Cold Work Strengthened Non-Quenched and Tempered Steel: A Green Manufacturing Solution for High-Strength Fasteners

Cold Work Strengthened Non-Quenched and Tempered Steel: A Green Manufacturing Solution for High-Strength Fasteners

Sustainable High-Strength Fasteners for Automotive, Construction, and Industrial Engineering Applications

In today’s fast-growing fastener industry, manufacturers are under increasing pressure to balance high mechanical performance, cost efficiency, and low-carbon manufacturing. In this context, a new generation of material, known as non-quenched and tempered steel (NQT steel), has emerged as a breakthrough solution.

Unlike conventional quenched and tempered steels used in high-strength fasteners, NQT steel eliminates multiple energy-intensive heat treatment steps while still achieving 10.9-grade high-strength fastener performance through advanced microalloying and controlled processing routes.

This article provides a detailed engineering analysis of the application of NQT steel in high-strength bolts, structural fasteners, and cold-formed components, focusing on materials science, processing routes, and industrial applications.

1. The Bottleneck of Conventional Heat Treatment in High-Strength Fasteners

Traditional manufacturing of high-strength bolts, such as ISO 898-1 10.9 and 12.9 grade fasteners, typically requires a full heat treatment route:

• Spheroidizing annealing

• Cold heading / cold forging

• Quenching + high-temperature tempering

Among these processes, quenching and tempering alone account for more than 40% of total energy consumption, making it the most expensive and carbon-intensive stage of production.

In addition, conventional heat treatment often introduces:

• Surface decarburization

• Oxidation scale formation

• Dimensional distortion

• Residual stress instability

For high-precision applications such as automotive and structural engineering, these issues directly affect fatigue reliability and dimensional consistency.

 The Breakthrough: Non-Quenched and Tempered Steel (NQT Steel)

Non-quenched and tempered steel eliminates the need for quenching and tempering by using:

• Microalloying with Nb, V, Ti

• Controlled rolling and controlled cooling (TMCP process)

• Precipitation strengthening during hot deformation

• Grain refinement strengthening

• Cold work strengthening via cold drawing or cold heading

This allows the material to achieve 10.9-grade mechanical properties without post-forging heat treatment, significantly improving production efficiency and environmental performance.

According to ISO 898-1 and ISO 898-2 fastener mechanical standards, NQT steel is increasingly recognized as a viable material pathway for high-strength cold-formed fasteners.

In China, standards such as (Cold heading and cold extrusion steel) define industrial-grade NQT steels, including MFT8, MFT9, and MFT10, enabling standardized industrial adoption.

 2. Microstructure Design: The Key to Strength Without Quenching

The performance of non-quenched and tempered steel depends entirely on its microstructural architecture, which can be classified into three main types:

 2.1 Ferrite–Pearlite NQT Steel (Most Widely Used)

This is the most mature industrial form of NQT steel.

Key features:

• Nb–V microalloy precipitation strengthening

• Fine ferrite + pearlite structure

• Stable mechanical properties

• Excellent cold heading performance

Engineering applications:

• ISO 898-1 8.8 to 10.9 grade fasteners

• Structural bolts and standard industrial screws

Research from Shanghai University and Yonggang Group shows that:

MFT9 steel after 50% cold reduction + 300°C aging achieves

• Tensile strength: ~1038 MPa

• Yield strength: ~978 MPa

Fully meeting 10.9-grade high-strength fastener requirements.

2.2 Bainitic NQT Steel (High Toughness Applications)

This grade uses:

• Low-carbon + high-manganese alloy design

• Controlled air cooling transformation

• Granular bainite microstructure

Advantages:

• Excellent low-temperature toughness

• High strength–ductility balance

• Improved impact resistance

Typical applications:

• Construction machinery fasteners

• Railway structural bolts

• Cold climate engineering systems

 2.3 Ferrite–Martensite Dual-Phase NQT Steel

A more advanced structure with:

• 10%–30% martensite phase

• Strong work-hardening capability

• Low yield ratio (improved safety margin)

Performance benefits:

• High strength equivalent to quenched-tempered steel

• Excellent deformation adaptability

• Superior fatigue resistance potential

 3. Cold Work Strengthening: The Core of NQT Steel Performance

Cold deformation (cold drawing or cold heading) is the most critical step in strengthening NQT fasteners.

Strengthening Mechanism

Cold deformation increases strength through:

• Dislocation multiplication

• Dislocation interaction and entanglement

• Strain hardening accumulation

As deformation increases, strengthening follows a nonlinear growth pattern.

 Engineering Data Example (MFT9 Steel)

After 50% reduction in cold drawing:

• Tensile strength: 715 MPa → 997 MPa (+282 MPa)

• Yield strength: 509 MPa → 910 MPa (+401 MPa)

• Hardness: 219 HV → 286 HV

Dislocation density increases from:

• 5.10 × 10¹⁴ cm⁻² → 1.14 × 10¹⁶ cm⁻²

This represents more than a 20× increase in dislocation density, which is directly responsible for the strength improvement.

 Low-Temperature Aging Stabilization

Aging treatment at 250–400°C plays a critical role:

• Carbon and nitrogen atoms diffuse and pin dislocations

• Formation of Cottrell atmospheres

• Stabilization of cold work structure

At optimal condition (300°C × 2h):

• Tensile strength increases further (~+41 MPa)

• Ductility improves (elongation restored from 11% to 17%)

However, overheating may cause over-aging and carbide coarsening, reducing strength.

 4. Industrial Production Control Requirements

To ensure stable industrial-scale production of NQT fasteners, strict control is required in three key areas:

 4.1 Steel Cleanliness Control

Impurity elements must be tightly controlled:

• S ≤ 0.008%

• P ≤ 0.018%

• Total oxygen ≤ 15 ppm

Microalloying elements:

• Nb + V ≥ 0.05%

These elements form nano-scale carbonitrides (~50 nm), providing:

• Precipitation strengthening

• Hydrogen trapping effect (improves delayed fracture resistance)      

 4.2 Microstructure Uniformity Control

• Banding structure ≤ Grade 3

• Hardness variation ≤ 2 HRC

• End quench hardenability fluctuation ≤ ±5 HBW

Uniformity is essential for consistent cold heading behavior.

 4.3 Grain Size and Texture Control

• Grain size requirement: ≥ ASTM grain size 8

• After heavy cold deformation:

• Grain orientation transforms into dominant 〈110〉 fiber       texture

• Improved structural alignment

• Enhanced load-bearing stability

This structural refinement significantly improves tensile strength and consistency in fastener manufacturing.

 5. Industrial Applications of Non-Quenched Fasteners

Non-quenched and tempered steel is now widely used in:

Automotive Industry

• Engine compartment bolts

• Suspension system fasteners

• Chassis structural connectors

Construction Engineering

• Steel structure bolts

• High-rise building connectors

• Preloaded structural fasteners

Heavy Machinery

• Excavator structural bolts

• Mining equipment connectors

• Hydraulic system fasteners

Rail and Energy Systems

• Wind turbine bolts

• Rail transit fastening systems

• Power plant structural assemblies

 Key Engineering Advantage

For large-diameter bolts (≥ M20) and long slender fasteners:

• Eliminates quenching deformation

• Improves dimensional stability

• Reduces production distortion risk

This makes NQT steel especially valuable for precision structural applications.

6. Future Development Trends

The next generation of non-quenched steel development focuses on:

• Higher strength grades (beyond 13.9 level performance      potential)

• Improved low-temperature toughness for extreme environments

• Reduced vanadium content for cost optimization

• Enhanced fatigue resistance through microstructure engineering

As global industries move toward low-carbon manufacturing and energy-efficient production, NQT steel is expected to become a mainstream material for high-strength fasteners.

 7. Conclusion: A Green Revolution in Fastener Manufacturing

Cold-worked, non-quenched, and tempered steel represents a major shift in fastener material engineering.

By combining:

• Controlled rolling and cooling (TMCP)

• Cold deformation strengthening

• Low-temperature aging stabilization

Manufacturers can achieve 10.9-grade high-strength fasteners without quenching and tempering, significantly reducing energy consumption and production cost.

For industrial buyers and engineering designers, this material system provides:

• Lower carbon footprint

• Higher production efficiency

• Stable mechanical performance

• Competitive lifecycle cost advantage

 Industrial Fastener Engineering Solutions

Guangzhou Juxin Development Co., Ltd. specializes in advanced high-strength fasteners, cold-formed bolts, and engineered fastening solutions, supporting global industries with ISO/DIN-standard compliant products designed for high-performance and sustainable manufacturing systems.

Packaging Standard

At Juxin Fasteners, we apply standardized export packaging to ensure product protection, traceability, and compliance with international logistics requirements.

1. Standard Export Packaging

Unless otherwise specified, all products will be packed according to our factory standard export packaging, which includes:

Moisture-resistant inner protection

Poly bag or small box packing as required

Reinforced export cartons

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Palletizing for sea or air shipment when necessary

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Private labeling

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All packaging processes are controlled under our ISO 9001 quality management system to ensure consistency, traceability, and product integrity throughout the supply chain.