What are the reasons for the low toughness and plasticity of medium carbon steel after normalizing?

author: YITECH

2025-01-18

Carbon steel1 is a popular choice for manufacturing due to its high tensile strength, low cost, and ability to be fabricated into various shapes and sizes.

What are the grades of medium carbon steel?

High quality carbon structural steel

30, 35, 40, 45, 50, 55, 60 steel: The number represents the average fraction of carbon content, such as 45 steel, indicating that the carbon content is about 0.45%, its strength and hardness is high, often used to manufacture high-strength mechanical parts, vehicle bearings, tools, etc.

30Mn, 40Mn, 50Mn, 60Mn, etc. : Adding a certain amount of manganese on the basis of medium carbon steel can improve the strength, hardness and wear resistance of steel, often used in the manufacture of parts requiring higher strength and wear resistance, such as gears, shaft parts, etc.

Ordinary carbon structural steel

Q235: The yield strength is about 235MPa, according to the quality grade is divided into A, B, C, D four grades, has good plasticity and welding performance, can be used to manufacture building structural parts, Bridges, ships and so on.

Q255, Q275: the strength is higher than Q235, but the plasticity and welding performance is slightly poor, often used to manufacture mechanical parts and tools with higher strength requirements.

Carbon tool steel

T8, T9, T10, etc. : The carbon content is between 0.8% and 1.0%, the hardness and wear resistance are high, mainly used for manufacturing tools, molds, measuring tools, etc., after quenching and tempering treatment, better cutting performance and service life can be obtained.

Medium carbon steel in international standard

AISI/SAE 1030, 1035, 1040, 1045, etc.: It is the medium carbon steel grade in the standard formulated by the American Institute of Iron and Steel and the Society of Automotive Engineers, and the corresponding grades of our country are relatively close in chemical composition and performance.

JIS S35C, S40C, S45C, S50C, etc.: It is the medium carbon steel grade in the Japanese industrial standard, of which S50C is equivalent to China's No. 50 high-quality carbon structural steel.

What is the reason why medium carbon steel is easy to break after normalizing?

Low impact toughness of medium carbon steel parts

Impact toughness is the ability of a material to resist impact loads. When the impact toughness of carbon steel is low, it cannot effectively absorb the impact energy when it is subjected to sudden external impact (such as smashing action). Because the material is difficult to buffer the external force through its own deformation, the stress will be quickly concentrated in the local area, and when the stress exceeds the strength limit of the material, it is easy to break. Just like the impact toughness of glass is low, it is easy to break when impacted by external forces.

Reasons for low impact toughness of medium carbon steel:

(1). Component factors.

The carbon content of medium carbon steel is between 0.25% and 0.60%, and the higher carbon content will form more cementite in the steel. Cementite has high hardness and great brittleness, it will reduce the toughness of steel in the matrix, especially the cementite distributed along the grain boundary, which is easy to become a channel for crack propagation, making the impact toughness decrease.

(2). The influence of impurity elements.

Impurity elements such as sulfur and phosphorus in steel will be polarized at the grain boundaries. For example, sulfur will form sulfide inclusions, and phosphorus will make steel cold and brittle, which will lead to the reduction of impact toughness of medium carbon steel.

(3). Reasons for organization.

Coarse pearlite appears in medium carbon steel after normalizing. Mainly because the cooling rate is too slow. Normalizing is to heat the steel above the critical temperature and cool it in the air after holding it for a certain time. If the cooling rate in the air is not fast enough, the austenite will be converted into pearlite in the relatively slow cooling process, and it is easy to form coarse pieces of pearlite. This coarse pearlite structure will cause the strength and toughness of the material to decrease. Because the lamellae spacing of coarse pearlite is large, the dislocation movement is relatively easy, and the material is prone to deformation and fracture when subjected to external forces. If this situation is to be improved, the cooling speed can be appropriately accelerated, but care should be taken to avoid quenching tissue such as martensite due to excessive cooling, otherwise it will lead to too high hardness and poor toughness of the material.

The appropriate value for impact toughness of medium carbon steel varies according to the specific application scenario.

In general mechanical structures, the impact toughness value (Charpy V-notch impact absorption energy) is more appropriate at about 27-50J. For example, when manufacturing ordinary shaft parts, the impact toughness of this range can ensure that the parts will not easily break when subjected to a certain impact load (such as accidental overload of equipment, minor collision).

If it is used to manufacture parts that bear high impact loads, such as the breaking hammer connecting rod in mining machinery, the impact toughness requirements are higher, and may need to reach more than 50J, or even higher values, in order to ensure that the parts work normally in frequent and high-strength impact environments.

DongGuan YiTai Electronic Technologies Co.,Ltd

DongGuan YiTai Electronic Technologies Co.,Ltd is a manufacturing service provider located in Dongguan, China.

YiTai specialized in CNC turning machining and sheet metal fabrication mainly. As a member of Hung Mou Group, we focus on the overseas marketing development. And based on our parent company’s manufacturing capability and resources, YiTai also expended machining services such as die casting, injection molding, aluminum profile extrusion, 3D printing, which are committed to providing customers with one-stop purchasing services and experience.

CNC MACHINING , CNC MILLING , CNC TURNING , SHEETMETAL , FASTENER , OTHERS