Steel Heat Treatment Processes: Annealing, Quenching & Tempering, Normalizing Guide

What Is Heat Treatment and Why Is It Necessary?

Heat treatment is the process of altering the mechanical properties of steel by heating it to specific temperatures, holding it at those temperatures, and cooling it in a controlled manner. Proper heat treatment allows you to achieve entirely different performance characteristics from the same steel grade. In this guide, we explain the most commonly applied heat treatment processes in industry in detail.

Annealing

Annealing is a heat treatment applied to soften steel and relieve internal stresses.

Soft Annealing

• Temperature: 680-720°C (below Ac1)
• Duration: 2-6 hours (depending on cross-section thickness)
• Cooling: Slow furnace cooling (~20°C/hour)
• Purpose: Maximum softness, improved machinability
• Result: Spheroidized pearlite structure, minimum hardness

Full Annealing

• Temperature: 820-900°C (above Ac3, depending on steel grade)
• Duration: 1-2 hours/25 mm cross-section
• Cooling: Slow furnace cooling
• Purpose: Homogeneous grain structure, uniform microstructure
• Result: Fine lamellar pearlite + ferrite

Stress Relief Annealing

• Temperature: 550-650°C
• Duration: 1 hour/25 mm cross-section
• Cooling: In furnace or still air
• Purpose: Relieving internal stresses after welding, machining, or cold forming
• Result: Dimensional stability, reduced cracking risk

Normalizing

Normalizing is the process of heating steel 30-50°C above the Ac3 temperature and cooling it in still air.

Parameters

• Temperature: 850-920°C (varies by steel grade)
• Duration: Until homogeneous austenitization is achieved
• Cooling: In still air

When Is It Applied?

• Correcting grain structure after rolling or forging
• Standardizing mechanical properties
• Providing homogeneous structure before welding
• Refining coarsened grain structure after hot working

Results

• Fine and homogeneous grain structure
• Reproducible mechanical properties
• Higher strength compared to annealing
• Improved machinability and weldability

Quenching & Tempering

Quenching & tempering is the heat treatment process in which a combination of high strength and toughness is achieved through hardening (quenching) followed by tempering.

Quenching

• Temperature: 830-880°C (depending on steel grade)
• Medium: Water, oil, or polymer solution
• Purpose: Martensite formation, maximum hardness

Tempering

• Low tempering (150-250°C): High hardness is maintained, stress is reduced
• Medium tempering (350-500°C): Generally avoided (temper embrittlement zone)
• High tempering (500-680°C): Optimum strength-toughness balance

Typical Quenching & Tempering Parameters

Steel Grade	Austenitization	Quenching Medium	Tempering Temperature
C45	830-860°C	Water/Oil	550-650°C
42CrMo4	840-880°C	Oil	540-680°C
34CrNiMo6	830-860°C	Oil	540-660°C

For conversions between hardness values, you can use our Hardness Converter tool.

Case Hardening (Carburizing)

Case hardening is the process of creating a hard surface layer by applying carbon diffusion to the surface of low-carbon steel.

Process Steps

1. Carburizing: 880-950°C, in carbon-rich atmosphere (4-12 hours)
2. Hardening: Direct quenching or reheating and quenching
3. Tempering: 150-200°C, low tempering

Suitable Steel Grades

• 16MnCr5: Automotive gears
• 20MnCr5: Heavy-duty gears
• 18CrNiMo7-6: Wind turbine gears

Results

• Surface hardness: 58-63 HRC
• Core hardness: 30-45 HRC
• Case depth: 0.5-2.0 mm (typical)

Selecting the Right Heat Treatment

Requirement	Recommended Process
Softening / Machinability	Soft Annealing
Homogeneous structure	Normalizing
High strength + toughness	Quenching & Tempering (Q+T)
Hard surface + tough core	Case Hardening
Stress relief	Stress Relief Annealing

Aktif Çelik Heat Treatment Support

At Aktif Çelik, we provide technical support to our customers before and after heat treatment. We supply steel bars in quenched & tempered, normalized, and soft annealed conditions. Our technical team can assist you in determining the appropriate steel grade and heat treatment combination for your application. You can examine the composition of your steel grades in detail with our Chemical Composition tool.