1. Overview of Carbon Steel Hardness
The hardness of carbon steel is one of the important indicators to measure its performance. Different types of carbon steel have different hardnesses. Understanding these differences will help you choose the right material in practical applications.
(I) Hardness range of ordinary carbon steel
The hardness of ordinary carbon steel is generally between 120 and 180HB, or the hardness is usually between 20 and 30 HRC. This is because the carbon content of ordinary carbon steel is relatively low, generally less than 0.25%. The low carbon content makes ordinary carbon steel have good plasticity and machinability, but relatively weak in hardness. In some occasions where the hardness requirement is not high, ordinary carbon steel can meet the demand, such as manufacturing some non-critical mechanical parts or structural parts.
(II) Examples of hardness of special carbon steel
Take 45 steel as an example, it shows different hardness under different treatment methods. After quenching, the hardness can reach HRC45 - 50; after carburizing, the surface hardness is HRC≥60; after nitriding, the surface hardness is HB≥600. This is because different treatment methods will change the structure of steel, thereby affecting its hardness. Quenching is to heat steel to a certain temperature and then cool it rapidly, so that the structure of the steel changes to martensite, thereby increasing the hardness. Carburizing is to penetrate carbon atoms into the surface of steel to form a high carbon layer and increase the surface hardness. Nitriding is to penetrate nitrogen atoms into the surface of steel to form a nitrided layer and increase the surface hardness.
(III) Hardness characteristics of 55Mn carbon steel
As a carbon steel, 55Mn has a hardness range of 180 - 220HB. The high hardness of 55Mn ensures its wear resistance and compression resistance in places with high strength requirements. This is because 55Mn contains a high amount of manganese, which can improve the strength and hardness of steel. At the same time, after proper heat treatment, the organizational structure of 55Mn is more uniform and the hardness is more stable. 55Mn is widely used in some places that require high strength and high wear resistance, such as ships, bridges, and automobiles.
2. Factors affecting the hardness of carbon steel
(I) The influence of carbon content
The carbon content of carbon steel has a significant effect on its hardness. Generally speaking, the higher the carbon content, the greater the hardness. When the carbon content is low, such as low carbon steel, the carbon content is usually between 0.0218% - 0.25%, and its hardness is relatively low. As the carbon content increases, entering the medium carbon steel range (carbon content 0.25% - 0.6%), the hardness gradually increases, and the plasticity begins to decrease. When the carbon content is further increased to high carbon steel (carbon content greater than 0.6%), the hardness increases significantly, but the plasticity also decreases further. For example, in the case of low carbon content, the structure is ferrite and pearlite. As the carbon content increases, the content of pearlite also increases, and the hardness of pearlite is higher than that of ferrite. When the carbon content reaches 0.8%, the structure is pearlite, and greater than 0.8% is pearlite and carbide. The hardness of carbide is very high, which makes the higher the carbon content, the greater the hardness.
(II) Effects of other elements
In addition to carbon content, elements such as manganese, silicon, sulfur, and phosphorus also affect the hardness of carbon steel. Manganese is a beneficial element in carbon steel. It can synthesize manganese sulfide with sulfide in steel, thereby reducing the harmful effects of sulfur. At the same time, most of the residual manganese can be dissolved in ferrite to improve the strength and hardness of steel. Generally, the manganese content is less than 1%. Silicon also has a similar effect. Residual silicon can dissolve in ferrite, strengthen ferrite, improve the strength and hardness of steel, and improve the liquid fluidity of steel, which is conducive to casting. Its content generally does not exceed 0.5%.
However, the effects of sulfur and phosphorus on the hardness of carbon steel are more complicated. Sulfur is insoluble in iron in steel, but exists in the form of FeS, forming a low melting point (985℃) eutectic with Fe. When the steel is hot-worked, it will cause cracking, which is called "hot brittleness". At the same time, it will reduce the strength and toughness of the steel. However, sulfur and manganese will form manganese sulfide, which can improve the machinability. The content should be strictly controlled to be generally less than 0.050%. Phosphorus can be completely dissolved in ferrite, improving the strength and hardness of steel, but it will cause the plasticity and toughness of steel to drop sharply below 100℃, that is, "cold brittleness". The higher the phosphorus content, the more serious the cold brittleness. However, when the content is 0.05%-0.15%, it can improve the machinability. At the same time, phosphorus will also deteriorate the welding performance of steel. The content should generally be less than 0.045%.
III. Differences in hardness of different types of carbon steel
(I) Classification by carbon content
- Low carbon steel: The carbon content is generally less than 0.25%. For example, No. 15 steel is a low carbon steel material with a carbon content between 0.12 and 0.18%. It has good ductility and plasticity, and a relatively low hardness. According to data, the hardness of low carbon steel is generally between 120 and 180 HB. Low carbon steel is easy to accept various processing such as forging, welding and cutting, and is often used to manufacture chains, rivets, bolts, shafts, etc., as well as various plates and containers. This type of steel is also often used to manufacture various carburized parts. After carburizing and quenching, the steel surface has high hardness and good wear resistance, while the core maintains a certain strength and toughness. It can be used to manufacture parts that withstand impact loads and wear-resistant parts, such as gears, short shafts, pins, etc.
- Medium carbon steel: The carbon content is between 0.25% and 0.60%. The strength and hardness are higher than those of low carbon steel, and the plasticity and toughness are lower than those of low carbon steel. Medium carbon steel has good hot processing and cutting performance, but poor welding performance. After quenching and tempering, medium carbon steel has good comprehensive mechanical properties, with a maximum hardness of about HRC55 (HB538) and σb of 600 - 1100MPa. For example, 45 steel, after quenching but before tempering, is qualified if the hardness is greater than HRC55 (up to HRC62), and the maximum hardness in actual application is HRC58 (high-frequency quenching). After quenching and tempering, the parts have good comprehensive mechanical properties and are widely used in various important structural parts, such as pistons of air compressors and pumps, impellers of steam turbines, shafts, worms, gears of heavy machinery, etc., as well as parts with wear-resistant surfaces, crankshafts, machine tool spindles, rollers, bench tools, etc.
- High carbon steel: Carbon content greater than 0.6%, high hardness and strength, but poor plasticity and toughness. High carbon steel is usually called tool steel, with a carbon content of 0.60% to 1.70%, which can be hardened and tempered. Hammers, crowbars, etc. are made of steel with a carbon content of 0.75%; cutting tools such as drills, taps, and reamers are made of steel with a carbon content of 0.90% to 1.00%. After proper heat treatment, high carbon steel has a very high hardness, generally between 65-75 HRC. However, due to its extremely poor welding performance, it is not used for general welding structures, but only for repair welding or surfacing of castings. After welding, the weldment should be tempered to eliminate stress, fix the structure, prevent cracks, and improve welding performance.
(II) Classification by quality
There may be some differences in hardness between ordinary carbon steel, high-quality carbon steel, and advanced high-quality carbon steel. The hardness of ordinary carbon steel is relatively general, and its relatively high impurity content may affect the stability of its hardness. High-quality carbon steel is more strictly controlled in terms of carbon content, and after proper heat treatment, its hardness may be higher than that of ordinary carbon steel, and its hardness stability is also better. Advanced high-quality carbon steel is more outstanding in all aspects of performance, and its hardness may be higher. At the same time, it also performs well in wear resistance and strength. For example, in some occasions with high requirements for hardness and quality, such as the manufacture of high-precision mechanical parts and cutting tools, advanced high-quality carbon steel may be preferred. In some occasions where the hardness requirement is not so high and the cost control is relatively strict, ordinary carbon steel may be more suitable.
IV. Practical application and selection of carbon steel hardness
(I) Application of carbon steel hardness in different fields
- Mechanical manufacturing
- In mechanical manufacturing, different parts have different requirements for carbon steel hardness. For example, key parts such as gears and crankshafts require higher hardness to withstand greater pressure and friction. After proper heat treatment, medium carbon steel and high carbon steel have higher hardness and strength, making them suitable for manufacturing these important mechanical parts.
- For some non-critical mechanical parts, such as bolts and nuts, although the hardness of low-carbon steel is low, it has good plasticity and machinability, which can meet its use requirements.
- Construction field
- In the construction field, carbon steel is mainly used to manufacture steel bars and supporting structures. Generally speaking, carbon steel used in construction needs to have a certain strength and hardness to ensure the stability and safety of the building structure. After proper processing and treatment, medium carbon steel and low carbon steel can meet the requirements of carbon steel hardness in the construction field.
- For example, in bridge construction, high carbon steel can be used to manufacture high-strength steel cables and connectors, while low carbon steel can be used to manufacture some auxiliary structural parts.
- Tool manufacturing field
- Tool manufacturing has very high requirements for the hardness of carbon steel. Due to its high carbon content, high carbon steel can reach a hardness of 65-75 HRC after heat treatment such as quenching, making it very suitable for making cutting tools.
- For example, cutting tools such as drills, taps, and reamers are usually made of high carbon steel with a carbon content of 0.90% to 1.00%, which can maintain a sharp blade during the cutting process and improve cutting efficiency and quality.
(II) How to choose carbon steel with appropriate hardness according to actual needs
- Consider the working environment
- If there is a large friction, impact or pressure in the working environment, then it is necessary to choose a carbon steel with a higher hardness. For example, in the fields of mining machinery and heavy machinery, the working environment is harsh and the parts need to withstand greater wear and impact. At this time, high carbon steel or specially treated medium carbon steel is a better choice.
- If the working environment is relatively mild and the hardness requirement is not high, then low carbon steel or ordinary carbon steel may be able to meet the needs.
- Consider requirements and expected performance
- If there are high requirements for the strength, wear resistance, toughness and other properties of the parts, then it is necessary to select carbon steel with appropriate hardness based on these requirements. For example, for parts that require high strength and high wear resistance, high carbon steel can be selected; for parts that require good plasticity and welding performance, low carbon steel or medium carbon steel can be selected.
- At the same time, it is also necessary to consider the expected service life and maintenance cost. Although carbon steel with higher hardness has excellent performance, it may be more expensive and the maintenance cost is relatively high. Therefore, when selecting carbon steel, it is necessary to comprehensively consider various factors to achieve the best cost-effectiveness.
In short, when selecting carbon steel, it is necessary to comprehensively consider factors such as working environment, requirements and expected performance, and select carbon steel with appropriate hardness to ensure the quality and performance of parts and meet the needs of actual applications. Understanding the importance of carbon steel hardness selection will help us better apply carbon steel in different fields and improve production efficiency and product quality.