Structural steel refers to steel that meets specific strength and formability grades. Formability is expressed as elongation after break in tensile test. Structural steel is generally used for load bearing and other applications where the strength of the steel is an important design criterion.
Structural steel can be subdivided into: carbon structural steel, high-quality carbon structural steel, low-alloy high-strength structural steel, alloy structural steel, spring steel, weathering structural steel, chip-free structural steel, non-quenched and tempered mechanical structural steel, etc.

4340 steel is nickel-chromium-molybdenum alloy steel. It will get high strength after the heat treat. And it has good toughness, fatigue resistance, impact toughness and etc. Its equivalent grade is JIS SNCM439 and DIN 1.6563. The properties and composition of 4340 steel round bar sheet plate as below.

It is an alloy structural grade which used by the SAE International(SAE), and it is chrome-nickel low alloy steel. 4340 steel round bar sheet plate will get high tensile strength, hardness and toughness after heat treatment. It is used widely in the transmission components of heavy chemical machinery. Different sections of each part of the casting which cause the temperature difference stress and phase transformation stress. If you can’t get their laws and characteristics, you can’t take the right rational processes. The consequence is that there is a risk of cracking the casting during the casting or heat treatment. So the control of chemical composition and the choice of the heat treatment process for the casting are really important.

1.6511 Steel Grade Equivalent Chart

4340 Steel Chemical Composition

Classification

Compared with ordinary carbon structural steel, high-quality carbon structural steel has lower content of sulfur, phosphorus and other non-metallic inclusions. According to the different carbon content and use, this kind of steel is roughly divided into three categories:
1. Less than 0.25% C is low-carbon steel, especially 08F, 08Al, etc. with carbon content less than 0.10%, which are widely used as deep-drawing parts such as automobile, manufacturing, etc. due to their good deep-drawing and weldability Tanks...etc, 20G is the main material for making ordinary boilers. In addition, low carbon steel is also widely used as carburizing steel for machinery manufacturing.
2. 0.25~0.60%C is medium carbon steel, which is mostly used in the quenched and tempered state to make parts for the machinery manufacturing industry. The amount of quenching and tempering is 22~34HRC, which can obtain comprehensive mechanical properties and is easy to cut.
3. More than 0.6% C is high carbon steel, which is mostly used to manufacture springs, gears, rolls, etc. According to the different manganese content, it can be divided into ordinary manganese content (0.25-0.8%) and higher manganese content (0.7 ~ 1.0% and 0.9 ~ 1.2%) two steel groups. Manganese can improve the hardenability of steel, strengthen ferrite, and improve the yield strength, tensile strength and wear resistance of steel. Usually, "Mn" is added after the grade of steel with high manganese content, such as 15Mn, 20Mn, to distinguish it from carbon steel with normal manganese content.

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