Normalizing vs. Annealing: Key Differences in HeatTreatment and When to Use Each

Table of Contents

• Introduction: Normalizing and Annealing – The “Twin Technologies” of Heat Treatment
• What are Normalizing and Annealing? Definitions and Heat
• Common Logic of Heat Treatment: Regulating Structure and Properties Through “Heating-Holding-Cooling”
• Annealing: Slow Cooling Creates “Soft Properties”
• Normalizing: Air Cooling Shapes “Balanced Properties”
• Normalizing vs. Annealing: Process Parameters and Cooling Methods
• Heating Parameters: Precise Control of Temperature and Holding Time
• Cooling Method: The “Key Step” Determining Performance
• Equipment and Cost: Trade-off Between Efficiency and Investment
• Core Performance Differences: Chain Reactions from Structure to Application
• Differences in Microstructure
• Differences in Mechanical Properties
• Scenario-Based Selection: Application Scope and Decision Logic of Normalizing and Annealing
• Core Scenarios for Prioritizing Annealing
• Core Scenarios for Prioritizing Normalizing
• Core Logic for Selection Decision: Four-Step Judgment Method
• Typical Application Cases: Practical Comparison Between Normalizing and Annealing
• Case 1: Heat Treatment Selection for Automobile Gearbox Gears
• Case 2: Heat Treatment Selection for High-Strength Bolts for Construction
• Case 3: Heat Treatment Selection for Large Machine Tool Beds
• Conclusion: Precise Selection is the “Core Competitiveness” of Heat Treatment

I. Introduction: Normalizing and Annealing – The “Twin Technologies” of Heat Treatment

In metal processing, heat treatment shapes material performance, and normalizing and annealing are its most widely used basic technologies.

What’s the real difference between normalizing and annealing, both using “heating-holding-cooling”?

This article clarifies their core differences from principles to applications, with international standards and cases, to guide precise process selection.

II. What are Normalizing and Annealing? Definitions and Heat

1.Common Logic of Heat Treatment: Regulating Structure and Properties Through “Heating-Holding-Cooling”

Normalizing and annealing have differences, but they follow the same logic of heat treatment.

They precisely control three key factors — heating temperature, holding time and cooling rate — to change the internal microstructure of metals and adjust material properties.

Their core goals fall into three categories:

• First, refine the grains. Get rid of structural defects from casting and forging.
• Second, remove internal stress. Stop the workpiece from deforming or cracking during processing or use. 
• Third, improve processing performance. Lay a good foundation for later processes like cutting, stamping and quenching. 

2. Annealing: Slow Cooling Creates “Soft Properties”

Annealing’s key feature is “slow cooling”. Originating in 19th-century steelmaking, it now has a full international standard system. ISO 14558 defines it as a metal heat treatment process.

Steps: Heat metal above its critical point (Ac3 for hypoeutectoid steel, Ac1 for hypereutectoid steel); hold at that temperature to homogenize structure; cool slowly in the furnace (≤50℃/h).

Annealing works by letting grains grow fully. Heating turns coarse grains to uniform austenite; holding enables full carbon diffusion and eliminates segregation; slow cooling transforms austenite to ferrite and pearlite, with uniform carbide precipitation and gradual stress release.

This process yields low hardness and high plasticity. Per ASTM A913, structural steel annealing achieves over 80% residual stress elimination and 30%–40% hardness reduction.

3. Normalizing: Air Cooling Shapes “Balanced Properties”

Normalizing aims to boost production efficiency. It improves annealing’s cooling method and delivers unique performance benefits.

ISO 14557 Specification for Normalizing of Steel defines it clearly: Heat metal to a higher critical temperature (Ac3 + 50–100℃ for hypoeutectoid steel, Accm for hypereutectoid steel), hold at that temperature for a set time, then cool naturally in still air at 100–300℃/h.

Fast air cooling makes normalizing special. Higher heating temperatures fully dissolve carbides into austenite. Rapid air cooling stops austenite from forming coarse grains, turning it into fine sorbite or fine pearlite instead.

This fine structure gives normalized workpieces balanced properties: low hardness like annealed parts, plus higher strength and toughness.

ASTM A387 data for alloy steel plates shows normalized steel has 5–10% higher tensile strength than same-material annealed steel. Its production cycle is nearly halved.

III. Normalizing vs. Annealing: Process Parameters and Cooling Methods

1.Heating Parameters: Precise Control of Temperature and Holding Time

Heating parameters are the first big difference between normalizing and annealing.They set the base for how the metal’s structure changes later.

(1)Temperature differences

Normalizing always uses higher temperatures than annealing.

• For hypoeutectoid steel (like 45# steel):
  • Annealing temperature: Ac3 + 30-50℃ (810-830℃)
  • Normalizing temperature: Ac3 + 50-100℃ (850-880℃)
• For hypereutectoid steel (like T12 steel):
  • Annealing uses Ac1 + 30-50℃. This keeps austenite grains from getting too big.
  • Normalizing uses temperatures above Accm. This makes sure all network cementite dissolves completely.

(2)Holding time differences

Holding time follows a simple rule: thick or complex parts need longer times; thin or simple parts need shorter times.

Annealing time is usually 1.5 to 2 times longer than normalizing time.

• Germany’s DIN 1702 Standard gives clear guidelines:
  • Annealing: 1-3 hours for every 100mm of thickness. It works well for high-alloy steel or large castings.
  • Normalizing: 0.5-1.5 hours for every 100mm of thickness. It only needs enough time to refine grains.
• Example: a 45# steel round bar with 100mm diameter
  • Annealing needs 2-3 hours of holding time.
  • Normalizing only needs about 1 hour.
  • This cuts down production time a lot.

2.Cooling Method: The “Key Step” Determining Performance

Heating parameters are the “foundation”. Cooling method is the key to performance differences between normalizing and annealing.

Annealing uses slow furnace cooling (or pit/sand cooling for special cases). For carbon steel, the rate is 20–30℃/h. It takes over 20 hours to cool from 800℃ to 200℃. This minimizes internal stress.

Normalizing uses air cooling. It is 3–5 times faster than annealing. You can adjust it slightly by stacking workpieces. A 100mm thick 45# steel piece only takes 4–6 hours to cool from 850℃ to room temperature.

Cooling rate directly affects hardness. Annealed 45# steel has a hardness of 150–180HB. Normalized 45# steel has 180–220HB. This meets cutting and load-bearing needs.

3.Equipment and Cost: Trade-off Between Efficiency and Investment

Process differences cause gaps in cost and efficiency.

Annealing uses furnaces for 8–12 hours per batch, including heating, holding and cooling. It uses more energy and reduces furnace turnover. The unit cost for 45# steel is about 1.2 yuan/kg.

Normalizing cuts batch time to 4–6 hours. Workpieces cool in air after heating and holding. This doubles furnace turnover with no extra energy cost. Its unit cost is 20–30% lower (0.8–1.0 yuan/kg for 45# steel). For mass-produced standard parts, it saves hundreds of thousands of yuan yearly. So it’s more popular for bulk production.

IV. Core Performance Differences: Chain Reactions from Structure to Application

1.Differences in Microstructure

Metal performance depends on its internal structure. You can easily see the structure differences between normalizing and annealing.

(1)Annealed microstructure

• Feature: Coarse but uniform.
• Hypoeutectoid steel (e.g., 45# steel): Has evenly mixed ferrite and pearlite. Grain size is about 10-12μm.
• Hypereutectoid steel (e.g., T12 steel): Tends to form network-like secondary cementite on pearlite. This makes the material less tough.
• Improvement method: Use spheroidizing annealing. It turns cementite into spherical shapes. This makes the material easier to process.

(1)Normalized microstructure

• Feature: Fine and dense.
• Grain size: 1-2 grades smaller than annealing. 45# steel after normalizing has grains only 6-8μm.
• Hypoeutectoid steel: Fine ferrite and fine pearlite are twisted together.
• Hypereutectoid steel: Carbides dissolve completely. Fast cooling forms scattered secondary cementite. No network structure remains.
• Key advantage: Fine grains make normalized workpieces stronger. This follows the “fine grain strengthening” rule. The Hall-Petch Equation says: Finer grains mean higher strength.

2.Differences in Mechanical Properties

Taking 45# steel, which is widely used, as an example, the mechanical properties of normalizing and annealing are significantly different. The specific data are shown in the following table:

Performance Index	Annealing (45# Steel Example)	Normalizing (45# Steel Example)	Core Reason for Difference
Hardness (HB)	150-180	180-220	Normalized structure is finer with higher pearlite content
Tensile Strength (MPa)	≥570	≥600	Fine grain strengthening effect, finer grains in normalized steel
Elongation (%)	≥16	≥14	Annealed structure is softer with better plasticity
Impact Toughness (J/cm²)	≥70	≥60	Annealing eliminates internal stress more thoroughly with better toughness

The data shows that annealed workpieces in plasticity and toughness, while normalized ones are stronger and harder.

This difference guides scenario-based selection: choose annealing for impact-loaded workpieces and normalizing for medium-load structural parts.

V. Scenario-Based Selection: Application Scope and Decision Logic of Normalizing and Annealing

1.Core Scenarios for Prioritizing Annealing

• Eliminate severe internal stress: Suitable for large castings (e.g., machine tool beds over 5m) and welded parts (e.g., pressure vessel heads). Mandatory stress relief per GB/T 15749 to avoid post-processing deformation or in-use cracking.
• Pretreatment for difficult cold working: Ideal for cold-drawn steel wire, deep-drawn steel plates, etc. Reduces hardness to below 180HB. Example: 5mm cold-drawn steel wire – elongation increases from 10% to 20%, fracture rate drops from 15% to 1%.
• Special performance needs: Enhances toughness to over 80J/cm² for low-temperature steel (e.g., polar ship hulls). Refines grains for tool steel before quenching to prevent cracking.

2.Core Scenarios for Prioritizing Normalizing

• Economical pretreatment for mass production: Cost-effective for batch-produced parts (e.g., auto gear blanks, bolts). Example: 20CrMnTi gear blanks – batch time cut from 10h to 5h, annual savings of 1.2 million yuan.
• Improve mechanical properties: Balances strength and toughness for medium-load parts (e.g., crane hooks, drive shafts). Example: Q355B drive shafts – tensile strength ≥600MPa, impact toughness ≥60J/cm² after normalizing.
• Eliminate defective structures: Fixes Widmanstatten structure in castings and coarse grains in forgings. Example: Aluminum alloy forgings – grains refined, surface roughness reduced from Ra12.5 to Ra6.3, machinability improved.

3.Core Logic for Selection Decision: Four-Step Judgment Method

Step 1: Clarify core needs

• Prioritize cost/efficiency: Choose normalizing (for mass-produced ordinary parts).
• Prioritize special performance: Choose annealing (for precision/large parts needing high flexibility and no stress).

Step 2: Analyze workpiece characteristics

• Size: Large parts → annealing.
• Material: High-carbon steel needs proper cementite shape. Hypereutectoid steel uses spheroidizing annealing.
• Subsequent process: Cold drawing/deep drawing → annealing. Rough turning/milling → normalizing.

Step 3: Refer to standards and experience

• Follow material specs: 45# steel structural parts → normalizing. Precision parts → annealing.
• Learn from others: Use mature experience of similar factories. Avoid repeated mistakes.

Step 4: Small-batch trial and error

• Key parts: Test small batches first.
• Check indicators: Hardness, mechanical properties.
• Mass production: Start only after meeting requirements. This avoids large-scale scrap.

VI. Typical Application Cases: Practical Comparison Between Normalizing and Annealing

1.Case 1: Heat Treatment Selection for Automobile Gearbox Gears

• Workpiece: 20CrMnTi alloy steel (needs pretreatment before carburizing/quenching + precision cutting; requires uniform carburization and low tool wear).
• Process: Spheroidizing annealing (860-880℃, 2h hold, furnace cool to 600℃ then air cool).
• Why: Hardness drops to 160-180HB; ensures uniform carburization and 40% less tool wear (surface roughness Ra1.6). Normalizing would cause high hardness and uneven carburization.

2.Case 2: Heat Treatment Selection for High-Strength Bolts for Construction

• Workpiece: Q355B steel, M24×150mm (1 million pieces/year; needs balanced strength and efficiency).
• Process: Normalizing (900-920℃, 1h hold, air cool).
• Why: Hardness 190-210HB (meets thread cutting); tensile strength ≥600MPa (complies with GB/T 1228). Cuts batch time from 8h to 4h, saves 300,000 yuan/year, and boosts yield strength by 5%.

3.Case 3: Heat Treatment Selection for Large Machine Tool Beds

• Workpiece: HT250 gray cast iron (5m×1.2m×0.8m; high casting stress, needs stable precision).
• Process: Stress relief annealing (550-600℃, 4h hold, furnace cool to 200℃ then remove).
• Why: Reduces residual stress by over 80%; guideway flatness error ≤0.02mm/m (stable after 1 year). Normalizing fails to eliminate stress and increases cracking risk.

VII. Conclusion: Precise Selection is the “Core Competitiveness” of Heat Treatment

Normalizing and annealing are complementary “twin technologies”.

Normalizing excels at efficiency, economy, and balanced performance (ideal for mass-produced ordinary parts).

Annealing offers low hardness, high plasticity, and thorough stress relief (perfect for precision/large parts).

The key is to match the process to workpiece characteristics and needs. As ASM International states in Materials Performance Handbook: “The best heat treatment process is always the one most suitable for specific scenarios.”