Hardenability and hardenability of steel
- Hardenability
Hardenability refers to the inherent property of a steel grade to obtain the depth of the hardened layer (martensite layer) during quenching under specified conditions.
Whether a steel part can be fully hardened is related to the critical cooling rate (vk) of the steel during quenching.
Measurement Indicator: Expressed by the effective hardened layer depth achievable when a standard test piece is quenched under certain conditions.
Depth of Hardened Layer: Refers to the distance from the surface of a steel part to the location where the martensitic structure accounts for 50% internally. The greater the depth of the hardened layer, the higher the hardenability; when the depth of the hardened layer reaches the core, the workpiece is fully hardened.
2. Influence of Hardenability on the Mechanical Properties of Steel
Hardenability has a significant impact on the mechanical properties of steel. If a workpiece is fully hardened, its surface properties are uniform and consistent, allowing the full potential of the steel's mechanical properties to be exerted. If not fully hardened, there will be differences in surface properties; especially after high-temperature tempering, the toughness of the core will be lower than that of the surface layer.
Under the same austenitizing conditions, the hardenability of the same type of steel is identical.
For the depth of the hardened layer: water quenching > oil quenching; small workpieces > large workpieces.
3. Factors Affecting Hardenability
Hardenability is an inherent property of steel, independent of external conditions of the steel (such as shape, size, surface area, and cooling medium), but closely related to its critical cooling rate. The smaller the critical cooling rate, the higher the hardenability of the steel.
All factors that affect the critical cooling rate (or the position of the C-curve) — such as chemical composition, quenching temperature, and holding time — will influence hardenability.
1）Chemical Composition
- Carbon Content: Among hypoeutectoid steel, eutectoid steel, and hypereutectoid steel, eutectoid steel has the smallest critical cooling rate and the highest hardenability among carbon steels. The hardenability of hypoeutectoid steel increases with the increase of carbon content. Within the normal quenching heating temperature range, the hardenability of hypereutectoid steel decreases as the carbon content increases.
- Alloying Elements: All alloying elements except cobalt shift the C-curve to the right, reduce the critical cooling rate, and improve hardenability.
2）Quenching Temperature and Holding Time
Increasing the heating temperature and extending the holding time can appropriately improve the hardenability of steel. However, this method will cause grain coarsening, so it is generally not adopted.
4. Determination of Hardenability: End Quench Test (Jominy End Quench Test)
5. Applications of Hardenability
1）Estimation of the Depth of the Hardened Layer
During part design, the known hardenability curve can be used to estimate the effective hardened layer depth of the part.
2）Material Selection Based on the Depth of the Hardened Layer
The effective hardened layer depth has a significant impact on the mechanical properties of the workpiece.
When the workpiece is fully hardened, a uniformly distributed structure can be obtained along the entire cross-section after tempering, and its mechanical properties are also consistent.
When not fully hardened, the mechanical properties of the workpiece's core are lower than those of the surface hardened layer.
When the stress of the workpiece is uniformly distributed along the cross-section and the mechanical properties on the cross-section are consistent, steel with high hardenability should be selected.
For parts subjected to bending or torsional loads (such as shafts), the surface stress is the highest while the core stress is very low, so steel with poor hardenability should be selected.
Under the same austenitizing conditions, the hardenability of the same type of steel is identical.
*Hardness after Quenching (Hardenability for Maximum Hardness)
Under normal quenching conditions, it refers to the maximum hardness achievable by obtaining a martensitic structure. Its main influencing factor depends on the carbon content in martensite, and has little to do with alloying elements. The higher the carbon content, the higher the hardness after quenching.
For example, low-carbon alloy steel has quite good hardenability but low hardness after quenching. Another example is high-carbon tool steel, which has poor hardenability but high hardness after quenching.
Typically, medium-carbon alloy steel 40Cr and carbon steel 45 are used for comparison. The former contains the alloying element chromium, so its hardenability is higher than that of the latter; however, its carbon content is lower than that of the latter, resulting in slightly lower hardness after quenching.
Note: Steel with high hardenability does not necessarily have high hardness after quenching, and vice versa.
Your message must be between 20-3,000 characters!