Basics of Quench Hardening - discussion of issues

Steel hardening is a heat treatment process consisting of two stages:

1. Heating the steel, causing ferrite and cementite to transform into austenite.
2. Rapid cooling, which converts austenite into martensite or bainite.

Austenite is a solid solution of carbon in gamma iron, present in most steels only at high temperatures. Martensite is the hardest yet most brittle form of steel, characterized by needle-like structures intersecting at 60° angles. When martensite lacks these characteristic needles, it is referred to as hardenite. Bainite resembles tempered martensite, offering high strength with reduced brittleness. Microscopic images of austenite and martensite are shown in Illustration No. 1.

Impact on mechanical properties: Hardened steel becomes extremely hard and strong but also more brittle and prone to fracture. To enhance its ductility, hardened steel undergoes tempering.

The material prepared for heat treatment is usually in a softened state (annealed) and has typically undergone cold plastic deformation.

Illustration 1 - Austenite at 100x magnification and martensite at 1000x magnification. Source: Metallographic Atlas of Structures.

Heat treatment types involving quench hardening

"Quench Hardening" is a broad term. Depending on the heating and cooling methods used, various types of quench hardening can be distinguished. Sometimes quenching and tempering are combined into a single heat treatment.

Based on the resulting microstructure, the following types exist:

• Martensitic transformation hardening
• Bainitic transformation hardening

Based on the heating method and austenization depth, quench hardening can be classified as:

• Hardening with through heating – The entire steel piece is heated to achieve a fully austenitic structure throughout its cross-section. The depth of martensitic or bainitic transformation depends on the cross-section size and the steel's physical properties. Typically, not the entire component undergoes hardening, meaning austenite does not transform into martensite across the entire cross-section. Thus, it is not precise to describe this as "through hardening".
• Surface hardening – Only the outer layer of the material is hardened, while the core retains a ferritic structure.

Further, volume hardening methods are categorized based on the cooling process:

• Customary quenching
• Marquenching
• Martempering
• Austempering

For surface hardening, cooling speed is less critical, and objects are typically quenched in water immediately after rapid heating. Surface hardening methods vary based on the heating technique:

• Flame hardening – Heating with gas burners.
• Induction hardening – Heating using electricity of high-frequency.
• Alloy quench, fused salt quench – Heating in a molten salt or lead bath.

Surface hardening differs significantly from volume hardening, requiring different equipment and techniques. In surface hardening, heating is extremely intense, whereas volume hardening is a slower, more uniform process—comparable to slow cooking versus searing on a hot pan. The relationships between these hardening methods are outlined in Table No. 1.

Depth of Austenitization	Resulting Structure	Cooling Method	Heat Transfer Method
Volume Hardening / Thorough Heating	Martensitic transformation hardening	Conventional Hardening
		Marquenching
	Bainitic transformation hardening	Martempering
		Austempering
Surface Hardening / Surface Heating	Martensitic transformation hardening		Flame Hardening
			Induction Hardening
			Alloy quench

Table No. 1 - Main Types of Hardening

Standard Quench Hardening with Through Heating

Heating temperatures are determined by steel mills and research institutions and are included in industry standards. Strict adherence to these standards is especially important for alloy steels. The following list provides some notable examples:

• Structural carbon steel C45: 820 - 860℃, cooling: water, oil
• Heat-treatable steel 34CrNiMo6 / 1.6582: 830 - 860℃, water, oil
• Carburizing steel 18CrNiMo7-6 / 1.6587: after initial carburizing: 840 - 870℃, after secondary carburizing: 800 - 830℃, water, oil
• Spring carbon steel C67S: 830 - 850℃, oil
• Spring alloy steel 51CrV4: 820 - 870℃, oil
• Boiler steel 21CrMoV5-7: 930 - 950℃ in water, 940 - 960℃ in oil
• Hot-work tool steel WWV, X30WCRV9-3, 1.2581: heat treatment with hardening at 1120 - 1160℃ followed by cooling and tempering in a salt bath at 450 - 550℃
• Cold-work tool steel NZ3: 850 - 880℃, oil

For carbon steels, the required heating temperature can be determined based solely on carbon content. Hypoeutectoid steels (<0.77% C) should be heated 30-50°C above the austenitic transformation temperature A₃. This ensures the complete dissolution of ferrite and cementite into austenite, maximizing martensite formation during cooling. Hypereutectoid steels (>0.77% C) should be heated to 760-780°C.

Heating time should be minimized while ensuring the desired material properties. This is important for both cost efficiency and final product quality. However, excessively rapid heating can cause thermal stress and cracking. For complex-shaped components with large surface areas, stepwise heating with intermediate soaking at multiple temperatures is recommended. This is also necessary for alloy steels, as alloying elements reduce thermal conductivity. For example, tool steels are often heated in two or three stages.

Example heating and soaking times for carbon steel when through heating:

• Cross-section 25 mm:
  • Gas-fired chamber furnaces - Heating: 20 min, Soaking: 5 min
  • Salt bath furnaces - Heating: 7 min, Soaking: 3 min
• Cross-section 50 mm:
  • GCF - H: 40 min, S: 10 min
  • SBF - H: 17 min, S: 8 min
• Cross-section 75 mm:
  • GCF - H: 60 min, S: 15 min
  • SBF - H: 24 min, S: 12 min
• Cross-section 100 mm:
  • GCF - H: 80 min, S: 20 min
  • SBF - H: 33 min, S: 17 min
• Cross-section 125 mm:
  • GCF - H: 100 min, S: 25 min
  • SBF - H: 40 min, S: 20 min
• Cross-section 150 mm:
  • GCF - H: 120 min, S: 30 min
  • SBF - H: 50 min, S: 25 min
• Cross-section 175 mm:
  • GCF - H: 140 min, S: 35 min
  • SBF - H: 55 min, S: 30 min
• Cross-section 200 mm:
  • GCF - H: 160 min, S: 40 min
  • SBF - H: 65 min, S: 35 min

For alloy steels, heating times should be increased by 25-40%.

Example heating and soaking times for taps, reamers, drills, round broaches, and solid tools during through heating in salt baths:

• First preheating stage to 550-600°C: t = bD
• Second preheating stage to 800-850°C: t = cD
• Final heating time to hardening temperature: t = aD

Where:

• t - heating time
• D - cutting part diameter of the tool in mm
• a: 0.09-0.12 for high-speed steel, 0.17-0.18 for high-alloy chromium steel, 0.15-0.20 for alloy steel
• b: 0.35-0.50 for high-speed steel, 0.30-0.40 for high-alloy chromium steel, 0.30-0.40 for alloy steel
• c: 0.30-0.35 for high-speed steel, 0.30-0.35 for high-alloy chromium steel

All data should be considered as guidelines. The optimal heating duration and temperature are determined experimentally through microscopic analysis. The presence of ferrite in carbon steel or excess carbides in alloy steel suggests either insufficient heating temperature or inadequate soaking time.

Customary Quenching

Customary Quenching (standard martensitic transformation quenching with continuous cooling) involves cooling at a rate higher than the critical cooling rate (CCR) without interruptions, down to the M_s temperature and sometimes even the M_f temperature. The next stage in the heat treatment process is tempering.

Cooling medium: In practice, carbon steels are quenched in water, while alloy steels can be cooled in oil or, in some cases, even in air.

Target microstructure: The resulting structure consists of martensite with retained austenite, as well as elements that do not undergo transformation, such as carbides and non-metallic inclusions.

Mechanical properties: Conventional hardening results in very high but non-uniform hardness, high strength, very low ductility, and thus high brittleness. Tempering is required to improve toughness and reduce brittleness.

Marquenching

Marquenching (also known as step complete quenching, martensitic transformation graduated quenching or hot quenching) is used for small, delicate objects as well as those with complex and varying cross-sections. The cooling process involves three phases:

• Cooling in a salt bath or oil at a temperature just above the martensitic transformation point.
• Soaking in the bath until the temperature is uniform throughout the cross-section. The soaking time is kept short to prevent bainitic transformation.
• Cooling to ambient temperature in air.

Target structure: Martensite, but with lower internal stresses and reduced distortion, as compared to customary quenching.

Martempering

Martempering (also called as step imcomplete quenching or bainitic transformation graduated quenching) involves cooling at a rate below the critical cooling rate. In practise, the process is similar to marquenching, but the soaking time is extended to allow bainitic transformation to begin.

Target structure: A combination of bainite, martensite, and retained austenite, as illustrated in Figure 2.

Mechanical properties: Greater ductility and impact resistance compared to typical quenched and tempered steel.

Austempering

Austempering  (also known as bainitic hardening with isothermal transformation or isothermal hardening) consists of three cooling phases:

• Cooling of supercooled austenite to a temperature below the pearlitic transformation range.
• Isothermal soaking in a cooling bath at 250-400°C until bainitic transformation is fully completed.
• Cooling to ambient temperature in air.

Target structure: Bainite. This process combines hardening and tempering in a single step. Additional tempering is not required.

Mechanical properties: Increased hardness and strength with significantly reduced internal stresses.

Figure 2 - Martensite with lower bainite. Source: Metallographic Atlas of Structures.

Cooling in Water, Oil, and Other Coolants

Quenching intensity is influenced by various factors, including the cooling medium (viscosity, temperature, thermal conductivity, specific heat, boiling point, and latent heat of vaporization), the movement of the object relative to the medium, surface condition, mass, and surface area.

Cooling media are classified into four groups, ranked from highest to lowest cooling intensity:

• Water and aqueous solutions
• Oils and fats
• Molten salts or metals
• Compressed air

Relative cooling rates in different media between 720-550°C (when water = 1):

• Water at 18°C: 1
• 10% NaOH: 2.05
• 10% NaCl: 1.96
• 30% Sn / 70% Cd alloy at 180°C: 0.77
• Mineral oil: 0.14 - 0.22
• Air: 0.028

Relative cooling rates in different media around 200°C (when water = 1):

• Water at 18°C: 1
• 10% NaOH: 1.36
• 10% NaCl: 0.98
• Mineral oil: 0.022
• Air: 0.007

Object movement relative to the cooling medium plays a crucial role and can significantly enhance quenching intensity.

Approximate relative hardening intensities in water based on movement:

• No movement: 1
• Slow movement: 1.0 - 1.3
• Moderate movement: 1.4 - 1.5
• Strong movement: 1.6 - 2.0
• Rapid movement: 4.0

Surface condition also affects cooling rates. Oxide layers, which have poor thermal conductivity, slow down cooling—particularly in oil. Objects heated in a salt bath tend to harden more effectively and uniformly than those heated in a chamber furnace.

Mass and surface area: Compact objects cool more slowly, while thin sections, edges, and protrusions cool faster, potentially causing distortion or cracks. The shape of a component is a key factor in selecting the appropriate steel grade for hardening.

Surface Hardening

In surface hardening, even a significant exceedance of the austenitization temperature may not have a detrimental effect on the hardening outcome, as the heating time is reduced to a few seconds. Various surface hardening techniques exist. Based on the heating device used, the following types are distinguished:

• Flame Hardening - heating using gas burners.
• Induction Hardening - heating with high-frequency currents.
• Bath Hardening / Alloy Hardening / Fused Salt Quenching - heating in a salt or lead bath.

Examples of surface hardening methods include simultaneous, continuous, and bath hardening.

Simultaneous Hardening Method involves heating the entire surface of the object simultaneously and then cooling it simultaneously upon reaching the required temperature. Heating coils with a profile matching the object can be used. This method is commonly used for hardening tools. A variant is the rotational flame hardening method, where the surface of a rapidly rotating object (75-100 rpm) is heated using a gas burner and then cooled simultaneously with a spray from all sides upon reaching the required temperature. This method is used for small-diameter objects, crankshafts, shaft journals, axles, and gears.

Continuous Hardening Method involves gradually heating the surface of a slowly moving object using a gas flame or induction coil, followed by a water spray for rapid cooling. This method is used for cooling very long elements such as rails, long shafts, and pipes.

Bath Hardening Method involves slowly heating the steel thoroughly in a molten salt bath to a temperature below the A₁ transformation, then rapidly heating the surface above A₃ by immersion in a salt or lead bath heated significantly above the A₃ temperature, holding it in the bath briefly, and then rapidly cooling it in a quenching medium.

An example of simultaneous induction hardening is shown in Illustration No. 3.

Illustration No. 3 - Simultaneous Induction Hardening. Source: Heat Treatment of Steel.

Advantages of Surface Hardening

Surface hardening is fast and repeatable. A major advantage is that it can be applied to already heat-treated components, as surface treatment does not damage the core's heat treatment. This allows the use of cheaper carbon steel instead of more expensive alloy steel.

Hardening Defects

Incomplete Hardening - If an object is heated to a temperature that is too low or is not held at the temperature long enough, ferrite will not fully dissolve into austenite, resulting in undissolved ferrite grains in a martensitic matrix after quenching. Steel with such a structure has reduced hardness and poorer mechanical properties.

Overheating of Steel - Proper hardening produces fine-grained martensite, which is less brittle. Overheating, meaning holding the steel at too high a temperature for too long or cooling it too aggressively, results in coarse-grained martensite with clearly defined needles in the austenitic background. Overheated steel is excessively brittle. A comparison of fine-grained and coarse-grained martensite is shown in Illustration No. 4. Normalizing Annealing removes steel overheating.

Illustration No. 4 - Fine-Needle and Coarse-Needle Martensite. Source: Metallurgy and Heat Treatment.

Burning is an extreme case of overheating. It occurs when internal oxidation at the grain boundaries takes place. Burning is an irreversible defect.

Excessively Fast Cooling results from using an overly aggressive quenching medium or improper immersion in the bath, leading to cracks and distortions.

Excessively Slow Cooling, on the other hand, leads to complete or partial failure of the quenching process.

Special Types of Hardening

Subzero Hardening is distinguished by an additional step after cooling the object to ambient temperature (sometimes after tempering), where the object is deep-frozen in liquid oxygen, a mixture of solid carbon dioxide and alcohol, or special freezers. The purpose of deep freezing is to increase hardness and other properties by further transforming retained austenite into martensite.

References

Mikołaj Scelina, Atlas Metalograficzny struktur, Wydawnictwa naukowo-techniczne, 1964

Leszek Adam Dobrzański, Metaloznawstwo i obróbka cieplna, Wydawnictwa szkolne i pedagogiczne, 1986

Kornel Wesołowski, Metaloznawstwo i obróbka cieplna, Państwowe wydawnictwa szkolnictwa zawodowego

Paweł Kosieradzki, Obróbka cieplna stali, Państwowe wydawnictwa techniczne, 1954

Marek Blicharski Inżynieria materiałowa STAL, Wydawnictwo WNT, ISBN 978-83-01-18955-6