In addition, according to the type of smelting furnace, steel is divided into open hearth steel (acid open hearth, basic open hearth), air converter steel (acid converter, basic converter, oxygen top-blown converter steel) and electric furnace steel. According to the degree of deoxidation during smelting, steel is divided into boiling steel (with incomplete deoxidation), killed steel (with relatively complete deoxidation) and semi-killed steel.

When naming steel products, steel mills often combine the three classification methods of use, composition, and quality. For example, the steel is called ordinary carbon structural steel, high-quality carbon structural steel, carbon spring steel, high-quality carbon tool steel, alloy structural steel, alloy tool steel, etc.

Mechanical properties of metal materials

The performance of metal materials is generally divided into two categories: process performance and use performance. The so-called process performance refers to the performance of metal materials under the specified cold and hot processing conditions in the processing and manufacturing process of mechanical parts. The process performance of metal materials determines its adaptability in the manufacturing process. Due to the different processing conditions, the required process performance is also different, such as casting performance, weldability, forgeability, heat treatment performance, machinability, etc. The so-called performance refers to the performance of metal materials under the conditions of use of mechanical parts, which include mechanical, physical, and chemical properties. The performance of the metal material determines its use range and service life.

In the machinery manufacturing industry, general mechanical parts are used in normal temperature, normal pressure and non-strongly corrosive media, and each mechanical part will bear different loads during use. The ability of metal materials to resist damage under load is called mechanical properties (or mechanical properties).

The mechanical properties of metal materials are the main basis for the design and selection of parts. The nature of the applied load is different (such as tension, compression, torsion, impact, cyclic load, etc.), and the required mechanical properties of the metal material will also be different. Commonly used mechanical properties include: strength, plasticity, hardness, impact toughness, multiple impact resistance and fatigue limit. The various mechanical properties will be discussed separately below.

1. Strength

Strength refers to the ability of metal materials to resist damage (excessive plastic deformation or fracture) under static load. Since the load acts in the form of tension, compression, bending, shearing, etc., the strength is also divided into tensile strength, compressive strength, bending strength, and shear strength. There is often a certain connection between various strengths, and tensile strength is generally used as the most basic strength indicator in use.

2. Plasticity

Plasticity refers to the ability of metal materials to produce plastic deformation (permanent deformation) without damage under load.

3. Hardness

Hardness is a measure of the hardness of metal materials. At present, the most commonly used method for measuring hardness in production is the indentation hardness method, which uses an indenter of a certain geometric shape to press into the surface of the metal material to be tested under a certain load, and the hardness value is determined according to the degree of indentation.

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

4. Fatigue

The strength, plasticity, and hardness discussed above are all indicators of the mechanical properties of metals under static load. In fact, many machine parts are working under cyclic loads, under which conditions the parts will fatigue.

5. Impact toughness

The load acting on the machine at a very large speed is called impact load, and the ability of metal springs to resist damage under impact load is called impact toughness.