NM360 vs NM400 – Composition, Heat Treatment, Properties, and Applications

Introduction

NM360 and NM400 are two commercially important abrasion-resistant (AR) steels used across mining, earthmoving, heavy machinery, and material handling sectors. Engineers and procurement professionals often face a classic selection dilemma: choosing a lower-cost, more formable grade versus a higher-hardness grade that extends wear life but can complicate fabrication and welding. Typical decision contexts include balancing lifecycle cost (wear life and replacement intervals) against fabrication complexity (welding pre-heat, post-weld treatment), and matching toughness to impact conditions.

The principal distinction between NM360 and NM400 is their target hardness level and the downstream implications of that difference. Both are high-strength, low-alloy steels designed for wear resistance, but NM400 is specified to a higher hardness class than NM360, which influences alloying, hardenability, mechanical properties, and fabrication practices. Because hardness correlates strongly with wear resistance and changes demands on welding, forming, and toughness, engineers commonly compare these two when optimizing equipment life and manufacturability.

1. Standards and Designations

• Common national and industry standards where equivalent grades or specifications appear:
• Chinese standards (NM-series): used in domestic and some international supply chains (NM360, NM400).
• EN / ISO: wear-resistant steels are often specified by hardness (e.g., HARDOX equivalents) rather than direct one-to-one designations.
• JIS and other national standards: similar approach, often referenced by nominal hardness.

• ASTM/ASME: no direct NM360/NM400 normative ASTM designation; AR (abrasion-resistant) steels are often supplied to proprietary or vendor standards, or referenced by hardness and chemical requirements.

• Material classification:

• Both NM360 and NM400 are low-alloy, high-strength abrasion-resistant steels (not stainless, not tool steels). They are typically considered HSLA (high-strength low-alloy) types formulated to deliver a wear-resistant microstructure through controlled chemistry and processing.

Note: Exact chemical and mechanical limits for NM grades may vary by producer and by national specification; always confirm with the mill certificate (MTC).

2. Chemical Composition and Alloying Strategy

Explanatory note: Suppliers vary exact chemistries to hit a target hardness (Brinell) and achieve desired toughness. NM400 typically contains adjustments (slightly higher carbon and/or microalloying and controlled alloying elements) to produce a harder, tougher martensitic or bainitic surface after rolling/heat treatment.

How alloying affects performance: - Carbon and alloying elements (Cr, Mo, Mn, B) increase hardenability and potential hardness, improving abrasive wear resistance. - Microalloying (V, Nb, Ti) refines grain and supports a balance between strength and toughness. - Excessive carbon or hardenability can increase risk of brittle behavior and create weldability constraints (requirement for preheat/interpass control, PWHT in thick sections).

3. Microstructure and Heat Treatment Response

• Typical microstructures:
• As-rolled and normalized NM360: a mix of tempered martensite, bainite, and some ferrite—designed to provide a combination of hardness and toughness.

• As-rolled and normalized NM400: higher proportion of martensitic/bainitic microstructure and finer grains due to increased hardenability and thermo-mechanical control, producing higher hardness.

• Heat treatment and processing routes:

• Normalizing: raises toughness and homogenizes microstructure; both grades respond well to normalization to reduce residual stresses and improve toughness.
• Quenching and tempering: used selectively for parts requiring higher surface hardness; straightforward quench hardening in thicker plates is limited by cracking risk—NM400 formulations are often tuned to reach target hardness with rolling and controlled cooling rather than aggressive post-heat treatment.
• Thermo-mechanical controlled processing (TMCP): commonly used to produce the desired combination of hardness and toughness in plate production for both NM360 and NM400. TMCP allows lower carbon content for a given hardness, improving weldability and toughness.

Processing implications: - NM400’s higher hardenability makes it more sensitive to cooling rates; faster cooling can form harder martensite requiring tempering to adjust toughness. - Lower-carbon NM360 can be easier to weld and form, but may offer shorter wear life under the same conditions.

4. Mechanical Properties

Interpretation: - NM400 is generally the stronger and harder plate; it will typically provide superior abrasive wear resistance due to increased hardness. - NM360 tends to be more ductile and can show better formability and easier weldability compared with NM400. - Actual tensile, yield, and impact values depend on plate thickness, heat treatment, and supplier-specific chemistry; always verify with mill test certificates and material certificates.

5. Weldability

Weldability depends on chemical composition (primarily carbon and equivalent parameters) and the component’s thickness and expected thermal cycles. Two common empirical indicators:

• IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• International Pcm (more conservative for PWHT considerations): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Higher $CE_{IIW}$ or $P_{cm}$ values indicate greater propensity to form hard martensite in the heat-affected zone (HAZ) and hence a higher risk of cold cracking—requiring preheat, controlled interpass temperature, and possibly PWHT. - Because NM400 typically targets higher hardness and higher hardenability, its carbon equivalent is often slightly higher than NM360; therefore NM400 can be more demanding to weld, particularly in thicker sections. - Practical guidance: - Use low-hydrogen consumables, appropriate preheat, and controlled interpass temperatures for both grades when thicknesses are significant. - For critical assemblies, perform welding procedure qualification (WPQR) and post-weld mechanical testing (e.g., HAZ toughness). - Where possible, select NM grades with optimized low-carbon chemistries and microalloying to reduce welding complexity.

6. Corrosion and Surface Protection

• These NM grades are non-stainless; corrosion resistance is typical of mild, low-alloy steels. Corrosion protection strategies include:
• Painting/coating systems (epoxy, polyurethane) for atmospheric protection.
• Hot-dip galvanizing is sometimes used for atmospheric corrosion protection, but galvanizing thick AR plate can be non-trivial; consider surface condition and dimensional tolerances.

• Metallurgical surface hardening or overlay welding (e.g., weld cladding) for both wear and corrosion in specific service environments.

• PREN (pitting resistance equivalent number) is specific to stainless alloys and is not applicable to NM360/NM400. For stainless alloys the index is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ Use PREN only when evaluating stainless materials; NM grades are evaluated for wear and corrosion separately.

7. Fabrication, Machinability, and Formability

• Cutting:
• Both grades can be plasma, oxy-fuel, or laser cut; NM400’s higher hardness may increase tool wear and require slower cutting parameters.
• Forming and bending:
• NM360 offers better cold formability and bendability; NM400 requires larger bend radii and may need special tooling or heating for tight bends.
• Machinability:
• Higher hardness in NM400 reduces machinability; expect faster tool wear and possibly lower feed rates or carbide tooling requirements.
• Surface finishing:
• Grinding, shot-blasting, and surface dressing are common; NM400 will generally demand more aggressive or durable abrasives.

Practical tip: When forming or bending, perform sample trials and account for springback and potential for cracking—especially with NM400 where localized hard zones can initiate cracks.

8. Typical Applications

Selection rationale: - Choose NM360 where forming, welding ease, or marginal cost advantages matter and where the wear regime is less severe or impact-dominated rather than sliding abrasion. - Choose NM400 where abrasive wear dominates, and extended in-service life from higher hardness justifies the higher procurement and fabrication controls.

9. Cost and Availability

• Relative cost: NM400 is typically more expensive per tonne than NM360 due to tighter chemistry control, processing to higher hardness, and potential premium for proven wear performance.
• Availability: Both grades are widely available from plate mills and specialty metal distributors. Availability by thickness, width, and plate finish can vary regionally—NM360 may be more available in wider product choices for forming applications, while NM400 is commonly stocked in standard plate sizes and thicknesses for heavy wear parts.
• Product form: Plates, fabricated liners, and machined cast replacements are common supply forms. Lead-times and MOQ (minimum order quantities) depend on producer and market demand.

10. Summary and Recommendation

Choose NM360 if: - Your application requires easier forming or more straightforward welding (lower fabrication cost). - Abrasion conditions are moderate or impact-dominant where ductility and toughness mitigate damage. - Lifecycle cost modeling shows replacement intervals under NM360 are acceptable.

Choose NM400 if: - Abrasive wear is the primary failure mode and maximizing in-service life is critical. - You can accommodate stricter welding and forming controls and accept higher upfront cost for reduced downtime and part replacement. - Design constraints permit larger bend radii or alternative fabrication strategies (e.g., using bolted liners instead of forming).

Final note: NM360 and NM400 are best selected after a site-specific assessment of wear mechanisms (sliding abrasion, gouging, impact), part geometry, and fabrication capabilities. Always request the mill test certificate (chemical and mechanical values) and, when welding critical components, qualify welding procedures and verify HAZ toughness for the exact plate batch.