Tempered steel’s working conditions, performance requirements, and characteristics

1. Working Conditions of Tempered Steel

Tempered steel is primarily used for manufacturing transmission and connecting components in machinery. These parts are subjected to complex stresses during operation, including rotational, tensile, compressive, bending, and impact stresses, as well as combinations of these. In some cases, they also experience significant friction. Due to these demanding conditions, tempered steel must possess the comprehensive mechanical properties required for various machine components.

Tempered steels

2. Performance Requirements for Tempered Steel

The performance of tempered steel must align with the working conditions of the components. It is essential that tempered steel exhibits high strength, sufficient plasticity, and toughness. The microstructure of tempered steel typically consists of tempered sorbite (or a combination of tempered sorbite and tempered troostite). This microstructure is achieved through a tempering process that involves quenching followed by high-temperature tempering.

Additionally, the depth of the hardened layer must be appropriate for the component’s working conditions. For parts subjected to significant tensile stress, complete hardening is required during quenching, ensuring that the martensite content in the core of the tempered part exceeds 95%. For shaft components that primarily endure rotational and bending stresses, where internal stress is concentrated at the surface, the hardened layer depth should be between 1/2 to 1/4 of the component’s radius.

3. Characteristics of Tempered Steel

(1) Carbon Content

The carbon content in tempered steel typically ranges from 0.25% to 0.5% by mass. An optimal carbon content enhances the steel’s hardenability and hardness. However, excessive carbon content can decrease the steel’s plasticity and toughness, while insufficient carbon content may result in inadequate hardness and strength. In carbon-tempered steels, the carbon content usually falls at the upper limit of this range, whereas in alloy-tempered steels, due to the strengthening effect of alloying elements, the carbon content is generally at the lower or middle end of this range.

(2) Alloying Elements in Tempered Steel

Tempered steel often contains alloying elements such as manganese (Mn), silicon (Si), chromium (Cr), nickel (Ni), molybdenum (Mo), tungsten (W), vanadium (V), and boron (B). Among these, Mn, Si, Cr, and Ni are added in larger amounts and are considered primary alloying elements. Mo, W, V, and titanium (Ti) are added in smaller quantities and are known as secondary alloying elements, tailored to complement the primary elements.
The primary functions of these alloying elements in tempered steel include enhancing hardenability (e.g., Mn, Mo, Cr, B), refining austenite grains (alloy elements other than Mn can impede austenite grain growth to varying degrees), and improving the strength and toughness of the steel matrix. Additionally, alloying elements such as V, W, Mo, and Cr form carbides that significantly increase tempering resistance, and certain elements like Mo or W can suppress secondary temper embrittlement, thus enhancing the overall stability of the steel.

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