NM400 vs HARDOX400 – Composition, Heat Treatment, Properties, and Applications

Introduction

Engineers, procurement managers, and production planners regularly face the choice between commercially available wear-resistant steels when designing components subject to abrasion, impact, and cyclic loading. The selection dilemma often centers on trade-offs such as guaranteed mechanical performance versus price and local availability, or weldability and fabrication ease versus long-term wear life.

NM400 and HARDOX400 appear side-by-side in many vendor quotes because they target the same service envelope: wear-resistant plate with a nominal "400" hardness class. The core practical distinction lies in provenance and supply model: HARDOX400 is a proprietary, tightly specified product from a global supplier with defined process control and documented mechanical guarantees; NM400 is a widely produced (non-proprietary) wear-resistant grade supplied by multiple mills with greater variability in chemistry and processing. That distinction drives differences in guaranteed properties, recommended fabrication procedures, and pricing.

1. Standards and Designations

• HARDOX400: Proprietary wear plate from SSAB (commonly referenced by producer name and the nominal hardness class). Classified as a quenched-and-tempered wear-resistant plate (HSLA-type, high hardness).
• NM400: Generic wear-resistant grade (NM = "wear" designation in some domestic standards). Typically supplied under national standards/specifications from regional mills; often considered a high-strength wear plate (HSLA-type).
• Other relevant standards where equivalent or similar materials appear: EN (European), JIS (Japanese), ASTM/ASME (US), GB (Chinese national standard) — note that direct one-to-one equivalence is not automatic because of differences in guaranteed mechanical properties, testing, and heat-treatment practice.
• Material type: Both are non-stainless, low-alloy, quenched-and-tempered wear steels (HSLA-like in application).

2. Chemical Composition and Alloying Strategy

Table: qualitative presence of common alloying elements

Element	HARDOX400 (typical strategy)	NM400 (typical strategy)
C	Low–moderate (controlled for toughness and weldability)
Mn	Low–moderate (for hardenability and tensile strength)
Si	Low (deoxidation; influences strength)
P	Trace/controlled (kept low for toughness)
S	Trace/controlled (kept low for machinability/toughness)
Cr	Low (may be present in small amounts to help hardenability/wear)
Ni	Low/trace (occasionally used to boost toughness)
Mo	Trace/low (to increase hardenability and temper resistance)
V	Trace (microalloying to refine grain)
Nb	Trace (microalloying to stabilize austenite grain size)
Ti	Trace (deoxidation, grain control)
B	Very low/trace (if used, improves hardenability at ppm levels)
N	Trace (controlled; influences inclusions, toughness)

Notes: - Exact chemistry varies by manufacturer and specification. Proprietary products typically maintain tighter limits and mill-to-mill consistency. - Alloying philosophy: both grades use low carbon with microalloying and small additions of Cr/Mo/Ni to achieve a martensitic or tempered martensitic matrix after quench-and-temper processing while keeping weldability and toughness acceptable. NM400 compositions are generally similar in type but can have broader ranges depending on the supplier.

How alloying affects properties: - Carbon and Mn increase hardenability and strength but raise susceptibility to cold cracking and reduce weldability if not controlled. - Microalloying elements (V, Nb, Ti) refine grain size and improve toughness without large increases to carbon equivalent. - Cr, Mo, and Ni at low levels increase hardenability and temper resistance, improving wear life at elevated stresses.

3. Microstructure and Heat Treatment Response

• Typical microstructure: Both grades are manufactured to produce a predominantly martensitic or tempered martensitic microstructure in the plate body. The microstructure is achieved by controlled quenching from the austenitizing temperature and subsequent tempering to balance hardness and toughness.
• HARDOX400: Produced using strictly controlled quench-and-temper cycles and, in some product families, thermo-mechanical rolling followed by quenching. The result is a fine-grained, uniformly tempered martensite with predictable transition properties through the plate thickness.
• NM400: Often produced by local mills with quench-and-temper or accelerated cooling; microstructure can be similar but may show greater variability in grain size or retained austenite depending on process control.
• Response to thermal processing:
• Normalizing will refine grain size and can improve toughness but will not typically produce the same hardness class as a proper quench-and-temper sequence.
• Quenching and tempering: Increases hardness and strength; tempering temperature controls final toughness/hardness balance—higher tempering temperature reduces hardness and raises toughness.
• Thermo-mechanical controlled processing (TMCP): Improves toughness at given hardness by producing favorable grain and dislocation structures; commonly used for high-quality proprietary wear plates.

4. Mechanical Properties

Table: comparative mechanical property descriptors

Property	HARDOX400	NM400
Tensile strength	High and tightly guaranteed; consistent across heat lots
Yield strength	High; producer often specifies minimums for plate thickness
Elongation	Moderate; retained ductility to resist brittle fracture
Impact toughness	Generally better controlled, with documented Charpy values at specified temperatures
Hardness	Nominal "400" class (producer-specified hardness target with tight tolerances); good through-thickness uniformity

Interpretation: - HARDOX400 is engineered and supplied with tighter, certified mechanical property windows. That typically translates into higher confidence that specified tensile, yield, and toughness values will be met in service. - NM400 aims at similar hardness and strength levels but can exhibit broader variability depending on the mill’s process control and certification level. - In practice, the two materials can be close in hardness, but HARDOX400 commonly offers superior guaranteed toughness and through-thickness consistency.

5. Weldability

Weldability depends on carbon content, carbon equivalent, and microalloying content. Use these common assessment formulas (interpret qualitatively):

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

$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Both grades have comparatively low carbon equivalents by design, but microalloying and small Cr/Mo additions increase hardenability. This can make the heat-affected zone (HAZ) susceptible to forming hard martensite and to cold cracking unless appropriate preheat, interpass temperature control, and post-weld heat treatment are applied. - HARDOX400 suppliers publish detailed welding instructions (recommended filler metals, preheat, interpass temperatures, and PWHT) because production batches are controlled and predictable. - NM400 may require more conservative welding practices (higher preheat, lower heat input, controlled interpass) if supplier data are limited. For critical welded structures, ask the mill for welding guidelines and qualification records. - Practical advice: use matching or slightly overmatching filler metals with good toughness, control heat input, and perform PWHT when required by the specific welding procedure specification (WPS).

6. Corrosion and Surface Protection

• Neither HARDOX400 nor NM400 is stainless steel; both are carbon-alloy wear steels and thus rely on surface protection for corrosion-sensitive environments.
• Typical surface protection strategies: hot-dip galvanizing (limited feasibility on large, thick plates), paint systems, epoxy linings, sacrificial coatings, or wear overlays (hardfacing).
• PREN (Pitting Resistance Equivalent Number) is not applicable to these non-stainless grades, but for completeness:

$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$

• Use PREN only when evaluating corrosion resistance of stainless alloys; for NM400/HARDOX400, focus on coating selection and environmental control (moisture, salt, acidic conditions).

7. Fabrication, Machinability, and Formability

• Cutting: Both grades are best cut with plasma, oxy-fuel (thicker sections), or laser for thinner plates; abrasive cutting is common for field trimming. Hardness increases tool wear—expect higher consumable replacement rates.
• Machinability: Hardness limits conventional machining; carbide tooling, rigid fixturing, and controlled feeds/speeds are required. HARDOX product data often provide guidance for machining allowances.
• Formability: Bending/forming at room temperature is limited by hardness/class; forming methods such as hot forming or pre-bend/heat-assisted techniques may be required. Both materials should be handled per supplier recommendations to avoid cracking.
• Finishing: Grinding, drilling, and tapping require specialized tooling and slower feed to avoid work hardening and tool failure.

8. Typical Applications

Table: typical uses

HARDOX400	NM400
Excavator buckets, dump truck bodies, wear liners, crusher jaws, chutes where guaranteed wear life and predictable performance are critical	Similar wear parts: buckets, liners, hoppers, screens; used where cost and local availability are primary drivers
High-value OEM components where supplier traceability, certification, and long-term service data are demanded	Local fabrication where competitive pricing and rapid supply are important
Applications requiring documented through-thickness toughness or specific Charpy energy values	Applications with less critical certification needs or shorter expected service life

Selection rationale: - Choose material based on wear mode (sliding vs impact), required toughness, weldability needs, supplier certification, and life-cycle cost. For abrasive sliding wear with predictable loading, either grade can be used; for impact-loaded components or safety-critical structures, the tighter property guarantees of HARDOX400 are often preferred.

9. Cost and Availability

• Cost: Proprietary, branded products (HARDOX400) commonly command a premium due to guaranteed properties, traceability, and global support. Generic grades (NM400) are often lower cost but may require additional inspection and qualification.
• Availability: NM400 is widely produced by regional mills and may be more readily available locally in certain markets with shorter lead times. HARDOX400 is globally available through the producer’s distribution but may involve longer lead times or higher logistics cost in some regions.
• Product forms: Both are available as plates in various thicknesses; HARDOX product families may offer broader options in plate thickness, quenched/tempered ranges, and fabricated components.

10. Summary and Recommendation

Table: high-level comparison

Attribute	HARDOX400	NM400
Weldability	Good with published welding procedures; supplier guidance available	Acceptable but may need more conservative WPS and verification
Strength–Toughness balance	Tightly controlled, consistent; better documented notch/impact performance	Comparable hardness but potentially more variability in toughness
Cost	Higher (premium for brand, certification, and consistency)	Lower (competitive local pricing, variable quality)

Conclusion — choose based on application risk profile: - Choose HARDOX400 if you require certified, repeatable wear performance with documented toughness and dimensional consistency, if the application is safety- or life-critical, or when long-term total cost of ownership (replacement downtime, labor costs) favors a higher-performance plate. - Choose NM400 if the project emphasizes lower initial material cost, fast local availability, or the application is less critical and can tolerate greater variability, provided you qualify the supplier, request material test reports, and adopt conservative fabrication practices.

Final practical recommendations: - Request mill certificates (chemical and mechanical), Charpy impact results where toughness matters, and welding guidelines before purchase. - For welded assemblies, qualify WPS with representative plate and joint geometry; use preheat and controlled heat input to avoid HAZ cracking. - Consider life-cycle costing: higher initial material cost may be offset by longer wear life and lower maintenance for high-quality, proprietary wear plates.