Hardenability, Quenching Media, and Tempering of Steel
Hardenability is a fundamental property of steel that describes its ability to develop hardness to a specified depth when quenched from the austenitizing temperature. It should not be confused with hardness or maximum hardness. Hardness is a measure of resistance to indentation (Brinell, Rockwell, Vickers, etc.), whereas maximum attainable hardness depends almost entirely on carbon content.
Maximum hardness is achieved only when the cooling rate during quenching is sufficiently rapid to produce a fully martensitic microstructure. While surface hardness is influenced by both carbon content and cooling rate, the depth over which hardness is maintained is governed by the steel’s hardenability.
Hardenability is primarily controlled by alloying elements, but it is also affected by austenite grain size, austenitizing temperature and time, and the prior microstructure. The level of hardenability required for a component depends on its size, geometry, and the magnitude and nature of service stresses.
For highly stressed components, the best combination of strength and toughness is typically obtained by full martensitic transformation followed by appropriate tempering. Conversely, components subjected mainly to surface wear or impact loading may perform better with shallow hardening and a relatively soft, tough core.
As section thickness increases, steels with higher hardenability are required to achieve uniform properties through the cross-section. Good engineering practice is to select the most economical steel grade that consistently meets performance requirements, avoiding excessive alloy content that can increase cost and susceptibility to quench cracking.
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Quenching Media
The selection of an appropriate quenching medium is a critical factor in achieving the desired hardness profile and hardenability. Quench severity can be adjusted by choosing the quenchant type, controlling bath agitation, and using additives that enhance heat extraction.
More severe quenchants permit the use of steels with lower hardenability but increase the risk of distortion and quench cracking. Water quenching is economical and aggressive, while oil quenching provides slower, more controlled cooling and reduced thermal stresses.
Salt baths and synthetic polymer quenchants are also widely used, but they typically require steels with higher alloy content. A fundamental guideline is that steel hardenability should not exceed what is necessary for the severity of the selected quenchant.
Carbon content should be limited to the minimum required to meet strength and hardness specifications, as higher carbon levels significantly increase crack sensitivity during quenching. Agitation of the quench bath also plays a major role—greater agitation improves heat transfer and increases quenching effectiveness.
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Hardenability Test Methods (Jominy End-Quench Test)
The most widely used method for evaluating steel hardenability is the Jominy end-quench test, standardized in SAE J406 and ASTM A255. In this test, a normalized cylindrical specimen is heated uniformly to the austenitizing temperature and quenched at one end with a controlled water jet.
After cooling, flat surfaces are ground on the specimen and Rockwell C hardness measurements are taken at fixed intervals from the quenched end. These values are plotted to form the Jominy curve, which characterizes the steel’s hardenability.
SAE and AISI publish standard “H-bands” that define acceptable hardness ranges for specific steel grades. The suffix H in an AISI/SAE designation indicates that the steel has been produced to controlled hardenability limits.
Cooling rates at various locations along the Jominy specimen correspond closely to those found at different depths in round bars of varying diameters. This relationship allows engineers to predict hardness distributions in real components based on Jominy test data.
Tempering of Steel
As-quenched steels are in a highly stressed and brittle condition and are seldom used without tempering. Tempering improves toughness and ductility by relieving residual stresses and modifying the martensitic microstructure.
Although tempering reduces hardness and tensile strength, the resulting increase in toughness is far more critical for most engineering applications. Alloy steels generally require higher tempering temperatures than carbon steels to achieve equivalent hardness levels.
Typical tempering temperatures range from 300°F to 1200°F. For many steels, the temperature range of 500–700°F is avoided due to blue brittleness, which leads to reduced impact strength.
Martensitic stainless steels should not be tempered in the range of 800–1100°F because this can result in poor toughness and reduced corrosion resistance. Maximum toughness is usually achieved at higher tempering temperatures.
Tempering should be performed as soon as possible after quenching, as any delay significantly increases the risk of cracking due to the high residual stresses present in the as-quenched condition.
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Source: Google Books
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