Steel Man: Quenched and Tempered Steel Performance Characteristics and Applications (Chapter One)

Quenched and Tempered Steel generally refers to medium carbon steel with a carbon content of 0.3-0.6%. The metallography structure after tempering is tempered sorbite. Tempered steel is used extensively for structural parts on various machines. The most widely used tempered steels are chromium tempered steels (such as 40Cr, 40CrSi), chromium manganese tempered steels (such as 40CrMn), chromium nickel tempered steels (such as 40CrNiMo, 37CrNi3A), and boron-containing tempered steels.

The heat treatment process is quenched into martensite after heating at a certain temperature above the critical point, and tempered at 500℃-650℃. The microstructure after heat treatment is tempered sorbite. The structure has a good combination of strength, plasticity and toughness.

The quality requirements of tempered steel, in addition to the general metallurgical aspects of low and high microstructure requirements, mainly for the mechanical properties of steel and cold brittleness transition temperature, fracture toughness and fatigue resistance closely related to working reliability and life. Under specific conditions, it is also required to have wear resistance, corrosion resistance and certain heat resistance. Because the tempered steel finally adopts high temperature tempering, the stress in the steel can be completely eliminated, and the hydrogen brittleness of the steel is small, the notch sensitivity is low, and the brittleness resistance is large, but there is also a unique high temperature tempering brittleness.

Most tempered steels are medium carbon alloy structures with yield strengths (σ0.2) in the 490-1200Mpa range. The tempered steel with welding performance as the outstanding requirement is low-carbon alloy structural steel, the yield strength (σ0.2) is generally 490-800Mpa, and has high plasticity and toughness. A small number of precipitation-hardened tempered steels with yield strength (σ0.2) can reach more than 1400Mpa, belonging to high strength and ultra-high strength tempered steels.

Commonly used Quenched and Tempered Steel is divided into 4 categories according to hardenability and strength:

① low hardenability steel mixing; ② medium hardenability tempered steel; ③ Tempered steel with high hardenability; ④ Tempered steel with high hardenability.

1. Effect of alloying elements on mechanical properties

By adjusting the content of alloying elements that increase the hardenability of steel, the same hardenability can be obtained and the same tensile strength and yield strength can be obtained. Therefore, when selecting alloying elements, priority should be given to elements that increase the hardenability and have a significant effect on the price, such as boron, manganese, chromium, etc. However, the tempering temperature used for steels with different alloying elements to be tempered to the same hardness is different, that is, the tempering resistance of various steels is different.

When the steel with the same hardenability is tempered to the same hardness, the tensile strength and yield strength are basically the same, but the brittleness failure tendency is very different, and the low temperature impact test is particularly obvious. The relationship between hardness and fatigue limit of tempered steel with different composition is different. When the hardness is below 35HRC, the fatigue limit and hardness are in a linear relationship, and the fluctuation range of fatigue limit is 130MPa. When the hardness exceeds 35HRC, the fluctuation range of fatigue limit becomes wider. For example, when the hardness is 55HRC, the fluctuation range of the fatigue limit is 380MPa.

2. Determination of hardness of tempered parts

The hardness of tempered parts can reflect the yield strength and tensile strength of the parts when the quenching conditions are the same.

The determination of hardness of tempered parts must take into account the requirements of the manufacturing process and the load conditions when used. From the manufacturing process, it is hoped that the parts are conditioned in the blank state, and then the cutting and assembly are carried out. In this way, the deformation and decarbonization caused by the heat treatment of the parts are eliminated in the later cutting process. However, the hardness of the parts using this manufacturing process cannot be too high, generally not more than 300HB, and the individual does not exceed 350HB, otherwise it is unfavorable to the cutting process. Parts that require higher hardness (such as some auto shafts requiring hardness of 341 ~ 415HB) can only be cut first, and then tempered, when the parts are heated, decarbonization and deformation should be prevented, and sometimes the straightening process should be increased after heat treatment. For parts produced in small batches or single pieces, the hardness allowed by cutting can be appropriately increased.

Determining the hardness of tempered parts must also take into account the characteristics of production, small batches of single production products, different parts can be selected for different hardness, mass production factories hope that the hardness range of most parts is consistent or fixed in several hardness ranges, which is very convenient for the organization of heat treatment production.

From the point of view of the use of parts, it is necessary to pay attention to the working conditions of parts and the shape of parts when determining the hardness of tempered parts. Generally speaking, the hardness value is high, the tensile strength, the yield strength and the fatigue strength of the smooth sample are high, but the plastic index is reduced, the brittleness failure tendency and the sensitivity of stress concentration are increased, therefore, when there is a notch on the part that plays the role of stress concentration (spline, groove or cross section change), in order to make the stress distribution uniform and reduce the stress concentration phenomenon. At this time, lower hardness can obtain higher fatigue properties.